Table of contents

We study the properties of cosmological shock waves identified in high-resolution, N-body/hydrodynamic simulations of a ΛCDM universe and their role on thermalization of gas and acceleration of nonthermal, cosmic-ray (CR) particles. External shocks form around sheets, filaments, and knots of mass distribution when the gas in void regions accretes onto them. Within those nonlinear structures, internal shocks are produced by infall of previously shocked gas to filaments and knots and during subclump mergers, as well as by chaotic flow motions. Due to the low temperature of the accreting gas, the Mach number of external shocks is high, extending up to M ~ 100 or higher. In contrast, internal shocks have mostly low Mach numbers. For all shocks of M ≥ 1.5, the mean distance between shock surfaces over the entire computed volume is ~4 h^-1 Mpc at present, or ~1 h^-1 Mpc for internal shocks within nonlinear structures. Identified external shocks are more extensive, with their surface area ~2 times larger than that of identified internal shocks at present. However, especially because of higher preshock densities but also due to higher shock speeds, internal shocks dissipate more energy. Hence, the internal shocks are mainly responsible for gas thermalization as well as CR acceleration. In fact, internal shocks with 2 ≲ M ≲ 4 contribute about one-half of the total dissipation. Using a nonlinear diffusive shock acceleration model for CR protons, we estimate the ratio of CR energy to gas thermal energy dissipated at cosmological shock waves to be about one-half through the history of the universe. Our result supports scenarios in which the intracluster medium contains energetically significant populations of CRs.

We calculate the global star formation rate density (SFRD) from z ~ 30-3 using a semianalytic model incorporating the hierarchical assembly of dark matter halos, gas cooling via atomic hydrogen, star formation, supernova feedback, and suppression of gas collapse in small halos due to the presence of a photoionizing background. We compare the results with the predictions of simpler models based on the rate of dark matter halo growth and a fixed ratio of stellar to dark mass, and with observational constraints on the SFRD at 3 ≲ z ≲ 6. We also estimate the star formation rate due to very massive, metal-free Population III stars using a simple model based on the halo formation rate, calibrated against detailed hydrodynamic simulations of Population III star formation. We find that the total production rate of hydrogen-ionizing photons during the probable epoch of reionization (15 ≲ z ≲ 20) is approximately equally divided between Population II and Population III stars, and that if reionization is late (z_reion ≲ 15, close to the lower limit of the range allowed by the Wilkinson Microwave Anisotropy Probe results), then Population II stars alone may be able to reionize the universe.

Reducing the power on small scales relative to the "standard" Λ cold dark matter (ΛCDM) model alleviates a number of possible discrepancies with observations and is favored by the recent analysis of the Wilkinson Microwave Anisotropy Probe (WMAP) plus galaxy and Lyα forest data. Here we investigate the epoch of reionization in several models normalized to WMAP on large scales and with sufficiently reduced power on small scales to solve the halo concentration and substructure problems. These include a tilted model, the WMAP running-index model, and a warm dark matter model. We assume that the universe was reionized by stellar sources composed of a combination of supermassive (~200 M_☉) Population III stars and Population II stars with a "normal" initial mass function (IMF). We find that in all of these models, structure formation and hence reionization occurs late, certainly at redshifts below 10, and more probably at z ≲ 6. This is inconsistent (at 2 σ) with the determination of z_reion ≃ 17 from the WMAP temperature-polarization data and is only marginally consistent with Sloan Digital Sky Survey quasar observations. The tension between the galactic-scale observations, which favor low-power models, and the early reionization favored by WMAP can only be resolved if the efficiency of Population III star formation is dramatically higher than any current estimate or if there is an exotic population of ionizing sources such as miniquasars. Otherwise, we may have to live with the standard ΛCDM power spectrum and solve the small-scale problems in some other way.

We place constraints on the redshift-averaged, effective value of the equation of state of dark energy, w, using only the absolute ages of Galactic stars and the observed position of the first peak in the angular power spectrum of the cosmic microwave background (CMB). We find w < -0.8 at the 68% confidence level. If we further consider that w ≥ -1, this finding suggests that within our uncertainties, dark energy is indistinguishable from a classical vacuum energy term. We detect a correlation between the ages of the oldest galaxies and their redshift. This opens up the possibility of measuring w(z) by computing the relative ages of the oldest galaxies in the universe as a function of redshift, dz/dt. We show that this is a realistic possibility by computing dz/dt at z ~ 0 from Sloan Digital Sky Survey (SDSS) galaxies and obtain an independent estimate for the Hubble constant, H₀ = 69 ± 12 km s^-1 Mpc^-1. The small number of galaxies considered at z > 0.2 does not yield, currently, a precise determination of w(z), but shows that the age-redshift relation is consistent with a standard ΛCDM universe with w = -1.

We have discovered six galaxies with spectroscopically confirmed redshifts of 4.8 < z < 5.8 in a single 44 arcmin² field deeply imaged in R, I, and z bands. All the spectra show an emission line in the region around 7000-8400 Å with a spectroscopically detected faint continuum break across the line. These six were drawn from 13 sources with I_AB < 26.2 and R_AB - I_AB > 1.5 in the field; this photometric cut is designed to select galaxies at z > 4.8. The line fluxes range between 0.2 and 2.5 × 10^-17 ergs cm^-2 s^-1, indicating luminosities of around 10⁴²-10⁴³ ergs s^-1 for Lyα, and their high emission line equivalent widths suggest very young ages (≲10⁸ yr). A further line-emitting object with no detectable continuum was serendipitously detected by spectroscopy. If this line is Lyα, then it is from a source at z = 6.6, making this the most distant galaxy known. However, the redshift cannot be considered secure as it is based on a single line. No broad emission line objects (quasars) were detected. The 13 sources at I_AB < 26.2 are less than that expected if the luminosity function of dropout galaxies remained unchanged between z = 3 and 6, although the deficit is not highly significant given possible cosmic variance. The UV luminosity density from galaxies brighter than our flux limit is considerably less than that necessary to keep the volume probed by our field at ~ 5.3 ionized. These galaxies are observed within several hundred megayears of the end of the epoch of reionization (z = 6-7), with little time for the luminosity function to evolve. This and the lack of detected quasars imply that the bulk of the UV flux that reionized the universe came from faint galaxies with M_AB(1700 Å) > -21.

The evolution of high-redshift galaxies in the two Hubble Deep Fields, HDF-N and HDF-S, is investigated using a cloning technique that replicates z ~ 2-3 U dropouts to higher redshifts, allowing a comparison with the observed B and V dropouts at higher redshifts (z ~ 4-5). We treat each galaxy selected for replication as a set of pixels that are k-corrected to higher redshift, accounting for resampling, shot noise, surface-brightness dimming, and the cosmological model. We find evidence for size evolution (a 1.7-fold increase) from z ~ 5 to 2.7 for flat geometries (Ω_M + Ω_Λ = 1.0). Simple scaling laws for this cosmology predict that size evolution scales as (1 + z)^-1, consistent with our result. The UV luminosity density shows a similar increase (×1.85) from z ~ 5 to 2.7, with minimal evolution in the distribution of intrinsic colors for the dropout population. In general, these results indicate less evolution than was previously reported, and therefore a higher luminosity density at z ~ 4-5 (~50% higher) than other estimates. We argue that the present technique is the preferred way to understand evolution across samples with differing selection functions, the most relevant difference here being the color cuts and surface brightness thresholds [e.g., due to the (1 + z)⁴ cosmic surface brightness dimming effect].

We investigate a hierarchical structure formation scenario in which galaxy stellar cores are created from the binding energy liberated by shrinking supermassive black hole (SMBH) binaries. The binary orbital decay heats the surrounding stars, eroding a preexisting stellar cusp ∝r^-2. We follow the merger history of dark matter halos and associated SMBHs via cosmological Monte Carlo realizations of the merger hierarchy from early times to the present in a ΛCDM cosmology. Massive black holes get incorporated through a series of mergers into larger and larger halos, sink to the center through dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, and form a binary system. Stellar dynamical processes drive the binary to harden and eventually coalesce. A simple scheme is applied in which the loss cone is constantly refilled and a constant density core forms because of the ejection of stellar mass. We find that a model in which the effect of the hierarchy of SMBH interactions is cumulative and cores are preserved during galaxy mergers produces at the present epoch a correlation between the "mass deficit" (the mass needed to bring a flat inner density profile to a r^-2 cusp) and the mass of the nuclear SMBH, with a normalization and slope comparable to the observed relation. Models in which the mass displaced by the SMBH binary is replenished after every major galaxy merger appear instead to underestimate the mass deficit observed in "core" galaxies.

We discuss the relation between the power carried by relativistic jets and the nuclear power provided by accretion for a group of blazars, including flat-spectrum radio quasars (FSRQs) and BL Lac objects. They are characterized by good-quality broadband X-ray data provided by the BeppoSAX satellite. The jet powers are estimated using physical parameters determined from uniformly modeling their spectral energy distributions. Our analysis indicates that for FSRQs the total jet power is of the same order as the accretion power. We suggest that blazar jets are likely powered by energy extraction from a rapidly spinning black hole (BH) through the magnetic field provided by the accretion disk. FSRQs must have large BH masses (10⁸-10⁹M_☉) and high, near-Eddington accretion rates. For BL Lac objects, the jet luminosity is larger than the disk luminosity. This can be understood within the same scenario if BL Lac objects have masses similar to FSRQs but accrete at largely subcritical rates, whereby the accretion disk radiates inefficiently. Thus, the "unification" of the two classes into a single blazar population, previously proposed on the basis of a spectral sequence governed by luminosity, finds a physical basis.

