Showing papers by "Space Telescope Science Institute published in 1996"

High-redshift galaxies in the hubble deep field : colour selection and star formation history to z 4

Abstract: The Lyman decrement associated with the cumulative effect of H I in QSO absorption systems along the line of sight provides a distinctive feature for identifying galaxies at z ≳ 2.5. Colour criteria, which are sensitive to the presence of a Lyman continuum break superposed on an otherwise flat UV spectrum, have been shown, through Keck spectroscopy, to successfully identify a substantial population of star-forming galaxies at 3 ≲ z ≲ 3.5. Such objects have proven to be surprisingly elusive in field galaxy redshift surveys; quantification of their surface densities and morphologies is crucial for determining how and when galaxies formed. The Hubble Deep Field (HDF) observations offer the opportunity to exploit the ubiquitous effect of intergalactic absorption and obtain useful statistical constraints on the redshift distribution of galaxies to considerably fainter limits than the current spectroscopic limits. We model the H I cosmic opacity as a function of redshift, including scattering in resonant lines of the Lyman series and Lyman continuum absorption, and use stellar population synthesis models with a wide variety of ages, metallicities, dust contents and redshifts to derive colour selection criteria that provide a robust separation between high-redshift and low-redshift galaxies. From the HDF images we construct a sample of star-forming galaxies at 2 ≲z ≲ 4.5. While none of the ∼ 60 objects in the HDF having known Keck/Low-Resolution Imaging Spectrograph (LRIS) spectroscopic redshifts in the range 0 ≲ z ≲1.4 is found to contaminate our high-redshift sample, our colour criteria are able to efficiently select the 2.6 ≲ z ≲ 3.2 galaxies identified by Steidel et al. The ultraviolet (and blue) dropout technique opens up the possibility of investigating cosmic star and element formation in the early Universe. We set a lower limit to the ejection rate of heavy elements per unit comoving volume from Type II supernovae at 〈z〉 = 2.75 of ≈ 3.6 × 10^(−4) M_⊙ yr^(−1) Mpc^(−3) (for q_0 = 0.5 and H_0 = 50 km s^(−1) Mpc^(−1)), which is 3 times higher than the local value but still 4 times lower than the rate observed at z ≈ 1. At 〈z〉 = 4, our lower limit to the cosmic metal ejection rate is ≈ 3 times lower than the 〈z〉 = 2.75 value. We discuss the implications of these results on models of galaxy formation, and on the chemical enrichment and ionization history of the intergalactic medium.

The Hubble Deep Field: Observations, data reduction, and galaxy photometry

Abstract: The Hubble Deep Field (HDF) is a Director’s Discretionary program on HST in Cycle 5 to image an undistinguished field at high Galactic latitude in four passbands as deeply as reasonably possible. These images provide the most detailed view to date of distant field galaxies and are likely to be important for a wide range of studies in galaxy evolution and cosmology. In order to optimize observing in the time available, a field in the northern continuous viewing zone was selected and images were taken for ten consecutive days, or approximately 150 orbits. Shorter 1-2 orbit images were obtained of the fields immediately adjacent to the primary HDF in order to facilitate spectroscopic follow-up by ground-based telescopes. The observations were made from 18 to 30 December 1995, and both raw and reduced data have been put in the public domain as a community service. We present a summary of the criteria for selecting the field, the rationale behind the filter selection and observing times in each band, and the strategies for planning the observations to maximize the exposure time while avoiding earth-scattered light. Data reduction procedures are outlined, and images of the combined frames in each band are presented. Objects detected in these images are listed in a catalog with their basic photometric parameters.

