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At least 613 records · Page 34
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NASA NTRS1988

The evoluation of young stellar object disks and their environment

By carrying out direct imaging and spectroscopic observations of young, pre-main sequence stars in nearby molecular clouds researchers have begun: to define the frequency with which disks of approximately solar system size and mass form around young stars, and to understand the timescale for disk evolution; to characterize the early radiation (ultraviolet and keV particle) environment of circumstellar disks through study of evolution of stellar winds, wind/disk interactions and the UV and optical emission characteristics of young stars; and to understand the evolution of the solid and gaseous constituents of disks through observations of absorption features in circumstellar gas, broad emission features produced by organic compounds on grain surfaces, and absorption features (e.g., ice) produced in grain mantles. These programs offer the possibilty of relating results from astrophysical studies of the environment to newly-formed stars to the record of planet formation preserved in the solar system. Researchers completed a spectroscopic survey of 30 T Tauri stars with ages ranging from approximately 2 x 10 to the 5th power to 3 x 10 to the 7th power years. From analysis of (OI) and (SII) emission lines, it was concluded that all but two of the stars in our sample are surrounded by optically opaque disks of dimension approximately 50 AU. The two remaining objects show evidence consistent with partial disk clearing (at an age approximately 3 x 10 to the 6th power year). R and I band CFHT observations yielded detections of disks of dimension approximately 100 AU surrounding 5 additional T Tauri stars.

Strom, Stephen E.↗
NASA NTRS1989

The stellar content of rich young clusters in the Large Magellanic Cloud

Initial mass functions (IMFs) of six young rich clusters in the LMC have been determined from star counts on photographic plates with different limiting magnitudes. Over the range of stellar masses from 1.5 to 6 solar masses, very flat IMF slopes between -0.2 and 0.8 are found. Possible explanations for the discrepancy between these results and those of Mateo (1988), who found step IMFs for a different sample of clusters in the Magellanic Clouds, are considered, and no obvious ones are found. The flat IMFs imply smaller mass-to-light ratios than previously assumed and reinforce previous conclusions that the clusters have unbound halos. Flat IMFs also imply that stellar winds would have been very important in restructuring and expelling the gas from a protocluster and that there would have been many supernovae early in the history of a supercluster.

Elson, Rebecca A. W.↗
NASA NTRS1989

Superbubble blowout dynamics

Multiple supernovae and stellar winds from OB associations carve large holes filled with hot gas in the galactic disk. These superbubbles sweep up H I into cold, thin, dense shells and eventually grow large enough to blow completely out of the galactic H I disk. When superbubbles blow out of the disk, they vent hot gas and supernova energy into the galactic corona. In this paper ZEUS, a two-dimensional hydrodynamics code, is used to model the blowout of a superbubble from exponential and Gaussian models for the vertical density stratification. The results are compared to those from the Kompaneets (thin-shell) approximation. It is found that this approximation works very well, and that most of the mass of the shell remains in the plane, with 5 percent of it accelerating upward. The venting of the hot gas and the stability of the shell depends strongly on the model of the density distribution. It is suggested that the low galactic halo actually consists of a froth of merged superbubbles.

Mac Low, Mordecai-Mark↗
NASA NTRS1989

Photospheres of hot stars. III - Luminosity effects at spectral type 09.5

Hydrogen and helium line profiles with high signal-to-noise ratios were obtained for four stars of spectral type 09.5 (Alpha Cam, Xi Ori A, Delta Ori A,AE Aur) that form a sequence in luminosity: Ia, Ib, II, V. The basic stellar parameters of these stars are determined by fitting the observed line profiles of weak photospheric absorption lines with profiles from models which include the effect of radiation scattered back onto the photosphere from their stellar winds, an effect referred to as wind blanketing. For these stars, the inclusion of wind blanketing is significant only for the most luminous star, Alpha Cam, for which the effective temperature was shifted about -2000 K relative to an unblanketed model.

