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It has recently been established that the filaments seen in the Las Campanas Redshift Survey (LCRS) are statistically significant at scales as large as 70-80 h^-1 Mpc in the δ = -3° slice and 50-70 h^-1 Mpc in the five other LCRS slices. The ability to produce such filamentary features is an important test of any model for structure formation. We have tested the ΛCDM model with a featureless, scale-invariant primordial power spectrum by quantitatively comparing the filamentarity in simulated LCRS slices with the actual data. The filamentarity in an unbiased ΛCDM model, we find, is less than the LCRS. Introducing a bias b = 1.15, the model is in rough consistency with the data, although in two of the slices the filamentarity falls below the data at a low level of statistical significance. The filamentarity is very sensitive to the bias parameter, and a high value (b = 1.5), which enhances filamentarity at small scales and suppresses it at large scales, is ruled out. A bump in the power spectrum at k ~ 0.05 h Mpc^-1 is found to have no noticeable effect on the filamentarity.

This paper explores the potential for statistical epoch of reionization (EOR) measurements using wide-field radio observations. New developments in low-frequency radio instrumentation and signal processing allow very sensitive EOR measurements, and the analysis techniques enabled by these advances offer natural ways of separating the EOR signal from the residual foreground emission. This paper introduces the enabling technologies and proposes an analysis technique designed to make optimal use of the capabilities of next-generation low-frequency radio arrays. The observations we propose can directly observe the power spectrum of the EOR using relatively short observations and are significantly more sensitive than other techniques that have been discussed in the literature. For example, in the absence of foreground contamination the measurements we propose would produce five 3 σ power spectrum points in 100 hr of observation with only 4 MHz bandwidth with LOFAR for simple models of the high-redshift 21 cm emission. The challenge of residual foreground removal may be addressed by the symmetries in the three-dimensional (two spatial frequencies and radio frequency) radio interferometric data. These symmetries naturally separate the EOR signal from most classes of residual unsubtracted foreground contamination, including all foreground continuum sources and radio line emission from the Milky Way.

We investigate the expected gravitational wave emission from coalescing supermassive black hole (SMBH) binaries resulting from mergers of their host galaxies. When galaxies merge, the SMBHs in the host galaxies sink to the center of the new merged galaxy and form a binary system. We employ a semianalytic model of galaxy and quasar formation based on the hierarchical clustering scenario to estimate the amplitude of the expected stochastic gravitational wave background due to inspiraling SMBH binaries and bursts due to the SMBH binary coalescence events. We find that the characteristic strain amplitude of the background radiation is h_c(f) ~ 10^-16(f/1 μHz)^-2/3 for f ≲ 1 μHz just below the detection limit from measurements of the pulsar timing provided that SMBHs coalesce simultaneously when host galaxies merge. The main contribution to the total strain amplitude of the background radiation comes from SMBH coalescence events at 0 < z < 1. We also find that a future space-based gravitational wave interferometer such as the planned Laser Interferometer Space Antenna might detect intense gravitational wave bursts associated with coalescence of SMBH binaries with total mass M_tot < 10⁷M_☉ at z ≳ 2 at a rate ~1.0 yr^-1. Our model predicts that burst signals with a larger amplitude h_burst ~ 10^-15 correspond to coalescence events of massive SMBH binary with total mass M_tot ~ 10⁸M_☉ at low redshift (z ≲ 1) at a rate ~0.1 yr^-1, whereas those with a smaller amplitude (h_burst ~ 10^-17) correspond to coalescence events of less massive SMBH binaries with total mass M_tot ~ 10⁶M_☉ at high redshift (z ≳ 3).

We investigate the interactions of high-redshift galaxy outflows with low-mass virialized clouds of primordial composition. While atomic cooling allows star formation in objects with virial temperatures above 10⁴ K, "minihalos" with virial temperatures below this threshold are generally unable to form stars by themselves. However, the large population of high-redshift starburst galaxies may have induced widespread star formation in neighboring minihalos, via shocks that caused intense cooling through both nonequilibrium H₂ formation and metal-line emission. Using a simple analytic model, we show that the resulting star clusters naturally reproduce three key features of the observed population of halo globular clusters (GCs). First, the 10⁴ K maximum virial temperature directly corresponds to the ~10⁶M_☉ upper limit on the stellar mass of such clusters, a feature that cannot be explained by any GC destruction mechanism. Secondly, the momentum imparted in such interactions is sufficient to strip the gas from its associated dark matter halo, explaining why GCs do not reside in the dark matter potential wells that are ubiquitous in galaxies. Finally, the mixing of ejected metals into the primordial gas provides a straightforward mechanism to explain the approximately 0.1 dex homogeneity of stellar metallicities within a given GC, while at the same time allowing for a large spread in metallicity between different clusters. To study the possibility of such "fine grained" mixing in detail, we use a simple one-dimensional numerical model of turbulence transport to simulate mixing in cloud-outflow interactions. We find that as the shock shears across the side of the cloud, Kelvin-Helmholtz instabilities arise, which cause turbulent mixing of enriched material into ≳20% of the cloud. Such estimates ignore the likely presence of large-scale vortices, however, which would further enhance turbulence generation. Thus, the global nature of mixing in these interactions is multidimensional, and quantitative predictions must await more detailed numerical studies.

Alfvén turbulence caused by statistically isotropic and homogeneous primordial magnetic field induces correlations in the cosmic microwave background (CMB) anisotropies. The correlations are specifically between the spherical harmonic modes a_l-1,m and a_l+1,m. In this paper we approach this issue from phase analysis of the CMB maps derived from the WMAP data sets. Using circular statistics and return phase mapping, we examine phase correlation of Δl = 2 for the primordial non-Gaussianity caused by the Alfvén turbulence at the epoch of recombination. Our analyses show that such specific features from the power-law Alfvén turbulence do not contribute significantly in the phases of the maps and could not be a source of primordial non-Gaussianity of the CMB.

Observations of the cosmic microwave background (CMB) can be contaminated by diffuse foreground emission from sources such as Galactic dust and synchrotron radiation. In these cases, the morphology of the contaminating source is known from observations at different frequencies, but not its amplitude at the frequency of interest for the CMB. We develop a technique for accounting for the effects of such emission in this case, and for simultaneously estimating the foreground amplitude in the CMB observations. We apply the technique to CMB data from the MAXIMA-1 experiment, using maps of Galactic dust emission from combinations of IRAS and DIRBE observations, as well as compilations of Galactic synchrotron emission observations. The spectrum of the dust emission over the 150-450 GHz observed by MAXIMA is consistent with preferred models, but the effect on CMB power spectrum observations is negligible.

We present H₀ results from Cosmic Background Imager (CBI) observations of the Sunyaev-Zel'dovich effect (SZE) in seven galaxy clusters, A85, A399, A401, A478, A754, A1651, and A2597. These observations are part of a program to study a complete, volume-limited sample of low-redshift (z < 0.1), X-ray-selected clusters. Our focus on nearby objects allows us to study a well-defined, orientation-unbiased sample, minimizing systematic errors due to cluster asphericity. We use density models derived from ROSAT imaging data and temperature measurements from ASCA and BeppoSAX spectral observations. We quantify in detail sources of error in our derivation of H₀, including calibration of the CBI data, density, and temperature models from the X-ray data, cosmic microwave background primary anisotropy fluctuations, and residuals from radio point source subtraction. From these seven clusters we obtain a result of H₀ = 67 km s^-1 Mpc^-1 for an unweighted sample average. The respective quoted errors are random and systematic uncertainties at 68% confidence. The dominant source of error is confusion from intrinsic anisotropy fluctuations.

Evidence from a large sample of quasar absorption-line spectra in damped Lyα systems has suggested a possible time variation of the fine-structure constant, α. The most statistically significant portion of this sample involves the comparison of Mg and Fe wavelength shifts using the many-multiplet (MM) method. However, the sensitivity of this method to the abundance of heavy isotopes, especially Mg, is enough to imitate an apparent variation in α in the redshift range 0.5 < z < 1.8. We implement recent yields of intermediate mass (IM) stars into a chemical evolution model and show that the ensuing isotope distribution of Mg can account for the observed variation in α provided the early initial mass function was particularly rich in IM stars or that the heavy Mg isotope yields from asymptotic giant branch stars are even higher than in present-day models. As such, these observations of quasar absorption spectra can be used to probe the nucleosynthetic history of low-metallicity damped Lyα systems in the redshift range 0.5 < z < 1.8. This analysis, in conjunction with other abundance measurements of low-metallicity systems, reinforces the mounting evidence that star formation at low metallicities may have been strongly influenced by a population of IM stars. Such IM stars have a significant influence on other abundances, particularly nitrogen. We constrain our models with independent measurements of N, Si, and Fe in damped Lyα systems as well as C/O in low-metallicity stars. In this way, we obtain consistent model parameters for this chemical evolution interpretation of the MM method results.

