Table of contents

We examine power spectra from the Abell/ACO rich cluster survey and the Two-Degree Field Galaxy Redshift Survey (2dFGRS) for observational evidence of features produced by the baryons. A nonnegligible baryon fraction produces relatively sharp oscillatory features at specific wavenumbers in the matter power spectrum. However, the mere existence of baryons will also produce a global suppression of the power spectrum. We look for both of these features using the false discovery rate statistic. We show that the window effects on the Abell/ACO power spectrum are minimal, which has allowed for the discovery of discrete oscillatory features in the power spectrum. On the other hand, there are no statistically significant oscillatory features in the 2dFGRS power spectrum, which is expected from the survey's broad window function. After accounting for window effects we apply a scale-independent bias to the 2dFGRS power spectrum, P_Abell(k) = b²P_2dF(k) and b = 3.2. We find that the overall shapes of the Abell/ACO and the biased 2dFGRS power spectra are entirely consistent over the range 0.02 ≤ k ≤ 0.15 h Mpc^-1. We examine the range of Ω_matter and baryon fraction, for which these surveys could detect significant suppression in power. The reported baryon fractions for both the Abell/ACO and 2dFGRS surveys are high enough to cause a detectable suppression in power (after accounting for errors, windows, and k-space sampling). Using the same technique, we also examine, given the best-fit baryon density obtained from big bang nucleosynthesis, whether it is possible to detect additional suppression due to dark matter-baryon interaction. We find that the limit on dark matter cross section/mass derived from these surveys is the same as those ruled out in a recent study by Chen, Hannestad, & Scherrer.

Recent observations of high-redshift quasar spectra reveal long gaps with little flux. A small or no detectable flux does not by itself imply that the intergalactic medium (IGM) is neutral. Inferring the average neutral fraction from the observed absorption requires assumptions about clustering of the IGM, which the gravitational instability model supplies. Our most stringent constraint on the neutral fraction at z ~ 6 is derived from the mean Lyβ transmission measured from the z = 6.28 Sloan Digital Sky Survey quasar of Becker and coworkers; the neutral hydrogen fraction at mean density has to be larger than 4.7 × 10^-4. This is substantially higher than the neutral fraction of ~(3-5) × 10^-5 at z = 4.5-5.7, suggesting that dramatic changes take place around or just before z ~ 6, even though current constraints are still consistent with a fairly ionized IGM at z ~ 6. These constraints also translate into constraints on the ionizing background, subject to uncertainties in the IGM temperature. An interesting alternative method to constrain the neutral fraction is to consider the probability of having many consecutive pixels with little flux, which is small unless the neutral fraction is high. It turns out that this constraint is slightly weaker than the one obtained from the mean transmission. We show that while the derived neutral fraction at a given redshift is sensitive to the power-spectrum normalization, the size of the jump around z ~ 6 is not. We caution that the main systematic uncertainties include spatial fluctuations in the ionizing background and the continuum placement. Tests are proposed. In particular, the sight line-to-sight line dispersion in mean transmission might provide a useful diagnostic. We express the dispersion in terms of the transmission power spectrum and develop a method to calculate the dispersion for spectra that are longer than the typical simulation box.

We create stacked composite absorption spectra from Hubble Space Telescope Faint Object Spectrograph data from four quasi-stellar objects to search for absorption lines in the extreme-ultraviolet wavelength region associated with Lyα forest absorbers in the redshift range 1.6 < z < 2.9. We successfully detect O V λ630 in Lyα absorbers throughout the 10¹³-10^16.2 cm^-2 column density range. For a sample of absorbers with 10^13.2 cm^-2 < N(H I) < 10^14.2 cm^-2, corresponding to gas densities ranging from around the universal mean to overdensities of a few, we measure an O V λ630 equivalent width of 10.9 ± 3.7 mÅ. We estimate that the detection is real with at least 99% confidence. We only detect O IV λ788, O IV λ554, O III λ833, and He I λ584 in absorbers with Lyα equivalent widths ≳0.6 Å, which are likely associated with traditional metal line systems. We find no evidence in any subsamples for absorption from N IV λ765, Ne V λ568, Ne VI λ559, Ne VIII λλ770, 780, or Mg X λλ610, 625. The measured equivalent widths of O V suggest values of ⟨O V/H I⟩ in the range -1.7 to -0.6 for 10^13.2 cm^-2 < N(H I) ≲ 10¹⁵ cm^-2. The lack of detectable O IV absorption except in the strongest absorption systems suggests a hard ionizing background similar to the standard Haardt & Madau spectrum. Using photoionization models, we estimate that the oxygen abundance in the intergalactic medium with respect to the solar value is [O/H] ≈ -2.2 to -1.3. Comparing to studies of C IV, we estimate [O/C] ≈ 0.3-1.2. The overabundance of oxygen relative to carbon agrees with other low-metallicity abundance measurements and suggests enrichment of the intergalactic medium by Type II supernovae.

In this paper, the continuum emission of active galactic nuclei is studied using broadband B, V, R, and I photometry. The nuclear contribution is estimated from the observations using two different approaches. In the first one, the images are deconvolved by the seeing profile and the corrected images are used to derive the nuclear contribution. In the second method, in order to extract the stellar contribution, a de Vaucouleurs brightness profile is assumed for the bulge. After subtraction of this component, the total nuclear emission is obtained from the corrected image. Both methods indicate that the stellar contribution is dominant. The average contribution of the featureless continuum to the total observed continuum is 30% in the four bands. We show that over 2 orders of magnitude the Hα emission-line luminosity correlates with the continuum emission in all the observed bands. As a consequence, the galaxies in the sample must have similar color index. This result provides a method to estimate the reddening correction for the nuclear continuum, which we found to be lower than the emission-line extinction correction estimated from the observed Hα/Hβ observed line ratio.

Black hole mass, along with mass accretion rate, is a fundamental property of active galactic nuclei (AGNs). Black hole mass sets an approximate upper limit to AGN energetics via the Eddington limit. We collect and compare all AGN black hole mass estimates from the literature; these 177 masses are mostly based on the virial assumption for the broad emission lines, with the broad-line region size determined from either reverberation mapping or optical luminosity. We introduce 200 additional black hole mass estimates based on properties of the host galaxy bulges, using either the observed stellar velocity dispersion or the fundamental plane relation to infer σ; these methods assume that AGN hosts are normal galaxies. We compare 36 cases for which black hole mass has been generated by different methods and find, for individual objects, a scatter as high as a couple of orders of magnitude. The less direct the method, the larger the discrepancy with other estimates, probably due to the large scatter in the underlying correlations assumed. Using published fluxes, we calculate bolometric luminosities for 234 AGNs and investigate the relation between black hole mass and luminosity. In contrast to other studies, we find no significant correlation of black hole mass with luminosity, other than those induced by circular reasoning in the estimation of black hole mass. The Eddington limit defines an approximate upper envelope to the distribution of luminosities, but the lower envelope depends entirely on the sample of AGNs included. For any given black hole mass, there is a range in Eddington ratio of up to 3 orders of magnitude.

Hubble Space Telescope (HST) ultraviolet Space Telescope Imaging Spectrograph (STIS) imaging and spectroscopy of the low-luminosity active galactic nucleus (LLAGN) NGC 4303 have identified the previously detected UV-bright nucleus of this galaxy as a compact, massive, and luminous stellar cluster. The cluster with a size (FWHM) of 3.1 pc and an ultraviolet luminosity log L_{1500 Å}(ergs s^-1 Å^-1) = 38.33 is identified as a nuclear super-star cluster (SSC) like those detected in the circumnuclear regions of spirals and starburst galaxies. The UV spectrum showing the characteristic broad P Cygni lines produced by the winds of massive young stars is best fitted by the spectral energy distribution of a massive cluster of 10⁵M_☉ (for a Salpeter initial mass function law with lower mass cutoff of 1 M_☉) generated in an instantaneous burst 4 Myr ago. The ionizing energy produced by this cluster exceeds the flux needed to explain the nuclear Hα luminosity. No evidence for an additional nonthermal ionizing source associated with an accreting black hole is detected in the ultraviolet. These new HST/STIS results unambiguously show the presence of a compact SSC in the nucleus of a low-luminosity AGN, which is also its dominant ionizing source. We hypothesize that at least some LLAGNs in spirals could be understood as the result of the combined ionizing radiation emitted by an evolving SSC (i.e., determined by the mass and age) and a black hole accreting with low radiative efficiency (i.e., radiating at low sub-Eddington luminosities) coexisting in the inner few parsecs region. Complementary multifrequency studies give the first hints of the very complex structure of the central 10 pc of NGC 4303, where a young SSC apparently coexists with a low-efficiency accreting black hole and with an intermediate/old compact star cluster and where, in addition, an evolved starburst could also be present. If structures such as those detected in NGC 4303 are common in the nuclei of spirals, the modeling of the different stellar components and their contribution to the dynamical mass has to be established accurately before deriving any firm conclusion about the mass of central black holes of few to several million solar masses.

