Showing papers on "Star formation published in 1982"
TL;DR: In this article, it was shown that the best model to fulfill these requirements involves cloud generation through disruption of red giants by stellar collisions, ionization by a population of stars which is affected either by enhanced metal abundances or the death of the most massive stars, and gas disposal by star formation.
Abstract: Observations of infrared fine-structure line emission from compact clouds of ionized gas in the galactic center have been reported by Lacy et al (1979, 1980). These observations suggest the existence of a central black hole of nearly 3,000,000 solar masses and require mechanisms to generate, ionize, and dispose of the gas clouds. It is found that the best model to fulfill these requirements involves cloud generation through disruption of red giants by stellar collisions, ionization by a population of stars which is affected either by enhanced metal abundances or the death of the most massive stars, and gas disposal by star formation. Although the existence of a massive black hole cannot be ruled out, it would play no necessary role in this model and may cause the tidal disruption of stars at a rate such that their accretion into the black hole would produce more radiation than is observed.
275 citations
TL;DR: In this article, the authors derived a simple set of equations which relate the enrichment (or depletion) of the interstellar matter (ISM) to the gas-to-total mass ratio, if a power-law dependence of the star formation rate (SFR) on the total mass of gas, dM∗dt ∝Mgk is adopted.
196 citations
134 citations
TL;DR: In this article, two-dimensional radiation-hydrodynamic calculations of the interaction of ionization and shock fronts with a geometrical inhomogeneity in a molecular cloud are presented.
Abstract: We present two-dimensional radiation-hydrodynamic calculations of the interaction of ionization and shock fronts with a geometrical inhomogeneity in a molecular cloud. These regions consist of low density clumps of masses < or =2 M/sub sun/, for which self-gravity is negligible. The radiation transport is both angle and frequency dependent. Ionizing stellar radiation and density variations in molecular cloud models are shown to produce a convergent shock that creates mass concentrations with densities significantly above those expected from one-dimensional compressions. The masses and time scale for concentrations to increase depend on the geometry of the initial density variations and the intensity of ionizing stellar radiation. The radiation of OB stellar associations is shown to be effective in forming dense mass globules from preexisting low density inhomogeneneities near the edges of H II regions. Our calculations suggest that the cloud environment surrounding the cluster may influence subsequent star formation within the cloud. We include an analytic treatment of isothermal interacting shocks. The possible relationship between the observed concentrations and ionization shock compression is discussed.
106 citations
TL;DR: In this paper, the authors made a well-sampled and deep survey for H..cap alpha.. emission objects in a region about 100 square degrees in area (about 5000 pc/sup 2/), centered on the lambda Orionis OB association.
Abstract: We have made a well-sampled and deep survey for H..cap alpha.. emission objects in a region about 100 square degrees in area (about 5000 pc/sup 2/), centered on the lambda Orionis OB association. We detected 83 objects, of which 62 were previously known from earlier, less extensive surveys. The majority of the H..cap alpha.. emission objects in this region are not projected against dark cloud material, an many appear to be within the central regions of the extended (i.e., about 6/sup 0/ in diameter) low density H II region excited by lambda/sup 1/ Orionis. The vast majority of the H..cap alpha.. emission objects form an elongated, barlike structure 2/sup 0/ x 8/sup 0/ in extent. These dimensions are similar to those of giant molecular cloud complexes which spawn OB and T associations. We suggest that the spatial distribution of H..cap alpha.. emission objects and OB association members in this region provides a fossil record of a preexistent giant molecular cloud complex. The gas from the original complex has been recently removed from the vicinity of these stars by the action of the lambda Orionis H II region and is presently detected as a massive shell surrounding the H II region.more » Estimates of the mass of the shell and of the mass of the H..cap alpha.. emission objects and OB stars in the complex enable a determination to be made of the total star-formation efficiency: which we find to be 0.2%: 0.3%. Such low efficiency may be characteristic of giant molecular cloud complexes. The low star-formation efficiency we find suggests that the vast majority of the original binding mass of the lambda Orionis complex has been dissipated, leaving recently formed stars behind. Consequently, we expect this association to be unbound and expanding.« less
95 citations
TL;DR: In this paper, the authors present optical and radio observations of global properties for a sample of noninteracting irregular galaxies and a few comparison objects, which are chosen primarily on the basis of their blue colors and thus may be expected to represent systems with high rates of star formation.
