Showing papers on "Star formation published in 1978"

A study of the Taurus dark cloud complex

Abstract: A near-infrared survey has been conducted of nearly 20 square degrees of the Taurus dark cloud complex. Additional observations have been made of selected objects detected in this survey. These observations show that recently formed stars are spread throughout the cloud and that these stars are primarily T Tauri stars or T Tauri-like stars. Two luminous objects are identified embedded in the reflection nebulae IC 359 and IC 2087. A new Herbig-Haro object is also described. The reddening law of the dark cloud material is discussed; it does not appear to be unusual in the infrared. Comparison of the young stellar population in Taurus with that in Ophiuchus suggests that the star formation mechanisms in the two regions are qualitatively different

An infrared study of the Ophiuchus dark cloud.

Abstract: A near-infrared survey has been conducted of nearly 18 square degrees of the Ophiuchus dark cloud complex. Additional observations have been made of selected objects found in this survey and of the brighter objects found by Grasdalen et al. and by Vrba et al. in this region in order to identify and study the young stars associated with the cloud. These observations show that very recent star formation has been largely restricted to a small region at the center of the dark cloud complex, no more than a few parsecs in extent. Most of the young stars do not appear to be main-sequence stars. At least three of these objects appear to be surrounded by infrared reflection nebulae. Many of the objects studied are background K and M giants which can be used to determine the near-infrared extinction due to the cloud.

On the bulk yields of nucleosynthesis from massive stars.

Abstract: Preliminary estimates are made of the absolute yields of abundant nuclei synthesized in observed stars. The compositions of nine helium stars of mass 3 or =10M/sub sun/ is estimated. A variety of choices for the initial mass function (IMF) are used to calculate the yield per stellar generation. For standard choices of the (IMF) the absolute and relative yields of /sup 12/C, /sup 16/O, /sup 20/Ne, /sup 24/Mg, the Si to Ca group, and the iron group agree with solar system values, to the accuracy of the calculations. The relative yields are surprisingly insensitive to the slope of the IMF. In a second approach, using standard estimates (Ostriker, Richstone, and Thuan) for the current rate of stellar death, I find the present rate of nucleosynthesis in the solar neighborhood to be about 10%of the average rate over galactic history. This result is consistent with many standard models of galactic evolution (for example, the Schmidt model in which star formation goes as gasmore » density squared). It appears that if the star formation rate is high enough to produce the stars we see around us, then the nucleosynthesis rate is large enough to produce the processed nuclei (except /sup 4/He) seen in those stars. The typical nucleosynthesis source is massive (Mapprox. =30 M/sub sun/); the death rate of such stars is a small fraction (3-10%) of recent estimates of the total rate of supernovae.« less

The neutrino radiation for a hot neutron star formation and the envelope outburst problem

Abstract: With the equations of neutrino heat conductivity being used, the neutrino light curve is calculated for the spherically symmetrical collapse of an iron-oxygen 2M ⊙ star (Figure 1) up to the formation of a hot hydrostatically equilibrium neutron star. The total energy, radiated in the form of muon and electron neutrinos, is 5.8×1053 erg (0.16Mc 2). The mean neutrino particle energy is ∼12 MeV for all the time the collapse proceeds. The maximum neutrino luminosity value is equal to 3×1053 erg s−1. For a 10M ⊙ star collapse, the luminosity maximum 3×1054 erg s−1 takes place just at the moment of the formation of a black hole inside the collapsing star. The total radiated energy in this case is about 0.08Mc 2. The set of calculations, allowing for the deposition of momentum by means of neutrino-nuclear coherent scattering, brings us to a conclusion that the envelope outburst is only possible if the scattering cross-section is 50 times larger than the value experimentally accepted (inequality 20)).

