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At least 1,279 records · Page 71

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NASA NTRS1990

Gamma-ray bursts from extinct neutron stars

The paper concentrates on disks around old extinct pulsars, that can produce gamma-ray bursts owing to viscous evolution of the disk bringing it into the near vicinity of the neutron star, with runaway ionization of the disk and simultaneous precipitation of this plasma onto the neutron star. An old extinct pulsar is modeled as a magnetized neutron star circled by a ring of cold dense matter with an orbital period approximately equal to the rotational period of the neutron star. The numerical estimates produced are found to be consistent with the observed properties of gamma-ray bursters.

Michel, F. C.↗

NASA NTRS1991

The rate of neutron star binary mergers in the universe - Minimal predictions for gravity wave detectors

Of the many sources which gravitational wave observatories might see, merging neutron star binaries are the most predictable. Their waveforms at the observable frequencies are easy to calculate. And three systems which will merge in less than a Hubble time have already been observed as binary pulsars: two in the disk of the Galaxy, and one in a globular cluster. From the lifetimes and positions of these, a lower limit to the merger rate in the Galaxy and globular cluster system are inferred with confidence. Taking the merger rate in other galaxies to scale with the star formation rate, the merger rate expected in the local universe is computed. An ultraconservative lower limit to the rate gives three per year within 1 Gpc. The best estimate, still conservative in that it considers only systems like those already observed, gives three per year within 200 Mpc. An upper limit of three mergers per year within 23/h Mpc is set by the rate of Type Ib supernovae. The rates of black hole binary mergers and black hole-neutron star binary mergers are model-dependent, but could be comparable to the given rate of neutron-star binary mergers.

Phinney, E. S.↗

NASA NTRS1990

Profuse central star formation in 'SO' galaxies

Radio continuum flux densities, far infrared flux densities, far infrared colors, and ultraviolet and optical spectra indicate that high rates of star formation are occurring in some SO galaxies, especially barred galaxies. Radio continuum and infrared images show that the star forming region is typically concentrated within the central few kpc of the galaxy. The high incidence of central star formation in barred galaxies suggests that bar-driven inflow of gas is fueling the star-forming regions in these galaxies.

Dressel, L. L.↗

NASA NTRS1992

Dust near luminous ultraviolet stars

More than 700 luminous stars in the infrared astronomical satellite (IRAS) Skyflux plates were examined for the presence of dust heated by a nearby star. This dust may be distinguished from the ubiquitous cool cirrus by its higher temperature and thus enhanced 60 micron emission. More than 120 dust clouds were found around only 106 of the stars with a volume filling factor of 0.006 and an intercloud separation of 46 pc. A region of dust smoothly distributed through the volume of space heated by the star could not be found and hence an upper limit of 0.05 cm(exp -3) is placed on the equivalent gas density in the intercloud regions. The clouds have an average density of 0.22 cm(exp -3) and a radius of 1.9 pc, albeit with wide variations in their properties. Two different scale heights of 140 and 540 pc were found. This was interpreted as evidence for different distributions of dust in and out of the galactic disk.

Henry, Richard C.↗

NASA NTRS1991

Recent star formation in the interacting galaxy system NGC 520

New optical spectroscopy, near-infrared spectroscopy, and mid-infrared imaging of the interacting system NGC 520 are presented. The data show that the heavily obscured primary nucleus has a current star-forming rate about 35 times the typical rate for a late-type galaxy nucleus, which may account for most of the large far-infrared luminosity of NGC 520. The secondary nucleus has little current star formation but has recently undergone a mild starburst. Optical colors and absorption line equivalent widths suggest that the extranuclear regions in both galaxies have experienced enhanced star formation roughly at the same time as the secondary nucleus, though at a lesser intensity. The extent of the extranuclear burst is large; at least half of the projected surface area of the two disks took part to varying degrees in the episode. Determining the exact age and strength of the recent star-formation episode in NGC 520 is uncertain. The best estimate for the initiation time of about few x 10 exp 8 yrs ago is consistent with the system-wide starburst being caused by the galaxy collision.

Stanford, S. A.↗

NASA NTRS1991

Photometry of astrometric reference stars

UBVRI, DDO, and uvby, H-beta photometry of astrometric reference stars is presented. Spectral types and luminosity classifications made from the colors are used to determine their spectroscopic parallaxes. In this paper, colors for 309 stars in 25 regions are given, and classifications for 210 stars have been made. These stars form reference frames in the Allegheny Observatory Multichannel Astrometric Photometer astrometric program, and in the Praesepe cluster reduced by Russell (1976). It is found that the present photometric spectral types are reliable to within 2.5 spectral subclasses.