The chemical composition of high-redshift galaxies is an important property that gives clues to their past history and future evolution and yet is difficult to measure with current techniques. In this paper we investigate new metallicity indicators based on the strengths of stellar photospheric features at rest-frame ultraviolet wavelengths. By combining the evolutionary spectral synthesis code Starburst99 with the output from the non-LTE model atmosphere code WM-basic, we have developed a code that can model the integrated ultraviolet stellar spectra of star-forming regions at metallicities between 1/20 and twice solar. We use our models to explore a number of spectral regions that are sensitive to metallicity and clean of other spectral features. The most promising metallicity indicator is an absorption feature between 1935 and 2020 Å, which arises from the blending of numerous Fe III transitions. We compare our model spectra with observations of two well-studied high-redshift star-forming galaxies, MS 1512-cB58 (a Lyman break galaxy at z_em = 2.7276) and Q1307-BM1163 (a UV-bright galaxy at z_em = 1.411). The profiles of the photospheric absorption features observed in these galaxies are well reproduced by the models. In addition, the metallicities inferred from their equivalent widths are in good agreement with previous determinations based on interstellar absorption and nebular emission lines. Our new technique appears to be a promising alternative, or complement, to established methods, which have only a limited applicability at high redshifts.

The goal of the Complete Optical and Radio Absorption Line System (CORALS) survey is to quantify the potential impact on QSO absorber statistics from dust in intervening galaxies. Dust may introduce a selection bias in surveys that are based on magnitude-limited QSO samples, leading to an underestimate of absorber number density, n(z). Here we present the results of the second phase of the CORALS survey, which extends our previous work on z > 1.8 damped Lyα systems (DLAs) to search for strong metal line systems (candidate DLAs) in the range 0.6 < z < 1.7. We have identified 47 Mg II systems with rest-frame equivalent widths EW(Mg λ2796) > 0.3 Å in our sample of 75 radio-selected quasars. The total redshift path covered by the survey is Δz = 35.2, 58.2, and 63.8 for EW(Mg λ2796) > 0.3, 0.6, and 1.0 Å thresholds, respectively (5 σ). Our principal and most robust result is that the n(z) value of low-redshift Mg II systems determined for the CORALS survey is in excellent agreement with that of optically selected, magnitude-limited QSO samples. We use empirically determined Mg II equivalent width statistics to estimate the likely number of DLAs in this sample. The statistically inferred number density of DLAs, n(z) = 0.16, is consistent with other low-redshift samples, although the large 1 σ error bars permit up to a factor of 2.5 more DLAs in CORALS. However, confirmation of the DLA candidates, precise evaluation of their n(z) values, and measurement of their H I column densities awaits UV observations with the Hubble Space Telescope. Finally, we report an excess of intermediate-redshift Mg II systems observed toward bright QSOs, which could be due to a lensing amplification bias. However, there is also evidence that this excess could simply be due to more sensitive EW detection limits toward brighter QSOs. We also emphasize that absorber statistics determined from magnitude-limited surveys reach a steady value if the completeness limit is significantly fainter than the fiducial value of the quasar luminosity function.

Recent improved determinations of the mass density ρ_BH of supermassive black holes (SMBHs) in the local universe have allowed accurate comparisons of ρ_BH with the amount of light received from past quasar activity. These comparisons support the notion that local SMBHs are "dead quasars" and yield a value ≳ 0.1 for the average radiative efficiency of cosmic SMBH accretion. BH coalescences may represent an important component of the quasar mass assembly and yet not produce any observable electromagnetic signature. Therefore, ignoring gravitational wave (GW) emission during such coalescences, which reduces the amount of mass locked into remnant BHs, results in an overestimate of . Here we put constraints on the magnitude of this bias. We calculate the cumulative mass loss to GWs experienced by a representative population of BHs during repeated cosmological mergers, using loss prescriptions based on detailed general relativistic calculations. Despite the possibly large number of mergers in the assembly history of each individual SMBH, we find that near-equal mass mergers are rare; therefore, the cumulative loss is likely to be modest, amounting at most to a 20% increase in the inferred value. Thus, recent estimates of ≳ 0.1 appear robust. The space interferometer LISA should provide empirical constraints on the dark side of quasar evolution by measuring the masses and rates of coalescence of massive BHs to cosmological distances.

The Far Ultraviolet Spectroscopic Explorer (FUSE) has surveyed a large sample (>100) of active galactic nuclei (AGNs) in the low-redshift universe (z < 1). Its response at short wavelengths makes it possible to measure directly the far-ultraviolet spectral properties of quasi-stellar objects (QSOs) and Seyfert 1 galaxies at z < 0.3. Using archival FUSE spectra, we form a composite extreme-ultraviolet (EUV) spectrum of QSOs at z ≤ 0.67. After consideration of many possible sources of systematic error in our analysis, we find that the spectral slope of the FUSE composite spectrum, α = -0.56 for F_ν ∝ ν^α, is significantly harder than the EUV (λ ≲ 1200 Å) portion of the composite spectrum of QSOs with z > 0.33 formed from archival Hubble Space Telescope (HST) spectra, α = -1.76 ± 0.12. We identify several prominent emission lines in the FUSE composite and find that the high-ionization O VI and Ne VIII emission lines are enhanced relative to the HST composite. Power-law continuum fits to the individual FUSE AGN spectra reveal a correlation between EUV spectral slope and AGN luminosity in the FUSE and FUSE+HST samples, in the sense that lower luminosity AGNs show harder spectral slopes. We find an anticorrelation between the hardness of the EUV spectral slope and AGN black hole mass, using estimates of this quantity found in the literature. We interpret these results in the context of the well-known anticorrelation between AGN luminosity and emission-line strength, the Baldwin effect, given that the median luminosity of the FUSE AGN sample is an order of magnitude lower than that of the HST sample.

We report on a short XMM-Newton observation of the radio-quiet narrow-line Seyfert 1 galaxy PG 1402+261. The EPIC X-ray spectrum of PG 1402+261 shows a strong excess of counts between 6 and 9 keV in the rest frame. This feature can be modeled by an unusually strong (equivalent width 2 keV) and very broad (FWHM velocity of 110,000 km s^-1) iron K-shell emission line. The line centroid energy at 7.3 keV appears blueshifted with respect to the iron Kα emission band between 6.4 and 6.97 keV, while the blue wing of the line extends to 9 keV in the quasar rest frame. The line profile can be fitted by reflection from the inner accretion disk, but an inclination angle of >60° is required to model the extreme blue wing of the line. Furthermore, the extreme strength of the line requires a geometry whereby the hard X-ray emission from PG 1402+261 above 2 keV is dominated by the pure-reflection component from the disk, while little or none of the direct hard power law is observed. Alternatively, the spectrum above 2 keV may be explained by an ionized absorber, if the column density is sufficiently high (N_H > 3 × 10²³ cm^-2) and if the matter is ionized enough to produce a deep (τ ~ 1) iron K-shell absorption edge at 9 keV. This absorber could originate in a large column density, high-velocity outflow, perhaps similar to those that appear to be observed in several other high accretion rate active galactic nuclei. Further observations, especially at higher spectral resolution, are required to distinguish between the accretion disk reflection and outflow scenarios.

We present integral field spectroscopy (IFS) of the central region of 3C 120. We have modeled the nuclear and host galaxy three-dimensional spectra using techniques normally applied to imaging, decoupling both components and obtaining a residual data cube. Using this residual data cube, we detected the extended emission line region associated with the radio jet. We obtained, for the first time, a clean spectrum of this region and found compelling evidence of a jet-cloud interaction. The jet compresses and splits the gas cloud, which is ionized by the active galactic nucleus (AGN) and/or by the strong local UV photon field generated by a shock process. We cannot confirm the detection of an extended emission line region associated with the counterjet reported by Axon and coworkers.

We report on archival data from the Chandra X-Ray Observatory for the radio jet of 3C 120. We consider the emission process responsible for the X-rays from four knots using spectra constructed from radio, optical, and X-ray intensities. While a simple synchrotron model is adequate for three of the knots, the fourth ("k25"), which was previously detected by ROSAT and is now well resolved with Chandra, still represents a problem for the conventional emission processes. If, as we argue, the flat X-ray spectra from two parts of k25 are synchrotron emission, then it appears that either the emission comes from an electron distribution spectrally distinct from that responsible for the radio emission or at the highest electron energies there is a significant deviation from the power law describing the electron distribution at lower energies.

We report on VLBI observations of M81*, the northeast-southwest oriented nuclear core-jet source of the spiral galaxy M81, at five different frequencies between 1.7 and 14.8 GHz. By phase referencing to supernova 1993J we can accurately locate the emission region of M81* in the galaxy's reference frame. Although the emission region's size decreases with increasing frequency while the brightness peak moves to the southwest, the emission region seems sharply bounded to the southwest at all frequencies. We argue that the core must be located between the brightness peak at our highest frequency (14.8 GHz) and the sharp bound to the southwest. This narrowly constrains the location of the core, or the purported black hole in the center of the galaxy, to be within a region of ±0.2 mas or ±800 AU (at a distance of ~4 Mpc). This range includes the core position that we determined earlier by finding the most stationary point in the brightness distribution of M81* at only a single frequency. This independent constraint therefore strongly confirms our earlier core position. Our observations also confirm that M81* is a core-jet source, with a one-sided jet that extends to the northeast from the core, on average curved somewhat to the east, with a radio spectrum that is flat or inverted near the core and steep at the distant end. The brightness peak is unambiguously identified with the variable jet rather than the core, which indicates limitations in determining the proper motion of nearby galaxies and in refining the extragalactic reference frame.