Abstract: We present optical and radio observations of global properties for a sample of noninteracting irregular galaxies and a few comparison objects. Program galaxies were chosen primarily on the basis of their blue colors and thus may be expected to represent systems with high rates of star formation. The data consist of 40'' aperture photometry through intermediate-band filters, 25'' aperture spectrophotometry in the region 3500--5900 A, spectrophotometry of individual H II regions primarily from 4500--7500 A;o, and 21 cm H I observations. From the filter photometry we determine a Mg index and a reddening-free color parameter Q. An iterative population synthesis technique is applied to the large aperture spectra in order to separate H..beta.. emission from absorption, and a star formation rate (SFR) is estimated from the number of ionizing photons coupled with a Miller and Scalo initial mass function. Line fluxes are used to determine the abundance ratios O/H and N/sup +//S/sup +/, and the radio data give information on gas masses and kinematics.
91 citations
TL;DR: In this article, the conditions under which massive protostellar objects may exhibit strong, rotationally driven winds were investigated, and an evolutionary scenario was sketched, which suggests that protstellar clouds may be expected to possess substantial magnetic fields when the clouds have contracted to radii approx. 10/sup 14/ cm.
Abstract: Recent radio observations have indicated that appreciable, continuing mass loss is occurring from regions of star formation. We investigate the conditions under which massive protostellar objects may exhibit strong, rotationally driven winds. For plausible rotational velocities and magnetic field strengths, ejection of approx.10/sup 4/ M/sub sun/ yr/sup -1/ at speeds approx.10/sup 1/--10/sup 2/ km s/sup -1/ can be maintained for time scales approx.10/sup -4/ yr; these values are insensitive to the protostellar luminosity. An evolutionary scenario is sketched which suggests that protostellar clouds may be expected to be rapidly rotating and to possess substantial magnetic fields when the clouds have contracted to radii approx. 10/sup 14/ cm. The centrifugally driven winds proposed present an alternative to fragmentation for providing the angular momentum loss required in the phase of rapid cloud contraction. Subject headings: stars: formation: stars: mass loss: stars: rotation
87 citations
TL;DR: In this article, it was shown that the high-pressure dust-free environment in the cooling flow lowers the Jeans mass and thereby favors the formation of lower-mass stars.
Abstract: The central galaxies in some clusters are observed to accrete cooling intracluster gas at rates of between 4 and 300 solar masses per year. The colors and luminosities of these galaxies show evidence for star formation at less than 10 per cent of those rates, if the stellar initial-mass-function is similar to that in the solar neighborhood. It is shown that the high-pressure dust-free environment in the cooling flow lowers the Jeans mass and thereby favors the formation of lower-mass stars. The lack of detectable H I in most of these clusters is also explained.
87 citations
TL;DR: A detailed review of the searches that have been made for infrared sources in star-forming regions and detailed studies of these sources and of the interstellar matter in their immediate vicinity can be found in this article.
Abstract: Protostars are the Holy Grail of infrared astronomy. In the 15 years since the discovery of the BN object in Orion by Becklin & Neugebauer (1967) and the strong infrared emission from R Mon by Mendoza (1966), infrared astronomers have devoted much of their efforts toward trying to understand the birth of stars. They have often hoped, and sometimes believed, that their studies would yield an unambiguous example of a protostar in Spitzer’s (1948) sense of an isolated interstellar cloud undergoing inexorable gravitational contraction to form a single star. These hopes were strongly encouraged by Larson’s (1969) theoretical models, which predicted that protostars would pass through a phase of high infrared luminosity during their accretion stage. The fact that no conclusive identification of such an object has yet been made stems not from a lack of candidate infrared sources, but from the difficulty of establishing the evolutionary status of the 30 or more objects that have already been found associated with dense molecular clouds. This review describes the searches that have been made for infrared sources in star-forming regions and the detailed studies that have been made of these sources and of the interstellar matter in their immediate vicinity. The emphasis is on those objects manifesting themselves almost exclusively at infrared wavelengths, rather than on the presumably more evolved objects such as compact H II regions and young, visible stars with infrared excesses. Recent work in these latter fields has been described by Habing & Israel (1979), Cohen & Kuhi (1979), and Hyland (1981). Other useful review articles can found in the conference proceedings on infrared astronomy (Wynn-Williams & Cruikshank 1981), interstellar molecules (Andrew 1980), and on various aspects of star formation (Gehrels 1978, Appenzeller et al. 1980, Roger Dewdney 1982). Infrared techniques are described by Soifer & Pipher (1978). The objects attracting the most attention from infrared astronomers almost all have bolometric luminosities in excess of 103 L®. Current theoretical rood-
73 citations
TL;DR: In this article, a review of radio-type heterodyne mixer receivers for millimeter-and sub-millimeter-wave astronomy is presented, with a focus on the use of these receivers for spectroscopy of the interstellar medium.
