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At least 1,261 records · Page 70
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NASA NTRS1989

The influence of grain growth in circumstellar dust envelopes on observed colors and polarization of some eruptive stars

R CrB stars are classical examples of stars where dust envelope formation takes place. Dust envelope formation was detected around the Kuwano-Honda object (PU Vul) in 1980 to 1981 when the star's brightness fell to 8(sup m). Such envelopes are also formed at nova outbursts. The process of dust envelope formation leads to appreciable variations in optical characteristics, which are seen in specific color and polarization variations in the course of light fading and the appearance of IR radiation. It is shown that the model of a circumstellar dust envelope with aligned particles of changing size can be successfully applied to explain most phenomena observed at the time of light minima for a number of eruptive stars. The polarization may arise in a nonspherical dust envelope or be produced by alignment of nonspherical particles.

Efimov, Yu. S.↗
NASA NTRS1989

Circumstellar grain extinction properties of recently discovered post AGB stars

The circumstellar grains of two hot evolved post asymptotic giant branch (post AGB) stars, HD 89353 and HD 213985 were examined. From ultraviolet spectra, energy balance of the flux, and Kurucz models, the extinction around 2175 A was derived. With visual spectra, an attempt was made to detect 6614 A diffuse band absorption arising from the circumstellar grains so that we could examine the relationship of these features to the infrared features. For both stars, we did not detect any diffuse band absorption at 6614 A, implying the carrier of this diffuse band is not the carrier of the unidentified infrared features not of the 2175 A bump. The linear ultraviolet extinction of the carbon-rich star HD 89353 was determined to continue across the 2175 A region with no sign of the bump; for HD 213985 it was found to be the reverse: a strong, wide bump in the mid-ultraviolet. The 213985 bump was found to be positioned at 2340 A, longward of its usual position in the interstellar medium. Since HD 213985 was determined to have excess carbon, the bump probably arises from a carbonaceous grain. Thus, in view of the ultraviolet and infrared properties of the two post AGB stars, ubiquitous interstellar infrared emission features do not seem to be associated with the 2175 A bump. Instead, the infrared features seem related to the linear ultraviolet extinction component: hydrocarbon grains of radius less than 300 A are present with the linear HD 89353 extinction; amorphous anhydrous carbonaceous grains of radius less than 50 A might cause the shifted ultraviolet extinction bump of HD 213985.

Buss, Richard H., Jr.↗
NASA NTRS1990

Submillimeter astronomy and low mass star formation

Surveys made at wavelengths shorter than 100 microns are criticized in that they are biased against finding the coldest, most deeply obscured, and possibly youngest protostars. Large scale mapping of star forming regions in the submillimeter continuum and in lines such as CS H2CO and NH3, that are excited at high density, is identified as crucial in order to find clouds in the earliest stages of collapse into stars. The use of photometry from 1 to 1000 microns in characterizing the disks around embedded stars and T Tauri stars is outlined.

Beichman, Charles A.↗
NASA NTRS1991

Dibaryons in neutron stars

The effects are studied of H-dibaryons on the structure of neutron stars. It was found that H particles could be present in neutron stars for a wide range of dibaryon masses. The appearance of dibaryons softens the equations of state, lowers the maximum neutron star mass, and affects the transport properties of dense matter. The parameter space is constrained for dibaryons by requiring that a 1.44 solar mass neutron star be gravitationally stable.

Olinto, Angela V.↗
NASA NTRS1989

Smithsonian Astrophysical Observatory (SAO) star catalog (Sao staff 1966, edition ADC 1989): Documentation for the machine-readable version

An updated, corrected, and extended machine readable version of the catalog is described. Published and unpublished errors discovered in the previous version were corrected, and multiple star and supplemental BD identifications were added to stars where more than one SAO entry has the same Durchmusterung number. Henry Draper Extension (HDE) numbers were added for stars found in both volumes of the extension. Data for duplicate SAO entries (those referring to the same star) were flagged. J2000 positions in usual units and in radians were added.

Roman, Nancy G.↗
NASA NTRS1990

On the Sandage period shift effect among field RR Lyrae stars

The origin of the disagreement between the results of the synthetic horizontal branch (HB) model of Lee et al. (1990) and the Sandage (1990) analysis of Lub's (1977) data on a sample of field RR Lyrae stars is investigated, extending the original synthetic HB calculations of Lee et al. to all HB types over the metallicity range of the Galactic halo. It is shown that the period shifts of the RR Lyrae stars in globular clusters depend strongly on their HB type and metallicity. Results suggest that the disagreement between the slopes, Delta logP(Teff)/Delta Fe/H, obtained by Lee et al. and the ones obtained by Sandage can be explained by the fact that Lub's sample may have contained many highly evolved stars in the metallicity range of Fe/H between -2.0 and -1.6, since Lee's analysis did not include these stars.

