Table of contents

We extend and apply a model-independent analysis method developed earlier by Daly & Djorgovski to new supernova, radio galaxy, and galaxy cluster samples to study the acceleration history of the universe and the properties of the dark energy. There is good agreement between results obtained with radio galaxies and supernovae, suggesting that both distance indicators are reliable. The deceleration parameter q(z) is obtained assuming only the validity of the FRW metric, allowing for a range of values of space curvature, and independent of a gravity theory and the physical nature of the contents of the universe. We show that q₀ is independent of space curvature, and obtain q₀ = − 0.48 ± 0.11. The transition redshift when q₀ = 0 is z_T = 0.78^{+ 0.08}_−0.27 for zero space curvature, and has a weak dependence on space curvature. We find good agreement between model-independent quantities and those predicted by general relativity, indicating that GR provides a good description of the data over look-back times of ten billion years.

We have performed narrowband NB973 (bandwidth 200 Å centered at 9755 Å) imaging of the Subaru Deep Field (SDF) and found two z = 7 Lyα emitter (LAE) candidates down to NB973 = 24.9. Carrying out deep follow-up spectroscopy, we identified one of them as a real z = 6.96 LAE. This has established a new redshift record, showing that galaxy formation was in progress just 750 Myr after the big bang. Meanwhile, the Lyα line luminosity function of LAEs is known to decline from z = 5.7 to 6.6 in the SDF; L^* at z = 6.6 is 40%-60% that at z = 5.7. We also confirm that the number density of z = 7 LAEs is only 17% of the density at z = 6.6 comparing the latest SDF LAE samples. This series of significant decreases in LAE density with increasing redshift could be the result of galaxy evolution during these epochs. However, using the UV continuum luminosity functions of LAEs and Lyman break galaxies, and a LAE evolution model based on hierarchical clustering, we find that galaxy evolution alone cannot entirely explain the decrease in density. This extra density deficit might reflect the attenuation of the Lyα photons from LAEs by the neutral hydrogen possibly left at the last stage of cosmic reionization at z ∼ 6-7.

The large majority of EGRET point sources remain without an identified low-energy counterpart, and a large fraction of these sources are most likely extragalactic. Whatever the nature of the extragalactic EGRET unidentified sources, faint unresolved objects of the same class must have a contribution to the diffuse extragalactic gamma-ray background (EGRB). Understanding this component of the EGRB, along with other guaranteed contributions from known sources, is essential if we are to use this emission to constrain exotic high-energy physics. Here, we follow an empirical approach to estimate whether a potential contribution of unidentified sources to the EGRB is likely to be important, and we find that it is. In addition, we show how upcoming GLAST observations of EGRET unidentified sources, as well as of their fainter counterparts, can be combined with GLAST observations of the Galactic and extragalactic diffuse backgrounds to shed light on the nature of the EGRET unidentified sources even without any positional association of such sources with low-energy counterparts.

Heckman and coworkers used the GALEX UV imaging survey to show that there exists a rare population of nearby compact UV-luminous galaxies (UVLGs) that closely resemble high-redshift Lyman break galaxies (LBGs). We present HST images in the UV, optical, and Hα and resimulate them at the depth and resolution of the GOODS/UDF fields to show that the morphologies of UVLGs are also similar to those of LBGs. Our sample of eight LBG analogs thus provides detailed insight into the connection between star formation and LBG morphology. Faint tidal features or companions can be seen in all of the rest-frame optical images, suggesting that the starbursts are the result of a merger or interaction. The UV/optical light is dominated by unresolved (~100-300 pc) super starburst regions (SSBs). A detailed comparison with the galaxies Haro 11 and VV 114 at z = 0.02 indicates that the SSBs themselves consist of diffuse stars and (super) star clusters. The structural features revealed by the new HST images occur on very small physical scales and are thus not detectable in images of high-redshift LBGs, except in a few cases where they are magnified by gravitational lensing. We propose, therefore, that LBGs are mergers of gas-rich, relatively low-mass (M_* ∼ 10¹⁰M_☉) systems, and that the mergers trigger the formation of SSBs. If galaxies at high redshifts are dominated by SSBs, then the faint-end slope of the luminosity function is predicted to have slope α ∼ 2. Our results are the most direct confirmation to date of models that predict that the main mode of star formation in the early universe was highly collisional.

We present a comparison of the properties of a giant radio galaxy and the ambient intergalactic medium, whose properties are inferred from the large-scale distribution in galaxies. The double lobes of the radio galaxy MSH 05-22 are giant—1.8 Mpc projected linear size—and interact with the environment outside the interstellar medium and coronal halo associated with the host galaxy. The radio lobes appear to be relicts, and the double structure is asymmetric. We have examined the large-scale structure in the galaxy distribution surrounding the radio source. The host galaxy of MSH 05-22 is associated with a small group that lies close to the boundary of sheetlike and filamentary density enhancements, and adjacent to a void. Assuming that the galaxies trace gas, the asymmetries in the radio morphology in this case study appear related to the anisotropy in the medium. However, the observed overdensities and structure formation models for the heating of the intergalactic medium (IGM) suggest a density-temperature product for the IGM environment that is an order of magnitude below that expected from the properties of the radio source. The discordance suggests that even sources like MSH 05-22, which are observed in the relatively low-density IGM environment associated with the filamentary large-scale structure and have multiple signatures of being relicts, may be overpressured and evolving toward an equilibrium relaxed state with the ambient IGM. Alternately, it is speculated that astrophysical feedback originating in galaxy overdensities observed 1-2 Mpc to the north and northeast of MSH 05-22 might be the mechanism for the heating of the ambient IGM gas.

We present extensive ground-based spectroscopy and HST imaging of 3C 79, an FR II radio galaxy associated with a luminous extended emission-line region (EELR). Surface brightness modeling of an emission-line-free HST R-band image reveals that the host galaxy is a massive elliptical with a compact companion 0.8^'' away and 4 mag fainter. The host galaxy spectrum is best described by an intermediate-age (1.3 Gyr) stellar population (4% by mass), superimposed on a 10 Gyr old population and a power law (α_λ = − 1.8); the stellar populations are consistent with supersolar metallicities, with the best fit given by the 2.5 Z_☉ models. We derive a dynamical mass of 4 × 10¹¹M_☉ within the effective radius from the velocity dispersion. The EELR spectra clearly indicate that the EELR is photoionized by the hidden central engine. Photoionization modeling shows evidence that the gas metallicity in both the EELR and the nuclear narrow-line region is mildly subsolar (0.3-0.7 Z_☉), significantly lower than the supersolar metallicities deduced from typical active galactic nuclei in the Sloan Digital Sky Survey. The more luminous filaments in the EELR exhibit a velocity field consistent with a common disk rotation. Fainter clouds, however, show high approaching velocities that are uncoupled from this apparent disk rotation. The striking similarities between this EELR and the EELRs around steep-spectrum radio-loud quasars provide further evidence for the orientation-dependent unification schemes. The metal-poor gas is almost certainly not native to the massive host galaxy. We suggest that the close companion galaxy could be the tidally stripped bulge of a late-type galaxy that is merging with the host galaxy. The interstellar medium of such a galaxy is probably the source for the low-metallicity gas in 3C 79.

Opacity effects in relativistic sources of high-energy gamma-rays, such as gamma-ray bursts (GRBs) or blazars, can probe the Lorentz factor of the outflow as well as the distance of the emission site from the source and, thus, help constrain the composition of the outflow (protons, pairs, magnetic field) and the emission mechanism. Most previous works consider the opacity in steady state. Here we study time-dependent effects of the opacity to pair production (γ γ → e⁺e⁻) in impulsive relativistic sources. We present a simple, yet rich, semianalytic model for the time and energy dependence of the optical depth, τ_{γ γ}, in which a thin spherical shell expands ultrarelativistically and emits isotropically in its own rest frame over a finite range of radii, R₀ ⩽ R⩽ R₀ + Δ R. This is particularly relevant for GRB internal shocks. We find that for impulsive sources (Δ R≲ R₀), while the instantaneous spectrum has an exponential cutoff above the photon energy ε₁(T) where τ_{γ γ}(ε₁) = 1, the time-integrated spectrum has a power-law high-energy tail above the photon energy ε_1* ∼ ε₁(Δ T) where Δ T is the duration of the emission episode. Furthermore, photons with ε > ε_1* should arrive mainly near the onset of the spike or flare corresponding to the short emission episode, since in impulsive sources it takes time to build up the (target) photon field, and thus, τ_{γ γ}(ε) initially increases with time and ε₁(T) correspondingly decreases with time, so that photons of energy ε > ε_1* are able to escape the source mainly very early on while ε₁(T) > ε . As the source approaches a quasi-steady state (Δ R≫ R₀), the time-integrated spectrum develops an exponential cutoff, while the power-law tail becomes increasingly suppressed.

We study the stellar and star formation properties of the host galaxies of 58 X-ray-selected AGNs in the GOODS portion of the Chandra Deep Field South (CDF-S) region at z ∼ 0.5-1.4. The AGNs are selected such that their rest-frame UV to near-infrared spectral energy distributions (SEDs) are dominated by stellar emission; i.e., they show a prominent 1.6 μm bump, thus minimizing the AGN emission "contamination." This AGN population comprises approximately 50% of the X-ray-selected AGNs at these redshifts. We find that AGNs reside in the most massive galaxies at the redshifts probed here. Their characteristic stellar masses (M_* ∼ 7.8 × 10¹⁰ and M_* ∼ 1.2 × 10¹¹M_☉ at median redshifts of 0.67 and 1.07, respectively) appear to be representative of the X-ray-selected AGN population at these redshifts and are intermediate between those of local type 2 AGNs and high-redshift (z ∼ 2) AGNs. The inferred black hole masses (M_BH ∼ 2 × 10⁸M_☉) of typical AGNs are similar to those of optically identified quasars at similar redshifts. Since the AGNs in our sample are much less luminous (L_{2–10 keV} < 10⁴⁴ erg s⁻¹) than quasars, typical AGNs have low Eddington ratios (η ∼ 0.01-0.001). This suggests that, at least at intermediate redshifts, the cosmic AGN "downsizing" is due to both a decrease in the characteristic stellar mass of typical host galaxies and less efficient accretion. Finally, there is no strong evidence in AGN host galaxies for either highly suppressed star formation (expected if AGNs played a role in quenching star formation) or elevated star formation when compared to mass-selected (i.e., IRAC-selected) galaxies of similar stellar masses and redshifts.

Type IIn supernovae (SNe IIn) dominate the brightest supernova events in observed FUV flux (~1200-2000 Å). We show that multiband, multiepoch optical surveys complete to m_r = 27 can detect the FUV emission of ~25 z > 2 SNe IIn deg⁻² yr⁻¹ (rest frame, or ~10 SNe IIn deg⁻² yr⁻¹ observed frame) to 4 σ using a technique that monitors color-selected galaxies. Moreover, the strength and evolution of the bright emission lines observed in low-redshift SNe IIn imply that the Lyα emission features in ~70% of z > 2 SNe IIn are above the spectroscopic thresholds of 8 m class telescopes for ~2 yr (rest frame). As a result, existing facilities have the capability to both photometrically detect and spectroscopically confirm z > 2 SNe IIn and pave the way for efficient searches by future 8 m class survey and 30 m class telescopes. The method presented here uses the sensitivities and wide-field capabilities of current optical instruments and exploits (1) the efficiency of z > 2 galaxy color selection techniques, (2) the intrinsic brightness distribution (⟨ M_B⟩ = − 19.0,σ ± 0.9) and blue profile of SN IIn continua, (3) the presence of extremely bright, long-lived emission features, and (4) the potential to detect blueshifted SN Lyα emission shortward of host galaxy Lyα features.

