Showing papers on "Star formation published in 1977"

Self-similar collapse of isothermal spheres and star formation.

Abstract: We consider the problem of the gravitational collapse of isothermal spheres by applying the similarity method to the gas-dynamic flow. We argue that a previous solution obtained by Larson and Penston to describe the stages prior to core formation is physically artificial; however, we find that the flow following core formation does exhibit self-similar properties.The latter similarity solution shows that the inflow in the dense central regions proceeds virtually at free-fall before the material is arrested by a strong radiating shock upon impact with the surface of the core. Two types of similarity solutions are obtained: one is the prototype for starting states which correspond to unstable hydrostatic equilibrium; the other, for states where the mass of the cloud slightly exceeds the maximum limit allowable for hydrostatic equilibrium. In both cases, an r/sup -2/ law holds for the density distribution in the static or nearly static outer envelope, and an r/sup -3///sup 2/ law holds for the freely falling inner envelope. Rapid infall is initiated at the head of the expansion wave associated with the dropping of the central regions from beneath the envelope. A numerical example is presented which is shown to be in good agreement with the envelopemore » dynamics obtained in previous studies of star formation using hydrodynamic codes.« less

The molecular complexes in Orion

Abstract: Line emission from CO at 2.6 mm is observed over an area of 28 square degrees in the Orion region. Most of the emission comes from two giant molecular complexes, roughly associated with Ori B and Ori A. The latter provides the best example of a giant molecular complex at the end of a sequence of OB association subgroups of decreasing age. This complex is apparently rotating with an angular velocity 4.5 by 10 to the -15th power per sec, but in a direction opposed to the Galactic rotation. The apparently denser parts of the cloud are rotating at a somewhat greater angular velocity. The total mass of the molecular gas derived from the observations is 200,000 solar masses, implying that roughly half the matter in this region is in the form of molecular hydrogen.

On the fragmentation of cosmic gas clouds. II - Opacity-limited star formation

Abstract: Opacity-limited fragmentation of gravitationally collapsing gas clouds is reexamined with inclusion of realistic dust-grain opacities and cooling rates. A minimum fragment size of approximately 0.01 solar mass is found in spherical collapse, but may be the same or larger for spheroidal collapse. A simple analytic expression is given for the characteristic protostellar mass above which further fragmentation can occur during the early opaque phases of dynamical protostellar collapse. The effects of rotation, fragment collisions, and magnetic fields are also discussed.

A connection between the rate of rotation of interstellar clouds, magnetic fields, ambipolar diffusion, and the periods of binary stars

Abstract: Observations do not show any interstellar clouds rotating much faster than the Galaxy. This can be attributed to magnetic braking, which the observed parameters of the interstellar medium show to be very efficient as long as the magnetic field is frozen in the matter. The critical density n/sub cr/ at which the field decouples from the matter determines the period of rotation of a binary star system, assumed to form by a collapsing (nearly nonmagnetic) fragment that conserves its angular momentum. The observed periods tau/sub b/ of binary stars (roughly 100 yr > or approx. = tau/sub b/> or approx. = 10 hr) may be accounted for if n/sub cr/ ranges from 7.5 x 10/sup 3/ cm/sup -3/ to 2.2 x 10/sup 6/ cm/sup -3/, respectively. We reexamine the process of ambipolar diffusion (by appropriately scaling the field strength to dense-cloud densities) and show that, at the above densities, it can decouple the field from the matter within 8.8 x 10/sup 5/ to 3.9 x 10/sup 7/ years. A scenario for star formation is outlined. (AIP)

Observational evidence for supernova-induced star formation: Canis Major R1.

Abstract: The R association CMa R1, which contains two classical Herbig emission stars (Z CMa and HD 53367) and several other extremely young stellar objects, is found to lie at the edge of a large-scale ring of emission nebulosity. The form of the ring, which is also seen at radio wavelengths, and the absence of luminous stellar objects at its center suggest that it may be a relatively old supernova remnant (SNR). This suggestion is greatly strengthened by the discovery of an expanding H I shell coincident with the optical feature and the discovery of a runaway star, HD 54662, in CMa OB1. An age of order 5 x 10/sup 5/ years is derived for the SNR by comparing its properties with theoretical expectation based on models of SNRs evolving in a uniform medium. The close agreement between the likely ages of the stars and the age of the SNR, as well as the location of the recently formed objects with respect to the supernova shell, strongly support the hypothesis that a supernova event triggered star formation in CMa R1. Several other cases where evidence exists for supernova-induced star formation are briefly discussed, the most interesting being the Orion region wheremore » the hypothesis may provide a simple explanation for such diverse features as the runaway stars, Barnard's loop, and the gas kinematics and recent star formation in the Trapezium region.« less