Lee, Young-Wook↗
NASA NTRS1990

Spectral synthesis in the ultraviolet. III - The spectral morphology of normal stars in the mid-ultraviolet

The morphology of 218 mid-UV spectra of stars ranging from O through K in spectral type is examined. Several new line and continuum indices are defined and their usefulness as temperature, luminosity, and metallicity discriminants is discussed. Mid-UV stellar continua are found to be markedly affected by abundance. A UV excess, delta(2600-V), is computed which is more sensitive by a factor of 10 to (Fe/H) than is delta(U-B). The relative strength of spectral lines in the mid-UV is not as strongly affected by abundance. Mid-UV spectra are much more sensitive to the temperature of the stellar population than to either metallicity or the dwarf/giant ratio. Mg II 2800 shows unexpected behavior, displaying no sensitivity to abundance for cool stars and a reversed sensitivity in FG dwarfs such that metal-poor stars have stronger Mg II strengths at the same temperature than more metal-rich stars.

Fanelli, Michael N.↗
NASA NTRS1990

Magnetically controlled circumstellar matter in the helium-strong stars

This paper reports the observation and interpretation of the ultraviolet spectrum variations of nine helium-strong stars: HD 36485, 37017, 37479, 37776, 58260, 60344, 64740, 96446, and 133518. A unified model is developed to account for the observed correlation among three stellar properties: the line profile characteristics of the C IV and Si IV resonance doublets, the variations in the strength of these lines, and the inferred magnetic field geometry. It is proposed that circumstellar plasma is trapped in the stellar magnetosphere near the magnetic equator or is channeled to form jetlike outflows from the magnetic polar regions. These results, together with those of a previous study of the helium-weak sn stars, show that both helium-weak and helium-strong stars can possess magnetospherically trapped plasma, notwithstanding their different photospheric properties. New results for radii and temperatures of the helium-strong stars in Ori OB 1 and for HD 64740 from low-dispersion IUE spectra are also presented.

Shore, Steven N.↗
NASA NTRS1990

A catalogue of QSOs and link stars for the HST link of the Hipparcos reference frame

A list of bright (less than 17 mag) southern QSOs and bright (less than 11 mag) stars that may be suitable for the Hubble Space Telescope link between the Hipparcos astrometric reference frame and the VLBI extragalactic frame is presented. The QSOs have been selected from various lists of radio objects and identifications. The stars have been selected from the Strasbourg (CDS) data base and from the Preliminary Second Cape Photographic Catalog, and supplemented with stars measured from the SERC IIIa-J Schmidt survey. These QSOs and stars have been included in the Hipparcos and HST schedule of observations.

White, Graeme L.↗
NASA NTRS1991

Nonthermal radio emission from the X-ray-bright K5 IV star 1E 1751 + 7046

VLA observations are analyzed of the K5 IV star 1E 1751 + 7046, which is known to be an X-ray source on the basis of Einstein observations and which shows some similarity to stars of the FK Comae type. Radio continuum emission has been detected at 3.6, 6, and 20 cm, and the spectral energy distribution is nearly flat. There is no evidence for circular polarization, and upper limits are given. The 6-cm flux is roughly one order of magnitude less than reported in a previous VLA observation, indicating that this star may flare at radio wavelengths. The inferred radio variability and the nearly flat spectral energy distribution suggest that the emission is of nonthermal origin. A variety of possible emission mechanisms is examined, and it is concluded that the emission is best explained in terms of a gyrosynchrotron/synchrotron model with the electrons following a power-law energy distribution. A brief comparison of the radio properties of 1E 1751 + 7046 with FK Comae-like stars and RS CVn systems is given.

Skinner, Stephen L.↗
NASA NTRS1991

A survey for infrared excesses among high galactic latitude SAO stars

This project involves extending the previous analysis of infrared excesses among a volume-limited sample of 134 nearby A-K main-sequence stars to a magnitude-limited sample of stars, culled from the SAO Catalog, with excesses determined from the IRAS Point Source Catalog flux density ratios. This new sample includes 5706 B-M type stars, 379 of which have infrared excesses. The objective involved use of a statistically complete survey of objects in a standard catalog in order to assess the frequency with which different physical processes can affect the infrared output of stars. These processes include, but are not limited to, orbiting cold particle clouds and the onset of rapid mass loss. It is concluded that cold disks are consistent with the infrared excesses found among A-G dwarfs and G-K giants in the sample.