We show that dynamical relaxation in the aftermath of a galactic merger and the ensuing formation and decay of a binary massive black hole (MBH) are dominated by massive perturbers (MPs) such as giant molecular clouds or clusters. MPs accelerate relaxation by orders of magnitude compared to two-body stellar relaxation alone, and efficiently scatter stars into the binary MBH's orbit. The three-body star-binary MBH interactions shrink the binary MBH to the point where energy losses from the emission of gravitational waves (GWs) lead to rapid coalescence. We model this process based on observed and simulated MP distributions and take into account the decreased efficiency of the star-binary MBH interaction due to acceleration in the galactic potential. We show that mergers of gas-rich galactic nuclei lead to binary MBH coalescence well within the Hubble time. Moreover, lower mass binary MBHs (<10⁸M_☉) require only a few percent of the typical gas mass in a postmerger nucleus to coalesce in a Hubble time. The fate of a binary MBH in a gas-poor galactic merger is less certain, although massive stellar structures (e.g., clusters, stellar rings) could likewise lead to efficient coalescence. These coalescence events are observable by their strong GW emission. MPs thus increase the cosmic rate of such GW events, lead to a higher mass deficit in the merged galactic core, and suppress the formation of triple-MBH systems and the resulting ejection of MBHs into intergalactic space.

Prior to the explosion of a carbon-oxygen white dwarf in a Type Ia supernova there is a long "simmering," during which the ¹²C +¹²C reaction gradually heats the white dwarf on a long (~10³ yr) timescale. Piro & Bildsten showed that weak reactions during this simmering set a maximum electron abundance Y_e at the time of the explosion. We investigate the nuclear reactions during this simmering with a series of self-heating, at constant pressure, reaction network calculations. Unlike in AGB stars, p captures onto ²²Ne and heavier trace nuclei do not play a significant role. The ¹²C abundance is sufficiently high that the neutrons preferentially capture onto ¹²C , rather than iron group nuclei. As an aid to hydrodynamical simulations of the simmering phase, we present fits to the rates of heating, electron capture, change in mean atomic mass, and consumption of ¹²C in terms of the screened thermally averaged cross section for ¹²C +¹²C . Our evaluation of the net heating rate includes contributions from electron captures into the 3.68 MeV excited state of ¹³C . This results in a slightly larger energy release, per ¹²C consumed, than that found by Piro & Bildsten, but less than that released for a burn to only ²⁰Ne and ²³Na . We compare our one-zone results to more accurate integrations over the white dwarf structure to estimate the amount of ¹²C that must be consumed to raise the white dwarf temperature, and hence to determine the net reduction of Y_e during simmering.

We present the results of an Hα near-infrared narrowband survey searching for star-forming galaxies at redshift z = 0.84. This work is an extension of our previous narrowband studies in the optical at lower redshifts. After removal of stars and redshift interlopers (using spectroscopic and photometric redshifts), we build a complete sample of 165 Hα emitters in the extended Groth strip and GOODS-N fields with L(H α) > 10⁴¹ ergs s⁻¹. We compute the Hα luminosity function at z = 0.84 after corrections for [N II] flux contamination, extinction, systematic errors, and incompleteness. Our sources present an average dust extinction of A(H α) = 1.5 mag. Adopting Hα as a surrogate for the instantaneous SFR, we measure an extinction-corrected SFR density of 0.17^{+ 0.03}_−0.03M_☉ yr⁻¹ Mpc⁻³. Combining this result to our prior measurements at z = 0.02, 0.24, and 0.40, we derive an Hα-based evolution of the SFR density proportional to (1 + z)^β with β = 3.8 ± 0.5. This evolution is consistent with that derived by other authors using different SFR tracers.

Large-scale asymmetries in the stellar mass distribution in galaxies are believed to trace nonequilibrium situations in the luminous and/or dark matter component. These may arise in the aftermath of events such as mergers, accretion, and tidal interactions. These events are key in the evolution of galaxies. In this paper we quantify the large-scale lopsidedness of light distributions in 25,155 galaxies at z < 0.06 from the Sloan Digital Sky Survey Data Release 4 using the m = 1 azimuthal Fourier mode. We show that the lopsided distribution of light is primarily due to a corresponding lopsidedness in the stellar mass distribution. Observational effects, such as seeing, Poisson noise, and inclination, introduce only small errors in lopsidedness for the majority of this sample. We find that lopsidedness correlates strongly with other basic galaxy structural parameters: galaxies with low concentration, stellar mass, and stellar surface mass density tend to be lopsided, while galaxies with high concentration, mass, and density are not. We find that the strongest and most fundamental relationship between lopsidedness and the other structural parameters is with the surface mass density. We also find, in agreement with previous studies, that lopsidedness tends to increase with radius. Both these results may be understood as a consequence of several factors. The outer regions of galaxies and low-density galaxies are more susceptible to tidal perturbations, and they also have longer dynamical times (so lopsidedness will last longer). They are also more likely to be affected by any underlying asymmetries in the dark matter halo.

We present results of integral field optical spectroscopy of five luminous blue compact dwarf galaxies. The data were obtained using the fiber system INTEGRAL attached to the William Herschel Telescope. The galaxies Mrk 370, Mrk 35, Mrk 297, Mrk 314, and III Zw 102 were observed. The central 33.6'' × 29.4'' regions of the galaxies were mapped with a spatial resolution of 2.7'' spaxel⁻¹, except for Mrk 314, in which we observed the central 16'' × 12'' region with a resolution of 0.9'' spaxel⁻¹. We use high-resolution optical images to isolate the star-forming knots in the objects; line ratios, electron densities, and oxygen abundances in each of these regions are computed. We build continuum and emission-line intensity maps, as well as maps of the most relevant line ratios: [O III] λ 5007/H β , [N II] λ 6584/H α , and H α /H β , which allow us to obtain spatial information on the ionization structure and mechanisms. We also derive the gas velocity field from the H α and [O III] λ 5007 emission lines. We find that all five galaxies are in the high end of the metallicity range of blue compact dwarfs with oxygen abundances varying from Z≃ 0.3 to 1.5 Z_☉. The objects show H II-like ionization in the whole field of view, except the outer regions of III Zw 102, whose large [N II] λ 6584/H α values suggest the presence of shocks. The five galaxies display inhomogeneous extinction patterns, and three of them have high H α /H β ratios, indicative of a large dust content; all of the galaxies display complex, irregular velocity fields in their inner regions.

Using the Gemini Near-Infrared Spectrograph (GNIRS), we have completed a near-infrared spectroscopic survey for K-bright galaxies at z ∼ 2.3 selected from the MUSYC survey. We derived spectroscopic redshifts from emission lines or from continuum features and shapes for all 36 observed galaxies. The continuum redshifts are driven by the Balmer/4000 Å break and have an uncertainty in Δ z/(1 + z) of <0.019. We use this unique sample to determine, for the first time, how accurately redshifts and other properties of massive high-redshift galaxies can be determined from broadband photometric data alone. We find that the photometric redshifts of the galaxies in our sample have a systematic error of 0.08 and a random error of 0.13 in Δ z/(1 + z) . The systematic error can be reduced by using optimal templates and deep photometry; the random error, however, will be hard to reduce below 5%. The spectra lead to significantly improved constraints for stellar population parameters. For most quantities this improvement is about equally driven by the higher spectral resolution and by the much reduced redshift uncertainty. Properties such as the age, A_V, current star formation rate, and the star formation history are generally very poorly constrained with broadband data alone. Interestingly, stellar masses and mass-to-light ratios are among the most stable parameters from broadband data. Nevertheless, photometric studies may overestimate the number of massive galaxies at 2 < z < 3 and thus underestimate the evolution of the stellar mass density. Finally, the spectroscopy supports our previous finding that red galaxies dominate the high-mass end of the galaxy population at z = 2-3.

K-band spectroscopic observations recorded with NIFS+ALTAIR on Gemini North are used to probe the central arcsecond of the compact elliptical galaxies NGC 4486B, NGC 5846A, and M32. The angular resolution of these data is ~0.1^'' FWHM; this corresponds to a spatial scale of 12 pc in NGC 5846A, which is the most distant galaxy in the sample. Indices that probe the strengths of various atomic and molecular features are measured. The central stellar contents of NGC 4486B and NGC 5846A are similar in the sense that they occupy the same regions of the (Ca I,¹²CO), (Na I,¹²CO), and (¹³CO,¹²CO) diagrams. The NGC 4486B and NGC 5846A observations depart from the sequence defined by solar neighborhood giants in the (Na I,¹²CO) diagram in a sense that is consistent with both galaxies having nonsolar chemical mixtures. For comparison, the M32 data are consistent with a chemical enrichment history like that in the Galactic disk; M32 could not have formed from the stripping of a larger elliptical galaxy. The behavior of the near-infrared line indices as a function of radius is also investigated. The stellar content in the central arcsecond of M32 appears to be well mixed. However, the radial behavior of the indices in NGC 4486B and NGC 5846A show complicated behavior, with the gradients that are present at large radii breaking down or reversing within a few tenths of an arcsec of the nucleus. Based on the age gradients predicted from visible wavelength spectra, coupled with the radial behavior of the ⟨Fe I⟩ and ¹²CO(2, 0) indices, it is suggested that the nuclear regions of NGC 4486B and NGC 5846A harbor intermediate-age populations.

Continuum observations at 350 μm of seven nearby elliptical galaxies for which CO gas disks have recently been resolved with interferometry mapping are presented. These SHARC II mapping results provide the first clearly resolved far-infrared (FIR)-to-submillimeter continuum emission from cold dust (with temperatures 31 K ≳ T≳ 23 K ) of any elliptical galaxy at a distance >40 Mpc. The measured FIR excess shows that the most likely and dominant heating source of this dust is not dilute stellar radiation or cooling flows, but rather star formation that could have been triggered by an accretion or merger event and fueled by dust-rich material that has settled in a dense region cospatial with the central CO gas disks. The dust is detected even in two cluster ellipticals that are deficient in H I, showing that, unlike H I, cold dust and CO in ellipticals can survive in the presence of hot X-ray gas, even in galaxy clusters. No dust cooler than 20 K, either distributed outside the CO disks or cospatial with and heated by the entire dilute stellar optical galaxy (or very extended H I), is currently evident.