We present a mosaic of XMM-Newton observations of the nearby major-merger cluster A754 that has either a wider field or better spectral or spatial resolution than previous observations. We construct maps of X-ray surface brightness and temperature integrated along the line of sight. From these two primary maps we derive pseudopressure and pseudoentropy maps. There is structure on a large range of scales in these maps, but the basic pattern is similar to numerical hydrodynamic simulations of cluster mergers. The high surface brightness eastern bar contains gas with the minimum entropy and temperature coupled with the highest iron abundance and density in the cluster. A new feature revealed by these observations is a plumelike structure that appears to emerge from the bar heading northwest. The diffuse radio source also occupies this region, and there is some correspondence between the two. Another new feature is a rim of hot gas to the east, south, and west. We interpret the bar as the core gas from the original main cluster flattened and displaced from the dark matter potential minimum by the merger. The hot rim is the outgoing forward shock from the merger. However, this and previous shocks were weak (M ≤ 2.25), so they are likely only small contributors to the radio-emitting particles. These observations lend support to the merger hypothesis in A754, but some of the parameters of existing models need modification.

Intracluster planetary nebulae are a useful tracer of the evolution of galaxies and galaxy clusters. We analyze our catalog of 318 intracluster planetary nebulae candidates found in 0.89 deg^-2 of the Virgo Cluster. We give additional evidence for the great depth of the Virgo Cluster's intracluster stellar population, which implies that the bulk of the intracluster stars come from late-type galaxies and dwarfs. We also provide evidence that the intracluster stars are clustered on the sky on arcminute scales, in agreement with tidal-stripping scenarios of intracluster star production. Although significant systematic uncertainties exist, we find that the average fraction of intracluster starlight in the Virgo Cluster is 15.8% ± 3.0% (statistical) ± 5.0% (systematic) and may be higher if the intracluster stars have a large spatial line-of-sight depth. We find that the intracluster star density changes little with radius or projected density over the range surveyed. These results, along with other intracluster star observations, imply that intracluster star production in Virgo is ongoing and consistent with the cluster's known dynamical youth.

The evolution in the comoving space density of the global average galaxy star formation rate (SFR) out to a redshift around unity is well established. Beyond z ≈ 1 there is growing evidence that this evolution is flat or even increasing, contrary to early indications of a turnover. Some recent analyses of z ≈ 6 photometric dropouts are suggestive of a decline from z = 3 to z ≈ 6, but there is still very little constraint on the extent of dust obscuration at such high redshifts. In less than a decade, numerous measurements of galaxy SFR density spanning z = 0 to as high as z ≈ 6 have rapidly broadened our understanding of galaxy evolution, and a summary of existing SFR density measurements is presented here. This global star formation history compilation is found to be consistent to within factors of about 3 over essentially the entire range 0 < z ≲ 6, and it can be used to constrain the evolution of the luminosity function (LF) for star-forming galaxies. The LF evolution for star-forming galaxies has been previously explored using optical source counts, as well as radio source counts at 1.4 GHz, and a well-known degeneracy between luminosity evolution [L ∝ (1 + z)^Q] and density evolution [ϕ ∝ (1 + z)^P] is found. Combining the constraints from the global SFR density evolution with those from the 1.4 GHz radio source counts at submillijansky levels allows this degeneracy to be broken and a best-fitting evolutionary form to be established. The preferred evolution in a H₀ = 70, Ω_M = 0.3, Ω_Λ = 0.7 cosmology from these combined constraints is Q = 2.70 ± 0.60, P = 0.15 ± 0.60.

Recent Chandra observations of nearby galaxies have revealed a number of ultraluminous X-ray sources (ULXs) with super-Eddington luminosities, away from the central regions of nonactive galaxies. The nature of these sources is still debated. We argue that a fraction of them could be young, Crab-like pulsars, the X-ray luminosity of which is powered by rotation. We use the pulsar birth parameters estimated from radio pulsar data to compute the steady state pulsar X-ray luminosity distribution as a function of the star formation rate (SFR) in the galaxy. We find that ~10% of optically dull galaxies are expected to have a source with L_X ≳ 10³⁹ ergs s^-1, while starburst galaxies should each have several of these sources. We estimate that the X-ray luminosity of a few percent of galaxies is dominated by a single bright pulsar with L_X ≳ 10³⁹ ergs s^-1, roughly independently of its SFR. We discuss observational diagnostics that can help distinguish the young pulsar population in ULXs.

We have obtained multiobject spectroscopy of H II regions in the spiral galaxy M51 with the Keck I telescope and the Low Resolution Imaging Spectrometer. For 10 objects we have detected the auroral line [N II] λ5755, while [S III] λ6312 has been measured in seven of these. This has allowed us to measure the electron temperature of the gas and to derive oxygen, sulfur, and nitrogen abundances for the 10 H II regions. Contrary to expectations from previous photoionization models of a few H II regions in M51 and from strong-line abundance indicators, the O/H abundance is below the solar value for most objects, with the most metal-rich H II regions, P203 and CCM 72, having log(O/H) = -3.16 [~1.4(O/H)_☉] and log(O/H) = -3.29 [~1.0 (O/H)_☉], respectively. The reduction of O/H by factors of up to 2 or 3 with respect to previous indirect determinations has important consequences for the calibration of empirical abundance indicators, such as R₂₃, in the abundance and excitation range found in the central regions of spiral galaxies. The abundance gradients in these galaxies can therefore be considerably flatter than those determined by using such empirical calibrations. The H II regions with a measured electron temperature span the range (0.19-1.04) R₀ in galactocentric radius and indicate a shallow abundance gradient for M51: -0.02 ± 0.01 dex kpc^-1. The S/O abundance ratio is found to be similar to previous determinations of its value in other spiral galaxies, log(S/O) ≈ -1.6. Therefore, we find no evidence for a variation in massive-star initial mass function or nucleosynthesis at high oxygen abundance. An overabundance of nitrogen is measured, with log(N/O) ≃ -0.6. On the basis of our new abundances, we revise the effective yield for M51, now found to be almost 4 times lower than previous estimates, and we discuss this result in the context of chemical evolution in galactic disks. Features from Wolf-Rayet stars (the blue bump at 4660 Å and the C III line at 5696 Å) are detected in a large number of H II regions in M51, with the C III λ5696 line found preferentially in the central, most metal-rich objects.

Archival XMM-Newton data of the central region of M31 were analyzed for diffuse X-ray emission. Point sources with 0.5-10 keV luminosity exceeding ~4 × 10³⁵ ergs s^-1 were detected. Their summed spectra are reproduced well by a combination of a disk blackbody component and a blackbody component, implying that the emission mainly comes from an assembly of luminous low-mass X-ray binaries. After excluding these point sources, spectra were accumulated over a circular region of 6' (1.2 kpc) centered on the nucleus. In the energy range above 2 keV, these residual spectra are understood to mainly be contributions from unresolved faint sources and spillover of photons from the excluded point sources. There is in addition a hint of ~6.6 keV line emission, which can be produced by a hot (temperature of several keV), thin, thermal plasma. Below 2 keV the spectra involve three additional softer components expressed by thin thermal plasma emission models, for which the temperatures are ~0.6, ~0.3, and ~0.1 keV. Their 0.5-10 keV luminosities within 6' are measured to be ~1.2 × 10³⁸, ~1.6 × 10³⁸, and ~4 × 10³⁷ ergs s^-1 in order of decreasing temperature. The archival Chandra data of the central region of M31 yielded consistent results. By incorporating different annular regions, all three softer thermal components were confirmed to be significantly extended. These results are compared with reports from previous studies. A discussion is presented on the origin of each thermal emission component.

The supermassive black hole at the Galactic center harbors a bound cluster of massive stars that should leave neutron star remnants. Extrapolating from the available data, we estimate that ~1000 radio pulsars may currently orbit Sgr A* with periods of ≲100 yr. Optimistically, 1-10 of the most luminous of these pulsars may be detectable with current telescopes in periodicity searches at frequencies near 10 GHz, where the effects of interstellar scattering are alleviated. Long-term timing observations of such a pulsar would clearly reveal its Keplerian motion and possibly show the effects of relativistic gravity. We briefly discuss how pulsar timing can be used to study the dynamical and interstellar environment of the central black hole and speculate on the prospects for astrometric observations of an orbiting pulsar.

We report the first measurement of the deuterium abundance in cosmic rays above 10 GeV nucleon^-1 of kinetic energy. The data were collected by the balloon-borne experiment CAPRICE98, which was flown on 1998 May 28-29 from Fort Sumner, New Mexico. The detector configuration included the NMSU-WiZard/CAPRICE superconducting magnet spectrometer equipped with a gas RICH detector, a silicon-tungsten calorimeter, and a time-of-flight system. By combining the information from the spectrometer and the RICH detector, it was possible to separate deuterons from protons in the kinetic energy range from 12 to 22 GeV nucleon^-1. In order to estimate the proton background and the deuteron selection efficiency, we developed an empirical model for the response of the instrument, based on the data collected in this experiment. The analysis procedure is described in this paper, and the result on the absolute flux of deuterium is presented. We found that the deuterium abundance at high energy is consistent with the hypothesis that the propagation mechanism of light nuclei is the same as that of heavier secondary components.

This paper presents archival ROSAT PSPC observations of the G65.2+5.7 supernova remnant (also known as G65.3+5.7). Little material obscures this remnant, and so it was well observed, even at the softest end of ROSAT's bandpass (~0.11-0.28 keV). These soft X-ray images reveal the remnant's centrally filled morphology, which, in combination with existing radio frequency observations, places G65.2+5.7 in the thermal composite (mixed-morphology) class of supernova remnants. Not only might G65.2+5.7 be the oldest known thermal composite supernova remnant, but owing to its optically revealed cool, dense shell, this remnant supports the proposal that thermal composite supernova remnants lack X-ray-bright shells because they have evolved beyond the adiabatic phase. These observations also reveal a slightly extended point source centered on R.A. 19^h36^m46^s, decl. 30°40'07'' and extending 65 in radius in the band 67 map. The source of this emission has yet to be discovered, as there is no known pulsar at this location.

