Showing papers on "Star formation published in 1986"

The birth of neutron stars

Abstract: Resultats de simulations numeriques detaillees de la phase de Kelvin Helmholtz de la naissance d'une etoile a neutrons

Candidate solar-type protostars in nearby molecular cloud cores

Abstract: IRAS data have been used to examine the vicinity of nearly 100 dense clumps of molecular gas observed in lines of CO and NH3. More than one-third of these molecular cloud 'cores' contain infrared sources that appear to be newly formed, or forming, low mass stars. While about one-third of the infrared sources are associated with visible stars and have properties that resemble T Tauri stars, the remainder of the infrared sources have no optical counterparts and are probably embedded within the molecular cloud itself. These invisible sources lie close to the molecular cloud peaks and have cold, massive shells of material around them. Some of the cores without infrared sources have gas properties similar to those with embedded sources and so may be on the verge of gravitational collapse. The selection of the IRAS sample and its properties are described. The spectral energy distributions of the sources and their nature are discussed along with their physical properties, the properties of cores with and without infrared sources, and the time scale for the evolution of the sources.

Timescales for Planetary Accretion and the Structure of the Protoplanetary Disk

Abstract: No self-consistent scenario for all stages of planetary accretion which satisfies observational constraints currently exists. An attempt is accordingly made here to resolve the timescale problems and to outline a planet formation scenario consistent with current theories of star formation as well as related models of the protoplanetary disk. For accretion to have proceeded in the manner presently hypothesized, the surface mass density of planetessimals would have had to to be relatively uniform in the Venus-Jupiter region of the protoplanetary disk, consistent with viscous accretion disk models of the solar nebula. The outer regions of the nebula would still have contained enough solid matter to account for the growth of Uranus and Neptune in 5 to 500 million years.

The IRAS Colors of Normal Galaxies

Abstract: In the IRAS color-color diagram, galaxies powered only by star formation are distributed in a band such that as systems get cooler at 12/25 microns they get warmer at 60/100 microns. A simple phenomenology is proposed which describes the emission from each galaxy as a combination of two components: cool, relatively constant 'cirrus' emission from the neutral medium, and from 'active' regions warmer emission whose colors depend on mean physical parameters in these regions. The model accounts for the IRAS colors of all galaxies detected as point sources in all four bands. The model implies that a galaxy's far-infrared flux as measured by IRAS is not simply proportional to the recent star formation rate, because only a (variable) fraction of the infrared is due to massive young stars, and only part of these stars' luminosity is reemitted in the infrared.

Infrared Spectra of Rotating Protostars

Abstract: Earlier calculations of the infrared emission expected from stars in the process of being made are corrected to include the most important observable effects of rotation and generalized. An improved version of the spherical model of a previous paper is developed, and the corresponding emergent spectral energy distributions are calculated for the theoretically expected mass infall rate in the cores of cool and quiescent molecular clouds. The dust grain opacity model and the temperature profile parameterization are improved. It is shown that the infrared spectrum of the IRAS source 04264+2426, which is associated with a Herbig-Haro object, can be adequately represented in terms of a rotating and accreting protostar. This strengthens the suggestion that collimated outflows in young stellar objects originate when a stellar wind tries to emerge and reverse the swirling pattern of infall which gave birth to the central star.

Bi-symmetric instabilities of the Kuz'min/Toomre disc

Abstract: Etude numerique de l'influence d'un mouvement aleatoire sur les modes insrables de formation des barres d'un type de modeles de disques stellaires. On montre que le taux de croissance des barres est une fonction simple du mouvement aleatoire et de la masse du halo

Molecular Clouds and Star Formation in the Inner Galaxy: A Comparison of CO, H ii, and Far-Infrared Surveys

Abstract: Surveys of the galactic plane over galactic latitudes from -1 degree to +1 degree and galactic longitudes from 12 degrees to 60 degrees are compared in the CO line at 2.6 mm, in the far-infrared (FIR) continuum at 150 micrometers and 250 micrometers, and in the radio continuum and H 110-alpha recombination line at 6 cm. The main purposes are to determine the degree of association between FIR sources, H II regions, and molecular clouds in the first quadrant and to describe and analyze the stellar content of these molecular clouds. Among the conclusions it is noted that most FIR sources coincide with HII regions, and nearly all H II regions coincide with molecular clouds, and that clouds in the inner galaxy are probably several tens of millions of years old and may have been producing O stars for only about the most recent 20 percent of their lives.

