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NASA NTRS1977

Stellar rotation and the thermomagnetic torque

It is noted that any reasonable assumption for the rotation rates of the interstellar clouds which must collapse to produce stars will result in stars which rotate much faster than suggested by observations if angular momentum is conserved throughout the collapse. The contribution of thermomagnetic torque, which can exist when a rarefied gas of polyatomic molecules is subjected to a magnetic field and a nonuniform temperature gradient, to the angular momentum of a collapsing protostellar object is examined in order to determine whether the magnitude of this torque is sufficient to slow down contracting protostellar clouds. Calculations are performed for two times during protostellar collapse when conditions may be favorable for thermomagnetic torque to play a role in angular-momentum considerations. The results indicate that this torque is apparently of no importance in determining or modifying stellar rotation during the early stages of stellar evolution.

Spear, G. G.↗

NASA NTRS2019

Energetic Particles of Cosmic Accelerators I: Galactic Accelerators

The high-energy universe has revealed that energetic particles are ubiquitous in the cosmos and play a vital role in the cultivation of cosmic environments on all scales. Our pursuit of more than a century to uncover the origins and fate of these cosmic energetic particles has given rise to some of the most interesting and challenging questions in astrophysics. Energetic particles in our own galaxy, galactic cosmic rays (GCRs), engage in a complex interplay with the interstellar medium and magnetic fields in the galaxy, giving rise to many of its key characteristics. For instance, GCRs act in concert with galactic magnetic fields to support its disk against its own weight. GCR ionization and heating are essential ingredients in promoting and regulating the formation of stars and protostellar disks. GCR ionization also drives astrochemistry, leading to the build up of complex molecules in the interstellar medium. GCR transport throughout the galaxy generates and maintains turbulence in the interstellar medium, alters its multi-phase structure, and amplifies magnetic fields. GCRs could even launch galactic winds that enrich the circumgalactic medium and alter the structure and evolution of galactic disks. As crucial as they are for many of the varied phenomena in our galaxy, there is still much we do not understand about GCRs. While they have been linked to supernova remnants (SNRs), it remains unclear whether these objects can fully account for their entire population, particularly at the lower (approximately less than 1 GeV per nucleon) and higher (~PeV) ends of the spectrum. In fact, it is entirely possible that the SNRs that have been found to accelerate CRs merely re-accelerate them, leaving the origins of the original GCRs a mystery. The conditions for particle acceleration that make SNRs compelling source candidates are also likely to be present in sources such as protostellar jets, superbubbles, and colliding wind binaries (CWBs), but we have yet to ascertain their roles in producing GCRs. For that matter, key details of diffusive shock acceleration (DSA) have yet to be revealed, and it remains to be seen whether DSA can adequately explain particle acceleration in the cosmos. This White Paper is the first of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV gamma-ray astronomy will bring to understanding their energetic particle phenomena. For the case of GCRs, MeV astronomy will: 1) Search for fresh acceleration of GCRs in SNRs; 2) Test the DSA process, particularly in SNRs and CWBs; 3) Search for signs of CR acceleration in protostellar jets and superbubbles.

Venters, Tonia M.↗

NASA NTRS2003

Protosteller Disks Under the Influence of Winds and UV Radiation

Star formation and the creation of protostellar disks generally occur in a crowded environment. Nearby young stars and protostars can influence the disks of their closets neighbors by a combination of outflows and hard radiation. The central stars themselves can have a stellar wind and may produce sufficient UV and X-ray to ultimately destroy their surrounding disks. Here we describe the results of numerical simulations of the influence that an external UV source and a central star's wind can have on its circumstellar disk. The numerical method (axial symmetry assumed) is described elsewhere. We find that protostellar disks will be destroyed on a relatively short time scale (~ 10(sup 5)yr) unless they are well shielded from O-stars. Initially isotropic T-Tauri winds do not significantly influence their disks, but instead are focused toward the rotation axis by the disk wind from photoevaporation.

formation↗

NASA NTRS1977

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

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.

Silk, J.↗

NASA NTRS1977

On the fragmentation of cosmic gas clouds. III - The initial stellar mass function

Radiation from the first opaque protostellar fragments to form in a nonhomologously collapsing molecular cloud can provide a significant heat source in regions where the matter remains relatively diffuse over the initial free-fall time scale. This energy input provides a negative feedback that gradually inhibits fragmentation as larger protostellar fragments form that are also more effective radiators. A highly simplified model is described that yields an expression for the initial mass function in stellar clusters. Dynamical dissipation, driven by the more massive protostars, tends to decouple the gas and grain temperatures and steepen the mass function. Fragmentation of less massive stars appears to proceed more efficiently when the heavy-element abundance is decreased, indicating the possibility of an inverse correlation between gradients in Z and M/L. Other possible observational tests of fragmentation are also suggested.