Stencel, Robert E.↗
NASA NTRS1991

Lithium in the most extreme halo stars - Trends with metallicity

A grid of extremely metal-poor stellar evolutionary tracks (Z = 0.00001) is constructed to compare the predicted lithium abundance depletion to observations of the lithium abundance in correspondingly metal-poor stars (Fe/H = -3.3), and, combined with an earlier work to investigate trends of lithium isochrones with metal abundance. Lithium abundances in the most extreme halo stars hint at the possibility of small downward slope toward lower T(eff) in the flat Li-T(eff) relation (the Spite lithium plateau) observed in slightly less extreme stars; the models also predict this. In the cool star region of the Li-T(eff) plane, higher metallicity lithium isochromes are cooler at fixed lithium abundance, consistent with the observations. Current data thus agree well with the lithium trends from standard model.

Deliyannis, Constantine P.↗
NASA NTRS1991

High-mass star-formation rates in M33

The H I, CO, and H-alpha data for M33 are analyzed to obtain high-mass star formation rates and efficiencies and to look for variations in these quantities within the inner disk of M33. Star formation rates and efficiencies are calculated using calibrated H-alpha data. The H-alpha emission in this region of the galaxy corresponds to high-mass and total star-formation rates of 0.007 and 0.04 solar mass/yr, respectively. These results agree reasonably well with the rates calculated from the 60 and 100-micron IRAS emission. The star formation rates obtained form FIR and H-alpha luminosities are compared with those obtained from optical photometry.

Wilson, Christine D.↗
NASA NTRS1991

Nitrogen abundances in carbon-rich stars that are progenitors to planetary nebulae

Published data for carbon stars losing more than 10 to the -6th solar mass/yr in the neighborhood of the sun are examined. The stars with higher terminal velocities have significantly higher fluxes of the J = 1-0 emission line of HCN relative to their infrared continuum emission at 60 microns. It is argued that this result is probably a reflection of a higher nitrogen abundance by perhaps a factor of 10 in these outflows. Such a high nitrogen abundance is consistent with the hypothesis that the high terminal velocity carbon stars have more massive main-sequence progenitors than average; these high terminal velocity carbon stars may, themselves, be the progenitors of type I planetary nebulae which display enhanced nitrogen abundances.

Jura, M.↗
NASA NTRS1991

DD 13 - A very young and heavily reddened early O star in the Large Magellanic Cloud

This paper investigates the Large Magellanic Cloud star DD 13, which is likely the major ionizing source of the nebula N159A. New optical spectroscopy and new estimates of the broadband photometric properties of DD 13 are obtained. A spectral type of O3-O6 V, E(B-V) = 0.64, and M(V) = -6.93 is found. The spectral type cannot be more precisely defined due to contamination of the spectral data by nebular emission, obliterating the important He I classification lines. These results, plus a published estimate of the Lyman continuum photon injection rate into N159A, suggest that DD 13 actually consists of about 2-4 young, early O stars still enshrouded by their natal dust cloud. The star DD 13 may be a younger example of the type of tight cluster represented by the LMC 'star' Sk-66 deg 41, recently revealed to be composed of six or more components.

Conti, Peter S.↗
NASA NTRS1991

Optical and ultraviolet spectroscopy of three F + B binary stars

Optical and ultraviolet spectroscopy is presented for three F + B objects that are members of the first group of strongly interacting, F II + B systems. The data obtained confirm that HD 59771, HD 242257, and CoD -30 5135 are all binary star systems consisting of a luminous F-type component and a B star. Strong, variable H-alpha emission is seen in all the stars. It is found that the UV spectrum of HD 59771 resembles the spectrum of HD 207739. CoD -30 5135 has the most dramatic mid-UV spectrum seen among the scores of observed cool + hot star systems.

Bopp, Bernard W.↗
NASA NTRS1991

Rotation and emission lines in stars and accretion disks

In the accretion disks of quiescent dwarf novae, Doppler mapping studies reveal that Balmer emission lines increase sharply toward the center of the disk, with surface brightnesses scaling roughly as R exp -3/2 varies as Omega(Kep). Similarly, among chromospherically active stars the H-alpha and Ca II H and K emission cores are stronger in the more rapidly rotating stars, with surface brightnesses scaling again roughly as Omega(rot). Since in both cases the emission lines scale linearly with the rotation frequency, it is proposed that the mechanism powering the emission lines in quiescent accretion disks is the same as that in chromospherically active stars, namely, the emergence of magnetic flux generated by the action of a dynamo, and its interaction with the atmosphere. If this empirical connection between disks and stars is in fact due to magnetic dynamos, the range of rotation rates available for testing dynamo theories expands from a factor of 1000 to 10 to the 7th.

Horne, Keith↗
NASA NTRS1990

Toward mapping the UV circumstellar shells of late-type stars

Detection of spatially resolved circumstellar matter surrounding selected cool stars, using off-source observations made with the International Ultraviolet Explorer satellite, is reported. The data demonstrate that: (1) the instrumental scattered light profile of IUE appears to depend on the ultraviolet color temperature of the star observed, and (2) certain red stars show signal in excess of the instrumental levels, at spatial offsets of 10 to 20 arc-seconds from the star.

Stencel, Robert E.↗