H II Regions and Star Formation in the Magellanic Clouds

Abstract: Photoelectrically calibrated maps of the H-alpha emission in the Magellanic Clouds have been used to measure integrated fluxes for several hundred H II regions and to study the properties of the H II region populations in the galaxies. The H II regions span a range of 10,000 in luminosity, from objects on the scale of the Orion Nebula to the 30 Doradus complex. The H-alpha luminosity function is well represented over this entire range by a power law function, indicating that there is no characteristic luminosity scale for the H II regions. The distributions of nebular diameters, on the other hand, are fitted well by exponential functions, with a scale length of 80 pc. Approximate fluxes for several of the extended filamentary networks in the galaxies have also been measured. This extended component probability contributes 15-25 percent of the total H-alpha luminosity of the galaxies. 42 references.

Stellar populations and star formation in irregular galaxies

Abstract: A review and intercomparison is conducted of data on various morphologically-chosen groups of irregular galaxies in order to understand the underlying physical mechanisms that differentiate these systems. Particular attention is given to the observational clues to the stellar content, star-formation processes, and star-formation histories of these galaxies, with an emphasis on the uncertainties and the many unanswered questions.

High-mass star formation due to cloud-cloud collisions

Abstract: Observational evidence is presented for the compression of molecular gas in the interface between colliding GMCs, and it is proposed that this is the dominant mode for high-mass star formation in the Galaxy. For a sample of 94 GMCs associated with high-luminosity radio H II regions, the efficiency of OB star formation decreases significantly with increasing cloud mass over the observed mass range. It is concluded that star formation is generally not stimulated by an internal mechanism. The formation of OB stars by cloud-cloud collisions is suggested by the observed quadratic dependence of the Galactic H II region distribution on the local density of H2. The preference for OB star formation in spiral arms is then naturally accounted for by orbit crowding and the increased collision frequency of clouds in the spiral arms.

The chemical evolution of the Galaxy

Abstract: The distribution of enriched material in the stars and gas of our Galaxy contains information pertaining to the chemical evolution of the Milky Way from its formation epoch to the present, providing general constraints on theories of galaxy formation. Detailed studies of the metallicities of well-defined samples of long-lived G-dwarf stars in the solar neighbourhood have ruled out the ‘simple closed-box’ model of galactic chemical evolution—too few very metal-poor stars are observed1–3. The weakest assumption inherent in this model is that the galactic disk formed and evolved as a closed system. Allowing accretion of gas, either metal-enriched or primordial (the latter requiring a particular dependence of star formation on gas density), can yield an improved fit to the observations3. However, secondary infall from the galactic extreme spheroid provides only a slight alleviation of this ‘G-dwarf problem’4. Here we show that consideration of the chemical properties of the thick-disk population of the Galaxy5,6 results in a self-consistent model for galactic chemical evolution.

The global evolution of the primordial solar nebula

Abstract: Complete radial, time-dependent calculations of the structure and evolution of the primordial solar nebula during the viscous diffusion stage are presented. The viscous stress is derived from analytic one-zone models of the vertical nebular structure based on detailed grain opacities. Comparisons with full numerical integrations indicate that the effective viscous alpha parameter is about 0.01. The evolution time of a minimum mass nebula is one-million yr or less. The flow pattern of fluid elements in the disk is examined and the implications the results have on the theory of the formation of the solar system are discussed.

Molecular gas in high-luminosity IRAS galaxies

Abstract: The paper reports observations of CO(J = 1-0) emission from an unbiased sample of the highest-luminosity IRAS galaxies with the aim of measuring their molecular gas content and determining whether star formation is a viable energy source for these high luminosities. All of the observed galaxies are rich in molecular gas with H2 masses in the range (4 x 10 to the 9th)-(4 x 10 to the 10th) solar masses. Their primary luminosity source appears to be star formation in molecular clouds. The majority, if not all, of the most luminous IRAS galaxies (L-FIR greater than 10 to the 11th solar luminosities) appear to be strongly interacting systems; those with the highest L-FIR/M(H2) ratios are mergers or close contact pairs.

