Showing papers on "Star formation published in 1991"

Galaxy formation through hierarchical clustering

Abstract: Analytic methods for studying the formation of galaxies by gas condensation within massive dark halos are presented. The present scheme applies to cosmogonies where structure grows through hierarchical clustering of a mixture of gas and dissipationless dark matter. The simplest models consistent with the current understanding of N-body work on dissipationless clustering, and that of numerical and analytic work on gas evolution and cooling are adopted. Standard models for the evolution of the stellar population are also employed, and new models for the way star formation heats and enriches the surrounding gas are constructed. Detailed results are presented for a cold dark matter universe with Omega = 1 and H(0) = 50 km/s/Mpc, but the present methods are applicable to other models. The present luminosity functions contain significantly more faint galaxies than are observed.

Fueling Starburst Galaxies with Gas-rich Mergers

Abstract: The dynamics of gas are modeled in a merger of two disk/halo galaxies of equal mass using a hybrid N-body/gasdynamics code. Violent tidal forces acting on the disks draw out extended tails and trigger the formation of central bars. As such bars form, gas in the inner half of each disk loses most of its angular momentum through gravitational torques and falls into a compact cloud within the center of the galaxy. These nuclear gas clouds merge when their parent galaxies do, resulting in the rapid assembly of about 5 billion solar masses of gas, which may plausibly be identified with the large central clouds seen in CO observations of galaxies such as NGC 520. Violent star formation in such central gas clouds, which seems inevitable, offers a likely explanation for luminous IRAS galaxies and may contribute significantly to the central stellar populations of merger remnants. If some of the nuclear gas can continue to lose angular momentum, it may be able to fuel or even form a central black hole, resulting in a radio galaxy or possibly even a quasar. 47 refs.

Molecular gas in galaxies

Abstract: Molecular gas is critical in determining both the morphology and evolution of galactic disks. It is within the giant molecular clouds that interstellar gas is cycled into the next generation of stars, and the most massive of these young stars produce a major part of the galactic luminosity. In addition, the dense interstellar medium, as it is highly dissipative, probably plays a fundamental part in determining the outcome of galactic interactions. Over the last two decades, CO observations have been used to probe the molecular component of hundreds of galaxies from the Local Group to the Virgo cluster, and in luminous galaxies with recession velocities up to cz = 45,000 km s^(-1).

The physics of star formation and early stellar evolution

Abstract: I: Physics of Giant Molecular Clouds: Origin, Structure, and Evolution. Star Forming Giant Molecular Clouds L. Blitz. The Origin and Evolution of Giant Molecular Clouds B.G. Elmegreen. Cosmic Magnetism and the Basic Physics of the Early Stages of Star Formation T.Ch. Mouschovias. II: The Physics of Star Formation. OB Associations and the Fossil Record of Star Formation A. Blaauw. Physical Conditions and Heating/Cooling Processes in High Mass Star Formation Regions R. Genzel. Newly Formed Massive Stars E. Churchwell. Masers and Star Formation N.D. Kylafis. The Physical Conditions of Low Mass Star Forming Regions J. Cernicharo. The Formation of Low Mass Stars: Observations C.J. Lada. The Formation of Low Mass Stars: Theory F.H. Shu. Numerical Studies of Cloud Collapse W.M. Tscharnutter. Binary Star Formation J.E. Pringle. Single-Stage Fragmentation and a Modern Theory of Star Formation T.Ch. Mouschovias. III: Physics of Early Stellar Evolution and Stellar Winds. Molecular Outflows: Observed Properties J. Bally, A.P. Lane. Herbig-Haro Objects B. Reipurth. The Physics of Disk Winds R.E. Pudritz, G. Gomez de Castro, A.I. Gomez de Castro. Ionized Winds from Young Stellar Objects N. Panagia. The Physics of Neutral Winds from Low Mass Young Stellar Objects A. Natta, C. Giovanardi. Eposodic Phenomena in Early Stellar Evolution L. Hartmann. Properties and Models of T Tauri Stars C. Bertout, G. Basri. The X-Ray and Radio Properties of Low-Mass Pre-Main Sequence Stars T. Montmerle. Polarization of Light and Models of the Circumstellar Environment of Young Stellar Objects P. Bastien. Index.

