Table of contents

N-body simulations sample their initial conditions on an initial particle distribution, which for cosmological simulations is usually a glass or grid, while a Poisson distribution is used for galaxy models, spherical collapse, etc. These pre-initial conditions have inherent correlations, noise due to discreteness, and preferential alignments, while the glass distribution is poorly defined and computationally expensive to construct. We present a novel particle distribution that can be useful as a pre-initial condition for N-body simulations, using a simple construction based on a "quaquaversal" tiling of space. This distribution has little preferred orientation (i.e., is statistically isotropic), has a rapidly vanishing large-scale power spectrum [P(k) ~ k⁴], and is trivial to create. It should be particularly useful for warm dark matter and cold collapse simulations.

A linear modulation of the primordial perturbations is proposed as an explanation for the observed asymmetry between the northern and southern hemispheres of the Wilkinson Microwave Anisotropy Probe (WMAP) data. A cut sky, reduced-resolution third-year "internal linear combination" (ILC) map was used to estimate the modulation parameters. A foreground template and a modulated plus unmodulated monopole and dipole were projected out of the likelihood. The effective χ² was reduced by 9 for three extra parameters. The mean Galactic colatitude and longitude of the modulation, with 68%, 95%, and 99.7% confidence intervals were 56 and 63. The mean percentage change of the variance across the poles of the modulation was 62. Implications of these results and possible generating mechanisms are discussed.

We analyze the 3 yr Wilkinson Microwave Anisotropy Probe (WMAP) temperature anisotropy data seeking to confirm the power spectrum and likelihoods published by the WMAP team. We apply five independent implementations of four algorithms to the power spectrum estimation and two implementations to the parameter estimation. Our single most important result is that we broadly confirm the WMAP power spectrum and analysis. Still, we do find two small but potentially important discrepancies. On large angular scales there is a small power excess in the WMAP spectrum (5%-10% at ℓ ≲ 30) primarily due to likelihood approximation issues between 13 ≤ ℓ ≲ 30. On small angular scales there is a systematic difference between the V- and W-band spectra (few percent at ℓ ≳ 300). Recently, the latter discrepancy was explained by Huffenberger et al. (2006) in terms of oversubtraction of unresolved point sources. As far as the low-ℓ bias is concerned, most parameters are affected by a few tenths of a σ. The most important effect is seen in n_s. For the combination of WMAP, ACBAR, and BOOMERANG, the significance of n_s ≠ 1 drops from ~2.7 σ to ~2.3 σ when correcting for this bias. We propose a few simple improvements to the low-ℓ WMAP likelihood code, and introduce two important extensions to the Gibbs sampling method that allows for proper sampling of the low signal-to-noise ratio regime. Finally, we make the products from the Gibbs sampling analysis publicly available, thereby providing a fast and simple route to the exact likelihood without the need of expensive matrix inversions.

Earlier papers introduced a method of accurately estimating the angular cosmic microwave background temperature power spectrum based on Gibbs sampling. Here we extend this framework to polarized data. All advantages of the Gibbs sampler still apply, and exact analysis of megapixel polarized data sets is thus feasible. These advantages may be even more important for polarization measurements than for temperature measurements. While approximate methods can alias power from the larger E-mode spectrum into the weaker B-mode spectrum, the Gibbs sampler (or equivalently, exact likelihood evaluations) allows for a statistically optimal separation of these modes in terms of power spectra. To demonstrate the method, we analyze two simulated data sets: (1) a hypothetical future CMBPol mission, with the focus on B-mode estimation; and (2) a Planck-like mission, to highlight the computational feasibility of the method.

Brighter Type Ia supernovae (SNe Ia) have broader, more slowly declining B-band light curves than dimmer SNe Ia. We study the physical origin of this width-luminosity relation (WLR) using detailed radiative transfer calculations of Chandrasekhar-mass SN Ia models. We find that the luminosity dependence of the diffusion time (emphasized in previous studies) is in fact of secondary relevance in understanding the model WLR. Instead, the essential physics involves the luminosity dependence of the spectroscopic/color evolution of SNe Ia. Following maximum light, the SN colors are increasingly affected by the development of numerous Fe II/Co II lines that blanket the B band and, at the same time, increase the emissivity at longer wavelengths. Because dimmer SNe Ia are generally cooler, they experience an earlier onset of Fe III to Fe II recombination in the iron group rich layers of ejecta, resulting in a more rapid evolution of the SN colors to the red. The faster B-band decline rate of dimmer SNe Ia thus reflects their faster ionization evolution.

We present the results of a survey for super Lyman limit systems (SLLSs; defined to be absorbers with 19.0 cm^-2 ≤ log N_{H I} ≤ 20.3 cm^-2) from a large sample of high-resolution spectra acquired using the Keck and Magellan telescopes. Specifically, we present 47 new SLLSs from 113 QSO sight lines. We focus on the neutral hydrogen frequency distribution f_{H I}(N, X) of the SLLS and its moments and compare these results with the Lyα forest and the damped Lyα systems (DLAs; absorbers with log N_{H I} ≥ 20.3 cm^-2). We find that the f_{H I}(N, X) of the SLLSs can be reasonably described with a power law of index α = -1.43 or -1.19 depending on whether we set the lower N_{H I} bound for the analysis at 10^19.0 or 10^19.3 cm^-2, respectively. The results indicate a flattening in the slope of f_{H I}(N, X) between the SLLSs and DLAs. We find little evidence for redshift evolution in the shape of f_{H I}(N, X) for the SLLSs over the redshift range of the sample 1.68 < z < 4.47 and only tentative evidence for evolution in the zeroth moment of f_{H I}(N, X), the line density ℓ_SLLS(X). We introduce the observable distribution function (N, X) and its moment, which elucidates comparisons of H I absorbers from the Lyα forest through to the DLA. We find that a simple three-parameter function can fit (N, X) over the range 17.0 cm^-2 ≤ log N_{H I} ≤ 22.0 cm^-2. We use these results to predict that f_{H I}(N, X) must show two additional inflections below the SLLS regime to match the observed f_{H I}(N, X) distribution of the Lyα forest. Finally, we demonstrate that SLLSs contribute a minor fraction (≈15%) of the universe's hydrogen atoms and, therefore, an even smaller fraction of the mass in predominantly neutral gas.

Using a sample of optically selected quasars from the Sloan Digital Sky Survey, we have determined the radio-loud fraction (RLF) of quasars as a function of redshift and optical luminosity. The sample contains more than 30,000 objects and spans a redshift range of 0 < z ≤ 5 and a luminosity range of -30 ≤ M_i < -22. We use both the radio-to-optical flux ratio (R parameter) and the radio luminosity to define radio-loud quasars. After breaking the correlation between redshift and luminosity due to the flux-limited nature of the sample, we find that the RLF of quasars decreases with increasing redshift and decreasing luminosity. The relation can be described in the form of log [RLF/(1 − RLF)] = b₀ + b_z log(1 + z) + b_M(M₂₅₀₀ + 26), where M₂₅₀₀ is the absolute magnitude at rest-frame 2500 Å, and b_z, b_M < 0. When using R > 10 to define radio-loud quasars, we find that b₀ = -0.132 ± 0.116, b_z = -2.052 ± 0.261, and b_M = -0.183 ± 0.025. The RLF at z = 0.5 declines from 24.3% to 5.6% as luminosity decreases from M₂₅₀₀ = -26 to -22, and the RLF at M₂₅₀₀ = -26 declines from 24.3% to 4.1% as redshift increases from 0.5 to 3, suggesting that the RLF is a strong function of both redshift and luminosity. We also examine the impact of flux-related selection effects on the RLF determination using a series of tests and find that the dependence of the RLF on redshift and luminosity is highly likely to be physical, and that the selection effects we considered are not responsible for the dependence.

We present X-ray observations of the narrow-line Seyfert 1 (NLS1) galaxy PKS 0558-504 obtained with RXTE during a 1 yr monitoring campaign. This source, which is one of the very few radio-loud NLS1 galaxies, shows strong X-ray flux variability on timescales of weeks to months, accompanied by spectral variability. The main goal of this study is to investigate the spectral variability with model-independent methods and time-resolved spectroscopy in order to shed light on the origin of the X-rays. The main results can be summarized as follows: (1) The flux typically changes by a factor of 1.5-2 on timescales of 10-30 days, with few extreme events where the flux increases by a factor of ~4 in 3 days. (2) We do not observe any large amplitude, flux-related spectral variations. During the flux variations, the spectrum varies mainly in normalization and not in shape. We do observe some small-amplitude spectral variations, which do not correlate with flux, although there is a hint of spectral hardening as the source brightens. (3) There is no evidence for reprocessing features, such as the Fe Kα line or a Compton hump. We argue that PKS 0558-504 is a peculiar object that appears to be different from most of the radio-quiet and radio-loud active galactic nuclei (AGNs). If a jet is responsible for the bulk of the X-rays, it must operate in an unusual way. If instead a corona is responsible for the X-rays, the system might be a large-scale analog of the Galactic black holes in the transient intermediate state.

We present dynamical models based on a study of high-resolution long-slit spectra of the narrow-line region (NLR) in NGC 1068 obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST). The dynamical models consider the radiative force due to the active galactic nucleus (AGN), gravitational forces from the supermassive black hole (SMBH), nuclear stellar cluster, and galactic bulge, and a drag force due to the NLR clouds interacting with a hot ambient medium. The derived velocity profile of the NLR gas is compared to that obtained from our previous kinematic models of the NLR, using a simple biconical geometry for the outflowing NLR clouds. The results show that the acceleration profile due to radiative line driving is too steep to fit the data and that gravitational forces alone cannot slow the clouds down, but with drag forces included, the clouds can slow down to the systemic velocity over the range 100-400 pc, as observed. However, we are not able to match the gradual acceleration of the NLR clouds from ~0 to 100 pc, indicating the need for additional dynamical studies.

