Table of contents

Sunyaev-Zeldovich effect (SZE) cluster surveys are anticipated to yield tight constraints on cosmological parameters, such as the equation of state of dark energy. In this paper, we study the effect of relativistic corrections of the thermal SZE on the cluster number counts expected from a cosmological model and thus, assuming that other cosmological parameters are known to high accuracies, on the determination of the w-parameter and σ₈ from an SZE cluster survey, where w = p/ρ (with p the pressure and ρ the density of dark energy) and σ₈ is the rms of the extrapolated linear density fluctuation smoothed over 8 Mpc h^-1. For the purpose of illustrating the effects of relativistic corrections, our analyses mainly focus on ν = 353 GHz and S_lim = 30 mJy, where ν and S_lim are the observing frequency and the flux limit of a survey, respectively. These observing parameters are relevant to the Planck survey. It is found that from two measurable quantities, the total number of SZE clusters and the number of clusters with redshift z ≥ 0.5, σ₈ and w can be determined to a level of ±1% and ±8%, respectively, with 1 σ uncertainties from a survey of 10,000 deg². Relativistic effects are important in determining the central values of σ₈ and w. If we choose the two quantities calculated relativistically from the flat cosmological model with σ₈ = 0.8284 and w = -0.75 as input, the derived σ₈ and w would be 0.819 and -0.81, respectively, if relativistic effects are wrongly neglected. The location of the resulting σ₈ and w in the σ₈-w plane is outside the 3 σ region around the real central σ₈ and w.

We show that the measurement of the baryonic acoustic oscillations in large high-redshift galaxy surveys offers a precision route to the measurement of dark energy. The cosmic microwave background provides the scale of the oscillations as a standard ruler that can be measured in the clustering of galaxies, thereby yielding the Hubble parameter and angular diameter distance as a function of redshift. This, in turn, enables one to probe dark energy. We use a Fisher matrix formalism to study the statistical errors for redshift surveys up to z = 3 and report errors on cosmography while marginalizing over a large number of cosmological parameters, including a time-dependent equation of state. With redshift surveys combined with cosmic microwave background satellite data, we achieve errors of 0.037 on Ω_X, 0.10 on w(z = 0.8), and 0.28 on dw(z)/dz for the cosmological constant model. Models with less negative w(z) permit tighter constraints. We test and discuss the dependence of performance on redshift, survey conditions, and the fiducial model. We find results that are competitive with the performance of future Type Ia supernova surveys. We conclude that redshift surveys offer a promising independent route to the measurement of dark energy.

Using a new, high mass resolution (Δm_b = 10^5.5M_☉) hydrodynamic simulation of a spatially flat ΛCDM cosmological model with detailed microphysics and galaxy formation, including radiation shielding, energy deposition, and metal enrichment from supernovae and associated metal cooling/heating, we compute the metallicity evolution of damped Lyα systems (DLAs) and find a reasonable agreement with observations. In particular, the observed slow evolution of the DLA metallicity occurs naturally in the simulation as a result of the combined effects of physical and observational selection. The slow metallicity evolution is caused by the steady transformation, with increasing time, of the highest metallicity systems to "galaxies," thus depleting this category, while all the lower metallicity systems show, individually, an increase in metallicity. Although the trend of DLA metallicity with redshift is in good agreement with observations, it appears that the average metallicity of simulated DLAs is higher than observed by 0.3-0.5 dex in the probed redshift range (z = 0-5). Our study indicates that this difference may be attributed to observational selection effects due to dust obscuration. If we allow for a dust obscuration effect, our model reproduces the observed metallicity evolution in both amplitude and slope. We find that DLAs are not a simple population but probe a range of different systems, and the mix changes with redshift. The median luminosity of a DLA, L_DLA(z), in units of typical galaxy luminosity at that redshift, L*(z), that is,_z, decreases from 1.1 to 0.5 as redshift declines from z = 3 to 0, but the absolute luminosity of the median DLA system increases in the same interval by a factor of 5 from 0.1L*(z = 0) to 0.5L*(z = 0). About 50% of all metals in the gaseous phase is in DLAs at all times from z = 5 to z = 1, making a rapid downturn at z ≤ 1 to ~20% by z = 0, as metals are swept into the hotter components of the intergalactic medium (IGM) as well as locked up in stars. While not the primary focus of this study, we find that the model provides good matches to observations with respect to column density distribution and evolution of neutral gas content, if the same dust obscuration is taken into account. We find Ω_DLA,comp = 1-3 × 10^-3, depending on the effect of dust obscuration.

We study contributions from inhomogeneous (patchy) reionization to arcminute-scale (1000 < l < 10,000) cosmic microwave background (CMB) anisotropies. We show that inhomogeneities in the ionization fraction, rather than in the mean density, dominate both the temperature and the polarization power spectra. Depending on the ionization history and the clustering bias of the ionizing sources, we find that rms temperature fluctuations range from 2 to 8 μK and the corresponding values for polarization are over 2 orders of magnitude smaller. Reionization can significantly bias cosmological parameter estimates and degrade gravitational lensing potential reconstruction from temperature maps but not from polarization maps. We demonstrate that a simple modeling of the reionization temperature power spectrum may be sufficient to remove the parameter bias. The high-l temperature power spectrum will contain some limited information about the sources of reionization.

Likelihood analyses of the COBE Differential Microwave Radiometer (DMR) sky maps are used to determine the normalization of the inverse-power-law potential scalar-field dark energy model. Predictions of the DMR-normalized model are compared with various observations to constrain the allowed range of model parameters. Although the derived constraints are restrictive, evolving dark energy density scalar-field models remains an observationally viable alternative to the constant cosmological constant model.

In order to constrain and possibly detect unusual physics during inflation, we allow the power spectrum of primordial matter density fluctuations, P_in(k), to be an arbitrary function in the estimation of cosmological parameters from data. The multiresolution and good localization properties of orthogonal wavelets make them suitable for detecting features in P_in(k). We expand P_in(k) directly in wavelet basis functions. The likelihood of the data is thus a function of the wavelet coefficients of P_in(k), as well as the Hubble constant H₀, baryon density Ω_bh², cold dark matter density Ω_ch², and the reionization optical depth τ_ri in a flat ΛCDM cosmology. We derive constraints on these parameters from cosmic microwave background anisotropy data (WMAP, CBI, and ACBAR) and large-scale structure data (2dFGRS and PSCZ) using the Markov chain Monte Carlo (MCMC) technique. The direct wavelet expansion method is different from and complementary to the wavelet band power method of Mukherjee & Wang, and results from the two methods are consistent. In addition, as we demonstrate, the direct wavelet expansion method has the advantage that once the wavelet coefficients have been constrained, the reconstruction of P_in(k) can be effectively denoised, i.e., P_in(k) can be reconstructed using only the coefficients that, say, deviate from zero at greater than 1 σ. In doing so, we retain the essential properties of P_in(k). The reconstruction also suffers much less from the correlated errors of binning methods. The shape of the primordial power spectrum, as reconstructed in detail here, reveals an interesting new feature at 0.001 ≲ k/Mpc^-1 ≲ 0.005. It will be interesting to see whether this feature is confirmed by future data. The reconstructed and denoised P_in(k) is favored over the scale-invariant and power-law forms at ≳1 σ.

Dust grains play a crucial role in the formation and evolution history of stars and galaxies in the early universe. We investigate the formation of dust grains in the ejecta of Population III supernovae, including pair-instability supernovae, which are expected to occur in the early universe, applying a theory of non-steady state nucleation and grain growth. Dust formation calculations are performed for core-collapse supernovae with progenitor mass M_pr ranging from 13 to 30 M_☉ and for pair-instability supernovae with M_pr = 170 and 200 M_☉. In the calculations, the time evolution of gas temperature in the ejecta, which strongly affects the number density and size of newly formed grains, is calculated by solving the radiative transfer equation, taking account of the energy deposition of radioactive elements. Two extreme cases are considered for the elemental composition in the ejecta, unmixed and uniformly mixed cases within the He core, and formation of CO and SiO molecules is assumed to be complete. The results of calculations for core-collapse supernovae and pair-instability supernovae are summarized as follows: in the unmixed ejecta, a variety of grain species condense, reflecting the difference of the elemental composition at the formation site in the ejecta; otherwise only oxide grains condense in the uniformly mixed ejecta. The average size of newly formed grains spans a range of 3 orders of magnitude, depending on the grain species and the formation condition, and the maximum radius is limited to less than 1 μm, which does not depend on the progenitor mass. The size distribution function of each grain species is approximately lognormal, except for Mg silicates, MgO, Si, and FeS in the unmixed case and Al₂O₃ in both cases. The size distribution function summed up over all grain species is approximated by a power-law formula whose index is -3.5 for the larger radius and -2.5 for the smaller one; the radius at the crossover point ranges from 0.004 to 0.1 μm, depending on the model of supernovae. The fraction of mass locked into dust grains increases with increasing the progenitor mass: 2%-5% of the progenitor mass for core-collapse supernovae and 15%-30% for pair-instability supernovae whose progenitor mass ranges from 140 to 260 M_☉. Thus, if very massive stars populated the first generation of stars (Population III stars), a large amount of dust grains would be produced in the early universe. We also discuss the dependence of the explosion energy and the amount of ⁵⁶Ni in the ejecta, as well as the efficiency of formation of CO and SiO molecules, on the formation of dust grains in the ejecta of supernovae.

We present a comprehensive lensing analysis of the rich cluster Cl 0024+1654 (z = 0.395) based on panoramic sparse-sampled imaging conducted with the WFPC2 and STIS cameras on board the Hubble Space Telescope. By comparing higher fidelity signals in the limited STIS data with the wider field data available from WFPC2, we demonstrate an ability to detect reliably weak-lensing signals to a cluster radius of ≃5 h Mpc, where the mean shear is around 1%. This enables us to study the distribution of dark matter with respect to the cluster light over an unprecedented range of cluster radii and environments. The projected mass distribution reveals a secondary concentration representing 30% of the overall cluster mass, which is also visible in the distribution of cluster member galaxies. We develop a method to derive the projected mass profile of the main cluster taking into account the influence of the secondary clump. We normalize the mass profile determined from the shear by assuming that background galaxies selected with 23 < I < 26 have a redshift distribution statistically similar to that inferred photometrically in the Hubble Deep Fields. The total mass within the central region of the cluster is independently determined from strong-lensing constraints according to a detailed model that utilizes the multiply imaged arc at z = 1.675. Combining strong and weak constraints, we are able to probe the mass profile of the cluster on scales of 0.1-5 Mpc, thus providing a valuable test of the universal form proposed by Navarro, Frenk, & White (NFW) on large scales. A generalized power-law fit indicates an asymptotic three-dimensional density distribution of ρ ∝ r^-n with n > 2.4. An isothermal mass profile is therefore strongly rejected, whereas an NFW profile with M₂₀₀ = 6.1 × 10¹⁴hM_☉ provides a good fit to the lensing data. We isolate cluster members according to their optical/near-infrared colors; the red cluster light closely traces the dark matter with a mean mass-to-light ratio of M/L_K = 40 ± 5 h₆₅M_☉/L_☉. Similar profiles for mass and light on 1-5 Mpc scales are expected if cluster assembly is largely governed by infalling groups.