Recent studies have indicated that the HCN-to-CO(J = 1–0) and HCO⁺-to-HCN(J = 1–0) ratios are significantly different between galaxies with AGN (active galactic nucleus) and SB (starburst) signatures. In order to study the molecular gas properties in active galaxies and search for differences between AGN and SB environments, we observed the HCN(J = 1–0), (J = 2–1), (J = 3–2), HCO⁺(J = 1–0), and HCO⁺(J = 3–2) emission with the IRAM 30 m in the center of 12 nearby active galaxies which either exhibit nuclear SB and/or AGN signatures. Consistent with previous results, we find a significant difference of the HCN(J = 2–1)-to-HCN(J = 1–0), HCN(J = 3–2)-to-HCN(J = 1–0), HCO⁺(J = 3–2)-to-HCO⁺(J = 1–0), and HCO⁺-to-HCN intensity ratios between the sources dominated by an AGN and those with an additional or pure central SB: the HCN, HCO⁺, and HCO⁺-to-HCN intensity ratios tend to be higher in the galaxies of our sample with a central SB as opposed to the pure AGN cases, which show rather low intensity ratios. Based on an LVG analysis of these data, i.e., assuming purely collisional excitation, the (average) molecular gas densities in the SB-dominated sources of our sample seem to be systematically higher than in the AGN sources. The LVG analysis seems to further support systematically higher HCN and/or lower HCO⁺ abundances as well as similar or higher gas temperatures in AGNs compared to the SB sources of our sample. In addition, we find that the HCN-to-CO ratios decrease with increasing rotational number J for the AGNs while they stay mostly constant for the SB sources.

A new type of compact stellar systems, labeled "ultracompact dwarf galaxies" (UCDs), was discovered in the last decade. Recent studies show that their dynamical mass-to-light ratios (M/L) tend to be too high to be explained by canonical stellar populations, being on average about twice as large as those of Galactic globular clusters of comparable metallicity. If this offset is caused by dark matter in UCDs, it would imply dark matter densities as expected for the centers of cuspy dark matter halos, incompatible with cored dark matter profiles. Investigating the nature of the high M/L ratios in UCDs therefore offers important constraints on the phase space properties of dark matter particles. Here we describe an observational method to test whether a bottom-heavy IMF may cause the high M/L ratios of UCDs. We propose to use the CO index at 2.3μm—which is sensitive to the presence of low-mass stars—to test for a bottom-heavy IMF. In the case that the high M/L ratios are caused by a bottom-heavy IMF, we show that the equivalent width of the CO index should be up to 30% weaker in UCDs compared to sources in the same metallicity range that have canonical IMFs. We find that these effects are well detectable with current astronomical facilities in a reasonable amount of time (a few hours to nights). In the Fornax and Virgo Clusters, all known UCDs can be mapped within a single square degree pointing. Therefore, the best observational approach is wide-field imaging with narrowband CO filters, complemented by precise measurements of [Fe/H] from optical multi-object spectroscopy. We conclude that measuring the CO index of UCDs is a promising tool to test whether their high M/L ratios are caused by a bottom-heavy IMF.

We present a new model of the three-dimensional distribution of molecular gas in the Milky Way Galaxy, based on CO line data. Our analysis is based on a gas-flow simulation of the inner Galaxy using smoothed particle hydrodynamics (SPH) and a realistic barred gravitational potential derived from the observed COBE DIRBE near-IR light distribution. The gas model prescribes the gas orbits much better than a simple circular rotation model and is highly constrained by observations, but it cannot predict local details. In this study, we provide a three-dimensional map of the observed molecular gas distribution using the velocity field from the SPH model. A comparison with studies of the Galactic center region suggests that the main structures are reproduced, but somewhat stretched along the line of sight, probably on account of limited resolution of the underlying SPH simulation. The gas model will be publicly available and may prove useful in a number of applications, among them the analysis of diffuse gamma-ray emission as measured with GLAST.

We provide a new distance estimate to the supernova remnant (SNR) Kes 73 and its associated anomalous X-ray pulsar (AXP) 1E 1841-045. 21 cm H I images and H I absorption/emission spectra from new VLA observations, and ¹³CO emission spectra of Kes 73 and two adjacent compact H II regions (G27.276+0.148 and G27.491+0.189) are analyzed. The H I images show prominent absorption features associated with Kes 73 and the H II regions. The absorption appears up to the tangent point velocity giving a lower distance limit to Kes 73 of 7.5 kpc, which has previously been given as the upper limit. In addition, G27.276+0.148 and G27.491+0.189 are at the far kinematic distances of their radio recombination line velocities. There is prominent H I emission in the range 80-90 km s⁻¹ for all three objects. The two H II regions show H I absorption at ~84 km s⁻¹, but there is no absorption in the Kes 73 absorption spectrum. This implies an upper distance limit of ~9.8 kpc to Kes 73. This corrected larger distance to Kes 73/AXP 1E 1841-045 system leads to a refined age of the SNR of 500-1000 yr, and a ~50% larger AXP X-ray luminosity.

We have observed the supernova remnant MSH 15-52 (G320.4-1.2), which contains the gamma-ray pulsar PSR B1509–58, using the CANGAROO-III imaging atmospheric Cerenkov telescope array from 2006 April to June. We detected gamma rays above 810 GeV at the 7 σ level during a total effective exposure of 48.4 hr. We obtained a differential gamma-ray flux at 2.35 TeV of (7.9 ± 1.5_stat± 1.7_sys) × 10⁻¹³ cm⁻² s⁻¹ TeV⁻¹, with a photon index of 2.21 ± 0.39_stat± 0.40_sys, which is compatible with that of the H.E.S.S. observation in 2004. The morphology shows extended emission compared to our point spread function. We consider the plausible origin of the high-energy emission based on a multiwavelength spectral analysis and energetics arguments.

Optical images and spectra, both ground based and taken by Hubble Space Telescope (HST), of the young, luminous O-rich supernova remnant in the irregular galaxy NGC 4449 are presented. HST images of the remnant and its local region were obtained with the ACS/WFC using filters F435W, F555W, F814W (B, V, and I, respectively), F502N ([O III]), F658N (Hα + [N II]), F660N ([N II]), and F550M (line-free continuum). These images show an unresolved remnant (FWHM < 0.05'') located within a rich cluster of OB stars which itself is enclosed by a nearly complete interstellar shell seen best in Hα + [N II] emission approximately 8'' × 6'' (150 pc × 110 pc ) in size. The remnant and its associated OB cluster are isolated from two large nearby H II regions. The ACS [O III] image shows the remnant may be partially surrounded by a clumpy ring of emission approximately 1'' (~20 pc) in diameter. Recent ground-based spectra of the remnant reveal (1) the emergence of broad, blueshifted emission lines of [S II] λλ6716, 6731, [Ar III] λ7136, and [Ca II] λλ7291, 7324 which were not observed in spectra taken in 1978-1980; (2) faint emission at 6540-6605 Å centered about Hα and [N II] λλ6548, 6583 with an expansion velocity of 500 ± 100 km s⁻¹; and (3) excess emission around 4600-4700 Å suggestive of a Wolf-Rayet population in the remnant's star cluster. We use these new data to re-interpret the origin of the remnant's prolonged and bright luminosity and propose that the remnant is strongly interacting with dense, circumstellar wind loss material from a ≳20 M_☉ progenitor star.

The dynamics and structure of accretion disks, which accumulate a vertical magnetic field in their centers, are investigated using two- and three-dimensional MHD simulations. The central field can be built up to the equipartition level, where it disrupts a nearly axisymmetric outer accretion disk inside a magnetospheric radius, forming a magnetically arrested disk (MAD). In the MAD, the mass accretes in the form of irregular dense spiral streams, and the vertical field, split into separate bundles, penetrates through the disk plane in low-density magnetic islands. The accreting mass, when spiraling inward, drags the field and twists it around the axis of rotation, resulting in collimated Poynting jets in the polar directions. These jets are powered by the accretion flow with an efficiency of up to ~1.5% (in units of _inc²). The spiral flow pattern in the MAD is dominated by modes with low azimuthal wavenumbers m ∼ 1–5 and can be a source of quasi-periodic oscillations in the outgoing radiation. The formation of the MAD and the Poynting jets can naturally explain the observed changes of spectral states in galactic black hole binaries. Our study is focused on black hole accretion flows; however, the results can also be applied to accretion disks around nonrelativistic objects, such as young stellar objects and stars in binary systems.

Fragmentation and binary formation processes are studied using three-dimensional resistive MHD nested grid simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate the cloud evolution from the molecular cloud core (n = 10⁴ cm ⁻³) to the stellar core (n≃ 10²² cm ⁻³), where n denotes the central density. We calculated 147 models with different initial magnetic, rotational, and thermal energies and the amplitudes of the nonaxisymmetric perturbation. In a collapsing cloud, fragmentation is mainly controlled by the initial ratio of the rotational to the magnetic energy, regardless of the initial thermal energy and amplitude of the nonaxisymmetric perturbation. The cloud rotation promotes fragmentation, while the magnetic field delays or in some cases suppresses fragmentation through all phases of cloud evolution. The results are categorized into three types. When the clouds have larger rotational energies in relation to magnetic energies, fragmentation occurs in the low-density phase (10¹² cm ⁻³≲ n≲ 10¹⁵ cm ⁻³) with separations of 3-300 AU. Fragments that appeared in this phase are expected to evolve into wide binary systems. On the other hand, when initial clouds have larger magnetic energies in relation to the rotational energies, fragmentation occurs only in the high-density phase (n≳ 10¹⁷ cm ⁻³) after the clouds experience a significant reduction of the magnetic field owing to the ohmic dissipation. Fragments appearing in this phase have mutual separations of ≲0.3 AU and are expected to evolve into close binary systems. No fragmentation occurs in the case of sufficiently strong magnetic field, in which single stars are expected to be born. Two types of fragmentation epoch reflect wide and close separations. We might be able to observe a bimodal distribution for the radial separation of the protostar in extremely young stellar groups.

Electronic transitions of MgC_2nH (n = 1-3) have been observed in the gas phase in the visible region. A mass-selective, resonant two-color two-photon ionization technique, coupled with a laser ablation source, was used. Ab initio calculations of the geometries, energies, and vertical electronic excitations have been carried out using hybrid density functional theory and the coupled-cluster approach. The spectra are assigned as the A²Π ← X²Σ⁺ electronic transition of linear chains. The origin bands are located at 4383, 4453, and 4525 Å for MgC₂H, MgC₄H, and MgC₆H, respectively, and provide a means of detecting such species in astronomical environments.

Observations of two H₂CO (3₀₃-2₀₂ and 3₂₁-2₂₀) lines and continuum emission at 1.3 mm toward Sgr B2(N) and Sgr B2(M) have been carried out with the SMA. The mosaic maps of Sgr B2(N) and Sgr B2(M) in both continuum and lines show a complex distribution of dust and molecular gas in both clumps and filaments surrounding the compact star formation cores. We have observed a decelerating outflow originated from the Sgr B2(M) core, showing that both the redshifted and blueshifted outflow components have a common terminal velocity. This terminal velocity is 58 ± 2 km s⁻¹. It provides an excellent method for determination of the systematic velocity of the molecular cloud. The SMA observations have also shown that a large fraction of absorption against the two continuum cores is redshifted with respect to the systematic velocities of Sgr B2(N) and Sgr B2(M), respectively, suggesting that the majority of the dense molecular gas is flowing into the two major cores where massive stars have been formed. We have solved the radiative transfer in a multilevel system with LVG approximation. The observed H₂CO line intensities and their ratios can be adequately fitted with this model for the most of the gas components. However, the line intensities between the higher energy level transition H₂CO(3₂₁-2₂₀) and the lower energy level transition H₂CO(3₀₃-2₀₂) is reversed in the redshifted outflow region of Sgr B2(M), suggesting the presence of inversion in population between the ground levels in the two K ladders (K₋₁ = 0 and 2). The possibility of weak maser processes for the H₂CO emission in Sgr B2(M) is discussed.