Voels, Stephen A.↗
NASA NTRS1989

No molecular gas disk in S106

The radio and optical bipolar H II region, S106, is bisected by a dark lane. The premain-sequence object, S106 IR, which is the source of a powerful ionized stellar wind, is the exciting source of this region and is found at the center of the equatorial emission gap. The existence of a massive, extended, molecular gas disk has previously been suggested as an explanation for the peculiar morphology of this source, and S106 has widely been quoted as the best example of theoretically posited accretion disks. The new, high-resolution, CS and (C-13)O observations presented show that the molecular emission, previously attributed to a disk structure, actually originates from distinct masses of molecular gas, swept up from the ambient cloud core by the ionized lobes.

Barsony, M.↗
NASA NTRS1989

An Exosat X-ray observation of one orbital cycle of 4U 1700-37/HD 153919

Exosat observations of close to one complete binary cycle of 4U 1700-37/HD 153919 are presented, and X-ray absorption variations are determined as a function of orbital phase in order to study the properties of the stellar wind of the companion HD 153919. The decrease noted in absorption between eclipse egress and phase 0.5 may be due to X-rays from a neutron star being absorbed by an inner wind with a radial velocity law close to that predicted by Castor et al. (1975). Under the assumption that the X-ray source is powered by material captured from the steller wind, the absorption variations with orbital phase and the time-average luminosity have been modeled to investigate the suppression of the radiative acceleration of the wind via the ionization of the wind by the X-ray source.

Haberl, F.↗
NASA NTRS1989

Circumstellar matter in Algols and Serpentids

A systematic search is reported for phenomena associated with circumstellar matter in interacting binary systems of the Algol and W Serpentis type. Ultraviolet emission lines have been detected by the author in 10 Algols and 6 Serpentids. No conspicuous difference exists between systems with disk accretion and those with direct stream impact. Evidence is indicated that the lines are formed predominantly by scattering in an induced stellar wind. Relative intensity of Fe II and Fe III emissions appears to be correlated with the spectral type of the gainer. It is suggested that in the Serpentids we see a thick disk nearly edge-on, but that the emission lines indicate the nature of the gainer inside.

Plavec, Mirek J.↗
NASA NTRS1989

Infrared diagnostics of interstellar shocks

The observations and theoretical models concerning interstellar shock waves are discussed. Interstellar shock waves are generated by the supersonic injection of mass into the interstellar medium by young stellar objects, by stellar winds, and by supernovae. Infrared emission lines from the heated, shocked gas provide valuable diagnostics of the density, temperature, velocity field, elemental abundances and extent of the emitting region. It is shown that the comparison of observed spectra from shocked regions with theoretical shock models can provide an estimate of the shock speed and the physical conditions in the preshock gas, including the ionization fraction and the magnetic field strength.

Hollenbach, D. J.↗
NASA NTRS1989

A close-up view of the S87 molecular outflow

Observations of the S87 star-forming region have been made at optical, far-infrared, centimeter, and millimeter wavelengths in order to explain the origin of the previously discovered supersonic molecular gas in this source. S87/IRS 1 is a massive, pre-main-sequence object still embedded in its parent molecular cloud, but disrupting its surroundings through the action of its powerful stellar wind. The shocked wind gas provides the force required to accelerate the surrounding molecular gas to supersonic velocities. An ordered, large-scale magnetic field is postulated to set up an initially anisotropic pressure distribution which channels the flow into two oppositely directed lobes and provides some further acceleration to the already supersonic molecular gas. Only 5 percent of the molecular gas actually reaches escape velocity.

