Showing papers on "Star formation published in 1983"

The rate of star formation in normal disk galaxies.

Abstract: Photometry of the integrated H..cap alpha.. emission in a large sample of field spiral and irregular galaxies has been used to obtain quantitative estimates of the total star formation rate (SFR) in the galaxies. The photoionization properties of a stellar population have been modeled for a variety of choices for the initial mass function (IMF). The observed UBV colors and H..cap alpha.. emission equivalent widths place tight constraints on the slope of the IMF between 1 M /sub sun/ and 50 M /sub sun/ in the galaxies; excellent agreement with the observed galaxy colors and H..cap alpha.. emission is obtained with models using an IMF slope close to Salpeter's original value. The properties of late-type galaxies are not well reproduced by the Miller-Scalo solar neighborhood IMF. The extinction-corrected star formation rates are large, as high as 20 M /sub sun/ yr/sup -1/ in giant Sc galaxies (H/sub 0/ = 50 km s/sup -1/ Mpc/sup -1/). The current rates in late-type galaxies are comparable to the past rates averaged over the age of the disk; late-type disk galaxies have evolved at a nearly constant rate, confirming earlier models by Searle, Sargent, and Bagnuolo. Little evidence is found for a strong correlationmore » between the SFR and average gas density; if the SFR proportional rho/sup n/, then the exponent n must be much less than 1, corroborating earlier studies of star formation in the solar neighborhood by Miller, Scalo, and Twarog. Comparison of the present SFRs with the remaining supply of interstellar gas yields consumption time scales of only a few times 10/sup 9/ years in most cases, in agreement with the model estimates of Larson, Tinsley, and Caldwell.« less