OH absorption against PSR B1718–35 at (l,b) = 351.688°, +0.671° has been discovered at 1665 and 1667 MHz using the Green Bank Telescope. The absorption appears to arise at the interface of an H II region and a molecular cloud that are likely associated with the high-mass star-forming region NGC 6334. Beam dilution is found to be the cause of differences between the opacity of the OH against the Galactic background continuum emission and against the pulsar. The OH cloud is approximately 3 × 1.3 pc and is located behind the H II region.

The results of a Hubble Space Telescope (HST) snapshot survey of post-AGB objects are shown. The aim of the survey is to complement existing HST images of protoplanetary nebulae and to connect various types of nebulosities with the physical and chemical properties of their central stars. Nebulosities are detected in 15 of 33 sources. Images and photometric and geometric measurements are presented. For sources with nebulosities we see a morphological bifurcation into two groups, DUPLEX and SOLE, as previous studies have found. We find further support for the previous results, suggesting that this dichotomy is caused by a difference in the optical thickness of the dust shell. The remaining 18 sources are classified as stellar post-AGB objects, because our observations indicate a lack of nebulosity. We show that some stellar sources may in fact be DUPLEX or SOLE objects based on their infrared colors. The causes of the differences among the groups are investigated. We discuss some evidence suggesting that high progenitor mass AGB stars tend to become DUPLEX post-AGB objects and intermediate progenitor mass AGB stars tend to become SOLE post-AGB objects. Most of the stellar sources probably have low-mass progenitors and do not seem to develop nebulosities during the post-AGB phase; therefore, they do not become planetary nebulae.

The origin and evolution of the X-ray emission in very young stellar objects (YSOs) are not yet well understood because it is very hard to observe YSOs in the protostellar phase. Using COUP data, we study the X-ray properties of stars in the ONC in different evolutionary classes: luminosities, hydrogen column densities N_H, effective plasma temperatures, and time variability are compared to understand if the interaction between the circumstellar material and the central object can influence the X-ray emission. We have assembled the deepest and most complete photometric catalog of objects in the ONC region from the UV to 8 μm using data from the HST Treasury Program; deep and almost simultaneous UBVI and JHK images taken, respectively, with WFI at ESO 2.2 m and ISPI at CTIO 4 m telescopes; and Spitzer IRAC imaging. We select high-probability candidate Class 0-I protostars, a sample of "bona fide" Class II stars, and a set of Class III stars with IR emission consistent with normal photospheres. Our principal result is that Class 0-Ia objects are significantly less luminous in X-rays, in both the total and hard bands, than the more evolved Class II stars with mass larger than 0.5 M_☉; the latter show X-ray luminosities similar to those of Class 0-Ib stars. This result supports the hypothesis that the onset of X-ray emission occurs at a very early stage of star formation. Spectral properties of Class 0-I stars are similar to those of the more evolved Class II and III objects, except for a larger absorption likely due to gas in the envelope or disk of the protostellar objects. Our data suggest that the three different classes have similar X-ray temporal variability.

Many models of gamma-ray bursts (GRBs) as well as of soft gamma repeaters (SGRs) involve a fireball—an optically thick concentration of radiation energy with a high ratio of energy density to rest mass. We study the asymptotic behavior of an ultrarelativistic fireball consisting of electron-positron pairs and photons. We show that in the ultrarelativistic limit, after photons decouple from the pairs, the photon distribution function remains a blackbody spectrum in some appropriate Lorentz frame, allowing us to define an effective Lorentz factor and temperature for the photon gas. We also study the freezing out of electron-positron pairs and their asymptotic Lorentz factor γ_∞. The dependence of these quantities on initial conditions can be described by simple scaling laws. We apply our results to SGR 1806-20 and find that the energy carried by electron-positron pairs is higher than calculated by former estimates, but is still an order of magnitude short of the minimum energy required to produce the observed afterglow. A viable solution of the energy budget is that the fireball is loaded by baryons or electromagnetic flux.

The chemical composition of the ultra-high-energy (UHE) cosmic rays serves as an important clue to their origin. Recent measurements of the elongation rates by the Pierre Auger Observatory hint at the possible presence of heavy or intermediate-mass nuclei in the UHE cosmic rays. Gamma-ray bursts (GRBs) and hypernovae have been suggested as possible sources of the UHE cosmic rays. Here we derive constraints on the physical conditions under which UHE heavy nuclei, if they are accelerated in these sources, can survive in their intense photon fields. We find that in the GRB external shock and hypernova scenarios, UHE nuclei can easily survive photodisintegration. In the GRB internal shock scenario, UHE nuclei can also survive, provided the dissipation radius and/or the bulk Lorentz factor of the relativistic outflow are relatively large, or if the low-energy self-absorption break in the photon spectrum of the prompt emission occurs above several keV. In internal shocks and in the other scenarios, intermediate-mass UHE nuclei have a higher probability of survival against photodisintegration than UHE heavy nuclei such as Fe.

We present the discovery and high signal-to-noise ratio spectroscopic observations of the optical afterglow of the long-duration gamma-ray burst GRB 070125. Unlike all previously observed long-duration afterglows in the redshift range 0.5 ≲ z ≲ 2.0, we find no strong (rest-frame equivalent width W_r≳ 1.0 Å) absorption features in the wavelength range 4000-10000 Å. The sole significant feature is a weak doublet that we identify as Mg II λλ2796 (W_r = 0.18 ± 0.02 Å), 2803 (W_r = 0.08 ± 0.01 Å) at z = 1.5477 ± 0.0001. The low observed Mg II and inferred H I column densities are typically observed in galactic halos, far away from the bulk of massive star formation. Deep ground-based imaging reveals no host directly underneath the afterglow to a limit of R > 25.4 mag. Either of the two nearest blue galaxies could host GRB 070125; the large offset (d ⩾ 27 kpc) would naturally explain the low column densities. To remain consistent with the large local (i.e., parsec scale) circumburst density inferred from broadband afterglow observations, we speculate that GRB 070125 may have occurred far away from the disk of its host in a compact star-forming cluster. Such distant stellar clusters, typically formed by dynamical galaxy interactions, have been observed in the nearby universe and should be more prevalent at z > 1, where galaxy mergers occur more frequently.

The properties of the highest velocity ejecta of normal Type Ia supernovae (SNe Ia) are studied via models of very early optical spectra of six SNe. At epochs earlier than 1 week before maximum, SNe with a rapidly evolving Si II λ6355 line velocity (HVG) have a larger photospheric velocity than SNe with a slowly evolving Si II λ6355 line velocity (LVG). Since the two groups have comparable luminosities, the temperature at the photosphere is higher in LVG SNe. This explains the different overall spectral appearance of HVG and LVG SNe. However, the variation of the Ca II and Si II absorptions at the highest velocities (v≳ 20,000 km s⁻¹) suggests that additional factors, such as asphericity or different abundances in the progenitor white dwarf, affect the outermost layers. The C II λ6578 line is marginally detected in three LVG SNe, suggesting that LVGs undergo less intense burning. The carbon mass fraction is small, only less than 0.01 near the photosphere, so that he mass of unburned C is only ≲0.01 M_☉. Radioactive ⁵⁶Ni and stable Fe are detected in both LVG and HVG SNe. Different Fe-group abundances in the outer layers may be one of the reasons for spectral diversity among SNe Ia at the earliest times. The diversity among SNe Ia at the earliest phases could also indicate an intrinsic dispersion in the LC width-luminosity relation.

We report the detections of the anticorrelated soft and hard X-rays, and the time lags of ~hectosecond from the neutron star low-mass X-ray binary Cyg X-2, a well-known Z-type luminous source. Both anticorrelation and the positive correlation were detected during the low-intensity states, while only the latter showed up during high-intensity states. Comparing with the lower part of normal branch and flaring branch, we find that more observations located on the horizontal and the upper normal branches are accompanied with anticorrelation, implying the occurrence of the anticorrelation when there is a low mass accretion rate. So far anticorrelated hard lags of 1000 s timescale are only reported from Galactic black hole candidates in their hard states. Here we provide the first evidence that a similar feature can also be established in neutron star systems such as Cyg X-2. Finally, the possible origins of the observed time lags are discussed under current LMXB models.

We study the effects of the sedimentation of the trace element ²²Ne in the cooling of white dwarfs. In contrast with previous studies—which adopted a simplified treatment of the effects of ²²Ne sedimentation—this is done self-consistently for the first time, using an up-to-date stellar evolutionary code in which the diffusion equation is coupled with the full set of equations of stellar evolution. Due the large neutron excess of ²²Ne, this isotope rapidly sediments in the interior of the white dwarf. Although we explore a wide range of parameters, we find that when using the most reasonable assumptions concerning the diffusion coefficient and the physical state of the white dwarf interior, the delay introduced by the ensuing chemical differentiation is minor for a typical 0.6 M_☉ white dwarf. For more massive white dwarfs, say M_WD ∼ 1.0 M_☉, the delay turns out to be considerably larger. These results are in qualitatively good accord with those obtained in previous studies, but we find that the magnitude of the delay introduced by ²²Ne sedimentation was underestimated by a factor of ~2. We also perform a preliminary study of the impact of ²²Ne sedimentation on the white dwarf luminosity function. Finally, we hypothesize on the possibility of detecting the sedimentation of ²²Ne using pulsating white dwarfs in the appropriate effective temperature range with accurately determined rates of change of the observed periods.

I investigate the discrepancy between the evolution and pulsation masses for Cepheid variables. A number of recent works have proposed that noncanonical mass loss can account for the mass discrepancy. This mass loss would be such that a 5 M_☉ star loses approximately 20% of its mass by arriving at the Cepheid instability strip; a 14 M_☉ star, none. Such findings would pose a serious challenge to our understanding of mass loss. I revisit these results in light of the Padova stellar evolutionary models and find evolutionary masses are (17 ± 5)% greater than pulsation masses for Cepheids between 5 < M/M_☉ < 14. I find that mild internal mixing in the main-sequence progenitor of the Cepheid are able to account for this mass discrepancy.

We present a geometrical methodology to interpret the periodical light curves of soft gamma repeaters based on the magnetar model and the numerical arithmetic of the three-dimensional magnetosphere model for the young pulsars. The hot plasma released by the starquake is trapped in the magnetosphere, and photons are emitted tangentially to the local magnetic field lines. The variety of radiation morphologies in the burst tails and the persistent stages could be well explained by the trapped fireballs on different sites inside the closed field lines. Furthermore, our numerical results suggest that the pulse profile evolution of SGR 1806–20 during the 2004 December 27 giant flare is due to a lateral drift of the emitting region in the magnetosphere.