Silk, J.↗

NASA NTRS1980

Clumpy molecular clouds - A dynamic model self-consistently regulated by T Tauri star formation

A model is proposed which can account for the longevity, energetics, and dynamical structure of dark molecular clouds. Recent observations of a high space density of T Tauri stars in some dark clouds provide the basis for the assertion that high-velocity winds from these low mass pre-main-sequence stars provide a continuous dynamic input into molecular clouds. It is postulated that as clumps are driven above the Jeans mass, both by coalescence and the enhancement of ram pressure through continuing acceleration by protostellar winds, collapse is followed by the formation of low-mass stars that generate additional protostellar winds. Finally, it is found that star formation occurs on a relatively slow time scale, comparable to the cloud lifetime

Norman, C.↗

NASA NTRS1981

Fragmentation in a rotating protostar - A re-examination of comparison calculations

The self-gravitating collapse of a rotating, isothermal protostellar cloud has been recalculated with two independent fluid-dynamic computer codes, in three space dimensions, with improved spatial resolution compared to previous calculations. The results again predict fragmentation and formation of a binary protostellar system with properties similar to those obtained in the lower-resolution calculations. The results, however, are in disagreement with those obtained with a particle-dynamic code by Gingold and Monaghan (1981) for the collapse of a cloud from the same initial conditions. Possible explanations for the divergent results are discussed.

Bodenheimer, P.↗

NASA NTRS1983

Models of the formation of the solar nebula

Protostellar cloud collapse and solar nebula formation models indicate that the size of the nebula produced will be larger in terms of both gas centrifugal balance R(CF) and collapse time diffusion length R(V). From this, it can be deduced that low mass nebulas are produced if (R(V)/R(CF))-squared is much greater than unity, while nebulas result for values lower than approximately unity. The total angular momentum value distinguishes most current models of the solar nebula. Analytic expressions for the surface density, nebular mass flux and photospheric temperature distributions during the formation stage are presented for simple modes illustrating and general properties of growing protostellar disks.

Cassen, P.↗

NASA NTRS1984

Submillimeter astronomy and the problem of star formation

Sources that have traditionally been called 'protostars,' because they were strong emitters of infrared radiation embedded in dust clouds, are now recognized to be 'newly formed' stars instead. Recent developments in submillimeter astronomy should permit a redoubling of efforts to find bodies that are the actual predecessors of newly formed stars. This renewed search for true protostars will be aided by advances that have occurred in submillimeter spectroscopy; these will permit an analysis of the physical conditions and chemical constitution of cooler protostellar clouds, and may provide insight into circumstances favoring protostellar collapse.

Harwit, M.↗

NASA NTRS1986

Primordial stellar evolution - The pre-main-sequence phase

The quasi-static contraction of primordial stars composed of pure hydrogen and helium gas is studied by following numerically the evolution of a star of five solar masses from the end of protostellar accretion to the onset of hydrogen burning. Although the protostellar core of this mass is radiatively stable and undergoing nonhomologous contraction, its large surface area and luminosity force the star to a partially convective, homologously contracting state within only 100 yr. Deuterium later ignites at an off-center temperature maximum but fails to produce interior convection. The star follows a conventional premain sequence track in the HR diagram, reaching the ZAMS after 1.2 million yr, with a luminosity of 880 solar luminosities and a radius of 1.2 solar radii.

Stahler, S. W.↗

NASA NTRS1987

Theory of collapse and protostar formation

Interstellar clouds must increase in density by a factor of more than 10 to the 20th in order to form stars. Because observations of the phases intermediate between dense interstellar clouds and pre-main-sequence stars are difficult, theoretical solutions presently provide the primary means for exploring the collapse phase of protostellar formation. The mathematical formulation of the protostellar collapse problem is presented, and various methods employed in solving the equations are outlined. This tutorial emphasizes the numerical approach to the study of the nonlinear time-dependent evolution of collapsing interstellar clouds, including self-gravitation, rotation, and radiative transfer. Results are summarized for the restricted cases of spherical and axisymmetric symmetry, as well as for fully three-dimensional evolutions, and briefly compared to observations of star formation.

Boss, A. P.↗

NASA NTRS1991

The dynamical evolution of the protosolar nebula

Evolutionary models for protostellar nebulae are calculated under the hypothesis that the only source for the turbulent viscosity is thermal convection. The viscous stress is approximated by an 'alpha' model, and the constant alpha is calculated in terms of the properties of turbulent thermal convection. A relatively sensitive dependence of the Rosseland mean opacity on temperature is needed for the vertical temperature gradient of the nebula to become convectively unstable. However, this requires that the vertical optical depth in the nebula is relatively sensitive to the total surface density, and as the disk is depleted of matter by protostellar accretion, the Rosseland mean optical depth drops and the disk must become convectively unstable. This limits the amount of mass that the nebula can accretionally process before the convective turbulence ceases. The resulting disk evolutionary properties are calculated and comparisons with the solar system are made.

Ruden, Steven P.↗

NASA NTRS1991

Far-infrared and submillimeter wavelength observations of star-forming dense cores. II - Images

This paper reports on far-infrared and submillimeter wavelength observations of low-mass protostellar candidates. The data set comprises emission maps of nine sources observed over a wavelength range 100-800 microns. The emission is extended at all wavelengths longer than 100 microns. The apparent size of the emission regions is weakly correlated with the beam size and is consistent with an underlying specific intensity profile which is scale-free. The observed emission maps are not circular, but have a mean aspect ratio of 1.3, and the position angles are similar to those determined from maps of molecular emission. The observational results are used in conjunction with theoretical considerations to constrain the physical properties of the putative protostellar envelopes.