The 158 micron forbidden C II line - A measure of global star formation activity in galaxies

Abstract: Some 158 micron (CII) fine structure line observations from a sample of fourteen gas rich galaxies are reported. These measurements confirm and generalize previous basic results that the (CII) line is bright amounting to approximately 0.1 to 1 percent of the Far Infra Red (FIR) luminosity of the nuclear regions of galaxies; the (CII) line is formed in the warm (temperature of the gas is greater than 200 K), dense (n sub H greater than 1000/cu cm) photodissociated gas at the interfaces between giant molecular clouds and ionized gas regions and is therefore associated with the molecular gas component in spiral galaxies; the (CII) line tracks the FIR continuum in a manner consistent with the PDR models; the integrated (CII) to isotope (12)CO (transition 1 to 0) line ratio is large (greater than or equal to 1000) in all galaxies studied, and is similarly large for galactic molecular clouds; the (CII) line is therefore energetically very important for the study of giant molecular clouds. Conclusions obtained from these results are given.

The Rate of Neutron Star Binary Mergers in the Universe: Minimal Predictions for Gravity Wave Detectors

Abstract: Of the many sources which gravitational wave observatories might see, merging neutron star binaries are the most predictable. Their waveforms at the observable frequencies are easy to calculate. And three systems which will merge in less than a Hubble time have already been observed as binary pulsars: two in the disk of the Galaxy, and one in a globular cluster. From the lifetimes and positions of these, a lower limit to the merger rate in the Galaxy and globular cluster system are inferred with confidence. Taking the merger rate in other galaxies to scale with the star formation rate, the merger rate expected in the local universe is computed. An ultraconservative lower limit to the rate gives three per year within 1 Gpc. The best estimate, still conservative in that it considers only systems like those already observed, gives three per year within 200 Mpc. An upper limit of three mergers per year within 23/h Mpc is set by the rate of Type Ib supernovae. The rates of black hole binary mergers and black hole-neutron star binary mergers are model-dependent, but could be comparable to the given rate of neutron-star binary mergers.

A 2.2 micron survey in the L1630 molecular cloud

Abstract: A 2.2 micron survey of a significant portion of the L1630 molecular cloud was carried out using the NOAO infrared array camera on the Kitt Peak 1.3 m telescope. This survey is estimated to be complete to 13th magnitude at K, which corresponds to a main-sequence star of about 0.6 solar mass. Therefore this survey has provided a sensitive census of the young stellar objects within this molecular cloud. As a result, 912 sources having m(K) less than 13.0 have been detected. It is estimated that 50 percent of these sources are associated with the molecular cloud. These sources are not distributed uniformly throughout the surveyed region but rather appear grouped or clustered. In fact, four embedded infrared clusters were identified. These clusters contain at least 58 percent but possibly as much as 96 percent of the total number of sources associated with this molecular cloud suggesting that the dominant mode of star formation in this region is in clusters.

Massive stars in Cyg OB2

Abstract: The stellar content of Cyg OB2 has been investigated using CCD photometry and spectroscopy The intrinsically bluest stars have been isolated for spectroscopy by using a reddening-free parameter The spectroscopy has made it possible to determine an improved distance modulus to the association (112 + or - 01) and has provided the means of accurately placing stars in the H-R diagram The H-R diagram shows a well-defined main sequence extending upward to 85 solar masses There are a few slightly evolved B supergiants of lower mass, suggesting that star formation did not take place strictly in a coeval manner The initial mass function of Cyg OB2 is found to have a slope of -01 + or - 01, much flatter than that generally found for massive stars in the Galaxy or Magellanic Clouds 48 refs

The chromospheric emission-age relation for stars of the lower main sequence and its implications for the star formation rate

Abstract: An attempt is made to formulate the relationship between age and chromospheric emission (CE) in late-type dwarf stars. Evidence is reviewed that a deterministic relationship of this type actually exists, and that for stars of known age, either a power-law relation or a curve corresponding to a constant star formation rate fits equally well. Further observations should be able to demonstrate either that there is a real excess of young stars near the sun or that the evolution of CE for a low-mass star goes through a slow initial decline, a rapid decline at intermediate ages, and finally a slow decline for old stars like the sun. 100 refs.