We investigate the relationship between accretion rates and the spectral energy distributions (SEDs) of BL Lac objects using a sample of objects for which published information on the host galaxies, emission-line luminosities, and peak frequencies and luminosities of their SEDs are available. The sample is composed of 43 BL Lac objects that have a relatively continuous distribution of peak frequencies. Under the assumption that the observed emission lines are photoionized by the central accretion disk, we use the line luminosities to estimate the accretion luminosities and hence accretion rates. We find that low-frequency-peaked BL Lac objects (LBLs) span a wide range of accretion rates, whereas high-frequency-peaked BL Lac objects (HBLs) cover a more restricted range of lower values. There appears to be a continuous distribution of accretion rates between the two subclasses of BL Lac objects. We find that the peak frequency of the SED, ν_pk, correlates with the accretion rate, approximately with the form ν_pk ∝ Λ^-3 in HBLs and ν_pk ∝ Λ^-0.25 in LBLs, where Λ ≡ L_lines/c². The peak luminosity of the SED is also correlated with Λ. These results suggest that the accretion rate influences the shape of the SED in BL Lac objects. They also support models that couple the jet and the accretion disk. We present a physical scenario to account for the empirical trends.

We report subarcsecond X-ray imaging spectroscopy of M87 and the core of the Virgo Cluster with the Chandra X-Ray Observatory. The X-ray morphology shows structure on arcsecond (~100 pc) to 10' (~50 kpc) scales, the most prominent feature being an "arc" running from the east, across the central region of M87, and off to the southwest. A ridge in the radio map, ending in an "ear-shaped" structure, follows the arc to the east. Depressions in the X-ray surface brightness correspond to the inner radio lobes, and there is no evidence of shock-heated gas surrounding them. There are also at least two approximately circular (centered near the nucleus) "edges" in the X-ray brightness distribution, the radii of which are slightly larger than the nuclear distances of the inner radio lobes and intermediate radio ridges. We speculate that these discontinuities may be spherical pulses or "fronts" driven by the same jet activity as is responsible for the radio structure; such pulses are found in recent numerical simulations. All these results provide good evidence that the nuclear activity affects the intracluster medium. We present a temperature map of the intracluster medium and obtain the temperature, pressure, and cooling time as a function of nuclear distance for the arcs and the ambient intracluster medium. We show that the gas in the arcs is cooler than, and probably overpressured with respect to, the ambient intracluster medium. The metal abundances of the cooler gas in the arc are somewhat enhanced relative to the ambient intracluster medium, favoring a "buoyant plume" origin for the X-ray arc, in which ambient gas near the nucleus is entrained by buoyant radio plasma and carried to larger nuclear distances. The gas within the inner front (nuclear distance ≃3.5 kpc ≃ 45'') is at least a two-temperature plasma, with the cool component at ≃1 keV. This cool region is concentrated to the north of the nucleus and is strongly correlated with the Hα+[N II] emission line distribution.

We investigate the predicted present-day temperature profiles of the hot, X-ray-emitting gas in galaxy clusters for two cosmological models—a current best-guess ΛCDM model and a standard cold dark matter (SCDM) model. Our numerically simulated catalogs of clusters are derived from high-resolution (15 h^-1 kpc) simulations which make use of a sophisticated, Eulerian-based, adaptive mesh-refinement code that faithfully captures the shocks that are essential for correctly modeling cluster temperatures. We show that the temperature structure on Mpc scales is highly complex and non-isothermal. However, the temperature profiles of the simulated ΛCDM and SCDM clusters are remarkably similar and drop off as T ∝ (1 + r/a_x)^-δ, where a_x ~ r_vir/1.5 and δ ~ 1.6. This decrease is in good agreement with the observational results of Markevitch et al. but diverges, primarily in the innermost regions, from their fit which assumes a polytropic equation of state. Our result is also in good agreement with a recent sample of clusters observed by BeppoSAX, though there is some indication of missing physics at small radii (r < 0.2 r_vir). We discuss the interpretation of our results and make predictions for new X-ray observations that will extend to larger radii than previously possible. Finally, we show that for r > 0.2 r_vir, our universal temperature profile is consistent with our most recent simulations, which include both radiative cooling and supernovae feedback.

The ESO Distant Cluster Survey (EDisCS) is a photometric and spectroscopic study of the galaxy cluster population at two epochs, z ≃ 0.5 and z ≃ 0.8, drawn from the Las Campanas Distant Cluster Survey (LCDCS). We report results from the initial candidate-confirmation stage of the program and use these results to probe the properties of the LCDCS. Of the 30 candidates targeted, we find statistically significant overdensities of red galaxies near 28. Of the 10 additional candidates serendipitously observed within the fields of the targeted 30, we detect red galaxy overdensities near six. We test the robustness of the published LCDCS estimated redshifts to misidentification of the brightest cluster galaxy (BCG) in the survey data and measure the spatial alignment of the published cluster coordinates, the peak red galaxy overdensity, and the BCG. We conclude that for LCDCS clusters out to z ~ 0.8, (1) the LCDCS coordinates agree with the centroid of the red galaxy overdensity to within 25'' (~150 h^-1 kpc) for 34 out of 37 candidates with 3 σ galaxy overdensities, (2) BCGs are typically coincident with the centroid of the red galaxy population to within a projected separation of 200 h^-1 kpc (32 out of 34 confirmed candidates), (3) the red galaxy population is strongly concentrated, and (4) the misidentification of the BCG in the LCDCS causes a redshift error greater than 0.1 in 15%-20% of the LCDCS candidates. These findings together help explain the success of the surface brightness fluctuations detection method.

The Rossi X-Ray Timing Explorer satellite observed the Coma Cluster for ~177 ks during 2000 November and December, a second observation motivated by the intriguing results from the first ~87 ks observation in 1996. Analysis of the new data set confirms that thermal emission from isothermal gas does not provide a good fit to the spectral distribution of the emission from the inner 1° radial region. While the observed spectrum may be fitted by emission from gas with a substantial temperature gradient, it is more likely that the emission also includes a secondary nonthermal component. If so, nonthermal emission comprises ~8% of the total 4-20 keV flux. Interpreting this emission as due to Compton scattering of relativistic electrons (which produce the known extended radio emission) by the cosmic microwave background radiation, we determine that the mean, volume-averaged magnetic field in the central region of Coma is B ~ 0.1-0.3 μG.

We present deep broadband R and narrowband Hα images of Stephan's Quintet. The observations in the R band show that the diffuse halo of Stephan's Quintet is larger than previously thought and extends out to NGC 7320C. However, we have not found emission connecting NGC 7331 and NGC 7320 to R ~ 26.7 mag arcsec^-2 (at more than a 3 σ level), so there is no direct evidence up to this limiting magnitude of a relation between the peculiar kinematical structure found in NGC 7331 and an ongoing or past interaction between this galaxy and NGC 7320. The Hα emission at high velocity (6000-7000 km s^-1) is distributed in a diffuse structure running north-south between NGC 7319 and NGC 7318B and in some other more concentrated features. Some of these are located in the tidal tails produced by the interaction between the galaxies of the group. With the Hα images we have made a two-dimensional velocity map that helps to identify the origin of each structure detected. This map does not show features at intermediate velocities between the high- and low-redshift members of the group. This is in agreement with the standard scenario in which the apparent proximity of NGC 7320 to the rest of the galaxies of the Quintet is merely a projection effect. The only point that is unclear in this interpretation is an Hα filament that is seen extending throughout NGC 7320 with velocity at 6500 km s^-1 instead of the 800 km s^-1 expected for this galaxy.