Cross Sections for Tidal Capture Binary Formation and Stellar Merger

Abstract: The cross sections for tidal capture binary formation are computed for polytropic models with indices n = 3/2, 2, and 3 to represent a wide range of stellar models. The computational procedure for the tidal capture cross sections is summarized, and the results of normal mode analyses and the capture cross sections are presented. It is estimated that 25 percent of the tidally captured binaries become mergers for a stellar system whose rms velocity is roughly 10 km/s (typical for globular clusters), while about 50 percent are expected to become mergers for rms velocity of roughly 300 km/s (typical for galactic nuclei). The implications of these results for the dynamical evolution of stellar systems influenced by tidally captured binaries are briefly discussed. 22 references.

The temperature structure in accretion flows onto massive protostars

Abstract: Radiation transfer problems involved in the infall of dust and gas during star formation are studied. Dust properties are discussed, and modifications of spherical radiative transfer equations are presented that permit forward scattering by dust to be treated for the small size of the star relative to the inner radius of the shell. A procedure for deriving the stellar radiation field incident on the inner edge of the shell is developed. The temperature correction procedure of Cassinelli and Hartmann (1975) for extended stellar atmospheres is modified so that the multitemperature nature of the grains in the cloud may be derived. Temperature distributions for three schematic models in which the density is prespecified are discussed. Radiative acceleration of grains is addressed, showing that the proper mean opacity differs by a large factor from the Rosseland mean opacity that is commonly used. Emergent fluxes for the models are given.

X-ray sources in regions of star formation. I - The naked T Tauri stars

Abstract: Einstein X-ray observations of regions of active star formation in Taurus, Ophiuchus, and Corona Australis show a greatly enhanced surface density of stellar X-ray sources over that seen in other parts of the sky. Many of the X-ray sources are identified with low-mass, pre-main-sequence stars which are not classical T Tauri stars. The X-ray, photometric, and spectroscopic data for these stars are discussed. Seven early K stars in Oph and CrA are likely to be 1-solar-mass post-T Tauri stars with ages of 10-million yr. The late K stars in Taurus are not post-T Tauri, but 'naked' T Tauri stars, which are coeval with the T Tauri stars, differing mainly in the lack of a circumstellar envelope.

CO observations of infrared bright galaxies - The efficiency of star formation

Abstract: CO emission has been detected in each of 14 of the IR-bright galaxies listed in IRAS Circular 15; for the nine galaxies of the largest angular size, the CO emission distributions along the major axis have been mapped. A strong correlation is noted between total CO luminosities and IR ones for galaxies in each of three ranges of dust temperature. The ratio of IR/CO luminosities increases with the ratio of 60/100-micron flux densities, consistent with emission of thermal origin at the characteristic temperature given by the dust temperature. If this luminosity ratio is a measure of the emergent stellar luminosity/unit molecular mass, or the efficiency of star formation, this efficiency varies over almost two orders of magnitude from one galaxy to another.

Metal-Enhanced Galactic Winds. I.

Abstract: Constraints on supernova-driven galactic winds from elliptical galaxies at the epoch of star formation are investigated. The occurrence of mass loss is found to depend critically on the supernova rate in the case of dwarf galaxies, while the depth of the potential well is the most important constraint for giant ellipticals. The smallest dwarf ellipticals must have evolved from significantly more massive progenitors in order to have sustained a wind that carried away most of their metal production.

Star formation in active dwarf galaxies

Abstract: Star formation and the ISM in active dwarf galaxies are studied based on data from the IRAS Point Source Catalog. The dwarf galaxies that show evidence of recent energetic star formation are generally also strong far-infrared emitters; thus, active current star formation is associated with a history of energetic star formation. A fraction of the galaxies considered here have a star formation rate that is significantly greater than the average rate in the recent past. Available gas is being consumed as efficiently in the presently active dwarfs as in the Milky Way. A primary difference between galaxies that are energetically forming stars and those that are not is the difference in gas mass, as well as the efficiency of star formation. The most important process may be the assembly of large amounts of gaseous material.

Do density waves trigger star formation

Abstract: A comparison of B - V colors, blue and far-IR surface brightnesses, metallicities, and star formation rates calculated from H-alpha and UV fluxes reveals no difference in the average star formation rates for galaxies with and without grand design spiral structure. This implies that strong density waves do not trigger a significant excess of star formation compared to that in similar galaxies without a wave. Density waves organize the gas and young stars into spiral arms because of the flow pattern, and they may influence the formation and destruction of the largest cloud complexes, but they appear to contribute less than 50 percent to the overall star formation rate. 46 references.