Spectroscopic Confirmation of a Population of Normal Star-forming Galaxies at Redshifts z > 3*

Abstract: We report the discovery of a substantial population of star-forming galaxies at 3.0 z 3.5. These galaxies have been selected using color criteria sensitive to the presence of a Lyman continuum break superposed on an otherwise very blue far-UV continuum, and then confirmed with deep spectroscopy on the W. M. Keck telescope. The surface density of galaxies brighter than = 25 with 3.0 z 3.5 is 0.4 ± 0.07 galaxies arcmin-2, approximately 1.3% of the deep counts at these magnitudes; this value applies both to "random" fields and to fields centered on known QSOs. The corresponding comoving space density is approximately half that of luminous (L L*) present-day galaxies. Our sample of z > 3 galaxies is large enough that we can begin to detail the spectroscopic characteristics of the population as a whole. The spectra of the z > 3 galaxies are remarkably similar to those of nearby star-forming galaxies, the dominant features being strong low-ionization interstellar absorption lines and high-ionization stellar lines, often with P Cygni profiles characteristic of Wolf-Rayet and O star winds. Lyα emission is generally weak (less than 20 A rest equivalent width) and is absent for more than 50% of the galaxies. We assign approximate mass scales to the galaxies using the strengths of the heavily saturated interstellar features and find that, if the line widths are dominated by gravitational motions within the galaxies, the implied velocity dispersions are 180 km s-1 ≤ σ ≤ 320 km s-1, in the range expected for massive galaxies. The star formation rates, which can be measured directly from the far-UV continua, lie in the range 4-25 h−250 M☉ yr-1 (for q0 = 0.5), with 8.5 h−250 M☉ yr-1 being typical. Together with the morphological properties of the z > 3 galaxy population, which we discuss in a companion paper, all of these findings strongly suggest that we have identified the high-redshift counterparts of the spheroid component of present-day luminous galaxies. In any case, it is clear that massive galaxy formation was already well underway by z ~ 3.5.

Mass Flow through Gaps in Circumbinary Disks

Abstract: We demonstrate through smoothed particle hydrodynamics simulations that a circumbinary disk can supply mass to the central binary through gas streams that penetrate the disk gap without closing it. The conditions for an efficient flow typically require the disk thickness-to-radius ratio z/r 0.05, if the turbulent viscosity parameter α is greater than 0.01. This mass flow may be important for both the individual systems and their statistics. It occurs preferentially onto the lower mass object and acts toward equalization of component masses. The less massive component may be more luminous and easier to detect, owing to its larger accretion luminosity. For eccentric binaries, the mass flow is strongly modulated in time, providing diagnostics for both the disk and the binary. In the protostellar disks, the flow could be detected as shock emission phased with the binary orbit, resulting from stream impact with the circumstellar disks and/or the young stars. In the (super)massive black hole binaries in nuclei of galaxies, the flow may result from the surrounding interstellar medium and produce nearly periodic emission, as observed in quasar OJ 287. For star-planet-disk systems, our results show that the opening of a gap around a planet is not always sufficient for the termination of its growth. This suggests that planets supplied by gas streams from protoplanetary disks may outgrow Jupiter to become "superplanets" with properties heretofore reserved for stars.

Spectroscopic Confirmation of a Population of Normal Star-Forming Galaxies at Redshifts z>3

Abstract: We report the discovery of a substantial population of star--forming galaxies at $3.0 \simlt z \simlt 3.5$. These galaxies have been selected using color criteria sensitive to the presence of a Lyman continuum break superposed on an otherwise very blue far-UV continuum, and then confirmed with deep spectroscopy on the W. M. Keck telescope. The surface density of galaxies brighter than R=25 with 3 3$ galaxies is large enough that we can begin to detail the spectroscopic characteristics of the population as a whole. The spectra of the $z>3$ galaxies are remarkably similar to those of nearby star-forming galaxies, the dominant features being strong low--ionization interstellar absorption lines and high--ionization stellar lines, often with P-Cygni profiles characteristic of Wolf-Rayet and O--star winds. Lyman $\alpha$ emission is generally weak ($ 50% of the galaxies. The star formation rates, measured directly from the far-UV continua, lie in the range 4-25 $h_{50}^{-2}$ M$\sun$ yr$^{-1}$ for $q_0=0.5$. Together with the morphological properties of the $z>3$ galaxy population, which we discuss in a companion paper (Giavalisco \et 1996), all of these findings strongly suggest that we have identified the high-redshift counterparts of the spheroid component of present--day luminous galaxies. In any case, it is clear that massive galaxy formation was already well underway by $z \sim 3.5$. (shortened abstract). arch-ive/yymmnnn