Abstract: This review is concerned with recent developments in the design and implementation of radio-type heterodyne mixer receivers for millimeter- and
submillimeter-wave astronomy. For the most part these receivers will be used for spectroscopy of the interstellar medium. Understanding of the nature of the interstellar medium has increased enormously in the last decade, in large part due to studies of interstellar molecule rotation transitions (Rank et al. 1971, Zuckerman & Palmer 1974, Thaddeus 1977). Line emission or absorption can be seen due to molecular clouds with hydrogen molecule densities ranging from 10^3-10^8 cm^(-3). This has led to improved understanding of galactic structure (Cohen et al. 1980), star formation (Elmegreen & Lada 1977), and stellar mass loss processes (Zuckerman 1980). Studies of molecule emission in external galaxies are also developing (Rickard et al. 1977). From a chemical
viewpoint there has been a similar surge forward in understanding, with the
identification of interstellar species such as HCO^+ (Hollis et al. 1975, Woods et al. 1975, Snyder et al . 1976) which occur in the theoretical ion-molecule reaction schemes (Herbst & Klemperer 1973, Watson 1975).
TL;DR: In this article, it was shown that star formation is inhibited by the high velocities, relative to the gas in the dust lanes, of the dense clouds whose orbits intersect the straight dust lanes.
Abstract: The lack of star formation associated with the straight dust lanes within the bar of barred spiral galaxies suggests that star formation is strongly inhibited there. I have shown that the inhibition may be an effect of the high velocities, relative to the gas in the dust lanes, of the dense clouds whose orbits intersect the dust lanes. These clouds, which presumably undergo gravitational collapse upon entering the dust lanes of normal spiral galaxies, are the precursors of star formation. In barred galaxies, the clouds in the bar enter the dust lanes at a velocity an order of magnitude higher than those in normal spirals. The high velocity can cause a quick compression of the clouds followed by a rapid expansion which disperses the majority of the cloud. In the outer parts of the barred spiral galaxy, and at the very center, the cloud velocities are lower, and star formation is not inhibited. This creates the typical appearance of a barred spiral galaxy, with star formation at the ends of the bar, and in the center, but not along the predominant straight dust lanes which lie along the bar. I have tested the effectiveness of this technique by calculating themore » orbits of the dense, prestellar clouds in the expected gravitational field of NGC 5383. A cloud is presumed to be destroyed if its velocity relative to the intercloud gas exceeds an arbitrary minimum velocity and is presumed to collapse into stars if the intercloud density around it is enhanced by an arbitrary minimum amount.« less
TL;DR: In this article, the authors assess whether B335 is unique among dark clouds or whether CO-12 pedestal features are quite common, 180 opacity class 5 and 6 Lynds clouds were surveyed.
Abstract: To assess whether B335 is unique among dark clouds or whether CO-12 pedestal features are quite common, 180 opacity class 5 and 6 Lynds clouds were surveyed. From this set of data, three additional sources were found to have pedestal features. These suggest the presence of embedded low-mass stars, though a hot differentially rotating disk cannot be excluded for B335. Estimates of the mass-loss rate required to produce stellar winds consistent with observations are comparable with mass-loss rates for T Tauri stars. Further, the pedestal feature formation rate is similar to the local low-mass star formation rate.
