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At least 397 records · Page 22

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NASA NTRS1986

Fe II emission line profiles in spectra of high luminosity non-coronal stars

The results of a program to study the winds of noncoronal late-type giant and supergiant stars with IUE are presented. The primary data for this study are profiles, fluxes, and velocities of the Fe II emission lines which occur throughout the 2200 to 3230 A region of the IUE long wavelength spectrographs. The apparent dependence of the Fe II profiles on stellar luminosity (surface gravity) and intrinsic line strength is discussed. The discovery of significant changes in the Fe II profiles in spectra of Gamma Crucis (M3 III), during the period 1978 to 1985, that suggest a substantial change in its outer atmosphere is noted.

Carpenter, Kenneth G.↗

NASA NTRS1987

VLA observations of a sample of galaxies with high far-infrared luminosities

Preliminary results are presented from a radio survey of galaxies detected by the IRAS minisurvey. It was found that the main difference between galaxies selected in the far infrared and those selected in the optical is that the former have higher radio luminosities and that the radio emission is more centrally concentrated. There is some evidence that the strong central radio sources in the galaxies selected in the infrared are due to star formation, the star formation rate divided by the volume in which the star formation is occuring is 100 to 1000 times greater in the galaxies selected in the infrared than in the disks of normal galaxies.

Eagles, S. A.↗

NASA NTRS1987

Summary of symposium: Low luminosity sources

The author summarized certain aspects of the conference. He shares this task with another colleague thereby breaking the task into more manageable proportions. The author covers the low luminosity sources. He begins his review with a summary of some major themes of the conference and ends with a few speculations on possible theoretical mechanisms.

Shu, Frank H.↗

NASA NTRS1986

New synchrotron luminosity distance limit for the 1979 March 5 gamma-ray event

New synchrotron luminosity estimates are provided for the March 5, 1979 gamma-ray event, based on the lack of low-frequency self-absorption cutoff. This leads to distance limits much lower than 55 kpc for a neutron-star emission area even in the most conservative case. The source is likely Galactic. If in addition the thermal self-Compton interpretation is invoked for the hard tail above 300 keV in the burst spectrum, then the source distance lies in the range 0.1-2 kpc for extreme ranges of the neutron-star emission area. This result strongly impacts current observational programs for this source.

Liang, E. P.↗

NASA NTRS1986

Bolometric luminosity evolution

The observations made by IRAS are capable of detecting evolution in the OB star formation rates of galaxies at cosmologically interesting redshifts if that evolution takes the form of star formation rates ten or more times higher than seen in nearby galaxies. Spectroscopic follow-up observations of IRAS-selected galaxies, and IRAS source counts, indicate that there may indeed exist a rare population of galaxies with such high star formation rates. The most luminous found so far approach quasars in luminosity.

Lonsdale, Carol J.↗

NASA NTRS1987

Redshifts of low-X-ray luminosity clusters of galaxies

Measurements of redshifts and velocity dispersions are presented for Abell clusters A539, A1185, and A1228, and the southern clusters S1840-623, S1904-618, S1908-566, and S2000-561. All these clusters have reported X-ray luminosities or upper limits. Finding charts for the clusters are presented, and the measured heliocentric redshifts are given along with redshifts obtained by other investigators. Comments are made about each cluster. The technique used to derive the redshifts is summarized.

Kowalski, M. P.↗

NASA NTRS1987

The low-luminosity X-ray sources in Omega Centauri

UBV observations of three X-ray sources in Omega Cen (sources A, D, and E of Hertz and Grindlay, 1983), obtained with a CCD detector on the 4-m telescope at CTIO in February and August 1985, are reported. The results are presented in images, graphs, and tables and characterized in detail. No optical cataclysmic variable candidates are found down to V = 24.5 mag, corresponding to M(V) = 10.6 in the cluster. It is inferred that these low-luminosity sources, located outside the globular cluster, may be superposed unrelated objects rather than close mass-exchange binaries.