Highly polarized QSOs discovered in the Two-Micron All Sky Survey (2MASS) have been observed to determine the source(s) of optical polarization in this near-infrared color-selected sample. Broad emission lines are observed in the polarized flux spectra of most objects, and the polarization of the lines is at about the same level and position angle as the continuum. Generally, the continuum is bluer and the broad-line Balmer decrement is smaller in polarized light than for the spectrum of total flux. Narrow emission lines are much less polarized than the broad lines and continuum for all polarized objects. These properties favor scattering by material close to a partially obscured and reddened active nucleus, but exterior to the regions producing the broad-line emission, as the source of polarized flux in 2MASS QSOs. The largely unpolarized narrow-line features require that the electrons or dust polarizing the light be located at distances from the nucleus not much greater than the extent of the narrow emission line region. The conclusion that the scattering material is located close to the nucleus is reinforced by the observation in four objects of changes in both the degree and position angle of polarization across the broad Hα emission-line profile, indicating that the broad emission line region (BLR) is at least partially resolved from distance of the scatterers. In addition to known high-polarization objects, four 2MASS QSOs with active galactic nucleus (AGN) spectral types of 1.9 and 2 were observed to search for hidden BLRs. Broad lines were detected in polarized light for two of these objects, and the polarizing mechanism appears to be the same for these objects as for the highly polarized QSOs in the sample that readily show broad emission lines in their spectra. The small observed sample of eight type 1 2MASS QSOs has weak [O III] emission in comparison to optically selected AGNs with similar near-infrared luminosity. The observations also show that starlight from the host galaxy contributes a significant amount of optical flux, especially for the narrow-line objects, and support the suggestion that many 2MASS QSOs are measured to have low polarization simply because of dilution of the polarized AGN light by the host galaxy.

It is generally argued that most clusters of galaxies host cooling flows in which radiative cooling in the center causes a slow inflow. However, recent observations by Chandra and XMM conflict with the predicted cooling flow rates. Among other mechanisms, heating by a central active galactic nucleus and thermal conduction have been invoked in order to account for the small mass deposition rates. Here we present a family of hydrostatic models for the intracluster medium where radiative losses are exactly balanced by thermal conduction and heating by a central source. We describe the features of this simple model and fit its parameters to the density and temperature profiles of Hydra A.

We present the analysis of the X-ray spectra of 18 distant clusters of galaxies with redshift 0.3 < z < 1.3. Most of them were observed with the Chandra satellite in long exposures ranging from 36 to 180 ks. For two of the z > 1 clusters, we also use deep XMM-Newton observations. Overall, these clusters probe the temperature range 3 keV ≲ kT ≲ 8 keV. Our analysis is aimed at deriving the iron abundance in the intracluster medium (ICM) out to the highest redshifts probed to date. Using a combined spectral fit of cluster subsamples in different redshift bins, we investigate the evolution of the mean ICM metallicity with cosmic epoch. We find that the mean Fe abundance at = 0.8 is Z = 0.25Z_☉, consistent with the local canonical metallicity value, Z ≃ 0.3 Z_☉, within the 1 σ confidence level (c.l.). Medium- and low-temperature clusters (kT < 5 keV) tend to have larger iron abundances than hot clusters. At redshift ~ 1.2 (four clusters at z > 1), we obtain a statistically significant detection of the Fe K line in only one cluster (Z > 0.10 Z_☉ at the 90% c.l.). Combining all the current data sets from Chandra and XMM at z > 1, the average metallicity is measured to be = 0.21Z_☉ (1 σ error), thus suggesting no evolution of the mean iron abundance out to z ≃ 1.2.

From the analysis of sensitive H I 21 cm line observations, we find evidence for vertically extended H I emission ( ≲ 2.4 kpc) in the edge-on, low surface brightness spiral galaxy UGC 7321. Three-dimensional modeling suggests that the H I disk of UGC 7321 is both warped and flared, but that neither effect can fully reproduce the spatial distribution and kinematics of the highest z-height gas. We are able to model the high-latitude emission as an additional H I component in the form of a "thick disk" or "halo" with a FWHM of ~3.3 kpc. We find tentative evidence that the vertically extended gas declines in rotational velocity as a function of z, although we are unable to completely rule out models with constant V. In spite of the low star formation rate of UGC 7321, energy from supernovae may be sufficient to sustain this high-latitude gas. However, alternative origins for this material, such as slow, sustained infall, cannot yet be excluded.

Three nearby galaxies that have abnormally high infrared-to-radio continuum ratios, NGC 1377, IC 1953, and NGC 4491, are investigated with a view to understanding the physical origin of their peculiarity. We review the existing data and present new radio continuum measurements along with near-infrared integral-field spectroscopy and molecular gas observations. The three galaxies have low luminosities but starburst-like infrared colors; in NGC 1377, no synchrotron emission is detected at any wavelength; in IC 1953, the observed synchrotron component is attributable to the spiral disk alone and is lacking in the central regions; and the radio spectrum of NGC 4491 is unusually flat. We also compare and contrast them with NGC 4418, a heavily extinguished galaxy that shares some attributes with them. After examining various scenarios, we conclude that these galaxies are most likely observed within a few megayears of the onset of an intense star formation episode after being quiescent for at least ≈100 Myr. This starburst, while heating the dust, has not produced optical signatures or a normal amount of cosmic rays yet. We briefly discuss the statistics of such galaxies and what they imply for star formation surveys.

We study the dynamical evolution of the M87 globular cluster system (GCS) with a number of numerical simulations. We explore a range of different initial conditions for the GCS mass function (GCMF), for the GCS spatial distribution, and for the GCS velocity distribution. Our simulations include the effects of two-body relaxation, dynamical friction, and mass loss due to stellar evolution. We first confirm that an initial power-law GCMF such as that observed in young cluster systems can be readily transformed through dynamical processes into a bell-shaped GCMF. However, only models with initial velocity distributions characterized by a strong radial anisotropy increasing with the galactocentric distance are able to reproduce the observed constancy of the GCMF at all radii. We show that such strongly radial orbital distributions are inconsistent with the observed kinematics of the M87 GCS. The evolution of models with a bell-shaped GCMF with a turnover similar to that currently observed in old GCSs is also investigated. We show that models with this initial GCMF can satisfy all the observational constraints currently available on the GCS spatial distribution, the GCS velocity distribution, and on the GCMF properties. In particular, these models successfully reproduce both the lack of a radial gradient of the GCS mean mass recently found in an analysis of Hubble Space Telescope images of M87 at multiple locations and the observed kinematics of the M87 GCS. Our simulations also show that evolutionary processes significantly affect the initial GCS properties by leading to the disruption of many clusters and changing the masses of those that survive. The preferential disruption of inner clusters flattens the initial GCS number density profile, and it can explain the rising specific frequency with radius; we show that the inner flattening observed in the M87 GCS spatial distribution can be the result of the effects of dynamical evolution on an initially steep density profile.

We study the dynamical evolution of globular clusters using our two-dimensional Monte Carlo code with the inclusion of primordial binary interactions for equal-mass stars. We use approximate analytical cross sections for energy generation from binary-binary and binary-single interactions. After a brief period of slight contraction or expansion of the core over the first few relaxation times, all clusters enter a much longer phase of stable "binary burning" lasting many tens of relaxation times. The structural parameters of our models during this phase match well those of most observed globular clusters. At the end of this phase, clusters that have survived tidal disruption undergo deep core collapse, followed by gravothermal oscillations. Our results clearly show that the presence of even a small fraction of binaries in a cluster is sufficient to support the core against collapse significantly beyond the normal core-collapse time predicted without the presence of binaries. For tidally truncated systems, collapse is easily delayed sufficiently that the cluster will undergo complete tidal disruption before core collapse. As a first step toward the eventual goal of computing all interactions exactly using dynamical three- and four-body integration, we have incorporated an exact treatment of binary-single interactions in our code. We show that results using analytical cross sections are in good agreement with those using exact three-body integration, even for small binary fractions, where binary-single interactions are energetically most important.

SN 2001el is the first normal Type Ia supernova to show a strong, intrinsic polarization signal. In addition, during the epochs prior to maximum light, the Ca II IR triplet absorption is seen distinctly and separately at both normal photospheric velocities and at very high velocities. The high-velocity triplet absorption is highly polarized, with a different polarization angle than the rest of the spectrum. The unique observation allows us to construct a relatively detailed picture of the layered geometrical structure of the supernova ejecta: in our interpretation, the ejecta layers near the photosphere (v ≈ 10,000 km s^-1) obey a nearly axial symmetry, while a detached, high-velocity structure (v ≈ 18,000-25,000 km s^-1) with high Ca II line opacity deviates from the photospheric axisymmetry. By partially obscuring the underlying photosphere, the high-velocity structure causes a more incomplete cancellation of the polarization of the photospheric light and so gives rise to the polarization peak and rotated polarization angle of the high-velocity IR triplet feature. In an effort to constrain the ejecta geometry, we develop a technique for calculating three-dimensional synthetic polarization spectra and use it to generate polarization profiles for several parameterized configurations. In particular, we examine the case in which the inner ejecta layers are ellipsoidal and the outer, high-velocity structure is one of four possibilities: a spherical shell, an ellipsoidal shell, a clumped shell, or a toroid. The synthetic spectra rule out the spherical shell model, disfavor a toroid, and find a best fit with the clumped shell. We show further that different geometries can be more clearly discriminated if observations are obtained from several different lines of sight. Thus, assuming that the high-velocity structure observed for SN 2001el is a consistent feature of at least a known subset of Type Ia supernovae, future observations and analyses such as these may allow one to put strong constraints on the ejecta geometry and hence on supernova progenitors and explosion mechanisms.