Highlights of astronomy

Abstract: Invited Discourses.- Pulsars and their Genesis.- Dark Matter in the Universe.- Venus.- Joint Discussions.- 1. Reference Frames.- Uses and Required Characteristics of Reference Frames for Galactic Astronomy.- Reference Frames for Minor Planets, Comets and Satellites.- Reference Frames and the Extragalactic Distance Scale.- Requirements for Earth Rotation Parameters.- Ephemerides and Celestial Mechanics.- Status Report on the Work on the FK5.- J2000.0.- Classical Absolute/Differential Programs.- The Background to the MERIT/COTES Recommendations on the Terrestrial and Celestial Reference Systems.- HIPPARCOS and Celestial Reference Frame.- The Use of the Hubble Space Telescope for Global Reference Frame Work.- Proper Motions Referred to Faint Galaxies.- Reference Frame Studies at JPL/ Caltech.- On the Use of Natural References.- Relativistic Reference Frames in Astrometry.- Comparison of the Optical and Radio Reference Frames.- The Promise of Optical/IR Interferometry and Space Astrometry.- Reference Frame/Coordinate System in General Relativity.- Discussions.- 2. Long-Period Eclipsing Binary Stars and Related Objects.- Photometry of the Recent Eclipse of Epsilon Aurigae.- A Spectroscopic View of Epsilon Aurigae.- Interpretation of the Data of Epsilon Aurigae.- Observation and Interpretation of Zeta Aurigae Stars.- Related Binaries, Including Symbiotic Stars.- Mass Transfer and Evolution in Long-Period Binary Systems.- Wind Accretion and Interaction in Long Period Binary Systems.- Summary and Critique of Joint Discussion II.- Extraction of the Shell Spectrum of Epsilon Aurigae.- Dynamics of the Epsilon Aurigae Ring Model.- Interim Discussion of the Orbit of ? Sagittae.- Evidence for the Roche Lobe Overflow in VV Cephei.- Infrared Michelson Interferometry of Zeta Aurigae Type Supergiants.- Very Long Period Supergiant Semidetached and Contact Systems.- 3. Solar and Stellar Nonradial Oscillations.- Progress and Problems in the Study of the Pulsating White Dwarf Stars.- The PG1159 Variables.- Non-Radial Oscillations in ? Scuti Stars and Rapidly Oscillating Ap Stars.- Beta Cephei Variables.- Nonradial and Radial Oscillations Observed in Non-Emission Line OB Dwarfs and Giants.- Non-Radial Pulsation in Be Stars.- Radial and Non-Radial Pulsations in Wolf-Rayet Stars and in Supergiants.- Solar and Solar-Like Oscillations: Theory.- 4. Radio Astronomy and Cosmology.- Cosmic Microwave Background Spectrum Measurements.- The Angular Distribution of the Cosmic Background Radiation.- Ovro Results on the Sunyaev-Zel'dovich Effect, 1983-1985.- The Microwave Background from Cambridge.- Searches for Primordial Pancakes.- Intergalactic Neutral Hydrogen.- The Nature of Faint Radio Sources.- Is the Upturn in the Source Counts Caused by Primeval Radio Galaxies ?.- A New Deep VLA Radio Survey at 6 cm.- Dependence of Linear Sizes and Spectral Indices of Extended Radio Galaxies on Redshift and Radio Luminosity.- Abundances of the Very Light Elements (D, 3He, 4He and 7Li) and Primordial Nucleosynthesis.- 5. Stellar Activity: Rotation and Magnetic Fields.- Stellar Dynamo Characteristics.- Stellar Activity Cycles.- On the Rotation-Activity Connection.- Convection as a Regulator of Dynamos.- The Measurement of Stellar Photospheric Magnetic Fields.- The Dichotomy Between CO Absorptions and Ca II Emissions in the Sun and Stars: An Indirect Diagnostic for Gas Disturbed by Magnetic Fields ?.- Starspots and Plages.- A Study of Three RS CVn-type Eclipsing Binaries.- Coronal Magnetic Fields.- Properties of Stellar Magnetic Fields in Close Binaries Deduced from Non-thermal Radio Observations.- Stellar X-ray and Radio Activities and Coronal Magnetic Field.- On Active Stars, Coronal Loops, Magnetic Braking and All That.- 6. Evolution in Young Populations in Galaxies.- The Concept of Young Populations.- Evolution of High Mass Stars.- The Dynamical Evolution of Young Clussters and Associations.- The Initial Mass Function in Young Star Clusters.- Discussion.- Structure and Evolution of Giant Molecular Clouds.- Molecular Clouds in M51 and in the Galaxy.- The Molecular Cloud Content of Spiral and Dwarf Galaxies.- Discussion.- Star Formation, Giant HII Regions and Spiral Structure.- Young Stars and Star Formation in Normal Irregular Galaxies.- Discussion.- Chemical Evolution of Galaxies.- Star Formation Bursts in Galaxies.- Discussion.- Summary.- 7. Supernovae.- Centenary of S Andromedae (SN 1885a).- Problems of Observing Nearby Galaxies Visually.- Some Possible Identification Between Chinese Guest Stars and Supernova Remnants.- Supernovae in Flocculent and Grand Design Galaxies.- Recent Optical Observations of Supernovae.- Supernovae and Stellar Mass Loss.- Models for the Early and Late Spectra of Supernovae.- NL TE-Effects in Supernovae Type II Photospheres.- Models of Type II Supernova Explosions.- Effects of Angular Momentum on Supernova Explosion and Stability of Rapidly Rotating Stellar Cores.- Gravitational Radiation by a Collapsing Rotating Stellar Core.- Phase Transitions of Superdense Matter and Supernova Explosion.- X-ray Emission from Supernova Remnants.- Supernovae and Cosmology.- VL BI Observations of the Compact Components in M82.- Radio Supernova Candidates in the M82 Starburst.- Joint Commission Meetings.- Hipparcos.- The ESA Space Astrometry Mission: Overview and Status.- Activities of the Input Catalogue Consortium.- Preparation of the Mission: Earth-based Photometry.- Preparation of the Mission: Earth-based Astrometry.- Activities of the Data Reduction Consortia.- Tycho: Photometry & Astrometry for more than 500 000 Stars.- The Tycho Input Catalogue.- Link with Extragalactic Reference Frames.- Coronal Activity and Interplanetary Disturbances.- Coronal Activity at Radio Wavelengths.- Coronal Response to Energy Release During Solar Flares.- Correlated Observations of Impulsive UV and Hard X-ray Bursts from the Solar Maximum Mission.- Coronal Mass Ejections.- The Sources of Major Heliospheric Disturbances.- The Response of Microwave Emission to the Development of Active Regions.- Coronal Holes and Flare Related Phenomena.- Coronal Structures Observed at Meter Wavelengths.- The Nancay Multifrequency Radioheliograph.- An Energy Storage Mechanism for a Solar Flare by Shearing the Magnetic Field.- Merit + Cotes.- Joint Summary Report of the IAU/IUGG Working Groups on the Rotation of the Earth and the Terrestrial Reference System.- Report of Joint Meeting Held on 1985 November 22 to consider the MERIT/COTES Recommendations for a New International Earth-rotation Service.- Synthetic Photometry.- Principles and Scope of Synthetic Photometry.- The Possibilities of Synthetic Photometry.- Passbands and Photometric Systems.- Observed Stellar Energy Distributions for Synthetic Photometry.- Theoretical Stellar Energy Distributions.- Synthetic Photometry and the Calibration of the Hubble Space Telescope.- Standard Stars.- The Microfiche of Standard Stars.- Energy Distribution, Photometry and Physical Characteristics of the Sun and Solar Analogs.- On the Absolute Energy Distributions of the Sun, of the "Solar Analogs" 16 Cyg B, 16 Cyg A, VB 64 and of Vega.- Note.- Additional Contributions.- New Radio Telescopes.- A Reconsideration of the Galactic Constants.- Author Index.