Hubble Space Telescope Observations of the Disk and Jet of HH 30

Abstract: HH 30 in Taurus has been imaged with the Hubble Space Telescope WFPC2. The images show in reflected light a flared disk with a radius of about 250 AU that obscures the protostar. The disk resembles detailed accretion disk models that constrain its density distribution and show that its inclination is less than 10 degrees. There are bipolar emission-line jets perpendicular to the disk, a very clear demonstration of the standard paradigm for accretion disk and jet systems. However, asymmetries in the light distribution show that the disk has not completely settled into a quasi-equilibrium accretion state, or that some of the observed scattering is from an asymmetric envelope. The emission-line jet itself is resolved into a number of knots with typical lengths and separations of 0".4, much smaller and more numerous than indicated by lower resolution ground-based studies. There are indications of still finer structures in the jet all the way to the resolution limit of 0".1. The knots have proper motions ranging from 100 to 300 km s^(-1) and are therefore generated at the surprisingly high rate of about 0.4 knots per jet per year. The jet appears to be collimated within a cone of opening angle 3o and can be seen to within 30 AU of the star. Both single- and multiple-scattering disk models have a range of possible solutions, but by requiring pressure support and temperature equilibrium, a self-consistent model emerges. There is evidence for pressure support because the disk appears to have a Gaussian height profile. The temperature at each point in the disk is determined by the disk geometry, which in turn fixes the temperature in a self-consistent manner. The extinction to the protostar is unknown but constrained to be greater than 24 mag. The optical properties of the scattering grains in the disk are determined and found to imply a large scattering asymmetry, but they seem to follow the interstellar reddening law. The absolute magnitude and colors of the unseen protostar, which has a brightness in the I bandpass of about 0.16 times solar and is very red, are obtained. The disk mass is about 0.006 times solar and has an expected lifetime of about 10^5 yr.

Tidal Decay of Close Planetary Orbits

Abstract: The 4.2-day orbit of the newly discovered planet around 51 Pegasi is formally unstable to tidal dissipation. However, the orbital decay time in this system is longer than the main-sequence lifetime of the central star. Given our best current understanding of tidal interactions, a planet of Jupiter’s mass around a solar-like star could have dynamically survived in an orbit with a period as short as ∼ 10 hr. Since radial velocities increase with decreasing period, we would expect to find those planets close to the tidal limit first and, unless this is a very unusual system, we would expect to find many more. We also consider the tidal stability of planets around more evolved stars and we re-examine in particular the question of whether the Earth can dynamically survive the red-giant phase in the evolution of the Sun. Subject headings: Planets and Satellites: General — Solar System: General — Stars: Planetary Systems — Sun: Solar-terrestrial Relations

Reddening and Star Formation in Starburst Galaxies

Abstract: The reddening properties and the star formation history of a sample of 19 starburst galaxies are investigated using multiwavelength spectroscopy and infrared broad band photometry. The difference in reddening between the ionized gas and the stars is explained as difference in the covering factors of the dust in front of the gas and of the stars. A ``template starburst spectrum'', derived by combining the reddening-corrected UV, optical, and infrared data of all the galaxies in the sample, is used to investigate the star formation history. Spectral synthesis models indicate that the observed UV emission can be attributed to a stellar population which is undergoing active star formation at a constant rate since ~ 20 Myr, in agreement with the supernova rates derived from the [FeII] emission line in the infrared. At least two, and probably more, intermediate age populations (age < 2 Gyr) contribute to the optical and infrared emission, while populations older than 2 Gyr do not contribute significantly to the template. Episodic star formation over the last Gyr is suggested, with star formation rates as large as or larger than the present rates. The synthetic stellar populations are generated according to an Initial Mass Function (IMF) with Salpeter slope (alpha=2.35) in the mass range 0.1--100 solar masses, and reproduce a number of observational constraints, such as the spectral energy distribution of the template spectrum, the equivalent width of the atomic hydrogen emission lines, and the mass-to-light ratios; the data, then, do not provide indication for a high-mass-star truncated or a low-mass-star deficient IMF in starburst galaxies.