We present the first high spectral resolution X-ray observation of the giant elliptical galaxy NGC 4636, obtained with the reflection grating spectrometer on board the XMM-Newton Observatory. The resulting spectrum contains a wealth of emission lines from various charge states of oxygen, neon, magnesium, and iron. Examination of the cross-dispersion profiles of several of these lines provides clear, unambiguous evidence of resonance scattering by the highest oscillator strength lines, as well as a weak temperature gradient in the inner regions of the interstellar medium. We invoke a sophisticated new Monte Carlo technique that allows us to properly account for these effects in performing quantitative fits to the spectrum. Our spectral fits are not subject to many of the systematics that have plagued earlier investigations. The derived metal abundances are higher than have usually been inferred from prior, lower spectral resolution observations of this source, but are still incompatible with conventional chemical-enrichment models of elliptical galaxies. In addition, our data are incompatible with standard cooling-flow models for this system; our derived upper limit to the mass deposition rate is below the predicted value by a factor of 3-5.

We study the angular power spectra of the polarized component of the Galactic synchrotron emission in the 28 deg² test region of the Southern Galactic Plane Survey at 1.4 GHz. These data were obtained by the Australia Telescope Compact Array and allow us to investigate angular power spectra down to arcminute scales. We find that, at this frequency, the polarization spectra for E- and B-modes seem to be affected by Faraday rotation produced in compact foreground screens. A different behavior is shown by the angular spectrum of the polarized intensity PI = (Q² + U²)^1/2. This is well fitted by a power law (C_PIℓ ∝ ℓ^−α_PI) with slope ~1.7, which agrees with higher frequency results and can probably be more confidently extrapolated to the cosmological window.

We report Sr, Pd, and Ag abundances for a sample of metal-poor field giants and analyze a larger sample of Y, Zr, and Ba abundances. The [Y/Zr] and [Pd/Ag] abundance ratios are similar to those measured for the r-process-rich stars CS 22892-052 and CS 31082-001. The [Pd/Ag] ratio is larger than predicted from the solar system r-process abundances. The constant [Y/Zr] and [Sr/Y] values in the field stars place strong limits on the contributions of the weak s-process and the main s-process to the light neutron-capture elements. Stars in the globular cluster M15 possess lower [Y/Zr] values than the field stars. There is a large dispersion in [Y/Ba]. Because the r-process is responsible for the production of the heavy elements in the early Galaxy, these dispersions require varying light-to-heavy ratios in r-process yields.

We use the classical r-process model to explore the implications of the recently reported first observation of U in the extremely metal-poor, r-process element-enriched halo star CS 31082-001 for U and Th cosmochronometry. Using updated nuclear physics input and performing a new, conservative, analysis of the remaining uncertainties in the classical r-process model, we confirm that U (together with Th) abundance observations in metal-poor stars are a promising tool for dating r-process events in the early Galaxy, independent of assumptions on Galactic chemical evolution. We show that nuclear physics uncertainties limit the present accuracy of estimated U/Th ages to about 2 Gyr. Critical nuclear data that are required to lower this uncertainty include β-delayed fission branchings and reliable predictions of the onset of deformation in the vicinity of the N = 184 shell closure around ²⁴⁴Tl, as both directly affect predicted U/Th ratios in r-process models. In this paper we apply, for the first time, the new HFBCS-1 mass model within the framework of the classical r-process model. We find that the predicted U and Th abundances are incompatible with the solar U and Th abundances and trace this back to a different prediction of the onset of deformation around ²⁴⁴Tl. In the case of CS 31082-001, we find it likely that the zero-age U and Th abundances were enhanced by about a factor of 2.5 compared to both (1) a theoretical extrapolation from the observed stable elements using the classical r-process model and (2) the zero-age abundances of Th and U in other r-process-enhanced, metal-poor halo stars. Although presently ad hoc, this "actinide boost" assumption solves the apparent problem of the relative age difference compared with other metal-poor halo stars and, at the same time, the problem of the inconsistency of ages based on U/(stable nucleus), Th/(stable nucleus) and U/Th ratios. There clearly exist differences, among some r-process-enhanced, metal-poor stars, in the level of the elemental abundances of actinides beyond the third r-process peak. Whether CS 31082-001 is a relatively rare case or commonplace awaits the identification of larger numbers of r-process-enhanced, metal-poor stars in which both U and Th can be measured. Using the U/Th ratio, we obtain a best age estimate for the r-process elements in CS 31082-001 of 15.5 ± 3.2 Gyr. Future observations of Pb and Bi and a better determination of the r-process contribution to solar Pb are needed to put the age estimates for this and other stars on a more solid basis. For our most likely scenario, we provide predictions of the expected upper and lower limits on the abundances of the elements Pb and Bi in CS 31082-001.

We present an analysis of the longest timescale microlensing events discovered by the MACHO Collaboration during a 7 year survey of the Galactic bulge. We find six events that exhibit very strong microlensing parallax signals due, in part, to accurate photometric data from the GMAN and MPS collaborations. The microlensing parallax fit parameters are used in a likelihood analysis, which is able to estimate the distances and masses of the lens objects based on a standard model of the Galactic velocity distribution. This analysis indicates that the most likely masses of five of the six lenses are greater than 1 M_☉, which suggests that a substantial fraction of the Galactic lenses may be massive stellar remnants. This could explain the observed excess of long-timescale microlensing events. The lenses for events MACHO-96-BLG-5 and MACHO-98-BLG-6 are the most massive, with mass estimates of M/M_☉ = 6 and M/M_☉ = 6, respectively. The observed upper limits on the absolute brightness of main-sequence stars for these lenses are less than 1 L_☉, so both lenses are black hole candidates. The black hole interpretation is also favored by a likelihood analysis with a Bayesian prior using a conventional model for the lens mass function. We consider the possibility that the source stars for some of these six events may lie in the foreground Galactic disk or in the Sagittarius (Sgr) dwarf galaxy behind the bulge, but we find that bulge sources are likely to dominate our microlensing parallax event sample. Future Hubble Space Telescope observations of these events can either confirm the black hole lens hypothesis or detect the lens stars and provide a direct measurement of their masses. Future observations of similar events by the Space Interferometry Mission or the Keck or VLT interferometers, as explained by Delplancke, Górski, & Richichi, will allow direct measurements of the lens masses for stellar remnant lenses as well.

Hubble Space Telescope imaging of M22 has allowed, for the first time, a detailed and uniform mapping of mass segregation in a globular cluster. Luminosity and mass functions from the turnoff down to the mid to lower main sequence are presented for M22 in annular bins from the center of the cluster out to five core radii. Within the core, a significant enhancement is seen in the proportion of 0.5-0.8 M_☉ stars compared with their numbers outside the core. Numerical modeling of the spatial mass spectrum of M22 shows that the observed degree of mass segregation can be accounted for by relaxation processes within the cluster. The global cluster mass function for M22 is flatter than the Salpeter initial mass function and cannot be represented by a single power law.

Extensive early observations proved that the ejecta of supernova 1987A (SN 1987A) are aspherical. The most important of these early observations include (1) the "Bochum event" that revealed small-scale spectroscopic structure indicating chemical inhomogeneities of the ejecta, (2) spectropolarimetry that showed deviations from spherical symmetry, and (3) speckle observations that revealed both the asymmetry of the ejecta and the "mystery spot" manifested as a secondary source off center from the bulk of the supernova ejecta. Fifteen years after the supernova explosion, the Hubble Space Telescope has resolved the rapidly expanding ejecta. The late-time images and spectroscopy provide a geometrical picture that is consistent with early observations and suggests a highly structured, axially symmetric geometry. We present here a new synthesis of the old and new data. We show that the Bochum event, presumably a clump of ⁵⁶Ni, and the late-time image, the locus of excitation by ⁴⁴Ti, are most naturally accounted for by sharing a common position angle of about 14°, the same as the mystery spot and early speckle data on the ejecta, and that they are both oriented along the axis of the inner circumstellar ring at 45° to the plane of the sky. We also demonstrate that the polarization represents a prolate geometry with the same position angle and axis as the early speckle data and the late-time image, and hence that the geometry has been fixed in time and throughout the ejecta. The Bochum event and the Doppler kinematics of the [Ca II]/[O II] emission in spatially resolved Hubble Space Telescope spectra of the ejecta can be consistently integrated into this geometry. The radioactive clump is deduced to fall approximately along the axis of the inner circumstellar ring and therefore to be redshifted in the north, whereas the [Ca II]/[O II] 7300 Å emission is redshifted in the south. We present a jet-induced model for the explosion and argue that such a model can account for many of the observed asymmetries. In the jet models, the oxygen and calcium are not expected to be distributed along the jet but primarily in an expanding torus that shares the plane and northern blue shift of the inner circumstellar ring.