Molecular clouds in the vicinity of W3, W4, and W5

Abstract: An extensive and well-sampled map of CO emission over the entire complex consisting of the H II regions W3 (IC 1795), W4 (IC 1805), and W5 (IC 1848) is presented which was obtained at an angular resolution of 8 arcmin. A contour map of integrated (C-12)O emission overlaid on the corresponding fields of the Palomar Sky Survey is provided along with a map of the peak (C-12)O brightness temperature for the W3-W4 region. Two large cloud complexes are found within the area surveyed, and three bright CO condensations are resolved in the peak-temperature map. Column densities and masses are estimated for the cloud complex in the W3-W4 region, and it is suggested that the ionization front of W4 and the molecular cloud are interacting physically. Two condensations associated, respectively, with W3 and W3 (OH) are shown to contain unmistakable evidence for recent OB star formation.

The stability of interstellar clouds containing magnetic fields.

Abstract: The stability of interstellar clouds against gravitational collapse and fragmentation in the presence of magnetic fields is investigated. A magnetic field can provide pressure support against collapse if it is strongly coupled to the neutral gas; this coupling is mediated by ion-neutral collisions in the gas. The time scale for the growth of perturbations in the gas is found to be a sensitive function of the fractional ion abundance of the gas. For a relatively large fractional ion abundance, corresponding to strong coupling, the collapse of the gas is retarded. Star formation is inhibited in dense clouds and the collapse time for diffuse clouds cn exceed the limit on their lifetime set by disruptive processes. For a small fractional ion abundance, the magnetic fields do not inhibit collapse and the distribution of the masses of collapsing fragments are likely to be quite different in regions of differing ion abundance. The solutions also predict the existence of large-scale density waves corresponding to two gravitational-magnetoacoustic modes. The conditions which best support these modes correspond to those found in the giant molecular clouds.

A study of the IC 5146 dark cloud complex

Abstract: The IC 5146 dark cloud complex has been studied in the infrared in order to identify and study associated young stellar objects. Most of the objects detected in the survey appear to be field stars, predominantly late-type giants. Three young objects were detected in the survey: the B0 star BD +46 deg 3474, the Ae star BD +46 deg 3471, and a previously unidentified object which appears to be a heavily obscured FU Orionis star. The properties of the last two objects are examined in detail, and an attempt is made to produce reasonable models for them. It is suggested that FU Orionis stars are binaries, and some consequences of this model are described. Photometry of the brighter stars in the IC 5146 cluster has been used to establish a distance to the cluster of 900 + or - 100 pc.

The fragmentation of isothermal rings and star formation.

Abstract: Numerical studies of the collapse of a rotating interstellar cloud predict the formation of a ring of matter embedded in the infalling gas cloud. The assumption of axisymmetry in those calculations most likely limits their range of validity to the early stages of collapse before nonaxisymmetric perturbations become important. In this paper we present the results of calculations on the stability of an isothermal ring to nonaxisymmetric perturbations by numerically integrating the equations of hydrodynamics in three space dimensions.We find that the ring fragments primarily into two or three bodies in less than one-half of a rotation period of the ring. The implication for the observational fact that most stars occur in multiple systems is clear. We find the mass and specific angular momentum for each of the resulting bodies to be roughly 10% of those quantities of the initial gas cloud. A ''cascading'' process in star formation is discussed where each of the resulting bodies would form rings in its collapse, which would fragment, etc.

Interstellar clouds and the formation of stars

Abstract: Part I gives a survey of the drastic revision of cosmic plasma physics which is precipitated by the exploration of the magnetosphere throughin situ measurements. The ‘pseudo-plasma formalism’, which until now has almost completely dominated theoretical astrophysics, must be replaced by an experimentally based approach involving the introduction of a number of neglected plasma phenomena, such as electric double layers, critical velocity, and pinch effect. The general belief that star light is the main ionizer is shown to be doubtful; hydromagnetic conversion of gravitational and kinetic energy may often be much more important. In Part II the revised plasma physics is applied to dark clouds and star formation. Magnetic fields do not necessarily counteract the contraction of a cloud; they may just as well ‘pinch’ the cloud. Magnetic compression may be the main mechanism for forming interstellar clouds and keeping them together. Part III treats the formation of stars in a dusty cosmic plasma cloud. Star formation is due to an instability, but it is very unlikely that it has anything to do with the Jeans instability. A reasonable mechanism is that the sedimentation of ‘dust’ (including solid bodies of different size) is triggering off a gravitationally assisted accretion. A ‘stellesimal’ accretion analogous to the planetesimal accretion leads to the formation of a star surrounded by a very low density hollow in the cloud. Matter falling in from the cloud towards the star is the raw material for the formation of planets and satellites. The study of the evolution of a dark cloud leads to a scenario of planet formation which is reconcilable with the results obtained from studies based on solar system data. This means that the new approach to cosmical plasma physics discussed in Part I logically leads to a consistent picture of the evolution of dark clouds and the formation of solar systems.