We study the role of massive perturbers (MPs) in deflecting stars and binaries to almost radial ("loss cone") orbits, where they pass near the central massive black hole (MBH), interact with it at periapse, and are ultimately destroyed. MPs dominate dynamical relaxation when the ratio of the second moments of the MP and star mass distributions, μ₂ ≡ N_p⟨M⟩/N_⋆⟨M⟩, satisfies μ₂ ≫ 1. We compile the MP mass function from published observations and show that MPs in the nucleus of the Galaxy (mainly giant molecular clouds), and plausibly in late-type galaxies generally, have 10² ≲ μ₂ ≲ 10⁸. MPs thus shorten the relaxation timescale by 10¹-10⁷ relative to two-body relaxation by stars alone. We show that this increases by 10¹-10³ the rate of large-periapse interactions with the MBH, where loss cone refilling by stellar two-body relaxation is inefficient. We extend the Fokker-Planck loss cone formalism to approximately account for relaxation by rare encounters with MPs. We show that binary star-MBH exchanges driven by MPs can explain the origin of the young main-sequence B stars that are observed very near the Galactic MBH and can increase by orders of magnitude the ejection rate of hypervelocity stars. In contrast, the rate of small-periapse interactions of single stars with the MBH, such as tidal disruption, is only increased by a factor of a few. We suggest that MP-driven relaxation plays an important role in the three-body exchange capture of stars on very tight orbits around the MBH. These captured stars may later be disrupted by the MBH via tidal orbital decay or direct scattering into the loss cone; captured compact objects may inspiral into the MBH by the emission of gravitational waves from zero-eccentricity orbits.

Stability properties of "magnetic tower" jets propagating in a gravitationally stratified background have been examined by performing three-dimensional magnetohydrodynamic simulations. The current-carrying, Poynting flux-dominated magnetic tower jet, which possesses a highly wound helical magnetic field, is subject to the current-driven instability (CDI). We find that, under general physical conditions including small perturbations in the initial background profiles, the propagating magnetic tower jets develop the nonaxisymmetric, m = 1 kink mode of the CDI. The kink mode grows on the local Alfvén crossing timescale. In addition, two types of kink modes appear in the system. At the central region where external thermal pressure confinement is strong, only the internal kink mode is excited and will grow. A large distance away from the central region where the external thermal pressure becomes low, the external kink mode is observed. As a result, the exterior of magnetic tower jets will be deformed into a large-scale wiggled structure. We also discuss extensively the different physical processes that contribute to the overall stability properties of the magnetic tower jets. Specifically, when the jet propagates in an initially unperturbed background, we find that it can survive the kink mode beyond the point predicted by the well-known Kruskal-Shafranov (K-S) criterion. The stabilization in this case comes mainly from the dynamical relaxation of magnetic twists during the propagation of magnetic towers; the magnetic pitch is reduced, and the corresponding K-S critical wavelength becomes longer as the tower jet proceeds. Furthermore, we show that the pressure-driven and Kelvin-Helmholtz instabilities do not occur in the magnetic tower jets. This strongly suggests that the CDI is the primary reason for the wiggling structures in jets.

We analyze XMM-Newton galaxy clusters Abell 1795, Abell S1101, Abell 1835, and MKW 3s in order to test whether their soft X-ray excess emission in the 0.2-0.5 keV band as reported by Kaastra et al. maintains after application of current knowledge of the XMM-Newton background and calibration. In this context, we examine the claim (Bregman & Lloyd-Davies) that the XMM-Newton sub-Galactic H I column density, and the accompanying soft excess continuum emission in the 0.2-0.5 keV band, is an artifact of an incorrect background subtraction. We show that since the cluster regions under scrutiny are within 500 kpc of the bright cluster center, the X-ray background level is negligible compared to the cluster emission level. Thus, at least in the central 500 kpc regions of the studied clusters, the reported soft excess is not a background artifact. We also study the possibility that the incomplete calibration information in the early phase of the XMM-Newton mission resulted in sub-Galactic H I column density, which was interpreted with the presence of a soft excess emitter. Our reanalysis of the four XMM-Newton observations with the MOS instruments yields evidence for the soft excess in all clusters. However, using the PN instrument, the best-fit H I column densities in Abell 1795, Abell 1835, and MKW 3s are at odds with the 2003 analysis of Kaastra et al. and in general agreement with those measured from 21 cm data in the direction of the clusters (Dickey & Lockman). Abell S1101 continues to feature a sub-Galactic NH also with the PN instrument, indicative of soft excess emission in both EPIC detectors. These differences are compatible with the current level of uncertainty in the calibration of both instruments.

The multiple images observed in the galaxy cluster Abell 1689 provide strong constraints not only on the mass distribution of the cluster, but also on the ensemble properties of the cluster galaxies. Using parametric strong-lensing models for the cluster, and by assuming well-motivated scaling laws between the truncation radius s and the velocity dispersion σ of a cluster galaxy, we are able to derive the sizes of the dark matter halos of cluster galaxies. For the scaling law expected for galaxies in the cluster environment (s ∝ σ), we obtain s = 64(σ/220 km s^-1) kpc. For the scaling law used for galaxies in the field with s ∝ σ², we find s = 66(σ/220 km s^-1)² kpc. Compared to halos of field galaxies, the cluster galaxy halos in Abell 1689 are strongly truncated.

We report on the discovery of a faint (M_V ~ -10.6 ± 0.2) dwarf spheroidal galaxy on deep F606W and F814W Hubble Space Telescope images of a Virgo intracluster field. The galaxy is easily resolved in our images, as our color magnitude diagram (CMD) extends ≳1 magnitude beyond the tip of the red giant branch (RGB). Thus, it is the deepest CMD for a small dwarf galaxy inside a cluster environment. Using the colors of the RGB stars, we derive a metal abundance for the dwarf of [M/H] = -2.3 ± 0.3 and show that the metallicity dispersion is less than 0.6 dex at 95% confidence. We also use the galaxy's lack of AGB stars and the absence of objects brighter than M_bol ~ -4.1 ± 0.2 to show that the system is old (t ≳ 10 Gyr). Finally, we derive the object's structural parameters and show that the galaxy displays no obvious evidence of tidal threshing. Since the tip of the red giant branch distance [(m - M)₀ = 31.23 ± 0.17 or D = 17.6 ± 1.4 Mpc] puts the galaxy near the core of the Virgo cluster, one might expect the object to have undergone some tidal processing. Yet the chemical and morphological similarity between the dwarf and the dSph galaxies of the Local and M81 Group demonstrates that the object is indeed pristine and not the shredded remains of a much larger galaxy. We discuss the possible origins of this galaxy and suggest that it is just now falling into Virgo for the first time.

We have used the Hubble Space Telescope's Advanced Camera for Surveys (ACS) to detect and measure ~5300 stars in a single intracluster field in the Virgo Cluster. By performing F606W and F814W photometry on these stars, we have determined their metallicity distribution function and constrained the types of stars present in this portion of Virgo's intracluster space. Based on the small number of stars detected that were brighter than the red giant branch (RGB) tip, we suggest that in this region, Virgo's intracluster stars are mostly old (≳10 Gyr). Through analysis of the RGB stars themselves, we determine that the population contains the full range of metallicities probed (-2.3 ≤ [M/H] ≤ 0.0). We also present evidence that the younger (≤10 Gyr) component of the population is more metal-rich, with [M/H] > -0.5. The spatial distribution of the most metal-poor stars in the field shows significantly more structure than that of the metal-rich stars, indicating that the intracluster population is not well mixed. We discuss the implications that these observations have for the production of intracluster stars and the dynamical evolution of the Virgo Cluster.

We present a sample of low-resolution 5-38 μm Spitzer IRS spectra of the inner few square kiloparsecs of 59 nearby galaxies spanning a large range of star formation properties. A robust method for decomposing mid-infrared galaxy spectra is described and used to explore the behavior of PAH emission and the prevalence of silicate dust extinction. Evidence for silicate extinction is found in ~ of the sample, at strengths that indicate that most normal galaxies undergo A_V ≲ 3 mag averaged over their centers. The contribution of PAH emission to the total infrared power is found to peak near 10% and extend up to ~20% and is suppressed at metallicities Z ≲ Z_☉/4, as well as in low-luminosity AGN environments. Strong interband PAH feature strength variations (2-5 times) are observed, with the presence of a weak AGN and, to a lesser degree, increasing metallicity shifting power to the longer wavelength bands. A peculiar PAH emission spectrum with markedly diminished 5-8 μm features arises among the sample solely in systems with relatively hard radiation fields harboring low-luminosity AGNs. The AGNs may modify the emitting grain distribution and provide the direct excitation source of the unusual PAH emission, which cautions against using absolute PAH strength to estimate star formation rates in systems harboring active nuclei. Alternatively, the low star formation intensity often associated with weak AGNs may affect the spectrum. The effect of variations in the mid-infrared spectrum on broadband infrared surveys is modeled and points to more than a factor of 2 uncertainty in results that assume a fixed PAH emission spectrum, for redshifts z = 0-2.5.