Weak-lensing measurements are starting to provide statistical maps of the distribution of matter in the universe that are increasingly precise and complementary to cosmic microwave background maps. The most common measurement is the correlation in alignments of background galaxies, which can be used to infer the variance of the projected surface density of matter. This measurement of the fluctuations is insensitive to the total mass content and is analogous to using waves on the ocean to measure its depths. However, when the depth is shallow, as happens near a beach, waves become skewed. Similarly, a measurement of skewness in the projected matter distribution directly measures the total matter content of the universe. While skewness has already been convincingly detected, its constraint on cosmology is still weak. We address optimal analyses for the Canada-France-Hawaii Telescope Legacy Survey in the presence of noise. We show that a compensated Gaussian filter with a width of 25 optimizes the cosmological constraint, yielding ΔΩ_m/Ω_m ~ 10%. This is significantly better than other filters that have been considered in the literature. This can be further improved with tomography and other sophisticated analyses.

We present a quantitative measure of the internal color dispersion within galaxies, which quantifies differences in galaxy morphology as a function of observed wavelength. We apply this statistic to a sample of local galaxies with archival images at 1500 and 2500 Å from the Ultraviolet Imaging Telescope and ground-based B-band observations in order to investigate how the internal dispersion between these colors relates to global galaxy properties (e.g., luminosity, color, and morphological type). In general, the dispersion in the internal galaxy colors correlates with transformations in the galaxy morphology as a function of wavelength; i.e., our internal color dispersion statistic quantifies the morphological K-correction. Mid-type spiral galaxies exhibit the highest dispersion in their ultraviolet-optical internal colors, which stems from differences in the stellar content that constitute the bulge, disk, and spiral-arm components. Irregular and late-type spiral galaxies show moderate internal color dispersion, although with lower values relative to the mid-type spirals. This implies that young stars generally dominate the ultraviolet-optical galaxy colors, modulo variations in the dust, gas, and stellar distributions. Elliptical, lenticular, and early-type spiral galaxies generally have low or negligible internal color dispersion, which indicates that the stars contributing to the ultraviolet-optical emission have a very homogeneous distribution. We discuss the application of the internal color dispersion to high-redshift galaxies in deep Hubble Space Telescope images. By simulating the appearance of the local galaxy sample at cosmological distances, many of the galaxies have luminosities that are sufficiently bright at rest-frame optical wavelengths to be detected within the limits of the currently deepest near-infrared surveys, even with no evolution. Under the assumption that the luminosity and color evolution of the local galaxies conform with the measured values of high-redshift objects, we show that galaxies' intrinsic internal color dispersion remains measurable out to z ~ 3.

We use the latest observations from the 2dF Galaxy Redshift Survey to fit the conditional luminosity function (CLF) formulation of the halo model for galaxies at z = 0. This fit is then used to test the extent of evolution in the halo occupation distribution to z = 0.8, by comparing the predicted clustering from this CLF to preliminary results from the DEEP2 Redshift Survey. We show that the current observations from the DEEP2 Redshift Survey are remarkably consistent with no evolution in the CLF from z = 0 to 0.8. This result is surprising, in that it suggests that there has been very little change in the way galaxies occupy their host dark matter halos over half the age of the universe. We discuss in detail the observational constraints we have adopted and also the various selection effects in each survey and how these affect the galaxy populations encountered in each survey.

We present the results of a high-resolution UV two-dimensional spectroscopic survey of star-forming galaxies observed with Hubble Space Telescope Space Telescope Imaging Spectrograph. Our main aim is to map the Lyα profiles to learn about the gas kinematics and its relation with the escape of Lyα photons and to detect extended Lyα emission due to scattering in gaseous halos. We have combined our data with previously obtained UV spectroscopy on three other star-forming galaxies. We find that the P Cygni profile is spatially extended, smooth, and spans several kiloparsecs covering a region much larger than the starburst itself. We propose a scenario whereby an expanding supershell is generated by the interaction of the combined stellar winds and supernova ejecta from the young starbursts, with an extended low-density halo. The variety of observed Lyα profiles both in our sample and in high-redshift starbursts is explained as phases in the time evolution of the supershell expanding into the disk and halo of the host galaxy. The observed shapes, widths, and velocities are in excellent agreement with the supershell scenario predictions and represent a time sequence. We confirm that among the many intrinsic parameters of a star-forming region that can affect the properties of the observed Lyα profiles, velocity and density distributions of neutral gas along the line of sight are by far the dominant ones, while the amount of dust will determine the intensity of the emission line, if any.

If enough of their Lyman-limit continuum escapes, star-forming galaxies could be significant contributors to the cosmic background of ionizing photons. To investigate this possibility, we obtained the first deep imaging in the far-ultraviolet of 11 bright blue galaxies at intermediate redshift (1.1 < z < 1.4) with the Space Telescope Imaging Spectrograph Far-Ultraviolet Multianode Microchannel Array detector on the Hubble Space Telescope. No Lyman continuum emission was detected. Sensitive, model-independent, upper limits of typically less than 10^-19 ergs cm^-2 s^-1 Å^-1 were obtained for the ionizing flux escaping from these normal galaxies. This corresponds to lower limits on the observed ratio of 1500-700 Å flux of 150 up to 1000. On the basis of a wide range of stellar synthesis models, this suggests that less than 6%, down to less than 1%, of the available ionizing flux emitted by hot stars is escaping these galaxies. The magnitude of this spectral break at the Lyman limit confirms that the basic premise of "Lyman break" searches for galaxies at high redshift can also be applied at intermediate redshifts. This implies that the integrated contribution of galaxies to the UV cosmic background at z ~ 1.2 is less than 15% and may be less than 2%.

We investigate the cosmological evolution of the hard X-ray luminosity function (HXLF) of active galactic nuclei (AGNs) in the 2-10 keV luminosity range of 10^41.5-10^46.5 ergs s^-1 as a function of redshift up to 3. From a combination of surveys conducted at photon energies above 2 keV with HEAO 1, ASCA, and Chandra, we construct a highly complete (>96%) sample consisting of 247 AGNs over the wide flux range of 10^-10 to 3.8 × 10^-15 ergs cm^-2 s^-1 (2-10 keV). For our purpose, we develop an extensive method of calculating the intrinsic (before absorption) HXLF and the absorption (N_H) function. This utilizes the maximum likelihood method, fully correcting for observational biases with consideration of the X-ray spectrum of each source. We find that (1) the fraction of X-ray absorbed AGNs decreases with the intrinsic luminosity and (2) the evolution of the HXLF of all AGNs (including both type I and type II AGNs) is best described with a luminosity-dependent density evolution (LDDE) where the cutoff redshift increases with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard X-ray background, thus solving its origin quantitatively. A combination of the HXLF and the N_H function enables us to construct a purely "observation-based" population synthesis model. We present basic consequences of this model and discuss the contribution of Compton-thick AGNs to the rest of the hard X-ray background.

We have carried out the first systematic survey of the submillimeter properties of broad absorption line (BAL) quasars. Thirty BAL quasars drawn from a homogeneously selected sample from the Sloan Digital Sky Survey at redshifts 2 < z < 2.6 were observed with the SCUBA array at the JCMT to a typical rms sensitivity of 2.5 mJy. Eight quasars were detected at greater than 2 σ significance, four of which are at greater than 3 σ significance. The far-infrared luminosities of these quasars are greater than 10¹³L_☉. There is no correlation of submillimeter flux with either the strength of the broad absorption feature or with absolute magnitude in our sample. We compare the submillimeter flux distribution of the BAL quasar sample with that of a sample of quasars that do not show BAL features in their optical spectra and find that the two are indistinguishable. BAL quasars do not have higher submillimeter luminosities than non-BAL quasars. These findings are consistent with the hypothesis that all quasars would contain a BAL if viewed along a certain line of sight. The data are inconsistent with a model in which the BAL phenomenon indicates a special evolutionary stage that coincides with a large dust mass in the host galaxy and a high submillimeter luminosity. Our work provides constraints on alternative evolutionary explanations of BAL quasars.

We present results from a 20 ks XMM-Newton observation of Mrk 231. The European Photon Imaging Camera (EPIC) spectral data reveal strong line emission due to Fe Kα, which has rarely been detected in this class, as broad absorption line quasars (BAL QSOs) are very faint in the X-ray band. The line energy is consistent with an origin in neutral Fe. The width of the line is equivalent to a velocity dispersion ~18,000 km s^-1, and thus the line may be attributed to transmission and/or reflection from a distribution of emitting clouds. If, instead, the line originates in the accretion disk, then the line strength and flat X-ray continuum support some contribution from a reflected component, although the data disfavor a model in which the hard X-ray band is purely reflected X-rays from a disk. The line parameters are similar to those obtained for the Fe Kα line detected in another BAL QSO, H1413+117.

We present Hubble Space Telescope/Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer spectra of the quasar RX J1230.8+0115 (V = 14.4,z = 0.117). In addition to Galactic, Virgo, and intervening absorption, this quasar is host to a remarkable intrinsic absorption complex. Four narrow absorption line systems, strong in C IV, N V, and O VI, lie within 5000 km s^-1 of the QSO redshift. Three of the systems appear to be line locked, two in N V and two in O VI, with the common system residing in between the other two (in velocity). All three systems show signs of an intrinsic origin—smooth windlike profiles, high ionization, and partial coverage of the central engine. The fourth system, which appears at the systemic redshift of the QSO, may originate from host galaxy or intervening gas. Photoionization analyses imply column densities in the range 19.1 < log N(H) < 21 and ionization parameters in the range -1.3 < log U < 0.3. Revisiting the issue of line locking, we discuss a possible model in the context of the accretion disk/wind scenario and point out several issues that remain for future simulations and observations.