Tidal Decay of Close Planetary Orbits

Abstract: The 4.2-day orbit of the newly discovered planet around 51~Pegasi is formally unstable to tidal dissipation. However, the orbital decay time in this system is longer than the main-sequence lifetime of the central star. Given our best current understanding of tidal interactions, a planet of Jupiter's mass around a solar-like star could have dynamically survived in an orbit with a period as short as $\sim10\,$hr. Since radial velocities increase with decreasing period, we would expect to find those planets close to the tidal limit first and, unless this is a very unusual system, we would expect to find many more. We also consider the tidal stability of planets around more evolved stars and we re-examine in particular the question of whether the Earth can dynamically survive the red-giant phase in the evolution of the Sun.

Notes on the eigensystem of magnetohydrodynamics

Abstract: The eigenstructure of the equations governing one-dimensional ideal magnetohy-drodynamics is examined, motivated by the wish to exploit it for construction of high-resolution computational algorithms. The results are given in simple forms that avoid indeterminacy or degeneracy whenever possible. The unavoidable indeterminacy near the magnetosonic (or triple umbilic) state is analysed and shown to cause no difficulty in evaluating a numerical flux function. The structure of wave paths close to this singularity is obtained, and simple expressions are presented for the structure coefficients that govern wave steepening.

H ii galaxies versus photoionization models for evolving starbursts

Abstract: We have constructed a grid of models representing an H II region produced by an evolving starburst embedded in a gas cloud of the same metallicity The models were produced with the spectral energy distribution from a stellar evolutionary synthesis code as input for a photoionization code that computes the emission-line strengths and equivalent widths Stellar evolution was assumed to proceed according to the models of Maeder The radiation -eld was computed using the Kurucz model atmospheres, supple- mented by the expanding non-LTE atmospheres of Schmutz et al for stellar evolutionary phases with strong winds, making a signi-cant improvement over previous works using classical static, plane-parallel model atmospheres Models for stellar interiors and atmospheres being still in a phase of continuous improvement, our population synthesis models reNect the state of the art in 1995 The models were used to analyze a sample of 100 H II galaxies for which both the Hb equivalent widths and the (O III) j4363 line intensities were available (the latter allowing a direct determination of the oxygen abundances based on measured electron temperatures) Because of these selection criteria, the results of our study are restricted to metal-poor objects with metallicities less than about one-half solar The confrontation of models with observations is presented in six diagnostic diagrams involving hydro- gen and oxygen lines Our approach is in many respects much more constraining for the models than previous studies on H II regions ionized by evolving starbursts We found that the standard starburst model (instantaneous burst of star formation with a Salpeter initial mass function and an upper cuto† mass of 100 reproduces the observational constraints pro- M _ ) vided by the nebular emission lines extremely well if selection e†ects are taken into account Models with a unique initial mass function are consistent with essentially all observational constraints over a metal- licity range from D0025 to D025 In contrast, models with a Salpeter-type initial mass function Z _ truncated at 50 are not consistent with the observations: they violate the observed distribution of M _ Hb equivalent widths The mean e†ective temperature of the ionizing star cluster declines from about 50,000 to 40,000 K during the time when the line (O III) j4363 is strong enough to be measurable W ithin the framework of our models, and in the abundance range where comparisons were made with observations, there is no signi-- cant evidence for a variation of the star cluster mean e†ective temperature with metallicity, other than the one generated by the metallicity-dependent stellar atmospheric and evolutionary models A very narrow range in ionization parameters is required to reproduce the observed line ratios This should set limits on the dynamical evolution of giant H II regions We -nd a large fraction of H II galaxies having (O I) j6300/Hb ratios larger than 002 Even models with the lowest ionization parameters considered do not produce these large ratios An approximate esti- mate of the mechanical energy released by winds and supernovae during later phases of the starburst leads to the suggestion that the (O I) j6300/Hb ratioEin contrast to other line ratios studiedEis signi-- cantly a†ected by shocks The small spread in the free parameters necessary to reproduce the emission-line properties of metal- poor H II galaxies allows us to propose a new indicator of the starburst age: the (O III) j5007 equivalent width is quite robust and can be used up to larger ages than the traditional Hb equivalent width for high signal-to-noise spectra This indicator should also prove useful for low signal-to-noise spectra of star-forming galaxies at higher redshift, because of the large value of (O III) j5007/Hb in starbursts younger than 5 Myr