Barsony, M.↗
NASA NTRS1989

The evolution of chromospheric activity of cool giant and subgiant stars

IUE spectra for a large sample of cool subgiant stars are examined, and evidence is found that subgiants in the mass range 1.2-1.6 solar masses undergo a sudden decline in UV transition region emission near B - V = 0.6, which corresponds to spectral type G0 IV. The decline in UV emission coincides with a sharp decrease in stellar rotation rates, and it is suggested that this decay in activity and rotation marks a transformation from acoustic heating in the early F stars to magnetic dynamo-driven activity in the cooler stars, resulting in a strong rotational braking action by stellar wind. For more massive giant stars, there is a similar transformation in the nature of chromospheric activity near B - V = 0.7, or spectral type G0 III, from acoustic heating in the F-type giants to a solarlike dynamo mechanism in the cooler giants. No sign of an abrupt drop in activity near spectral type G5 III at the location of Gray's proposed rotational boundary line is seen.

Simon, Theodore↗
NASA NTRS1988

The environments of cool stars

This review describes recent conclusions about the physical environment of red giant and supergiant stars. This includes coronae, chromospheres, dust formation and stellar winds. This knowledge can provide the boundary conditions for considering what role such objects play as members of binary star systems, where tidal forces and companion behavior alter observed characteristics.

Stencel, Robert E.↗
NASA NTRS1990

Transition regions in solar system and astrophysical plasmas

A brief review is presented of basic particle and field characteristics of plasmas observed within the solar system, especially near transition regions, and their parameter ranges are compared with those inferred for stellar winds and the interstellar medium. Parameter ranges for solar system and astrophysical plasmas are found to have considerable overlap. In addition, astrophysics provides unique, global perspectives of large-scale systems, whereas solar-system space physics provides for direct quantitative testing of physical processes. Astrophysics and solar-system space physics studies thus have complementary and synergistic roles.

Eastman, Timothy E.↗
NASA NTRS1990

The mass of the O6Iaf star HD 153919

The mass of the O component in the massive X-ray Binary 4U 1700-37 is reexamined, using the modern theory of stellar winds and applying it to X-ray light curves for the system and to UV wind line profiles. It is found that the mass of the O star is about 50 solar masses, in accord with the masses of other stars of similar spectral type. Thus, the problematic nature of the undermassive massive X-ray binaries appears to be resolved in favor of higher masses.

Heap, S. R.↗
NASA NTRS1990

Ultraviolet gas absorption and dust extinction toward M8

Interstellar absorption lines are analyzed using high-resolution IUE spectra of 11 stars in the young cluster NGC 6530 located in the M8 region. High-velocity clouds at -35 km/s and -60 km/s are seen toward all cluster stars. The components arise in gases that are part of large interstellar bubbles centered on the cluster and driven by stellar winds of the most luminous members. Absorption lines of species of different ionization states are separated in velocity. The velocity stratification is best explained as a 'champagne' flow of ionized gas away from the cluster. The C IV/Si IV ratios toward the hotter cluster members are consistent with simple photoionization models if the gas-phase C/Si ratio is increased by preferential accretion onto dust grains. High ion column densities in the central cluster decline with distance from W93, suggesting that radiation from a hot source near W93 has photoionized gas in the central cluster.

Boggs, Don↗
NASA NTRS1989

IUE observations of variability in the WN6 star HD 192163

Results are presented of a study of 77 high-resolution IUE spectra of the WN6 star HD 192163, for which new IUE observations obtained over 48 hrs in 1987 were combined with data drawn from the IUE archive, covering various epochs over the 1978-1987 period. The 1987 series of 48 spectra were found to exhibit significant variability over a period of 24 hrs in the N IV 1718-A, C IV 1550-A, and He II 1640-A P Cygni profiles. The possible causes of these variabilities are considered in terms of binary-induced changes due to a compact companion and intrinsic stellar wind variations.