We present two-dimensional line profiles of high-velocity (~±12,000 km s^-1) Lyα and Hα emission from supernova remnant 1987A obtained with the Space Telescope Imaging Spectrograph between 1997 September and 2001 September (days 3869-5327 after the explosion). This emission comes from hydrogen in the debris that is excited and ionized as it passes through the remnant's reverse shock. We use these profiles to measure the geometry and development of the reverse-shock surface. The observed emission is confined within ~±30° about the remnant's equatorial plane. At the equator, the reverse shock has a radius of ~75% of the distance to the equatorial ring. We detect marginal differences (6% ± 3%) between the location of the reverse-shock front in the northeast and southwest parts of the remnant. The radius of the reverse shock surface increases for latitudes above the equator, a geometry consistent with a model in which the supernova debris expands into a bipolar nebula. Assuming that the outer supernova debris has a power-law density distribution, we can infer from the reverse-shock emission light curve an expansion rate (in the northeast part of the remnant) of 3700 ± 900 km s^-1, consistent with the expansion velocities determined from observations in radio (Manchester et al.) and X-ray (Park et al.; Michael et al.) wavelengths. However, our most recent observation (at day 5327) suggests that the rate of increase of mass flux across the northeast sector of the reverse shock has accelerated, perhaps because of deceleration of the reverse shock caused by the arrival of a reflected shock created when the blast wave struck the inner ring. Resonant scattering within the supernova debris causes Lyα photons created at the reverse shock to be directed preferentially outward, resulting in a factor of ~5 difference in the observed brightness of the reverse shock in Lyα between the near and far sides of the remnant. Accounting for this effect, we compare the observed reverse-shock Lyα and Hα fluxes to infer the amount of interstellar extinction by dust as E(B - V) = 0.17 ± 0.01 mag. We also notice extinction by dust in the equatorial ring with E(B - V) ≈ 0.02-0.08 mag, which implies dust-to-gas ratios similar to that of the LMC. Since Hα photons are optically thin to scattering, the observed asymmetry in brightness of Hα from the near and far sides of the remnant represents a real asymmetry in the mass flux through the reverse shock of ~30%. We discuss future observational strategies that will permit us to further investigate the reverse-shock dynamics and resonant scattering of the Lyα line and to constrain better the extinction by dust within and in front of the remnant.

Based on fractional Brownian motion (fBm) simulations of three-dimensional gas density and velocity fields, we present a study of the statistical properties of spectroimagery observations (channel maps, integrated emission, and line centroid velocity) in the case of an optically thin medium at various temperatures. The power spectral index γ_W of the integrated emission is confirmed to be that of the three-dimensional density field (γ_n) provided that the medium's depth is at least of the order of the largest transverse scale in the image, and the power spectrum of the centroid velocity map is found to have the same index γ_C as that of the velocity field (γ_v). Further tests with non-fBm density and velocity fields show that this last result holds and is not modified by the effects of density-velocity correlations. A comparison is made with the theoretical predictions of Lazarian & Pogosyan.

We have searched for interstellar conformer I glycine (NH₂CH₂COOH), the simplest amino acid, in the hot molecular cores Sgr B2(N-LMH), Orion KL, and W51 e1/e2. An improved search strategy for intrinsically weak molecular lines, involving multisource observations, has been developed and implemented. In total, 82 spectral frequency bands, in the millimeter-wave region, were observed over a 4 yr period; 27 glycine lines were detected in 19 different spectral bands in one or more sources. The rotational temperatures derived from "rotation diagrams" are 75 K for Sgr B2(N-LMH), 141 K for Orion KL, and 121 K for W51 e1/e2. The total column densities inferred are 4.16 × 10¹⁴ cm^-2 for Sgr B2, 4.37 × 10¹⁴ cm^-2 for Orion, and 2.09 × 10¹⁴ cm^-2 for W51. Production of interstellar glycine by both gas-phase ion-molecule reactions and by ultraviolet photolysis of molecular ices is briefly discussed. The discovery of interstellar glycine strengthens the thesis that interstellar organic molecules could have played a pivotal role in the prebiotic chemistry of the early Earth.

Several nearby individual low column density interstellar cloudlets have been identified previously on the basis of kinematical features evident in high-resolution Ca⁺ observations near the Sun. One of these cloudlets, the "Apex Cloud" (AC), is within 5 pc of the Sun in the solar apex direction. The question of which interstellar cloud will constitute the next Galactic environment of the Sun can, in principle, be determined from cloudlet velocities. The interstellar absorption lines toward α Cen (the nearest star) are consistent within measurement uncertainties with the projected "G" cloud (GC) and AC velocities, and also with the velocity of the cloud inside of the solar system (the local interstellar cloud [LIC]), provided a small velocity gradient is present in the LIC. The high GC column density toward α Oph compared to α Aql suggests that α Aql may be embedded in the GC so that the AC would be closer to the Sun than the GC. This scenario favors the AC as the next cloud to be encountered by the Sun, and the AC would have a supersonic velocity with respect to the LIC. The weak feature at the AC velocity toward 36 Oph suggests that the AC cloud is either patchy or does not extend to this direction. Alternatively, if the GC is the cloud that is foreground to α Cen, the similar values for N(H⁰) in the GC components toward α Cen and 36 Oph indicate this cloud is entirely contained within the nearest ~1.3 pc, and the Ca⁺ GC data toward α Oph would then imply a cloud volume density of ~5 cm^-3, with dramatic consequences for the heliosphere in the near future.

We present the first high spatial resolution X-ray images of two high-mass star forming regions, the Omega Nebula (M17) and the Rosette Nebula (NGC 2237-2246), obtained with the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer instrument. The massive clusters powering these H II regions are resolved at the arcsecond level into more than 900 (M17) and 300 (Rosette) stellar sources similar to those seen in closer young stellar clusters. However, we also detect soft diffuse X-ray emission on parsec scales that is spatially and spectrally distinct from the point-source population. The diffuse emission has luminosity L_X ≃ 3.4 × 10³³ ergs s^-1 in M17 with plasma energy components at kT ≃ 0.13 and ≃0.6 keV (1.5 and 7 MK), while in Rosette it has L_X ≃ 6 × 10³² ergs s^-1 with plasma energy components at kT ≃ 0.06 and ≃0.8 keV (0.7 and 9 MK). This extended emission most likely arises from the fast O star winds thermalized either by wind-wind collisions or by a termination shock against the surrounding media. We establish that only a small portion of the wind energy and mass appears in the observed diffuse X-ray plasma; in these blister H II regions, we suspect that most of it flows without cooling into the low-density interstellar medium. These data provide compelling observational evidence that strong wind shocks are present in H II regions.

The molecular evolution that occurs in collapsing prestellar cores is investigated. To model the dynamics, we adopt the Larson-Penston solution and analogs with slower rates of collapse. For the chemistry, we utilize the new standard model with the addition of deuterium fractionation and grain-surface reactions treated via the modified rate approach. The use of surface reactions distinguishes the present work from our previous model. We find that these reactions efficiently produce H₂O, H₂CO, CH₃OH, N₂, and NH₃ ices. In addition, the surface chemistry influences the gas-phase abundances in a variety of ways. For example, formation of molecular nitrogen on grain surfaces followed by desorption into the gas enhances the abundance of this gas-phase species and its daughter products N₂H⁺ and NH₃. The current reaction network along with the Larson-Penston solution allows us to reproduce satisfactorily most of the molecular column densities and their radial distributions observed in L1544. The agreement tends to worsen with models that include strongly delayed collapse rates. Inferred radial distributions in terms of fractional abundances are somewhat harder to reproduce. In addition to our standard chemical model, we have also run a model with the UMIST gas-phase chemical network. The abundances of gas-phase sulphur-bearing molecules such as CS and CCS are significantly affected by uncertainties in the gas-phase chemical network. In all our models, the column density of N₂H⁺ monotonically increases as the central density of the core increases during collapse from 3 × 10⁴ to 3 × 10⁷ cm^-3. Thus, the abundance of this ion can be a probe of evolutionary stage. Molecular D/H ratios in assorted cores are best reproduced in the Larson-Penston picture with the conventional rate coefficients for fractionation reactions. If we adopt the newly measured and calculated rate coefficients, the D/H ratios, especially N₂D⁺/N₂H⁺, become significantly lower than the observed values.

Using the Very Large Array, we have detected weak OH maser emission near the Turner-Welch protostellar source in the W3 OH region. Unlike typical interstellar OH masers, which are associated with ultracompact H II regions, our measured positions and proper motions (from very long baseline interferometry) indicate that these OH masers are associated with a bipolar outflow traced by strong H₂O masers. These OH masers may be part of a class of interstellar OH masers that are associated with very young stars that have yet to, or may never, create ultracompact H II regions. This class of OH masers appears to form near the edges of very dense material (within which H₂O masers form), where total densities drop precipitously and interstellar UV radiation is sufficient to dissociate the H₂O molecules. Observations of this class of OH masers may be an important way to probe the distribution of this important molecule in interstellar shocks at arcsecond resolution or better.

The light curves of hypernovae, i.e., very energetic supernovae with E₅₁ ≡ E/10⁵¹ ergs ≳5-10, are characterized by a phase of linear decline at epochs of a few months. Classical, one-dimensional explosion models fail to simultaneously reproduce the light curve near peak and at the linear decline phase. The evolution of these light curves may, however, be explained by a simple model consisting of two concentric components. The outer component is responsible for the early part of the light curve and for the broad absorption features observed in the early spectra of hypernovae, similar to the one-dimensional models. In addition, a very dense inner component is added, which reproduces the linear decline phase in the observed magnitude versus time relation for SN 1998bw, SN 1997ef, and SN 2002ap. This simple approach does contain one of the main features of jet-driven, asymmetric explosion models, namely, the presence of a dense core. Although the total masses and energies derived with the two-component model are similar to those obtained in previous studies that also adopted spherical symmetry, this study suggests that the ejecta are aspherical, and thus, the real energies and masses may deviate from those derived assuming spherical symmetry. The supernovae that were modeled are divided into two groups according to the prominence of the inner component: the inner component of SN 1997ef is denser and more ⁵⁶Ni-rich, relative to the outer component, than the corresponding inner components of SN 1998bw and SN 2002ap. These latter objects have a similar inner-to-outer component ratio, although they have very different global values of mass and energy.

