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At least 685 records · Page 38
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NASA NTRS1982

Tidal stability of gas clouds in the Large Magellanic Cloud and M101

The tidal stability of the giant atomic gas clouds in the Large Magellanic Cloud (LMC) and in M101 is examined. The giant atomic clouds in the Large Magellanic Cloud are found to be unstable against tidal disruption by the gravity of the LMC, but the clouds in M101 are in approximate tidal balance. It is unlikely that there is sufficient unobserved molecular gas to stabilize the LMC clouds, although they may have substantial molecular cores which are tidally stable. The time scale for tidal disruption of the LMC clouds is of the order of 20-30 million years. Because most of the clouds have associated H II regions, the onset of star formation in these clouds must occur rapidly. In order to balance the loss due to tidal disruption, the clouds would have to be forming at a rate of approximately 0.09 solar masses/year per sq kpc, a value comparable to that derived for the Milky Way.

Blitz, L.↗
NASA NTRS1982

The interstellar spectrum of the supernova 1980k in NGC 6946

Bright supernovae in external galaxies offer a rare opportunity to probe the intervening interstellar media over very extended path-lengths, including the disk and halo of the Galaxy and the parent galaxy of the supernova. SN 1980k was discovered in NGC 6946 on 1980 October 29. In the first month after discovery, high resolution optical and ultraviolet observations were obtained to exploit the supernova as a probe of the intervening interstellar media. The obtained data are discussed. In the spectra, absorption lines were observed of Mg I, Mg II, Fe II, Mn II, Ca II and Na I. The absorption lines are attributed to the intervening interstellar media distributed over the very extended line of sight sampled, of order 7 Mpc. These lines are wider and stronger than any previously measured in the quiescent interstellar gas in the halo and disk of the Milky Way and of external galaxies

Pettini, M.↗
NASA NTRS1983

OSS-1/STS-3 Shuttle induced atmosphere experiment

Direct light form the Sun and the sunlit Earth, and indirect light from these same sources reflected off parts of the orbiter and its payload were the two major sources of light seen in the bay during spacecraft day. Brightness arising from sunlight reflected off particulates originating from the spacecraft (corona or induced atmosphere) were tentatively identified. Sources of light observed during spacecraft night include large scale diffuse glows associated with Vernier thruster firings, surface glows on the orbiter in the direction of orbiter n motion, and periodic sky brightness structures observed primarily at 4200 A and 6300 A. Some information was obtained on the size and trajectories of individual contaminant particulates. Astronomical data were obtained from large regions of the Milky Way and zodiacal light, including large regions to within 35 deg of the Sun and possibly closer. Coordinated and sometimes simultaneous observations were successfully made from Hawaii and from STS-3 to provide unique information on atmospheric sources and sinks of radiation.

Weinberg, J. L.↗
NASA NTRS1983

Variability and mass loss in IA O-B-A supergiants

Recently completed catalogs of MK spectral types and UBV photometry of 1227 OB stars in the southern Milky Way have been analyzed to investigate brightness and color variability among the Ia supergiants. It is found that brightness variability is common among the O9-B1 supergiants with typical amplitudes about 0.1 and time scales longer than a week and shorter than 1000 days. Among the A supergiants fluctuations in U-B color are found on similar time scales and with amplitude about 0.1. For many early Ia supergiants there is a poor correlation between Balmer jump and spectral type, as had been known previously. An attempt to correlate the Balmer jump deficiency with mass loss rate yielded uncertain results.

Schild, R. E.↗
NASA NTRS1982

Faint-object spectrograph optical bench

The Faint-Object Spectrograph (FOS) is one of five scientific instruments under development for use with the Optical Telescope Assembly (OTA) of NASA's Space Telescope. It is a dual-channel spectrograph operating with two independent 512-channel pulse-counting Digicons. The FOS will be employed in connection with the study of scientific questions associated with quasars, active galaxies, normal distant and local group galaxies, a wide class of objects within the Milky Way Galaxy and neighboring galaxies, and objects within the solar system. The FOS contains an optical bench which supports all optical elements. Dimensional stability was the primary design requirement for the optical bench. This led to the selection of a graphite/epoxy structure using laminates with very low coefficients of thermal expansion and high stiffness. The technical requirements are considered and details of fabrication are discussed.

Toth, J. M., Jr.↗
NASA NTRS1981

The circumstellar envelopes of F- and G-type supergiants in the Large Magellanic Cloud

The outer atmospheres of four F- and G-type supergiants in the LMC are compared with those of their Milky Way counterparts by means of 2.5 and 5.1 A/mm high dispersion Echelle spectra. Na I D line doubling indicates extensive circumstellar envelopes, mass loss rates greater than 0.00001 solar masses/year, and outflow velocities of 10-60 km/sec. The Ca II H and K lines yield new data on extragalactic star chromospheres.