Dense cores in dark clouds. VI - Shapes

Abstract: A comparison is presently conducted between 48 line-intensity maps of 16 dense cores in dark clouds, based on observations in the 13-mm line of NH3 and the 3-mm lines of CS and C(O-18). Core elongation in this sample does not differ significantly between cores with and without embedded stars; elongation appears to be a condition prior to star formation, rather than the consequence of formation. The characteristic elongation of dense cores implies that models of equilibrium between self-gravity and isotropic random motions are incomplete. The observed elongation of dense cores in projection is modeled as arising from a group of either prolate or oblate spheroids. 50 refs.

A cluster of He I emission-line stars in the Galactic center

Abstract: We report 1″.9/300 km s −1 resolution imaging spectroscopy of He I n=2 1 P→n=2 1 S 2.06 μm line emission in the Galactic center. There is a group of about a dozen compact, broad-line He I emission-line stars most of which are positionally coincident with compact 2 μm continuum sources. IRS 16 and its broad-line region is probably a central concentration of this 1 pc diameter He I star cluster. The He I stars may be blue supergiants with heavy mass loss. The high He I/H I (Brα, Brγ) ratios suggest an overabundance of helium in their atmospheres indicative of a post-main-sequence phase.

A Catalog of Bright-rimmed Clouds with IRAS Point Sources: Candidates for Star Formation by Radiation-driven Implosion. I. The Northern Hemisphere

Abstract: Forty-four bright-rimmed clouds associated with IRAS sources have been selected from the Palomar Sky Survey prints. They are good candidates for the sites of star formation induced by radiation-driven implosions. Nine of the bright-rimmed clouds are known to be associated with molecular outflows and two (including one with an outflow) with HH objects. Most of their sizes are ≃ 1 pc, similar to those of Bok globules. The luminosities of the associated IRAS sources are relatively large compared to those of the IRAS sources associated with dark globules or dense cores in quark cloud complexes. IRAS luminosity to cloud mass ratios are significantly greater than those in dark globules or in dense cores of dark cloud complexes

Magnetic braking, ambipolar diffusion, cloud cores, and star formation: Natural length scales and protostellar masses

Abstract: Magnetic braking is essential for cloud contraction and star formation. Ambipolar diffusion is inavoidable in self-gravitating, magnetic clouds and leads to single-stage (as opposed to hierarchical) fragmentation (or core formation) and protostar formation. Magnetic forces dominate thermal-pressure and centrifugal forces over scales comparable to molecular cloud radii. Numerical calculations based on new adaptative-grid techniques, that follow the formation of fragments by ambipolar diffusion and their subsequent collapes up to an enhancement in central density above its initial equilibrium value by a factor ∼ 10, are presented.

Modeling Galaxy Formation in Evolving Dark Matter Halos

Abstract: A simple semianalytic model is used to study the radiative cooling of gas in an evolving distribution of cold dark matter halos. Two related models are used to describe the evolving distribution of collisionless dark matter: the Press-Schechter formalism and a recently developed Monte Carlo model. The Monte Carlo method follows the merging of halos and hence extends the fully analytic treatment in several ways. First, plausible estimates of halo lifetimes are made. Second, the local depletion of the primordial gas is followed in each halo as progressively more gas cools and forms stars. Finally, limits are set on the merging of protogalaxies. It is shown that, in the absence of any form of energy input, an unacceptably large fraction of the baryons will cool and condense in low-mass halos at high redshift. In order to counteract efficient cooling at early times, it is postulated that energy input by supernovae associated with the first stars formed in these halos is sufficient to unbind the gas in halos with circular velocities less than a critical value V(crit). Estimates of the present-day luminosity function calculated using this model suggest V(crit) is about 200 km/s or more, and hence that self-regulating or self-limitingmore » star formation operates even in bright L(asterisk) galaxies. 33 refs.« less

Star formation and the nature of bipolar outflows

Abstract: This paper presents a simple physical model for the bipolar molecular outflows that frequently accompany star formation. The model forges an intrinsic link between the bipolar flow phenomenon and the process of star formation, and it helps to explain many of the systematics known for existing sources.

Origin of kinematic subsystems in elliptical galaxies

Abstract: The formation of a counterrotating central gas disk in a merger of two gas-rich disk galaxies of equal mass is demonstrated. Such a structure may well account for the unusual gas kinematics found in the merger remnant NGC7252. Continued star formation in such gaseous disks may produce central components with decoupled kinematics resembling the cores of some elliptical galaxies.