We report sensitive Australia Telescope Compact Array radio continuum observations toward IRAS 15596-5301 and 16272-4837, two luminous objects (L > 2 × 10⁴L_☉) thought to represent massive star-forming regions in early stages of evolution (due to previously undetected radio emission at the 1 σ level of 2 mJy beam^-1). Also reported are 1.2 mm continuum and a series of molecular line observations made with the Swedish ESO Submillimeter Telescope. The radio continuum observations toward IRAS 15596-5301 reveal the presence of three distinct compact sources, with angular sizes of 27-88 (FWHM), all located within a region of 30'' in diameter. Assuming that these are regions of ionized gas, we find that they have diameters of 0.06-0.2 pc and electron densities of 8 × 10²-2 × 10³ cm^-3 and that they are excited by early B-type stars. The 1.2 mm observations show that the dust emission arises from a region of 42'' × 25'' (FWHM) with a total flux of 5.8 Jy, implying a mass of 1.4 × 10³M_☉. The line observations indicate that IRAS 15596-5301 is associated with a molecular cloud with a FWHM angular size of 37'' (~0.4 pc radius at the distance of 4.6 kpc), a molecular hydrogen density of ~4 × 10⁵ cm^-3, and a rotational temperature of ~27 K. We suggest that the massive dense core associated with IRAS 15596-5301 contains a cluster of B stars that are exciting compact H II regions that are in pressure equilibrium with the dense molecular surroundings. No radio continuum emission was detected from IRAS 16272-4837 up to a 3 σ limit of 0.2 mJy. However, the 1.2 mm observations show strong dust emission arising from a region of 41'' × 25'' (FWHM) with a total flux of 13.8 Jy, implying a mass of 2.0 × 10³M_☉. The line observations indicate the presence of an elongated molecular cloud with FWHM major and minor axes of 61'' and 42''(0.50 × 0.35 pc in radius at the distance of 3.4 kpc), a molecular hydrogen density of ~2 × 10⁵ cm^-3, and a rotational temperature of ~27 K. The high luminosity (2.4 × 10⁴L_☉) and lack of radio emission from this massive core suggest that it hosts an embedded young massive protostar that is still undergoing an intense accretion phase. This scenario is supported by the observed characteristics of the line profiles and the presence of a bipolar outflow detected from observations of the SiO emission. We suggest that IRAS 16272-4837 is a bona fide massive star-forming region in a very early evolutionary stage, being the precursor of an ultracompact H II region.

We present narrowband Hα, [S II], and [O I] Hubble Space Telescope images of the young planetary nebula GL 618. This object is a compact, bipolar nebula that is currently undergoing the transition from asymptotic giant branch star to planetary nebula. Our images confirm the presence of at least three highly collimated outflows emanating from the central regions of GL 618. We also detect Hα emission close to the central dust lane and in an extended scattered light halo. The three outflows are occurring simultaneously in this object as opposed to being the result of a precessing jet. We derive an inclination for the brightest outflow in the east lobe of 39° ± 4°. This differs from the previous estimate of 45°. In addition, our results indicate that the outflows seen in GL 618 are probably not coplanar. Line strengths derived from the narrowband images indicate a shock velocity in the range of 40-100 km s^-1. Based on the shock velocity, we estimate that the age of the outflows is less than 500 yr. The outflows seen in the optical images of GL 618 are related to features seen in near-IR, CO, and CS maps of this object. This relationship indicates that the outflows are playing a major role in the morphological evolution of this young planetary nebula, interacting with and shaping the neutral envelope surrounding GL 618. We discuss the implications of these jets and their interaction with the neutral envelope in the context of current models of planetary nebula formation.

Long-baseline optical interferometers can now detect and resolve hot dust emission thought to arise at the inner edge of circumstellar disks around young stellar objects (YSOs). We argue that the near-infrared sizes being measured are closely related to the radius at which dust is sublimated by the stellar radiation field. We consider how realistic dust optical properties and gas opacity dramatically affect the predicted location of this dust destruction radius, an exercise routinely done in other contexts but so far neglected in the analysis of near-infrared sizes of YSOs. We also present the accumulated literature of near-infrared YSO sizes in the form of a size-luminosity diagram and compare with theoretical expectations. We find evidence that large (≳1.0 μm) dust grains predominate in the inner disks of T Tauri and Herbig Ae/Be stars, under the assumption that the innermost gaseous disks are optically thin at visible wavelengths.

The realization that gamma-ray bursts (GRBs) release a constant amount of energy implies that post-jet-break afterglow evolution is largely universal. For a given redshift, all afterglows should be detected up to a fixed observer angle. We estimate the observed magnitude and the implied detectability of orphan afterglows. We show that for reasonable limiting magnitudes (m_lim = 25), orphan afterglows will typically be detected from small (~10°) angles away from the GRB jet axis. A detected orphan afterglow generally corresponds to a "near miss" of a GRB whose jet is pointing just slightly away from us. With our most optimistic parameters, we expect that 15 orphan afterglows will be recorded in the Sloan Digital Sky Survey, and 35 transients will be recorded in a dedicated 2 m class telescope operating full time for a year in an orphan afterglow search. The rate is smaller by a factor of 15 for our "canonical" parameters. We show that for a given facility, an optimal survey should be shallower, covering a larger area, rather than deeper. The limiting magnitude should not be, however, lower than ~23, as in this case, more transients from on-axis GRBs will be discovered than orphan afterglows. About 15% of the transients could be discovered with a second exposure of the same area provided that it follows after 3, 4, and 8 days for m_lim = 23, 25, and 27, respectively.

Many models of gamma-ray bursts (GRBs) invoke a central engine consisting of a black hole of a few solar masses accreting matter from a disk at a rate of a fraction to a few solar masses per second. Popham et al. and Narayan et al. have shown that, for ≳ 0.1 M_☉ s^-1, accretion proceeds via neutrino cooling and neutrinos can carry away a significant amount of energy from the inner regions of the disks. We improve on these calculations by including a simple prescription for neutrino transfer and neutrino opacities in such regions. We find that the flows become optically thick to neutrinos inside a radius R ~ 6R_S-40R_S for in the range of 0.1-10 M_☉ s^-1, where R_S is the black hole Schwarzchild radius. Most of the neutrino emission comes from outside this region, and the neutrino luminosity stays roughly constant at a value L_ν ~ 10⁵³ ergs s^-1. We show that, for ≳ 1 M_☉ s^-1, neutrinos are sufficiently trapped that energy advection becomes the dominant cooling mechanism in the flow. These results imply that ν annihilation in hyperaccreting black holes is an inefficient mechanism for liberating large amounts of energy. Extraction of rotational energy by magnetic processes remains the most viable mechanism.