Dense cores in dark clouds. II. NH3 observations and star formation.

Abstract: We present partial results of a survey of approx.100 visually opaque regions in nearby dark clouds in the 1.3 cm (J,K) = (1,1) line of NH/sub 3/, with mapping of all strong sources (''dense cores'') and (2,2) line observations in selected positions. For 27 dense cores with distance estimates, mean properties are: FWHM diameter, 0.1 pc; density, 3 x 10/sup 4/ cm/sup -3/; mass, 4 M/sub sun/; kinetic temperature, 11 K; and FWHM velocity width, 0.3 km s/sup -1/. We compare line shapes with cloud motion models, and source density, size, and temperature with equilibrium and stability requirements. These indicate that most dense cores are in the early stages of collapse or in near-critical equilibrium; if in equilibrium, they are probably supported by a combination of thermal and subsonic turbulent motions. In Taurus-Auriga, positions of the 10 known dense cores are well correlated with positions of emission-line star groups. In the next dense core free-fall time, 2 x 10/sup 5/ yr, the Taurus-Auriga complex is expected to form 25--50 emission-line stars. This is consistent with the estimated number of dense cores, 25. Taken together, these results suggest that most of the dense cores described here will form low-mas stars inmore » the next approx.10/sup 6/ yr.« less

Primordial star formation - The role of molecular hydrogen

Abstract: We investigate the thermal and chemical evolution of a collapsing spherical cloud composed of pure hydrogen gas. The cloud is assumed to be in pressure-free collapse. Over a wide range of initial conditions, virtually all the gas is converted to molecular form by a density n = 10/sup 12/ cm/sup -3/. The most effective reactions are the three-body ones: H+H+H..-->..H/sub 2/+H and H+H+H/sub 2/..-->..2H/sub 2/. As a result of significant cooling from the molecules, the temperature rise is slowed, and the Jeans mass eventually falls below 0.1 M/sub sun/ for clouds less massive than 100 M/sub sun/. Such clouds should therefore be capable of fragmenting into low-mass stars. This conclusion is strengthened if angular momentum slows the collapse. We also include in a heuristic manner the effect of shock heating from colliding fragments in a turbulent collapsing cloud. With substantial heating, the Jeans mass cannot drop as far, owing to the early destruction of hydrogen molecules. The primordial stellar mass spectrum may therefore be a sensitive function of the degree and effectiveness of intercloud collisions.

Dense cores in dark clouds. III. Subsonic turbulence.

Abstract: In 43 dark clouds with narrow molecular lines, significant correlations exist between line width and map size of the form ..delta..vproportionalR/sup 0.5/, and between mean density and map size of the form nproportionalR/sup -1/; and the law of virial equilibrium is closely satisfied. These relations tend to confirm those found earlier for larger clouds, and extend them into the regime of subsonic turbulence. If these relations for large and small clouds reflect the same process, it is still unclear whether this process is a turbulent energy cascade, as in Kolmogorov turbulence, or simply the tendency of clouds with nproportionalR/sup -1/ to be in virial equilibrium. For the smallest clouds (''dense cores'') there is weak support for the turbulent cascade picture. In this picture viscous dissipation of turbulence is likely to play an important role in heating and star formation. The time scale for the free decay of turbulence in dense cores is < or approx. =5 x 10/sup 5/ yr, comparable to the free-fall time. Dense cores can thus form low-mass stars on free-fall time scales if their input of fresh turbulence is sufficiently reduced. This reduction may occur as a result of their small size, or their high densitymore » contrast with surrounding gas. Observed distributions of dense core properties in Taurus, Ophiuchus, and other dark cloud regions appear consistent with dense core evolution toward star formation via dissipation of turbulence. In complexes which are vigorously forming low-mass stars, dense cores are more prevalent, smaller, denser, and have narrower lines than in regions with less star formation.« less

Dense cores in dark clouds. II. NH3 observations and star formation.