The ⟨V/V_max⟩ of the cosmological X-ray flashes detected by the Wide Field Cameras on BeppoSAX is calculated theoretically in a simple jet model. The total emission energy from the jet is assumed to be constant. We find that if the jet opening half-angle is smaller than 0.03 radian, the theoretical ⟨V/V_max⟩ for fixed opening half-angle is less than ~0.4, which is consistent with the recently reported observational value of 0.27 ± 0.16 at the 1 σ level. This suggests that the off-axis gamma-ray burst jet with the small opening half-angle at the cosmological distance can be identified as the cosmological X-ray flash.

We present models for reprocessing of an intense flux of X-rays and gamma rays expected in the vicinity of gamma-ray burst sources. We consider the transfer and reprocessing of the energetic photons into observable features in the X-ray band, notably the K lines of iron. Our models are based on the assumption that the gas is sufficiently dense to allow the microphysical processes to be in a steady state, thus allowing efficient line emission with modest reprocessing mass and elemental abundances ranging from solar to moderately enriched. We show that the reprocessing is enhanced by down-Comptonization of photons whose energy would otherwise be too high to absorb in iron and that pair production can have an effect on enhancing the line production. Both "distant" reprocessors, such as supernova or wind remnants, and "nearby" reprocessors, such as outer stellar envelopes, can reproduce the observed line fluxes with Fe abundances 30-100 times above solar, depending on the incidence angle. The high incidence angles required arise naturally only in nearby models, which for plausible values can reach Fe line-to-continuum ratios close to the reported values.

The behavior of the afterglow (AG) of gamma-ray bursts (GRBs) directly provides, in the cannonball (CB) model, information about the environment of their progenitor stars. The well-observed early temporal decline of the AG of GRB 021211 is precisely the one predicted in the presence of a progenitor's "wind," which resulted in a density profile ∝1/r² around the star. The subsequent fast fading—which makes this GRB "quasi-dark"—is the one anticipated if, farther away, the interstellar density is roughly constant and relatively high. The CB model fit to the AG clearly shows the presence of an associated supernova akin to SN 1998bw and allows even for the determination of the broadband spectrum of the host galaxy. GRB 990123 and GRB 021004, whose AGs were also measured very early, are also discussed.

While the origin of r-process nuclei remains a long-standing mystery, recent spectroscopic studies of extremely metal poor stars in the Galactic halo strongly suggest that it is associated with core-collapse supernovae. In this study we examine r-process nucleosynthesis in a "prompt supernova explosion" from an 8-10 M_☉ progenitor star as an alternative scenario to the "neutrino wind" mechanism, which has also been considered a promising site of the r-process. In the present model, the progenitor star has formed an oxygen-neon-magnesium (O-Ne-Mg) core (of mass 1.38 M_☉) at its center. Its smaller gravitational potential, as well as the smaller core that is in nuclear statistical equilibrium at the time of core bounce, as compared with the iron cores in more massive stars, may allow the star to explode hydrodynamically rather than by delayed neutrino heating. The core-collapse simulations are performed with a one-dimensional, Newtonian hydrodynamic code. We obtain a very weak prompt explosion in which no r-processing occurs. We further simulate energetic prompt explosions by enhancement of the shock-heating energy in order to investigate conditions necessary for the production of r-process nuclei in such events. The r-process nucleosynthesis is calculated using a nuclear reaction network code including relevant neutron-rich isotopes with reactions among them. The highly neutronized ejecta (Y_e ≈ 0.14-0.20) lead to robust production of r-process nuclei; their relative abundances are in excellent agreement with the solar r-process pattern. Our results suggest that prompt explosions of 8-10 M_☉ stars with O-Ne-Mg cores can be a promising site of r-process nuclei. The mass of the r-process material per event is about 2 orders of magnitude larger than that expected from Galactic chemical evolution studies. We propose, therefore, that only a small fraction of r-process material is ejected via "mixing-fallback" mechanism of the core matter, wherein most of the r-process material falls back onto the proto-neutron star. A lower limit on the age of the universe is derived by application of the uranium-thorium (U-Th) chronometer pair by comparison with the observed ratio of these species in the highly r-process-enhanced, extremely metal poor star CS 31082-001. The inferred age is 14.1 ± 2.4 Gyr—the same as that obtained previously based on the neutrino wind scenario with the same nuclear mass formula. This suggests that chronometric estimates obtained using the U-Th pair are independent of the astrophysical conditions considered.

It has been suggested that quasi-periodic oscillations of accreting X-ray sources may relate to the modes named in the title. We consider nonaxisymmetric linear perturbations to an isentropic, isothermal, unmagnetized thin accretion disk. The radial wave equation, in which the number of vertical nodes (n) appears as a separation constant, admits a wave action current that is conserved except, in some cases, at corotation. Waves without vertical nodes amplify when reflected by a barrier near corotation. Their action is conserved. As was previously known, this amplification allows the n = 0 modes to be unstable under appropriate boundary conditions. In contrast, we find that waves with n > 0 are strongly absorbed at corotation rather than amplified; their action is not conserved. Therefore, nonaxisymmetric p-modes and g-modes with n > 0 are damped and stable even in an inviscid disk. This eliminates a promising explanation for quasi-periodic oscillations in neutron star and black hole X-ray binaries.

We examine the small-scale dynamics of black hole accretion disks in which radiation pressure exceeds gas pressure. Local patches of disk are modeled by numerically integrating the equations of radiation MHD in the flux-limited diffusion approximation. The shearing-box approximation is used, and the vertical component of gravity is neglected. Magnetorotational instability (MRI) leads to turbulence in which accretion stresses are due primarily to magnetic torques. When radiation is locked to gas over the length and timescales of fluctuations in the turbulence, the accretion stress, density contrast, and dissipation differ little from those in the corresponding calculations, with radiation replaced by extra gas pressure. However, when radiation diffuses each orbit a distance that is comparable to the rms vertical wavelength of the MRI, radiation pressure is less effective in resisting squeezing. Large density fluctuations occur, and radiation damping of compressive motions converts PdV work into photon energy. The accretion stress in calculations having a net vertical magnetic field is found to be independent of opacity over the range explored and approximately proportional to the square of the net field. In calculations with zero net magnetic flux, the accretion stress depends on the portion of the total pressure that is effective in resisting compression. The stress is lower when radiation diffuses rapidly with respect to the gas. We show that radiation-supported Shakura-Sunyaev disks accreting via internal magnetic stresses are likely to have radiation marginally coupled to turbulent gas motions in their interiors.

We report on the long-term monitoring of X-ray dips from the ultracompact low-mass X-ray binary (LMXB) XB 1916-053. Roughly one-month interval observations were carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996, during which the source varied between dim, hard states and more luminous, soft states. The dip spectra and dip light curves were compared against both the broadband luminosity and the derived mass accretion rate (). The dips spectra could be fitted by an absorbed blackbody plus cutoff power-law nondip spectral model, with additional absorption ranging from 0 to > 100 × 10²² cm^-2. The amount of additional blackbody absorption was found to vary with the source luminosity. Our results are consistent with an obscuration of the inner disk region by a partially ionized outer disk. The size of the corona, derived from the dip ingress times, was found to be ~10⁹ cm. The corona size did not correlate with the coronal temperature, but seemed to increase when also increased. We discuss our findings in the context of an evaporated accretion disk corona model and an ADAF-type model.

We present a new model for the synchrotron compact nebular emissions for Vela and PSR B1706-44, and derive fundamental pulsar/plerion parameters such as the pair production multiplicity, M, and wind magnetization parameter, σ. The pair cascade above the pulsar polar cap, combined with the energy from the pulsar wind, injects a pair plasma into the surrounding environment. This wind, consisting of particles and fields, is shocked by the environment, resulting in synchrotron emission from "thermalized" pairs in a compact nebula. The broadband nebular spectrum depends crucially on the spin-down power, distance, pair multiplicity, pulsar wind shock radius, and σ at this shock. We construct such a model for the particle spectra in the compact X-ray nebulae of Vela and PSR B1706-44. Fits to the multiwavelength spectra of these sources indicate that 300 < M < 1000, whereas 0.05 < σ < 0.5 for Vela. The same σ interval (as for Vela) was independently derived from the radial gradients of the X-ray compact nebular emission from both Vela and PSR B1706-44, giving us confidence in our results. The M we derive for Vela is too small to be explained by curvature losses in the open magnetosphere of Vela, but the presence of optical pulsed photons could modify the predicted multiplicity.

Blackbody fits to the soft X-ray spectra of cooling neutron stars will, in general, overestimate the surface temperature of the source. We examine blackbody fits to the spectra predicted by light-element neutron star model atmospheres folded with the ACIS response matrices and quantify the inferred temperature error for a range of magnetic field strengths representative of the broad populations of isolated cooling neutron stars. Results for the Position Sensitive Proportional Counter are derived for comparative purposes. In each example, the blackbody temperature systematically overestimates the underlying flux temperature of the model and is an unreliable gauge by which to estimate the surface properties of neutron stars. We demonstrate a linear relationship between flux temperature and peak energy, similar to Wien displacement, as a function of magnetic field strength for the model atmospheres. We calculate the excess optical magnitude predicted from the model atmospheres with respect to the Rayleigh-Jeans portion of the fitted blackbody. We detail the nature and magnitude of these discrepancies and identify their origin in the opacity of the atmospheric plasma, the magnitude of interstellar absorption, and the instrumental response function.