It has recently been argued that the HCN J = 1-0 line emission may not be an unbiased tracer of dense molecular gas (n ≳ 10⁴ cm^-3) in luminous infrared galaxies (LIRGs; L_FIR > 10¹¹L_☉) and that HCO⁺J = 1-0 may constitute a better tracer instead, casting doubt onto earlier claims supporting the former as a good tracer of such gas. In this paper new sensitive HCN J = 4-3 observations of four such galaxies are presented, revealing a surprisingly wide excitation range for their dense gas phase that may render the J = 1-0 transition from either species a poor proxy of its mass. Moreover, the well-known sensitivity of the HCO⁺ abundance to the ionization degree of molecular gas (an important issue omitted from the ongoing discussion about the relative merits of HCN and HCO⁺ as dense gas tracers) may severely reduce the HCO⁺ abundance in the star-forming and highly turbulent molecular gas found in LIRGs, while HCN remains abundant. This may result in the decreasing HCO⁺/HCN J = 1-0 line ratios with increasing IR luminosity found in LIRGs, and it casts doubts on HCO⁺ rather than HCN as a good dense molecular gas tracer. Multitransition observations of both molecules are needed to identify the best such tracer and its relation to ongoing star formation, and to constrain what may be a considerable range of dense gas properties in such galaxies.

In order to test the silicate-core/organic-mantle model of galactic interstellar dust, we have performed spectropolarimetry of the 3.4 μm C–H bond stretch that is characteristic of aliphatic hydrocarbons, using the nucleus of the Seyfert 2 galaxy, NGC 1068, as a bright, dusty background source. Polarization calculations show that if the grains in NGC 1068 had the properties assigned by the core-mantle model to dust in the Galactic diffuse interstellar medium (ISM), they would cause a detectable rise in polarization over the 3.4 μm feature. No such increase is observed. We discuss modifications to the basic core-mantle model, such as changes in grain size or the existence of additional nonhydrocarbon aligned grain populations, that could better fit the observational evidence. However, we emphasize that the absence of polarization over the 3.4 μm band in NGC 1068—and, indeed, in every line of sight examined to date—can be readily explained by a population of small, unaligned carbonaceous grains with no physical connection to the silicates.

We have studied the mass distributions in the bright E0 galaxy NGC 1407 and its associated group by analyzing Chandra and ROSAT data. To probe the stellar mass distribution, we calculated the stellar mass-to-light ratios by comparing the observed line-strength indices and multicolor photometric data with different stellar synthesis models. We find that the gas is single phase, with a temperature of ≃0.7 keV within 1R_e. Outside 1R_e, the gas temperature increases quickly outward to >1 keV, indicating its group origin. We reveal that the X-ray surface brightness profile shows a central excess in the innermost region, and on both the total mass and dark matter profiles there is a flattened feature at about ≲1R_e, where the gas temperature increases rapidly. This may be a mark of the boundary between the galaxy and group halos. The total mass and dark matter distributions within 0.85R_e are cuspy and can be approximated by power-law profiles with indices of ≃2, which are marginally consistent with the generalized NFW profiles with ζ = 2. The mass in outer regions can be well fitted by a single NFW profile, and the derived concentration parameter c (18.6 ± 1.5) is larger than the 68% upper limit for a halo at z = 0 with the given M_vir. We find that the NGC 1407 group has a baryon-dominated core, while the mass in the >1R_e region is dominated by dark matter. At the virial radius r₂₀₀ = 572 ± 118 kpc, the inferred mass and mass-to-light ratio are M₂₀₀ = 2.20 ± 0.42 × 10¹³M_☉ and M_vir/L_B = 311 ± 60 M_☉/L_B,☉, respectively, showing that the NGC 1407 group is an extremely dark system, similar to many clusters of galaxies.

We present Spitzer IRAC observations at 3.6 and 4.5 μm along with optical data from the Local Group Galaxies Survey to investigate the evolved stellar population of the Local Group dwarf irregular galaxy WLM. These observations provide a nearly complete census of the AGB stars. We find that 39% of the infrared-detected AGB stars are not detected in the optical data, even though our 50% completeness limit is 3 mag fainter than the red giant branch tip. An additional 4% of the infrared-detected AGBs are misidentified in the optical, presumably due to reddening by circumstellar dust. We also compare our results with those of a narrowband optical carbon star survey of WLM and find the latter study sensitive to only 18% of the total AGB population. We detect objects with infrared fluxes consistent with them being mass-losing AGB stars and derive a present-day total mass-loss rate from the AGB stars of = (0.7-2.4) × 10^-3M_☉ yr^-1. The distributions of mass-loss rates and bolometric luminosities of AGBs and red supergiants are very similar to those in the LMC and SMC, and the empirical maximum mass-loss rate observed in the LMC and SMC is in excellent agreement with our WLM data.

We present the properties of the central stars from a sample of 54 planetary nebulae (PNe) observed in the Large Magellanic Cloud (LMC) with the Hubble Space Telescope Imaging Spectrograph (STIS). The Hubble Space Telescope's spatial resolution allows us to resolve the central star from its nebula (and line-of-sight stars) at the distance of the LMC, eliminating the dependency on photoionization modeling in the determination of the stellar flux. For the PNe in which the central star is detected, we obtain the stellar luminosities by directly measuring the stellar fluxes through broadband imaging and the stellar temperatures through Zanstra analysis. From the position of the central stars in the H-R diagram with respect to theoretical evolutionary tracks, we are able to determine reliable core masses for 21 central stars. By including the central star masses determined in this paper with the 16 obtained previously using the same technique, we have increased the sample of central star masses in the LMC to 37, for which we find a non-Gaussian mass distribution. The average central star mass for this sample is ⟨m_CS,LMC⟩ = 0.65 ± 0.07 M_☉, slightly higher than the one reported in the literature for both white dwarfs and the central stars of PNe in the Galaxy. If significant, this higher average central star mass in the LMC can be understood in terms of a metallicity dependency on mass-loss rates during the asymptotic giant branch, since the LMC has on average half the metallicity of the Galaxy. Finally, for the 37 objects analyzed in the LMC, we do not find any significant correlation between the mass of the central star and the morphology of the nebula.

The recent discovery of diffuse, very high energy (VHE) γ-radiation from the Galactic center ridge by the HESS telescope allows for the first time the direct determination of the parameters of Galactic cosmic-ray propagation models. Whereas this discovery showed that the diffuse γ-radiation can be explained by the interaction of VHE cosmic-ray (CR) protons with the interstellar gas located in several giant molecular clouds, we show in this paper that the associated diffusion coefficient for the protons depends on the epoch of activity of the central source of protons: Assuming that the supernova remnant (SNR) Sgr A East was responsible for the particle acceleration, we infer a diffusion coefficient for the Galactic center region of κ = 1-5 kpc² Myr^-1 for a mean proton energy of ~3 TeV. More specifically, for impulsive injection in a 5-10 kyr SNR, we infer a value of κ = 1-2 kpc² Myr^-1, whereas for source activity timescales equal to the age of the SNR, the diffusion coefficient would increase to κ ~ 5 kpc² Myr^-1. These values are smaller than those inferred from local CR abundances. Finally, the above-mentioned values of κ for impulsive injection are equally valid if the required transient source of protons was due to an earlier epoch of stellar infall into the black hole Sgr A*.

The inner couple of hundred parsecs of our Galaxy are characterized by a significant amount of synchrotron-emitting gas. Many of the best studied sources in this region exhibit a mixture of 6.4 keV Fe Kα emission, molecular line emission, and nonthermal radio continuum radiation. The spatial correlation between fluorescent Fe Kα line emission at 6.4 keV and molecular line emission from Galactic center molecular clouds has been explained as reflected X-rays from a past outburst of Sgr A*. Here we present a multiwavelength study of this region and find a correlation between the nonthermal radio filaments and the X-ray features. This correlation, when combined with the distribution of molecular gas, suggests against the irradiation model. Instead, we account for this distribution in terms of the impact of the relativistic particles from local (nonthermal filaments) and extended sources with diffuse neutral gas producing both nonthermal bremsstrahlung X-ray continuum emission and diffuse 6.4 keV line emission. The production rate of Fe Kα photons associated with the injection of electrons into a cloud as a function of column density is calculated. The required energy density of low-energy cosmic rays associated with the synchrotron-emitting radio filaments or extended features is estimated to be in the range between 20 and ~10³ eV cm^-3 for Sgr C, Sgr B1, Sgr B2, and "the 45 and -30 km s^-1" clouds. We also generalize this idea to explain the cosmic-ray heating of molecular gas, the interstellar cosmic-ray ionization, the pervasive production of the diffuse Kα line, and TeV emission from the Galactic center molecular clouds. In particular, we suggest that inverse Compton scattering of the submillimeter radiation from dust by relativistic electrons may contribute substantially to the large-scale diffuse TeV emission observed toward the central regions of the Galaxy.

The recent progress made in Galactic γ-ray astronomy using the High Energy Stereoscopic System (H.E.S.S.) instrument provides for the first time a population of Galactic TeV γ-rays, and hence potential neutrino sources, for which the neutrino flux can be estimated. Using the energy spectra and source morphologies measured by H.E.S.S., together with new parameterizations of pion production and decay in hadronic interactions, we estimate the signal and background rates expected for these sources in a first-generation water Cerenkov detector (ANTARES) and a next-generation neutrino telescope in the Mediterranean Sea, KM3NeT, with an instrumented volume of 1 km³. We find that the brightest γ-ray sources produce neutrino rates above 1 TeV, comparable to the background from atmospheric neutrinos. The expected event rates of the brightest sources in the ANTARES detector make a detection unlikely. However, for a 1 km³ KM3NeT detector, event rates of a few neutrinos per year from these sources are expected, and the detection of individual sources seems possible. Although generally these estimates should be taken as flux upper limits, we discuss the conditions and type of γ-ray sources for which the neutrino flux predictions can be considered robust.