Saint-Louis, N.↗
NASA NTRS1990

The infrared continuum spectrum of x ray illuminated molecular gas

In starburst galaxies, active galaxies, and the mysterious ultraluminous infrared galaxies, x rays are likely to interact with molecular gas and dust, thereby inducing infrared emission. X ray heated thermal dust will emit the IR continuum, and x ray photoelectrons will excite an IR emission-line spectrum. Here, researchers model the IR continuum emission characteristic of some selected x ray spectral fluxes, in particular the x ray bremsstrahlung characteristic of supernova and stellar wind bubble shocks in dense media and the power law spectra characteristic of active galactic nuclei. These models are part of a larger project to determine the complete IR spectra, lines plus continuum, of x ray sources embedded in molecular gas. They modeled the thermal emission from grains by calculating a grain temperature/size/composition distribution function, f(T,a,Comp.), which accounts for temperature fluctuations by averaging over all grain thermal histories. In determining the grain thermal distribution, researchers account for both direct grain heating (by x ray absorption and subsequent electron energy deposition) and indirect grain heating (by absorption of the UV emission stimulated by non-thermal photo- and Auger electrons in the gas phase). We let the grain size distribution be proportional to a(exp -3.5), and they consider two types of grain composition: graphites, which we assume to be pure carbon, and silicates, which contain all other depleted heavy elements. They derive the grain composition distribution function from solar abundances and interstellar depletion data.

Voit, G. Mark↗
NASA NTRS1990

Evolution of a superbubble blastwave in a magnetized medium

Researchers investigate the effects of interstellar magnetic fields on the evolution and structure of interstellar superbubbles, using both analytic and numerical magnetohydrodynamic (MHD) calculations. These cavities of hot gas, surrounded by shells of cold dense material preceded by a shock wave result from the combined action of stellar winds and supernova explosions in OB associations. If the medium in which a superbubble goes off is homogeneous and unmagnetized, the blast wave expands isotropically. As the interstellar gas flows through the shock, it cools significantly and gets strongly compressed such that thermal pressure remains approximately equal to ram pressure. Hence, the swept up material is confined to a very thin shell. However, if the ambient medium is permeated by a uniform magnetic field B sub o approx. 3 mu G (typical value for the interstellar matter (ISM)), the configuration loses its spherical symmetry, and, due to magnetic pressure, the shell of swept up material does not remain thin. Researchers found the following qualitative differences: (1) Except in the immediate vicinity of the magnetic poles, the shell is supported by magnetic pressure. (2) The refraction of field lines at the shock and the thermal pressure gradient along the shell both contribute to accelerating the gas toward the equator. The resulting mass flux considerably decreases the column density at the magnetic poles. (3) Away from the poles, magnetic tension in the shell causes the field lines (particularly the inner boundary) to elongate in the direction of B sub o. In contrast, the shock wave radius increases with increasing theta. (4) The reduced inertia of a parcel in the polar neighborhood makes it easier to decelerate, and accounts for the dimple which appears at the poles in numerical simulations. This dimple also results from the necessity to call on intermediate shocks in order to insure a smooth transition between a purely thermal shock at the poles and a magnetic shock in the rest of the shell. (5) The shock wave propagates faster than in the absence of magnetic field, except near the poles where the reduced mass of the shell allows it to be more efficiently decelerated.

Ferriere, Katia M.↗
NASA NTRS1989

Sublimating comets as the source of nucleation seeds for grain condensation in the gas outflow from AGB stars

A growing amount of observational and theoretical evidence suggests that most main sequence stars are surrounded by disks of cometary material. The dust production by comets in such disks is investigated when the central stars evolve up the red giant and asymptotic giant branch (AGB). Once released, the dust is ablated and accelerated by the gas outflow and the fragments become the seeds necessary for condensation of the gas. The origin of the requisite seeds has presented a well known problem for classical nucleation theory. This model is consistent with the dust production observed in M giants and supergiants (which have increasing luminosities) and the fact that earlier supergiants and most WR stars (whose luminosities are unchanging) do not have significant dust clouds even though they have significant stellar winds. Another consequence of the model is that the spatial distribution of the dust does not, in general, coincide with that of the gas outflow, in contrast to the conventional condensation model. A further prediction is that the condensation radius is greater that that predicted by conventional theory which is in agreement with IR interferometry measurements of alpha-Ori.

Whitmire, D. P.↗