The galactic density wave, molecular clouds, and star formation

Abstract: A dynamical model has been devised for the galactic orbits of molecular clouds after their birth in the galactic density wave. This model suggests a picture in which young protoclusters or associations and their associated placental interstellar material actively radiate /sup 12/C/sup 16/O spectral line radiation at 115 GHz until about 30 million years after birth, at which time there is an abrupt cutoff of that spectral line radiation.A survey of 63 young open clusters reveals associated 115 GHz emission if the cluster or association contains main-sequence O stars, but the associated emission is not present for clusters whose earliest main-sequence star is B0 or later, confirming the cutoff predicted by the dynamical model.This association between /sup 1/2C/sup 16/O spectral line radiation and open clusters containing young stars has allowed us to use the surface density of /sup 12/C/sup 16/O to determine the surface density of young clusters, and we have used the dynamical model to determine their corresponding birth locations and birthrates. We also derive the birthrate of open clusters averaged over the Galaxy.In addition, it has been suggested that if supernovae in the cluster are capable of ridding the cluster of its interstellar gas, thereby stopping the COmore » spectral line radiation, then these observations give an upper mass limit for stars that become supernovae.Next we discuss recombination line observations of H II regions and compare the predictions of our dynamical model for them against the predictions for CO molecular clouds.Finally we compare our independent time scale from the dynamical model with conventional stellar evolution time scales. They agree if about 20 million years elapse between the time and interstellar cloud passes through the spiral-shock wave and the time when stars begin to form in the cloud.« less

Dark nebulae, globules, and protostars

Abstract: Some of the more important aspects of research on dark nebulae, globules, and protostars are reported. Included are the birth of a star in the galaxy, optical and radio evidence for large globules and their evolutionary status, and star formation in the star clouds of Magellan. 51 references. (JFP)

The local complex of O and B stars. II - Kinematics

Abstract: The observed space velocities of O-B5 stars lying within about 1 kpc of the sun are analyzed. Motions examined include local solar motion, differential galactic rotation, mean peculiar stellar velocities, and systematic stellar motions parallel and perpendicular to the galactic plane. Kinematical determinations of the Oort constant and the K term for stars previously classified as members of the Gould and galactic belts are found to confirm earlier results that the Gould belt (or at least its youngest component) comprises a local perturbation in the general galactic velocity field. It is shown that if the Gould belt is expanding as a unit, its expansion age must be about 70 million years. The analyses also indicate negligibly small translational motion of the Gould belt relative to the galactic belt and parallel to the galactic plane as well as an approximate 'seesaw' motion of the Gould belt about the Cassiopeia-Carina line in the galactic plane. The age of the Gould belt is inferred to be either 20 or 60 million years from the vertical motions, which would imply the existence of either one or two waves of star formation as the Gould belt passed through the galactic belt either once or twice.

Pregalactic nucleosynthesis

Abstract: We discuss the behavior of density fluctuations in an expanding universe and show that these should lead to the early formation of pregalactic hydrogen-helium stars of several hundred to several thousand solar masses. These stars flood the universe with radiation having a color temperature ≳105 K; this terminates star formation but permits galaxy formation to continue. About 10−2 of the mass of the galaxies is converted into heavy elements by pregalactic nucleosynthesis, with an error factor of a few.

Infrared studies of star formation.

Abstract: The lifetime of a massive star before it begins to deplete its nuclear fuel is less than 5 million years (1). In contrast, the age of the galaxy is more than 10 billion years (2); nevertheless, many massive and short-lived stars have been identified by observations at optical wavelengths. These massive stars must have formed very recently relative to the age of the galaxy, and therefore star formation must be occurring in the galaxy at the present time.

Is star formation bimodal? ii. the nearest early-type stars.

Abstract: The discussion of the evidence in Paper I concerning the possibility that star formation is bimodal is extended to include all of the stars in the Bright Star Catalogue (B SC) with luminosity between Mv = - 1 and + and spectral type earlier than about A2. All of these objects are probably older than about 1O years. Photometry is also presented from some more distant and younger clusters that may contain mode B (> 59 "o) stars only. Key words: stellar evolution - stellar associations - photometry

How do neutron stars evolve

Abstract: A possible evolutionary history of neutron stars is delineated, ranging from star formation through supernova explosions, X-ray and pulsar stages to an eventual pulsar turnoff via spin alignment, or magnetic field decay. New is the emphasis that there should be two populations of pulsars, the slow and the fast one, according to their successive formation in a binary system. The two pulsar populations are related to two phenotypes of supernova remnants, of which Cas A and the Crab are the best known examples.

Contraction of the Orion Nebula cluster-molecular cloud A complex

Abstract: Observational evidence is presented in support of the view that the width of the molecular lines in Orion is produced by a contraction of the cloud with a velocity directly proportional to the distance from the cloud center. The main support is provided by close agreement between the motions of the cloud and that of the stars embedded in it. A further implication of this agreement is that star formation occurred throughout the Orion cloud, rather than only in the dense center as predicted by the conventional picture of star formation.

Relation between metallicity and multiplicity for solar type stars

Abstract: IT has been suggested that the ratio of heavy elements to hydrogen (metallicity) in a contracting interstellar cloud might affect the process of star formation. There is now statistical evidence for solar type stars that the metallicity of a forming stellar system is correlated with the multiplicity of the resultant system and may actually determine whether the system will be multiple or single.

New evidence supports supernova origin of solar system

Abstract: Measurements of isotopic anomalies in meteorites have stimulated theoreticians to consider new models for the formation of the solar system. Several groups suggest that a supernova explosion was the trigger. Observational evidence for a supernova having triggered star formation provides support for this view.

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.

Infrared Observations of Star Formation Regions

Abstract: This review is divided into three parts. The first section gives a brief introduction to the different infrared wavelength ranges and to the various kinds of infrared objects seen in regions of star formation. The second section reviews the recent progress in infrared observations, concentrating on the three years since the review by Wynn-Williams and Becklin (1974) was written. The third section describes in more detail four varied examples of star formation regions.