Resent observations and theoretical interpretations suggest that intermediate-mass black holes (IMBHs; 100-10⁵M_☉) are formed in the centers of young and compact star clusters born close to the center of their parent galaxy. Such a star cluster would sink toward the center of the galaxy, and at the same time stars are stripped out of the cluster by the tidal field of the parent galaxy. We investigated the orbital evolution of the IMBH, after its parent cluster is completely disrupted by the tidal field of the parent galaxy, by means of large-scale N-body simulations. We constructed a model of the central region of our galaxy, with a supermassive black hole (SMBH; ≥10⁶M_☉) and Bahcall-Wolf stellar cusp, and placed an IMBH in a circular orbit of radius 0.086 pc. The IMBH sinks toward the SMBH through dynamical friction, but dynamical friction becomes ineffective when the IMBH reaches the radius inside which the initial stellar mass is comparable to the IMBH mass. This is because the IMBH kicks out the stars. This behavior is essentially the same as the loss-cone depletion observed in simulations of massive SMBH binaries. After the evolution through dynamical friction stalled, the eccentricity of the orbit of the IMBH goes up, resulting in the strong reduction of the merging timescale through gravitational wave radiation. Our result indicates that the IMBHs formed close to the galactic center can merge with the central SMBH in short time. The number of merging events detectable with DECIGO or BBO is estimated to be around 50 yr^-1. The event rate for LISA would be similar or less, depending on the growth mode of IMBHs.

We present here a simple approach to understanding the gas cloud mass distribution function by simulating formation and destruction of gas clouds and gas clumps in the ISM. We include as relevant processes coagulation to form bigger clouds, as well as disruption by collisions and the removal of gas by collapse to form stars. We evolve initial sets of preexisting gas clumps with a range of initial distribution functions (flat, Gaussian, fractal) for their physical parameters and with different geometrical forms (spherical or elongated) for the individual clouds, and constrain them within an imaginary box representing gravitational bounding, applying the kinematic laws of nonelastic collisions. The results agree well with observations of the mass distribution function of Galactic giant gas clouds if we choose a Gaussian for the initial distribution function, and initial gas clouds which are quasi-spherical.

We present Arecibo L-band Feed Array 21 cm observations of a subcomplex of HVCs at the tip of the anticenter complex. These observations show morphological details that point to interaction with the ambient halo medium and differential drag within the cloud subcomplex. We develop a new technique for measuring cloud distances, which relies on these observed morphological and kinematic characteristics, and show that it is consistent with Hα distances. These results are consistent with distances to HVCs and halo densities derived from models in which HVCs are formed from cooling halo gas.

New observations at high latitudes above the H II region W4 show that the structure formerly identified as a chimney candidate, an opening to the Galactic halo, is instead a superbubble in the process of fragmenting and possibly evolving into a chimney. Data at high Galactic latitudes (b > 5°) above the W3/W4 star-forming region at 1420 and 408 MHz Stokes I (total power) and 1420 MHz Stokes Q and U (linear polarization) reveal an egg-shaped structure with morphological correlations between our data and the Hα data of Dennison, Topasna, and Simonetti. Polarized intensity images show depolarization extending from W4 up the walls of the superbubble, providing strong evidence that the radio continuum is generated by thermal emission coincident with the Hα emission regions. We conclude that the parts of the H II region hitherto known as W4 and the newly revealed thermal emission are all ionized by the open cluster OCl 352. At an assumed distance of 2.35 kpc, the ovoid structure is 164 pc wide and extends 246 pc above the midplane of the Galaxy. The shell's emission decreases in total intensity and polarized intensity in various locations, appearing to have a break at its top and another on one side. Using a geometric analysis of the depolarization in the shell's walls, we estimate that a magnetic field line-of-sight component of 3-5 μG exists in the shell. We explore the connection between W4 and the Galactic halo, considering whether sufficient radiation can escape from the fragmenting superbubble to ionize the kiloparsec-scale Hα loop discovered by Reynolds, Sterling and Haffner.

While studying extraplanar neutral hydrogen in the disk-halo transition of the inner Galaxy, we have discovered what appears to be a huge superbubble centered around l ≈ 30°, whose top extends to latitudes >25° at a distance of about 7 kpc. It is detected in both H I and Hα. Using GBT, we have measured more than 220,000 H I spectra at 9' angular resolution in and around this structure. The total H I mass in the system is ≈10⁶M_☉, and it has an equal mass in H⁺. The plume of H I capping its top is 1.2 × 0.6 kpc in l and b and contains 3 × 10⁴M_☉ of H I. Despite its location (the main section is 3.4 kpc above the Galactic plane), the kinematics of the plume appears to be dominated by Galactic rotation, but with a lag of 27 km s^-1 from corotation. At the base of this structure there are "whiskers" of H I several hundreds of parsecs wide, reaching more than 1 kpc into the halo; they have a vertical density structure suggesting that they are the bubble walls and have been created by sideways rather than upward motion. They resemble the vertical dust lanes seen in NGC 891. From a Kompaneets model of an expanding bubble, we estimate that the age of this system is ≈30 Myr and its total energy content ~10⁵³ ergs. It may just now be at the stage where its expansion has ceased and the shell is beginning to undergo significant instabilities. This system offers an unprecedented opportunity to study a number of important phenomena at close range, including superbubble evolution, turbulence in an H I shell, and the magnitude of the ionizing flux above the Galactic disk.

We present VLBA observations of 1667 MHz OH maser emission from the massive star formation region G5.89-0.39. The observations were phase-referenced, allowing the absolute positions of the masers to be obtained. The 1667 MHz masers have radial velocities that span ~50 km s^-1 but show little evidence of tracing the bipolar molecular outflow, as has been claimed in previous studies. We identify 23 Zeeman pairs through comparison of masers in left and right circular polarization. Magnetic field strengths range from -2 to +2 mG, and an ordered reversal in magnetic field direction is observed toward the southern region of the UC H II region. We suggest that the velocity and magnetic field structure of the 1667 MHz masers can be explained in the context of a model in which the masers arise in a neutral shell just outside a rapidly expanding ionized shell.

We report on a second epoch of VLBA observations of the 1665 and 1667 MHz OH masers in the massive star-forming region W75N. We find evidence to confirm the existence of very strong (~40 mG) magnetic fields near source VLA 2. The masers near VLA 2 are dynamically distinct and include a very bright spot apparently moving at 50 km s^-1 relative to those around VLA 1. This fast-moving spot may be an example of a rare class of OH masers seen in outflows in star-forming regions. Due to the variability of these masers and the rapidity of their motions, tracking these motions will require multiple observations over a significantly shorter time baseline than obtained here. Proper motions of the masers near VLA 1 are more suggestive of streaming along magnetized shocks rather than Keplerian rotation in a disk. The motions of the easternmost cluster of masers in W75N(B) may be tracing slow expansion around an unseen exciting source.

We simulate the early stages of the evolution of turbulent, virialized, high-mass protostellar cores, with primary attention to how cores fragment and whether they form a small or large number of protostars. Our simulations use the Orion adaptive mesh refinement code to follow the collapse from ~0.1 pc scales to ~10 AU scales, for durations that cover the main fragmentation phase, using three-dimensional gravito-radiation hydrodynamics. We find that for a wide range of initial conditions radiation feedback from accreting protostars inhibits the formation of fragments, so that the vast majority of the collapsed mass accretes onto one or a few objects. Most of the fragmentation that does occur takes place in massive, self-shielding disks. These are driven to gravitational instability by rapid accretion, producing rapid mass and angular momentum transport that allows most of the gas to accrete onto the central star rather than forming fragments. In contrast, a control run using the same initial conditions but an isothermal equation of state produces much more fragmentation, both in and out of the disk. We conclude that massive cores with observed properties are not likely to fragment into many stars, so that, at least at high masses, the core mass function probably determines the stellar initial mass function. Our results also demonstrate that simulations of massive star-forming regions that do not include radiative transfer, and instead rely on a barotropic equation of state or optically thin heating and cooling curves, are likely to produce misleading results.

We present mid-infrared Spitzer IRS spectra of the UX Orionis star VV Ser, combined with interferometric and spectroscopic data from the literature covering UV to submillimeter wavelengths. The full set of data are modeled by an axisymmetric Monte Carlo radiative transfer code to test the prediction of Dullemond et al. that disks around UX Orionis stars are self-shadowed and seen nearly edge-on. Our model is consistent with all the available observational constraints, providing strong support for this interpretation. The mid-infrared SED is declining and exhibits weak silicate emission features, consistent with a self-shadowed geometry. MIPS imaging shows that the disk has a small grain dust mass as low as 0.8 × 10^-7M_☉, which may be due to strong grain growth and settling. The grains in the upper layers of the puffed-up inner rim must be small (0.01-0.4 μm) to reproduce the colors (R_V ~ 3.6) of the optical light curve, while the silicate emission features indicate that grains in the outer disk (>1-2 AU) are somewhat larger (0.3-3.0 μm). If grains in the inner disk are small, the location of the puffed-up inner rim is estimated to be at 0.7-0.8 AU. This is almost twice the rim radius estimated from near-infrared interferometry. Since larger (more gray) grains are able to penetrate closer to the star for the same dust sublimation temperature, we suggest a model in which large grains in the disk midplane reach to within 0.25 AU of the star, while small grains in the disk surface create a puffed-up rim at ~0.7-0.8 AU.