The appearance of the young supernova remnant SN 1006 is dominated by emission from nonradiative shocks in the northeast and northwest regions. At X-ray energies the northeast shock exhibits predominantly nonthermal synchrotron emission, while the northwest shock exhibits a thermal spectrum. We present far-ultraviolet spectra of the northeast (NE) and northwest (NW) portions of SN 1006 acquired with the Far Ultraviolet Spectroscopic Explorer (FUSE). We have detected emission lines of O VI λλ1032, 1038 and broad Lyβ λ1025 in the NW filament but detect no emission lines in the NE region down to a level of 4.7 × 10^-17 ergs cm^-2 s^-1 arcsec^-2. We observed in the NW an O VI intensity of 2.0 ± 0.2 × 10^-16 ergs cm^-2 s^-1 arcsec^-2 and measured an O VI line width of 2100 ± 200 km s^-1 at a position where the Hα width was measured to be 2290 ± 80 km s^-1. This implies less than mass-proportional heating of the ions. Using the ratio of intensities I(NW)/I(NE) ~ n(NW)/n(NE), the density ratio of the two regions is found to be ≥4, a value that is consistent with the uncertainties of the ratio of 2.5 measured in 2003 by Long and coworkers. The derived O VI kinetic temperature is compared to previous estimates of electron, proton, and ion temperatures in the remnant to study the relative heating efficiency of various species at the shock front. The degree of postshock temperature equilibration may be crucial to particle acceleration, since the temperature of each species determines the number of high-speed particles available for injection into an acceleration process that could produce Galactic cosmic rays.

In the framework of the transition matrix approach, we calculate the relevant optical properties of cosmic dust grains of amorphous carbon and astronomical silicates, modeled as aggregates of spherical monomers. Two mechanisms of aggregation were considered, producing clusters with different structure and degree of fluffiness: ballistic particle-cluster aggregation (BPCA) and ballistic cluster-cluster aggregation (BCCA). Our results are very different from those obtained through computational approaches based on effective medium theories and might have major implications both on the modeling procedure and on the dust-mass balance in the interstellar medium.

The polarized thermal radiation at far-infrared and submillimeter wavelengths from dust grains in interstellar clouds with irregular magnetic fields is simulated. The goal is to determine to what degree irregularity in the magnetic fields can be consistent with the observation that the maps of the polarization vectors are relatively ordered. Detailed calculations are performed for the reduction in the fractional polarization and the dispersion in position angles as a function of the ratio of the irregular to the uniform magnetic field and as a function of the relevant dimensions measured in correlation lengths of the field. We show that the polarization properties of quiescent clouds and of star-forming regions are consistent with Kolmogorov-like turbulent magnetic fields that are comparable in magnitude to the uniform component of the magnetic fields. If the beam size is much smaller than the correlation length, L_corr, of the fields, the calculated percentage polarization, p, decreases to an asymptotic value when the number of correlation lengths, N_corr, through the cloud exceeds a few × 10. For these values of N_corr, the dispersion in the position angles, σ_α, is still appreciable, decreasing to only ~20°. However, when the finite size of a telescope beam is taken into account the asymptotic value of p is reached for fewer correlation lengths (smaller N_corr) because of averaging over the beam; σ_α becomes much smaller and is consistent with the observational data. The smoothing of the polarization properties due to the combined effect of the thickness of the cloud and the finite size of the beam can be described by a single variable that we designate as the generalized number of correlation lengths. In addition, we study various factors that may contribute to the decrease in the linear polarization percentage with increasing intensity that is observed at submillimeter and far-infrared wavelengths in many, although not all, dark clouds (the "polarization hole" effect). Depolarization due to a density cutoff in the polarizing effect of dust, thermalization, and correlations between the density and the properties of the magnetic field are considered.

We have used observations taken under the Far Ultraviolet Spectroscopic Explorer (FUSE) S405/505 channel realignment program to explore the diffuse far-ultraviolet (FUV; 1000-1200 Å) radiation field. Of the 71 independent locations in that program, we have observed a diffuse signal in 32, ranging in brightness from 1600 to a maximum of 2.9 × 10⁵ photons cm^-2 sr^-1 s^-1 Å^-1 in Orion. The FUSE data confirm that the diffuse FUV sky is patchy with regions of intense emission, usually near bright stars, but also with dark regions, even at low Galactic latitudes. We find a weak correlation between the FUV flux and the 100 μm ratio but with wide variations, perhaps due to differences in the local radiation field.

We present high signal-to-noise ratio echelle spectra of the compact high surface brightness, low-ionization planetary nebula (PN) IC 418. These reveal 807 emission lines down to intensities less than 10^-5 that of Hβ for which we determine widths and relative intensities. We show that line profiles are a valuable parameter for making line identifications and in constraining the excitation mechanism of the lines. We present evidence that indicates that many supposed high-level recombination lines may in fact be excited by a process other than recombination. We contend from the detection of dielectronic recombination lines that their relatively low intensities argue against their making a significant contribution to level populations of the heavy ions in this object. Following similar analyses of other PNe we find that IC 418 shows a small discrepancy in ion abundances derived from forbidden versus recombination lines of the heavy elements.

Absorption and emission lines originating from the ν₃ C–H stretching manifold of gas-phase CH₄ were discovered in the high-resolution (R = 25,000) infrared L-band spectrum along the line of sight toward NGC 7538 IRS 9. These observations provide a diagnostic of the complex dynamics and chemistry in a massive star-forming region. The line shapes resemble P Cygni profiles with the absorption and emission components shifted by ~7 km s^-1 with respect to the systemic velocity. Similar velocity components were observed in CO at 4.7 μm, but in contrast to CH₄, the CO shows deep absorption due to a high-velocity outflow as well as absorption at the systemic velocity due to the cold outer envelope. It is concluded that the gas-phase CH₄ abundance varies by an order of magnitude in this line of sight: it is low in the envelope and the outflow (X[CH₄] < 0.4 × 10^-6) and at least a factor of 10 larger in the central core. The discovery of solid CH₄ in independent ground- and space-based data sets shows that methane is nearly entirely frozen onto grains in the envelope. It thus appears that CH₄ is formed by grain surface reactions, evaporates into the gas phase in the warm inner regions of protostellar cores, and is efficiently destroyed in shocks related to outflows.

We report the detection of complex molecules (HCOOCH₃, HCOOH, and CH₃CN), signposts of a hot core-like region, toward the low-mass Class 0 source NGC 1333 IRAS 4A. This is the second low-mass protostar in which such complex molecules have been searched for and reported, the other source being IRAS 16293-2422. It is therefore likely that compact (a few tens of AU) regions of dense and warm gas, where the chemistry is dominated by the evaporation of grain mantles and where complex molecules are found, are common in low-mass Class 0 sources. Given that the chemical formation timescale is much shorter than the gas hot-core crossing time, it is not clear whether the reported complex molecules are formed on the grain surfaces (first-generation molecules) or in the warm gas by reactions involving the evaporated mantle constituents (second-generation molecules). We do not find evidence for large differences in the molecular abundances, normalized to the formaldehyde abundance, between the two solar-type protostars, suggesting perhaps a common origin.

We derive a relation between the flux density F_ν,j at the light-curve break of a gamma-ray burst (GRB) afterglow and the break time t_j. The break is due to the transition from the spherical-like to jet-like evolution of the afterglow, when the Lorentz factor of the jet equals the inverse of the initial half-opening angle, i.e., γ = 1/θ₀. We show that this relation indeed behaves as F_ν,j ∝ t among GRBs for the slow-cooling case, where p is the power-law index of electron distribution. A statistical analysis of the optical jet breaks of nine GRBs gives p = 2.10 ± 0.21, which is consistent with the shock acceleration theory. The value of p derived in this way is different from the observed temporal index α₂ (F_ν ∝ t) of the late-time light curve after t_j, which suffers several uncertainties from the unclear hydrodynamics of the sideways expansion and exhibits a large dispersion. Our results not only confirm that the remnants of GRBs are standard candles, but also provide the first evidence that the physical parameters of relativistic shocks are universal, with the favored values _e ~ 0.1 and _B ~ 10^-3.

The origin and structure of magnetic fields in gamma-ray burst (GRB) fireball plasmas are two of the most important open questions in all GRB models. It has been claimed that recent measurements of γ-ray polarization suggest the presence of a uniform field originating in the compact object driving the outflow. This interpretation is, however, controversial, since a high degree of linear polarization is also possible in the presence of a random magnetic field, arguably originating in electromagnetic instabilities that develop at the collisionless shock. We show that the structure and strength of the magnetic field may be constrained by radio and IR observations of the early afterglow, where plasma effects on the polarization of the propagating radiation are significant. We calculate these propagation effects for cold and relativistic plasmas, and find that in the presence of a uniform equipartition field the degree of linear polarization is suppressed and circular polarization prevails at low frequencies, below ~1-3 GHz (~a few × 10¹⁴ Hz) in the forward (reverse) shock, at the onset of fireball deceleration. At higher frequencies linear polarization dominates. At the frequency of the transition between circular and linear polarization, the net level of polarization is minimal, ~10%-20%. These features are nearly independent of the density of the environment into which the fireball expands. When the uniform field is much weaker than the equipartition value, the transition frequency is smaller by an order of magnitude. Depending on the geometry of the emitting region, oscillations of the polarization position angle may be observed from the optical reverse-shock emission, provided that the strength of the magnetic field is close to equipartition. The dependence of these results on viewing geometry, outflow collimation, and magnetic field orientation is discussed. When the field is entangled over length scales much smaller than the extent of the emitting plasma, the aforementioned effects should not be observed, and a linear polarization at the few percent level is expected. Polarimetric observations during the early afterglow, and particularly of the reverse-shock emission, may therefore place strong constraints on the structure and strength of the magnetic field within the fireball plasma.