On the masses of giant molecular cloud complexes

Abstract: A method of mass estimation for molecular clouds is presented which is based on approximate balance in the outer cloud layers between the cloud's gravitation, the galactic tide, and internal pressure. The largest observed clouds, which have greatest linear extents of 100 pc, are found to have masses of at least 200,000 solar masses. The cloud masses cannot exceed this lower limit by more than a factor of 3, or the velocity distributions of disk stars would be more relaxed than is actually observed. This implied upper limit to cloud masses combined with the galactic tide may be related to the absence of clouds at galactocentric radii less than 4 kpc. If Sagittarius B2 is bound, its mass must be more than 50 million solar masses.

The origin of the solar system: Implications for transneptunian planets and the nature of the long-period comets

Abstract: If the solar system origin is considered within the framework of the author's hypothesis on the binary stars formation as a result of rotational-exchange break-up of the rotating protostar, then difficulties involved in the usual nebular hypotheses are automatically removed (unclear aspects of the possibility of formation of the gas disc proper, the problems of the angular momentum including slow rotation of the Sun and coplanarity of the planetary orbits, of differences in planetary masses and composition, the need, for the disc remnants to be swept out, the long time of planetary formation as compared with the possible lifetime of a turbulized disc etc.).

Star formation in the NGC 7129 region - A CO molecular-line and far-infrared continuum study

Abstract: Results are presented for a joint radio/IR study of the region around NGC 7129, a reflection nebula seen against a large dark cloud. The results reported include high-angular-resolution (C-12)O and (C-13)O line measurements, high-angular-resolution observations in the range from 35 to 200 microns, and observations at 10 to 20 microns of selected objects in the nebula. Important constraints are placed on the total stellar luminosities, dust densities and temperatures, and heating and cooling processes occurring in the nebula. Specifically, it is found that: (1) the (C-13)O and dust column densities increase significantly from west to east across the nebula; (2) the (C-13)O column density in the vicinity of the two brightest visible stars decreases relative to the column density north and south of those stars; (3) kinetic temperature increases along the whole boundary of the nebulosity; and (4) the strongest far-IR source in NGC 7129 is coincident with the pre-main-sequence star LkH-alpha 234.

Star formation in molecular clouds

Abstract: A general overview is presented of the findings of molecular line and infrared astronomy regarding star formation. Beginning with the statement that stars, and therefore, planets, form from molecular clouds, it is argued that the growing body of data on conditions in molecular clouds should serve as input parameters for theoretical calculations of star and planet formation. The fact that the gravitational potential energy of molecular clouds far exceeds their thermal energy implies that the required rate of star formation is easily explained. Indeed the most important problem in this field is that the rate of star formation predicted from molecular cloud studies is too high. Until this problem is understood, additional modes of inducing star formation, while they may be occurring, only add to the existing difficulties.

CO and H/sub 2/O observations of the H II region NGC 281

Abstract: CO observations of the region around NGC 281 show two molecular cloud fragments that appear to be separated by a channel of ionized gas at the southern edge of the H II region. An H/sub 2/O maser was discovered at the northern edge of one cloud, near the interface between the cloud and the H II region. Near the maser position, /sup 12/CO temperatures and line widths reach a maximum value. A ridge of enhanced continuum emission from the H II region is also found to be coincident with the maser position, and this result suggests that the maser source is adjacent to an ionization front which is progressing into the back side of the molecular cloud. The dynamical history of the region is considered. It is found that the H II region has probably had enough time to move neutral material over distnces that are consistent with the projected separation between the clouds and the exciting star cluster. It is also shown that pressure from the H II region could have triggered gravitational instabilities in a layer at the edge of the molecular cloud. The proximity of the maser to this edge seems to indicate that such gravitational instabilitiesmore » may have already resulted in star formation. 2 figures, 1 table.« less

Star formation in or very close to a southern globule.