Direct time-resolved spectral fitting has been performed on continuous Rossi X-Ray Timing Explorer monitoring of seven Seyfert 1 galaxies to study their broadband spectral variability and Fe Kα variability characteristics on timescales of days to years. Variability in the Fe Kα line is not detected in some objects but is present in others; e.g., in NGC 3516, NGC 4151, and NGC 5548 there are systematic decreases in line flux by factors of ~2-5 over 3-4 years. The Fe Kα line varies less strongly than the broadband continuum, but, like the continuum, exhibits stronger variability toward longer timescales. Relatively less model-dependent broadband fractional variability amplitude (F_var) spectra also show weaker line variability compared with the continuum variability. Comparable systematic long-term decreases in the line and continuum are present in NGC 5548. Overall, however, there is no evidence for correlated variability between the line and continuum, severely challenging models in which the line tracks continuum variations modified only by a light-travel time delay. Local effects such as the formation of an ionized skin at the site of line emission may be relevant. The spectral fitting and F_var spectra both support spectral softening as continuum flux increases.

We study the evolution of the broad, double-peaked Hα emission-line profile of the LINER/Seyfert 1 nucleus of NGC 1097, using 24 spectra obtained over a time span of 11 years—from 1991 November through 2002 October. While in the first 5 years the main variation was in the relative intensity of the blue and red peaks, in the last years we have also observed an increasing separation between the two peaks, at the same time as the integrated flux in the broad line has decreased. We propose a scenario in which the emission originates in an asymmetric accretion disk around a supermassive black hole, whose source of ionization is getting dimmer, causing the region of maximum emission to come closer to the center (and thus to regions of higher projected velocity). We use the observations to constrain the evolution of the accretion disk emission and to evaluate two models: the elliptical-disk model previously found to reproduce the observations from 1991 to 1996 and a model of a circular disk with a single spiral arm. In both models the peak emissivity of the disk drifts inward with time, while the azimuthal orientation of the elliptical-disk or the spiral pattern varies with time. In the case of the spiral-arm model, the whole set of data is consistent with a monotonic precession of the spiral pattern, which has completed almost two revolutions since 1991. Thus, we favor the spiral-arm model, which, through the precession period, implies a black hole mass that is consistent with the observed stellar velocity dispersion. In contrast, the elliptical-disk model requires a mass that is an order of magnitude lower. Finally, we have found tentative evidence of the emergence of an accretion disk wind, which we hope to explore further with future observations.

We present a 60 ks Chandra ACIS-S observation of the isolated edge-on spiral galaxy NGC 3556, together with a multiwavelength analysis of various discrete X-ray sources and diffuse X-ray features. Among 33 discrete X-ray sources detected within the I_B = 25 mag arcsec^-2 isophote ellipse of the galaxy, we identify a candidate for the galactic nucleus, an ultraluminous X-ray source that might be an accreting intermediate-mass black hole, a possible X-ray binary with a radio counterpart, and two radio-bright giant H II regions. We detect large amounts of extraplanar diffuse X-ray emission, which extend about 10 kpc radially in the disk and ≳4 kpc away from the galactic plane. The diffuse X-ray emission exhibits significant substructures, possibly representing various blown-out superbubbles or chimneys of hot gas heated in massive star-forming regions. This X-ray-emitting gas has temperatures in the range of ~(2-7) × 10⁶ K and has a total cooling rate of ~2 × 10⁴⁰ ergs s^-1. The energy can be easily supplied by supernova blast waves in the galaxy. These results show NGC 3556 to be a galaxy undergoing vigorous disk-halo interaction. The halo in NGC 3556 is considerably less extended, however, than that of NGC 4631, in spite of many similarities between the two galaxies. This may be due to the fact that NGC 3556 is isolated, whereas NGC 4631 is interacting. Thus, NGC 3556 presents a more pristine environment for studying the disk-halo interaction.

This paper describes the Chandra observation of the diffuse emission in the face-on spiral NGC 6946. Overlaid on optical and Hα images, the diffuse emission follows the spiral structure of the galaxy. An overlay on a 6 cm polarized radio intensity map confirms the phase offset of the polarized emission. We then extract and fit the spectrum of the unresolved emission with several spectral models. All model fits show a consistent continuum thermal temperature with a mean value of 0.25 ± 0.03 keV. Additional degrees of freedom are required to obtain a good fit, and any of several models satisfy that need; one model uses a second continuum component with a temperature of 0.70 ± 0.10 keV. An abundance measure of 3 for Si differs from the solar value at the 90% confidence level; the net diffuse spectrum shows that the line lies above the instrumental Si feature. For Fe the abundance measure of 0.67 ± 0.13 is significant at 99%. Multiple Gaussians also provide a good fit. Two of the fitted Gaussians capture the O VII and O VIII emission; the fitted emission is consistent with an XMM-Newton RGS spectrum of diffuse gas in M81. The ratio of the two lines is less than 0.6-0.7 and suggests the possibility of nonequilibrium ionization conditions existing in the interstellar medium of NGC 6946. An extrapolation of the point-source luminosity distribution shows that the diffuse component is not the sum of unresolved point sources; their contribution is at most 25%.

The POINT-AGAPE collaboration is currently searching for massive compact halo objects (MACHOs) toward the Andromeda galaxy (M31). The survey aims to exploit the high inclination of the M31 disk, which causes an asymmetry in the spatial distribution of M31 MACHOs. Here, we investigate the effects of halo velocity anisotropy and flattening on the asymmetry signal using simple halo models. For a spherically symmetric and isotropic halo, we find that the underlying pixel lensing rate in far-disk M31 MACHOs is more than 5 times the rate of near-disk events. We find that the asymmetry is further increased by about 30% if the MACHOs occupy radial orbits rather than tangential orbits, but it is substantially reduced if the MACHOs lie in a flattened halo. However, even for halos with a minor- to major-axis ratio of q = 0.3, the number of M31 MACHOs in the far side outnumber those in the near side by a factor of ~2. There is also a distance asymmetry, in that the events on the far side are typically farther from the major axis. We show that, if this positional information is exploited in addition to number counts, then the number of candidate events required to confirm asymmetry for a range of flattened and anisotropic halo models is achievable, even with significant contamination by variable stars and foreground microlensing events. For pixel lensing surveys that probe a representative portion of the M31 disk, a sample of around 50 candidates is likely to be sufficient to detect asymmetry within spherical halos, even if half the sample is contaminated, or to detect asymmetry in halos as flat as q = 0.3, provided less than a third of the sample comprises contaminants. We also argue that, provided its mass-to-light ratio is less than 100, the recently observed stellar stream around M31 is not problematic for the detection of asymmetry.

We present Space Telescope Imaging Spectrograph (STIS) broadband imagery and optical slitless spectroscopy of three young star clusters in the Small Magellanic Cloud (SMC). MA 1796 and MG 2 were previously known as planetary nebulae and were observed as such in our Hubble Space Telescope (HST) survey. With the HST spatial resolution, we show that they are instead H II regions, surrounding very young star clusters. A third compact H II region, MA 1797, was serendipitously observed by us since it falls in the same frame of MA 1796. A limited nebular analysis is presented as derived from the slitless spectra. We find that MA 1796 and MG 2 are very heavily extincted, with c ≥ 1.4, defining them as the most extincted optically discovered star-forming regions in the SMC. MA 1796 and MG 2 are extremely compact (less than 1 pc across), while MA 1797, with diameter of about 3 pc, is similar to the ultracompact H II regions already known in the SMC. Stellar analysis is presented, and approximate reddening correction for the stars is derived from the Balmer decrement. Limited analysis of their stellar content and their ionized radiation shows that these compact H II regions are ionized by small stellar clusters whose hottest stars are at most of the B0 class. These very compact, extremely reddened, and probably very dense H II regions in the SMC offer insight into the most recent star formation episodes in a very low metallicity galaxy.

We present a new method based on H I column densities for determination of distances within the disk of the Galaxy. The technique is useful for all Galactic plane objects, including H II regions and supernova remnants (SNRs), provided a line-of-sight velocity can be assigned to the object. Our method uses 21 cm spectral-line data to find the atomic hydrogen column density to an object, and beyond it to the Galactic edge. A model of the smooth large-scale Galactic distribution of H I material seen in emission (which principally traces the smooth structure of the Galaxy) is constructed. Our model accounts for scale-height flaring with increasing Galactocentric radius and includes the Galactic warp, which is prominent in the first and second quadrants of the Galaxy. The model's ability to trace the observed distribution of H I is demonstrated on lines of sight toward SNR DA 530 (l = 933, b = 7^°) and H II region Sh 121 (l = 902, b = 17). We then apply the new technique to 29 Sharpless H II regions with known photometric distances across the second quadrant. We measure line-of-sight velocities for the H II regions from associated ¹²CO emission, using 1' resolution ¹²CO (J = 1-0) data from the Canadian Galactic Plane Survey. Our distance method yields distances to these objects that are consistent with their photometric distances and which are markedly smaller than the kinematic distances found from a flat Galactic rotation curve.

Scattering and absorption properties at optical and ultraviolet wavelengths are calculated for an interstellar dust model consisting of carbonaceous grains and amorphous silicate grains. Polarization as a function of scattering angle is calculated for selected wavelengths from the infrared to the vacuum ultraviolet. The widely used Henyey-Greenstein phase function provides a good approximation for the scattering phase function at wavelengths between ~0.4 and 1 μm but fails to fit the calculated phase functions at shorter and longer wavelengths. A new analytic phase function is presented. It is exact at long wavelengths and provides a good fit to the numerically calculated phase function for λ > 0.27 μm. Observational determinations of the scattering albedo and ⟨cos θ⟩ show considerable disagreement, especially in the ultraviolet. Possible reasons for this are discussed.

Scattering and absorption of X-rays by interstellar dust is calculated for a model consisting of carbonaceous grains and amorphous silicate grains. The calculations employ realistic dielectric functions with structure near X-ray absorption edges, with resulting features in absorption, scattering, and extinction. Differential scattering cross sections are calculated for energies between 0.3 and 10 keV. The median scattering angle is given as a function of energy, and simple but accurate approximations are found for the X-ray scattering properties of the dust mixture, as well as for the angular distribution of the scattered X-ray halo for dust with simple spatial distributions. Observational estimates of the X-ray scattering optical depth are compared to model predictions. Observations of X-ray halos to test interstellar dust grain models are best carried out using extragalactic point sources.

