Showing papers on "Star formation published in 1992"

Formation of very strongly magnetized neutron stars - Implications for gamma-ray bursts

Abstract: It is proposed that the main observational signature of magnetars, high-field neutron stars, is gamma-ray bursts powered by their vast reservoirs of magnetic energy. If they acquire large recoils, most magnetars are unbound from the Galaxy or reside in an extended, weakly bound Galactic corona. There is evidence that the soft gamma repeaters are young magnetars. It is argued that a convective dynamo can also generate a very strong dipole field after the merger of a neutron star binary, but only if the merged star survives for as long as about 10-100 ms. Several mechanisms which could impart a large recoil to these stars at birth, sufficient to escape from the Galactic disk, are discussed.

Abundances of the heavy elements in the Magellanic Clouds. III - Interpretation of results

Abstract: Chemical and structural evolutionary models for the Magellanic Clouds assuming bimodal star formation and gas infall are presented. The models are discussed in relation to the observed chemical abundances of the Clouds and are compared with our own Galaxy. The detailed abundances derived from previous work are investigated for any obvious trends with metallicity or differences compared with the Galaxy. Considering all the data, conclusions are drawn on the possible star formation histories of the Magellanic Clouds and the implications for our own Galaxy

MG and Fe absorption features in elliptical galaxies

Abstract: Fe and Mg indices from two homogenous collections of nuclear elliptical galaxy spectra are compared with model indices. In the average giant elliptical, the [Mg/Fe] ratio exceeds that of the most metal-rich stars in the solar neighborhood by ∼0.2-0.3 dex, with a large spread about this mean. This result implies a variable «enrichment effectiveness» of Type II supernovae (SNs) compared with Type Ia SNs in the evolution of ellipticals, caused perhaps by differences in star formation time scales, the initial mass function, or the amount of Fe versus Mg ejected in galactic winds

On dwarf elliptical galaxies and the faint blue counts

Abstract: The nature of the faint blue objects seen in deep images of the sky has been a source of great puzzlement. Their high surface density and weak clustering argues against their being either the progenitors or the merging components of the present-day bright galaxies. The faint blue counts are interpreted as being due to dwarf elliptical galaxies undergoing their initial starburst at z∼1. The starburst epoch is determined by the decline in the UV background, which results in the stably confined photoionized gas in dark haloes of M ∼ 10 9 M ⊙ being able to cool and settle in the centre of the haloes and undergo star formation

Planetary Camera observations of NGC 1275 - Discovery of a central population of compact massive blue star clusters

Abstract: We have discovered a population of bright blue pointlike sources within 5 kpc of the nucleus of NGC 1275 using HST Planetary Camera observations. The typical object has M_v~- 12 to - 14 (H_0 = 75 km s^(-1) Mpc^(-1); the brightest has M_v~-16. They are all blue, with V- R≾0.3. The color distribution and lack of excess Ha emission are consistent with nearly all being continuum sources. Many of the sources are unresolved even with the HST and consequently have sizes of ≾ 15 pc. We suggest that these are young star clusters that will evolve to look like globular clusters. They are bluer than any clusters seen in the Milky Way or M87, and brighter than the blue clusters seen in the LMC. We derive ages of several hundred million years or less and corresponding masses of 10^5-10^8 M_☉. The existence of these young clusters may be connected with a current or previous interaction with another galaxy, with the cooling flow in NGC 1275, or with some combination. Structure is detected in the underlying galaxy light that is suggestive of a merge between NGC 1275 and a second galaxy some 10^8 yr ago. If this merger triggered star formation, it would naturally account for the observed uniformity of cluster colors. Steady-state star formation in the x-ray cooling flow would imply a wider range in cluster age and color than is seen, unless the clusters disrupt. An interaction with the projected high-velocity, infalling system cannot explain the observations because this system has not yet reached the center of NGC 1275 where the clusters are concentrated, and because it has a total interaction time that is far too short for either the observed cluster lifetimes or the dynamical lifetime of structure in the galaxy. If the presence of recently formed protoglobulars around NGC 1275 is related to a previous merger, this would remove an important objection to the merger hypothesis for elliptical galaxy origins, provided that adequate gas is available in the merger for their formation.