We present mid-infrared Spitzer IRAC and MIPS images of the UX Orionis star VV Ser and the surrounding cloud. The 5.6-70 μm images show bright, localized, and nebulous emission extended over 4' centered on VV Ser. This nebulosity is due to transiently heated grains excited by UV photons emitted by VV Ser. Imprinted on the nebulosity is a wedge-shaped dark band, centered on the star. We interpret this as the shadow cast by the inner regions of a near-edge-on disk, allowing the PAHs to be excited only outside of this shadow. We extend an axisymmetric radiative transfer model of the VV Ser disk described in a companion paper to include quantum-heated PAH molecules and very small grains (VSGs) in the thermal cooling approximation. The presence of a disk shadow strongly constrains the inclination as well as the position angle of the disk. The nebulosity at 5.6-8.0 μm and the 2175 Å absorption feature seen in an archival spectrum from the IUE can be fit using only PAHs, consistent with the main carrier of the 2175 Å feature being due to the graphite-like structure of the PAHs. The PAH component is found to be relatively smoothly distributed in the cloud, while the population of VSGs emitting at 20-70 μm is strongly concentrated ~50'' to the southeast of VV Ser. Depending on the adopted PAH opacity, the abundance of PAHs in the surrounding cloud is constrained to 5% ± 2% of the total dust mass. Although relatively rare, quantum-heated nebulosities surrounding single, well-defined stars are well-suited for gaining unique insights into the physics of very small particles in molecular clouds.

We have conducted a thorough and blind search for emission lines in >70 Swift X-ray afterglows of total exposure ~10⁷ s. We find that most afterglows are consistent with pure power laws plus extinction. Significant outliers to the population exist at the 5%-10% level and have anomalously soft, possibly thermal spectra. Four bursts are singled out via possible detections of two to five lines: GRB 060218, GRB 060202, GRB 050822, and GRB 050714B. Alternatively, a blackbody model with kT ~ 0.1-0.5 keV can describe the soft emission in each afterglow. The most significant soft-component detections in the full data set of ~2000 spectra correspond to GRB 060218/SN 2006aj, with line significances ranging up to ~20 σ. A thermal plasma model fit to the data indicates that the flux is primarily due to L-shell transitions of Fe at roughly solar abundance. We associate (>4 σ significant) line triggers in the three other events with K-shell transitions in light metals. We favor a model where the possible line emission in these afterglows arises from the mildly relativistic cocoon of matter surrounding the GRB jet as it penetrates and exits the surface of the progenitor star. The emitting material in each burst is at a similar distance ~10¹²-10¹³ cm, a similar density ~10¹⁷ cm^-3, and subject to a similar flux of ionizing radiation. The lines may correlate with the X-ray flaring. For the blackbody interpretation, the soft flux may arise from breakout of the GRB shock or plasma cocoon from the progenitor stellar wind, as recently suggested for GRB 060218 (Campana et al. 2006). Due to the low z of GRB 060218, bursts faint in gamma rays with fluxes dominated by this soft X-ray component could outnumber classical GRBs 100 to 1.

With an eye toward application to the theory of core-collapse supernovae, we perform a global linear analysis of the stability of spherically symmetric accretion flows through a standing shock wave onto a proto-neutron star. For the unperturbed flows, we adopt spherically symmetric, steady solutions obtained with a realistic equation of state and realistic neutrino reaction rates. These solutions are characterized by the mass accretion rate and neutrino luminosity. Then we solve the equations for linear perturbations numerically, obtaining the eigenfrequencies and eigenfunctions. We find the following: (1) The flows are stable for all modes if the neutrino luminosity is lower than a certain value, e.g., ~1 × 10⁵² ergs s^-1 for = 1.0 M_☉ s^-1. (2) For higher luminosities, nonradial instabilities are induced, probably through advective-acoustic cycles. Interestingly, modes with l = 2 and l = 3 first become unstable for relatively low neutrino luminosities, e.g., ~(2-3) × 10⁵² ergs s^-1 for the same accretion rate, whereas the l = 1 mode is the most unstable for higher luminosities, ~(3-7) × 10⁵² ergs s^-1. These are all oscillatory modes. (3) For still larger luminosities, ≳7 × 10⁵² ergs s^-1 for = 1.0 M_☉ s^-1, nonoscillatory modes, both radial and nonradial, become unstable. These nonradial modes are identified as convective, and their growth rates have a peak at l = 5-11, depending on the luminosity. We confirm the result from numerical simulations that the instabilities induced by advective-acoustic cycles are more important than convection for lower neutrino luminosities. Furthermore, we investigate changing the inner boundary conditions and find that while the effects are nonnegligible, the existence of the instabilities does not qualitatively change for a variety of conditions.

We follow up on our (Radhakrishnan & Deshpande) radically different interpretation of the observed structures and morphologies in the X-ray observations of the nebulae around young pulsars (PWNe). In our general model for PWNe (Radhakrishnan & Deshpande), originally motivated by the Chandra observations of the Vela X-ray nebula, the bright arcs, the jetlike feature, and the diffuse components in such nebulae can be explained together in detail, wherein the arcs are understood as traces of the particle beams from the two magnetic poles at the shock front. We consider this as important evidence for collimated particle beams from pulsars' magnetic poles. In this paper we discuss the variability in the features in the Vela X-ray nebula observed by Pavlov and coworkers and assess the relevance and implication of our model to the observations on the Crab and other remnants. Our basic picture after incorporating the signatures of free precession of the central compact object can readily account for the variability and significant asymmetries, including the bent jetlike features, in the observed morphologies. The implications of these findings are discussed.

We investigate the polarization of the high-energy emission from the Crab pulsar within the framework of the outer gap accelerator, following previous studies by Cheng and coworkers. A recent version of the outer gap, in which the gap extends from inside the null charge surface to the light cylinder, is used to examine the synchrotron radiation from the secondary and tertiary pairs that are produced outside the gap. We are able to simultaneously reproduce the light curve, the spectrum, and the polarization characteristics by taking into account the gyration of the particles. The polarization position angle curve and the degree of polarization are calculated and compared with the Crab optical data. We demonstrate that the radiation from inside the null charge surface produces the outer wing and off-pulse portions of the light curve and that the tertiary pairs contribute to the bridge emission. The emission from the secondary pairs explains the main features of the observed light curve and spectrum. On the other hand, the emissions both from inside the null charge surface and from the tertiary pairs are required in order to explain the optical polarization behavior of the Crab pulsar. The energy dependence of the polarization features is predicted by the model. The polarization position angle curve indicates that our viewing angle as measured from the pulsar's rotational axis is greater than 90°.

In an earlier paper, the general formulation and results for photon reprocessing (downscattering) that included recoil and Comptonization effects due to divergence of the flow were presented. In a second paper we showed the Monte Carlo (MC) simulated continuum and line spectra. We also provided an analytical description of the simulated continuum spectra using the diffusion approximation. We have simulated the propagation of monochromatic and continuum photons in a bulk outflow from a compact object. Electron scattering of the photons within the expanding flow leads to a decrease of their energy, which is of first order in V/c (where V is the outflow velocity). The downscattering effect of first order in V/c in the diverging flow is explained by semianalytical calculations and confirmed by MC simulations. We conclude that redshifted lines and downscattering bumps are intrinsic properties of the powerful outflows for which the Thomson optical depth is greater than 1. We fitted our model line profiles to the observations using four free parameters, β = V/c, the optical depth of the wind τ, the wind temperature kT_e, and the original line photon energy E₀. We show how the primary spectrum emitted close to the black hole is modified by reprocessing in the warm wind. In the framework of our wind model, the fluorescent iron line Kα is formed in the partly ionized wind as a result of illumination by central source continuum photons. The demonstrated application of our outflow model to the XMM-Newton observations of MCG -6-30-15 and to the ASCA observations of GRO J1655-40 points out a potential powerful spectral diagnostic for probes of the outflow-central object connection in Galactic and extragalactic black hole sources.

We have developed a systematic procedure to study the disks in Algol-type binaries using spectroscopic analysis, synthetic spectra, and tomography. We analyzed 119 Hα spectra of TT Hya, an Algol-type eclipsing interacting binary, collected from 1985 to 2001. The new radial velocities enabled us to derive reliable orbital elements, including a small nonzero eccentricity, and to improve the accuracy of the absolute dimensions of the system. High-resolution IUE spectra were also analyzed to study the formation of the UV lines and continuum. Synthetic spectra of the iron curtain using our new SHELLSPEC program enabled us to derive a characteristic disk temperature of 7000 K. We have demonstrated that the UV emission lines seen during total primary eclipse cannot originate from the accretion disk but most likely arise from a hotter disk-stream interaction region. The synthetic spectra of the stars, disk, and stream allowed us to derive a mass transfer rate ≥2 × 10^-10M_☉ yr^-1. Doppler tomography of the observed Hα profiles revealed a distinct accretion disk. The difference spectra produced by subtracting the synthetic spectra of the stars resulted in an image of the disk, which virtually disappeared once the composite synthetic spectra of the stars and disk were used to calculate the difference spectra. An intensity enhancement of the resulting tomogram revealed images of the gas stream and an emission arc. We successfully modeled the gas stream using SHELLSPEC and associated the emission arc with an asymmetry in the accretion disk.