A survey of the ISM in early-type galaxies. I. The ionized gas

Abstract: We present results of a CCD optical imaging survey of the ionized gas in 73 luminous elliptical and lenticular galaxies, selected from the RC3 catalog to represent a broad variety of X-ray, radio, infrared and kinematical properties. For each galaxy we have used broad-band R images and narrow-band images centered at the and emission lines to derive the luminosity and distribution of the ionized gas. We found that a large fraction of E (72%) and S0 (85%) galaxies in our sample contain ionized gas. The gas morphology appears to be rather smooth for most galaxies; however of the sample galaxies show a very extended filamentary structure. According to the morphology and size of the gas distribution, the galaxies have been classified into three broad groups, named small disk (SD), regular extended (RE) and filamentary structure (F). The mean diameter of the emitting region ranges between 1 and 10 kpc; the derived mass of the ionized gas ranges between 103 and 105 solar masses. A significant correlation between and X-ray luminosities is found for those galaxies (27% of the sample) for which we have detected ionized gas and are also listed as X-ray sources. However, there are relatively strong X-ray emitting galaxies for which we have not detected emission and objects which show emission-lines but are not listed either in the EINSTEIN or in the ROSAT databases. The distribution of datapoint and upper limits in this diagram suggests that galaxies with warm gas are also X-ray emitters, while there are X-ray emitters without measurable emission. Similar characteristics are present in the correlation between the infrared luminosity in the 12 band and ; correlations with other infrared wavelengths are weaker. A strong correlation was also found between the luminosity and the luminosity in the B band inside the region occupied by the line-emitting gas. We use these correlations to discuss the possible mechanisms responsible for the gas ionization and excitation, analyzing in particular the role of the post-AGB stars and the thermal conduction from the X-ray halo in providing the necessary source of ionization.

Far-Ultraviolet Imaging of Jupiter's Aurora and the Io “Footprint”

Abstract: Far-ultraviolet images of Jupiter from the Hubble Space Telescope Wide Field Planetary Camera 2 reveal polar auroral emissions at 300 kilometer resolution and three times higher sensitivity than previously achieved. Persistent features include a main oval containing most of the emission and magnetically connected to the middle magnetosphere, diffuse and variable emissions poleward of the main oval, and discrete emission from Io's magnetic footprint equatorward of the oval. The auroral emissions are variable, exhibit magnetic conjugacy, and are visible above the planet limb. All emissions approximately co-rotate with Jupiter except the Io “footprint,” which is fixed along Io's magnetic flux tube.

A Database for Galaxy Evolution Modeling

Abstract: This paper represents a collective effort to provide an extensive electronic data base useful for the interpretation of the spectra and evolution of galaxies. A broad variety of empirical and theoretical data is discussed here, and the data are made fully available in the AAS CD-ROM Series, Vol. 7. Several empirical stellar libraries are part of this data base. They cover the ultraviolet spectral range observed with IUE, optical data from different ground-based telescopes, and ground-based infrared data. Spectral type coverage depends on the wavelength, but is mostly complete for types O to M and luminosity classes V to I. A large metallicity range is covered as well. Theoretical libraries of selected spectral indices of cool stars and of stellar continuum fluxes in the temperature range 2000-50,000 K as well as Wolf-Rayet energy distributions are presented. Several libraries of star clusters and early-type galaxies have been selected for this data base. We discuss an extensive set of empirical spectral templates covering the wavelength region from 1200 to 9800 Angstrom, as well as narrow-band line indices in a large number of passbands. Bench-mark spectra of nearby galaxies for model tests are included as well. We compiled numerous evolutionary models and isochrones for stars of all mass ranges of interest, wide metallicity range, and for all evolutionary phases, including the pre-main-sequence phase. The majority of the models have been computed by the Geneva and Padova groups. Evolutionary synthesis models computed by several independent groups are made available. They can be applied to old and young systems, and are optimized with respect to different aspects of input physics. The model predictions include stellar (colors, magnitudes, absorption features) and nebular (emission-line fluxes) properties. Finally, we present models of ionized gas to be used for the interpretation of active galactic nuclei and young star-forming galaxies. The community is encouraged to make use of this electronic data base and to perform a critical comparison between the individual datasets.