Dense Molecular Gas and Starbursts in Ultraluminous Galaxies

Abstract: With the IRAM 30 m telescope we observed HCN(1→0) emission from five ultraluminous galaxies, three lower luminosity interacting systems, and two gas-rich normal galaxies. There are huge masses of high-density gas (2x10 10 M ⊙ ) in the ultraluminous galaxies, which shows star formation, rather than active galactic nuclei, generates their infrared luminosity. HCN traces H 2 at a much higher density, ∼10 4 cm −3 , than CO (∼500 cm −3 )

Spectroscopy of galaxies in distant clusters. IV - A catalog of photometry and spectroscopy for galaxies in seven clusters with z in the range of 0.35 to 0.55

Abstract: Photometry and spectroscopy for seven deep fields containing distant clusters of galaxies with z in the range of 0.35 to 0.55 are presented. Positions and photometric parameters, including r-magnitudes g - r and r - i colors, surface brightnesses, and photometric profile types are given for about 2000 galaxies. Low-resolution spectroscopy is obtained from which redshifts are determined for 289 objects, of which 190 are cluster members. These are classified according to dominant spectral features, and examples are plotted in each cluster. Color-magnitude and color-color diagrams are formed which show trends in the cluster populations, and maps are made of the cluster field using the color-color relations to increase the contrast of cluster over field. Galaxies with spectra typical of old stellar populations cluster most strongly, with active galaxies, those with recent or ongoing star formation, or an active nucleus, distributed more diffusely. The g - r color is well correlated with active star formation as judged from spectral features.

Binaries as Tracers of Stellar Formation

Abstract: 1. N-body simulations of primordial binaries and tidal capture in open clusters 2. When and how can binary data test similar models? 3. Statistical analysis of single-lined red giant spectroscopic binaries 4. The formation of binary stars 5. Distribution and evolution of orbital elements for 1 M primaries 6. Tidal circularization of short period binaries 7. Composite-spectrum binaries 8. Orbital elements for field late-type binaries 9. Evidences for interaction among wide binary systems: to Ba or not to Ba? 10. Spectroscopic binaries in the open cluster M67 11. Spectroscopic binaries in the halo 12. Eccentricity evolution of a binary embedded in a disk 13. The eccentricity distribution of pre-main sequence binaries 14. A new algorithm to derive the mass-ratio distribution of spectroscopic binaries in open clusters 15. Are barium dwarfs progenitors of barium giants 16. RZ Eridani as a constraint on synchronization and circularization times 17. Ekman layers and tidal synchronization of binary stars 18. The distribution of mass ration in late-type main-sequence binary systems 19. The dynamical evolution of G-type main sequence binaries 20. Present state of the tidal theory 21. Infrared companions: clues to binary star formation 22. The distribution of cutoff periods with age: an observational constraint on tidal circularization theory.

Model scattering envelopes of young stellar objects. I - Method and application to circumstellar disks

Abstract: We describe a Monte Carlo code that accurately treats multiple scattering, absorption, and polarization by dust, and use this code to calculate images of dusty disks around young stellar objects. We present some approximate analytic results that describe the behavior of the Monte Carlo calculations. A geometrically thin disk illuminated by a central T Tauri star scatters very little light at distances of many AU from the star. Viewed at any inclination, the flux scattered by such a disk at the distance to the nearest star-forming region will be overwhelmed by the stellar image. An optically thick disk that has a flaring surface may be observable, especially if viewed nearly edge-on so that the stellar source becomes occulted. An optically thin disk with a finite opening angle, similar to the one surrounding beta Pictoris, is about as observable as the typical flared optically thick disk at a similar distance from the earth. The polarization position angle is perpendicular to the disk plane in all of the models, in contrast to observations of many young stellar objects which have the position angle oriented parallel to the presumed disk plane. We suggest that the scattered light structures observed around many premain-sequence objects are dusty envelopes rather than disks.