An extensive grid of synthetic mid- and far-ultraviolet spectra for accretion disks in cataclysmic variables has been presented by Wade & Hubeny. In those models, the disk was assumed to be in steady state; that is, T_eff(r) is specified completely by the mass M_WD and radius R_WD of the accreting white dwarf star and the mass transfer rate , which is constant throughout the disk. In these models, T_eff(r) ∝ r^-3/4, except as modified by a cutoff term near the white dwarf. Actual disks may vary from the steady state prescription for T_eff(r), however, as a result of, for example, outburst cycles in dwarf novae ( not constant with radius) or irradiation (in which case T_eff in the outer disk is raised above T_steady). To show how the spectra of such disks might differ from the steady case, we present a study of the ultraviolet (UV) spectra of models in which power-law temperature profiles T_eff(r) ∝ r^-γ with γ < are specified. Otherwise, the construction of the models is the same as in the Wade & Hubeny grid, to allow comparison. We discuss both the UV spectral energy distributions and the appearance of the UV line spectra. We also briefly discuss the eclipse light curves of the nonstandard models. Comparison of these models with UV observations of nova-like variables suggests that better agreement may be possible with such modified T_eff(r) profiles.

We present optical and ultraviolet photometry and spectroscopy and optical circular spectropolarimetry of the ultramassive, rapidly rotating, high-field magnetic white dwarf EUVE J0317-85.5. From the combined data sets, we establish an ephemeris for the photometric, spectroscopic, and polarimetric variations with a spin period of 725.7277 s, the fastest yet measured for an isolated white dwarf. We build a series of far-ultraviolet (FUV) and optical flux spectra as well as optical polarization spectra that describe the atmospheric properties as a function of rotation phase. Twelve components of the Lyman line series are identified in the FUV: seven of them are forbidden lines enforced by the electric field that permeates the strongly magnetized photosphere. Attempts at modeling the FUV and optical data with two classes of models indicate an unusual field structure on the star. Both offset-dipole and multipolar expansion models fail to adequately reproduce the spectroscopic and polarimetric properties simultaneously, and we postulate that the surface of EUVE J0317-85.5 may be punctuated by a high-field magnetic spot.

In this paper, we present the first theoretical γ Doradus instability strip. We find that our model instability strip agrees very well with the previously established, observationally based, instability strip of Handler & Shobbrook. We stress, as do Guzik et al., that the convection zone depth plays the major role in the determination of our instability strip. Once this depth becomes too deep or too shallow, the convection zone no longer allows for pulsational instability. Our theoretical γ Dor instability strip is bounded by ~6850 and 7360 K at the red and blue edge, respectively, on the zero-age main sequence and by ~6560 and 7000 K at the red and blue edge, respectively, approximately 2 mag more luminous. This theoretical strip, transformed to the observer's color-magnitude diagram, overlays the region where most of the 30 bona fide γ Dor stars are found.

We model the nucleosynthesis during the thermal pulse phase of a rotating, solar metallicity, asymptotic giant branch (AGB) star of 3 M_☉, which was evolved from a main-sequence model rotating with 250 km s^-1 at the stellar equator. Rotationally induced mixing during the thermal pulses produces a layer (~2 × 10^-5M_☉) on top of the CO core where large amounts of protons and ¹²C coexist. With a postprocessing nucleosynthesis and mixing code, we follow the abundance evolution in this layer, in particular that of the neutron source ¹³C and of the neutron poison ¹⁴N. In our AGB model mixing persists during the entire interpulse phase because of the steep angular velocity gradient at the core-envelope interface, thereby spreading ¹⁴N over the entire ¹³C-rich part of the layer. We follow the neutron production during the interpulse phase and find a resulting maximum neutron exposure of τ_max = 0.04 mbarn^-1, which is too small to produce any significant s-process. In parametric models, we then investigate the combined effects of diffusive overshooting from the convective envelope and rotationally induced mixing. Just adding the overshooting and leaving the rotational mixing unchanged results in a small maximum neutron exposure (0.03 mbarn^-1). Models with overshoot and weaker interpulse mixing—as perhaps expected from more slowly rotating stars—yield larger neutron exposures. In a model with overshooting without any interpulse mixing a neutron exposure of up to 0.72 mbarn^-1 is obtained, which is larger than required by observations. We conclude that the incorporation of rotationally induced mixing processes has important consequences for the production of heavy elements in AGB stars. While through a distribution of initial rotation rates, it may lead to a natural spread in the neutron exposures obtained in AGB stars of a given mass in general—as appears to be required by observations—it may moderate the large neutron exposures found in models with diffusive overshoot in particular. Our results suggest that both processes, diffusive overshoot and rotational mixing, may be required to obtain a consistent description of the s-process in AGB stars that fulfills all observational constraints. Finally, we find that mixing due to rotation within our current framework does increase the production of ¹⁵N in the partial mixing zone. However, this increase is not large enough to boost the production of fluorine to the level required by observations.

We investigate the sensitivity to temperature and gravity of the strong absorption features in the J- and K-band spectra of substellar objects. We compare the spectra of giants and young M dwarfs (of low gravity) to field M and L dwarfs (of high gravity) and to model spectra from the Lyon group. We find that low-resolution spectra of M4-M9 stars and young brown dwarfs at R ~ 350 and signal-to-noise ratios greater than 70 can determine the spectral type to a precision of ±1 subtype using the H₂O and CO bands and can measure the surface gravity to ±0.5 dex using the atomic lines of K I and Na I. This result points toward the development of photometric spectral indices to separate low-mass members from foreground and background objects in young clusters and associations. We also emphasize the complexity of the interpretation of the empirical quantities (e.g., spectral types) in terms of the physical variables (e.g., temperature, opacities) in the cool atmospheres of young brown dwarfs.

We present a new census of the stellar and substellar members of the young cluster IC 348. We have obtained images at I and Z for a 42' × 28' field encompassing the cluster and have combined these measurements with previous optical and near-infrared photometry. From spectroscopy of candidate cluster members appearing in these data, we have identified 122 new members, 15 of which have spectral types of M6.5-M9, corresponding to masses of ~0.08-0.015 M_☉ by recent evolutionary models. The latest census for IC 348 now contains a total of 288 members, 23 of which are later than M6 and thus are likely to be brown dwarfs. From an extinction-limited sample of members (A_V ≤ 4) for a 16' × 14' field centered on the cluster, we construct an initial mass function (IMF) that is unbiased in mass and nearly complete for M/M_☉ ≥ 0.03 (≲M8). In logarithmic units where the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_☉, and then declines into the substellar regime. In comparison, the similarly derived IMF for Taurus from Briceño et al. and Luhman et al. rises quickly to a peak near 0.8 M_☉ and steadily declines to lower masses. The distinctive shapes of the IMFs in IC 348 and Taurus are reflected in the distributions of spectral types, which peak at M5 and K7, respectively. These data provide compelling, model-independent evidence for a significant variation of the IMF with star-forming conditions.

We present a method for calculating the radiative transfer on a protoplanetary disk perturbed by a protoplanet. We apply this method to determine the effect on the temperature structure within the photosphere of a passive circumstellar disk in the vicinity of a small protoplanet of up to 20 Earth masses. The gravitational potential of a protoplanet induces a compression of the disk material near it, resulting in a decrement in the density at the disk's surface. Thus, an isodensity contour at the height of the photosphere takes on the shape of a well. When such a well is illuminated by stellar irradiation at grazing incidence, it results in cooling in a shadowed region and heating in an exposed region. For typical stellar and disk parameters relevant to the epoch of planet formation, we find that the temperature variation due to a protoplanet at 1 AU separation from its parent star is about 4% (5 K) for a planet of 1 Earth mass, about 14% (19 K) for planet of 10 Earth masses, and about 18% (25 K) for planet of 20 Earth masses, We conclude that even such relatively small protoplanets can induce temperature variations in a passive disk. Therefore, many of the processes involved in planet formation should not be modeled with a locally isothermal equation of state.

In recent years several pairs of extrasolar planets have been discovered in the vicinity of mean-motion commensurabilities. In some cases, such as the GJ 876 system, the planets seem to be trapped in a stationary solution, the system exhibiting a simultaneous libration of the resonant angle θ₁ = 2λ₂ - λ₁ - ϖ₁ and of the relative position of the pericenters. In this paper we analyze the existence and location of these stable solutions, for the 2 : 1 and 3 : 1 resonances, as functions of the masses and orbital elements of both planets. This is undertaken via an analytical model for the resonant Hamiltonian function. The results are compared with those of numerical simulations of the exact equations. In the 2 : 1 commensurability, we show the existence of three principal families of stationary solutions: (1) aligned orbits, in which θ₁ and ϖ₁ - ϖ₂ both librate around zero, (2) antialigned orbits, in which θ₁ = 0 and the difference in pericenter is 180°, and (3) asymmetric stationary solutions, in which both the resonant angle and ϖ₁ - ϖ₂ are constants with values different from 0° or 180°. Each family exists in a different domain of values of the mass ratio and eccentricities of both planets. Similar results are also found in the 3 : 1 resonance. We discuss the application of these results to the extrasolar planetary systems and develop a chart of possible planetary orbits with apsidal corotation. We estimate, also, the maximum planetary masses in order for the stationary solutions to be dynamically stable.