We report the carbon monoxide isotope ratio in local molecular clouds toward LkHα 101, AFGL 490, and Mon R2 IRS 3. The vibrational transition bands of ¹²CO ν = 2 ← 0 and ¹³CO ν = 1 ← 0 were observed with high-resolution near-infrared spectroscopy (R = 23,000) to measure the ¹²CO/¹³CO ratio. The isotopic ratios are ¹²CO/¹³CO = 137 ± 9 (LkHα 101), 86 ± 49 (AFGL 490), and 158 (Mon R2 IRS 3), which are 1.5-2.8 times higher than the local interstellar medium value of ¹²CO/¹³CO = 57 ± 5 from millimeter C¹⁸O emission observations. This is not easily explained by saturation of the ¹³CO absorption. It is also questionable whether the selective photodestruction of ¹³CO can account for the difference between the Galactic trend and the present observation, because the molecular clouds are with high visible extinction (A_V = 10-70 mag), well shielded from destructive FUV radiation. The molecular gas associated with AFGL 490 and Mon R2 IRS 3 consists of multiple temperature components lying in the lines of sight. In the cool component (T_ex < 100 K), the excitation temperature of ¹²CO is twice that of ¹³CO. We attribute the temperature discrepancy to the photon-trapping effect, which makes the radiative cooling of the main isotopomer less effective.

Using data from the Southern Galactic Plane Survey (SGPS) we analyze an H I self-absorption cloud centered on l = 3180, b = -05 with velocity v = -1.1 km s^-1. The cloud was observed with the Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope and is at a near-kinematic distance of ≲400 pc, with derived dimensions of ≲5 × 11 pc. We apply two different methods to find the optical depth and spin temperature. In both methods we find upper limit spin temperatures ranging from 20 to 25 K and lower limit optical depths of ~1. We look into the nature of the H I emission and find that 60%-70% originates behind the cloud. We analyze a second cloud at the same velocity, centered on l = 319^°, b = 04, with an upper limit spin temperature of 20 K and a lower limit optical depth of 1.6. The similarities in spin temperature, optical depth, velocity, and spatial location are evidence that the clouds are associated, possibly as one large cloud consisting of smaller clumps of gas. We compare H I emission data with ¹²CO emission and find a physical association of the H I self-absorption cloud with molecular gas.

The 6 cm formaldehyde (H₂CO) maser sources in the compact H II regions NGC 7538 IRS 1 and G29.96-0.02 have been imaged at high resolution (θ_beam < 50 mas). Using the Very Long Baseline Array and MERLIN, we find the angular sizes of the NGC 7538 masers to be ~10 mas (30 AU), corresponding to brightness temperatures ~10⁸ K. The angular sizes of the G29.96-0.02 masers are ~20 mas (130 AU), corresponding to brightness temperatures ~10⁷ K. Using the VLA, we detect 2 cm formaldehyde absorption from the maser regions. We detect no emission in the 2 cm line, indicating the lack of a 2 cm maser and placing limits on the 6 cm excitation process. We find that both NGC 7538 maser components show an increase in intensity on 5-10 yr timescales while the G29.96-0.02 masers show no variability over 2 yr. A search for polarization provides 3 σ upper limits of 1% circularly polarized and 10% linearly polarized emission in NGC 7538 and of 15% circularly polarized emission in G29.96-0.02. A pronounced velocity gradient of 28 km s^-1 arcsec^-1 (1900 km s^-1 pc^-1) is detected in the NGC 7538 maser gas.

Over the past years observations of young and populous star clusters have shown that the stellar initial mass function (IMF) appears to be an invariant featureless Salpeter power law with an exponent α = 2.35 for stars more massive than a few M_☉. A consensus has also emerged that most, if not all, stars form in stellar groups and star clusters and that the mass function of young star clusters in the solar neighborhood and in interacting galaxies can be described, over the mass range of a few 10 to 10⁷M_☉, as a power law with an exponent β ≈ 2. These two results imply that galactic-field IMFs for early-type stars cannot, under any circumstances, be a Salpeter power law, but that they must have a steeper exponent, α_field ≳ 2.8. This has important consequences for the distribution of stellar remnants and for the chemodynamical and photometric evolution of galaxies.

We present model spectral energy distributions (SEDs), colors, polarization, and images for an evolutionary sequence of a low-mass protostar from the early collapse stage (Class 0) to the remnant disk stage (Class III). We find a substantial overlap in colors and SEDs between protostars embedded in envelopes (Class 0-I) and T Tauri disks (Class II), especially at mid-IR wavelengths. Edge-on Class I-II sources show double-peaked SEDs, with a short-wavelength hump due to scattered light and a long-wavelength hump due to thermal emission. These are the bluest sources in mid-IR color-color diagrams. Since Class 0 and I sources are diffuse, the size of the aperture over which fluxes are integrated has a substantial effect on the computed colors, with larger aperture results showing significantly bluer colors. Viewed through large apertures, the Class 0 colors fall in the same regions of mid-IR color-color diagrams as Class I sources and are even bluer than Class II-III sources in some colors. It is important to take this into account when comparing color-color diagrams of star formation regions at different distances or different sets of observations of the same region. However, the near-IR polarization of the Class 0 sources is much higher than the Class I-II sources, providing a means to separate these evolutionary states. We varied the grain properties in the circumstellar envelope, allowing for larger grains in the disk midplane and smaller grains in the envelope. In comparing with models with the same grain properties throughout, we find that the SED of the Class 0 source is sensitive to the grain properties of the envelope only—that is, grain growth in the disk in Class 0 sources cannot be detected from the SED. Grain growth in disks of Class I sources can be detected at wavelengths greater than 100 μm. Our image calculations predict that the diffuse emission from edge-on Class I and II sources should be detectable in the mid-IR with the Space Infrared Telescope Facility (SIRTF) in nearby star-forming regions (out to several hundred parsecs).

We present deep 3.6 cm radio continuum observations of the H II region NGC 2024 in Orion B obtained using the Very Large Array in its A configuration, with 02 angular resolution. We detect a total of 25 compact radio sources in a region of 4' × 4'. We discuss the nature of these sources and its relation to the infrared and X-ray objects in the region. At least two of the radio sources are obscured proplyds whose morphology can be used to restrict the location of the main ionizing source of the region. This cluster of radio sources is compared with others that have been found in regions of recent star formation.

We present subarcsecond (FWHM ~ 05) J, H, K, and L' images of a young stellar cluster associated with a candidate massive protostar IRAS 22134+5834. The observations reveal a centrally symmetric, flattened cluster enclosing a central dark region. The central dark region is possibly a cavity within the flattened cluster. It is surrounded by a ring composed of five bright stars and the candidate massive protostar IRAS 22134+5834. We construct JHKL' color-color and HK color-magnitude diagrams to identify the young stellar objects and estimate their spectral types. All the bright stars in the ring are found to have intrinsic infrared excess emission and are likely to be early- to late-B type stars. We estimate an average foreground extinction to the cluster of A_v ~ 5 mag and individual extinctions to the bright stars in the range A_v ~ 20-40 mag, indicating possible cocoons surrounding each massive star. This ring of bright stars is devoid of any H II region. It is surrounded by an embedded cluster, making this an example of a (proto)cluster that is in one of the dynamically least relaxed states. These observations are consistent with the recent nonaxisymmetric calculations of Li & Nakamura, who present a star formation scenario in which a magnetically subcritical cloud fragments into multiple magnetically supercritical cores, leading to the formation of small stellar groups.

We present a near-infrared extinction study of the dark globule L694-2, a starless core that shows strong evidence for inward motions in the profiles of molecular spectral lines. The J-, H-, and K-band data were taken using the European Southern Observatory New Technology Telescope. The best-fit simple spherical power-law model has index p = 2.6 ± 0.2, over the ~0.036-0.1 pc range in radius sampled in extinction. This power-law slope is steeper than the value of p = 2 for a singular isothermal sphere, the initial condition of the inside-out model for protostellar collapse. Including an additional component of extinction along the line of sight further steepens the inferred profile. A fit for a Bonnor-Ebert sphere model results in a supercritical value of the dimensionless radius ξ_max = 25 ± 3. This unstable configuration of material in the L694-2 core may be related to the observed inward motions. The Bonnor-Ebert model matches the shape of the observed density profile but significantly underestimates the amount of extinction observed in the L694-2 core (by a factor of ~4). This discrepancy in normalization has also been found for the nearby protostellar core B335 (Harvey and coworkers). A cylindrical model with scale height H = 0.0164 ± 0.002 pc (135 ± 5'') viewed at a small inclination to the axis of the cylinder provides an equally good radial profile as a power-law model, and it also reproduces the asymmetry of the L694-2 core remarkably well. In addition, this model provides a possible basis for understanding the discrepancy in the normalization of the Bonnor-Ebert model, namely, that L694-2 has prolate structure, with the full extent (mass) of the core being missed by analysis that assumes symmetry between the profiles of the core in the plane of the sky and along the line of sight. If the core is sufficiently magnetized, then fragmentation may be avoided, and later evolution might produce a protostar similar to B335.

We present here mid-infrared images of seven sites of water maser emission thought to be associated with the hot molecular core (HMC) phase of massive star formation. We have detected mid-infrared emission from the locations of two of these HMC candidates, G11.94-0.62 and G45.07+0.13. From our observations we derived lower limit estimates of the luminosities for the exciting sources in these two HMC candidates. We find that the estimates are consistent with the hypothesis that the HMCs are internally heated by massive B- and O-type stars. We observed two sites with HMCs previously claimed to be detected in the mid-infrared, G19.61-0.23 and G34.26+0.15; however, we did not detect mid-infrared emission from either HMC location. We place new upper limits on the mid-infrared flux densities for these HMCs that are much lower than their previously reported flux densities. We were also able to obtain extremely accurate astrometry for our mid-infrared observations of G9.62+0.19 and conclude that the mid-infrared emission previously thought to be coming from the HMC in this field is in fact coming from a different source altogether.

Berkely-Illinois-Maryland Association (BIMA) array observations of the Orion nebula discovered a giant flare from a young star previously undetected at millimeter wavelengths. The star briefly became the brightest compact object in the nebula at 86 GHz. Its flux density increased by more than a factor of 5 on a timescale of hours, to a peak of 160 mJy. This is one of the most luminous stellar radio flares ever observed. Remarkably, the Chandra X-Ray Observatory was in the midst of a deep integration of the Orion nebula at the time of the BIMA discovery; the source's X-ray flux increased by a factor of 10 approximately 2 days before the radio detection. Follow-up radio observations with the VLA and BIMA showed that the source decayed on a timescale of days, then flared again several times over the next 70 days, although never as brightly as during the discovery. Circular polarization was detected at 15, 22, and 43 GHz, indicating that the emission mechanism was cyclotron. VLBA observations 9 days after the initial flare yield a brightness temperature T_b > 5 × 10⁷ K at 15 GHz. Infrared spectroscopy indicates that the source is a K5 V star with faint Br γ emission, suggesting that it is a weak-line T Tauri object. Zeeman splitting measurements in the infrared spectrum find B ~ 2.6 ± 1.0 kG. The flare is an extreme example of magnetic activity associated with a young stellar object. These data suggest that short observations obtained with the Atacama Large Millimeter Array will uncover hundreds of flaring young stellar objects in the Orion region.