Ultraluminous IRAS galaxies: Formation of elliptical galaxies by merger-induced dissipative collapse

Abstract: Evidence that ultraluminous IRAS galaxies are elliptical galaxies forming by merger-induced dissipative collapse are summarized. Tidal tails and shells show that they are recent merger remnants. Near the center, the mass in gas is comparable to the mass in stars within the same radius; since no progenitors are this gas-rich, gas must have collapsed toward the center. Typical central gas densities ∼10 2 M ⊙ pc -3 are as high as stellar mass densities in ellipticals; this solves the well-known problem of producing the observed high densities

The Formation of Stars

Abstract: Our galaxy is composed of billions of stars concentrated in the galactic plane, surrounded by interstellar gas, which is mainly composed of hydrogen. About half of the hydrogen is distributed uniformly in the galaxy; the other half is concentrated in interstellar clouds. We find atomic hydrogen in diffuse clouds (with a particle number density n = 10 cm−3); we find molecular hydrogen (H2) in dark clouds (n = 103 cm−3). Finally we have interstellar molecular clouds (n = 106 cm−3) which also contain more complex molecules.

Faint galaxies: evolution and cosmological curvature

Abstract: RECENT observations of faint galaxies at near-infrared wavelengths1–3 reveal a surprisingly low surface density when compared to the excess of blue galaxies seen at optical wavelengths4. Attempts to determine the cosmological curvature from the asymptotic surface density of faint galaxies thus produce conflicting results3,5–7. We propose to resolve this conflict with an evolutionary model in which galaxies merge at recent look-back times. The contrast between optical and infrared galaxy counts then follows from the very different lifetimes of stellar types contributing to emission in the galactic rest-frame. Together with evidence we present an increased star formation rate in galaxies at moderate redshift, the merging model can account for both the number–magnitude relations and available redshift distributions. A clear prediction is that there should be an absence of high-redshift galaxies in deep infrared-selected surveys. If correct, the model confirms earlier suspicions that galaxies cannot be used as reliable tracers of the geometry of the Universe.

NGC 1705. I. Stellar populations and mass loss via a galactic wind

Abstract: A detailed multi-waveband study of the galaxy NGC 1705 is presented. The data include optical and ultraviolet spectroscopy, optical and infrared photoelectric aperture photometry, narrowband and broadband images, H I spectra, and data from the IRAS point source catalog. NGC 1705 is shown to be a nearby nucleated blue compact dwarf galaxy. It contains two stellar populations, distinct in structure and color : an inner high surface brightness population with a radius of 500 pc, and a relatively low surface-brightness population extending out to a radius of at least 1.56 kpc

A triggering mechanism for enhanced star formation in colliding galaxies

Abstract: A physical mechanism is proposed to explain the origin of the intense burst of massive-star formation seen in colliding/merging, gas-rich, field spiral galaxies. We explicitly take account of the different parameters for the two main mass components, H 2 and H I, of the interstellar medium within a galaxy and follow their consequent different evolution during a collision between two galaxies. It is also noted that, in a typical spiral galaxy-like our galaxy, the Giant Molecular Clouds (GMCs) are in a near-virial equilibrium and form the current sites of massive-star formation, but have a low star formation rate

The collapse of our Galaxy and the formation of the galactic disk

Abstract: The self-regulated chemical and dynamical evolution of an initially hot, gaseous protodisk is examined in detail, taking into account both star formation and heating and cooling processes in a multicomponent interstellar medium. It is shown that one can understand the observed structure of the galactic disk in a natural way as a consequence of the gravitational settling of the protodisk. During the first 4-6 × 10 9 yr, stars form in a thick disk and «old thin disk» component, with kinematical and chemical properties which are in agreement with the observations

The stellar content of LH 9 and 10 (N11) in the LMC - A case for sequential star formation

Abstract: The young OB associations Lucke-Hodge 9 and 10 are studied with UBV photometry that is independent of reddening to determine the IMF directly from star counts. The temperature and reddening of the stars are determined which, in conjunction with the spectroscopic classification of the earliest stars, is employed to place the stellar groups on the theoretical H-R diagram. Observations are also presented of the highly compact H II region/knot N11A and the multiple system HD 32228, and LH 9 and 10 are compared. The Lyman ionizing flux calculated at 4.9-7.2 x 10 exp 50/s agrees well with flux required to generate the H-alpha luminosity of the H II region. LH 10 has a much flatter slope, a higher ratio of higher-mass to lower-mass stars, and greater reddening than LH 9, and LH 10 contains all of the O stars earlier than O6. It is concluded that LH 9 is older than LH 10 and probably contributed to the initiation of star formation in LH 10.