Castelaz, Michael W.↗

NASA NTRS1991

A model for the infrared emission from an OB star cluster environment

A model for the infrared emission from the neighborhood of an OB star cluster is described. The distribution of gas and dust around the stars, properties of the dust, and the cluster and interstellar radiation fields are variable. The model can be applied to regions around clusters embedded to various degrees in their parental molecular clouds (i.e., compact H II regions, blister-type H II regions, and the tenuous H II regions ionized by naked O stars). The model is used to simulate IRAS observations of a typical blister H II region. Infrared surface brightness and spectral energy distributions are predicted and the impact of limited spatial resolution is illustrated. The model results are shown to be consistent with observations of the exemplary outer Galaxy OB cluster NGC 7380. It is planned to use the model as a diagnostic tool to probe the physical conditions and dust properties in star-formation regions and, ultimately, in an interpretation of the spectral energy distributions of spiral galaxies.

Leisawitz, D.↗

NASA NTRS1991

New candidates for carbon stars with silicate features

All stars in the General Catalog of Cool Galactic Carbon Stars with IRAS 12-micron fluxes greater than 10 Jy were searched for Low-Resolution-Spectrometer (LRS) spectra in the IRAS LRS data base. Out of the 532 spectra examined, 11 were found to show the 9.7-micron silicate emission feature. Four of these are identified for the first time. This group of carbon stars may represent transition objects between oxygen-rich and carbon-rich stars on the asymptotic giant branch.

Chan, S. J.↗

NASA NTRS1992

Non-LTE, line-blanketed model atmospheres for late O- and early B-type stars

The use of non-LTE line-blanketed model atmospheres to analyze the spectra of hot stars is reported. The stars analyzed are members of clusters and associations, have spectral types in the range O9-B2 and luminosity classes in the range III-IV, have slow to moderate rotation, and are photometrically constant. Sampled line opacities of iron-group elements were incorporated in the radiative transfer solution; solar abundances were assumed. Good to excellent agreement is obtained between the computed profiles and essentially all the line profiles used to fix the model, and reliable stellar parameters are derived. The synthetic M II 5581 equivalent widths agree well with the observed ones at the low end of the temperature range studied, but, above 25,000 K, the synthetic line is generally stronger than the observed line. The behavior of the observed equivalent widths of N II, N III, C II and C III lines as a function of Teff is studied. Most of the lines show much scatter, with no consistent trend that could indicate abundance differences from star to star.

Grigsby, James A.↗

NASA NTRS1992

Stellar occultation candidates from the guide star catalog. I - Saturn, 1991-1999

A list of 203 potential occultations by Saturn and its ring of stars from the HST Guide Star Catalog (GSC) during the years 1991-1999 is presented. This list features many fainter candidates than do current occultation candidate lists for Saturn; these fainter stars can also provide a high signal-to-noise ratio if observed with a large telescope or in the IR where Saturn and its rings have absorption bands. The occultation circumstances are listed, as well as star information found in the GSC.

Bosh, A. S.↗

NASA NTRS1992

Tidal capture of stars by a massive black hole

The processes leading to tidal capture of stars by a massive black hole and the consequences of these processes in a dense stellar cluster are discussed in detail. When the amplitude of a tide and the subsequent oscillations are sufficiently large, the energy deposited in a star after periastron passage and formation of a bound orbit cannot be estimated directly using the linear theory of oscillations of a spherical star, but rather numerical estimates must be used. The evolution of a star after tidal capture is discussed. The maximum ratio R of the cross-section for tidal capture to that for tidal disruption is about 3 for real systems. For the case of a stellar system with an empty capture loss cone, even in the case when the impact parameter for tidal capture only slightly exceeds the impact parameter for direct tidal disruption, tidal capture would be much more important than tidal disruption.

Novikov, I. D.↗

NASA NTRS1992

Rosat X-ray All-Sky Survey observations of hybrid stars

Data from the Rosat All-Sky Survey for nine hybrid stars, objects showing spectroscopic evidence for cool massive winds and 500,000-K material, are presented. Two of the nine stars were detected above a limiting flux threshold of 2 x 10 exp -13 ergs/sq cm s. The K3 III star Delta And was detected just at this threshold. The K4 III star Alpha TrA was measured at 8 x 10 exp -13 ergs/sq cm s. Since these detections were made in both low- and high-energy bands of the Rosat 0.1-2.4-keV passband, it is suggested that the emissions originate in coronae of about 10 exp 7 K.

Haisch, Bernhard↗

NASA NTRS1992

Evolved stars in Omega Centauri. I - Radial distribution of blue subdwarfs

A U - V color-magnitude diagram containing over 5000 stars from 10 fields in Omega Centauri at or brighter than the main-sequence turnoff is presented. Completeness corrections are obtained as a continuous function of magnitude and radial distance are obtained and used to show that the blue subdwarfs are centrally concentrated with respect to subgiants and stars on the horizontal branch proper. The chance probability of this result is less than 1 percent. Since the blue subdwarfs probably consist of helium-burning cores of about 0.5 solar mass surrounded by a thin hydrogen envelope, mass segregation could not produce this result if these stars had evolved singly. Two kinds of possible precursor systems are considered: pairs of degenerate dwarfs which subsequently merge to form helium-burning stars, and moderately wide binaries in which mass transfer is initiated shortly before the helium flash is ignited.