The Type Ic supernova (SN) 2002ap is an interesting event with very broad spectral features like the famous energetic SN 1998bw associated with a gamma-ray burst (GRB) 980425. Here we examine the jet hypothesis from SN 2002ap recently proposed based on the redshifted polarized continuum found in a spectropolarimetric observation. We show that jets should be moving at about 0.23c to a direction roughly perpendicular to us, and the degree of polarization requires a jet kinetic energy of at least 5 × 10⁵⁰ ergs, a similar energy scale to the GRB jets. The weak radio emission from SN 2002ap has been used to argue against the jet hypothesis, but we argue that this is not a problem because the jet is expected to be freely expanding and unshocked. However, the jet cannot be kept ionized because of adiabatic cooling without external photoionization or a heating source. We explored various ionization possibilities and found that only the radioactivity of ⁵⁶Ni is a plausible source, indicating that the jet is formed and ejected from the central region of the core collapse, not from the outer envelope of the exploding star. Then we point out that, if the jet hypothesis is true, the jet will eventually sweep up enough interstellar medium and generate shocks in a few to 10 yr, producing strong radio emission that can be spatially resolved, giving us a clear test for the jet hypothesis. Discussions are also given on what the jet would imply for the GRB-SN connection, when it is confirmed. We suggest the existence of two distinct classes of GRBs from similar core-collapse events but by completely different mechanisms. Cosmologically distant GRBs having an energy scale of ~10⁵⁰-10⁵¹ ergs are collimated jets generated by the central activity of core collapses, associated with ⁵⁶Ni ejection along with the jets. SN 2002ap can be considered as a failed GRB of this type with large baryon contamination. On the other hand, much less energetic ones including GRB 980425 are rather isotropic, which may be produced by hydrodynamical shock acceleration at the outer envelope. We propose that the radioactive ionization for the SN 2002ap jet may give a new explanation also for the X-ray line features often observed in GRB afterglows.

Hydrodynamics and explosive nucleosynthesis in bipolar supernova explosions are examined to account for some peculiar properties of hypernovae as well as peculiar abundance patterns of metal-poor stars. The explosion is assumed to be driven by bipolar jets that are powered by accretion onto a central remnant. The energy injection rate by the jets is assumed to be proportional to the accretion rate, i.e., Ė_jet = αc². We explore the features of the explosions with varying progenitors' masses and jet properties. The outcomes are different from conventional spherical models. (1) In the bipolar models, Fe-rich materials are ejected at high velocities along the jet axis, while O-rich materials occupy the central region, whose density becomes very high as a consequence of continuous accretion from the side. This configuration can explain some peculiar features in the light curves and the nebular spectra of hypernovae. (2) Production of ⁵⁶Ni tends to be smaller than in spherical thermal bomb models. To account for a large amount of ⁵⁶Ni observed in hypernovae, the jets should be initiated when the compact remnant mass is still smaller than 2-3 M_☉, or they should be very massive and slow. (3) Ejected isotopes are distributed as follows in order of decreasing velocities: ⁶⁴Zn, ⁵⁹Co, ⁵⁶Fe, ⁴⁴Ti, and ⁴He at the highest velocities; ⁵⁵Mn, ⁵²Cr, ³²S, and ²⁸Si at the intermediate velocities; and ²⁴Mg and ¹⁶O at the lowest velocities. (4) The abundance ratios (Zn, Co)/Fe are enhanced while the ratios (Mn, Cr)/Fe are suppressed. This can account for the abundance pattern of extremely metal-poor stars. These agreements between the models and observations suggest that hypernovae are driven by bipolar jets and have significantly contributed to the early Galactic chemical evolution.

We present a new model of high-energy light curves from rotation-powered pulsars. The key ingredient of the model is the gap region (i.e., the region where particle acceleration is taking place and high-energy photons originate) that satisfies the following assumptions: (1) the gap region extends from each polar cap to the light cylinder; (2) the gap is thin and confined to the surface of last open magnetic-field lines; (3) photon emissivity is uniform within the gap region. The model light curves are dominated by strong peaks (either double or single) of caustic origin. Unlike other pulsar models with caustic effects, the double peaks arise from a crossing two caustics, each of which is associated with a different magnetic pole. The generic features of the light curves are consistent with the observed characteristics of pulsar light curves: (1) the most natural (in terms of probability) shape consists of two peaks (separated by 0.4 to 0.5 in phase for large viewing angles); (2) the peaks possess well-developed wings; (3) there is a bridge (interpeak) emission component; (4) there is a nonvanishing off-pulse emission level; (5) the radio pulse occurs before the leading high-energy peak. The model is well suited for four gamma-ray pulsars—Crab, Vela, Geminga, and B1951+32—with double-peak light curves exhibiting the peak separation of 0.4 to 0.5 in phase. Here we apply the model to the Vela pulsar. Moreover, we indicate the limitation of the model in accurate reproducing of the light curves with single pulses and narrowly separated (about 0.2 in phase) pulse peaks. We also discuss the optical polarization properties for the Crab pulsar in the context of the two-pole caustic model.

The observed spectra and X-ray luminosities of millisecond pulsars in 47 Tuc can be interpreted in the context of theoretical models based on strong, small-scale, multipole fields on the neutron star surface. For multipole fields that are relatively strong compared to the large-scale dipole field, the emitted X-rays are thermal and likely result from polar cap heating associated with the return current from the polar gap. On the other hand, for weak multipole fields, the emission is nonthermal and results from synchrotron radiation of e^± pairs created by curvature radiation. The X-ray luminosity L_X is related to the spin-down power L_sd, expressed in the form L_X ∝ L with β ~ 0.5 and ~1 for strong and weak multipole fields, respectively. If the polar cap size is of the order of the length scale of the multipole field s, the polar cap temperature is ~3 × 10⁶ K(L_sd/10³⁴ ergs s^-1)^1/8(s/3 × 10⁴ cm)^-1/2. A comparison of the X-ray properties of millisecond pulsars in globular clusters and in the Galactic field suggests that the emergence of relatively strong small-scale multipole fields from the neutron star interior may be correlated with the age and evolutionary history of the underlying neutron star.

We discuss the mass-radius (M-R) relations for low-mass (M < 0.1 M_☉) white dwarfs (WDs) of arbitrary degeneracy and evolved (He, C, O) composition. We do so with both a simple analytical model and models calculated by integration of hydrostatic balance using a modern equation of state valid for fully ionized plasmas. The M-R plane is divided into three regions where either Coulomb physics, degenerate electrons, or a classical gas dominates the WD structure. For a given M and central temperature T_c, the M-R relation has two branches differentiated by the model's entropy content. We present the M-R relations for a sequence of constant-entropy WDs of arbitrary degeneracy parameterized by M and T_c for pure He, C, and O. We discuss the applications of these models to the recently discovered accreting millisecond pulsars. We show the relationship between the orbital inclination for these binaries and the donor's composition and T_c. In particular, we find from orbital inclination constraints that the probability XTE J1807-294 can accommodate a He donor is approximately 15%, while for XTE J0929-304 it is approximately 35%. We argue that if the donors in ultracompact systems evolve adiabatically, there should be 60-160 more systems at orbital periods of 40 minutes than at orbital periods of 10 minutes, depending on the donor's composition. Tracks of our mass-radius relations for He, C, and O objects are available in the electronic version of this paper.

We investigate the effect of rotation on the evolution of double-degenerate white dwarf systems, which are possible progenitors of Type Ia supernovae. We assume that prior to merging, the two white dwarfs rotate synchronously at the orbital frequency and that in the merger process, the lighter white dwarf is transformed into a thick disk from which the more massive white dwarf initially accretes at a very high rate (~10^-5M_☉ yr^-1). Because of the lifting effect of rotation, the accreting white dwarf expands until the gravitational acceleration and centripetal acceleration required for binding at the surface become equal, initiating a Roche instability. The white dwarf continues to accrete matter from the disk, but at a rate that is determined by the balance between two competing processes operating in outer layers: (1) heating, expansion, and spin-up due to accretion and (2) cooling and contraction due to thermal diffusion. The balance produces an accretion rate such that the angular velocity of the white dwarf ω_WD and the break-up angular velocity ω_cr remain equal. Because of the deposition of angular momentum by accreted matter and the contraction of the accreting star, ω_WD increases continuously until the rotational energy reaches about 14% of the gravitational binding energy; then, another instability sets in: the structure is forced to adopt an elliptical shape and emit gravitational waves. Thereafter, a balance between the rate of deposition of angular momentum by accreted matter and the rate of loss of angular momentum by gravitational waves produces a nearly constant or "plateau" accretion rate of ~4 × 10^-7M_☉ yr^-1. The mass of the accreting white dwarf can increase up to and beyond the Chandresekhar mass limit for nonrotating white dwarfs before carbon ignition occurs. Independent of the initial value of the accretion rate, the physical conditions suitable for carbon ignition are achieved at the center of the accreting white dwarf and, because of the high electron degeneracy, the final outcome is an event of SN Ia proportions. Our results apply to merged binary white dwarf systems which, at the onset of explosive carbon ignition, have a total mass in the range 1.4-1.5 M_☉.

Proton capture by ¹⁷F plays an important role in the synthesis of nuclei in nova explosions. A revised rate for this reaction, based on a measurement of the ¹H(¹⁷F, p)¹⁷F excitation function using a radioactive ¹⁷F beam at Oak Ridge National Laboratory's Holifield Radioactive Ion Beam Facility, is used to calculate the nucleosynthesis in nova outbursts on the surfaces of 1.25 and 1.35 M_☉ ONeMg white dwarfs and a 1.00 M_☉ CO white dwarf. We find that the new ¹⁷F (p, γ)¹⁸Ne reaction rate changes the abundances of some nuclides (e.g., ¹⁷O) synthesized in the hottest zones of an explosion on a 1.35 M_☉ white dwarf by more than a factor of 10⁴ compared to calculations using some previous estimates for this reaction rate, and by more than a factor of 3 when the entire exploding envelope is considered. In a 1.25 M_☉ white dwarf nova explosion, this new rate changes the abundances of some nuclides synthesized in the hottest zones by more than a factor of 600, and by more than a factor of 2 when the entire exploding envelope is considered. Calculations for the 1.00 M_☉ white dwarf nova show that this new rate changes the abundance of ¹⁸Ne by 21% but has negligible effect on all other nuclides. Comparison of model predictions with observations is also discussed.