V794 Aql was observed in a high state with the Hubble Space Telescope Space Telescope Imaging Spectrograph (HST STIS) on 2003 August 28, and with the Far Ultraviolet Spectroscopic Explorer (FUSE) on 2004 May 13. We present here a spectral analysis of the FUSE and HST STIS spectra. For a 0.9 M_☉, the best fit is an accretion disk with a mass accretion rate = 10^-8.5 to 10^-8.0M_☉ yr^-1 with an inclination of 60° when assuming E(B - V) = 0.2. The corresponding distance to the system is d = 690 pc. A single white dwarf model leads to a rather hot temperature (between 30,000 and 55,000 K depending on the assumptions) but does not provide a fit as good as the accretion disk model. The same disk model is the best fit to the FUSE spectrum, the HST STIS spectrum, and the combined FUSE + HST STIS spectrum, implying therefore that the disk model is the best fit not only in the least χ² sense, but also as a consistent solution across a large-wavelength span of observation. We find that the model fits are in much better agreement with the dereddened spectra when E(B - V) is large, as excess emission in the longer wavelengths renders the slope of the observed spectra almost impossible to fit, unless E(B - V) = 0.2 . A large reddening value is in agreement with the hydrogen column density we find, N(H I) = 4.5 × 10²⁰ cm^-2 and N(H₂) = 3 × 10¹⁷ cm^-2 and with the E(B - V) value derived from the existing archival International Ultraviolet Explorer spectra.

We examine the effects of significant electron antineutrino fluxes on hydrogen burning. Specifically, we find that the bottleneck weak nuclear reactions in the traditional p-p chain and the hot CNO cycle can be accelerated by antineutrino capture, increasing the energy generation rate. We also discuss how antineutrino capture reactions can alter the conditions for break out into the rp-process. We speculate on the impact of these considerations for the evolution and dynamics of collapsing very massive and supermassive compact objects.

We present millimeter interferometric observations of the molecular envelope around the red supergiant VY CMa with the Submillimeter Array (SMA). The high angular resolution (<2'') allows us to derive the structure of the envelope as observed in the 1.3 mm continuum, ¹²CO(2-1), ¹³CO(2-1), and SO(6₅-5₄) lines emission. The circumstellar envelope is resolved into three components: (1) a dense, compact, and dusty central component, embedded in (2) a more diffuse and extended envelope, and (3) a high-velocity component. We construct a simple model, consisting of a spherically symmetric slowly expanding envelope and bipolar outflows with a wide opening angle (~120°) viewed close to the line of sight (i = 15°). Our model can explain the main features of the SMA data and previous single-dish CO multiline observations. An episode of enhanced mass loss along the bipolar direction is inferred from our modeling. The SMA data provide a better understanding of the complicated morphology seen in the optical/IR high-resolution observations.

We present the first direct magnetic field measurements on M dwarfs cooler than spectral class M4.5. Utilizing a new method based on the FeH band near 1 μm, we categorize the integrated surface magnetic flux as low (well under 1 kG), intermediate (between 1 and about 2.5 kG), or strong (greater than about 3 kG) for a set of more than 20 stars ranging from M2 down to M9. Along with the field, we also measure the rotational broadening (v sin i) and Hα emission strength. Our goal is to advance the understanding of how dynamo field production varies with stellar parameters for very low mass stars, how the field and emission activity are related, and whether there is a connection between the rotation and magnetic flux. We find that fields are produced throughout the M dwarfs. In the mid-M stars, there is a clear connection between slow rotation and weak fields. In the late-M stars, rotation is always measurable, and the strongest fields are associated with with the most rapid rotators. Interestingly, these very cool rapid rotators appear to have the largest magnetic flux in the whole sample (greater than in the classical dMe stars). Hα emission is found to be a good proxy for magnetic fields, although the relation between the fractional emission and the magnetic flux varies with effective temperature. The drop-off in this fractional emission near the bottom of the main sequence is not accompanied by a drop-off in magnetic flux. It is clear that the methodology we have developed can be further applied to discover more about the behavior of magnetic dynamos and activity in cool and fully convective objects.

We present new infrared spectra of the T8 brown dwarf 2MASS J04151954-0935066: 2.9-4.1 μm spectra obtained with the Infrared Camera and Spectrograph on the Subaru Telescope, and 5.2-14.5 μm spectra obtained with the Infrared Spectrograph on the Spitzer Space Telescope. We use these data and models to determine an accurate bolometric luminosity of log L_bol/L_☉ = -5.67 and to constrain the effective temperature, gravity, mass, and age to 725-775 K, log g = 5.00-5.37, M = 33-58 M_Jup, and age = 3-10 Gyr. We perform the same analysis using published 0.6-15 μm spectra for the T7.5 dwarf 2MASS J12171110-0311131, for which we find a metal-rich composition ([Fe/H] ~ 0.3), and log L_bol/L_☉ = -5.31, T_eff = 850-950 K, log g = 4.80-5.42, M = 25-66 M_Jup, and age = 1-10 Gyr. These luminosities and effective temperatures straddle those determined with the same method and models for Gl 570D by Saumon et al. and make 2MASS J04151954-0935066 the coolest and least luminous T dwarf with well-determined properties. We find that synthetic spectra generated by the models reproduce the observed red through mid-infrared spectra of 2MASS J04151954-0935066 and 2MASS J12171110-0311131 very well, except for known discrepancies that are most likely due to the incomplete CH₄ opacities. Both objects show evidence of departures from strict chemical equilibrium, and we discuss this result in the context of other late T dwarfs in which disequilibrium phenomena have been observed.

We present high angular resolution images of the post-AGB nebula IRAS 18276-1431 (also known as OH 17.7-2.0) obtained with the Keck II adaptive optics (AO) system in its natural guide star (NGS) mode in the K_p, L_p, and M_s near-infrared bands. We also present supporting optical F606W and F814W HST images as well as interferometric observations of the ¹²CO J = 1-0, ¹³CO J = 1-0, and 2.6 mm continuum emission with OVRO. The envelope of IRAS 18276-1431 displays a clear bipolar morphology in our optical and NIR images with two lobes separated by a dark waist and surrounded by a faint 4.5'' × 3.4'' halo. Our K_p-band image reveals two pairs of radial "searchlight beams" emerging from the nebula center and several intersecting, arclike features. From our CO data we derive a mass of M > 0.38 (D/3 kpc)²M_☉ and an expansion velocity V_exp = 17 km s^-1 for the molecular envelope. The density in the halo follows a radial power law ∝r^-3, which is consistent with a mass-loss rate increasing with time. Analysis of the NIR colors indicates the presence of a compact central source of ~300-500 K dust illuminating the nebula in addition to the central star. Modeling of the thermal IR suggests a two-shell structure in the dust envelope: (1) an outer shell with inner and outer radius R_in ~ 1.6 × 10¹⁶ and R_out ≳ 1.25 × 10¹⁷ cm, dust temperature T_dust ~ 105-50 K, and a mean mass-loss rate of ≈ 10^-3M_☉ yr^-1; and (2) an inner shell with R_in ~ 6.3 × 10¹⁴ cm, T_dust ~ 500-105 K, and ~ 3 × 10^-5M_☉ yr^-1. An additional population of big dust grains (radius a ≳ 0.4 mm) with T_dust = 150-20 K and mass M_dust = (0.16-1.6) × 10^-3 (D/3 kpc)²M_☉ can account for the observed submillimeter and millimeter flux excess. The mass of the envelope enclosed within R_out = 1.25 × 10¹⁷ cm derived from SED modeling is ~1 (D/3 kpc)²M_☉.

Magnetic helicity is a useful quantity in characterizing the magnetic systems of solar active regions. The purpose of the present work is to check for consistency between the local correlation tracking (LCT) method used to measure helicity injection through the photosphere, and the linear force-free field (LFFF) method used to determine helicity in the corona, based on the principle of helicity conservation in the solar corona. We have calculated the amount of magnetic helicity injected through the photosphere during the first disk passage of AR 10696 using the LCT method initially described by Chae. We have also measured the coronal magnetic helicity as a function of time using the LFFF method. With a value for the force-free α, the coronal field is constructed from the extrapolation of the Solar and Heliospheric Observatory (SOHO) MDI magnetograms, then compared with the coronal loops in the EUV images taken by the SOHO EIT. The force-free α that best fits the loops is used to calculate the coronal helicity. From a careful comparison of different helicity measurements during each time interval, we have reached the core conclusion that our measurements follow the helicity conservation principle with an uncertainty of ~15% and hence support the consistency between the two different methods with the same amount of uncertainty.

Statistical tests based on linear discriminant analysis are applied to numerous photospheric magnetic parameters, continuing toward the goal of identifying properties important for the production of solar flares. For this study, the vector field data are University of Hawai`i Imaging Vector Magnetograph daily magnetograms obtained between 2001 and 2004. Over 1200 separate magnetograms of 496 numbered active regions comprise the data set. At the soft X-ray C1.0 level, 359 magnetograms are considered "flare productive" in the 24 hr postobservation. Considering multiple photospheric variables simultaneously indicates that combinations of only a few familiar variables encompass the majority of the predictive power available. However, the choice of which few variables is not unique, due to strong correlations among photospheric quantities such as total magnetic flux and total vertical current, two of the most powerful predictors. The best discriminant functions result from combining one of these with additional uncorrelated variables, such as measures of the magnetic shear, and successfully classify over 80% of the regions. By comparison, a success rate of approximately 70% is achieved by simply classifying all regions as "flare quiet." Redefining "flare-productive" at the M1.0 level, parameterizations of excess photospheric magnetic energy outperform other variables. However, the uniform flare-quiet classification rate is approximately 90%, while incorporating photospheric magnetic field information results in at most a 93% success rate. Using nonparametric discriminant analysis, we demonstrate that the results are quite robust. Thus, we conclude that the state of the photospheric magnetic field at any given time has limited bearing on whether that region will be flare productive.