A study of the stellar population in the Chamaeleon dark clouds

Abstract: The properties of the stellar population in the Chamaeleon dark clouds are discussed. Spectral energy distributions, based on the extant photometric and spectroscopic data base and IRAS fluxes measured from coadded data taken at the position of each star, and spectral types allow placement of the stars in an H-R diagram. The age and mass distributions and the luminosity function for the Chamaeleon stars are compared to those in the Taurus-Auriga dark clouds and are found to be similar. A small subsample (eight of 36) of the Chamaeleon stars show unusual spectral energy distributions which seem best interpreted as arising from circumstellar disks whose inner regions (R(in)) is less than 30-50 stellar radii) area devoid of material. The X-ray properties of this sample of premain-sequence objects are compared to those of other premain-sequence samples, as well as to the Hyades and the Pleiades main-sequence stars.

Two-Stage Starbursts in the Large Magellanic Cloud: N11 as a Once and Future 30 Doradus

Abstract: A recent, detailed study of the stellar content in the LMC giant shell H II region N11 has revealed a distinct, dual structural morphology, which is remarkably analogous to that emerging from current IR imaging of 30 Doradus. In both regions, the energetic stellar activity from an initial, massive, centrally condensed starburst has apparently triggered a secondary burst around its periphery about 2 × 10 6 years later, but in N11 the entire process is more advanced than in 30 Dor by 2 × 10 6 years. That is, N11 appears to be an evolved 30 Dor

On the Use of Far-Infrared Luminosity as a Star Formation Indicator in Galaxies

Abstract: By comparing the H-alpha and far-infrared (FIR) luminosities for an optically selected and magnitude-limited (m B is not greater than 14.5) sample of galaxies, a strong nonlinearity is found in the log L(H-alpha)-log L(FIR) correlation, and a systematic decrease of the L(FIR)/L(H-alpha) ratio from early- to late-type spirals. It is shown that these trends cannot be interpreted in the frame of a single-component model for the FIR emission of galaxies, with the star formation rate as the only parameter, but are consistent with a two-component model, with the cirrus fraction as the second parameter. A decreasing contribution of the cirrus component to L(FIR) toward later types, going from about 86 percent for Sa galaxies to about 3 percent for Sdm galaxies was found. If L(FIR) is to be used as a star formation indicator in galaxies, it needs to corrected, to first order, for the cirrus component, taking into account the morphological type of the galaxy.

The Star Formation History of the Large Magellanic Cloud

Abstract: Deep photometric observations of stars in three fields of the LMC are presented, and these data are interpreted using synthetic CMDs and LFs generated from overshoot models. The field CMDs and LFs with a star formation rate that experienced a large increase (4 +/- 0.5) x 10 exp 9 yr ago is successfully modeled. The precise age of this 'burst' depends sensitively on the characteristics of the models. Classical (i.e., nonovershoot) models yield a burst age about 2 x 10 exp 9 yr younger than the value obtained. An initial mass function with slope of 2.35 (the Salpeter value) and a mean field star metallicity of Fe/H of about -0.7 are consistent with the photometric data and LFs. It is suggested that the star formation rate in the LMC was globally quite low during at least the first half of its lifetime, and that a major event triggered a substantial and relatively sudden increase in the star formation rate throughout the entire LMC which persisted for several 10 exp 9 yr and even up to the present epoch in some parts of that galaxy.

Towards understanding the stellar initial mass function

Abstract: It is suggested that the stellar initial mass function (IMF) is closely related to the geometrical structure of star-forming clouds, and that the power-law form of the upper IMF results from accretion processes in hierarchical groupings of forming stars. If the stars in these groupings form from linear cloud structures such as filaments or strings of clumps, and if the overall structure of star-forming clouds is sheet-like or two-dimensional, then the mass of the most massive star that can form in each grouping is predicted to increase as the square root of the group mass; this in turn implies an IMF with a slope of x = 2 , in acceptable agreement with observations