Detection of Ozone on Ganymede

Abstract: An absorption band at 260 nanometers on the trailing hemisphere of Ganymede, identified as the Hartley band of ozone (O3), was measured with the Hubble Space Telescope. The column abundance of ozone, 4.5 × 1016 per square centimeter, can be produced by ion impacts or by photochemical equilibrium with previously detected molecular oxygen (O2). An estimated number density ratio of [O3]/[O2] ≈ 10−4 to 10−3 requires an atmospheric density orders of magnitude higher than upper limits from spacecraft occultation experiments. Apparently, this O2-O3 “atmosphere” is trapped in Ganymede9s surface ice, an inference consistent with the shift and broadening of the band compared with the gas-phase O3 band.

HST Snapshot Survey of 3CR Radio Source Counterparts. I. Intermediate Redshifts

Abstract: We have obtained images of 267 3CR radio galaxies and quasars by conducting a snapshot survey with HST's WFPC2 through a broadband red (F702W) filter. This is the first in a series of papers resulting from this survey, describing and presenting the basic data. Here we focus on the 77 radio galaxies within the redshift range 0.1 < z < 0.5 (91% of the total number of 3CR radio galaxies within this redshift range). The images show that on the 01 scale of the HST observations there is a wide variety of structure in radio galaxy morphology. At least 30% of the galaxies show dust, either well-defined dust lanes or chaotically distributed throughout the galaxy. We find disturbed morphologies, such as multiple nuclei and tails of emission, that could indicate merging systems. There is an obvious optical synchrotron jet in 3C 346, and another eight jet candidates are present.

The White Dwarf Distance to the Globular Cluster NGC 6752 (and Its Age) with the HUBBLE SPACE TELESCOPE

Abstract: Deep Hubble Space Telescope (HST) observations with WFPC2 of the nearby globular cluster NGC 6752 have allowed us to obtain accurate photometry for the cluster white dwarfs (WDs). A sample of local WDs of known trigonometric parallax and mass close to that of the cluster WDs have also been observed with WFPC2. Matching the cluster and the local WD sequences provides a direct measure of the distance to the cluster: (m - M)0 = 13.05, with an uncertainty less than ±0.1 mag, which allows a substantial reduction in the uncertainty in the age of the cluster. Indeed, coupling this value of the cluster distance to the cluster metallicity, helium abundance, and α-element enhancement [α/Fe] = 0.5 yields an age of 15.5 and 14.5 Gyr using evolutionary models that do not include or do include helium diffusion, respectively. The uncertainty affecting these age determinations is ~10%. The majority of the cluster WDs appear to be of the DA variety, while the color-magnitude location of two WDs is consistent with the DB type. This suggests a cluster DB/DA ratio similar to that of WDs in the solar neighborhood.

Polarimetry and modelling of narrow-line active galaxies

Abstract: iDivision of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts ALI0 9AB 2Department of Astronomy and McDonald Observatory, RLM 15.308, University of Tems at Austin, Austin, TX 78712, USA 3Anglo-Australian Observatory, PO Box 296, Epping, NSW 2121, Australia 'University of Oxford, Nuclear and Astrophysics Laboratory, Keble Road, Oxford OXI 3RH sAffiliated to the Space Sciences Division of ESA, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Wuffield RDdio Astronomy Laboratory, University of Manchester, Jodrell Bank, Macclesfield, Cheshire SKI 1 9DL