A linear analysis of the thermal stability of rotating low-mass stars evolving from the zero-age main sequence (ZAMS) to the red giant branch (RGB) tip has been carried out. Two stellar models are considered: one for 1.2 M_☉ with solar metallicity (Z = 0.0188), and the other for 0.8 M_☉ with Z = 0.0005. An instability in a thermonuclear burning shell on the RGB in the first of these cases could potentially be related to the phenomenon of Li-rich red giants, while a hydrogen shell instability in the second model may help to explain the observed chemical abundance anomalies in globular cluster red giants. A range of surface rotational velocities in MS stars has been considered, and we have assumed that, beginning at the ZAMS, the stars (including their convective envelopes) rotate differentially with depth. This assumption, which leads to significantly higher centrifugal accelerations in the H-burning shell than with the case of solid-body rotation on the MS, is expected to favour the development of thermal instabilities. However, all of our models for rotating low-mass stars—even those that had much higher rotation rates on the MS than those observed for F-, G-, and K-type dwarfs—were found to be thermally stable throughout their evolution from the ZAMS to the RGB tip. Only when helium burning was ignited to end the first ascent of the giant branch did we find a thermal instability (the helium flash).

We present a 2.218 μm image from the Hubble Space Telescope/Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and a 55 μm image from ISOPHOT of the dust ring surrounding the luminous blue variable (LBV) candidate HD 168625, together with new temperature and optical depth maps derived from mid-IR images. The shell is detached from the star in the near-IR, and substructure in the overall toroidal shell is visible. The far-IR image constrains the extent of the dust shell to ~25'' in diameter, providing an upper radius limit for modeling. The temperature maps and the NICMOS image show evidence for very small transiently heated dust grains in the shell. The opacity maps show higher optical depth in the limbs, consistent with interpretation of the dust shell as an equatorially enhanced torus inclined ~60° with respect to the observer. An overall trend in the dust emission location with wavelength is observed and interpreted as a variation with respect to location in the nebula of either the dust grain size distribution or gas-to-dust mass ratio. Radiative transfer calculations using 2-DUST indicate that a mass-loss event occurred ~5700 yr ago with a rate of (1.9 ± 0.1) × 10^-4M_☉ yr^-1, creating a dust torus that currently has a τ_V ~ 0.22 in the equatorial plane and a dust mass of (2.5 ± 0.1) × 10^-3M_☉. Using published values for the gas mass, we find a gas-to-dust mass ratio of 840, which is ~4 times higher than current estimates for the interstellar medium. In addition to a high equator-to-pole density ratio (~31) torus, an elliptical midshell is needed to reproduce the appearance and spectral energy distribution of the dust. Therefore, HD 168625 is an excellent example of proposed models of LBV nebulae in which a stellar wind interacts with a preexisting density contrast and creates a blowout in the polar direction perpendicular to the equatorial ring. The circumstellar shell is much lower in mass than that of LBV η Carinae, suggesting that HD 168625 had a lower mass progenitor.

A survey of 30 nearby M6.0-M7.5 dwarfs with K_s < 12 mag utilizing the Hokupa`a adaptive optics system at the Gemini North Telescope has discovered three new binary systems. All three systems have separations between 012 and 029 (3-10 AU) with similar mass ratios (q > 0.8, ΔK_s < 0.7). This result gives further support to the suggestion that wide (a > 20 AU), very low mass (M_tot < 0.185 M_☉) binary systems are exceedingly rare or perhaps even nonexistent. The semimajor axis distribution of these systems peaks at ~5 AU, tighter than more massive M and G binary distributions, which have a broad peak at separations of ~30 AU. We find a sensitivity-corrected binary fraction in the range 5% for M6.0-M7.5 stars with separations a > 3 AU. This binary frequency is less than the ~32% measured among early M dwarfs over the same separation range. Two of the low-mass binaries are probable Hyades open cluster members based on proper motions, cluster membership probabilities, radial velocities, and near-IR photometry. LP 415-20 has the distinction of being the tightest (3.6 AU) multiple system ever spatially resolved in the cluster, and the companions of LP 415-20 and LP 475-855 are among the least massive objects ever resolved in the Hyades, with estimated masses of 0.081 and 0.082M_☉.

High-resolution X-ray spectroscopy with the diffraction gratings of Chandra and XMM-Newton offers new chances to study a large variety of stellar coronal phenomena. A popular X-ray calibration target is Capella, which has been observed with all gratings with significant exposure times. We gathered together all available data of the High Energy Transmission Grating Spectrometer (HETGS; 155 ks), Low Energy Transmission Grating Spectrometer (LETGS; 219 ks), and Reflection Grating Spectrometer (RGS; 53 ks) for comparative analysis, focusing on the Ne IX triplet at around 13.5 Å, a region that is severely blended by strong iron lines. We identify 18 emission lines in this region of the High-Energy Grating (HEG) spectrum, including many from Fe XIX, and find good agreement with predictions from a theoretical model constructed using the Astrophysical Plasma Emission Code. The model uses an emission measure distribution derived from Fe XV to Fe XXIV lines. The success of the model is due in part to the inclusion of accurate wavelengths from laboratory measurements. While these 18 emission lines cannot be isolated in the LETGS or RGS spectra, their wavelengths and fluxes as measured with HEG are consistent with the lower resolution spectra. In the Capella model for HEG, the weak intercombination line of Ne IX is significantly blended by iron lines, which contribute about half the flux. After accounting for blending in the He-like diagnostic lines, we find the density to be consistent with the low-density limit (n_e < 2 × 10¹⁰ cm^-3); however, the electron temperature indicated by the Ne IX G-ratio is surprisingly low (~2 MK) compared to the peak of the emission measure distribution (~6 MK). Models show that the Ne IX triplet is less blended in cooler plasmas and in plasmas with an enhanced neon-to-iron abundance ratio.

With the help of the Laplace-Lagrange solution of the secular perturbation theory in a double-planet system, we study the occurrence and the stability of apsidal secular resonance between the two planets. The explicit criteria for predicting whether two planets are in apsidal resonance is derived, which shows that the occurrence of the apsidal resonance depends only on the mass ratio (m₁/m₂), semimajor axis ratio (a₁/a₂), initial eccentricity ratio (e₁₀/e₂₀), and the initial relative apsidal longitude (ϖ₂₀ - ϖ₁₀) between the two planets. The probability of two planets falling in apsidal resonance is given in the initial element space. We verify the criteria with numerical integrations for the HD 12661 system and find they give good predictions except at the boundary of the criteria or when the planet eccentricities are too large. The nonlinear stability of the two planets in HD 12661 system are studied by calculating the Lyapunov exponents of their orbits in a general three-body model. We find that two planets in large-eccentricity orbits could be stable only when they are in aligned apsidal resonance. When the planets are migrated under the planet-disk interactions, for more than half of the studied cases, the configurations of the apsidal resonances are preserved. We find the two planets of the HD 12661 system could be in aligned resonance and thus more stable, provided they have Ω₂ - Ω₁ ≈ 180^°. The applications of the criteria to the other multiple planetary systems are discussed.

We study the motions of small solids, ranging from micron-sized dust grains to meter-sized objects, in the vicinity of local pressure enhancements of a gaseous nebula. Integrating numerically, we show that as a result of the combined effect of gas drag and pressure gradients, solids tend to accumulate at the locations where the pressure of the gas maximizes. The rate of migration of solids varies with their sizes and densities and also with the physical properties of the gas. The results of our numerical simulations indicate that such migrations are most rapid for meter-sized objects. The applicability of the results to the enhancement of the collision and coagulation of solids and also to the growth rate of planetesimals is discussed.

In our previous paper we showed that the currently determined orbital parameters placed four recently announced planetary systems (HD 12661, HD 38529, HD 37124, and HD 160691) in very different situations from the point of view of dynamical stability. In the present paper we deal with the last of these systems, whose orbital parameters of the outer planet are yet uncertain. We discover a stabilizing mechanism that could be the key to its existence. The paper is devoted to the study of this mechanism by a global dynamics analysis in the orbital parameter space related to the HD 160691 system. We obtained our results using a new technique called the mean exponential growth factor of nearby orbits (MEGNO), and verified them with the fast Lyapunov indicator technique (FLI). In order to be dynamically stable, the HD 160691 planetary system has to satisfy the following conditions: (1) it should have a 2 : 1 mean motion resonance, (2) combined with an apsidal secular resonance, (3) in a configuration P_c(ap)-S-P_b(ap) (which means that the planets c and b may be considered as initially located at their apoastron around the central star S), and (4) it must satisfy specific conditions for the respective sizes of the eccentricities. High eccentricity for the outer orbit (e_c > 0.52) is the most probable necessary condition, while the eccentricity of the inner orbit e_b becomes relatively unimportant when e_c > 0.7. We also show that there is an upper limit for planetary masses (in the interval permitted by the undetermined line-of-sight inclination factor sin i_l) due to the dynamical stability mechanism. More generally, in this original orbital topology, where the resonance variables θ₁ and θ₃ librate about 180° while θ₂ librates about 0°, the HD 160691 system and its mechanism have revealed aspects of the 2 : 1 orbital resonances that have not been observed nor analyzed before. The present topology with antialigned apsidal lines, combined with the 2 : 1 resonance, is indeed more wide-ranging than the particular case of the HD 160691 planetary system. It is a new theoretical possibility that is suitable for a stable regime despite relatively small semimajor axes with respect to the important masses in interactions.

This paper describes a model that can explain the observed clumpy structures of debris disks. Clumps arise because after a planetary system forms, its planets migrate because of angular momentum exchange with the remaining planetesimals. Outward migration of the outermost planet traps planetesimals outside its orbit into its resonances, and resonant forces cause azimuthal structure in their distribution. The model is based on numerical simulations of planets of different masses, M_pl, migrating at different rates, _pl, through a dynamically cold (e < 0.01) planetesimal disk initially at a semimajor axis a. Trapping probabilities and the resulting azimuthal structures are presented for a planet's 2 : 1, 5 : 3, 3 : 2, and 4 : 3 resonances. Seven possible dynamical structures are identified from migrations defined by μ = M_pl/M_* and θ = _pl (a/_*)^1/2. Application of this model to the 850 μm image of Vega's disk shows that its two clumps of unequal brightness can be explained by the migration of a Neptune-mass planet from 40 to 65 AU over 56 Myr; tight constraints are set on possible ranges of these parameters. The clumps are caused by planetesimals in the 3 : 2 and 2 : 1 resonances; the asymmetry arises because of the overabundance of planetesimals in the 2 : 1(u) over the 2 : 1(l) resonance. The similarity of this migration to that proposed for our own Neptune hints that Vega's planetary system may be much more akin to the solar system than previously thought. Predictions are made that would substantiate this model, such as the orbital motion of the clumpy pattern, the location of the planet, and the presence of lower level clumps.