Observations of solar wind electron velocity distribution functions (VDFs) reveal pronounced deviations from a Maxwellian with enhanced numbers of suprathermal electrons. It is shown that these suprathermal tails of the electron VDFs can originate from the solar corona. A model for the acceleration of suprathermal electrons based on resonant interaction with whistler waves is presented. Quasi-linear theory describes this interaction as pitch-angle diffusion in the reference frame of the waves. The waves are assumed to be generated below the coronal base and to propagate antisunward through the corona. Under plasma conditions with high whistler wave phase speeds, the resonant interaction causes electrons to be accelerated from relatively small sunward velocities parallel to the background magnetic field to high speeds perpendicular to the magnetic field. Such plasma conditions are found in the solar corona. A kinetic model is developed to study this acceleration mechanism and the evolution of an electron VDF from the coronal base up into interplanetary space. The kinetic model includes not only the resonant interaction with whistler waves but also Coulomb collisions and the mirror force the electrons experience in the opening magnetic structure of a coronal funnel. The wave absorption of the electrons is also considered to guarantee energy conservation. Kinetic results for the coronal funnel and solar wind are presented. The electron VDFs show deviations from a Maxwellian that are in coincidence with theoretical expectations. The whistler waves generate suprathermal electrons. Toward interplanetary space, the mirror force focuses the electrons toward a narrow "strahl." A comparative study without whistler waves shows that the waves considerably enhance the suprathermal electron fluxes in interplanetary space at 1 AU, as they are observed in the solar wind.

We report on observations acquired by the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO), from 2000 June 10 to June 17 at the time of a SOHO-Sun-Ulysses quadrature. UVCS took data at 1.6 and 1.9 R_☉ with a slit normal to the solar radius and centered along the radial to Ulysses. A streamer complex was sampled by UVCS throughout the quadrature campaign, giving us the opportunity to derive plasma parameters in different streamers and to compare them with plasma properties measured in situ. Large Angle Spectroscopic Coronagraph images above 2 R_☉ helped us understand the temporal evolution of the streamer complex. We derive densities, temperatures, and elemental abundances in two streamers, which have different temperatures and element abundances. In spite of these differences, both structures have the same first ionization potential (FIP) bias. The Fe/O ratio, which may be considered a proxy for the FIP effect, was measured in situ by the Solar Wind Ion Composition Spectrometer aboard the Ulysses spacecraft. Values of Fe/O measured in the corona at the sites where in situ plasma originated agree with in situ Fe/O values.

Observations made with TRACE have detected a class of persistent active region loops that have flat 195/171 Å filter ratios. The intensity of these loops implies a density that is as much as 3 orders of magnitude larger than the densities of static solutions to the hydrodynamic equations. It has recently been suggested that these loops are bundles of impulsively heated strands that are cooling through the TRACE passbands. This scenario implies that the loops would appear in the hotter (Fe XV 284 Å or Fe XII 195 Å) TRACE filter images before appearing in the cooler (Fe IX/X 171 Å) TRACE filter images. In this paper, we test this hypothesis by examining the temporal evolution of five active region loops in multiple TRACE EUV filter images. We find that all the loops appear in the hotter filter images before appearing in cooler filter images. We then use the measured delay to estimate a cooling time and find that four of the five loops have lifetimes greater than the expected lifetime of a cooling loop. These results are consistent with the hypothesis that each apparent loop is a bundle of sequentially heated strands; other explanations will also be discussed. To facilitate comparisons between these loops and hydrodynamic simulations, we use a new technique to estimate the loop length and geometry.

Observations with the Transition Region and Coronal Explorer (TRACE) have revealed a new class of active region loops. These loops have relatively flat filter ratios, suggesting approximately constant temperatures near 1 MK along much of the loop length. The observed apex intensities are also higher than static, uniformly heated loop models predict. These loops appear to persist for much longer than a characteristic cooling time. Recent analysis has indicated that these loops first appear in the hotter Fe XV 284 Å or Fe XII 195 Å filters before they appear in the Fe IX/Fe X 171 Å filter. The delay between the appearance of the loops in the different filters suggests that the loops are impulsively heated and are cooling when they are imaged with TRACE. In this paper we present time-dependent hydrodynamic modeling of an evolving active region loop observed with TRACE. We find that by modeling the loop as a set of small-scale, impulsively heated filaments we can generally reproduce the spatial and temporal properties of the observed loop. These results suggest that both dynamics and filamentation are crucial to understanding the observed properties of active region loops observed with TRACE.

This paper discusses constraints on the magnetic field geometry of solar prominences derived from one-dimensional modeling and analytic theory of the formation and support of cool coronal condensations. In earlier numerical studies we identified a mechanism—thermal nonequilibrium—by which cool condensations can form on field lines heated at their footpoints. We also identified a broad range of field line shapes that can support condensations with the observed sizes and lifetimes: shallowly dipped to moderately arched field lines longer than several times the heating scale. Here we demonstrate that condensations formed on deeply dipped field lines, as would occur in all but the near-axial regions of twisted flux ropes, behave significantly differently than those on shallowly dipped field lines. Our modeling results yield a crucial observational test capable of discriminating between two competing scenarios for prominence magnetic field structure: the flux rope and sheared-arcade models.

Solar type IV radio bursts present a theoretical challenge because they are composed of both continuum emission and fine structures. The latter include "zebra bursts," which appear as harmonically spaced multiplets that shift in frequency with time. Similarities between these features and terrestrial auroral emissions suggest a new model to explain zebra-structured type IV emissions. In this model, the basic generation mechanism is identical with that proposed by Winglee and Dulk: mode conversion of Z-mode waves generated by the cyclotron maser mechanism under the condition f_uh = Nf_ce, with N an integer; however, we propose a twist on this model whereby the "zebra bursts" do not arise from multiple N-values. Rather, the presence of localized density irregularities within the type IV source region leads to trapping of the upper hybrid Z-mode waves in density enhancements, which results in a discrete spectrum of upper hybrid modes with nearly constant frequency spacing. The number m of quasi-harmonics is limited by the trapping (quantization) conditions. The problem is described by an equivalent Schrödinger equation for the trapped mode, which is solved for an (idealized) cylindrical square density irregularity. In this model, the eigenfrequency spacing matches the observed type IV frequency spacings for less than 10% density enhancements with individual scale sizes of 30-1000 thermal electron gyroradii, corresponding to 1-100 m scales in coronal loops. To produce the observed emitted power for a reasonable (<1%) efficiency requires a large number of such individual microscopic sources occurring over a portion of a magnetic type IV loop at a restricted altitude within which the magnetic field and density are approximately constant. The loop plasma in the zebra emission source is thus highly turbulent in the sense that it contains a large number of density fluctuations. In this case transition radiation can effectively contribute to the radiation background and may also provide the wave power required in the upper hybrid range for generating zebra emissions.

Magnetic fields in the solar corona are most likely the dominant source of energy that powers coronal mass ejections (CMEs). Such energy must be above and beyond that of a potential (current-free) magnetic field, and thus the pre-CME coronal magnetic field should contain significant electric currents. Given the diffuse nature of the corona, the coronal magnetic field is likely to be largely force free, implying that electric currents are closely aligned with the field itself. In this work we explore such force-free fields, in the spherical geometry appropriate to the solar corona, with the aim of understanding how the magnetic flux distribution at the coronal base affects the storage of magnetic energy. We find that energy storage is enhanced when a region of strong potential field overlies a nonpotential field whose footpoints are confined to low solar latitudes. Furthermore, those flux distributions consistent with strong overlying potential fields may enable larger energy buildup, when examined in the context of limits imposed by the scalar virial theorem and the Aly-Sturrock theorem. Finally, we demonstrate the existence of force-free fields containing detached flux ropes, with energies that lie above the Aly-Sturrock limit.

We study the evolution of twist and magnetic helicity in the coronal fields of active regions as they emerge. We use multiday sequences of Solar and Heliospheric Observatory Michelson Doppler Interferometer magnetograms to characterize the region's emergence. We quantify the overall twist in the coronal field, α, by matching a linear force-free field to bright coronal structures in EUV images. At the beginning of emergence, all regions studied have α ≃ 0. As the active region grows, α increases and reaches a plateau within approximately 1 day of emergence. The inferred helicity transport rate is larger than differential rotation could produce. Following the 2000 work of Longcope & Welsch, we develop a model for the injection of helicity into the corona by the emergence of a twisted flux tube. This model predicts a ramp-up period of approximately 1 day. The observed time history α(t) is fitted by this model assuming reasonable values for the subphotospheric Alfvén speed. The implication is that helicity is carried by twisted flux tubes rising from the convection zone and transported across the photosphere by spinning of the poles driven by magnetic torque.

We report the detection of free-free (bremsstrahlung) emission near 1200 Å from a flare at the solar limb observed with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The observations consist of a time series of slit spectra at a fixed pointing that lasted almost 2 hr, during which the observed solar region produced a C8 flare. Using the free-free continuum intensities in conjunction with intensities of high-temperature (10⁶-10⁷ K) emission lines that appear in the same wavelength range, we derive the flare plasma electron density, electron temperature, emission measure, and nonthermal mass motions before, during, and after the flare. We describe a new diagnostic method for determining the temperature of cooling plasmas. Because the free-free radiation is emitted primarily by the interaction of electrons with nuclei of H and He atoms, we are also able to derive the Fe/H, Al/H, and Ca/H abundance ratios from the line intensities of highly ionized Fe, Al, and Ca lines and the intensities of the free-free emission, assuming a He abundance. The present work demonstrates the exceptional plasma diagnostic potential of ultraviolet free-free continuum radiation when coupled with emission-line intensities. We demonstrate that a similar technique could be employed to diagnose plasma properties of stellar flares using a high-resolution spectrometer with a sufficiently large effective collecting area.