Abstract: We present partial results of a survey of approx.100 visually opaque regions in nearby dark clouds in the 1.3 cm (J,K) = (1,1) line of NH/sub 3/, with mapping of all strong sources (''dense cores'') and (2,2) line observations in selected positions. For 27 dense cores with distance estimates, mean properties are: FWHM diameter, 0.1 pc; density, 3 x 10/sup 4/ cm/sup -3/; mass, 4 M/sub sun/; kinetic temperature, 11 K; and FWHM velocity width, 0.3 km s/sup -1/. We compare line shapes with cloud motion models, and source density, size, and temperature with equilibrium and stability requirements. These indicate that most dense cores are in the early stages of collapse or in near-critical equilibrium; if in equilibrium, they are probably supported by a combination of thermal and subsonic turbulent motions. In Taurus-Auriga, positions of the 10 known dense cores are well correlated with positions of emission-line star groups. In the next dense core free-fall time, 2 x 10/sup 5/ yr, the Taurus-Auriga complex is expected to form 25--50 emission-line stars. This is consistent with the estimated number of dense cores, 25. Taken together, these results suggest that most of the dense cores described here will form low-mas stars inmore » the next approx.10/sup 6/ yr.« less

The birthline for low-mass stars

Abstract: Using the results of protostar theory, the locus in the Hertzsprung-Russell diagram is found where pre-main-sequence stars of subsolar mass should begin their quasi-static contraction phase and first appear as visible objects. This 'birthline' is in striking agreement with observations of T Tauri stars, providing a strong confirmation of the fact that these stars are indeed contracting along Hayashi tracks. The assumption that most T Tauri stars first appear along this line forces a recalibration of their ages. This recalibration removes the puzzling dip in present-day star formation seen in age histograms of several cloud complexes. Since the underlying protostar calculation assumes that the parent cloud was only thermally supported prior to its collapse, the observed location of the birthline places severe restrictions on the degree of extrathermal support provided by rotation, magnetic fields, or turbulence. In addition, the hypothesis that the collapse from thermally supported clouds to low-mass stars proceeds through protostellar disks appears untenable, since the disk accretion process almost certainly produces pre-main-sequence stars with radii well below the observed birthline.

On the evolution of the spiral galaxies in the Virgo cluster.

Abstract: Integrated Balmer emission line fluxes have been measured for 26 spiral galaxies in the core of the Virgo cluster. They are combined with published 21-cm and UBV color data in order to compare the disk properties of the cluster galaxies with galaxies in the field. Virgo spirals of a given Hubble type possess less star formation, less neutral hydrogen, and are redder than their field counterparts. Various possible selection effects are examined, but they cannot explain the differences. The relationships between color, M/sub H/ /L and H..cap alpha.. strengths, however, are indistinguishable in Virgo and the field. Some of the Virgo spirals may simply be normal spirals of slightly earlier type, but at least a few appear to be genuinely anemic galaxies, as suggested by van den Bergh.

VLA observations of massive star formation in spiral nuclei.

Abstract: We utilize both the spectral and spatial information in high-resolution radio continuum maps to decouple thermal emission due to H II regions from the extended, strong synchrotron emission found in galactic cores. A first-order analysis, assuming constant spectral indices across the maps, yields thermal fluxes to within a factor of 2. We thus confirm the presence of large numbers of H II regions and young stars in the nuclear regions of three nearby spiral galaxies.

Dynamical constraints on star formation efficiency.

Abstract: Dynamical constraints are placed on the local star formation efficiency during the formation of star clusters. Virial models are used to examine the expected changes in cluster velocity dispersion, density and radius during the period when the gas not converted into stars is removed from the system. Comparison of observed initial and final states of open clusters indicate local efficiencies of about 30% if the gas dispersal is slow relative to the dynamical crossing time and 55% if the gas loss is rapid. Efficiencies are somewhat dependent on support mechanisms of molecular clouds, but it is argued that observations of young clusters are in accord with the assumptions of the models used here.