We determine the distribution of circumstellar disk masses in the young (~0.3 Myr) cluster NGC 2024 by imaging a 25 × 25 region in 3 mm continuum emission to an rms noise level of ~0.75 mJy beam^-1 with the Owens Valley Millimeter Array. The mosaic encompasses 147 K-band sources, as well as the molecular ridge seen previously in dust continuum emission. We detect 10 pointlike sources in 3 mm continuum emission above the level of 5 σ within the unit gain region of the mosaic. One of these sources corresponds to the near-IR source IRS 2, an early B-type star. Two other sources are tentatively associated with low-mass near-IR cluster members, and the remaining seven sources have no K-band counterparts. Assuming the millimeter continuum point sources represent emission from circumstellar disks and/or envelopes, then ~6% of the total population (infrared and millimeter sources) in the NGC 2024 mosaic have a circumstellar mass in excess of ~0.06 M_☉. We obtain further constraints on the average circumstellar disk mass by considering the mean millimeter continuum flux observed toward a sample of 140 K-band sources that likely have stellar masses ≲1-2 M_☉. While none of these sources are detected individually above the 3 σ limit of ~0.035 M_☉, the ensemble of sources are detected in the mean at the 5 σ level with a mean disk mass of ~0.005 M_☉. Compared with the older (~2 Myr) cluster IC 348, NGC 2024 contains a higher frequency of massive disks or envelopes and has a higher mean disk mass by a factor of 2.5 ± 1.3 among K-band sources, suggesting that the mean circumstellar mass is decreasing with cluster age. We also compare the results for the NGC 2024 and IC 348 clusters with those for the lower density Taurus star-forming region. Finally, we compare our detection limits with the minimum mass estimate for the proto-solar nebula and discuss possible implications for planet formation.

The radial velocities of ~1800 nearby Sun-like stars are currently being monitored by eight high-sensitivity Doppler exoplanet surveys. Approximately 90 of these stars have been found to host exoplanets massive enough to be detectable. Thus, at least ~5% of target stars possess planets. If we limit our analysis to target stars that have been monitored the longest (~15 years), ~11% possess planets. If we limit our analysis to stars monitored the longest and whose low surface activity allows the most precise velocity measurements, ~25% possess planets. By identifying trends of the exoplanet mass and period distributions in a subsample of exoplanets less biased by selection effects and linearly extrapolating these trends into regions of parameter space that have not yet been completely sampled, we find that at least ~9% of Sun-like stars have planets in the mass and orbital period ranges M sin i > 0.3M_Jup and P < 13 years and at least ~22% have planets in the larger range M sin i > 0.1M_Jup and P < 60 years. Even this larger area of the log(mass)-log(period) plane is less than 20% of the area occupied by our planetary system, suggesting that this estimate is still a lower limit to the true fraction of Sun-like stars with planets, which may be as large as ~100%.

A semiempirical, axisymmetric model of the solar minimum corona is developed by solving the equations for conservation of mass and momentum with prescribed anisotropic temperature distributions. In the high-latitude regions, the proton temperature anisotropy is strong and the associated mirror force plays an important role in driving the fast solar wind; the critical point where the outflow velocity equals the parallel sound speed (v = c_∥) is reached already at 1.5 R_☉ from Sun center. The slow wind arises from a region with open-field lines and weak anisotropy surrounding the equatorial streamer belt. The model parameters were chosen to reproduce the observed latitudinal extent of the equatorial streamer in the corona and at large distance from the Sun. We find that the magnetic cusp of the closed-field streamer core lies at about 1.95 R_☉. The transition from fast to slow wind is due to a decrease in temperature anisotropy combined with the nonmonotonic behavior of the nonradial expansion factor in flow tubes that pass near the streamer cusp. In the slow wind, the plasma β is of order unity and the critical point lies at about 5 R_☉, well beyond the magnetic cusp. The predicted outflow velocities are consistent with O⁵⁺ Doppler dimming measurements from UVCS/SOHO. We also find good agreement with polarized brightness (pB) measurements from LASCO/SOHO and H I Lyα images from UVCS/SOHO.

One of the proposed damping mechanisms of coronal (transverse) loop oscillations in the kink mode is resonant absorption as a result of the Alfvén speed variation at the outer boundary of coronal loops. Analytical expressions for the period and damping time exist for loop models with thin nonuniform boundaries. They predict a linear dependency of the ratio of the damping time to the period on the thickness of the nonuniform boundary layer. Ruderman and Roberts used a sinusoidal variation of the density in the nonuniform boundary layer and obtained the corresponding analytical expression for the damping time. Here we measure the thickness of the nonuniform layer in oscillating loops for 11 events, by forward-fitting of the cross-sectional density profile n_e(r) and line-of-sight integration to the cross-sectional fluxes F(r) observed with TRACE 171 Å. This way we model the internal (n_i) and external electron density (n_e) of the coronal plasma in oscillating loops. This allows us to test the theoretically predicted damping rates for thin boundaries as a function of the density ratio χ = n_e/n_i. Since the observations show that the loops have nonuniform density profiles, we also use numerical results for damping rates to determine the value of χ for the loops. We find that the density ratio predicted by the damping time, χ_LEDA = 0.53 ± 0.12, is a factor of ≈1.2-3.5 higher than the density ratio estimated from the background fluxes, χ = 0.30 ± 0.16. The lower densities modeled from the background fluxes are likely to be a consequence of the neglected hotter plasma that is not detected with the TRACE 171 Å filter. Taking these corrections into account, resonant absorption predicts damping times of kink-mode oscillations that are commensurable with the observed ones and provides a new diagnostic of the density contrast of oscillating loops.

We use soft X-ray and magnetic field observations of the Sun (quiet Sun, X-ray bright points, active regions, and integrated solar disk) and active stars (dwarf and pre-main-sequence) to study the relationship between total unsigned magnetic flux, Φ, and X-ray spectral radiance, L_X. We find that Φ and L_X exhibit a very nearly linear relationship over 12 orders of magnitude, albeit with significant levels of scatter. This suggests a universal relationship between magnetic flux and the power dissipated through coronal heating. If the relationship can be assumed linear, it is consistent with an average volumetric heating rate ~ /L, where is the average field strength along a closed field line and L is its length between footpoints. The Φ-L_X relationship also indicates that X-rays provide a useful proxy for the magnetic flux on stars when magnetic measurements are unavailable.

The Large Angle and Spectrometric Coronagraph Experiment (LASCO) C2 and C3 coronagraphs recorded a unique coronal mass ejection (CME) on 1999 April 2. The event did not have the typical three-part CME structure and involved a small-filament eruption without any visible overlying streamer ejecta. The event exhibited an unusually clear signature of a wave propagating at the CME flanks. The speed and density of the CME front and flanks were consistent with the existence of a shock. To better establish the nature of the white-light wave signature, we employed a simple MHD simulation using the LASCO measurements as constraints. Both the measurements and the simulation strongly suggest that the white-light feature is the density enhancement from a fast-mode MHD shock. In addition, the LASCO images clearly show streamers being deflected when the shock impinges on them. It is the first direct imaging of this interaction.

Recently, it has been found that the inferred injection times of >25 keV electrons are up to 30 minutes later than the start times of the associated type III radio bursts at the Sun. Thus, it has been suggested that the electrons that produce type III bursts do not belong to the same population as those observed above 25 keV. This paper examines the characteristics and circumstances of 79 solar electron beam events measured on the Advanced Composition Explorer (ACE) spacecraft. Particular attention is paid to the very low frequency emissions of the associated radio bursts and the ambient conditions at the arrival times of the electrons at the spacecraft. It is found that the inferred >25 keV electron injection delays are correlated with the times required for the associated radio bursts to drift to the lowest frequencies. This suggests that the electrons responsible for the radio emission, and those observed above 25 keV, are part of a single population and that the electrons both above and below 25 keV are delayed in the interplanetary medium. Further evidence for a single population is the general correspondence between electron and local radio intensities and temporal profiles. It is found that the delays increase with the ambient solar wind density, consistent with the propagation times of the electrons being determined by the characteristics of the interplanetary medium. However, it is known that particle arrival times at 1 AU are a linear function of inverse particle speed. Conventionally, such a relationship is taken to indicate scatter-free propagation when inferred path lengths lie close to 1.2 AU, as they do for the electron events studied here. These conflicting interpretations require further investigation.

We present a measurement of cosmic shear on scales ranging from 10'' to 2' in 347 Wide Field Planetary Camera 2 (WFPC2) images of random fields. Our result is based on shapes measured via image fitting and on a simple statistical technique; careful calibration of each step allows us to quantify our systematic uncertainties and to measure the cosmic shear down to very small angular scales. The WFPC2 images provide a robust measurement of the cosmic shear signal decreasing from 5.2% at 10'' to 2.2% at 130''.

The Sunyaev-Zel'dovich (SZ) effect was previously measured in the Coma Cluster by the Owens Valley Radio Observatory and Millimeter and IR Testa Grigia Observatory experiments and recently also with the W ilkinson Microwave Anisotropy Probe satellite. We assess the consistency of these results and their implications on the feasibility of high-frequency SZ work with ground-based telescopes. The unique data set from the combined measurements at six frequency bands is jointly analyzed, resulting in a best-fit value for the Thomson optical depth at the cluster center, τ₀ = (5.35 ± 0.67) × 10^-3. The combined X-ray and SZ determined properties of the gas are used to determine the Hubble constant. For isothermal gas with a β density profile we derive H₀ = 84 ± 26 km (s Mpc)^-1; the (1 σ) error includes only observational SZ and X-ray uncertainties.

We show a possible way to measure the column density of free electrons along the light path, the so-called dispersion measure (DM), from the early [~415(ν/1 GHz)^-2(DM/10⁵ pc cm^-3) s] radio afterglows of the gamma-ray bursts. We find that the proposed Square Kilometer Array can detect bright radio afterglows around the time ~10³(ν/160 MHz)^-2 s to measure the intergalactic DM (≳6000 pc cm^-3 at redshift z > 6) up to z ~ 30, from which we can determine the reionization history of the universe and identify the missing warm-hot baryons if many DMs can be measured. At low z, the DM in the host galaxy may reach ~10⁵ pc cm^-3 depending on the burst environment, which may be probed by the current detectors. Free-free absorption and diffractive scattering may also affect the radio emission at a high density.