Ladd, E. F.↗

NASA NTRS1992

Evidence for a wind-swept cavity in HH 34?

High-resolution HCO(+) observations of two sections of the HH 34 system are presented. The emission is seen in association with, and to the north of, the exciting star of the HH 34 jet as well as to the south of the HH object HH 34. The emission to the north and northwest of HH 34 IR is in two ridge formed in the shape of a U. These two ridges are probably the limb-brightened walls of a cavity. The data suggest that the cavity was cleared by a protostellar wind with mass loss and momentum flux similar to the neutral wind in HH 7-11. HCO(+) emission is detected in association with the source IRS 5 and to the south of HH 34, with a gap between the HH object and the emission of 15-40 arcsec. These results confirm that the emission traces dense circumstellar gas an both low and high velocity gas involved in the interaction between fast protostellar winds and their surroundings.

Rudolph, Alexander↗

NASA NTRS1992

Dynamics of binary and planetary-system interaction with disks - Eccentricity changes

Protostellar and protoplanetary systems, as well as merging galactic nuclei, often interact tidally and resonantly with the astrophysical disks via gravity. Underlying our understanding of the formation processes of stars, planets, and some galaxies is a dynamical theory of such interactions. Its main goals are to determine the geometry of the binary-disk system and, through the torque calculations, the rate of change of orbital elements of the components. We present some recent developments in this field concentrating on eccentricity driving mechanisms in protoplanetary and protobinary systems. In those two types of systems the result of the interaction is opposite. A small body embedded in a disk suffers a decrease of orbital eccentricity, whereas newly formed binary stars surrounded by protostellar disks may undergo a significant orbital evolution increasing their eccentricities.

Atrymowicz, Pawel↗

NASA NTRS1991

Circumstellar chemistry

Recent theoretical studies of circumstellar chemistry are discussed for both red-giant and protostellar winds. The generalized photochemical model is able to account for the recently discovered silicon-bearing molecules in the prototypical, C-rich, AGB star IRC + 10216. The surprising occurrence of CO in protostellar winds that are largely atomic is interpreted to be the result of the high density and the rapid decrease of the temperature with distance that is expected for such winds.

Glassgold, A. E.↗

NASA NTRS1994

The jet model of chondrule formation

We estimate the size range of particles that are ejected from a protostellar accretion disk by a protostellar jet. An n-body code is used to determine the subsequent motion of the ejected particles, where the particles are subject to two forces: the gravitational attraction from the protostar; and the gas drag from the halo gas of the accretion disk.

Liffman, K.↗

NASA NTRS1995

Collapse and fragmentation of molecular cloud cores. 2: Collapse induced by stellar shock waves

The standard scenario for low-mass star formation involves 'inside-out' collapse of a dense molecular cloud core following loss of magnetic field support through ambipolar diffusion. However, isotopic anomalies in presolar grains and meteoritical inclusions imply that the collapse of the presolar cloud may have been triggered by a stellar shock wave. This paper explores 'outside-in' collapse, that is, protostellar collapse initiated directly by the compression of quiescent dense cloud cores impacted by relatively slow stellar shock waves. A second-order accurate, gravitational hydrodynamics code has been used to study both the spherically symmetrical and three-dimensional evolution of initially centrally condensed, isothermal, self-gravitating, solar-mass cloud cores that are struck by stellar shock waves with velocities up to 25 km/s and postshock temperatures of 10 to 10,000 K. The models show that such mild shock waves do not completely shred and destroy the cloud, and that the dynamical ram pressure can compress the cloud to the verge of self-gravitational collapse. However, compression caused by a high postshock temperature is a considerably more effective means of inducing collapse. Shock-induced collapse produces high initial mass accretion rates (greater than 10(exp -4) solar mass/yr in a solar-mass cloud) that decline rapidly to much lower values, depending on the presence (approximately 10(exp -6) solar mass/yr) or absence (approximately 10(exp -8) to 10(exp -7) solar mass/yr) of an infinite reservoir of mass. Stellar mass accretion rates approximately 10(exp -7) solar mass/yr have been previously inferred from the luminosities of T Tauri stars; balanced mass accretion (stellar rate = envelope rate) at approximately 10(exp -7) solar mass/yr could then be possible if accretion occurs from a finite mass reservoir. Fluid tracers are used to determine what fraction of the stellar shock material is incorporated into the resulting protostellar object and disk; roughly half the impinging material is injected into the collapsing cloud core when there is a high postshock temperature. The models are consistent with a scenario where an AGB star wind triggered the collapse of the presolar cloud while injecting about 0.01 solar mass of matter derived from the AGB star envelope, as has been separately inferred on the basis of nucleosynthesis calculations.

Boss, Alan P.↗