The evolution of intermediate-mass protostars. I, Basic results

Abstract: A numerical calculation of the structure and evolution of Population I protostars in the range from 1 to 8 solar masses is presented. The protostar is modeled as being built up from a spherically symmetric accretion flow, with a constant mass accretion rate of 0.00001 solar mass/yr. The protostar is initially fully convective due to the steady state burning of deuterium near its center. The mass-radius relation during this phase is derived analytically. At 2.4 solar masses, a radiative barrier appears which prevents the convective transport of deuterium to the center. The subsurface shell is ignited by the deuterium, which causes the protostar to swell. The star joins the zero-age main sequence by the time it reaches 8 solar masses. It is found that over a significant mass range, protostars contain a substantial amount of deuterium, and it is inferred that the ignition of this fuel during subsequent premain-sequence contraction could drive the surface convection that is apparently present in many Herbig Ae and Be stars. 35 refs.

Chemical evolution with bursts of star formation - Element ratios in dwarf galaxies

Abstract: It is demonstrated that galaxies in which star formation proceeds in a small number of bursts evolve their chemical elements in ratios which are very different from those in galaxies with continuing star formation. Systematic changes of element ratios with overall chemical abundance are determined to a large extent by the onset and duration of the star formation bursts and can be very different from those seen in the solar neighborhood. In particular, it is shown that an underabundance of oxygen relative to iron, such as is observed in the Large Magellanic Cloud, occurs naturally when star formation proceeds in a small number of widely separated bursts, as is inferred from the age distribution of LMC field stars and clusters. There is no need to invoke either variations in the stellar initial mass function or metal-enhanced winds. 25 refs.

Chemical Evolution of the Galaxy

Abstract: Since the discovery of the cosmic microwave background ( 1 84), it is gen­ erally assumed that the universe originated from a hot big bang (78). Detailed nucleosynthesis calculations based on the hot big bang model (e.g. 4, 1 82, 221 ) showed that no element heavier than 9Be could have been synthesized primordially with an abundance exceeding 101 4 by mass fraction. However, in the universe that we live in today, there are appreci­ able amounts of elements heavier than helium, called heavy elements or metals in general. These elements-accounting for about 2% of the visible mass-are believed to have been synthesized in stars or starlike objects (cf 26). Cosmic gas fragmented into huge gas clouds, forming galaxies­ further fragmentation of which possibly led to the formation of stars. Stars evolve on time scales ranging from several millions to tens of billions of years, synthesize elements in their central parts, and emit the processed elements into the interstellar medium (ISM) at various stages. The ISM is thus enriched with heavy elements, and the stars formed thereafter are born with a higher metallicity. This cycle should go on until all gas in the interstellar medium is exhausted (200a, 2 12a). Thus the study of chemical evolution of the galaxies involves understanding the spatial distribution and temporal evolution of various elements in the galaxies by taking into

Subdwarf studies. II - Abundances and kinematics from medium resolution spectra. III - The halo metallicity distribution

Abstract: Stars previously identified as having UV excesses are observed at 1-A resolution in the Ca II K-line region. Comparisons of these data with other samples and with Monte Carlo simulations involving a single component halo have yielded estimates of halo velocity dispersions and rotation velocity, corrected for the kinematic biases in the sample. It is suggested that the data are not consistent with a model in which the halo formed from star formation in a dissipating, collapsing cloud; they are, however, reconcilable with the formation of the halo stars by numerous, independently evolving gas clouds. The metallicity distribution of a sample of 372 kinematically selected halo stars is then constructed, with a view to selection effects in the data. Good agreement is noted between the globular cluster metallicity distribution and a stochastic model with a mean of 10 enrichments/fragment. 153 refs.

Does accretion cease when a star approaches breakup

Abstract: A steady-state thin accretion disk is modeled around a uniformly rotating unmagnetized star in order to investigate whether disk accretion can continue as the accreted angular momentum spins the central star to near breakup. A mapping between the specific angular momentum (SAM) added to the star and the stellar rotation rate (SRR) is obtained. When SRR is somewhat less than the breakup rotation rate of the star, a class of solutions is found where the angular velocity of the disk attains a maximum close to the star and then decreases rapidly in a boundary layer to match SRR. If SRR is near breakup, a second class of solutions is found where the disk angular velocity has no maximum but increases monotonically all the way down to the stellar surface. SAM decreases very rapidly with increasing SRR and even takes on fairly large negative values. The spin-up of an accreting star slows down and eventually stops at a rotation rate near breakup. Beyond this point, the star can continue to accrete any amount of matter without actually breaking up. 18 refs.