The second Born corrections to the electrical and thermal conductivities are calculated for the dense matter in the liquid metal phase for various elemental compositions of astrophysical importance. Inclusion up to the second Born corrections is sufficiently accurate for the Coulomb scattering of the electrons by the atomic nuclei with Z≲ 26. Our approach is semianalytical and is in contrast to that of the previous authors who have used fully numerical values of the cross section for the Coulomb scattering of the electron by the atomic nucleus. The merit of the present semianalytical approach is that this approach allows us to obtain the results with a reliable Z dependence and ρ dependence. The previous fully numerical approach has made use of the numerical values of the cross section for the scattering of the electron off the atomic nucleus for a limited number of Z-values, Z = 6, 13, 29, 50, 82, and 92, and for a limited number of electron energies, 0.05, 0.1, 0.2, 0.4, 0.7, 1, 2, 4, and 10 MeV. Our study, however, has confirmed that the previous results are sufficiently accurate. They are recovered if the terms higher than the second Born terms are taken into account. We make a detailed comparison of the present results with those of the previous authors. The numerical results are parameterized in the form of analytic formulae that would facilitate practical use of the results. We also extend our calculations to the case of mixtures of nuclear species.

We report on new and archival X-ray and near-IR observations of the anomalous X-ray pulsar 1E 1048.1–5937 performed between 2001 and 2007 with the Rossi X-Ray Telescope Explorer (RXTE), the Chandra X-Ray Observatory, the Swift Gamma-Ray Burst Explorer, the Hubble Space Telescope (HST), and the Very Large Telescope. Monitoring with RXTE revealed that following its ~2001-2004 active period, 1E 1048.1–5937 entered a phase of timing stability; at the same time, simultaneous observations with Chandra and HST in 2006 showed that its X-ray and near-IR radiative properties, all variable prior to 2005, stabilized. Specifically, the 2006 X-ray spectrum is consistent with a two-component blackbody plus power law, with an average kt = 0.52 keV and Γ = 2.8, at a mean flux level of ~6.5 × 10⁻¹² erg cm⁻² s⁻¹ (2–10 keV). The near-IR counterpart in 2005-2006 was detected at H ∼ 22.7 mag and K_s ∼ 21.0 mag, considerably fainter than previously measured. In 2007 March, this newfound quiescence was interrupted by sudden X-ray flux, spectral, and pulse morphology changes, simultaneous with a large glitch and near-IR enhancement. Our RXTE observations revealed a factor of ~3 increase in pulsed flux (2-10 keV), while observations with Chandra and Swift saw the total X-ray flux increase much more than the pulsed flux, reaching a peak value of >7 times the quiescent value (2-10 keV). We find a strong anticorrelation between X-ray flux and pulsed fraction, and a correlation between X-ray spectral hardness and flux. Simultaneously with the radiative and timing changes, we observed the X-ray pulse profile change significantly from nearly sinusoidal to having multiple peaks. We compare these remarkable events with other magnetar outbursts and discuss implications in the context of AXP emission models.

SWIFT J1756.9–2508 is one of the few accreting millisecond pulsars (AMPs) discovered to date. We report here the results of our analysis of its aperiodic X-ray variability, as measured with the Rossi X-Ray Timing Explorer during the 2007 outburst of the source. We detect strong (~35%) flat-topped broadband noise throughout the outburst with low characteristic frequencies (~0.1 Hz). This makes SWIFT J1756.9–2508 similar to the rest of AMPs and to other low-luminosity accreting neutron stars when they are in their hard states, and enables us to classify this AMP as an atoll source in the extreme island state. We also find a hard tail in its energy spectrum extending up to 100 keV, fully consistent with such source and state classification.

Numerical simulations including magnetic fields have become important in many fields of astrophysics. Evolution of magnetic fields by the constrained-transport algorithm preserves magnetic divergence to machine precision and thus represents one preferred method for the inclusion of magnetic fields in simulations. We show that constrained transport can be implemented with volume-centered fields and hyperresistivity on a high-order finite-difference stencil. In addition, the finite-difference coefficients can be tuned to enhance high-wavenumber resolution. Similar techniques can be used for the interpolations required for dealiasing corrections at high wavenumber. Together, these measures yield an algorithm with a wavenumber resolution that approaches the theoretical maximum achieved by spectral algorithms. Because this algorithm uses finite differences instead of fast Fourier transforms, it runs faster and is not restricted to periodic boundary conditions. In addition, since the finite differences are spatially local, this algorithm is easily scalable to thousands of processors. We demonstrate that, for low-Mach number turbulence, the results agree well with a high-order, non-constrained-transport scheme with Poisson divergence cleaning.

We study the role of the guide field in relativistic magnetic reconnection in a Harris current sheet of pair (e^±) plasmas, using linear theories and particle-in-cell (PIC) simulations. Two-dimensional PIC simulations exhibit the guide field dependence to the linear instabilities; the tearing or reconnection modes are relatively insensitive, while the relativistic drift-kink instability (RDKI), the fastest mode in a relativistic current sheet, is stabilized by the guide field. Particle acceleration in the nonlinear stage is also investigated. A three-dimensional PIC simulation demonstrates that the current sheet is unstable to the RDKI, although a small reconnection occurs in the deformed current sheet. Another three-dimensional PIC simulation with a guide field demonstrates a completely different scenario. Secondary magnetic reconnection is triggered by nonlinear coupling of oblique instabilities, which we call the relativistic drift-sausage tearing instability. Therefore, particle acceleration by relativistic guide field reconnection occurs in three-dimensional configuration. Based on the plasma theories, we discuss an important role of the guide field: to enable nonthermal particle acceleration by magnetic reconnection.

We present a set of Spitzer 24 μm MIPS time series observations of the M dwarf eclipsing binary star GU Boötis. Our data cover three secondary eclipses of the system: two consecutive events and an additional eclipse 6 weeks later. The study's main purpose is the long-wavelength (and thus limb-darkening-independent) characterization of GU Boo's light curve, allowing for independent verification of the results of previous optical studies. Our results confirm previously obtained system parameters. We further compare GU Boo's measured 24 μm flux density to the value predicted by spectral fitting and find no evidence for circumstellar dust. In addition to GU Boo, we characterize (and show examples of) light curves of other objects in the field of view. Analysis of these light curves serves to characterize the photometric stability and repeatability of Spitzer's MIPS 24 μm array over short (days) and long (weeks) timescales at flux densities between approximately 300 and 2000 μJy. We find that the light-curve rms about the median level falls into the 1%-4% range for flux densities higher than 1 mJy. Finally, we comment on the fluctuations of the 24 μm background on short and long timescales.

A detailed high-resolution spectroscopic analysis is presented for the carbon-rich low-metallicity Galactic halo object CS 22964–161. We have discovered that CS 22964–161 is a double-lined spectroscopic binary and have derived accurate orbital components for the system. From a model atmosphere analysis we show that both components are near the metal-poor main-sequence turnoff. Both stars are very enriched in carbon and in neutron-capture elements that can be created in the s-process, including lead. The primary star also possesses an abundance of lithium close to the value of the "Spite plateau." The simplest interpretation is that the binary members seen today were the recipients of these anomalous abundances from a third star that was losing mass as part of its AGB evolution. We compare the observed CS 22964–161 abundance set with nucleosynthesis predictions of AGB stars, discuss issues of envelope stability in the observed stars under mass transfer conditions, and consider the dynamical stability of the alleged original triple star. Finally, we consider the circumstances that permit survival of lithium, whatever its origin, in the spectrum of this extraordinary system.

Chemically peculiar stars define a class of stars that show unusual elemental abundances due to stellar photospheric effects and not due to natal variations. In this paper, we compare the elemental abundance patterns of the ultra-metal-poor stars with metallicities [ Fe/H ] ∼ − 5 to those of a subclass of chemically peculiar stars. These include post-AGB stars, RV Tauri variable stars, and the Lambda Bootis stars, which range in mass, age, binarity, and evolutionary status, yet can have iron abundance determinations as low as [ Fe/H ] ∼ − 5. These chemical peculiarities are interpreted as due to the separation of gas and dust beyond the stellar surface, followed by the accretion of dust-depleted gas. Contrary to this, the elemental abundances in the ultra-metal-poor stars are thought to represent yields of the most metal-poor supernovae and, therefore, observationally constrain the earliest stages of chemical evolution in the universe. Detailed chemical abundances are now available for HE 1327–2326 and HE 0107–5240, the two extreme ultra-metal-poor stars in our Galaxy, and for HE 0557–4840, another ultra-metal-poor star found by the Hamburg/ESO survey. There are interesting similarities in their abundance ratios to those of the chemically peculiar stars; e.g., the abundances of the elements in their photospheres are related to the condensation temperature of that element. If these three stars are chemically peculiar, then their CNO abundances suggest true metallicities of [ X/H ] ∼ − 2 to –4. It is important to establish the nature of these stars, since they are used as tests of the early chemical evolution of the Galaxy.

Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low-mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the ³He(³He,2p)⁴He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of ³He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low-mass giants, a problem of more than three decades standing. This mixing mechanism, which we call "δ μ mixing," operates rapidly (relative to the nuclear timescale of overall evolution, ~10⁸ yr) once the hydrogen-burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Population I stars between 0.8 and 2.0 M_☉ develop ¹²C/¹³C ratios of 14.5 ± 1.5, while Population II stars process the carbon to ratios of 4.0 ± 0.5. In stars less than 1.25 M_☉, this mechanism also destroys 90%-95% of the ³He produced on the main sequence.

We report the results of our Hubble Space Telescope (HST) snapshot survey with the ACS HRC PR200L prism, designed to measure the near-UV emission in a sample of nearby M dwarfs. Thirty-three stars were observed, spanning the mass range from 0.1 to 0.6 solar masses (T_eff ∼ 2200-4000 K) where the UV energy distributions vary widely between active and inactive stars. These observations provide much needed constraints on models of the habitability zone and the atmospheres of possible terrestrial planets orbiting M dwarf hosts and will be useful in refining the target selection for future space missions such as Terrestrial Planet Finder (TPF). We compare our data with a new generation of M dwarf atmospheric models and discuss their implications for the chromospheric energy budget. These NUV data will also be valuable in conjunction with existing optical, FUV, and X-ray data to explore unanswered questions regarding the dynamo generation and magnetic heating in low-mass stars.

Both core accretion and disk instability appear to be required as formation mechanisms in order to explain the entire range of giant planets found in extrasolar planetary systems. Disk instability is based on the formation of clumps in a marginally gravitationally unstable protoplanetary disk. These clumps can only be expected to contract and survive to become protoplanets if they are able to lose thermal energy through a combination of convection and radiative cooling. Here we present several new three-dimensional, radiative hydrodynamics models of self-gravitating protoplanetary disks, where radiative transfer is handled in the flux-limited diffusion approximation. We show that while the flux-limited models lead to higher midplane temperatures than in a diffusion approximation model without the flux limiter, the difference in temperatures does not appear to be sufficiently high to have any significant effect on the formation of self-gravitating clumps. Self-gravitating clumps form rapidly in the models both with and without the flux limiter. These models suggest that the reason for the different outcomes of numerical models of disk instability by different groups cannot be attributed solely to the handling of radiative transfer, but rather appears to be caused by a range of numerical effects and assumptions. Given the observational imperative to have disk instability form at least some extrasolar planets, these models imply that disk instability remains as a viable giant planet formation mechanism.