The first stars

Abstract: Primordial clouds are likely to be remarkably uniform over stellar mass-scales in the absence of a pre-existing generation of stars. Thermal instability is found to occur during the collapse of a primordial cloud when the H2 abundance is rising and the H2 optical depth is of order unity. The e-folding rate for fluctuation growth exceeds the free-fall collapse rate by an order of magnitude. Large density fluctuations of mass-scale 0.1 solar mass arise in any collapsing cloud with metallicity not greater than 0.001 of the solar value. Gravitational instability ensures that many of the clumps coagulate to form protostars of masses extending up to the initial Jeans mass when the fluctuations develop, namely 100 solar masses. The primordial IMF should therefore have spanned the mass range from 0.1 to 100 solar masses, but may have been dominated by the more massive stars.

Far-infrared detection of low-luminosity star formation in the Bok globule B335

Abstract: Measurements of far-infrared intensities in the Bok globule B335, made with higher spatial and spectral resolution than two earlier studies, are described. The NASA 3-m Infrared Telescope Facility was used in February 1981 and the Kuiper Airborne Observatory 0.9-m telescope was used in October 1981 and August 1982 for the observations. The far-infrared source in B335 was revealed to be more compact than previously assumed, ruling out the interstellar radiation field as the dominant heat source. The new far-infrared size, luminosity, and temperature derived suggest that these may be the first observations of low-mass star formation embedded deeply within a Bok globule. The far-infrared source does not have a near-infrared or radio continuum counterpart. High angular resolution at submillimeter wavelengths will be critical in further studies of low-mass star formation.

Are interstellar toroids the focusing agent of the bipolar molecular outflows

Abstract: The phenomenon of bipolar mass outflow in regions of star formation is an important phase in the early stages of stellar evolution In this paper we address observationally the nature of the focusing agent We searched the (J,K) = (1,1) inversion transition of ammonia in 10 sources of bipolar outflow and detected and mapped the emission in seven of them Since ammonia is a tracer of high-density gas, n(H/sub 2/)> or approx =5 x 10/sup 3/ cm/sup -3/, we conclude that bipolar molecular outflows are commonly associated with dense condensations These condensations have internal velocity dispersions larger than those of dark clouds; however, they are in approximate virial equilibrium The ammonia data show the correlation between internal velocity dispersion and cloud size and mass previously obtained from carbon monoxide observations The molecular condensations associated with the bipolar outflows are generally elongated and engulf the compact object suspected to be the source of energy of the system For nine sources where clear orientations could be assigned to both the major axis of the condensation and the direction of the outflow, we found seven cases where these axes were nearly perpendicular This suggests that these elongted clouds may be toroid-shaped with dimensionsmore » of tenths of parsecs and may provide the focusing mechanism in bipolar outflows In this model, the stellar wind of the central object has created a bipolar cavity in an originally pancake-shaped cloud The wind, which could be isotropic near the star, is stopped in the plane of the dense toroid but escapes along the poles, accelerating the surrounding lower density molecular gas« less

Toward explaining Seyfert galaxies

Abstract: Galaxies with extensive star formation in their nuclei (star-burst galaxies) produce up to 10/sup 9/ M/sub sun/ of compact stellar remnants in their nuclei. Data for the kinematics of these remnants imply that a significant fraction can settle into nuclear volumes of radii 1 pc. It is suggested that this mechanism provides a source for large masses of accretors in galactic nuclei that can power Seyfert 1 galaxies. In the scenario presented. Seyfert 1 galaxies, and conceivably even luminous quasars, are powered not by a single massive black hole but by large numbers of small accretors, black holes or neutron stars, left after processes of stellar evolution.

Observations of suspected low-mass post-T Tauri stars and their evolutionary status

Abstract: The results of a study of five X-ray discovered weak emission pre-main-sequence stars in the Taurus-Auriga star formation complex are presented. All are of spectral type K7-M0, and about 1-2 mag above the main sequence. One is a double-lined spectroscopic binary, the first spectroscopic binary PMS star to be confirmed. The ages, masses, and radii of these stars as determined by photometry and spectroscopy are discussed. The difference in emission strength between these and the T Tauri stars is investigated, and it is concluded that these 'post-T Tauri' stars do indeed appear more evolved than the T Tauri stars, although there is no evidence of any significant difference in ages.