TL;DR: In this paper, optical data from which they derive properties of star-forming regins in a sample of 15 irregulars which are actively forming stars is used to investigate the physical processes controlling the star formation histories and evolution of noninteracting irregular galaxies.
Abstract: In order to investigate the physical processes controlling the star formation histories and evolution of noninteracting irregular galaxies, we present optical data from which we derive properties of star-forming regins in a sample of 15 irregulars which are actively forming stars. Kinematics, sizes, and distributions of H II are obtained from H..cap alpha.. velocity profiles from high dispersion echelle spectra and images taken with the video camera through b, y, I, and H..cap alpha.. passbands.
TL;DR: In this paper, the role of turbulence in establishing the initial stellar mass function was examined and it was shown that the flattening and/or turnover at the low mass end may be a signature of interstellar turbulence.
Abstract: It is shown that the Jeans mass for gravitational collapse can be very much reduced by the influence velocity fields, even when allowance is made for non-isothermal gas behavior. We examine the role of turbulence in establishing the initial stellar mass function and show that the flattening and/or turnover at the low mass end may be a signature of interstellar turbulence. We consider also the implications of primordial turbulence for the formation of stars in the early universe.
TL;DR: In this article, numerical models of galactic chemical evolution have been computed under different assumptions on the star formation and infall rates, and the best fit to the observed data is obtained for a model with a constant star formation rate and a uniform infall rate of 5 x 10/sup -3/ M/sub sun/ kpc/sup-2/ yr/sup −1/.
Abstract: In order to quantitatively reproduce the chemical abundance gradient and the age-metallicity relation observed in the disk of the Galaxy, numerical models of galactic chemical evolution have been computed under different assumptions on the star formation and infall rates. The best fit to the observed data is obtained for a model with a constant star formation rate and a uniform infall rate of 5 x 10/sup -3/ M/sub sun/ kpc/sup -2/ yr/sup -1/. Models with other star formation laws do not reproduce the observed data as well.
15 Oct 1982
TL;DR: In this article, the Orion nebula's cloak was used as a model for gas super-shells around OB associations, optical and UV data concerning the nebula physical conditions, the presence of atomic carbon in Orion, large scale distribution of far-IR and sub-mm line emission, star formation studies in the IR, gas dynamics in the circumstellar nebula of the Becklin-Neugebauer source, maser sources and far IR CO line emission in Orion-KL, and synthesis maps of mm molecular lines.
Abstract: The present conference on astronomical studies of the Orion Nebula covers molecular clouds in Orion, the use of the nebula's cloak as a model for gas super-shells around OB associations, optical and UV data concerning the nebula's physical conditions, the presence of atomic carbon in Orion, large scale distribution of far-IR and sub-mm line emission, star formation studies in the IR, gas dynamics in the circumstellar nebula of the Becklin-Neugebauer source, maser sources and far-IR CO line emission in Orion-KL, and synthesis maps of mm molecular lines. Also discussed are Orion's star distributions, core region nebular condensations, energetic molecular flows in star-forming cloud cores, IR observations of HH objects, compact continuum radio sources, the SiO maser, shock waves, and the chemical evolution of OB associations.
TL;DR: In this paper, UBV photometry is given for 39 photoelectrically observed and 66 photographically observed stars lying in the open cluster Berkeley 87, and an evolutionary age of about one to two million years is derived from star counts delineating cluster boundaries and from the ZAMS fitting for faint cluster stars and MK spectral types for four bright members.
Abstract: UBV photometry is given for 39 photoelectrically observed and 66 photographically observed stars lying in the open cluster Berkeley 87. A cluster distance of 946 + or - 26 pc and an evolutionary age of about one to two million years, are derived from star counts delineating cluster boundaries and from the ZAMS fitting for faint cluster stars and MK spectral types for four bright members. A physical association is suggested for Berkeley 87 with the molecular clouds, compact H II regions and maser sources belonging to the ON2 star formation complex. The nature of the reddening and obscuration within the cluster can be used to demonstrate that Berkeley 87 is located on the western edge of one of the giant molecular clouds belonging to the Cygnus X complex. The BC Cyg and HDE 229059 supergiants are found to be likely members of the cluster corona.