Four Ophiuchus binaries, two Class I systems and two Class II systems, with separations of ~450-1100 AU, were observed with the Owens Valley Radio Observatory (OVRO) millimeter interferometer. In each system, the 3 mm continuum maps show dust emission at the location of the primary star, but no emission at the position of the secondary. This result is different from observations of less evolved Class 0 binaries, in which dust emission is detected from both sources. The nondetection of secondary disks is, however, similar to the dust distribution seen in wide Class II Taurus binaries. The combined OVRO results from the Ophiuchus and Taurus binaries suggest that secondary disk masses are significantly lower than primary disk masses by the Class II stage, with initial evidence that massive secondary disks are reduced by the Class I stage. Although some of the secondaries retain hot inner disk material, the early dissipation of massive outer disks may negatively impact planet formation around secondary stars. Masses for the circumprimary disks are within the range of masses measured for disks around single T Tauri stars and, in some cases, larger than the minimum mass solar nebula. More massive primary disks are predicted by several formation models and are broadly consistent with the observations. Combining the 3 mm data with previous 1.3 mm observations, the dust opacity power-law index for each primary disk is estimated. The opacity index values are all less than the scaling for interstellar dust, possibly indicating grain growth within the circumprimary disks.

We report detection of cool dust surrounding solar-type stars from observations performed as part of the Spitzer Legacy Science Program FEPS. From a sample of 328 stars having ages ~0.003-3 Gyr we have selected sources with 70 μm flux densities indicating excess in their SEDs above expected photospheric emission. Six strong excess sources are likely primordial circumstellar disks, remnants of the star formation process. Another 25 sources having ≥3 σ excesses are associated with dusty debris disks, generated by collisions within planetesimal belts that are possibly stirred by existing planets. Six additional sources with ≥2 σ excesses require confirmation as debris disks. In our analysis, most (>80%) 70 μm excess sources have ≥3 σ excesses at 33 μm as well, while only a minority (<40%) have ≥3 σ excesses at 24 μm. The rising SEDs toward (and perhaps beyond) 70 μm imply dust temperatures < 45–85 K for debris in equilibrium with the stellar radiation field. From fitted single-temperature blackbody models we infer bulk dust properties such as characteristic temperature, location, fractional luminosity, and mass. For > of the debris sources we find that multiple temperature components are suggested, implying a dust distribution extending over many tens of AU. Because the disks are dominated by collisional processes, the parent body (planetesimal) belts may be extended as well. Preliminary assessment of the statistics of cold debris around Sun-like stars shows that ~10% of FEPS targets with masses between 0.6 and 1.8 M_☉ and ages between 30 Myr and 3 Gyr exhibit excess 70 μm emission. We find that fractional excess amplitudes appear higher for younger stars and that there may be a trend in 70 μm excess frequency with stellar mass.

We report the discovery of a massive planet (M_psin i = 13.02 ± 0.64 M_J; total mass = 13.25 ± 0.64 M_J), large (1.95 ± 0.16 R_J) planet in a transiting, eccentric orbit (e = 0.260 ± 0.017) around a 10th magnitude F5 V star in the constellation Camelopardalis. We designate the planet XO-3b and the star XO-3, also known as GSC 03727–01064. The orbital period of XO-3b is 3.1915426 ± 0.00014 days. XO-3 lacks a trigonometric parallax; we estimate its distance to be 260 ± 23 pc. The radius of XO-3 is 2.13 ± 0.21 R_☉, its mass is 1.41 ± 0.08 M_☉, its vsin i = 18.54 ± 0.17 km s⁻¹, and its metallicity is [ Fe/H ] = − 0.177 ± 0.027. This system is unusual for a number of reasons. XO-3b is one of the most massive planets discovered around any star for which the orbital period is less than 10 days. The mass is near the deuterium-burning limit of 13 M_J, which is a proposed boundary between planets and brown dwarfs. Although Burrows et al. propose that formation in a disk or formation in the interstellar medium in a manner similar to stars is a more logical way to differentiate planets and brown dwarfs, our current observations are not adequate to address this distinction. XO-3b is also unusual in that its eccentricity is large given its relatively short orbital period. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. Analysis of the transit light curve of XO-3b suggests that the spectroscopically derived parameters may be overestimated. Though relatively noisy, the light curves favor a smaller radius in order to better match the steepness of the ingress and egress. The light curve fits imply a planetary radius of 1.25 ± 0.15 R_J, which would correspond to a mass of 12.03 ± 0.46 M_J. A precise trigonometric parallax measurement or a very accurate light curve is needed to resolve the uncertainty in the planetary mass and radius.

Previous investigations of magnetic field line random walk and charged particle transport in turbulence mostly employed an axisymmetric turbulence model. However, real turbulence is assumed to be nonaxisymmetric. In this article we employ a simple model for nonaxisymmetric turbulence based on a very recent article. By analytically describing the random walk of magnetic field lines and by combining these results with a general compound transport model, we derive the cosmic-ray diffusion tensor. As demonstrated, the nonaxisymmetry leads to different components of the cosmic-ray diffusion tensor in the perpendicular direction. These new results are essential for several applications such as solar modulation studies.

Electron distributions with various degrees of asymmetry associated with the energetic tail population are commonly detected in the solar wind near 1 AU. By numerically solving one-dimensional electrostatic weak turbulence equations the present paper demonstrates that a wide variety of asymmetric energetic tail distributions may result. It is found that a wide variety of asymmetric tail formation becomes possible if one posits that the solar wind electrons are initially composed of thermal core plus field-aligned counterstreaming beams, instead of the customary thermal population plus a single beam. It is shown that the resulting nonlinear wave-wave and wave-particle interactions lead to asymmetric nonthermal tails. It is found that the delicate difference in the average beam speeds associated with the forward versus backward components is responsible for the generation of asymmetry in the energetic tail.

We report on the analysis of a fast coronal mass ejection (CME)-driven shock observed on 2002 July 23 with the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO). The CME was first detected in white light by the Large Angle and Spectrometric Coronagraph Experiment (LASCO), and shock-associated type II metric emission was recorded by several ground-based radio spectrographs. The evolution of the excess broadening of the O VI λ1032 line profiles observed by UVCS at 1.63 R_☉ is consistent with the passage of a CME-driven shock surface enveloping a bubble-type, conically expanding CME, and its dynamics is found to be well associated with the complex, multiple type II radio emission detected in the metric band. Our results suggest that there might be a deficiency of ion heating in the present event with respect to what was observed in previous CME shocks detected by UVCS, and that this paucity might be attributed to different local plasma conditions, such as higher ambient coronal plasma β. We conclude that plasma β could be an important parameter in determining the effect of ion heating at collisionless shock fronts in the solar corona.

The origin and magnetic topology of impulsive solar energetic particle (SEP) events (also called solar ³He-rich events) are numerically investigated by using a three-dimensional axisymmetric time-dependent self-consistent magnetohydrodynamic (MHD) model. The results indicate that when a counterclockwise (or normal) magnetic flux rope is emerged from the photosphere at the open field region near the closed magnetic field lines, the magnetic topology produced by the MHD simulation is that proposed by Reames to lead to impulsive SEP events. The flux emergence initiates the magnetic configuration which produces the magnetic reconfiguration and reconnection at the coronal base. The magnetic reconnection at the coronal base strongly disturbs the magnetic fields in the solar corona and interplanetary space, and generates fast jetlike plasma outflows (or non-flux-rope coronal mass ejections). The magnetic field line disturbances could scatter charged particles and therefore accelerate them to high energies through the Fermi acceleration mechanism.

We present, to our knowledge for the first time, a rare observation of direct magnetic interaction between a transequatorial jet and interconnecting loops (IL) in the southern hemisphere. The jet originated from a flare and appeared to move outward along open field lines, but it passed so close to the IL that its edge met with one of the IL ends. As a result, the IL began to erupt, weak brightenings appeared at the meeting site, and a nearby dark feature was disturbed. After the eruption, in addition to a looplike dimming due to the disappearance of the IL, a dimming region was formed around its another end, which was very probably caused by the expansion or opening of its field lines and represented its evacuated feet. Two coronal mass ejections (CMEs) were observed within 2 hr in association with the event. One was related to the flare and the jet, while the other was due to the IL eruption. These observations suggest that a sole flare can not only trigger a CME but also simultaneously trigger an IL eruption by means of its interaction with a jet, so can lead to two interdependent CMEs, i.e., a sympathetic CME pair physically connected by the jet/IL interaction.

We present the first statistical analysis of the thermal and nonthermal X-ray emission of all 25,705 microflares (RHESSI) observed between 2002 March and 2007 March. These events were found by searching the 6-12 keV energy range (see Paper I) and are small active region flares, from low (GOES) C class to below A class. Each microflare is automatically analyzed at the peak time of the 6-12 keV emission: the thermal source size is found by forward-fitting the complex visibilities for 4-8 keV, and the spectral parameters (temperature, emission measure, power-law index) are found by forward-fitting a thermal plus nonthermal model. The resulting wealth of information we determine about the events allows a range of the thermal and nonthermal properties to be investigated. In particular, we find that there is no correlation between the thermal loop size and the flare magnitude, indicating that microflares are not necessarily spatially small. We present the first thermal energy distribution of RHESSI flares and compare it to previous thermal energy distributions of transient events. We also present the first nonthermal power distribution of RHESSI flares and find that a few microflares have unexpectedly large nonthermal powers up to 10²⁸ erg s⁻¹. The total microflare nonthermal energy, however, is still small compared to that of large flares as it occurs for shorter durations. These large energies and difficulties in analyzing the steep nonthermal spectra suggest that a sharp broken power law and thick-target bremsstrahlung model may not be appropriate for microflares.

We study the statistical significance of observed temporal variations of the solar active-region hemispheric helicity rule, as measured by the latitudinal gradient of the best-fit linear force-free-field parameter, dα/dφ . Using data from four different vector magnetographs, we compute and compare average annual dα/dφ values for these instruments for 19 years from solar cycles 21, 22, and 23. We find that although every instrument shows the "wrong" sign for the hemispheric rule in some years, there is no agreement among the instruments on which years are abnormal. None of the four data sets shows annual values of dα/dφ departing from the hemispheric helicity rule by more than 3 σ. We conclude that because the hemispheric helicity rule is a weak tendency with significant scatter, an annual subset of active regions is likely to produce statistically unreliable results.

We present a model for the total solar irradiance. The model takes the observed location, timing, and area of emerging active regions as input and produces a time-evolving size distribution of magnetic structures over the solar surface. We assume that the bright magnetic structures (faculae), which counteract the irradiance deficit caused by sunspots, consist of the products of active region decay. We simulate the decay process as a combination of fragmentation and boundary erosion of large-scale magnetic structures. The model has several adjustable parameters that control the decay processes and the irradiance contribution from the quiet Sun and the small-scale magnetic elements that are produced during the decay process. We use a genetic algorithm to estimate these parameters by fitting to the observed irradiance and daily sunspot area time series over the 1978-2007 time interval. Given the simplifications associated with the model, the resultant parameter values are well constrained within the imposed ranges. In addition, the irradiance and daily sunspot area time series produced by the best-fit models agree very well with the observations, although the sunspot area fits tend to perform better than the irradiance fits. However, it is evident that the model is neglecting a significant source of excess brightness, which manifests itself in two ways. First, the optimal values for the lifetime and intensity contrast of the bright, small-scale flux elements are both larger than expected. Second, the synthetic irradiance consistently underestimates the observations during the ascending phase of a cycle, despite the daily sunspot area fitting the observations quite well during these times. We also show that this genetic forward modeling approach can be used to detect a long-term trend of decadal timescale in the quiet-Sun irradiance. Assuming a constant quiet-Sun irradiance, we reconstruct the total solar irradiance over the 1874-1978 time interval, for which observational data of emerging active regions are available.