We analyze the possible existence of a pulsational instability excited by the -mechanism during the last few centuries of evolution of a Chandrasekhar-mass white dwarf prior to its explosion as a Type Ia supernova. Our analysis is motivated by the temperature sensitivity of the nuclear energy generation rate (~T²³) in a white dwarf whose structural adiabatic index is near 4/3. Based on a linear stability analysis, we find that the fundamental mode and higher order radial modes are indeed unstable and that the fundamental mode has the shortest growth timescale. However, the growth timescale for such instability never becomes shorter than the evolutionary timescale. Therefore, even though the star is pulsationally unstable, we do not expect these radial modes to have time to grow and to affect the structure and explosion properties of Type Ia supernovae.

The origin and nature of ultraluminous X-ray sources (ULXs) is a contentious and controversial topic. There are ongoing debates about the masses of the objects responsible, their sources of mass for accretion, and their relation to stellar populations in galaxies. A new picture of these objects is proposed in which they are intermediate-mass black holes confined to the disks of their host galaxies and accreting from the interstellar medium. They are then preferentially found in or near molecular clouds. This model correctly predicts the shape of the observed luminosity function and requires only a very small fraction of the baryonic mass of a galaxy to be in the form of intermediate-mass black holes. Because the X-rays they produce strongly heat nearby interstellar gas and because they move relatively rapidly in and out of dense regions, ULXs are predicted to have brief episodes of high luminosity, perhaps ~10⁵ yr in duration, but they may recur many times.

We have performed 2.5-dimensional general relativistic magnetohydrodynamic (MHD) simulations of collapsars including a rotating black hole. This paper is an extension of our previous paper. The current calculation focuses on the effect of black hole rotation using general relativistic MHD with simplified microphysics; i.e., we ignore neutrino cooling, physical equation of state, and photodisintegration. Initially, we assume that the core collapse has failed in this star. A rotating black hole of a few solar masses is inserted by hand into the calculation. We consider two cases, a corotating case and a counterrotating case with respect to the black hole rotation. Although the counterrotating case may be unrealistic for collapsars, we perform it as the maximally dragging case of a magnetic field. The simulation results show the formation of a disklike structure and the generation of a jetlike outflow near the central black hole. The jetlike outflow propagates outwardly with the twisted magnetic field and becomes collimated. We have found that the jets are generated and accelerated mainly by the magnetic field. The total jet velocity in the rotating black hole case is comparable to that of the nonrotating black hole case, ~0.3c. When the rotation of the black hole is faster, the magnetic field is twisted strongly owing to the frame-dragging effect. The magnetic energy stored by the twisting magnetic field is directly converted to kinetic energy of the jet rather than propagating as an Alfvén wave. Thus, as the rotation of the black hole becomes faster, the poloidal velocity of the jet becomes faster. In the rapidly rotating black hole case the jetlike outflow can be produced by the frame-dragging effect only through twisting of the magnetic field, even if there is no stellar rotation.

A soft component of thermal emission is very commonly observed from the surfaces of quiescent, accreting neutron stars. We searched with Chandra for such a surface component of emission from the dynamical black hole candidate XTE J1118+480 (=J1118), which has a primary mass M₁ ≈ 8 M_☉. None was found, as one would expect if the compact X-ray source is a bona fide black hole that possesses an event horizon. The spectrum of J1118 is well fitted by a simple power-law model that implies an unabsorbed luminosity of L_X ≈ 3.5 × 10³⁰ ergs s^-1 (0.3-7 keV). In our search for a thermal component, we fitted our Chandra data to a power-law model (with slope and N_H fixed) plus a series of nine hydrogen atmosphere models with radii ranging from 9/8 to 2.8 Schwarzschild radii. For the more compact models, we included the important effect of self-irradiation of the atmosphere. Because of the remarkably low column density to J1118, N_H ≈ 1.2 × 10²⁰ cm^-2, we obtained very strong limits on a hypothetical thermal source: kT_∞ < 0.011 keV and L_∞,th < 9.4 × 10³⁰ ergs s^-1 (99% confidence level). In analogy with neutron stars, there are two possible sources of thermal radiation from a hypothetical surface of J1118: deep crustal heating and accretion. The former mechanism predicts a thermal luminosity that exceeds the above luminosity limit by a factor of ≳25, which implies that either one must resort to contrived models or, as we favor, J1118 is a true black hole with an event horizon. In addition to neutron stars, we also consider emission from several exotic models of compact stars that have been proposed as alternatives to black holes. As we have shown previously, accreting black holes in quiescent X-ray binaries are very much fainter than neutron stars. One potential explanation for this difference is the larger and hence cooler surface of an 8 M_☉ compact object that might be masked by the interstellar medium. However, our upper limit on the total luminosity of J1118 of 1.3 × 10³¹ ergs s^-1 is far below the luminosities observed for neutron stars. This result strengthens our long-held position that black holes are faint relative to neutron stars because they possess an event horizon.

We report on the timing analysis of RXTE observations of the Galactic microquasar GRS 1915+105 performed in 2003. Out of a total of six ~20 ks observations, we focus here only on the three during which GRS 1915+105 is found in a steady C-state (referred to as class χ), resulting in a total of ~50 ks. During these observations, we detect low-frequency quasi-periodic oscillations (QPOs) with high (~14%) rms amplitude in the 2-40 keV energy range. Contrary to what is usually observed in GRS 1915+105, in most of our observations the QPO frequency presents no correlation with the RXTE PCA count rate, nor with the RXTE ASM count rate. We present, for the first time, high-resolution (22 spectral channels) 2-40 keV spectral fits of the energy dependence of the QPO amplitude ("QPO spectra"). The QPO spectra are well modeled with a cutoff power law except on one occasion in which a single power law gives a satisfactory fit (with no cutoff at least up to ~40 keV). The cutoff energy evolves significantly from one observation to another, from a value of ~21.8 to ~30 keV in the other observations in which it is detected. We discuss the possible origin of this behavior and suggest that the compact jet detected in the radio contributes to the hard X-ray (≥20 keV) mostly through synchrotron emission, whereas the X-rays emitted below 20 keV would originate through inverse Compton scattering. The dependence of the QPO amplitude on the energy can be understood if the modulation of the X-ray flux is contained in the Comptonized photons and not in the synchrotron ones.

We present spectroscopy of the microquasar SS 433 obtained near primary eclipse and disk precessional phase Ψ = 0.0, when the accretion disk is expected to be most "face-on." The likelihood of observing the spectrum of the mass donor is maximized at this combination of orbital and precessional phases, since the donor is in the foreground and above the extended disk believed to be present in the system. The spectra were obtained over four different runs centered on these special phases. The blue spectra show clear evidence of absorption features consistent with a classification of A3-7 I. The behavior of the observed lines indicates an origin in the mass donor. The observed radial velocity variations are in antiphase to the disk, the absorption lines strengthen at mideclipse when the donor star is expected to contribute its maximum percentage of the total flux, and the line widths are consistent with lines created in an A supergiant photosphere. We discuss and cast doubt on the possibility that these lines represent a circumstellar shell spectrum rather than the mass donor itself. We reevaluate the mass ratio of the system and derive masses of 10.9 ± 3.1 and 2.9 ± 0.7 M_☉ for the mass donor and compact object plus disk, respectively. We suggest that the compact object is a low-mass black hole. In addition, we review the behavior of the observed emission lines from both the disk/wind and high-velocity jets and show that the current orbital ephemeris and disk precession/nodding model parameters are still valid.

We present radio imaging observations of the 1998 outburst of the peculiar emission-line star CI Cam, taken ~1, 4, 75, 82, 93, 163, and 306 days after the beginning of the 1998 March 31.64 X-ray flare. The first two epochs show a resolved but compact (no larger than 12 mas) radio source that becomes optically thin at frequencies higher than 5 GHz. The spectrum and brightness temperatures are consistent with synchrotron self-absorption, although free-free absorption may also play a role. The later images show a large (120-350 mas) oval-shaped or double-ring remnant. The radio spectrum combined with the high brightness temperature indicates that the emission is synchrotron, while the morphology suggests that this is powered by a decelerating shock moving through dense circumstellar material produced by a strong stellar wind. The radio images of CI Cam are equally well fitted by an expanding ellipsoid or two expanding rings; the former gives Θ ≈ 4.2(t - 50,904.1)^0.77, with Θ the major axis in milliaarcseconds and t the Modified Julian Date (MJD). The corresponding expansion speed in the plane of the sky was ~12,000 km s^-1 over the first few days (for an assumed distance of 5 kpc), slowing by a factor of ~3 by the time of the last observation almost a year later. The radio emission from all other X-ray binary transients is either unresolved or takes the form of highly collimated relativistic jets. We suggest that CI Cam represents a rare case in which these jets were smothered early on by the unusually dense circumstellar medium. In this model, CI Cam is the analog to extragalactic supernovae formed by the collapsar mechanism, while the more usual X-ray binaries with relativistic jets are analogous to the jets that escape those supernovae to form a subset of γ-ray bursts.