We investigate the effect of sound-speed perturbations on the characteristics of helioseismic signals in order to demonstrate the feasibility and desirability of wave field helioseismology. The oscillatory nature of the signals with varying frequency content allows us to characterize the waveform of the signals by the amplitudes of the peaks and troughs as well as by the time lags between successive peaks and troughs. Sinusoidal sound-speed perturbations with an amplitude of 1% of the local sound speed produce more than ±10% changes in the amplitudes of the peaks and troughs. The same sound-speed perturbations produce changes in the time lags between successive peaks and troughs of the signals that are comparable with the variations of first-break travel times of the oscillatory wave packet. The vertical and horizontal sound-speed perturbations result in distinct patterns of changes in the shape of the helioseismic signals. These changes in the amplitudes and travel-time lags are differential for the successive peaks and troughs: the magnitude is higher for the later peaks and troughs. We also observe that these variations in the amplitude and travel-time lags are more sensitive to shorter wavelength sound-speed perturbations at relatively small source-receiver offsets. Therefore, our results indicate that the waveform of the signals is variable and sensitive to sound-speed perturbations. These observations can be explained by the more dispersive nature of the low-frequency components in the seismic signals at shallow depths. The relatively high sensitivity of these perturbations in the waveform of the signals to sound speed, density, and vertical density gradient at shallow depths due to dispersion is an important modeling consideration.

In low-temperature plasmas, X-ray line emission is dominated by radiative cascades following recombination and photoexcitation, with a negligible contribution from collisional excitation. In this paper, we present theoretical line formation rate coefficients for all n → 2 (3 ≤ n ≤ 5) transitions of Fe XVII-XXIV in the temperature range between 1 and 500 eV. Both radiative recombination (RR) and ΔN = 0 dielectronic recombination (DR) are taken into account. In addition, the rate coefficients for radiative recombination continua onto n = 2 and 3 states of Fe XVII-XXIV are tabulated. The total RR rate coefficients of Fe XVIII-XXV and total ΔN = 0 DR rate coefficients of Fe XVIII-XXIV are presented as well, and compared with existing experimental measurements and theoretical calculations.

We derive the evolution of the linear bias factor, b(z), in cosmological models driven by an exotic fluid with an equation of state p_x = wρ_x, where -1 ≤ w < 0 (quintessence). Our aim is to put constrains on different cosmological and biasing models by combining the recent observational clustering results of optical (2dF) galaxies (Hawkins et al.) with those predicted by the models. We find that when fitted to the 2dF clustering results, our bias model predicts different bias evolution for different values of w. The models that provide the weak biasing (b₀ ~ 1.1) of optical galaxies found in many recent observational studies are flat, Ω_m = 0.3 with w ≤ -0.9. These models, however, predict a weak redshift evolution of b(z), not corroborated by N-body simulations.

We report on XMM-Newton spectroscopic observations of the luminous radio-quiet quasar PDS 456. The hard X-ray spectrum of PDS 456 shows a deep absorption trough (constituting 50% of the continuum) at energies above 7 keV in the quasar rest frame, which can be attributed to a series of blueshifted K-shell absorption edges due to highly ionized iron. The higher resolution soft X-ray Reflection Grating Spectrometer spectrum exhibits a broad absorption line feature near 1 keV, which can be modeled by a blend of L-shell transitions from highly ionized iron (Fe XVII-Fe XXIV). An extreme outflow velocity of ~50,000 km s^-1 is required to model the K- and L-shell iron absorption present in the XMM-Newton data. Overall, a large column density (N_H = 5 × 10²³ cm^-2) of highly ionized gas (log ξ = 2.5) is required in PDS 456. A high-mass outflow rate of ~10 M_☉ yr^-1 (assuming a conservative outflow covering factor of 0.1 sr) is derived, which is of the same order as the overall mass accretion rate in PDS 456. The kinetic energy of the outflow represents a substantial fraction (~10%) of the quasar energy budget, while the large column and outflow velocity place PDS 456 toward the extreme end of the broad absorption line quasar population.

We report the detection of very broad H I absorption against the central regions of the radio galaxy 3C 293. The absorption profile, obtained with the Westerbork Synthesis Radio Telescope, has a full width at zero intensity of about 1400 km s^-1, and most of this broad absorption (~1000 km s^-1) is blueshifted relative to the systemic velocity. This absorption represents a fast outflow of neutral gas from the central regions of this active galactic nucleus. Possible causes for such an outflow are discussed. We favor the idea that the interaction between the radio jet and the rich interstellar medium produces this outflow. Some of the implications of this scenario are considerebd.

We report the discovery of eight new extremely metal-poor galaxies [XMPGs; 12 + log(O/H) < 7.65] and the recovery of four previously known or suspected XMPGs (I Zw 18, HS 0822+3542, HS 0837+4717, and A1116+517) using Sloan Digital Sky Survey (SDSS) spectroscopy. These new objects were identified after an analysis of 250,000 galaxy spectra within an area of ~3000 deg² on the sky. Our oxygen abundance determinations have an accuracy of ≤0.1 dex and are based on the temperature-sensitive [O III] λ4363 line and on the direct calculation of the electron temperature. We briefly discuss a new method of oxygen abundance determinations using the [O II] λλ7319, 7330 lines that is particularly useful for SDSS emission-line spectra with redshifts ≤0.024 since the [O II] λ3727 emission line falls outside of the SDSS wavelength range. We detect XMPGs with redshifts ranging from 0.0005 to 0.0443 and M_g luminosities from -12.4 to -18.6 mag. Our eight new XMPGs increase the number of known metal-deficient galaxies by approximately one-quarter. The estimated surface density of XMPGs is 0.004 deg^-2 for r ≤ 17.77 mag.

Deep infrared observations and long-term monitoring programs have provided dynamical evidence of a supermassive black hole of mass 3 × 10⁶M_☉ associated with the radio source Sagittarius A* at the center of our Galaxy. The brightest stars orbiting within 0.1 pc of the black hole appear to be young, massive main-sequence stars, in spite of an environment near the black hole that is hostile to star formation. We discuss mechanisms by which stars born outside the central parsec can sink toward the black hole and conclude that the drag coming from plausible stellar populations does not operate on the short timescales required by the stellar ages. We propose that these stars were dragged in by a second black hole of mass ~10³-10⁴M_☉, which would be classified as an intermediate-mass black hole. We discuss the implications for the stellar populations and the kinematics in the Galactic center. Finally, we note that continued astrometric monitoring of the central radio source offers us the prospect for a direct detection of such objects.

We calculate the time evolution of the flux, apparent size, and image centroid motion of gamma-ray burst (GRB) radio jets and show that they can be resolved by the Very Long Baseline Array (VLBA) at distances of hundreds of megaparsecs. We find that GRB 030329, which showed spectroscopic evidence for an associated Type Ic supernova (SN) at a distance of ≈800 Mpc, might just be resolvable by VLBA after several months. The prospects are much better for jets that are oriented sideways in similar SNe with no GRB counterpart; in particular, the motion of the flux centroid in such jets can be detected by the VLBA up to z ~ 1, even when the jet cannot be resolved. If most GRBs are accompanied by a Type Ib/c SN, then there should be a few SN/GRB jets per year within a distance ≲200 Mpc, and most of them would be oriented sideways with no gamma-ray or X-ray precursor. Detection of these jets can be used to calibrate the fraction of all core-collapse SNe that produce relativistic outflows and determine the local GRB rate. Overall, the rate of Type Ib/c SNe that do not produce a GRB at all, but rather make relativistic radio jets with an initial Lorentz factor of a few, may be larger by up to 2 orders of magnitude than the rate of those that produce GRBs.

A radio afterglow was detected following the 1998 August 27 giant flare from the soft gamma repeater (SGR) 1900+14. Its short-lived behavior is quite different from the radio nebula of SGR 1806-20, but very similar to radio afterglows from classic gamma-ray bursts (GRBs). Motivated by this, we attempt to explain it with the external shock model as invoked in the standard theory of GRB afterglows. We find that the light curve of this radio afterglow is not consistent with the forward shock emission of an ultrarelativistic outflow, which is suggested to be responsible for the initial hard spike of the giant flare. Nevertheless, shock emission from a mildly or subrelativistic outflow expanding into the interstellar medium could fit the observations. The possible origin for this kind of outflow is discussed, based on the magnetar model for SGRs. Furthermore, we suggest that the presence of an ultrarelativistic fireball from SGR giant flares could be tested by rapid radio to optical follow-up observations in the future.

We present the result of astrometric observations of the radio pulsar PSR B0656+14, made using the Very Long Baseline Array. The parallax of the pulsar is π = 3.47 ± 0.36 milliarcsec, yielding a distance 288 pc. This independent distance estimate has been used to constrain existing models of thermal X-ray emission from the neutron star's photosphere. Simple blackbody fits to the X-ray data formally yield a neutron star radius R_∞ ~ 7-8.5 km. With more realistic fits to a magnetized hydrogen atmosphere, any radius between ~13 and ~20 km is allowed.

We show that the giant flares of soft gamma-ray repeaters (E ~ 10⁴⁴ ergs) can push the inner regions of a fall-back disk out to larger radii by radiation pressure, while matter remains bound to the system for plausible parameters. The subsequent relaxation of this pushed-back matter can account for the observed enhanced X-ray emission after the August 27 giant flare of SGR 1900+14. Based on the results of our models, we estimate that the ratio of the fluences of the enhanced X-ray emissions to that of the preceding bursts remains constant for a particular SGR with similar preburst inner-disk conditions, which is consistent with the four different burst observations of SGR 1900+14.

This work was carried out using the Caltech Submillimeter Observatory and presents the observational study of HDS and D₂S toward a sample of Class 0 sources and dense cores. We report the first detection of doubly deuterated hydrogen sulfide (D₂S) in two dense cores and analyze the chemistry of these molecules, aiming to help understand the deuteration processes in the interstellar medium. The observed values of the D₂S/HDS ratio and upper limits require an atomic D/H ratio in the accreting gas of 0.1-1. The study presented in this Letter supports the hypothesis that formaldehyde, methanol, and hydrogen sulfide are formed on the grain surfaces, during the cold prestellar core phase, where the CO-depleted gas has large atomic D/H ratios. The high values for the D/H ratios are consistent with the predictions of a recent gas-phase chemical model that includes H and its deuterated isotopomers, H₂D⁺, D₂H⁺, and D (Roberts, Herbst, & Millar).