Hagen, W.↗
NASA NTRS1983

Observations of neutral hydrogen in radio-loud and interacting galaxies

The results of a survey of H I in radio-loud and interacting galaxies is presented. Four cases of H I absorption and five of emission are reported. The interesting features found for individual galaxies are described, and the systematic properties are discussed. Column densities of absorbing gas generally exceed those expected for a 'Milky Way' H I disk by more than an order of magnitude. The absorbing gas must have a flattened, disklike morphology oriented roughly parallel to the optical disk of the galaxy. Turbulent noncircular gas motions are evidently present, which are shown to be almost certainly induced by galaxy-galaxy interactions. The set of galaxies in which H I absorption has been detected is dominated by morphologically peculiar objects. It is concluded that the detection of H I seen in absorption against a nuclear radio source permits direct determination of the sense of radial flow of extranuclear material, and is direct evidence that potential 'food' for a compact object in the nucleus exists in the galaxy.

Heckman, T. M.↗
NASA NTRS1983

Stellar populations in local group dwarf elliptical galaxies. I - NGC 147

A color-magnitude diagram of NGC 147 to an I magnitude of 23 is presented. The stellar population in the outer parts of this elliptical galaxy resembles that of the globular clusters of the Milky Way. Quantitative comparison of the giant branch with those of globular clusters yields a mean metallicity of -1.2 + or - 0.2, making NGC 147 a part of the general correlation between mass and metallicity seen in ellipticals. The giant branch appears to be broad, which suggests a metallicity dispersion. The absence of asymptotic giant branch stars at luminosities above that of the red giant branch tip sets an upper limit of 10 percent for the fraction of stars in this NGC 147 field that have ages less than 12 Gyr. This result contrasts with the situation in some of the related, but less massive, dwarf spheroidal systems. If the choice is made to assume, rather than determine the stellar content of NGC 147, a distance of 630 + or - 50 kpc is derived, similar to that of M31.

Mould, J. R.↗
NASA NTRS1984

Submillimeter wavelength survey of the galactic plane from l = -5 deg to l = +62 deg: Structure and energetics of the inner disk

Results from a large scale survey of the first quadrant of the Milky Way galactic plane at wavelengths of 150, 250, and 300 microns with a 10x10 arcmin beam are presented. The emission detected in the survey arises from compact sources, most of which are identified with known peaks of 5 GHz and/or CO emission, and from an underlying diffuse background with a typical angular width of approximately 0.9 deg (FWHM) which accounts for most of the emission. A total of 80 prominent discrete sources were identified and characterized, of which about half were not previously reported at far infrared wavelengths. The total infrared luminosity within the solar circle is approximately 1 to 2x10 to the 10th power L sub 0, and is probably emitted by dust that resides in molecular clouds.

Hauser, M. G.↗
NASA NTRS1983

Self-regulated star formation in the galaxy

Assuming that star formation regions are supported against gravity by winds from low mass young objects, the stellar birthrate obtained for winds interacting in the momentum conservation stage is correlated with the molecular gas density of the parent fragment as n to the 13/8 power or n to the 5/8 power, respectively, for rates/unit volume or rates/unit mass. Birthrates derived from protostellar rotationally driven winds are in good agreement with the observed star production in the cloud B18. With the aid of observed Taurus-Auriga complex properties, the present model is extrapolated to the Galaxy as a whole, yielding a Milky Way predicted average rate that is in good agreement with standard estimates based on observations of the solar neighborhood.

Franco, J.↗
NASA NTRS1983

Photometry of resolved galaxies. II - Sextans A

Luminosity functions and color-magnitude diagrams are given for 652 stars in the Sextans A dwarf irregular galaxy, and they are compared with other Local Group galaxies. The brightest red and blue stars presently determined are in agreement with those of Sandage and Carlson (1982), and a group of red stars is identified which may be analogous to the bright carbon star population recently found in the Magellanic Clouds. While the similarity of Sextans A's luminosity function with those of the LMC and the Milky Way is in keeping with a universal luminosity function, stellar mass function and birthrate resemble those of the SMC. Per unit gas mass, Sextans A has a birthrate almost identical to the SMC, suggesting that the mean age of stars in Sextans A is significantly less than of those in the Magellanic Clouds.