We present a model for the high-energy γ-ray emission from the outer gap of soft gamma-ray repeaters (SGRs) during their quiescent states. In this model, X-rays come from the stellar surface, but the emerging X-ray spectrum will have a power-law tail because of the multiple scattering at the cyclotron resonance in the magnetosphere, as pointed out by Thompson, Lyukitov, & Kulkarni. The outer gap is sustained by the collision between these X-rays with the high-energy photons produced in the outer gap through the photon-photon pair production. We have taken a magnetic dipole geometry into account in estimating the fractional size of the outer gap. The fractional size of the outer gap depends on the period, surface magnetic field, average X-ray energy, and the magnetic inclination angle of the neutron star. After the average fractional size outer gap is determined, the spectrum and luminosity of high-energy photons from the outer gap can be calculated. We apply this model to some SGRs, such as SGR 1806-20 and SGR 1900+14, and compare the expected integral fluxes with the sensitivities of EGRET, GLAST, MAGIC, and VERITAS. We predict that the integral flux of SGR 1900+14 may be greater than the sensitivity of GLAST, and especially that the integral flux for a large magnetic inclination angle (say 80°) may be greater than the sensitivities of GLAST and MAGIC. However, we predict that SGR 1806-20 would not be detected by GLAST because its distance is about 3 times of that of SGR 1900+14.

Bipolar outflows are present in many disk-accreting astrophysical systems. In disk-accreting cataclysmic variables (CVs), these outflows are responsible for most of the strong features in the ultraviolet spectra of these systems. However, there have been few attempts to model these features quantitatively. Here we describe a new, hybrid Monte Carlo/Sobolev code, which allows us to synthesize the complete spectrum of a disk-dominated, mass-losing CV. The line profiles that we calculate for C IV resemble those calculated by previous workers when an identical geometry is assumed. However, our synthetic spectra exhibit not only the well-known resonance lines of O VI, N V, Si IV, and C IV but, with an appropriate choice of mass-loss rate and wind geometry, also many lines originating from excited lower states. Many of these lines have already been seen in the far-ultraviolet spectra of CVs obtained with the Hopkins Ultraviolet Telescope, the Far Ultraviolet Spectroscopic Explorer, and the Hubble Space Telescope. In order to illustrate the degree to which we are currently able to reproduce observed spectra, we finally present a preliminary fit to the Hopkins Ultraviolet Telescope spectrum of the dwarf nova Z Cam in outburst.

Using extensive Hubble Space Telescope imaging, combined with Chandra X-ray and Parkes radio data, we have detected the optical binary companion to a second millisecond pulsar (MSP) in the globular cluster 47 Tucanae. This faint (V = 22.3) blue (V-I = 0.7) star shows a large-amplitude (60%-70%) sinusoidal variation in both V and I. The period (3.19066 hr) and phase of the variation match those of the MSP 47 Tuc W (which does not have an accurate radio timing position) to within 0.5 s and 1.2 minutes, respectively, well within the 1 σ errors. The phase dependence of the intensity and color implies that heating of a tidally locked companion causes the observed variations. The eclipsing nature of this MSP in the radio, combined with the relatively large companion mass (greater than 0.13 M_☉) and the companion's position in the color-magnitude diagram, suggests that the companion is a main-sequence star, a rare circumstance for an MSP companion. This system is likely to have had a complex evolution and represents an interesting case study in MSP irradiation of a close companion. We present evidence for another optical variable with similar properties to the companion of 47 Tuc W. This variable also may be an MSP companion, although no radio counterpart has yet been detected.

We present results from the imaging portion of a far-ultraviolet (FUV) survey of the core of 47 Tucanae. We have detected 767 FUV sources, 527 of which have optical counterparts in archival HST/WFPC2 images of the same field. Most of our FUV sources are main-sequence (MS) turnoff stars near the detection limit of our survey. However, the FUV/optical color-magnitude diagram (CMD) also reveals 19 blue stragglers (BSs), 17 white dwarfs (WDs), and 16 cataclysmic variable (CV) candidates. The BSs lie on the extended cluster MS, and four of them are variable in the FUV data. The WDs occupy the top of the cluster cooling sequence, down to an effective temperature of T_eff ≃ 20,000 K. Our FUV source catalog probably contains many additional, cooler WDs without optical counterparts. Finally, the CV candidates are objects between the WD cooling track and the extended cluster MS. Four of the CV candidates are previously known or suspected cataclysmics. All of these are bright and variable in the FUV. Another CV candidate is associated with the semidetached binary system V36 that was recently found by M. D. Albrow and coworkers. V36 has an orbital period of 0.4 or 0.8 days, blue optical colors, and is located within 1'' of a Chandra X-ray source. A few of the remaining CV candidates may represent chance superpositions or SMC interlopers, but at least half are expected to be real cluster members with peculiar colors. However, only a few of these CV candidates are possible counterparts to Chandra X-ray sources. Thus, it is not yet clear which, if any, of them are true CVs, rather than noninteracting MS/WD binaries or helium WDs.

We examine the AAVSO light curve of U Gem from 1908 to 2002, with particular focus on the 1985 October outburst. This outburst was longer than any other seen in U Gem by about a factor of 2 and appears to be unique among all dwarf nova outbursts seen in systems with orbital periods longer than 3 hr in that one can measure the decay timescale during the initial slow decay. This rate is ~26 ± 6 days mag^-1. Using estimates of the rate of accretion during outburst taken from Froning et al., one can show that ~10²⁴ g of gas was accreted onto the white dwarf during the outburst, which constrains the surface density in the outer accretion disk to be ~600 g cm^-2. The observed timescale for decay is consistent with that expected in U Gem, given its orbital period and disk mass at the time the outburst began. The data are not of sufficient quality to be able to ascertain a deviation from exponentiality in the decay light curve (as in the SU UMa stars' superoutbursts). When coupled with the viscous time inferred from the (short orbital period) SU UMa stars, the U Gem viscous timescale lends support to the standard model in which the decays in dwarf novae can either be viscous or thermal, with the ratio between them being roughly h/r, where h is the vertical pressure scale height in the disk. Indeed, dwarf novae are the only systems for which one can be reasonably certain of the identification of "viscous" and "thermal" decays.

We present new X-ray and radio observations of the Wolf-Rayet star EZ CMa (HD 50896) obtained with XMM-Newton and the Very Large Array (VLA). This WN4 star exhibits optical and UV variability at a period of 3.765 days whose cause is unknown. Binarity may be responsible, but the existence of a companion has not been proven. The radio spectral energy distribution of EZ CMa determined from VLA observations at five frequencies is in excellent agreement with predictions for free-free wind emission, and the ionized mass-loss rate allowing for distance uncertainties is = 3.8 (±2.6) × 10^-5M_☉ yr^-1. The CCD X-ray spectra show prominent Si XIII and S XV emission lines and can be acceptably modeled as an absorbed multitemperature optically thin plasma, confirming earlier ASCA results. Nonsolar abundances are inferred with Fe notably deficient. The X-ray emission is dominated by cooler plasma at a temperature kT_cool ≈ 0.6 keV, but a harder component is also detected, and the derived temperature is kT_hot ≈ 3.0-4.2 keV if the emission is thermal. This is too high to be explained by radiative wind shock models, and the X-ray luminosity of the hard component is 3 orders of magnitude lower than expected for accretion onto a neutron star companion. We show that the hard emission could be produced by the Wolf-Rayet wind shocking onto a normal (nondegenerate) stellar companion at close separation. Finally, using comparable data sets we demonstrate that the X-ray and radio properties of EZ CMa are strikingly similar to those of the WN5-6 star WR 110. This similarity points to common X-ray and radio emission processes in WN stars and discredits the idea that EZ CMa is anomalous within its class.

We have undertaken quantitative analysis of four LMC and SMC O4-9.7 extreme supergiants using far-ultraviolet FUSE, ultraviolet IUE/Hubble Space Telescope, and optical Very Large Telescope UV-Visual Echelle Spectrograph spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing, developed by Hillier & Miller, are used to analyze wind and photospherics spectral features simultaneously. Using Hα to constrain mass-loss rates, He I-He II photospheric lines reveal stellar temperatures that are systematically (5-7.5 kK) and substantially (15%-20%) lower than previously derived from unblanketed, plane-parallel, non-LTE photospheric studies. We have confidence in these revisions since derived temperatures generally yield consistent fits across the entire 912-7000 Å observed spectral range. In particular, we are able to resolve the UV-optical temperature discrepancy identified for AzV 232 (O7 Iaf⁺) in the SMC by Fullerton and coworkers. The temperature and abundance sensitivity of far-ultraviolet, UV, and optical lines are discussed. "Of" classification criteria are directly linked to (strong) nitrogen enrichment (via N III λ4097) and (weak) carbon depletion (via C III λλ4647-4651), providing evidence for mixing of unprocessed and CNO-processed material at their stellar surfaces. Oxygen abundances are more difficult to constrain, except via O II lines in the O9.7 supergiant, for which it is also found to be somewhat depleted. Unfortunately, He/H is very difficult to determine in individual O supergiants because of uncertainties in microturbulence and the atmospheric scale height. The effect of wind clumping is also investigated, for which P V λλ1118-1128 potentially provides a useful diagnostic in O star winds, unless phosphorus can be independently demonstrated to be underabundant relative to other heavy elements. Revised stellar properties affect existing calibrations of (1) Lyman continuum photons—a factor of 2 lower for the O4 supergiant—and (2) kinetic energy released into the ISM by O supergiants. Our results also have importance for the calibration of the wind momentum-luminosity relationship for OB stars, particularly since the stars studied here are among the visually brightest OB stars in external galaxies.