We report the detection of continuum emission at λ = 850 and 450 μm from disks around four classical T Tauri stars in the MBM 12 (L1457) young association. Using a simple model, we infer masses of 0.0014-0.012 M_☉ for the disk of LkHα 263 ABC, 0.005-0.021 M_☉ for S18 ABab, 0.03-0.18 M_☉ for LkHα 264 A, and 0.023-0.23 M_☉ for LkHα 262. The disk mass found for LkHα 263 ABC is consistent with the 0.0018 M_☉ inferred from the scattered-light image of the edge-on disk around component C. Comparison to earlier ¹³CO line observations indicates CO depletion by up to a factor of 300 with respect to dark-cloud values. The spectral energy distributions (SEDs) suggest grain growth, possibly to sizes of a few hundred microns, but our spatially unresolved data cannot rule out opacity as an explanation for the SED shape. Our observations show that these T Tauri stars are still surrounded by significant reservoirs of cold material at an age of 1-5 Myr. We conclude that the observed differences in disk mass are likely explained by binary separation affecting the initial value. With available accretion rate estimates we find that our data are consistent with theoretical expectations for viscously evolving disks having decreased their masses by ~30%.

We present the serendipitous detection of an extreme-ultraviolet flare on EUVE J0613-23.9B. The flare showed over a 200-fold increase above the quiescent emission in the DS/Lexan 60-200 Å wavelength band. Optical spectroscopy revealed that the event was associated with an active dM3.5e star. The EUVE spectra are dominated by emission lines formed at temperatures in excess of 10⁷ K. The observation is unique as we have detected, for the first time, a strong Lyman continuum in the EUVE long-wavelength range (320-650 Å). The flare in the continuum (T ≈ 20,000-30,000 K) was extremely short, lasting for less than 500 s, while in the DS (T ≈ 10⁷ K) its duration was ≈28 ks. The total energy of the flare in the DS is ~3 × 10³⁴ ergs. We have made a fit to the continuum using semiempirical model atmospheres and derived the time-averaged temperature and density structures.

We present high-resolution optical spectroscopy of four candidate members of the nearby TW Hydrae young association including three brown dwarfs (2MASS 1207-3932, 2MASS 1139-3159, and TWA 5B) and one T Tauri multiple star (TWA 5A). Using echelle spectra from the Magellan Baade 6.5 m telescope, we confirm the pre-main-sequence status and cluster membership of the substellar candidates, through the detection of Li I, Na I consistent with low gravity, and radial velocity. Given their late spectral type (~M8) and the youth of the association (age ~ 10 Myr), cluster membership certifies these three objects as young brown dwarfs. One of them (2MASS 1207-3932) shows strong emission both in the hydrogen Balmer series (Hα to H) and in He I (4471, 5876, 6678, and 7065 Å), compared with other young brown dwarfs of similar spectral type. The Hα line is also relatively broad (10% width ~200 km s^-1) and asymmetric. These characteristics suggest that 2MASS 1207-3932 is a (weak) accretor. While we cannot rule out activity, comparison with a flaring field dwarf implies that such activity would have to be quite anomalous. The verification of accretion would make it the oldest actively accreting brown dwarf known to date, suggesting that inner-disk lifetimes in substellar objects can be comparable to those in stars, consistent with a similar formation mechanism. The close triple TWA 5A also appears to be a variable accretor, implying that long-lived disks can exist in multiple systems.

We present intermediate-resolution (R = 1500) near-infrared spectroscopy from 1.17 to 1.37 μm of the spectral type T planetary candidate member in the σ Orionis cluster S Ori 70 reported by Zapatero Osorio et al. The new data have been obtained with NIRSPEC at the Keck II telescope. The best fit of our mid-resolution spectrum of S Ori 70 with theoretical spectra gives log g = 3.5 ± 0.5 cm s^-2 and T_eff = 1100 K. The low gravity of this object derived from spectral synthesis supports its youth and membership to the young σ Orionis cluster. Using evolutionary models for an age of 3 Myr, we obtain a mass of 3 M_Jup and a radius of 0.16 R_☉, independent of the distance to the object. Our analysis confirms that S Ori 70 is the lowest mass cluster planet so far identified in the galaxy.

We propose that the large photometric variations of KH 15D are due to an eclipsing swarm of solid particles, trapped in a giant gaseous vortex rotating at ~0.2 AU from the star. The efficiency of the capture-in-vortex mechanism easily explains the observed large optical depth. The weaker opacity at mid-eclipse is consistent with a size segregation and a slow concentration of the particles toward the center of the vortex. This dusty structure must extend over ~ of an orbit to account for the long eclipse duration. The estimated size of the trapped particles is found to range from 1 to 10 cm, consistent with the gray extinction of the star. The observations of KH 15D support the idea that giant vortices can grow in circumstellar disks and play a central role in planet formation.

Over the last decade, the pre-main-sequence star KH 15D has exhibited periodic eclipses that are surprisingly deep (~3 mag) and long-lasting (~40% of the 48.4 day period). The cause of the eclipses is unknown, but it could be a feature in a nearly edge-on protoplanetary disk. Here we report on an analysis of archival photographs of KH 15D from the Harvard College Observatory plate collection, most of which were taken during the years 1913-1951. During this time range, the data are consistent with no eclipses; the duty cycle of 1 mag eclipses was less than 20%. The decadal timescale of this change in eclipse behavior is compatible with the expected timescale of protoplanet/disk interactions. Archival images from more recent epochs should reveal the onset of the eclipses.

Ground-based searches for transiting Jupiter-sized planets have so far produced few detections of planets but many of stellar systems with eclipse depths, durations, and orbital periods that resemble those expected from planets. The detection rates prove to be consistent with our present knowledge of binary and multiple-star systems and of Jovian-mass extrasolar planets. Space-based searches for transiting Earth-sized planets will be largely unaffected by the false alarm sources that afflict ground-based searches, except for distant eclipsing binaries whose light is strongly diluted by that of a foreground star. A by-product of the rate estimation is evidence that the period distribution of extrasolar planets is depressed for periods between 5 and 200 days.

Nucleobases are nitrogen heterocycles (N-heterocycles) that are essential components of the genetic material in all living organisms. Extraterrestrial nucleobases have been found in several carbonaceous chondrites, but only in traces. No astronomical data on these complex molecules are currently available. A large fraction of the cosmic carbon is known to be incorporated into aromatic material, and given the relatively high abundance of cosmic nitrogen, the presence of N-heterocycles can be expected. We present infrared spectroscopic laboratory data of adenine and uracil under simulated space conditions. At the same time we tested the stability of these nucleobases against ultraviolet (UV) irradiation at 12 K. Our experimental results indicate that gas-phase adenine and uracil will be destroyed within hours in the Earth's vicinity. In dense interstellar clouds exposed to UV radiation, only adenine could be expected to survive for a few million years. We discuss possible formation routes to purines and pyrimidines in circumstellar environments and in meteorite parent bodies.

We studied solar coronal X-ray jets by MHD numerical simulations with thermal conduction effects based on the magnetic reconnection model. Key physical processes are included, such as the emergence of magnetic fluxes from the convection zone, magnetic reconnection with the coronal magnetic fields, heat conduction to the chromosphere, and chromospheric evaporation. High-density evaporation jets were successfully reproduced in the simulations. The mass of the evaporation jets M is described as M = 6.8 × 10¹² g(B/10 G)^15/7(T_cor/10⁶ K)^5/14 × (s_flare/5000 km)^12/7(t/400 s), where B is the magnetic field strength, T_cor is the coronal temperature, s_flare is the loop height, and t is the duration of the ejection.

We present the study of an eruption from the low solar atmosphere (photosphere/chromosphere) as seen in Transition Region and Coronal Explorer 1600 Å images and with the Solar and Heliospheric Observatory Michelson Doppler Imager. The eruption reached its maximum at 20:08 UT on 2002 July 15 in the NOAA Active Region 10030 (N19°, W01°), accompanied by an X3 flare and followed by a fast-halo coronal mass ejection. The main observational results from the data are as follows: (1) the erupting plasma was in a rapidly rising, twisted ropelike structure; (2) the eruption occurred just preceding the onset of its driven flare; and (3) the morphology and magnetic flux of one slender footpoint (~9000 km in length) of the rope developed rapidly on the photosphere. This structure disappeared in white light and in the magnetograms within 60 minutes. This evidence supports the erupting flux rope model. Our data favor the idea that a catastrophic loss of MHD equilibrium can be the primary driving mechanism for the rapid ejection of a flux rope. This conclusion is based on the judgment that the ambient fields of the flux rope were partly opened as a result of the magnetic reconnection.

The pure rotational spectrum of FeN in its X²Δ_i ground state has been recorded using millimeter-wave direct absorption techniques in the range 198-525 GHz. New measurements have also been carried out for FeC (X³Δ_i), in particular of the Ω = 1 fine-structure component and the ⁵⁴FeC isotopomer. These molecules were created by the reaction of iron vapor with either CH₄ (FeC) or N₂ (FeN) in a DC discharge. Eight rotational transitions were recorded for FeN in its lowest-lying spin component, Ω = 5/2, and multiple transitions were measured for FeC in all three of its spin-orbit ladders, as well as for ⁵⁴FeC (Ω = 3). These data have been analyzed, and precise spectroscopic constants for both radicals have been determined. The fine structure in FeC was found to exhibit an irregular pattern, indicating that higher order spin-orbit perturbations are occurring in this molecule. Although only one spin component was observed for FeN, the bond length established from the Ω = 5/2 data is consistent with a ²Δ ground state, as indicated by theory. Fe-bearing species are relevant to many astrophysical topics, including astrochemistry, dust grains composition, nucleosynthesis, and mass loss from asymptotic giant branch stars.