The extragalactic background light at 4400 A

Abstract: Space based photometric determinations of the extragalactic background light (EBL) are compared with galaxy count determinations of EBL brightness. The results of various EBL studies at blue and visual wavelengths are summarized, and an upper limit of 3.9 S-sub-10 (V)-sub-G2V at the 2 sigma level is placed on the brightness of the extragalactic background at 4400 A. EBL data in regions near the north and south galactic poles are also presented, the first such data obtained from heliocentric distances where the zodiacal light is negligible. Several paradigms consistent with the inferred EBL limits are mentioned, including less galactic evolution or masking of galactic evolution, galaxy formation at very high redshifts, and dusty primeval galaxies decreasing optical EBL and increasing infrared EBL. Models promulgating all primeval galaxies as high-luminosity objects undergoing bursts of star formation are incompatible with the EBL observations.

Spiral Structure and Star Formation. I. Formation Mechanisms and Mean Free Paths

Abstract: The wide variety of optical appearances observed in spiral galaxies has encouraged the growth of two theoretical approaches to explain the spiral patterns exhibited by such young objects as OB associations, giant H II regions, and dark dust lanes. These approaches are related to the density wave theory of spiral structure and 'stochastic, self-propagating star formation'. Levinson and Roberts (1981) tried to reconcile these two theoretical approaches, and considered a disk filled with discrete gas clouds. The present investigation is concerned with refinements and extensions of the model of Levinson and Roberts. Attention is given to gravitational forces and dynamical propagation, cloud-cloud collisions, supernova interactions, computational models, a theory concerning the interactions of the gas cloud and stellar association systems, the time evolution of the gas cloud-stellar association systems, and aspects of collisionally triggered star formation.

White dwarfs and neutron stars in globular cluster X-ray sources

Abstract: We predict here that globular clusters contain at least as many cataclysmic variables, which contain white dwarfs, as bright (Lx>1036 erg s−1) X-ray sources, which contain neutron stars. Globular clusters contain many more white dwarfs than neutron stars, but the capture mechanisms for the formation of binaries are more efficient for neutron stars. We point out here the consequences of the frequent formation of temporarily bound triple systems (resonance scattering). These are: an extra enhancement of neutron star capture with respect to white dwarf capture, a considerable probability of actual collisions between stars, and the presence of cataclysmic variables in the outer regions of globular clusters.

A multifrequency study of star formation in the blue compact dwarf galaxy-i zw 36

Abstract: Radio, near IR, optical, and UV observations of I Zw 36 = Mrk 209 = Haro 29 are reported. The H I distribution shows a core-halo structure, the core containing half of the mass and showing systematic motions; the halo is diffuse and contains several H I clumps. The visible star formation region is associated with the core but is shifted slightly with respect to the H I peak column density; and the virial mass is 5 to 7 times the H I mass. Star formation models with an initial mass function of slope 1.5 (the Salpeter value being 1.35) and a burst age or duration of a few million years fit well the optical spectrophotometric measurements. The data also suggest that the column density of molecular hydrogen in I Zw 36 is 6 + or - 3 times that of the neutral hydrogen, about the right amount to account for the virial mass.

The star-forming history of the young cluster NGC 2264.

Abstract: UBVRI H-alpha photographic photometry was obtained for a sample of low-mass stars in the young open cluster NGC 2264 in order to investigate the star-forming history of this region. A theoretical H-R diagram was constructed for the sample of probable cluster members. Isochrones and evolutionary tracks were adopted from Cohen and Kuhi (1979). Evidence for a significant age spread in the cluster was found amounting to over ten million yr. In addition, the derived star formation rate as a function of stellar mass suggests that the principal star-forming mass range in NGC 2264 has proceeded sequentially in time from the lowest to the highest masses. The low-mass cluster stars were the first cluster members to form in significant numbers, although their present birth rate is much lower now than it was about ten million yr ago. The star-formation rate has risen to a peak at successively higher masses and then declined.