Hoessel, J. G.↗
NASA NTRS1984

Detection of far-infrared forbidden O I and forbidden O III emission from the galaxy M82

Observations in M82 of the lowest-lying transitions of neutral and doubly ionized oxygen, forbidden O I 63.2 microns and forbidden O III 88.4 microns, which have wavelengths at which the extinction toward M82 is negligible, are reported. For O III, the +220 km/s velocity of the line center with respect to the local standard of rest, the 300 km/s intrinsic line width, and the asymmetry of the profile are consistent with the systemic velocity and rotational broadening seen in other ionic lines in the nucleus of M82. The fraction of oxygen that is present in doubly ionized form, indicates that the ionization state is similar to that found in H II regions in the disk of the Milky Way Galaxy. The central velocity of the O I line is +130 km/s, and no broadening in excess of instrumental resolution is seen. This result appears to rule out association of this emission with the periphery of the central H II region.

Watson, D. M.↗
NASA NTRS1984

Ultraviolet spectroscopy of the planetary nebula in the Fornax galaxy

The planetary nebula in the Fornax dwarf spheroidal galaxy is carbon rich, according to calculations based on measurements of the 1909 A emission line doublet of semiforbidden C III obtained with the International Ultraviolet Explorer. The ratio N(C)/N(O) is approximately equal to 3.7, comparable to the largest reliably determined carbon/oxygen ratios in high excitation planetaries of the Milky Way. The present result is based on four low-dispersion spectra with a combined exposure time of 27.2 hours; the Fornax planetary nebula is probably the most distant known planetary that can be observed with IUE. The IUE data were analyzed together with visible-wavelength emission-line fluxes reported by Danziger et al. (1978) to compute abundances for various elements. In terms of chemical composition, the Fornax nebula resembles planetary nebulae in the Magellanic Clouds more closely than it does typical or carbon-rich planetaries in the Galaxy.

Maran, S. P.↗
NASA NTRS1984

Chemical evolution in space

Most of the complex molecules in interstellar space ar probably contained in small, frozen interstellar dust grains which are about as old as the earth and have been photochemically converted into large organic molecules. These molecules' maximum molecular weight is limited only by the approximately 0.1-micron grain size. Their evolution leads from cool, evolved stellar atmospheres' formation of seedlings to destruction through incorporation into the material of new stars. Organic dust constitutes about 0.1 percent of the total mass of the Milky Way, far outweighing any estimates of total planetary mass in the Galaxy. Because comets may be virtually pure, aggregated interstellar dust, they offer a source of interstellar organic material for detailed study.

Greenberg, J. M.↗
NASA NTRS1984

Interstellar absorption lines in the spectrum of supernova Evans in M83 (NGC 5236)

It is pointed out that supernovae in other galaxies create spontaneous opportunities to probe spectroscopically the gaseous material in galactic halos. Because of their brightness, supernovae offer an unparalleled chance to obtain spectra with high wavelength resolution and/or signal-to-noise ratio. The present investigation is concerned with observations of visual interstellar lines in the spectrum of a supernova discovered on July 3, 1983, and identified as Type I. The supernova was located 124 arc sec south and 122 arc sec west of the nucleus of M83. Evidence is obtained that for velocities near that of M83, the line of sight toward the supernova contains a complex array of absorption features of Ca II. In both Ca II and Na I, the absorptions associated with M83 are far more prominent than those halfway through the disk (and halo) of the Milky Way Galaxy. It is felt that many of the displaced velocity components in the M83 system arise from a gaseous halo.

Rodgers, A. W.↗
NASA NTRS1984

Molecular Clouds Within 100 Pc

Observations at the 2.6 mm line of CO reveal the presence of a large number of molecular clouds at high galactic latitude. If the velocity dispersion of the clouds is a measure of their scale height, the mean distance of the ensemble detected is 100 pc. The clouds are unusual in that either they are not gravitationally bound or they are very deficient in CO relative to molecular hydrogen. These clouds represent a heretofore unrecognized component of the local interstellar medium. If they are pervasive in the Milky Way, they probably represent the small molecular cloud component of the interstellar medium.

Blitz, L.↗
NASA NTRS1985

Constraints on the galactic distribution of cosmic rays from the COS-B gamma-ray data

The diffuse component of the galactic high energy gamma rays results mainly from the interaction of CR nuclei and electrons with the nuclei of the interstellar gas. An additional contribution is obtained from the interaction of CR electrons with the interstellar photons through the inverse-Compton (IC) process. Gamma ray astronomy therefore offers an excellent means to study the distribution of CR particles throughout the Galaxy, but it is essential to know the distribution of the target interstellar gas particles, the major constituents being atomic and molecular hydrogen. Large scale millimeter wave surveys of the CO molecule covering more than half of the Milky Way, obtained with the Columbia 1.2 m telescopes, are currently available and are used to trace the H2; the COS-B observations have sufficient resolution and sensitivity to constrain the relation between the integrated CO line intensity and the molecular hydrogen column density.

Bloemen, J. B. G. M.↗