WFPC2 Studies of the Crab Nebula. III. Magnetic Rayleigh-Taylor Instabilities and the Origin of the Filaments

Abstract: Recently obtained Hubble Space Telescope WFPC2 images of the Crab Nebula show that the emission-line filaments are dominated by structures that morphologically appear to be the result of magnetic Rayleigh Taylor (R-T) instabilities at the interface between the pulsar-driven synchrotron nebula and a shell of swept up ejecta. We replace this morphological argument with a quantitative treatment of the growth rate and characteristic wavelength of such instabilities. Using published data on the rate of expansion of the synchrotron nebula and the density of the ejecta, together with a wavelength for the instability measured from the WFPC2 images, we calculate a magnetic field strength of ~540 µG. This is within a factor of 2 of the canonical minimum energy equipartition field of 300 µG, and probably closer than that to a more realistic estimate of the field at the edge of the Crab. Comparison of the detailed morphology and ionization structure of the R-T fingers in the Crab with recent magnetohydrodynamical simulations which follow the development of magnetic R-T instabilities into the nonlinear regime is used to establish a sequence of filament properties which are determined by the density of the shell of swept-up ejecta at the edge of the synchrotron nebula. When the density is below a critical value, the interface is stable. For somewhat higher densities R-T instabilities grow, but the field, which becomes aligned along the length of the R-T fingers, is strong enough to prevent the development of secondary Kelvin Helmholtz (K-H) instabilities as the finger falls through the lighter medium. At higher densities these K-H instabilities develop, but the field is still strong enough to maintain a long streamer-like connection between the head of the filament and the shell. In a few cases, the density of the shell is high enough that the magnetic field is unable to prevent the fragmentation of R-T fingers, and the structure becomes more characteristic of a nonmagnetic R-T instability. The magnetic field is oriented along the length of an R-T finger, so material is free to "pour" into the finger from above. In equilibrium, gradients in thermal pressure and effective gravity must balance along field lines. As a result, loss of pressure support in the fingers due to cooling enhances the flow of material into the fingers, "siphoning" gas into the finger from above. If an extended remnant of ejecta surrounds the visible extent of the Crab, as has been suggested frequently, then the synchrotron nebula is expanding through this extended remnant, sweeping up ejecta as it goes. R-T instabilities channel this swept-up ejecta into the hierarchy of dense visible filaments. It seems likely that the current system of filaments originated as a result of R-T instabilities as the synchrotron nebula expanded out through more uniformly distributed ejecta. If an extended remnant remains today, then filament formation is an ongoing process. The ionization structure of filaments is also found to change in a systematic way as a function of the relative importance of the magnetic field and the mass density. Filaments which are dominated by the magnetic field are confined by the field and have sharp, well defined edges. Filaments in which the magnetic field is less dominant consist of high-density, low-ionization cores embedded within more extended high-ionization material. This confirms a previous suggestion that variations in magnetic confinement are an important caveat to published interpretations of spectra of Crab filaments.

The Jet of M87 at Tenth-Arcsecond Resolution: Optical, Ultraviolet, and Radio Observations*

Abstract: The European Space Agency's Faint Object Camera (FOC) on the Hubble Space Telescope (HST) was used to obtain optical and UV imaging of the jet of M87 prior to the HST refurbishment. We present a detailed comparison to new VLA radio observations at similar spatial resolution and flux information for the radio, optical, and UV bands.While the radio and optical images present a remarkable degree of similarity, there are nevertheless significant differences. The optical/UV images show intrinsically higher contrast than the radio, with compact regions of emission localized within the knots. The jet is narrower in the optical/UV; the emission is more concentrated to the jet's center in the optical/UV than in the radio band. The radio-to-optical spectral index of the interknot regions is steeper than that of the knots themselves. There are also differences in the detailed knot structure of the optical emission compared to the radio, and there is a weak overall spectral steepening with distance from the nucleus beyond knot A.The jet does not show pronounced limb brightening in the optical/UV. This indicates that the emission occurs within the jet and not in a thin boundary around the jet, as in some jet models.We explore an idealized synchrotron model of jet emission and derive volume-deprojected physical parameters for the model. While the knots themselves are overpressured with respect to the surroundings, the pressure of the interknot regions in this observationally based model drops to the ambient external pressure, as in theories that invoke jet recollimation at shocks. Alternatively, internal shocks may be triggered by boundary instabilities or time-dependent power output from the nucleus. Downstream from knot A, the situation is less ordered, although in situ acceleration near the jet's center line also seems to be required, as the optical jet remains narrower than the radio jet there.