In this paper we investigate the formation of the white-light (WL) continuum during solar flares and its relationship to energy deposition by electron beams inferred from hard X-ray emission. We analyze nine flares spanning GOES classifications from C4.8 to M9.1, seven of which show clear cospatial RHESSI hard X-ray and TRACE WL footpoints. We characterize the TRACE WL/UV continuum energy under two simplifying assumptions: (1) a blackbody function, or (2) a Paschen-Balmer continuum model. These set limits on the energy in the continuum, which we compare with that provided by flare electrons under the usual collisional thick-target assumptions. We find that the power required by the white-light luminosity enhancement is comparable to the electron beam power required to produce the HXR emission only if the low-energy cutoff to the spectrum is less than 25 keV. The bulk of the energy required to power the white-light flare (WLF) therefore resides at these low energies. Since such low-energy electrons cannot penetrate deep into a collisional thick target, this implies that the continuum enhancement is due to processes occurring at moderate depths in the chromosphere.

We present a detailed study of Hα surges from cotemporal high-resolution multiwavelength images of NOAA AR 8227 obtained by the 50 cm Swedish Vacuum Solar Telescope (formerly situated on La Palma, Spain) and TRACE. We find that two kinds of collisions between opposite polarity magnetic flux produce the surges. First, one edge of an emerging flux region (EFR) collides with the preexisting magnetic field and causes continual surge activities, which have already been named EFR surges by previous authors. Secondly, moving magnetic features (MMFs), which emerge near the sunspot penumbra, pass through the ambient plasma and eventually collide with the opposite polarity magnetic field of the EFR. During their passage from the sunspot penumbra to the EFR, the MMFs constantly interacted with other magnetic elements and had a close relationship and showed similar flow patterns to Ca II K bright points. These brightenings were located at the leading edges of the MMFs. Cancellation of opposite polarity magnetic flux at the surge footpoint is observed, accompanied by chromospheric and coronal brightenings. We explain the evolutionary and morphological characteristics of the multiwavelength features associated with the Hα surges in both cases by the extension of previous 2D schematic models of reconnection in surges. Furthermore, by measuring the expansion velocity and photospheric magnetic field around the surge footpoint, we estimate a dimensionless reconnection rate of 0.04 (ratio of inflow velocity to Alfvén velocity). This is sufficient to produce a significant surge that heats the chromospheric plasma to coronal temperatures.

Noble gas isotopes in presolar silicon carbide (SiC) dust grains from primitive meteorites provide, together with major element isotopic compositions, insight into the nucleosynthetic output of different types of evolved stars >4.5 Gyr ago. We report here new results from helium and neon isotopic analyses of single presolar SiC grains with sizes between 0.6 and 6.3 μm using an ultrahigh sensitivity mass spectrometer. These noble gas studies were complemented by an ion microprobe study (NanoSIMS) of Si, C, and N isotopic compositions of the same grains. About 40%, or 46 of the 110 grains analyzed, contain nucleosynthetic ²²Ne and/or ⁴He from their parent stars above our mass spectrometer's detection limit. We discuss the possible stellar sources using isotopic ratios as constraints combined with new model predictions for low- to intermediate-mass (1.5, 2, 3, and 5 M_☉) asymptotic giant branch (AGB) stars of different metallicities (1, 1/2, 1/3, and 1/6 Z_☉). Most SiC grains are of the mainstream type and originated in low-mass AGB stars. We find a higher-than-expected percentage of A/B type grains, with some containing ²²Ne and/or ⁴He. In addition, we find one noble gas-rich nova grain candidate, one supernova grain (X-type grain), and one ²²Ne-rich X- or Z-type grain candidate.

Raster ion imaging of the oxygen isotopes with the NanoSIMS ion microprobe has been used to identify presolar grains in two primitive meteorites. Eleven presolar silicates and eight presolar oxides were identified in the primitive carbonaceous chondrite Acfer 094 for abundances of 325 and 360 parts per million (ppm), respectively. In addition, nine presolar silicates and five presolar oxide grains were identified in the CO3 chondrite ALHA 77307, for abundances of 320 and 200 ppm, respectively. These abundances, which are matrix-normalized and corrected for instrumental detection efficiencies, are much higher than those of other presolar phases, with the exception of nanodiamonds, although the latter may not all be presolar. The chemical compositions of six presolar silicate grains from ALHA 77307 were elucidated by Auger spectroscopy. Transmission electron microscopy (TEM) analysis of one presolar silicate grain revealed a nonstoichiometric composition and an amorphous structure as indicated by the diffuse electron diffraction pattern. The oxygen isotopic compositions of the presolar silicates indicate origins in red giant and asymptotic giant branch stars. Analysis of the Si isotopic compositions of 10 presolar silicates provides further constraints on the effects of Galactic chemical evolution.

We compute the luminosity function (LF) and the formation rate of long gamma-ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by BATSE in three different scenarios: (1) GRBs follow the cosmic star formation, and their LF is constant in time; (2) GRBs follow the cosmic star formation, but the LF varies with redshift; and (3) GRBs form preferentially in low-metallicity environments. We find that the differential peak flux number counts obtained by BATSE and by Swift can be reproduced using the same LF and GRB formation rate, indicating that the two satellites are observing the same GRB population. We then check the resulting redshift distributions in light of Swift 2 year data, focusing in particular on the relatively large sample of GRBs detected at z > 2.5. We show that models in which GRBs trace the cosmic star formation and are described by a constant LF are ruled out by the number of high-z Swift detections. This conclusion does not depend on the redshift distribution of bursts that lack optical identification, nor on the existence of a decline in star formation rate at z > 2, nor on the adopted faint end of the GRB LF. Swift observations can be explained by assuming that the LF varies with redshift and/or that GRB formation is limited to low-metallicity environments.

Long-duration gamma-ray bursts (GRBs) have eight luminosity relations where observable burst properties can yield the burst luminosity and hence distance. This turns GRBs into useful tools of cosmology. Recently, two tests have been proposed (by Nakar & Piran and by Li) for which one of the eight relations is claimed to have significant problems. In this Letter, we generalize these tests and apply them to all eight GRB luminosity relations. (1) All eight relations pass the Nakar & Piran test after accounting for the uncertainties on the data and the dispersions of the correlations. (2) All eight relations are good when the GRB redshifts are known, for example, for calibration of the relations and for GRB Hubble diagram purposes. (3) We confirm the earlier results that the E_{γ, iso}-E_peak Amati relation must produce very large error bars whenever an unknown redshift being sought is ≳1.4. (4) The E_γ-E_peak relation of Ghirlanda et al. must produce very large error bars whenever an unknown redshift being sought is ≳3.4. (55) The other six relations have no problem at all from the ambiguity test of Li.

In the double neutron star merger or neutron star-black hole merger model for short gamma-ray bursts, the outflow launched might be mildly magnetized and neutron-rich. The magnetized neutron-rich outflow will be accelerated by the magnetic and thermal pressure and may finally form a two-component jet, as suggested by Vlahakis, Peng, & Königl. We show in this work that such a two-component jet model could well reproduce the multiwavelength afterglow light curves, in particular the X-ray flat segment, of short GRB 051221A. In this model, the central engine need not be active much longer than the prompt γ-ray emission.

We present a combined analysis of the kinematics of the Large Magellanic Cloud through its H I gas, carbon stars, and red supergiant stars. After correcting the line-of-sight velocities for the recent accurate measurement of the LMC's space motion, we find that each kinematic tracer clearly defines a flat rotation curve with similar shape but different amplitude for each tracer: 61 km s^-1 for the carbon stars, 80 km s^-1 for the H I, and 107 km s^-1 for the red supergiants. Previously identified tidal H I features are seen to harbor numerous carbon stars, with the tidally disturbed stars comprising 7%-15% of the total sample. This discovery implies that we cannot depend on the carbon star sample alone to construct a reliable model of the LMC's gravitational potential. We also find red supergiants with peculiar kinematics, but their association with tidal features is unclear and may instead be interacting with supergiant H I shell SGS4. In addition, although the local velocity dispersion of the red supergiants is small, ~8 km s^-1, their velocity dispersion about the carbon star rotation solution is 17 km s^-1, equal to the velocity dispersion of the carbon stars themselves. We thus appear to be witnessing the tidal heating of the LMC's stellar disk.