We observed the Seyfert I active/broad-line radio galaxy 3C 120 with the Chandra high-energy transmission gratings and present an analysis of the soft X-ray spectrum. We identify the strongest absorption feature (detected at >99.9% confidence) with O VIII Lyα (FWHM = 1010 km s^-1), blueshifted by -5500 ± 140 km s^-1 from systemic velocity. The absorption may be due to missing baryons in a warm/hot intergalactic medium (WHIGM) along the line of sight to 3C 120 at z = 0.0147 ± 0.0005, or it could be intrinsic to the jet of 3C 120. Assuming metallicities of ~0.1 Z_☉, we estimate an ionic column density of N_{O VIII} > 3.4 × 10¹⁶ cm^{- 2} for the WHIGM and a filament depth of less than 19 h Mpc. We find a baryon overdensity greater than 56 relative to the critical density of a Λ-dominated cold dark matter universe, which is in reasonable agreement with WHIGM simulations. We detect, at marginal significance, absorption of O VIII Lyα at z ~ 0 due to a hot medium in the Local Group. We also detect an unidentified absorption feature at ~0.71 keV. Absorption features that might be expected along with O VIII Lyα were not significant statistically. Relative abundances of metals in the WHIGM and local absorbers may therefore be considerably different from solar.

We introduce our project on galaxy evolution in the environment of rich clusters, aiming at disentangling the importance of specific interaction and galaxy transformation processes from the hierarchical evolution of galaxies in the field. Emphasis is laid on the examination of the internal kinematics of disk galaxies through spatially resolved multiobject spectroscopy with FORS at the Very Large Telescope. First results are presented for the clusters MS 1008.1-1224 (z = 0.30), Cl 0303+1706 (z = 0.42), and Cl 0413-6559 (F1557.19TC; z = 0.51). Out of 30 cluster members with emission lines, 13 galaxies exhibit a rotation curve of the universal form rising in the inner region and passing over into a flat part. The other members have either intrinsically peculiar kinematics (4), too strong geometric distortions (9), or a too low signal-to-noise ratio (4) for a reliable classification of their velocity profiles. The 13 cluster galaxies for which a maximum rotation velocity could be derived are distributed in the Tully-Fisher diagram very similar to field galaxies from the FORS Deep Field that have corresponding redshifts and do not show any significant luminosity evolution with respect to local samples. The same is true for the seven galaxies observed in the cluster fields that turned out not to be members. The mass-to-light ratios of the 13 Tully-Fisher cluster spiral galaxies cover the same range as the distant field population, indicating that their stellar populations were not dramatically changed by possible cluster-specific interaction phenomena. The cluster members with distorted kinematics may be subject to interaction processes, but it is impossible to determine whether or not these processes also lead to changes in the overall luminosity of their stellar populations.

In this Letter, we present results from our exploratory mid-IR study of Centaurus A circumnuclear environment using high angular resolution imaging at the Magellan 6.5 m telescope with the MIRAC/BLINC camera. We detected emission from a compact region surrounding the nuclear source and obtained photometry at 8.8 μm and in the N band. Our analysis suggests that the nuclear region is resolved with a size of ≈3 pc. The mid-IR emission from this region is likely associated with cool dust with an estimated temperature of ~160 K, surrounding the central "hidden" active galactic nucleus (AGN). We discuss the characteristics of this emission in relation to other mid-IR observations and the implications on models of dust formation in AGNs.

0103-72.6, the second brightest X-ray supernova remnant (SNR) in the Small Magellanic Cloud (SMC), has been observed with the Chandra X-Ray Observatory. Our Chandra observation unambiguously resolves the X-ray emission into a nearly complete, remarkably circular shell surrounding bright clumpy emission in the center of the remnant. The observed X-ray spectrum for the central region is evidently dominated by emission from reverse-shock-heated metal-rich ejecta. Elemental abundances in this ejecta material are particularly enhanced in oxygen and neon, while less prominent in the heavier elements Si, S, and Fe. We thus propose that 0103-72.6 is a new "oxygen-rich" SNR, making it only the second member of the class in the SMC. The outer shell is the limb-brightened soft X-ray emission from the swept-up SMC interstellar medium. The presence of O-rich ejecta and the SNR's location within an H II region attest to a massive star core-collapse origin for 0103-72.6. The elemental abundance ratios derived from the ejecta suggest an ~18 M_☉ progenitor star.

We present a self-consistent mean field theory of the dynamo in three dimensions and turbulent diffusion in two dimensions in weakly ionized gases. We find that in three dimensions, the back-reaction does not alter the β-effect while it suppresses the α-effect when the strength of a mean magnetic field exceeds the critical value B_c ~ ^1/2. Here, ν_in is the ion-neutral collision frequency, τ the correlation time of ions, and R_m the magnetic Reynolds number. These results suggest that a mean field dynamo operates much more efficiently in a weakly ionized gas where ν_inτ ≫ 1 than in a fully ionized gas. Furthermore, we show that in two dimensions, the turbulent diffusion is suppressed by the back-reaction when a mean magnetic field reaches the same critical strength B_c, with the upper bound on the turbulent diffusion given by its kinematic value. Astrophysical implications are discussed.

Halo globular clusters pose four succinct issues that must be solved in any scenario of their formation: single-age, single-metallicity stellar populations; a lower limit ([Fe/H] ~ -2.5) to their average metallicity; comprising only 1% of the stellar halo mass; and being among the oldest stars in our Galaxy. New spectra are presented of Galactic stars and integrated spectra of Galactic globular clusters that extend to 3250 Å. These spectra show that the most metal-poor and among the best-studied Galactic globular clusters show strong NH 3360 Å absorption, even though their spectral energy distributions in the near-UV are dominated by blue horizontal-branch AF-type stars. These strong NH features must be coming from the main-sequence stars in these clusters. These new data are combined with existing data on the wide range of carbon and nitrogen abundance in very metal-poor ([Fe/H] < -3.5) halo giant and dwarf stars, together with recent models of zero-metal star formation, to make a strawman scenario for globular cluster formation that can reproduce three of the above four issues, as well as two of the three related issues pertaining to nitrogen overabundance. This strawman proposal makes observational and theoretical predictions that are testable, needing specific help from the modelers to understand all of the elemental constraints on globular cluster and halo formation.

The 2000 outburst of V445 Puppis shows unique properties, such as the absence of hydrogen, the enrichment of helium and carbon, and the slow development of the light curve with a small amplitude that does not resemble any classical novae. This object has been suggested to be the first example of a helium nova. We calculate theoretical light curves of helium novae and reproduce the observational light curve of V445 Pup. Modeling indicates a very massive white dwarf (WD), more massive than 1.3 M_☉. The companion star is possibly either a helium star or a helium-rich main-sequence star. We estimate the ignition mass as several times 10^-5M_☉, the corresponding helium accretion rate as several times 10^-7M_☉ yr^-1, and the recurrence period as several tens of years. These values suggest that the WD is growing in mass and ends up either a Type Ia supernova or an accretion-induced collapse to a neutron star.

We present mid-infrared nulling interferometric and direct imaging observations of the Herbig Ae star HD 100546 obtained with the Magellan I (Baade) 6.5 m telescope. The observations show resolved circumstellar emission at 10.3, 11.7, 12.5, 18.0, and 24.5 μm. Through the nulling observations (10.3, 11.7, and 12.5 μm), we detect a circumstellar disk, with an inclination of 45^° ± 15^° with respect to a face-on disk, a semimajor axis position angle of 150^° ± 10^° (east of north), and a spatial extent of about 25 AU. The direct images (18.0 and 24.5 μm) show evidence for cooler dust with a spatial extent of 30-40 AU from the star. The direct images also show evidence for an inclined disk with a similar position angle as the disk detected by nulling. This morphology is consistent with models in which a flared circumstellar disk dominates the emission. However, the similarity in relative disk size that we derive for different wavelengths suggests that the disk may have a large inner gap, possibly cleared out by the formation of a giant protoplanet. The existence of a protoplanet in the system also provides a natural explanation for the observed difference between HD 100546 and other Herbig Ae stars.

We present Very Long Baseline Array proper-motion measurements of water masers toward two young stellar objects (YSOs) of the W75 N star-forming region. We find that these two objects are remarkable for having a similar spectral type, being separated by 07 (corresponding to 1400 AU), and sharing the same environment, but with a strikingly different outflow ejection geometry. One source has a collimated, jetlike outflow at a 2000 AU scale, while the other has a shell outflow at a 160 AU scale expanding in multiple directions with respect to a central compact radio continuum source. This result reveals that outflow collimation is not only a consequence of ambient conditions but is something intrinsic to the individual evolution of stars and brings to light the possibility of noncollimated outflows in the earliest stages of YSOs.

Past studies addressing the thermal atmospheric escape of hydrogen from "hot Jupiters" have been based on the planet's effective temperature, which, as we show here, is not physically relevant for loss processes. In consequence, these studies led to significant underestimations of the atmospheric escape rate (≤10³ g s^-1) and to the conclusion of long-term atmospheric stability. From more realistic exospheric temperatures, determined from X-ray and extreme-ultraviolet (XUV) irradiation and thermal conduction in the thermosphere, we find that energy-limited escape and atmospheric expansion arise, leading to much higher estimations for the loss rates (≈10¹² g s^-1). These fluxes are in good agreement with recent determinations for HD 209458b based on observations of its extended exosphere. We also show that for young solar-type stars, which emit stronger XUV fluxes, the inferred loss rates are significantly higher. Thus, hydrogen-rich giant exoplanets under such strong XUV irradiances may evaporate down to their core sizes or shrink to levels at which heavier atmospheric constituents may prevent hydrodynamic escape. These results could explain the apparent paucity of exoplanets so far detected at orbital distances less than 0.04 AU.

A two-fluid dynamic model of long-lived coronal loops is presented, whereby heating of the confined plasma is achieved by turbulence-driven Alfvén waves. It is assumed that the nonthermal motions inferred from spectral line observations in the transition region are due to Alfvén waves. It is also assumed that the turbulence is already fully developed when the waves are injected at the footpoint of the loop while the wave/turbulence energy is readily absorbed by the proton gas. The Coulomb coupling between protons and electrons subsequently heats the electron gas. The model produces a fairly uniform electron temperature in the coronal segment of the loop even though the heating is nonuniform. The model also reproduces electron densities of (1-4) × 10⁹ cm^-3, in the range inferred from observations, as well as a moderate flow speed around 10 km s^-1 along the loop. The turbulence heating mechanism adopted in this Letter, however, cannot produce stable loops with temperatures T ≤ 1.3 × 10⁶ K.