We study the effect of stellar rotation on the carbon ignition in a carbon-oxygen white dwarf accreting CO-rich matter. Including the effect of the centrifugal force of rotation, we have calculated evolutionary models up to the carbon ignition for various accretion rates. The rotational velocity at the stellar surface is set to the Keplerian velocity. The angular velocity in the stellar interior is determined by taking into account the transport of angular momentum due to turbulent viscosity. We have found that an off-center carbon ignition occurs even when the effect of stellar rotation is included if the accretion rate is sufficiently high; the critical accretion rate for the off-center ignition is hardly changed by the effect of rotation. Rotation, however, delays the ignition, i.e., the mass coordinate of the ignition layer and the mass of the white dwarf at the ignition are larger than those for the corresponding nonrotating model. The result supports our previous conclusion that a double-white dwarf merger would not be a progenitor of a Type Ia supernova (SN Ia).

We analyze the light curve of the microlensing event OGLE-2003-BLG-175/MOA-2003-BLG-45 and show that it has two properties that, when combined with future high-resolution astrometry, could lead to a direct, accurate measurement of the lens mass. First, the light curve shows clear signs of distortion due to the Earth's accelerated motion, which yields a measurement of the projected Einstein radius _E. Second, from precise astrometric measurements, we show that the blended light in the event is coincident with the microlensed source to within about 15 mas. This argues strongly that this blended light is the lens and hence opens the possibility of directly measuring the lens-source relative proper motion μ_rel and so the mass M = (c²/4G)μ_relt_EE, where t_E is the measured Einstein timescale. While the light-curve-based measurement of _E is, by itself, severely degenerate, we show that this degeneracy can be completely resolved by measuring the direction of proper motion μ_rel.

The mechanism responsible for the natal kicks of neutron stars continues to be a challenging problem. Indeed, many mechanisms have been suggested, and one hydrodynamic mechanism may require large initial asymmetries in the cores of supernova progenitor stars. Goldreich and coworkers suggested that unstable g-modes trapped in the iron (Fe) core by the convective burning layers and excited by the -mechanism may provide the requisite asymmetries. We perform a modal analysis of the last minutes before collapse of published core structures and derive eigenfrequencies and eigenfunctions, including the nonadiabatic effects of growth by nuclear burning and decay by both neutrino and acoustic losses. In general, we find two types of g-modes: inner core g-modes, which are stabilized by neutrino losses, and outer core g-modes, which are trapped near the burning shells and can be unstable. Without exception, we find at least one unstable g-mode for each progenitor in the entire mass range we consider, 11-40 M_☉. More importantly, we find that the timescales for growth and decay are an order of magnitude or more longer than the time until the commencement of core collapse. We conclude that the -mechanism may not have enough time to significantly amplify core g-modes prior to collapse.

We discuss an interesting feature of the distribution of luminous blue variables (LBVs) on the H-R diagram, and we propose a connection with the bistability jump seen in the winds of early-type supergiants. There appears to be a deficiency of quiescent LBVs on the S Doradus instability strip at luminosities between log(L/L_☉) ≃ 5.6 and 5.8. The upper boundary, interestingly, is also where the temperature-dependent S Doradus instability strip intersects the bistability jump at about T_eff ≃ 21,000 K. Because of increased opacity, winds of early-type supergiants are slower and denser on the cool side of the bistability jump, and we postulate that this may trigger optically thick winds that inhibit quiescent LBVs from residing there. We conduct numerical simulations of radiation-driven winds for a range of temperatures, masses, and velocity laws at log(L/L_☉) = 5.7 to see what effect the bistability jump should have. We find that for relatively low stellar masses, the order-of-magnitude increase in the wind density at the bistability jump leads to the formation of a modest to strong pseudophotosphere that might alter a star's apparent position on the H-R diagram. The effect is strongest for LBVs approaching 10 M_☉, where the pseudophotospheres are sufficiently extended to make an early B-type star appear as a yellow hypergiant. Thus, the proposed mechanism will be most relevant for LBVs that are post-red supergiants [curiously, the upper boundary at log(L/L_☉) ≃ 5.8 coincides with the upper luminosity limit for red supergiants]. Further work is obviously needed, especially with regard to a possible evolutionary connection between the "missing" LBVs and the most luminous red supergiants and yellow hypergiants. Specifically, yellow hypergiants such as IRC +10420 and ρ Cas occupy the same luminosity range as the missing LBVs and show apparent temperature variations at constant luminosity. If these yellow hypergiants do eventually become Wolf-Rayet stars, we speculate that they may skip the normal LBV phase, at least as far as their apparent positions on the H-R diagram are concerned.

We investigate the spectral energy distributions (SEDs) of oxygen-rich asymptotic giant branch (AGB) stars at different pulsation phases using infrared observational data, including data from the Infrared Space Observatory. Comparing the results of detailed radiative transfer model calculations with observations, we explore the changes of the relevant parameters of the dust shells and central stars depending on the pulsation phase. We find that the schemes of the SED changes for low mass-loss rate O-rich AGB (LMOA) stars are quite different from those for high mass-loss rate O-rich AGB (HMOA) stars. For LMOA stars, we find that the dust formation temperature is much lower than 1000 K, the stronger stellar winds produce more dust grains, and the dust shell optical depth increases at the maximum phase. For HMOA stars, the deep silicate absorption features show significant variations depending on the pulsation phase, mainly due to changes in the properties of the dust shells. Considering the dust formation and evaporation processes in HMOA stars, we propose three possible dust models to explain the SED changes. Contrary to previous investigations, we find that the models that do not require dust evaporation at the maximum phase produce SEDs similar to the observations.

Lines from HC₃N and isotopic substituted species in ground and vibrationally excited states produce crowded millimeter and submillimeter wave spectra in the C-rich protoplanetary nebula CRL 618. The complete sequence of HC₃N rotational lines from J = 9-8 to J = 30-29 has been observed with the IRAM 30 m telescope toward this object. Lines from a total of 15 different vibrational states (including the fundamental), with energies up to 1100 cm^-1, have been detected for the main HC₃N isotopomer. In addition, the Caltech Submillimeter Observatory telescope has been used to complement this study in the range J = 31-30 to J = 39-38, with detections in five of these states, all of them below 700 cm^-1. Only the rotational lines of HC₃N in its ground vibrational state display evidence of the well-known CRL 618 high-velocity outflow. Vibrationally excited HC₃N rotational lines exhibit P Cygni profiles at 3 mm, evolving to pure emission line shapes at shorter wavelengths. This evolution of the line profile shows little dependence on the vibrational state from which the rotational lines arise. The absorption features are formed against the continuum emission, which has been successfully characterized in this work as a result of the large frequency coverage. The fluxes range from 1.75 to 3.4 Jy in the frequency range 90-240 GHz. These values translate to an effective continuum source with a size between 022 and 027, an effective temperature at 200 GHz ranging from 3900 to 6400 K, and a spectral index between -1.15 and -1.12. We have made an effort to simultaneously fit a representative set of observed HC₃N lines through a model with an expanding shell around the central star and its associated H II region, assuming that LTE prevails for HC₃N. The simulations show that the slowly expanding inner envelope has expansion and turbulence velocities of ~5-18 and ~3.5 km s^-1, respectively, and that it is possibly elongated. Its inclination with respect to the line of sight has also been explored. The HC₃N column density in front of the continuum source has been determined by comparing the output of an array of models with the data. The best fits are obtained for column densities in the range × 10¹⁷ cm^-2, consistent with previous estimates from Infrared Space Observatory (ISO) data, and T_K in the range 250-275 K, in very good agreement with estimates made from the same ISO data.

It is well established that certain detached eclipsing binary stars exhibit apsidal motions whose values are in disagreement with calculated deviations from Keplerian motion based on tidal effects and the general theory of relativity. Although many theoretical scenarios have been demonstrated to bring calculations into line with observations, all have seemed unlikely for various reasons. In particular, it has been established that the hypothesis of a third star in an orbit almost perpendicular to the orbital plane of the close binary system can explain the anomalous motion at least in some cases. The stability of triple star systems with highly inclined orbits has been in doubt, however. We have found conditions that allow the long-term stability of such systems, so that the third-body hypothesis now seems a likely resolution of the apsidal motion problem. We apply our stability criteria to the cases of AS Cam and DI Her and recommend observations at the new Keck interferometer, which should be able to directly observe the third bodies in these systems.