TL;DR: In this paper, the luminosity function has been derived in an approximate manner to M/sub v/ = +5; since the cluster is too young to have lost a significant number of stars by evaporation, these data may be used to derive an initial mass function (IMF).
Abstract: UBV photographic photometry to Vapprox.16.5 (M/sub v/ = +3.5) has been obtained for over 350 stars in the field of the young cluster NGC 3293. The main sequence of the cluster is populated with stars at least as faint as M/sub v/ = 2.8 +- 0.2, implying a contraction age (tau/sub c/) of at least 25 +- 10 million years. This is significantly greater than its nuclear age (tau/sub N/) of 6 +- 2 million years, implying that (if tau/sub c/ and tau/sub N/ are correctly given by the stellar models): (1) there is an age spread of at least 20 +- 10 million years, and (2) the lower-mass stars formed first. The luminosity function has been derived in an approximate manner to M/sub v/ = +5; since the cluster is too young to have lost a significant number of stars by evaporation, these data may be used to derive an initial mass function (IMF). This shows that the cluster has an excess of high-mass stars by comparison with the IMF of field stars. The age spread, color-magnitude diagram, and IMF of NGC 3293 are probably typical of galactic clusters, suggesting that: (1) clusters with O-star members are already moremore » than 10 or 20 million years old, (2) star formation in clusters is a gradual process which is probably driven by the gravitational collapse of a clumpy, turbulent molecular cloud, and terminated by the formation of massive stars, and (3) the initial mass function varies with time during the cluster formation period.« less
Book•
01 Jan 1982
TL;DR: The Submillimeter Wave Astronomy Conference as discussed by the authors was held at Queen Mary College, London, in September 1981 and the major subject areas covered are large-scale structure and radiative transfer within interstellar clouds, spectroscopic observations of molecular sources, interstellar chemistry, and submillimeter (SM) instrumentation.
Abstract: Observations in the 100-1000-micron band and the instruments used to obtain them are discussed in contributions to the Submillimeter Wave Astronomy Conference held at Queen Mary College, London, in September 1981. The major subject areas covered are large-scale structure and radiative transfer within interstellar clouds, spectroscopic observations of molecular sources, interstellar chemistry, and submillimeter (SM) instrumentation. Reports are included on the formation of giant cloud complexes, cool molecular clouds, models for hot-centered and externally heated clouds, dust in Bok globules, airborne FIR and SM spectroscopy, rotational transitions of CH3OH and NH2 near 1.2 mm, high-velocity flows and molecular jets, FIR emissions from late-type galaxies, ion-grain collisions as a source of interstellar molecules, bandpass filters for SM astronomy, the SM receiver of the future, HF techniques in heterodyne astronomy, and the mm-wave cosmic background.
TL;DR: In this paper, it was shown that the magnetic field strength at the center of a collapsing cloud is strongly amplified during collapse even for values of q of about 1, despite extremely low values of fractional ionization.
Abstract: The gravitational collapse of isothermal, nonrotating magnetic gas clouds have been calculated numerically, including the effects of ambipolar diffusion. The fractional ionization in the clouds is approximated by a power-law function of the gas density, f = K/n to the q-power, where K and q are adjustable parameters. Eleven numerical experiments were run, and the results indicate that the asymptotic character of collapse is determined mainly by the value of q and is largely independent of the other parameters characterizing a cloud (e.g., K, cloud mass). In particular, there is nearly a one-to-one correspondence between q and the slope, x, of the central magnetic field strength-gas density relationship. If q is no more than 0.8, a cloud collapses asymptotically, as though the magnetic field were 'frozen' to the neutral matter. The magnetic field strength at the center of a collapsing cloud is strongly amplified during collapse even for values of q of about 1, despite extremely low values of fractional ionization. A discussion of the theoretical basis for this unexpected behavior is given. Possible implications of our results for the problems of magnetic braking of rotating protostars and star formation in general are also presented.