In deeply convective stars, the nonthermal energy required to heat the chromosphere ultimately is supplied by turbulent magnetoconvection. Because the early and middle A stars have very shallow convective layers, they are not expected to produce enough magnetoconvective power to sustain luminous chromospheres or hot coronae. Here we describe a search for chromospheric emission in the far-ultraviolet (905-1185 Å) spectra of seven main-sequence A stars, based on observations from the Far Ultraviolet Spectroscopic Explorer (FUSE) telescope. Our survey spans the interval in effective temperature along the main sequence over which powerful subsurface convection zones and hence chromospheric emission are expected to vanish. The presence or absence of high-temperature emissions in our FUSE spectra therefore can be used to identify the locus for the transition from convective to radiative envelopes—a change in stellar structure that is difficult to assess by other means. We present our observations and analysis of the subcoronal emission lines of C III λλ977, 1175 and O VI λλ1032, 1037, which bracket a range in formation temperatures from 50,000 to 300,000 K. To supplement our FUSE observations, we also report Goddard High Resolution Spectrograph measurements of Si III λ1206 and H I Lyα λ1215, obtained from archival observations of the Hubble Space Telescope, as well as X-ray measurements from previous ROSAT survey and pointed observations. We detected C III and O VI emission features in the FUSE spectra of the coolest stars of our sample, at T_eff ≲ 8200 K. When normalized to the bolometric luminosities, the detected emission-line fluxes are comparable to solar values. We detected none of the hotter stars in our survey at T_eff ≥ 8300 K. Upper limits on the normalized flux in some instances approach 40 times less than solar. Within an uncertainty in the effective temperature scale of up to several hundred kelvins, our FUSE observations indicate that the transition between convective and radiative stellar envelopes takes place at, or very near, the point along the main sequence where stellar structure models predict and, moreover, that the changeover occurs very abruptly, over a temperature interval no greater than ~100 K in width. Our FUSE sample also includes two binary stars. In both cases, the narrow UV line profiles we have observed suggest that the high-temperature emission is most likely associated with the late-type companions rather than the A stars themselves.

We suggest a mechanism for enhancing magnetic activity in tidally interacting binaries. We suppose that the deviation of the primary star from spherical symmetry due to the tidal influence of the companion leads to stellar pulsation in its fundamental mode. It is shown that stellar radial pulsation amplifies torsional Alfvén waves in a dipole-like magnetic field, buried in the interior, according to the recently proposed swing wave-wave interaction. Then amplified Alfvén waves lead to the onset of large-scale torsional oscillations, and magnetic flux tubes arising toward the surface owing to magnetic buoyancy diffuse into the atmosphere producing enhanced chromospheric and coronal emission.

We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N type. Abundances of Sr, Y, Zr (low-mass s-elements, or ls), Ba, La, Nd, Sm, and Ce (high-mass s-elements, or hs) are derived using the spectral synthesis technique from high-resolution spectra. The N stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study (Utsumi 1985), and also smaller than those found in supergiant post-asymptotic giant branch (post-AGB) stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M → MS → S → SC → C during the AGB phase. We compare the observational data with recent s-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low-mass AGB star models (M ≲ 3 M_☉) and s-element observations. In low-mass AGB stars, the ¹³C(α, n)¹⁶O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of ¹³C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colors, and the theoretical relations between stellar mass, metallicity, and the final C/O ratio, we conclude that most (or maybe all) of the N stars studied in this work are intrinsic, thermally pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.

¹²C and ¹³C abundances have been measured in 11 bright giant members of the globular cluster ω Centauri via observations of the first-overtone CO bands near 2.3 μm. The stars in this sample were selected to span a substantial fraction of the range of iron abundances found in this cluster. In addition, the sample covers a range of [O/Fe], [Na/Fe], and [Al/Fe] abundance ratios derived in previous studies. In all ω Cen giants the ¹²C/¹³ C abundance ratio is found to be quite low, indicating deep mixing in these red giants. The mean value for the entire sample is ⟨¹²C/¹³C⟩ = 4.3 ± 0.4 (σ = 1.3), with nine stars equal, within the errors, to the equilibrium ratio ¹²C/¹³C = 3.5 and two stars having slightly higher values. There is no correlation between the ¹²C/¹³ C and the abundance of iron. In addition, no correlation of ¹²C/¹³C with [¹²C/Fe] is found (all giants are deeply mixed), although the derived abundances of [¹²C/Fe] show a positive correlation with [O/Fe] and an anticorrelation with [Na/Fe] (with the oxygen and sodium abundances taken from previous studies in the literature). A comparison of the isotopic carbon ratios in ω Cen with those from other globular clusters (M4, M71, NGC 6752, and 47 Tuc) and with literature oxygen abundances, may reveal a slight trend of decreasing ¹²C/¹³C ratios with decreasing [O/Fe] in the entire globular cluster sample of red giants. A comparison between ¹²C/¹³C and both [Na/Fe] and [Al/Fe], however, reveals no trend.

The time histories of near-relativistic scatter-free impulsive beamlike electron events (38-315 keV, 0.4 < v/c < 0.8) measured at 1 AU provide unique information on their solar acceleration and release. We have identified 79 such impulsive events from 1997 August through 2000 September. Detailed statistical analysis of the timing between the near-relativistic electron injection and the soft X-ray, microwave, chromospheric Hα, and metric and decametric type III radio emission shows that the near-relativistic electrons measured by the Advanced Composition Explorer/Electron, Proton, and Alpha Monitor in orbit around the Earth's first Lagrangian point are associated with western hemisphere events and are injected with a median delay of ~10 minutes after the start of the electromagnetic emissions (including metric and decametric type III events). The delayed injection, as well as there being only a weak statistical correlation between the intensities of the near-relativistic electrons and the characteristics of the electromagnetic emissions, indicates that the escaping near-relativistic electron populations are not directly related to those that generate the prompt flare-related emissions. The observations are consistent with acceleration of the escaping near-relativistic electrons by an outgoing coronal shock (V ~ 1000 km s^-1) launched near the time of the prompt electromagnetic emissions.

We have examined transient coronal activity observed by the Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph Experiment (LASCO) around the release time of beams of near-relativistic electrons (~40-300 keV) observed by the Advanced Composition Explorer (ACE)/Electron, Proton, and Alpha Monitor (EPAM) at 1 AU. The electron beams are strongly associated with coronal mass ejections (CMEs) emitted from near the Sun's western limb, although the converse is not true; many more west hemisphere CMEs are seen by LASCO than are relativistic electron beams seen by ACE/EPAM. However, when they are present, strong beams of electrons may be used to infer reliably the actual solar release time as they propagate scatter free. Fifty-two electron beams were observed from 1997 September to 2000 September at times of LASCO observations. Of these, there were 47 observations of an associated coronal transient, of which 33 were "classical" CMEs, as distinct from blobs and jets. We were able to extrapolate the height-time plot back to a nominal 1 R_☉, to estimate a CME launch time. All but two of the CMEs were seen in projection off the west solar limb. For 37 events there was an associated soft X-ray event, the majority of which were from western active regions. All electron events were accompanied by decametric type III emission consistent with electron beams having exciter energies of around a few keV. The near-relativistic electron injection time was typically delayed by ~20 minutes from the CME launch time and greater than ~10 minutes after the onset of the electromagnetic radio and X-ray signatures of the flare (when present). Therefore, the near-relativistic electrons that must be present to produce the chromospheric electromagnetic emission do not escape promptly (at least at detectable intensities). The radial distance of most CMEs at the electron release time was between 1.5 and 3.5 R_☉. Both the peak electron intensity and the spectral hardness of the electrons were positively correlated with the CME speed, a signature of shock acceleration. We therefore suggest that most of the near-relativistic electrons seen by ACE/EPAM are accelerated by the shock driven by the coronal transient and are released at a radial distance around 2-3 R_☉.