We constrain the properties of the mechanism(s) responsible for the bulk of the heating of the corona of the Sun by simulating, for the first time, the appearance of the entire solar corona. Starting from full-sphere magnetic field maps for 2000 December 1 and 8, when the Sun was moderately active, we populate nearly 50,000 coronal field lines with quasi-static loop atmospheres. These atmospheres are based on heating flux densities F_H that depend in different ways on the loop half-length L, the field strength B at the chromospheric base, the loop expansion with height, and the heating scale height. The best match to X-ray and EUV observations of the corona over active regions and their environs is found for F_H ≈ 4 × 10¹⁴B^1.0±0.3/L^1.0±0.5 (in ergs cm^-2 s^-1 for B in Mx cm^-2 and L in cm), while allowing for substantial loop expansion with increasing height, and for a heating scale height that is at least a sizeable fraction of the loop length. This scaling for coronal heating points to DC reconnection at tangential discontinuities as the most likely coronal heating mechanism, provided that the reconnection progresses proportional to the Alfvén velocity. The best-fit coronal filling factor equals unity, suggesting that most of the corona is heated most of the time. We find evidence that loops with half-lengths exceeding ~100,000 km are heated significantly more than suggested by the above scaling, possibly commensurate with the power deposited in the open field of coronal holes.

Using meter-wave solar radio spectral observations of the 2001 September 28 flare, we discuss simultaneously appearing type II-like bursts observed at 40-80 MHz and ≈300 MHz as radio signatures of the upper and lower reconnection outflow termination shock (TS). The features are identified during the impulsive phase but well after the regular traveling-shock type II burst and during the rise of a coronal mass ejection. The upper and lower TS features reveal a tendency for flux anticorrelation over time. We use radio imaging data (Nançay Radio Heliograph) and Yohkoh soft X-ray images to support the TS hypothesis. Assuming shock drift acceleration, we compute the flux of accelerated electrons for lower TS conditions that lead to an electron population with energies of ≈15 keV.

We present a numerical model for the simulation of water line emission in cometary coma. The model is based on a spherically symmetric density distribution with a constant expansion velocity (Haser model) and the Monte Carlo radiative transfer code published by Hogerheijde & van der Tak. It includes the seven lowest rotational levels of ortho-water, which are the primarily populated levels in the rotationally cold gas of the coma. We discuss the main excitation mechanisms for ortho-water in the coma and study their relative contribution as a function of distance from the comet nucleus. The model is used to derive the water production rate from observations made with the Submillimeter Wave Astronomy Satellite toward comet C/1999 T1 (McNaught-Hartley). They differ from the water production rates derived with an independent model by less than 20% and thus agree within the larger uncertainty due to the limited signal-to-noise ratio of the observations. We give predictions for spectral line observations of H₂O and H₂¹⁸O in comets with present and future airborne and space observatories, including ESA's Herschel Space Observatory and the Stratospheric Observatory for Infrared Astronomy (SOFIA). These models cover a range of water vapor production rates (10²⁷-10²⁹ s^-1) and heliocentric distances (1-3 AU) and demonstrate that water line emission can be easily detected with Herschel.

We propose a novel multivariate Monte Carlo method as an efficient and flexible approach to analyzing extended X-ray sources with the reflection grating spectrometers (RGS) on XMM-Newton. A multidimensional interpolation method is used to efficiently calculate the response function for the RGS in conjunction with an arbitrary spatially varying spectral model. Several methods of event comparison that effectively compare the multivariate RGS data are discussed. The use of a multidimensional instrument Monte Carlo method also creates many opportunities for the use of complex astrophysical Monte Carlo calculations in diffuse X-ray spectroscopy. The methods presented here could be generalized to other X-ray instruments as well.

We present yet another new family of masks for high-contrast imaging, as required for the planned Terrestrial Planet Finder space telescope. We call these masks "checkerboard" masks. They consist of two "bar-code" masks, one rotated 90° with respect to the other. Each bar-code mask provides contrast to the 10^-5 level. The checkerboard mask then achieves a 10^-10 level of contrast everywhere except along the two axes of symmetry, where the contrast remains at the 10^-5 level. With these designs, we are able to reduce the inner working angle to 2λ/D for each bar code, which translates to 2λ/D along the diagonal of the associated checkerboard mask. We show that by combining a Lyot-plane checkerboard mask with an image-plane occulter we can achieve even tighter inner working angles, although as with occulting designs in general pointing error and stellar size become nontrivial issues. Checkerboard masks can be thought of as the binary-mask analog of Nisenson's apodized square aperture concept.

The performance of high-contrast imaging systems is very often limited by the presence of speckles in the point-spread function (PSF) of the central source. Since this unwanted light is coherent with the central source, it is possible to make it interfere with light extracted from the center of the PSF. The light of a faint companion, however, will not interfere with the central source. By taking advantage of this fundamental difference, it is possible to detect faint companions superposed on speckles more than 100 times brighter, even if the speckles are rapidly "boiling." Possible optical designs to use this technique on interferometers or imaging telescopes are shown, and a data analysis algorithm is proposed. Synchronous interferometric speckle suppression greatly improves the performance of ground-based telescopes with adaptive optics systems for direct imaging of faint companions. On a space telescope, where the speckle lifetime is expected to be longer, companions 10⁴ times fainter than the speckle halo can be detected in a few minutes.

We present a deep H-band image of the field of a candidate z = 10 galaxy magnified by the foreground (z = 0.25) cluster A1835. The image was obtained with NIRI on Gemini-North to better constrain the photometry and investigate the morphology of the source. The image is approximately 1 mag deeper and has better spatial resolution (seeing was 04-05) than the existing H-band image obtained with ISAAC on the Very Large Telescope by Pelló et al. The object is not detected in our new data. Given the published photometry (H_AB = 25.0), we would have expected it to have been detected at more than ~7 σ in a 14 diameter aperture. We obtain a limit of H_AB > 26.0 (3 σ) for the object. A major part of the evidence that this object is at z = 10 was the presence of a strong continuum break between the J and H bands, attributed to absorption of all continuum shortward of 1216 Å in the rest frame of the object. Our H-band nondetection substantially reduces the magnitude of any break and therefore weakens the case that this object is at z = 10. Without a clear continuum break, the identification of an emission line at 1.33745 μm as Lyα at z ≈ 10 is less likely. We show that the width and flux of this line are consistent with an alternative emission line such as [O III] λ5007 from an intermediate-redshift H II/dwarf galaxy.

Periodicities in blazar light curves may be related to helical trajectories in extragalactic radio jets by differential Doppler boosting effects. We consider ballistic and nonballistic (i.e., radial) trajectories and discuss three possible periodic driving mechanisms for the origin of helical jet paths, namely, orbital motion in a binary black hole system, jet precession, and intrinsic jet rotation. It is shown that precessional-driven ballistic motion is unlikely to result in observable periods of less than several tens of years. We demonstrate that for nonballistic helical motion the observed period is generally strongly shortened relative to the real physical driving period because of light-travel time effects. Internal jet rotation may thus account for observed periods P_obs ≲ 10 days. Periodicity due to orbital-driven (nonballistic) helical motion, on the other hand, is usually constrained to periods of P_obs ≳ 10 days, while Newtonian-driven precession is unlikely to be responsible for periodicity on a timescale P_obs ≲ 100 days but may well be associated with periods of P_obs ≳ 1 yr.

The properties of relativistic radio jets are thought to be closely connected with the properties of accretion disks in active galactic nuclei. We explore this issue using a sample of 35 blazars with very long baseline observations, for which we can estimate the kinetic powers of their relativistic jets, the bolometric luminosity of their accretion disks, and the masses of their black holes. Contrary to previous claims, we find that the jet kinetic power is significantly correlated with the disk luminosity. This supports the notion that the disk is somehow coupled to the jet, even on parsec scales. Moreover, we show that the correlation improves by including the black hole mass as a second parameter. The dominance of the jet, parameterized as the ratio of the jet kinetic power to the disk luminosity, is largely controlled by and is inversely correlated with the Eddington ratio of the accretion disk. This empirical relation should serve as a useful guide for theoretical models for jet formation.

Motivated by recent observations of high-velocity, highly ionized winds in several QSOs, models of purely continuum-driven winds launched from ~200GM_BH/c² are presented. Launching conditions are investigated, as well as the observational signatures for a variety of initial conditions and illuminating continua. While we verify that continuum-driven highly ionized outflows reach the observed velocities for L/L_Edd ≥ 1 independent of the incident spectral shape, such winds are too highly ionized to exhibit the observed absorption features when launched with an active galactic nucleus continuum (in fact, such winds are so ionized that they are driven primarily by electron scattering). If the wind is instead illuminated with a blackbody continuum originating from an optically thick shield, the gas is too weakly ionized and does not produce high-energy absorption features. If high-velocity high-ionization winds are truly launched from very near the black hole, such winds must be launched under other conditions or via other processes; we summarize some possibilities.

We present high-resolution Very Large Array (VLA) observations of the molecular gas in the host galaxy of the highest redshift quasar currently known, SDSS J1148+5251 (z = 6.42). Our VLA data of the CO (3-2) emission have a maximum resolution of 017 × 013 (≤1 kpc) and enable us to resolve the molecular gas emission both spatially and in terms of velocity. The molecular gas in J1148+5251 is extended to a radius of 2.5 kpc, and the central region shows two peaks separated by 03 (1.7 kpc). These peaks account for about half of the total emission, while the remainder is more extended. Each of these unresolved peaks contains a molecular gas mass of ~5 × 10⁹M_☉ (similar to the total mass found in nearby ultraluminous infrared galaxies) and has an intrinsic brightness temperature of ~35 K (averaged over the 1 kpc-sized beam), comparable to what is found in nearby starburst centers. Assuming that the molecular gas is gravitationally bound, we estimate a dynamical mass of ~4.5 × 10¹⁰M_☉ within a radius of 2.5 kpc (~5.5 × 10¹⁰M_☉ if corrected for a derived inclination of i ~ 65°). This dynamical mass estimate leaves little room for matter other than the detected molecular gas; in particular, the data are inconsistent with a ~10¹²M_☉ stellar bulge that would be predicted based on the M_BH-σ_bulge relation. This finding may indicate that black holes form prior to the assembly of the stellar bulges and that the dark matter halos are less massive than those predicted on the basis of the black hole/bulge mass relationship.