OB Associations and the Fossil Record of Star Formation

Abstract: Properties of OB Associations within 1.5 kpc are reviewed as “fossils” of local star formation, and as a reference frame for the more detailed discussions, elsewhere in this volume, of star formation occurring within the sample. Special attention is paid to the process of sequential star formation. For the nearest associations this can be traced back to the formation of the Gould Belt between 30 and 50 million years ago. A few remarks on future research conclude the review.

NGC 1333 IRAS 4 : a very young, low-luminosity binary system

Abstract: IRAS 4 is a cold, low luminosity source in the active star formation region NGC 1333. The present sub-mm photometry and high spatial resolution 800- and 450-micron maps resolve this source into a binary system whose components are separated by about 31 arcsec and connected by a bridge of faint dust emission. The dust masses associated with the binary component are by far the largest seen toward any star of comparable luminosity, and are a factor of 10 higher than predicted by protostar models. It is suggested that these observations indicate a very young stellar system, possibly an actual protostar, in which most luminosity is generated by accretion. 27 refs.

He II emission in extragalactic H II regions

Abstract: Spectroscopic observations confirming the presence of nebular He II 4686-A emission in the SMC H II region N76 are presented, and it is demonstrated that the He II emission associated with the WO star in IC 1613 is also extended. The properties of four H II regions are discussed. The close correlation of the emission with specific stars indicates that photoionization by the stars themselves is the excitation mechanism, and it is proposed that this may be true for those nebulae ionized by WO stars and some rare high-excitation WN stars. The existence of these nebulae with He II emission increases the likelihood that the 4686-A emission frequently observed in dwarf emission-line galaxies is nebular in origin. This prompts the conclusion that the radiation field associated with star-forming regions can be harder than previously suspected, and reopens the issue of whether photoionization by stars in young galaxies can account for the ionization observed in QSO absorption-line systems. 42 refs.

The starburst ring around the Seyfert nucleus in NGC 7469

Abstract: High-resolution radio continuum, optical emission line, and optical continuum images of the luminous Seyfert plus circumnuclear starburst hybrid galaxy NGC 7469 are presented. The radio emission from the starburst is largely confined to a ring of diameter 3 arcsec, or 1.0 kpc. There is clear evidence for optical continuum emission and probably optical line emission from the ring. The disks of NGC 1068 and NGC 7469 exhibit a strong similarity in almost all observed properties. Current star formation models can account for the luminous far-infrared emission and the strength of the nonthermal radio emission in NGC 7469, with an expected supernova rate of about one per year. The starburst ring is probably a result of concentration of gas into a ring via resonances between orbital motion and a rotating barlike or ovally distorted potential.

On the dispersion in direction of interstellar polarization

Abstract: The spatial distribution of polarization angle is summarized for 15 dark clouds that lack prominent star formation, for four star clusters embedded in dark clouds, for one evolved cluster with little associated extinction, and for six dark cloud complexes. Most clouds have either one well-defined mean direction over their spatial extent or two or three spatial zones having noticeably different mean directions. Nearly all regions have a single local maximum in their number distribution of polarization angle. Clouds with embedded clusters have a more complex distribution of polarization direction than do clouds without clusters. It is proposed that the enhanced dispersion of polarization angle in clusters may be more closely associated with young stars and/or with dense gas, than simply with the number of stars in the cluster. The greatest dispersion in direction among clouds without clusters, 0.7 radians, occurs in L1689 in Ophiuchus. 54 refs.

Frontiers of stellar evolution

Abstract: The present conference discusses theoretical and observational views of star formation, spectroscopic constraints on the evolution of massive stars, very low mass stars and brown dwarfs, asteroseismology, globular clusters as tests of stellar evolution, observational tests of stellar evolution, and mass loss from cool evolved giant stars. Also discussed are white dwarfs and hot subdwarfs, neutron stars and black holes, supernovae from single stars, close binaries with evolved components, accretion disks in interacting binaries, supernovae in binary systems, stellar evolution and galactic chemical evolution, and interacting binaries containing compact components.