Two of the most attractive spectral windows for spectropolarimetric investigations of the physical properties of the plasma structures in the solar chromosphere and corona are the ones provided by the spectral lines of the He I 10830 and 5876 Å (or D₃) multiplets, whose polarization signals are sensitive to the Hanle and Zeeman effects. However, in order to be able to carry out reliable diagnostics, it is crucial to have a good physical understanding of the sensitivity of the observed spectral line radiation to the various competing driving mechanisms. Here we report a series of off-the-limb non-LTE calculations of the He I D₃ and 10830 Å emission profiles, focusing our investigation on their sensitivity to the EUV coronal irradiation and the model atmosphere used in the calculations. We show in particular that the intensity ratio of the blue to the red components in the emission profiles of the He I 10830 Å multiplet turns out to be a good candidate as a diagnostic tool for the coronal irradiance. Measurements of this observable as a function of the distance to the limb and its confrontation with radiative transfer modeling might give us valuable information on the physical properties of the solar atmosphere and on the amount of EUV radiation at relevant wavelengths penetrating the chromosphere from above.

The minimum energy fit (MEF), a velocity inversion technique, infers all components of the photospheric velocity that are consistent with the induction equation. From the set of consistent velocity fields, it selects the smallest overall flow speed by minimizing a kinetic energy functional. If partial velocity information is available from other measurements, it can be incorporated into the MEF methodology by minimizing the squared difference from that data. We incorporate the partial velocity information provided by local correlation tracking (LCT) technique and Doppler velocity measurements. We test the incorporation of these auxiliary velocity fields using the simulated magnetograms and velocitygrams. To the known velocity field we compare the results obtained from the MEF alone, the MEF with LCT constraints, and the MEF with LCT and Doppler information. We find that the combination of MEF with LCT and vertical velocity yields the best agreement. We also apply these three methods to actual vector magnetograms of AR 8210 obtained by the Imaging Vector Magnetograph. The results suggest that in this active region the helicity and energy fluxes are dominated by the horizontal rather than the vertical components of the velocity.

Recent observations of coronal-loop waves by TRACE and within the corona as a whole by CoMP clearly indicate that the dominant oscillation period is 5 minutes, thus implicating the solar p modes as a possible source. We investigate the generation of tube waves within the solar convection zone by the buffeting of p modes. The tube waves—in the form of longitudinal sausage waves and transverse kink waves—are generated on the many magnetic fibrils that lace the convection zone and pierce the solar photosphere. Once generated by p-mode forcing, the tube waves freely propagate up and down the tubes, since the tubes act like light fibers and form a waveguide for these magnetosonic waves. Those waves that propagate upward pass through the photosphere and enter the upper atmosphere, where they can be measured as loop oscillations and other forms of propagating coronal waves. We treat the magnetic fibrils as vertically aligned, thin flux tubes and compute the energy flux of tube waves that can be generated and driven into the upper atmosphere. We find that a flux in excess of 10⁵ ergs cm⁻² s⁻¹ can be produced, easily supplying enough wave energy to explain the observations. Furthermore, we compute the associated damping rate of the driving p modes and find that the damping is significant compared to observed line widths only for the lowest order p modes.

We present a well-calibrated EUV spectrum of a solar coronal bright point observed with the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket instrument on 2006 April 12. The coronal bright point brightened around 06:30 UT during a period of emerging magnetic flux and remained bright at least until the rocket flight around 18:12 UT, while the magnetic flux merged and canceled. Density-sensitive line intensity ratios yield mutually consistent coronal electron densities (N_e in cm⁻³) of log N_e ≈ 9.4. The differential emission measure (DEM, in cm⁻⁵ K⁻¹) curve derived from the spectrum yields a peak of log DEM ≈ 20.70 at log T ≈ 6.15 and a local minimum of log DEM ≈ 20.15 at log T ≈ 5.35. Photospheric (not coronal) element abundances are required to achieve equality and consistency in the DEM derived from lines of Mg V, Mg VI, Mg VII, and Ca VII (with a low first ionization potential, or FIP) and lines from Ne IV and Ne V (with a high FIP) formed at transition region temperatures. The bright point's photospheric abundance is likely produced by reconnection-driven chromospheric evaporation, a process that is not only central to existing bright point models, but also consistent with measurements of relative Doppler velocities.

We present the first vertical ion density profiles of Jupiter's upper atmosphere derived directly from ground-based observations. Observations of infrared H⁺₃ emissions in Jupiter's auroral/polar regions were collected by the high-resolution spectrometer NIRSPEC on the Keck II telescope. We have calculated vertical density profiles for a latitude in the southern auroral region using the measured column densities and a shell model of the Jovian ionospheric H⁺₃ emission. We compare our resultant profiles to those generated by a recent one-dimensional model in both local thermodynamic equilibrium (LTE) and non-LTE conditions. We find good agreement with the model profiles up to 1800 km. Above that, however, our measurements show that more H⁺₃ is produced than is predicted by the model. Our observational method is a new tool for probing Jupiter's upper atmosphere from Earth and can possibly be extended to the study of other gas giant planets.

Comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR) passed within ~0.3 AU of Earth in April and May of 2004. Their tails were observed by the Earth-orbiting Solar Mass Ejection Imager (SMEI) during this period. A time series of photometric SMEI sky maps displays the motions and frequent disruptions of the comet plasma tails. Ephemerides are used to unfold the observing geometry; the tails are often seen to extend ~0.5 AU from the comet nuclei. Having selected 12 of the more prominent motions as "events" for further study, we introduce a new method for determining solar wind radial velocities from these SMEI observations. We find little correlation between these and the changing solar wind parameters as measured close to Earth, or with coarse three-dimensional reconstructions using interplanetary scintillation data. A likely explanation is that the transverse sizes of the solar wind perturbations responsible for these disruptions are small, ≲0.05 AU. We determine the radial velocities of these events during the disruptions, using a technique only possible when the observed comet tails extend over a significant fraction of an AU. We find typical radial velocities during these events of 50-100 km s⁻¹ lower than before or afterward. Time durations of such events vary, typically from 3 to 8 hr, and correspond to comet traversal distances ~10⁶ km (0.007 AU). We conclude that these large disturbances are primarily due to ubiquitous solar wind flow variations, of which these measured events are a subset.

In recent years, there has been a proliferation of wide-field sky surveys to search for a variety of transient objects. Using relatively short focal lengths, the optics of these systems produce undersampled stellar images often marred by a variety of aberrations. As participants in such activities, we have developed a new algorithm for image subtraction that no longer requires high-quality reference images for comparison. The computational efficiency is comparable with similar procedures currently in use. The general technique is cross-convolution: two convolution kernels are generated to make a test image and a reference image separately transform to match as closely as possible. In analogy to the optimization technique for generating smoothing splines, the inclusion of an rms width penalty term constrains the diffusion of stellar images. In addition, by evaluating the convolution kernels on uniformly spaced subimages across the total area, these routines can accommodate point-spread functions that vary considerably across the focal plane.

Assuming that dark matter is dominated by WIMPs, it accretes by gravitational attraction and scattering over baryonic material and annihilates inside celestial objects, giving rise to a "dark luminosity" which may potentially affect the evolution of stars. We estimate the dark luminosity achieved by different kinds of stars in a halo with DM properties characteristic of the ones where the first star formation episode occurs. We find that both massive, metal-free and small, galactic-like stars can achieve dark luminosities comparable to or exceeding those due to their nuclear burning. This might have dramatic effects over the evolution of the very first stars, known as Population III.

Using deep near-infrared spectroscopy, Kriek et al. found that ~45% of massive galaxies at z ∼ 2.3 have evolved stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star (LGS)-assisted Keck/adaptive optics (AO). Considering that their median stellar mass is 1.7 × 10¹¹M_☉, the galaxies are remarkably small, with a median effective radius r_e = 0.9 kpc. Galaxies of similar mass in the nearby universe have sizes of ≈5 kpc and average stellar densities that are 2 orders of magnitude lower than the z ∼ 2.3 galaxies. These results extend earlier work at z ∼ 1.5 and confirm previous studies at z > 2 that lacked spectroscopic redshifts and imaging of sufficient resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most ~10% of the population of K-selected quiescent galaxies at z ∼ 2.3, effectively ruling out simple monolithic models for their formation. The galaxies must evolve significantly after z ∼ 2.3, through dry mergers or other processes, consistent with predictions from hierarchical models.

In low-collisionality plasmas heat flows almost exclusively along magnetic field lines and the condition for stability to convection is modified from the standard Schwarzschild criterion. We present local two- and three-dimensional simulations of a new heat-flux-driven buoyancy instability (the HBI) that occurs when the temperature in a plasma decreases in the direction of gravity. We find that the HBI drives a convective dynamo that amplifies an initially weak magnetic field by a factor of ~20. In simulations that begin with the magnetic field aligned with the temperature gradient, the HBI saturates by rearranging the magnetic field lines to be almost purely perpendicular to the initial temperature gradient. This magnetic field reorientation results in a net heat flux through the plasma that is less than 1% of the field-free (Spitzer) value. We show that the HBI is likely to be present in the cool cores of clusters of galaxies between ~0.1-100 kpc, where the temperature increases outward. The saturated state of the HBI suggests that inward thermal conduction from large radii in clusters is unlikely to solve the cooling flow problem. Finally, we also suggest that the HBI may contribute to suppressing conduction across cold fronts in galaxy clusters.

Using the APEX submillimeter telescope we have investigated the ¹²CO(3-2) emission in five face-on nearby barred spiral galaxies, where three of them are high surface brightness galaxies (HSBs) lying at the Freeman limit, and two are low surface brightness galaxies (LSBs). We have positive detections for two of three HSB spirals and nondetections for the LSBs. For the galaxies with positive detection (NGC 0521 and PGC 070519), the emission is confined to their bulges, with velocity dispersions of ~90 and ~73 km s^-1 and integrated intensities of 1.20 and 0.76 K km s^-1, respectively. For the nondetections, the estimated upper limit for the integrated intensity is ~0.54 K km s^-1. With these figures we estimate the H₂ masses as well as the atomic-to-molecular mass ratios. Although all the galaxies are barred, we observe ¹²CO(3-2) emission only for galaxies with prominent bars. We speculate that bars could dynamically favor the ¹²CO(3-2) emission, as a second parameter after surface brightness. Therefore, secular evolution could play a major role in boosting collisional transitions of molecular gas, such as ¹²CO(3-2), especially in LSBs.