Accretion-driven star formation in central dominant galaxies in x-ray clusters

Abstract: We argue that low-mass stars are being formed from the large amounts of gas being accreted by central dominant galaxies in some X-ray clusters. We suggest that low-mass star formation may be favored because the Jeans mass decreases as the ambient pressure increases, and the pressures observed in these accretion flows are four orders of magnitude higher than those in the local interstellar medium. We have calculated the spectra for the inner parts of these galaxies including accretion-driven star formation, and we show that the colors of these galaxies may be affected at an observable level. We suggest observations to search for these color anomalies. The unusual A-type spectrum and blue colors of NGC 1275 may be explained by accretion-driven star formation, as first suggested by Kent and Sargent. The luminosity evolution due to accretion by first brightest cluster galaxies can affect determinations of q/sub 0/ which use these galaxies, although this effect is probably not as large as other evolutionary changes. We speculate that the blue stellar spectra and giant-elliptical light profiles observed in the faint optical extensions around several quasars might indicate that similar star formation is occurring in these quasars, although it is unlikely that they are more » accreting intracluster gas. « less

Atlas of Main-line OH Masers in the Galactic Longitude Range 3° to 60°

Abstract: We tabulate all 55 OH main-line masers discovered to date in the galactic plane between longitude 3° and 60°. For most of these we show current spectra, which have been taken with the Parkes 64 m radio telescope, in both senses of circular polarization on the 1665 MHz and/or 1667 MHz transitions; for some sources we give new position estimates, and several sources are reported for the first time. We discuss many sources individually and note that while most of the main-line OH masers probably pinpoint compact HII regions and sites of current star formation, a few appear to be unusual varieties occurring in circumstellar shells, possibly associated with late-type stars.

Supernovae. Part II: The aftermath

Abstract: Part I [Rev Mod Phys 54, 1183 (1982)] explored stellar evolution leading up to supernovae and observations and models of the events themselves Part II addresses the aftermath: supernova remnants, products, and by-products, including nucleosynthesis, and the future of supernova research Some of the important questions are: (1) How close is the association among supernova events, pulsar production, and remnant production? (2) Where does most of the energy from neutron star formation go? and (3) How do supernovae interact with the rest of the universe, for instance in heating and stirring the interstellar medium, accelerating cosmic rays, and triggering or inhibiting star formation? © 1983 The American Physical Society

Self-regulated star formation in the galaxy

Abstract: Assuming that star formation regions are supported against gravity by winds from low mass young objects, the stellar birthrate obtained for winds interacting in the momentum conservation stage is correlated with the molecular gas density of the parent fragment as n to the 13/8 power or n to the 5/8 power, respectively, for rates/unit volume or rates/unit mass. Birthrates derived from protostellar rotationally driven winds are in good agreement with the observed star production in the cloud B18. With the aid of observed Taurus-Auriga complex properties, the present model is extrapolated to the Galaxy as a whole, yielding a Milky Way predicted average rate that is in good agreement with standard estimates based on observations of the solar neighborhood.

Infrared observations of OB star formation in NGC 6334

Abstract: Infrared photometry and maps from 2 to 100 microns are presented for three of the principal far infrared sources in NGC 6334. Each region is powered by two or more very young stars. The distribution of dust and ionized gas is probably strongly affected by the presence of the embedded stars; one of the sources is a blister H II region, another has a bipolar structure, and the third exhibits asymmetric temperature structure. The presence of protostellar objects throughout the region suggests that star formation has occurred nearly simultaneously in the whole molecular cloud rather than having been triggered sequentially from within.

Formation of OB clusters: W33 complex

Abstract: OB star formation in the W33 complex is studied using radio continuum interferometry and the spectral lines of the NH/sub 3/ molecule. Although OB star formation is extended on the scale of 10 pc, the most recent episode of star formation producing the most massive stars in the region is confined to a 1 pc core within the dense molecular gas. The clumpy appearance of the H II region is consistent with the interpretation that a cluster of OB stars has formed recently in the core of the molecular cloud. Although the radio structures are well correlated with observed 20 ..mu..m structures, the latter observation underestimates the intrinsic luminosities of the exciting sources. Far-infrared measurements, however, do provide a reliable indication of the total luminosity of the region. If a cluster of OB stars has formed in the core of W33, the observed total luminosity suggests that this cluster does not extend below O8. The location of the most massive and youngest stars toward the core of this region supports the traditional notion that star formation proceeds by a slow collection of the gas, followed by fragmentation, collisions, and subsequent coagulations.