We report on the discovery of a surprising observed correlation between the slope of the low-mass stellar global mass function (GMF) of globular clusters (GCs) and their central concentration parameter c = log(r_t/r_c), i.e., the logarithmic ratio of tidal and core radii. This result is based on the analysis of a sample of 20 Galactic GCs with solid GMF measurements from deep HST or VLT data. All the high-concentration clusters in the sample have a steep GMF, most likely reflecting their initial mass function. Conversely, low-concentration clusters tend to have a flatter GMF, implying that they have lost many stars via evaporation or tidal stripping. No GCs are found with a flat GMF and high central concentration. This finding appears counterintuitive, since the same two-body relaxation mechanism that causes stars to evaporate and the cluster to eventually dissolve should also lead to higher central density and possibly core collapse. Therefore, more concentrated clusters should have lost proportionately more stars and have a shallower GMF than low-concentration clusters, contrary to what is observed. It is possible that severely depleted GCs have also undergone core collapse and have already recovered a normal radial density profile. It is, however, more likely that GCs with a flat GMF have a much denser and smaller core than that suggested by their surface brightness profile and may well be undergoing collapse at present. In either case, we may have so far seriously underestimated the number of post-core collapse clusters, and many may be lurking in the Milky Way.

On the basis of 3 years of deep observations of the Galactic center with the Chandra X-Ray Observatory, we report the discovery of changes in the intensities and morphologies of two hard X-ray nebulosities. The nebulosities are dominated by fluorescent iron emission and are coincident with molecular clouds. The morphological changes are manifest on parsec scales, which requires that these iron features are scattered X-rays from a 2 or 3 year long outburst of a point source (either Sgr A* or an X-ray binary) with a luminosity of at least 10³⁷ ergs s^-1. The variability precludes the hypotheses that these nebulae either are produced by keV electrons bombarding molecular clouds or are iron-rich ejecta from supernovae. Moreover, the morphologies of the reflection nebulae implies that the dense regions of the clouds are filamentary, with widths of ≈0.3 pc and lengths of ≈2 pc.

Observations of planetary nebulae (PNe) in the work of Sterling, Dinerstein, & Bowers have revealed abundances in the neutron-capture element germanium (Ge) from solar to factors of 3-10 above solar. The enhanced Ge is an indication that the slow neutron-capture process (the s-process) operated in the parent star during the thermally pulsing asymptotic giant branch (TP-AGB) phase. We compute the detailed nucleosynthesis of a series of AGB models to estimate the surface enrichment of Ge near the end of the AGB. A partial mixing zone of constant mass is included at the deepest extent of each dredge-up episode, resulting in the formation of a ¹³C pocket in the top approximately one-tenth of the He-rich intershell. All of the models show surface increases of [Ge/Fe] ≲ 0.5, except the 2.5 M_☉, Z = 0.004 case, which produced a factor of 6 enhancement of Ge. Near the tip of the TP-AGB, a couple of extra thermal pulses (TPs) could occur to account for the composition of the most Ge-enriched PNe. Uncertainties in the theoretical modeling of AGB stellar evolution might account for larger Ge enhancements than we predict here. Alternatively, a possible solution could be provided by the occurrence of a late TP during the post-AGB phase. Difficulties related to spectroscopic abundance estimates also need to be taken into consideration. Further study is required to better assess how the model uncertainties affect the predictions and, consequently, if a late TP should be invoked.

The mass-loss rates of hot, massive, luminous stars are considered a decisive parameter in shaping the evolutionary tracks of such stars and influencing the interstellar medium on galactic scales. The small-scale structures (clumps) that are omnipresent in such winds may reduce empirical estimates of mass-loss rates by an evolutionarily significant factor of ≥3. So far, there has been no direct observational evidence that wind clumping may persist at the same level in environments with a low ambient metallicity, where the wind-driving opacity is reduced. Here we report the results of time-resolved spectroscopy of three presumably single Population I Wolf-Rayet stars in the Small Magellanic Cloud, where the ambient metallicity is ~Z_☉. We detect numerous small-scale emission peaks moving outward in the accelerating parts of the stellar winds. The general properties of the moving features, such as their velocity dispersions, emissivities, and average accelerations, closely match the corresponding characteristics of small-scale inhomogeneities in the winds of Galactic Wolf-Rayet stars.

We investigate the morphology and the evolutionary stage of the hypercompact H II region M17-UC1 using observations at infrared wavelengths and NIR radiative transfer modeling. For the first time, this region is resolved into two emission areas separated by a dark lane reminiscent of an obscuring silhouette caused by a circumstellar disk. So far, the observational data as well as model calculations suggest that M17-UC1 is surrounded by a disk of cool dust. This direct detection of a circumstellar disk candidate around a hypercompact H II region is in agreement with the expectations of the disk accretion model for high-mass star formation.

At the onset of high-mass star formation, accreting protostars are deeply embedded in massive cores made of gas and dust. Their spectral energy distribution is still dominated by the cold dust and rises steeply from near- to far-infrared wavelengths. The young massive star-forming region IRDC 18223-3 is a prototypical infrared dark cloud with a compact millimeter continuum core that shows no protostellar emission below 8 μm. However, based on outflow tracers, early star formation activity was previously inferred for this region. Here we present recent Spitzer observations from the MIPSGAL survey that identify the central protostellar object for the first time at 24 and 70 μm. Combining the mid- to far-infrared data with previous millimeter continuum observations and the upper limits below 8 μm, one can infer the physical properties of the central source. At least two components with constant gas mass M and dust temperature T are necessary: one cold component (~15 K and ~576 M_☉) that contains most of the mass and luminosity, and one warmer component (≥51 K and ≥0.01 M_☉) to explain the 24 μm data. The integrated luminosity of ~177 L_☉ can be used to constrain additional parameters of the embedded protostar from the turbulent core accretion model for massive star formation. The data of IRDC 18223-3 are consistent with a massive gas core harboring a low-mass protostellar seed of still less than half a solar mass with high accretion rates of the order 10^-4M_☉ yr^-1. In the framework of this model, the embedded protostar is destined to become a massive star at the end of its formation processes.

The gas equation of state may be one of the critical factors for the disk instability theory of gas giant planet formation. This Letter addresses the treatment of H₂ in hydrodynamic simulations of gravitationally unstable disks. In our discussion, we point out possible consequences of erroneous specific internal energy relations, approximate specific internal energy relations with discontinuities, and assumptions of constant Γ₁. In addition, we consider whether the ortho/para ratio for H ₂ in protoplanetary disks should be treated dynamically as if the species are in equilibrium. Preliminary simulations indicate that the correct treatment is particularly critical for the study of gravitational instability when T = 30-50 K.

In a disk around DM Tau, previous observation of ¹³CO (J = 2-1 and 1-0 transitions) derived a ¹³CO gas temperature of ~13-20 K, which is lower than the sublimation temperature of CO (20 K). We argue that the existence of such cold CO can be explained by a vertical mixing of disk material. As the gas is transported from a warm layer to a cold layer, CO is depleted onto dust grains with a timescale of ~10³ yr. Because of the steep temperature gradient in the vertical direction, an observable amount of CO is still in the gas phase when the fluid parcel reaches the layer of ~13 K. Apparent temperature of CO decreases as the maximum grain size increases from micron to millimeter size.

Near-infrared and optical images of HD 32297 indicate that it has an edge-on debris disk, similar to β Pic. I present high-resolution optical spectra of the Na I doublet toward HD 32297 and stars in close angular proximity. A circumstellar absorption component is clearly observed toward HD 32297 at the stellar radial velocity, which is not observed toward any of its neighbors, including the nearest only 0.9' away. An interstellar component is detected in all stars >90 pc, including HD 32297, likely due to the interstellar material at the boundary of the Local Bubble. Radial velocity measurements of the nearest neighbors, BD +07 777s and BD +07 778, indicate that they are unlikely to be physically associated with HD 32297. The measured circumstellar column density around HD 32997, log N ~ 11.4, is the strongest Na I absorption measured toward any nearby main-sequence debris disk, even the prototypical edge-on debris disk β Pic. Assuming that the morphology and abundances of the gas component around HD 32297 are similar to β Pic, I estimate an upper limit to the gas mass in the circumstellar disk surrounding HD 32297 of ~0.3 M_⊕.

We investigate the properties of two "classical" EUV Imaging Telescope (EIT) coronal waves. The two source regions of the associated coronal mass ejections (CMEs) possess opposite helicities, and the coronal waves display rotations in opposite senses. We observe deep core dimmings near the flare site and also widespread diffuse dimming, accompanying the expansion of the EIT wave. We also report a new property of these EIT waves, namely, that they display dual brightenings: persistent ones at the outermost edge of the core dimming regions and simultaneously diffuse brightenings constituting the leading edge of the coronal wave, surrounding the expanding diffuse dimmings. We show that such behavior is consistent with a diffuse EIT wave being the magnetic footprint of a CME. We propose a new mechanism where driven magnetic reconnections between the skirt of the expanding CME magnetic field and quiet-Sun magnetic loops generate the observed bright diffuse front. The dual brightenings and the widespread diffuse dimming are identified as innate characteristics of this process.

We report the discovery of a new radio feature associated with coronal mass ejection (CME) events. The feature is a low-frequency (<1 MHz), relatively wide (~300 kHz) continuum that appears just after the main phase of the eruptive event, lasts for several hours, and exhibits a slow negative frequency drift. So far, we have identified this radio signature in a handful of CME events and suspect it might be a common occurrence. The radio continuum starts almost simultaneously with the commonly observed decimetric type IV stationary continuum (also called flare continuum), but the two seem unrelated. The emission mechanism, whether plasma emission or gyroresonance, is unclear at the moment. On the basis of our preliminary analysis, we interpret this radio continuum as the lateral interaction of the CME with magnetic structures. Another possibility is that this continuum traces the reconfiguration of large-scale loop systems, such as streamers. In other words, it could be the large-scale counterpart of the post-CME arcades seen over active region neutral lines after big CME events. This Letter aims to bring attention to this feature and attract more research into its nature.