First Image of the Surface of a Star with the Hubble Space Telescope

Abstract: A direct image of the surface of a star, Betelgeuse (=α Ori; M2 Iab), has been obtained with the Faint Object Camera on the Hubble Space Telescope Images in two ~300 A-wide bands centered at 2550 and 2800 A cover ~10 resolution elements on the stellar disk The ultraviolet diameter of Betelgeuse of 125 ± 5 mas at 2500 A (corresponding to 108 ± 4 mas, FWHM) is a factor of 22 larger than the optical diameter, thus indicating a substantially extended chromosphere in this supergiant A single bright, unresolved area is present in the southwest quadrant of the disk (PA = 235°) in both images, with a peak amplitude of 13-18 as compared to the surrounding disk and a temperature differential in excess of 200 K This feature may be the result of magnetic activity, atmospheric convection, or global pulsations and shock structures that heat the chromosphere Spatially resolved spectroscopy of the Mg II λλ2795, 2802 doublet with the Goddard High Resolution Spectrograph suggests complicated dynamics, with outflowing material in the chromosphere indicated by the Mg II emission

On the Formation and Evolution of Common Envelope Systems

Abstract: We discuss the formation of a common envelope system following dynamically unstable mass transfer in a close binary and the subsequent dynamical evolution and final fate of the envelope. We base our discussion on new three-dimensional hydrodynamic calculations that we have performed for a close binary system containing a 4 M☉ red giant with a 0.7 M☉ main-sequence star companion. The initial parameters are chosen to model the formation of a system resembling V471 Tau, a typical progenitor of a cataclysmic variable binary. The calculations are performed using the smoothed particle hydrodynamics (SPH) method with up to 5 × 104 particles. As initial condition we use an exact hydrostatic equilibrium configuration at the onset of dynamically unstable mass transfer. The nonlinear development of the instability is followed using SPH until a quasi-static common envelope configuration is formed. In our highest resolution calculation, we find evidence for a corotating region of gas around the central binary. This is in agreement with the theoretical model proposed by Meyer & Meyer-Hofmeister for the evolution of common envelope systems, in which this central corotating region is coupled to the envelope through viscous angular momentum transport only. We also find evidence that the envelope is convectively unstable, in which case the viscous dissipation time could be as short as ~100 dynamical times, leading to rapid ejection of the envelope. For V471 Tau our results, and the observed parameters of the system, are entirely consistent with rapid envelope ejection on a timescale ~1 yr and an efficiency parameter αCE 1.

Ultraviolet Extinction by Interstellar Dust in External Galaxies: M31

Abstract: Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra of stars in OB associations of M31 are used to derive the UV extinction by interstellar dust in M31 by three different methods: (1) comparing spectra of M31 star pairs, (2) comparing spectra of M31 stars to those of Galactic standard stars, and (3) comparing M31 star spectra to atmosphere models. The derived intrinsic M31 extinction curve has an overall wavelength dependence very similar to that of the average Galactic extinction curve but possibly has a weaker 2175 A bump, however, with a significance of only 1 σ. This result is different from the LMC (30 Dor)-like curves published earlier, which contained both intrinsic M31 extinction and "foreground" extinction, and were based either on low-signal IUE spectra, or on FOS data affected by inaccuracy in the preliminary flux calibration, and were not computed with the pair method used in this work. In this work, the foreground extinction component from the Galactic halo is also investigated. The foreground curve is consistent with the standard curve. While there is a slight indication for a steeper foreground curve than the standard one, the difference is not significant considering the data uncertainties.