Engineering topics

Murray, Stephen D.

Publications and source records attributed to Murray, Stephen D..

11 records

NASA NTRS1994

Accretion disk coronae in high-luminosity systems

We present the results of self-consistent models of Compton-heated accretion disk coronae. The models are calculated using a new method for computing monochromatic radiative transfer n two dimensions. The method splits the radiation into direct and scattered components. The direct radiation is computed by calculating the optical depth along rays, while transfer of the scattered radiation is approximated by flux-limited diffusion. The resulting code agrees with more accurate treatments to within 50%, and is highly efficient, making it practical for use in large hydrodynamic simulations. The coronal models are used to confirm the results of earlier work, and to extend it to higher luminosities. In contrast to earlier work, which found the outer disks to be shadowed by the inner corona at high luminosities, we find our results to form an almost continuous extension of the models at lower luminosities. This is due to the presence of multiply scattered radiation, which acts to partially offset the loss of direct radiation from the central source. Although the analytic methods derived at lower luminosities cannot be used to derive the coronal structure for L/L(sub Edd) approx. greater than 0.1, the results of the models are amenable to semiempirical fits. We also discuss possible observational consequences of the results for coronal veiling and line fluorescence from the disk.

Murray, Stephen D.↗

NASA NTRS1992

Globular cluster formation - The fossil record

Properties of globular clusters which have remained unchanged since their formation are used to infer the internal pressures, cooling times, and dynamical times of the protocluster clouds immediately prior to the onset of star formation. For all globular clusters examined, it is found that the cooling times are much less than the dynamical times, implying that the protoclusters must have been maintained in thermal equilibrium by external heat sources, with fluxes consistent with those found in previous work, and giving the observed rho-T relation. Self-gravitating clouds cannot be stably heated, so that the Jeans mass forms an upper limit to the cluster masses. The observed dependence of protocluster pressure upon galactocentric position implies that the protocluster clouds were in hydrostatic equilibrium after their formation. The pressure dependence is well fitted by that expected for a quasi-statically evolving background hot gas, shock heated to its virial temperature. The observations and inferences are combined with previous theoretical work to construct a picture of globular cluster formation.

Murray, Stephen D.↗

NASA NTRS1992

Star formation in protogalactic clouds

The physical processes determining the rate of star formation in protogalactic clouds and the resulting effects on the structure of the galaxies are examined with reference to analytical results and numerical simulations. For spheroidal systems, it is found that rapid star formation cannot be prevented once the density exceeds a typical value. The corresponding length scale is comparable to the length scales of spheroidal systems. The gas is converted into stars at a time scale of 2 Gyr or less, during which the total luminosity of the protogalaxy in ionizing photons is 10 exp 43-44 ergs/s, and the gas is enriched to solar abundance with a steep gradient. A simplistic analysis of the resulting stellar distribution finds that de Vaucouleurs profiles may naturally result. The Faber-Jackson relation is also found to follow naturally. The Fischer-Tully relation follows by an analysis similar to that used to derive the Faber-Jackson relation, provided that the peak rotational velocity is determined by the bulge mass.

Lin, Douglas N. C.↗

NASA NTRS1992

The formation of coronal regions in accretion disks

A mechanism for heating the hot low-density gas or winds above accretion disks, which is similar to that used to model chromospheric heating in the sun and other stars, is proposed. Sound waves propagating through an accretion disk are refracted away from the central plane by the strong density gradient. As they move into regions of lower density, the sound waves accelerate to form shocks, which heat the gas, leading to the formation of a hot low-density region. The steeper density gradients present in disks make the process more efficient than in stellar atmospheres. Results of hydrodynamical simulations show that waves with frequencies similar to the local Keplerian frequency lead to the most efficient heating. For the optically thin region modeled in the present study, the result is the formation of coronal regions with densities less than about 10 exp 10/cu cm, and the temperatures ranging from 10 exp 4 to 6 K over a few scale heights. It is argued that the hot low-density gas which results from the shock heating is responsible for the observed UV lines from cataclysmic variables, as well as the spatially coincidental lines of H and He I.

Murray, Stephen D.↗

NASA NTRS1991

The formation of primordial binaries in globular clusters by star-disk interactions

The formation of primordial binaries in globular clusters is examined using simple numerical models. Clusters of protostars collapse until their velocity dispersion rises sufficiently to reverse the infall and the cluster reaches equilibrium. During the collapse, interactions between stars and protostellar disks lead to stellar capture. It is found that binary fraction of a few percent typically result. Binary formation is terminated when the velocity dispersion rises to a point at which most encounters result in disk destruction rather than capture. As a result, much gas is returned to the cluster ISM, limiting the star formation efficiency to a value significantly below 100 percent.