Margon, Bruce↗

NASA NTRS1987

Observations of jets from low-luminosity stars. II - SVS 12, HH 30, near HL Tauri, and HH 34

Spatially resolved spectral scans were obtained for four Herbig-Haro jets emanating from low-luminosity premain-sequence stars. There appears to be a general tendency for the excitation and electron density to diminish along these jets. For three jets, the electron density dependence is close to 1/r. HH 30's scattered stellar continuum showed much weaker Fe II lines in 1985 than in 1979, indicative of variable stellar activity. The most distant knot in HH 34's jet has the lowest excitation of any known HH object, with S II-line/H-alpha = 12.

Cohen, Martin↗

NASA NTRS1988

Aperture synthesis mapping of molecular gas in high-luminosity IRAS galaxies

The Owens Valley millimeter-wave interferometer has been used for high-resolution mapping of the 2.6 mm CO emission from the high-luminosity infrared galaxies NGC 520 (Arp 157), NGC 7469 (Arp 298), and Arp 55. Assuming the same empirical relationship between CO brightness and molecular hydrogen surface mass density as has been found for giant molecular clouds in the Milky Way, it is found that the masses of H2 gas in these concentrations are 10 to the 9th - 10 to the 10th solar masses, typically one-third of the total molecular gas content of these galaxies. The interferometric sizes correspond to radii of 0.8 kpc (NGC 520), 1.4 kpc (NGC 7469), and less than 2.8 kpc (Arp 55). For the same regions the dynamical masses estimated from the CO line width and size of the emission region are only a factor of 3-5 higher. The mean molecular gas surface densities averaged over these regions are in the range 610-825 solar masses/sq pc, a factor of 10 brighter than those obtained for corresponding regions in the nucleus of the Milky Way. The high mass fractions obtained for the interstellar medium in the central regions of these three galaxies strongly suggest that large-scale gravitational instability in the gas may play an important role in the further concentration of the gas in the nucleus and in possibly precipitating a large-scale burst of star formation.

Sanders, D. B.↗

NASA NTRS1988

Low luminosity radio galaxies - Effects of gaseous environment

X-ray data from the Einstein Observatory have been collected for a number of low luminosity B2 radio galaxies, in order to study the effect of gas pressure on radio sources. Generally, the thermal pressure of the X-ray emitting gas is sufficient to confine most of the radio components. Various possibilities are given for sources where thermal pressure significantly exceeds the nonthermal pressure. For radio lobes, an inverse correlation is found between the central densities of the cluster gas and the size of the radio structures. For jets, a study of the pressure ratios is given for different distances from the radio core. In a few cases, pressure imbalance is found in the outer regions of the jets. However, from depolarization data, this is unlikely to be produced by a large thermal component inside the radio jets.

Morganti, R.↗

NASA NTRS1988

On the redistribution of OB star luminosity and the warming of nearby molecular clouds

IRAS observations of the neighborhoods of six H II regions in the outer Galaxy are correlated with CO maps of the same regions. It is found that more than half of the far-IR luminosity from within about 25 to 75 pc of the ionizing stars is contributed by dust associated with molecular clouds. It is shown that a major and increasing fraction of the OB cluster starlight is not absorbed locally. The observations suggest that the H II regions are not radiation-bounded and have ion densities of about 1/cu cm. A model describing the interstellar environment of OB star clusters as a function of time is presented.

Leisawitz, D.↗

NASA NTRS1988

Observed variability of the solar luminosity

Data on solar luminosity variations are compiled in graphs and analyzed, with a focus on the SMM satellite bolometric measurements and other observations made since 1983. Consideration is given to total irradiance measurements in integrated light; emission-line variability in the visible, UV, and IR; mechanisms contributing to the solar variability (sunspots, faculae, rotation, oscillations, granulation, and active networks); the role of the solar magnetic cycle; and other observable parameters indicating variability. Also discussed are the effects of solar variations on the earth climate and the implications of the observations for theoretical models of convective-envelope processes (thermal diffusion near the surface and magnetic pressure in the interior).