On the basis of Yohkoh Hard X-Ray Telescope data and the magnetograms taken by the SOHO Michelson Doppler Imager and the Solar Magnetic Field Telescope at Huairou Solar Observing Station, we suggest an interpretation of the well-observed "Bastille Day 2000" flare. The large-scale structure and dynamics of the flare, as seen in hard X-rays, can be explained in terms of the three-dimensional reconnection at a separator in the corona. More specifically, we suggest that before occurrence of two-ribbon flares with significant decrease of a distance between the hard X-ray (HXR) footpoints, like the Bastille Day flare, the bases of magnetic field separatrices are moved by the large-scale photospheric flows of two types. First, the shear flows, which are parallel to the photospheric neutral line, increase the length of field lines in the corona and produce an excess of magnetic energy. Second, the converging flows, i.e., the flows directed to the neutral line, create the preflare current layers in the corona and provide an excess of energy sufficient to produce a large flare. During the flare, both excesses of magnetic energy are released completely or partially. In the Bastille Day flare, the model describes two kinds of apparent motions of the HXR kernels. One is an increase of a distance between the flare ribbons in which the HXR kernels appear. The effect results from fast reconnection in a coronal current layer. The second effect is a decrease of the distance between the kernels moving to each other as a result of relaxation of magnetic tensions generated by the photospheric shear flows.

This paper describes coronal inflows observed with the Large Angle Spectrometric Coronagraph (LASCO). The inflows are not seen above 5.5 R_☉, which appears to be a "point of no return" for the Sun's plasmas and fields. Below this height, most inflows seem to indicate magnetic flux that is returning to the Sun after its reconnection at sector boundaries. Some inflows have characteristics (like fast, oppositely directed ejections of material) that are easily interpreted in terms of conventional models of field line reconnection. However, the overwhelming majority of coronal inflows have a more complex behavior that typically includes the following characteristics:

The Ulysses trajectory provides a unique opportunity to study the propagation of MeV electrons in a wide range of heliographic latitudes and during varying conditions in the inner heliosphere. From the Ulysses launch up to the beginning of 1998, the 3-10 MeV electron count rate of the COSPIN/KET instrument has been consistently described by modulation models taking into account Galactic cosmic rays as well as Jovian electrons. In this paper we focus on the MeV electron observations from 1998 onward, covering Ulysses' second out-of-ecliptic path, which was performed under solar maximum conditions. In contrast to our expectations, the electron intensity stayed at approximately the same level as the one observed in 1991 when Ulysses was magnetically well connected to Jupiter. In this paper we report on the discovery of short-term 3-10 MeV and 7-20 MeV electron intensity increases at the highest southern heliographic latitudes, which are not correlated with solar particle events.

Size histograms of all Arecibo ultra-high-frequency radar micrometeors detected in 1997-1998 whose radii were measured by atmospheric drag are presented. Most can be fitted with either a lognormal function or, alternatively, one or more power-law functions. Either form is indicative of significant fragmentation. The interplanetary dust particle (IDP) histogram results are discussed and compared with those considered to be extrasolar particles, including a subset of those deemed to be true interstellar particles (ISPs). The Arecibo IDP power-law results are shown to agree well with those derived from IRAS dust bands and Long-Duration Exposure Facility cratering, thus confirming the applicability of the sample to the derivation of mass estimates. A dichotomy between size histograms of particles with preperihelion Earth encounters and those with postperihelion encounters is evidence that significant size histogram change occurs when the smallest particles, including all ISPs, pass close to the Sun, even if only once. A small sample of previously undetected Arecibo postperihelion ISPs coming from the direction of the known Ulysses gas and dust flow are shown to have a size distribution and solar system dynamical properties similar to other Arecibo ISPs and therefore can be combined with previous ISP results to obtain a more robust sample. Derived mass flux points for the Arecibo ISPs agree well (over 5 orders of magnitude of mass) with a previously derived mass flux distribution function for Ulysses/Galileo spacecraft dust. This combined spacecraft and ground-based mass flux function is then used to infer a number of interesting mass-related solar system and astrophysical quantities.

We use the frequency map analysis method to identify for Trojan orbits of Saturn the regions in the proper orbital element phase space characterized by higher stability. We find that Trojan orbits with proper eccentricity around 0.05, libration amplitude of about 80°, and inclination lower than 15° show a slow diffusion in the proper frequency of the longitude of perihelion , which indicates long-term stability. Numerical integration of some of these stable orbits indicates a half-life of about 2.5 Gyr. Orbits with inclination of about 20° are destabilized by a secular resonance with the forcing term 2g₆ - g₅. At higher inclinations Saturn Trojan orbits are unstable on a short timescale (a few × 10⁵ yr). Applying the frequency map analysis to the numbered Jupiter Trojans, we find that the size of the stability region is much larger for Jupiter Trojans than for Saturn Trojans. Moreover, the diffusion rate is significantly lower, suggesting that the dynamical lifetimes of Jupiter Trojans are considerably longer. The frequency analysis method allows us to separate the proper and forced components of the eccentricity of Trojans. A semianalytical model for secular motion of Saturn Trojans is presented.

The spectrum of the solar corona and plasma spectra in astrophysical objects and fusion devices exhibit forbidden lines in alkali-like ions. Ions belonging to the iron group are particularly important in this respect. The electric quadrupole (E2) transitions for Fe XVI are computed using the highly correlated relativistic wave function obtained by using coupled cluster theory including all single, double, and some triple excitations from the core. Term values of the present ion obtained by our method are compared with the available experimental and theoretical data. The detailed highly accurate relativistic data for line strengths and transition probabilities are presented for a large number of transitions of low transition rates, and a few of both of them are compared with existing data as samples.

We report first results for the cosmic infrared background (CIB) fluctuations at 1.25, 1.65, and 2.17 μm obtained from long exposures constructed from Two Micron All Sky Survey standard star fields. We have co-added and analyzed scans from one such field with a total exposure time greater than 1 hr and removed sources and other artifacts. The stars and galaxies were clipped out to K_s ≃ 19 mag, leaving only high-z galaxies (or possibly local low surface brightness systems). The angular power spectrum of the remaining diffuse emission on scales from a few arcseconds to a few arcminutes has a power-law slope consistent with emission produced by clustered galaxies. The noise (and residual artifacts) contribution to the signal is small, and the colors of the signal are very different from Galactic stars or airglow. We therefore identify the signal as CIB fluctuations from the faint unresolved galaxies. We show that the present-day galaxies with no evolution would produce a significant deficit in the observed CIB fluctuations. Thus, the dominant contribution to the observed signal must come from high z and may indicate high rates of star formation at those epochs.

The optical light curve of GRB 010222 exhibited one of the slowest decays of any gamma-ray burst to date. Its broadband properties have been difficult to explain with conventional afterglow models, as they require either the power-law index of the underlying electron energy distribution to be low, p < 2, or the outflow to be quasi-spherical, thus reviving the energy problem. We argue that the slow decay of GRB 010222 and a linear polarization of 1.36% ± 0.64% are naturally explained by a jet model with continuous energy injection. The electron energy distribution then has p = 2.49 ± 0.05, fully consistent with the expectation from detailed modeling of acceleration in relativistic shocks that p > 2, thus alleviating the "p-problem."

We suggest that the collapsing core of a massive rotating star may fragment to produce two or more compact objects. Their coalescence under gravitational radiation gives the resulting black hole or neutron star a significant kick velocity, which may explain those observed in pulsars. A gamma-ray burst can result only when this kick is small. Thus, only a small fraction of core-collapse supernovae produce gamma-ray bursts. The burst may be delayed significantly (hours to days) after the supernova, as suggested by recent observations. If our picture is correct, core-collapse supernovae should be significant sources of gravitational radiation with a chirp signal similar to a coalescing neutron star binary.