Bipolar Molecular Outflows

Abstract: Stars form by the gravitational collapse of dense gas in molecular clouds, so one would naively expect that infall motions dominate the gas kinematics in star-forming regions. Observations, however, show this not to be the case, and that outflow motions — not infall — is what prevails around young stellar objects (YSOs). This apparent contradiction was fully realized about 20 years ago, when the first systematic molecular observations of star-forming regions were done, and bipolar molecular outflows were found towards almost every region of star formation (e.g, Snell et al. 1980, Bally & Lada 1983). The discovery of outflows has deeply changed our picture of how stars are born; their observational study has dominated the field of star formation research, and their theoretical understanding has posed a serious challenge to theorists of stellar birth. After 20 years of intense outflow research, enormous progress has been done, but some basic questions still remain unanswered. In this review we attempt to briefly describe what we know about molecular outflows, and also show the challenges that lie ahead in this exciting field of star formation.

Multiplicity among the young stars in Taurus

Abstract: Twenty-eight young stars systems in the northern Taurus star-forming region are surveyed for multiplicity. These observations, made by infrared lunar occultation and imaging techniques, can identify binaries in the angular separation range 0.005-10 arcsec. Of the 28 systems, 11 are binaries and two are triples, giving an observed multiplicity of 1.5 stars per system. The binary frequency and distribution of separations are similar to those of the main-sequence F and G spectral-type stars. Stars that have detected companions and those without are indistinguishable in H-alpha equivalent width, indicating that stars with active inner disks are represented in both groups. The multiple systems are in general weaker 1.3 mm continuum emitters than the single stars, indicating that the large-scale disks in these systems are less massive. However, there are important exceptions so the evolution of disks in multiple systems is not clear.

Formation and evolution of the Galactic Bulge and Spheroid: Where did the Spheroid Gas Go?

Abstract: The chemical abundance distribution of the Galactic spheroid is most simply explained if most - perhaps as much as 90% - of the mass originally associated with the spheroid was removed during the period of star formation. The fate of this gas is largely determined by its angular momentum content and cooling processes. The central Galactic bulge is a likely sink. We discuss several possible models for evolution of the Galactic r 1/4 spheroid and bulge, to investigate how the fate of the missing spheroid mass can be constrained, together with the rate of star formation in the bulge

Companions to white dwarfs : very low-mass stars and the brown dwarf candidate GD 165B

Abstract: We estimate the initial mass function (IMF) for star formation at the bottom of the main sequence by measuring the number of low-luminosity companions to white dwarfs. A histogram of the number of companions versus luminosity indicates that the IMF is flat or increasing with decreasing stellar mass down to, at least, 0.1 M ⊙ .

Models of Star Formation in Interacting and Merging Disk Galaxies

Abstract: We present a series of models of interactions between star-forming disk galaxies. The models employ an N-body code to calculate the gravitational dynamics, a discrete cloud model to govern the ISM dynamics, and a modified Schmidt law to describe star formation. We model both merging galaxies and more distant flyby encounters, with differing disk orientations. We find that the global star formation rates (SFRs) in merging galaxies can be increased by an order of magnitude for several hundred million years, with the vast majority of the star formation arising in the central regions of the merging galaxies

An Instability in Neutron Stars at Birth

Abstract: Calculations with a two-dimensional hydrodynamic simulation show that a generic Raleigh-Taylor-like instability occurs in the mantles of nascent neutron stars, that it is possibly violent, and that the standard spherically symmetric models of neutron star birth and supemova explosion may be inadequate. Whether this "convective" instability is pivotal to the supemova mechanism, pulsar magnetic fields, or a host of other important issues that attend stellar collapse remains to be seen, but its existence promises to modify all questions concerning this most energetic of astronomical phenomena.

The Edge of the Galactic Disk

Abstract: As part of a stellar population sampling program, a series of photometric probes at various field sizes and depths have been obtained in a low-extinction window in the Galactic anticenter direction. Such data set strong constraints on the radial structure of the disk. At the forefront of this «drilling» program, very deep CCD frames probe the most external parts of the disk. Over the whole effective magnitude range (18-25), all contributions in the statistics which should be expected from old disk stars beyond 6 kpc vanish, although such stars dominate by far at distances less than 5 kpc