TL;DR: Far-infrared maps and multifrequency photometry are presented for M51 and its companion galaxy NGC 5195 in this paper, showing that roughly 70% of this luminosity is produced by the 5 arcmin (approximately 15 kpc) wide far infrared disk of M51.
Abstract: Far-infrared maps and multifrequency photometry are presented for M51 and its companion galaxy NGC 5195. Dust reradiates about half the starlight of the M51 + NGC 5195 system to produce the observed far-infrared (80-200 microns) luminosity of 3 x 10 to the 10th solar luminosities. Observed with 49-arcsec resolution and covering 37 arcmin-squared, the maps show that roughly 70% of this luminosity is produced by the 5 arcmin (approximately 15 kpc) wide far-infrared disk of M51. Comparable luminosities emitted from the central regions (diameters = 49 arcsec or 2.3 kpc) of M51 and NGC 5195 contribute the remaining 30% (approximately 1 x 10 to the 10th solar luminosities). The photometry reveals a difference in energetics at the galactic scale; dust producing the central luminosity of NGC 5195 classified as Ip-Ep, Irr II or SBa(r) is much less abundant and considerably warmer than the approximately 20 K dust producing the central luminosity of M51 (Sc I). Far-infrared properties are given for several galactic scale complexes of star formation in M51. Relationships are discussed for selected maps of optical starlight and emissions from these components of M51's interstellar medium: dust grains, CO molecules, hydrogen atoms, electrons, and H II regions.
TL;DR: In this paper, it was shown that if the mass profile changes homologously, the scale size of the galaxy does also, under conventional assumptions about galaxy age and initial mass function, this bloating is smaller than the differences between open and flat cosmologies at z = 0.5.
Abstract: Galactic mass loss due to gas production by an evolving stellar population and removal due to supernovae or some other energy source can alter the size of a galaxy over a long time through the adiabatic invariants of the orbits. Numerical and analytic results show that if the mass profile changes homologously, the scale size of the galaxy does also. Under conventional assumptions about galaxy age and initial mass function, this bloating is smaller than the differences between open and flat cosmologies at z = 0.5. If, however, some elliptical galaxies maintain star formation until recent epochs and then lose a (correspondingly) larger fraction of their mass, the effect may be important. It may also be important in decreasing the size of cD galaxies as a result of gas accretion.
TL;DR: In this article, color-magnitude diagrams for resolved stars in the Pegasus dwarf galaxy in the green, red, and infrared passbands of the extended Gunn photometric system are presented.
Abstract: Color-magnitude diagrams for resolved stars in the Pegasus dwarf galaxy in the green, red, and infrared passbands of the extended Gunn photometric system are presented. The evolved nature of the upper main sequence and the lack of luminous red supergiants indicate that recent star formation in Pegasus has been very subdued. Three star clusters are identified. Their red colors indicate they are of intermediate age or older. Two features of the color-magnitude diagrams are of interest: a group of red stars with I at about 21.3, which may be the counterparts of the carbon stars found in the Magellanic Clouds, and a clump in the (I, R-I)-diagram, which is probably the tip of the old giant branch. A provisional distance estimate for Pegasus of 1.7 Mpc is derived, placing it at the outer margins of the Local Group.
TL;DR: In this paper, the authors show that the appearance of an outflow is strongly affected by the environment in which it is embedded, and that the outflow in Cepheus A may not be very well collimated.
Abstract: The velocity extents, the spatial distributions, and the masses, deduced from CO observations, are very different for the red and blue outflows in Cepheus A. These properties can be modeled by placing the source of outflow amidst an anisotropic density distribution. The red outflow expands in the direction of lower density while the blue outflow encounters greater resistance as it expands toward the density peak in the cloud, as delineated by the NH/sub 3/ emission. The ratio of the masses and the total mass swept up by the outflow can be estimated by a simple model. The observed self-absorption in the line profile is probably due to absorption by the ambient cloud against a hot and optically thick component in the red outlow. We conclude that the appearance of an outflow is strongly affected by the environment in which it is embedded. In particular, the outflow in Cepheus A may not be very well collimated. The general attributes of the model are shown to be applicable to other regions of outflow. The apparently bipolar nature of outflows in star formation regions is discussed.