We have combined very long baseline interferometry (VLBI) data from several programs in order to resolve differences in reported parsec-scale jet speeds for the TeV gamma-ray source Markarian 501. Data from the Very Long Baseline Array (VLBA) 2 cm survey, and 8 and 15 GHz data from the Radio Reference Frame Image Database, have been combined with data from a 5 GHz VLBI Space Observatory Programme observation to determine the apparent motions of jet components in this source. The combined data set consists of 12 observations between 1995 April and 1999 July. Four jet components are detected at most epochs, all of which are clearly subluminal (i.e., with apparent speeds less than the speed of light) and two of which appear stationary. The established TeV gamma-ray sources Mrk 501 and Mrk 421 thus both have subluminal parsec-scale jets, in contrast to the apparently superluminal jets of the majority of GeV sources detected by EGRET. No new VLBI component has emerged from the core following the extended TeV high state in 1997, again in contrast to the general behavior of GeV gamma-ray sources.

Obscured active galactic nuclei, which are classified optically as type 2 (narrow line) Seyfert galaxies in the local universe, are by far the most promising candidates for the origin of the hard (2-10 keV) X-ray background radiation. However, optical follow-up observations of faint X-ray sources in deep Chandra images have revealed surprising numbers of apparently normal galaxies at modest redshift. Such objects represent ~40%-60% of the sources classified in deep Chandra surveys, raising the possibility that the X-ray galaxy population has evolved with cosmic time. Alternatively, most of the faint X-ray galaxies in question are so distant that their angular diameters are comparable to the slit widths used in ground-based spectroscopic observations; thus, their nuclear spectral features may be overwhelmed ("hidden") by host galaxy light. To test this hypothesis, we have obtained integrated spectra of a sample of nearby, well-studied Seyfert 2 galaxies. The data, which accurately simulate observations of distant Chandra sources, demonstrate convincingly that the defining spectral signatures of Seyfert 2s can be hidden by light from their host galaxies. In fact, 60% of the observed objects would not be classified as Seyfert 2s on the basis of their integrated spectra, similar to the fraction of faint X-ray sources identified with "normal" galaxies. Thus, the numbers of narrow-line active galaxies in deep Chandra surveys (and perhaps all ground-based spectroscopic surveys of distant galaxies) are likely to have been underestimated.

We present UV images and spectra of the starburst galaxy I Zw 18, taken with the Space Telescope Imaging Spectrograph. The high spatial resolution of these data allows us to isolate clusters containing Wolf-Rayet stars of the subtype WC. Our far-UV spectra clearly show C IV λλ1548, 1551 and He II λ1640 emission of WC stars in two clusters: one within the bright (northwest) half of I Zw 18 and one on the outskirts of this region. The latter spectrum is unusual because the C IV is seen only in emission, indicating a spectrum dominated by WC stars. These data also demonstrate that the H I column in I Zw 18 is strongly peaked in the fainter (southeast) half of I Zw 18, with a column depth far larger than that reported in previous analyses.

We present Very Large Telescope observations of the interacting system AM 1353-272. Using the FORS2 instrument, we studied the kinematics of the ionized gas along its prominent tidal tails and discovered strikingly large velocity gradients associated with seven luminous tidal knots. These kinematical structures cannot be caused by streaming motion and most likely do not result from projection effects. More probably, instabilities in the tidal tails have led to the formation of kinematically decoupled objects that could be the progenitors of self-gravitating tidal dwarf galaxies.

Using the new 23 GHz receivers at the Very Large Array (VLA), we have detected NH₃ (6, 6) emission (ν = 25.056025 GHz) from hot (>150 K) molecular clouds in the central 10 pc of the Galaxy. This is the first successful detection of NH₃ (6, 6) with the VLA. The brightest emission comes from a region interior to the "circumnuclear disk" (CND), less than 1.5 pc in projected distance from Sagittarius A*. This region does not show molecular emission from lower energy transitions such as NH₃ (1, 1) and (2, 2), HCN (1-0), and HCO⁺ (1-0). Line ratios of NH₃ (6, 6) and (3, 3) emission as well as NH₃ (6, 6) line widths have peak values within 1.5 pc of Sgr A*, indicating that the gas is physically close to the nucleus. NH₃ (6, 6) is also detected toward many features outside the CND observed in NH₃ (1, 1), (2, 2), and (3, 3). These features tend to lie along ridges of gas associated with Sgr A East or the massive "molecular ridge" that connects the "20 km s^-1" and "50 km s^-1" giant molecular clouds.

We present the first detection of unidentified infrared (UIR) emission features at ~6.4 and 7.9 μm in the spectrum of the dusty WC8 Wolf-Rayet star WR 48a. Based on the H-deficient nature of WC stars, we attribute the emission features to large carbonaceous molecules or amorphous carbon dust grains in the circumstellar environment of WR 48a. The 6.4 μm feature resembles the emission feature seen toward H-deficient planetary nebulae (PNe), while the 7.9 μm profile resembles that of some PNe with H-deficient WC10 central stars. These similarities point toward a similar origin of the dust in these H-deficient environments and highlights the apparent sensitivity of the UIR bands to physical conditions. In the case of WR 48a and the [WC10] PNe, shock processing may play a major role in dust formation.

We present observations of the W3 complex of massive star formation regions using the Chandra X-Ray Observatory. In the W3 core region, our observations resolve the emission previously observed with ASCA into 101 compact sources. The integrated spectrum is consistent with that observed earlier by ASCA and shows significant emission at energies larger than 2.5 keV. Comparing our Chandra data with existing near-infrared images, we find X-ray counterparts for W3 IRS 2, W3 IRS 2a, and W3 IRS 3a that are believed to be the ionizing stars for the H II regions W3A and W3B. We also detect X-ray emission coincident with the massive protostar candidate W3 IRS 5. Comparing our Chandra data with existing radio continuum data, we find that for radio continuum components A, B, C, D, G, and H, one or several X-ray sources are located at the peak radio position and/or the geometric center of the H II region. We postulate that the X-ray sources are the young massive stars that are also responsible for the ionization of the compact and ultracompact H II regions in the W3 core. Our observations show that very young massive stars are emitters of relatively hard X-rays and that they can be detected with Chandra even in a high-density environment.

We report estimates of the masses of the component stars in the pre-main-sequence spectroscopic binary UZ Tau E. These results come from the combination of our measurements of the mass ratio, M₂/M₁ = 0.28 ± 0.01, obtained using high-resolution H-band spectroscopy, with the total mass of the system, (1.31 ± 0.08)(D/140 pc) M_☉, derived from millimeter observations of the circumbinary disk (Simon, Dutrey, & Guilloteau). The masses of the primary and secondary are (1.016 ± 0.065)(D/140 pc) M_☉ and (0.294 ± 0.027)(D/140 pc) M_☉, respectively. Using the orbital parameters determined from our six epochs of observation, we find that the inclination of the binary orbit, 598 ± 44, is consistent with that determined for the circumbinary disk from the millimeter observations, indicating that the disk and binary orbits are probably coplanar.

In photoionized plasmas, X-ray line emission is dominated by radiative cascades following recombination and photoexcitation, with a negligible contribution from collisional excitation due to the relatively low electron temperatures. Most existing photoionized plasma emission codes include only radiative recombination (RR) when calculating the level populations. Such models have been shown to be inadequate to explain the X-ray emission from H-like and He-like ions in the spectra of some Seyfert 2 active galactic nuclei and X-ray binaries obtained by the grating spectrometers on board Chandra and XMM-Newton. It has been shown that photoexcitation must be included to obtain theoretical models consistent with observations. In this Letter we show that for X-ray emission from iron L-shell ions, one must also include dielectronic recombination (DR) as a level population mechanism. As an example, the well-known 3 → 2 lines of Fe XVII have been analyzed in detail. The 3F line at 16.78 Å is most affected by DR of Fe XVIII at low temperatures, while it is negligibly weak in purely RR models.