Surface brightness and color distributions in blue compact dwarf galaxies. I - Haro 2, an extreme example of a star-forming young elliptical galaxy

Abstract: The first results of a large-scale program to study the morphology and structure of blue compact dwarf galaxies from CCD observations are presented. The observations and reduction procedures are described, and surface brightness and color profiles are shown. The results are used to discuss the morphological type of Haro 2 and its stellar populations. It is found that Haro 2 appears to be an extreme example of an elliptical galaxy undergoing intense star formation in its central regions, and that the oldest stars it contains were made only about four million yr ago. The 'missing' mass problem of Haro 2 is also discussed.

Enhanced star formation ― The importance of bars in spiral galaxies

Abstract: It was found that among an IR luminous subset of nearby spiral galaxies, nearly all of the systems with IRAS colors and luminosities indicative of enhanced star formation are barred. Radio continuum and IR spectroscopic results support the hypothesis that this emission originates within the central 2 kpc; possibly in a circumnuclear ring. It was also found that outer rings are over represented among these barred systems and suggest possible reasons for this phenomena.

The Phase Space Density in Elliptical Galaxies

Abstract: The high core densities of elliptical galaxies place a significant constraint on conditions at the time of star formation. Phase mixing in cores is investigated with a small series of one-dimensional, gravitational collapse experiments. It is found that very cold collapses, ones in which the initial random velocity is less than 20 percent of the virial velocity, do undergo a reduction in the core phase space density from the initial value. Consequently, purely stellar mergers are unable to produce the normal ellipticals. Mergers are allowed for the most luminous ellipticals, and with the controlled aid of a high-density bulge component or gas dissipating into the core, mergers could work for the lower mass ellipticals as well. 24 references.

Protostellar formation in rotating interstellar clouds. V - Nonisothermal collapse and fragmentation

Abstract: Numerical calculations are presented for rigorous models spanning a four-dimensional parameter space of initial conditions of the three-dimensional collapse of rotating protostellar clouds, encompassing radiative transfer in the Eddington approximation and detailed thermodynamical relations. It is found that protostellar formation may involve a few stages of hierarchical fragmentation terminated by increased thermal pressure in the nonisothermal regime, that high thermal energy clouds remain nearly axisymmetric during the first dynamic collapse phase, and that very slowly rotating clouds can fragment. The presolar nebula was probably formed from a cloud with very little initial rotation.

The Pattern of H i Deficiency in the Virgo Cluster

Abstract: A sample of 160 galaxies in the Virgo region, including 16 new 21-cm profiles in the Virgo 5-degree core obtained with the 305-m Arecibo telescope, are examined to investigate the severe depletion of interstellar H I within spiral galaxies in the Virgo cluster core. A similar and non-Gaussian distribution is found for the distribution of H I deficiencies of both faint galaxies and brighter spirals, and populations of galaxies with normal abundances of interstellar H I, and those of gas poor objects exhibiting a late-type morphology, are both noted. One-sixth of the sample within the Virgo 5-degree core have lost more than 90 percent by mass of their original neutral hydrogen, and three quarters of the galaxies found within 2.5 degrees of M87 are H I poor by more than a factor of three. The most deficient galaxies are also found to be the ones with the smallest ratios of H I to optical disk size, and H I poor galaxies are redder than normal, indicating that star formation has been quenched. 57 references.

Multicolor photometry of field galaxies to B = 24

Abstract: The redshifts and colors of nearly 10,000 galaxies at galactic latitudes SA57 and SA68 were determined by photometric scans which included UV and near-IR bands, covering 0.5 sq deg of sky. Slopes were calculated for UV, IR, red and blue counts of galaxies with respect to the brightness detected. It was found that galaxies of magnitudes over 20 show increasingly greater UV excess, which may be indicative of star formation regions. Future applications of the data generated for examining galaxy clustering patterns and evolutionary scenarios are discussed.

The molecular content of interacting and isolated galaxies - the effect of environment on the efficiency of star formation

Abstract: Molecular gas observations of merging/interacting and isolated galaxies are presented in order to study the relationship between environment and the efficiency of star formation. The two galaxy samples differ primarily in their IR properties and are quite similar in their molecular gas contents. The ratios of IR luminosity to H2 mass have a mean value of 78 and 12 solar luminosity/solar mass for interacting and isolated galaxies, respectively. The highest star formation efficiencies (SFEs) appear to occur in the merging and interacting pairs. The SFE in merging/interacting galaxies is greater than that found in the spiral arms of M51 and may be roughly proportional to the rate of cloud-cloud collisions in the interacting systems.