We report the first measurement of the X-ray spectrum of the z = 5.99 quasar SDSSp J130608.26+035626.3 from a 120 ks observation by the Chandra ACIS-S instrument. Between 0.5 and 7 keV, corresponding to 3.5-49 keV in the quasar rest frame, we find an energy index of 0.86 ± 0.2, consistent with the typical indices found for radio-quiet quasars at lower redshifts and inconsistent with the index required to match the diffuse X-ray background. We have a weak indication of a redshifted Fe K line. In comparing the counting rate between an earlier short observation and the longer observation reported here, we find evidence for source variability at the 99.9% confidence level. We note that other nearby X-ray sources would bias the measured α_ox = 1.70 by -0.09 if the X-ray flux were determined from within a 60'' extraction circle. Our results for the energy index and the α_ox are consistent with no strong evolution in the active galactic nucleus emission mechanism with redshift out to z ≈ 6 and therefore with the picture that massive black holes have already formed less than 1 Gyr after the big bang.

We present two Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array (PCA) observations of the type 2 Seyfert galaxy NGC 4388 caught in an unusual low X-ray absorption state. The observations were triggered by a detection in the 1.5-3 keV band of the RXTE all-sky monitor. NGC 4388 was found at a somewhat high continuum level [f(2-10 keV) = 8 × 10^-11 ergs cm^-2 s^-1] and with a column density N_H ~ 3 × 10²² cm^-2, a factor of ~10 lower than normal. The second PCA observation, 4 hr later, gave N_H < 2 × 10²¹ cm^-2 indicating, at the 3.1 σ level, variability so rapid it puts the absorber on a few 100 Schwarzschild radii scale, similar to the broad emission line region or smaller. This small scale creates difficulties for the parsec-scale obscuring torus paradigm of unified schemes for type 1 and type 2 active galactic nuclei.

We present new sensitive wide-band measurements of the fine-structure line ³P₁ → ³P₀ (J = 1-0, 492 GHz) of neutral atomic carbon (C I) in the two typical ultraluminous infrared galaxies (ULIRGs) NGC 6240 and Arp 220. We then use them along with several other C I measurements in similar objects found in the literature to estimate their global molecular gas content under the assumption of a full C I-H₂ concomitance. We find excellent agreement between the H₂ gas mass estimated with this method and the standard methods using ¹²CO. This may provide a new way to measure H₂ gas mass in galaxies and one that may be very valuable in ULIRGs since in such systems the bright ¹²CO emission is known to systematically overestimate the gas mass while their ¹³CO emission (an often-used alternative) is usually very weak. At redshifts z ≥ 1 the C I J = 1-0 line shifts to much more favorable atmospheric windows and can become a viable alternative tracer of the H₂ gas, fueling starburst events in the distant universe.

The lithium abundance in 62 halo dwarfs is determined from accurate equivalent widths reported in the literature and an improved infrared flux method temperature scale. The Li abundance of 41 plateau stars (those with T_eff > 6000 K) is found to be independent of temperature and metallicity, with a star-to-star scatter of only 0.06 dex over a broad range of temperatures (6000 K < T_eff < 6800 K) and metallicities (-3.4 < [Fe/H] < -1), thus imposing stringent constraints on depletion by mixing and production by Galactic chemical evolution. We find a mean Li plateau abundance of A_Li = 2.37 dex (⁷Li/H = 2.34 × 10^-10), which, considering errors of the order of 0.1 dex in the absolute abundance scale, is just in borderline agreement with the constraints imposed by the theory of primordial nucleosynthesis and Wilkinson Microwave Anisotropy Probe data (2.51 dex < A < 2.66 dex).

According to common assumptions, matter in the mass range A ≥ 20 is processed through the so-called NeNa cycle during hydrogen-burning nucleosynthesis. The existence of such a reaction cycle implies that the (p, α) reaction on ²³Na is more likely to occur than the competing (p, γ) reaction. However, recently evaluated thermonuclear rates for both reactions carry relatively large uncertainties and allow for both possibilities, i.e., a "closed" and an "open" NeNa cycle. We measured the ²³Na(p, γ)²⁴Mg reaction at the Laboratory for Experimental Nuclear Astrophysics. The present experimental results, obtained with our sensitive γ-ray detection apparatus, reduce the ²³Na + p reaction rate uncertainties significantly. We demonstrate that a closed NeNa cycle does not exist at stellar temperatures of T = 0.2-0.4 GK. The new results have important implications for the nucleosynthesis in classical novae, including the amount of ²⁶Al ejected by the thermonuclear explosion, the elemental abundances of Mg and Al observed in nova shells, and observations of Mg and Al isotopic ratios in primitive meteorites.

We present numerical studies of three-dimensional electron magnetohydrodynamic (EMHD) turbulence. We investigate the cascade timescale and anisotropy of freely decaying, strong EMHD turbulence with zero electron skin depth. The cascade time scales with k^-4/3. Our numerical results clearly show scale-dependent anisotropy. We find that the observed anisotropy is consistent with k_∥ ∝ k, where k_∥ and k_⊥ are wavenumbers parallel and perpendicular to (local) mean magnetic field, respectively.

The universality of interstellar turbulence is examined from observed structure functions of 27 giant molecular clouds and Monte Carlo modeling. We show that the structure functions, δv = v_ol^γ, derived from wide-field imaging of ¹²CO J=1-0 emission from individual clouds are described by a narrow range in the scaling exponent, γ, and the scaling coefficient, v_o. The similarity of turbulent structure functions emphasizes the universality of turbulence in the molecular interstellar medium and accounts for the cloud-to-cloud size/line width relationship initially identified by Larson. The degree of turbulence universality is quantified by Monte Carlo simulations that reproduce the mean squared velocity residuals of the observed cloud-to-cloud relationship. Upper limits to the variation of the scaling amplitudes and exponents for molecular clouds are ~10%-20%. The measured invariance of turbulence for molecular clouds with vastly different sizes, environments, and star formation activity suggests a common formation mechanism such as converging turbulent flows within the diffuse interstellar medium and a limited contribution of energy from sources within the cloud with respect to large-scale driving mechanisms.

The relation between the zero-age main-sequence mass of a star and its white dwarf remnant (the initial-final mass relation) is a powerful tool for the exploration of mass-loss processes during stellar evolution. We present an empirical derivation of the initial-final mass relation based on spectroscopic analysis of seven massive white dwarfs in NGC 2168 (M35). Using an internally consistent data set, we show that the resultant white dwarf mass increases monotonically with progenitor mass for masses greater than 4 M_☉, one of the first open clusters to show this trend. We also find two massive white dwarfs foreground to the cluster that are otherwise consistent with cluster membership. These white dwarfs can be explained as former cluster members moving steadily away from the cluster at speeds of ≲0.5 km s^-1 since their formation and may provide the first direct evidence of the loss of white dwarfs from open clusters. Based on these data alone, we constrain the upper mass limit of white dwarf progenitors to be ≧5.8 M_☉ at the 90% confidence level for a cluster age of 150 Myr.

L and M band observations of the nova-like variable V4332 Sgr are presented. Two significant results are obtained, viz., the unusual detection of water ice at 3.05 μm and the fundamental band of ¹²CO at 4.67 μm in emission. The ice feature is a first detection in a nova-like variable, while the CO emission is rarely seen in novae. These results, when considered together with other existing data, imply that V4332 Sgr could be a young object surrounded by a circumstellar disk containing gas, dust, and ice. The reason for a nova-like outburst to occur in such a system is unclear. But since planets are believed to form in such disks, it appears plausible that the enigmatic outburst of V4332 Sgr could be due to a planetary infall. We also give a more reliable estimate for an epoch of dust formation around V4332 Sgr that appears to have taken place rather late in 1999—nearly 5 years after its outburst.

We calculate helicities of solar active regions based on the idea that poloidal flux lines get wrapped around a toroidal flux tube rising through the convection zone, thereby giving rise to the helicity. Rough estimates based on this idea compare favorably with the observed magnitude of helicity. We use our solar dynamo model based on the Babcock-Leighton α-effect to study how helicity varies with latitude and time. At the time of solar maximum, our theoretical model gives negative helicity in the northern hemisphere and positive helicity in the south, in accordance with observed hemispheric trends. However, we find that during a short interval at the beginning of a cycle, helicities tend to be opposite of the preferred hemispheric trends.

Simultaneous spectral differential imaging is a high-contrast technique by which subtraction of simultaneous images reduces noise from atmospheric speckles and optical aberrations. Small non-common-wave-front errors between channels can seriously degrade its performance. We present a new concept, a multicolor detector assembly, which can eliminate this problem. The device consists of an infrared detector and a microlens array onto the flat side of which a checkerboard pattern of narrowband microfilters is deposited, each microfilter coinciding with a microlens. Practical considerations for successful implementation of the technique are mentioned. Numerical simulations predict a noise attenuation of 10^-3 at 05 for a 10⁵ s integration on a m_H = 5 star of Strehl ratio 0.9 taken with an 8 m telescope. This reaches a contrast of 10^-7 at an angular distance of 05 from the center of the star image.