Hudson, H. S.↗

NASA NTRS1987

Synthetic horizontal branch models for globular clusters - The luminosity of the horizontal branch and the Oosterhoff effect

The variation of horizontal-branch (HB) luminosities with metal abundances is analyzed on the basis of HB models synthesized from theoretical HB evolutionary tracks. The focus is on the Oosterhoff effect, as related to period shifts in globular-cluster RR Lyr variables. The construction of the models and the Oosterhoff period groups is explained in detail, and the implications for globular-cluster ages are considered. The ratio of Delta M(bol) (RR) to Delta Fe/H for the HB is calculated as 0.24, slightly steeper than that found by Sandage (1981 and 1982).

Lee, Young-Wook↗

NASA NTRS1989

The far-infrared luminosity of molecular clouds in the Galaxy

A detailed analysis is presented of the IR emission and CO properties of a large sample of giant molecular clouds (GMCs) with and without H II regions. It is found that high-mass star-forming clouds have significantly higher peak dust temperatures, but averaged over the entire cloud volume, their IR color temperatures are not very different from those of the general GMC population and the mean disk temperature. The results indicate that the bulk of the IR emission in GMCs is excited by embedded stars within the clouds rather than the exterior interstellar radiation field, that only a small fraction of the IR luminosity originates from stars which ionize the H II region. An overall higher level of star-formation activity for the H II region GMCs than the general population is indicated.

Scoville, N. Z.↗

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

The winds of high luminosity late-type bright stars

The occurrence and characteristics of the Fe II line asymmetries were studied to determine the radial dependence of the wind velocity for each star. The dependence of the Fe II profiles on spectral type and luminosity class and thus the variation of the velocity fields with stellar type was also investigated. This allows the generality of the results reported for alpha Ori by Carpenter (1984b) to be judged. In addition, new atomic data was used along with observations of the C II (UV 0.01) multiplet to estimate N(sub e) in the stellar winds. Measures of relative Fe II fluxes can be used in a probability-of-escape model to determine the opacity and hydrogen column density versus height in the chromosphere of each star. Finally, analysis of the fluorescent Fe II lines (pumped by Ly alpha) near 2507 A will yield estimates of the intrinsic stellar Ly alpha flux that cannot be measured directly because of interstellar and circumstellar absorption. One important goal of the effort was to acquire high resolution spectra of the whole 2300 to 3200 A region of 13 luminous K and M stars as a data base that will be enormously valuable in planning observations with the Hubble Space Telescope High Resolution Spectrograph. It is also proposed to follow up the recent discovery of significant variations in the Fe II chromospheric emission line profiles from the M-giant Gamma Cru for the purpose of determining the underlying cause of the variations.

Stencel, Robert E.↗

NASA NTRS1990

The deep luminosity function of the globular cluster M30

An effort is made to determine the mass function in low-metallicity, postcore-collapse globular cluster M30, as well as to analyze its radial trend, using new deep CCD photometry. It is confirmed that oxygen-enhanced isochrones yield a good representation of the color-magnitude diagrams. Luminosity functions are constructed and corrected for for field-object contamination and incompleteness. The data presented agree with the power-law trend ascertained by Pryor et al. (1986) for a multimass King-Michie model, yielding a global slope of 0.7.

Piotto, Giampaolo↗

NASA NTRS1990

Luminosity-velocity diagrams for Virgo Cluster spirals. I - Inner rotation curves

Optical rotation curves are presented for the innermost portions of nine spiral galaxies in the Virgo Cluster. The emission-line (H-alpha and forbidden N II) velocity data are to be used in combination with new CCD photometry to construct luminosity-velocity diagrams, in a continuing investigation of an apparent initial linear branch and its potential as a distance indicator. Compared to recent H I data, the present optical rotation curves generally show systematically steeper inner gradients. This effect is ascribed to the poorer resolution of the H I data and/or to holes in the gas distribution.

Woods, David↗