We report the discovery of a coherent magnetic spiral structure within the nearby ringed Sab galaxy NGC 4736. High-sensitivity radio polarimetric data obtained with the VLA at 8.46 and 4.86 GHz show a distinct ring of total radio emission precisely corresponding to the bright inner pseudoring visible in other wavelengths. However, unlike the total radio emission, the polarized radio emission reveals a clear pattern of ordered magnetic field of spiral shape, emerging from the galactic center. The magnetic vectors do not follow the tightly wrapped spiral arms that characterize the inner pseudoring, but instead cross the ring with a constant and large pitch angle of about 35°. The ordered field is thus not locally adjusted to the pattern of star formation activity, unlike what is usually observed in grand-design spirals. The observed asymmetric distribution of Faraday rotation suggests the possible action of a large-scale MHD dynamo. The strong magnetic total and regular field within the ring (up to 30 and 13 μG, respectively) indicates that a highly efficient process of magnetic field amplification is under way, probably related to secular evolutionary processes in the galaxy.

Superbubbles (SBs) are among the greatest injectors of energy into the Galaxy, and have been proposed to be the acceleration site of Galactic cosmic rays. They are thought to be powered by the fast stellar winds and powerful supernova explosions of massive stars in dense stellar clusters and associations. Observations of the SB "DEM L192" in the neighboring Large Magellanic Cloud (LMC) show that it contains only about one-third the energy injected by its constituent stars via fast stellar winds and supernovae. It is not yet understood where the excess energy is going—thus, the so-called energy-crisis. We show here that it is very likely that a significant fraction of the unaccounted for energy is being taken up in accelerating cosmic rays, thus bolstering the argument for the SB origin of cosmic rays.

One of the possible origins of short gamma-ray bursts (SGRBs) is merging of compact binaries, and the effect of large kick velocity is a signature that can be used as an observational test for this hypothesis. Intracluster SGRBs that escaped from a host galaxy in a galaxy cluster are interesting in this context, since they would escape more easily by cluster tidal force, and would have brighter afterglow luminosity by dense intracluster gas, than those in general field galaxies. Here we calculate the escape fraction of compact binaries from their host galaxies in a galaxy cluster, and discuss some observational implications. We find that the escape fraction strongly depends on the nature of dark matter subhalos associated with member galaxies. If the amount of dark matter around member galaxies is not large and the gravitational potential for an escaping binary is determined mostly by stellar mass, most of SGRBs should escape and be observed as hostless, which is a much higher fraction than those in the field. Hence, statistics of intracluster SGRBs could give important information about the dark matter distribution in galaxy clusters, as well as hints for the origin of SGRBs.

We report the discovery of a bright transient X-ray source, CXOU J132518.2–430304, toward Centaurus A (Cen A) using six new Chandra X-Ray Observatory observations in 2007 March-May. Between 2003 and 2007, its flux has increased by a factor of >770. The source is likely a low-mass X-ray binary in Cen A with unabsorbed 0.3-10 keV band luminosities of (2-3) × 10³⁹ergs s⁻¹ and a transition from the steep-power-law state to the thermal state during our observations. CXOU J132518.2–430304 is the most luminous X-ray source in an early-type galaxy with extensive timing information that reveals transience and a spectral state transition. Combined with its luminosity, these properties make this source one of the strongest candidates to date for containing a stellar-mass black hole in an early-type galaxy. Unless this outburst lasts many years, the rate of luminous transients in Cen A is anomalously high compared to other early-type galaxies.

Recent observations show that hypernovae may deposit some fraction of their kinetic energy in mildly relativistic ejecta. In the dissipation process of such ejecta in a stellar wind, cosmic-ray protons can be accelerated up to ~10¹⁹ eV. We discuss the TeV to MeV gamma-ray and the X-ray photon signatures of cosmic rays accelerated in hypernovae. Secondary X-ray photons, emitted by electron-positron pairs produced via cascade processes due to high-energy protons, are the most promising targets for X-ray telescopes. Synchrotron photons emitted by protons can appear in the GeV band, requiring nearby (<40 Mpc) hypernovae for detection with GLAST. In addition, air Cerenkov telescopes may be able to detect regenerated TeV photons emitted by electron-positron pairs generated by CMB attenuation of π⁰-decay photons.

We present evidence that PG 1159 stars could harbor He-rich envelopes substantially thinner than those predicted by current evolutionary models with current estimates of mass loss, which may be attributable to an extensive mass-loss episode during the born-again AGB phase. Specifically, we show that the models with thin He-rich envelopes predict remarkably large magnitudes of the rates of period change of the trapped and untrapped modes observed in the pulsating star PG 1159–035. This is a consequence of the much shorter evolutionary timescale of the models with thin He-rich envelopes during the low-gravity PG 1159 regime. Our findings are particularly interesting in view of the suggestion of an evolutionary link between the helium-deficient PG 1159 star H1504+65 and the recently discovered white dwarfs with almost pure carbon atmospheres.

We report the detection of 18 infrared reflection nebulae (IRNe) in the J, H, and K_s linear polarimetric observations of the NGC 6334 massive star-formation complex, of which 16 IRNe are new discoveries. Our images cover ~180 arcmin², some of the widest near-infrared polarization data in star-formation regions so far. These IRNe are most likely associated with embedded young OB stars at different evolutionary phases, showing a variety of sizes, morphologies, and polarization properties, which can be divided into four categories. We argue that the different nebula characteristics are a possible evolutionary sequence of circumstellar structures around young massive stars.

The pulsating white dwarf G29-38 possesses a dust disk and metal lines attributed to the accretion of its disk material. Von Hippel and Thompson have reported variability in the equivalent width of G29-38's Ca II K line on a timescale of days. We use high-resolution optical spectroscopy of G29-38's Ca II K line to test this observation. Over 6 days spanning 2007 June to 2007 October we see no evidence for variability in the equivalent width of the Ca II K line. We also sample the variability of the Ca II K line over integrated timescales of ~100-500 s, where errors from incomplete coverage of pulsation modes are predicted to be ~8%-15%. We find that the scatter of the equivalent widths over this time period is consistent with measurement errors at the 7% level, slightly weaker than predicted but within the uncertainties of predictions. Weaker Ca and Mg lines observed show no significant variability on yearly timescales over 10 years based on our data and other high-resolution spectra. We conclude that further study is warranted to verify whether the accretion onto G29-38 is variable.

The presence of intergalactic star-forming regions within galaxy clusters shows that stars may form well away from a host galaxy. On larger scales, the existence of distorted isolated galaxies hosting star formation, apparently triggered by unseen companions, challenges our understanding of galaxy interactions and evolution. Moreover, most galaxies contain one or more massive central black holes that can be ejected by gravitational-radiation-induced recoil or by gravitational slingshot. In this Letter we examine the effect of a runaway supermassive black hole (RSMBH) passage through the intergalactic medium using the impulse approximation. Even if ejected at high speed, our analytical estimate indicates that SMBHs are able to ignite star formation efficiently in the wake of their trajectories. Cluster-bound RSMBHs may control intergalactic star formation and have substantial influence on the evolution of low-mass galaxies. Star formation induced by RSMBHs or, as we dub it, the invisible-hand mechanism may have had a very relevant role during the quasar epoch when most SMBHs formed.

Using the longest optical-interferometeric baselines currently available, we have detected strong near-infrared (NIR) emission from inside the dust destruction radius of Herbig Ae stars MWC 275 and AB Aur. Our submilliarcsecond resolution observations unambiguously place the emission between the dust destruction radius and the magnetospheric corotation radius. We argue that this new component corresponds to hot gas inside the dust sublimation radius, confirming recent claims based on spectrally resolved interferometry and dust evaporation front modeling.

I propose a method to detect planets around compact binaries that are strong sources of gravitational radiation. This approach is to measure gravitational wave phase modulations induced by the planets, and its prospect is studied with a Fisher matrix analysis. I find that, using LISA, planets can be searched for around ~3000 Galactic double white dwarfs with detection limit ≳ 4 M_J (M_J ∼ 2 × 10³⁰ g: the Jupiter mass). With its follow-on missions, planets with mass ≳ 1 M_J might be detected around double neutron stars even at cosmological distances z ∼ 1. In this manner, gravitational wave observation has the potential to make interesting contributions to extrasolar planetary science.

Most of the presently identified exoplanets have masses similar to that of Jupiter and therefore are assumed to be gaseous objects. With the ever-increasing interest in discovering lower mass planets, several of the so-called super-Earths (1 M_⊕ < M < 10 M_⊕), which are predicted to be rocky, have already been found. Here we report the possible discovery of a planet around the M-type star GJ 436 with a minimum mass of 4.7 ± 0.6 M_⊕ and a true mass of ~5 M_⊕, which would make it the least massive planet around a main-sequence star found to date. The planet is identified from its perturbations on an inner Neptune-mass transiting planet (GJ 436b), by pumping eccentricity and producing variations in the orbital inclination. Analysis of published radial velocity measurements indeed reveals a significant signal corresponding to an orbital period that is very close to the 2:1 mean motion resonance with the inner planet. The near-grazing nature of the transit makes it extremely sensitive to small changes in the inclination.

We have conducted a morphological analysis of the dust features generated during the first 5 weeks after the 2007 October outburst of comet 17P/Holmes. We used a computer simulation technique to generate synthetic images of dust ejecta released from discrete sources on a rotating nucleus. We found that the main "jets" are caused by discrete sources located near the south pole of the nucleus, while particle ejection is otherwise produced on the whole sunlit southern hemisphere, causing the observed dust shell in the sunward direction. The rotational parameters we derived (Φ = 213° and I = 90°, or the north pole pointing to R.A. = 38.8°, decl. = +35.1°) indicate that the subsolar latitude was near –90° at the outburst time. Assuming the same dust distribution function everywhere on the comet surface, and the same particle size dependence on the velocity, the ejection velocities for particles forming the "jets" must be remarkably different, by a factor of ~5 slower, than the background hemispherical particle emission. Based on a comparison of synthetic images with cometary images obtained during the 1892 outburst, we show that the rotational parameters are remarkably consistent during both 1892 and 2007 apparitions.

Saturn's largest satellite, Titan, has a thick atmosphere dominated by nitrogen and methane. The dense orange-brown smog hiding the satellite's surface is produced by photochemical reactions of methane, nitrogen, and their dissociation products with solar ultraviolet, which lead primarily to the formation of ethane and heavier hydrocarbons. In the years prior to the exploration of Titan's surface by the Cassini-Huygens spacecraft, the production and condensation of ethane was expected to have formed a satellite-wide ocean 1 km in depth, assuming that it was generated over the solar system's lifetime. However, Cassini-Huygens observations failed to find any evidence of such an ocean. Here we describe the main cause of the ethane deficiency on Titan: cryovolcanic lavas regularly cover its surface, leading to the percolation of the liquid hydrocarbons through this porous material and its accumulation in subsurface layers built up during successive methane outgassing events. The liquid stored in the pores may, combined with the ice layers, form a stable ethane-rich clathrate reservoir, potentially isolated from the surface. Even with a low open porosity of 10% for the subsurface layers, a cryovolcanic icy crust less than 2300 m thick is required to bury all the liquid hydrocarbons generated over the solar system's lifetime.

Solar wind plasma is known to cool down more slowly while it is blown away from the Sun than expected from an adiabatic spherical expansion. Some source of heating is thus needed to explain the observed temperature radial profile. The presence of a nonlinear turbulent magnetohydrodynamic energy cascade has been recently observed in solar wind plasma. This provides for the first time a direct estimation of the turbulent energy transfer rate, which can contribute to the in situ heating of the wind. The value of such contribution is shown to represent an important fraction (from 5% to 100%) of the total heating, and is strongly correlated with the wind temperature.