Murray, Stephen D.↗

NASA NTRS1991

Thermal instabilities in proto-globular clusters resulting from time-dependent potentials

In this paper a scenario is proposed to explain the lack of young globular clusters in the Galaxy. It is argued that the formation of the Galactic disk would induce tidal compressions on protoglobular clouds during passage through the Galactic disk. Such a compression would trigger the onset of thermal instability in typical protoglobular cluster clouds, leading to rapid cooling and contraction and subsequent star formation. The clouds are thus unlikely to survive several passages through the Galactic disk following its formation. Instabilities preferentially occur in clouds with relatively high metallicity and with orbits lying more nearly in the plane of the Galaxy, possibly leading to the formation of a population of clusters with small orbital inclination, similar to the population of disk clusters observed in the Galaxy.

Murray, Stephen D.↗

NASA NTRS1991

Star formation in globular clusters and dwarf galaxies and implications for the early evolution of galaxies

Based upon the observed properties of globular clusters and dwarf galaxies in the Local Group, we present important theoretical constraints on star formation in these systems. These constraints indicate that protoglobular cluster clouds had long dormant periods and a brief epoch of violent star formation. Collisions between protocluster clouds triggered fragmentation into individual stars. Most protocluster clouds dispersed into the Galactic halo during the star formation epoch. In contrast, the large spread in stellar metallicity in dwarf galaxies suggests that star formation in their pregenitors was self-regulated: we propose the protocluster clouds formed from thermal instability in the protogalactic clouds and show that a population of massive stars is needed to provide sufficient UV flux to prevent the collapsing protogalactic clouds from fragmenting into individual stars. Based upon these constraints, we propose a unified scenario to describe the early epochs of star formation in the Galactic halo as well as the thick and thin components of the Galactic disk.

Lin, Douglas N. C.↗

NASA NTRS1990

On the fragmentation of protogalactic clouds

After its initial collapse, a protogalactic cloud is expected to be shock heated to its virial temperature of about 1 million K. This temperature lies near a very unstable region of the cooling curve, so that density enhancements will be expected to grow rapidly as the protogalaxy cools to lower temperatures. In this contribution, the dynamical effects on the growth of perturbations are examined. It is shown that perturbations with large initial amplitudes lead to the formation of H2, which results in rapid cooling to 100 K, and the formation of population III stars. Perturbations with small initial amplitudes, however, do not cool significantly ahead of the background, do not develop strong contrasts with the background, and so cannot induce the formtion of H2. They thus remain near 10,000 K, forming a population of protoclusters with masses of about 10 to the 7th solar masses, somewhat larger than typical globular cluster masses. Contamination by metals from a primordial generation of stars may lead to cooling and a significant reduction of the Jeans mass. Photodissociation induced by a background source of hard photons can also prevent cooling below 10,000 K, enhancing the formation of protoclusters relative to stars.

Murray, Stephen D.↗

NASA NTRS1990

On the origin of metal homogeneities in globular clusters

Various transport processes which may have affected the chemical homogeneity in protocluster clouds are examined. It is shown that the characteristic diffusion time scale associated with collisions between grains and gas atoms is considerably longer than that on which star formation is expected to occur. Collisions between large grains and gas atoms lead to mass segregation and metallicity gradients on a time scale comparable to the crossing time of the clusters in the Galaxy. One possible mechanism for inducing and maintaining chemical homogeneity is turbulent diffusion in the clouds. The mixing time scale required in this case is comparable to several internal dynamical time scales, longer than the evolutionary time scale of the most massive stars, and shorter than the Galactic orbital time scale of the clouds. Thus, metals in presently observed stars probably did not originate from upper main-sequence stars of a coeval generation.

Murray, Stephen D.↗

NASA NTRS1989

The fragmentation of proto-globular clusters. II - Gravitational instabilities

The remarkable homogeneity in the metal abundances within a typical globular cluster may be a consequence of the fragmentation of the proto-globular clouds. In this paper, the late stages of fragmentation are investigated by examining the growth of gravitational instabilities in the shell. It is found that the short dynamical time scale of the shell leads to the rapid growth of gravitational instabilities, so that the entire process of fragmentation to stellar mass scales can be completed on a time scale much less than the dynamical time scale of the parent cloud.

Murray, Stephen D.↗

NASA NTRS1989

The fragmentation of proto-globular clusters. I - Thermal instabilities

The metal abundances among the stars within a typical globular cluster are remarkably homogeneous. This indicates that star formation in these systems was a globally coordinated event which occurred over a time span less than or comparable to the collapse time scale of the cluster. This issue is addressed by assuming that the fragmentation of a proto-globular cluster cloud proceeded in two steps. In the first step, thermal instability led to the rapid growth of initial fluctuations. This led to a large contrast in the dynamical time scales between the perturbations and the parent cloud, and the perturbations then underwent gravitational instabilities on short time scales. This process is modeled using one-dimensional hydrodynamic simulations of clouds both with and without external heat sources and self-gravity. The models include the effects of a non-equilibrium H2 abundance. The results indicate that fragmentation can occur on time scales significantly less than the dynamical time scale of the parent cloud.

Murray, Stephen D.↗