Bailyn, Charles D.↗

NASA NTRS1992

Candidates for extreme carbon stars

A group of candidates for extreme carbon stars has been identified based on their IRAS low-resolution spectra being similar to those of AFGL 3068 and IRC +10 deg 216, which are prototypes of highly evolved carbon stars with very high mass-loss rates. Ground-based photometry has been obtained for five of these sources in order to determine the peaks of their energy distributions. All five sources have color temperatures between 300 and 500 K, and have energy distributions very similar to AFGL 3068. The observed data are fitted with radiative transfer models with optical depths at 11.3 microns ranging from 1.4 to 8. These results suggest that these stars have among the most optically thick circumstellar envelopes of all carbon stars observed to date.

Volk, Kevin↗

NASA NTRS1992

Magnitude calibration of a fixed head star tracker using Astro-1 flight data

The Astro-1 UV astronomy mission was hampered by the failures of the automatic star acquisition procedure. The acquisition procedure depended on the Instrument Pointing Subsystem's Fixed Head Star Trackers (FHST) to acquire, track and identify guidestars of known visual magnitude. During the Astro-1 mission it was suspected that the star magnitudes measured by the FHST were much lower than predicted. A postflight investigation of the Astro-1 flight data confirmed and quantified this suspicion. Star magnitude calibration curves computed from the flight data depict the variance from the preflight calibration curves. These results are helping engineers to plan improvements to the acquisition procedure for the upcoming Astro-2 mission.

Rakoczy, John M.↗

NASA NTRS1992

The reddening to the galaxy IC 342 from CCD BV photometry of its brightest stars

An attempt was made to estimate the reddening for the galaxy IC 342 on the basis of CCD BV photometry obtained for 117 stars brighter than V = 22.0 mag in a field centered near a spiral arm. At V = 19.4 mag, the plume of the main sequence and slightly evolved blue supergiant stars were detected. The plume is centered at a color of (B-V) = 0.54 mag. A comparison with similar color-magnitude-diagram data for the brightest stars in IC 1613 suggests that these stars in IC 342 are reddened by E(B-V) = 0.79 +/- 0.05 mag (A sub B = 3.32 +/- 0.21 mag).

Madore, Barry F.↗

NASA NTRS1992

Active galactic nuclei. IV - Supplying black hole clusters by tidal disruption and by tidal capture of stars

The extent to which individual holes in a cluster of black holes with a mass spectrum can liberate and accrete the resulting material by tidally disrupting stars they encounter, or by capturing stars as binary companions is studied. It is found that the smaller black holes in 'the halo' of such clusters can adequately supply themselves to the level M-dot sub h or greater than 0.0001(M-dot sub h) sub crit, and up to 0.05(M-dot sub h)sub crit for the smallest holes, by tidal disruption, as long as the cluster is embedded in a distribution of stars of relatively high density (not less than 0.1M sub cl/cu pc), and as long as the entire cluster of stars is not too compact (not less than 0.5 pc). Consideration is given to modifications this 'internal' mode of supply introduces in the spectrum emitted by such black hole clusters, and to the current status of their viability as models for AGN and QSOs in light of dynamical studies by Quinlan and Shapiro (1987, 1989).

Stoeger, W. R.↗

NASA NTRS1992

Line profile asymmetries in chromospherically active stars

A powerful, new probe of chromospheric activity, cross-correlation, has been developed and applied to a variety of stars. In this particular application, an entire CCD spectrum of an active star is correlated with the spectrum of a narrow-line, inactive star of similar spectral type and luminosity class. Using a number of strong lines in this manner enables the detection of absorption profile asymmetries at moderate resolution (lambda/Delta lambda about 40,000) and S/N 150:1. This technique has been applied to 14 systems mostly RS CVn's, with 10 not greater than nu sin i not greater than 50 km/s and P not less than 7 d. Distortions were detected for the first time in five systems: Sigma Gem, IM Peg, GX Lib, UV Crb, and Zeta And. Detailed modeling, incorporating both spectral line profiles and broad-band photometry, is applied to Sigma Gem. Profile asymmetries for this star are fitted by two high-latitude spots covering 5 percent of the stellar surface. The derived spot temperature of 3400 K is lower than found in previous studies. In addition, two well-known systems have been studied: HD 199178 and V711 Tau. Polar spots are found on both.

Dempsey, Robert C.↗