Abstract: A pair of globules near a - 8h25m = ' has special interest because two Herbig-Haro objects are located in the western globule of the pair. The visual absorption in the core of each globule is at least 20 magnitudes, the diameter of each is 0.3 parsecs, and the estimated total mass of each is 25 solar masses. Key words: globules-Herbig-Haro objects-star formation

Conference on Protostars and Planets, University of Arizona, Tucson, Ariz., January 3-7, 1978, Proceedings

Abstract: These papers deal with star formation, the origin of the solar system, and the possible formation of planets associated with other stars Specific topics include chemical and isotopic anomalies in meteorites, laboratory analogs of interstellar dust, properties of dark globules, locations of newly formed stars in molecular clouds, O-star formation, observations of star-formation regions near supernova remnant W44, and equilibrium condensation of supernova ejecta Other contributions discuss radio observations of molecular clouds associated with T Tauri stars, rotational velocities of pre-main-sequence stars, observational studies of star formation, characteristics of a possible preplanetary disk around the pre-main-sequence star MWC 349, calculations of the collapse of a rotating dusty protostellar cloud, a theory on the dynamics of dust particles in an incompressible turbulent fluid, and an analysis of the dynamical collapse of a model presolar nebula

Dynamic structure of dark molecular clouds

Abstract: Theoretical CO line profiles are calculated for dark molecular clouds in the case where the cloud is resolved. Assuming that the density and velocity structure of the cloud can be approximated by power laws, both crude analytical and detailed numerical calculations indicate that realistic profiles can be produced only under certain restricted combinations of density and velocity laws. The probable presence of density gradients implies that interpretation of CO profiles on the basis of the assumption of uniform density can lead to serious errors in the estimate of cloud masses. It is suggested that dark clouds and molecular clouds associated with H II regions may represent two consecutive evolutionary stages of stellar formation and that L134 is in the process of evolving from the former to the latter.

Far-infrared observations of H II regions near the galactic center

Abstract: This paper presents and discusses far-infrared observations of nine H II regions within 1 deg of the galactic center, including Sgr A, Sgr B2, Sgr C, and G0.5-0.0. The far-infrared luminosity, color temperature, and optical depth of these regions and the ratio of infrared flux to radio-continuum flux lie in the range characteristic of spiral-arm H II regions. The far-infrared results are therefore consistent with the idea that the galactic-center H II regions are ionized by luminous early-type stars. Steep systematic gradients in far-infrared color temperature and optical depth are seen along the galactic plane between Sgr B2 and G0.5-0.0; the appearance of this area is similar to that of regions of star formation in the spiral arms.

Star formation in the bright-rimmed molecular cloud IC 1848 A

Abstract: A bright IR source has been detected within a bright-rimmed dust cloud at the edge of the IC 1848 H II region. The source appears to be an early-type star with a circumstellar dust shell typical of protostars. This star is associated with the position of greatest CO excitation in a dense molecular cloud. The contours of CO emission correspond to those of the bright-rimmed dust cloud, showing that the star formed within the bright rim. Formaldehyde observations at 6 cm, 2 cm, and 2 mm are used to determine the density of the layer between the star and the ionized gas of the bright H..cap alpha.. rim. The location of this star, with respect to the dense molecular cloud which is subject to the external pressure of HII region, indicates the possible role of the expansion of IC 1848 in triggering star formation in dense regions at the perimeter of the H II region. The observed CO emission is used to determine the required luminosity of the embedded star. An early-type star of this luminosity should be detectable as a compact continuum source.