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At least 613 records · Page 34
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NASA NTRS1987

An X-ray survey for pre-main-sequence stars in the Taurus-Auriga and Perseus molecular cloud complexes

Seventy-five fields from the Einstein X-Ray Observatory IPC detector between 3h and 5h right ascension and 15 deg and 35 deg declination are examined to search for X-ray-emitting low-mass pre-main-sequence (PMS) stars. Six such stars were previously found; they appear to be similar to T Tauri stars but without dense circumstellar envelopes or winds. Finding charts are presented for 59 X-ray sources that may be PMS stars. While some are likely to be spurious X-ray sources, chance coincidences with unrelated stars, or non-PMS stellar sources such as RS CVn type binaries, approximately half are probably X-ray-selected PMS stars.

Feigelson, E. D.↗
NASA NTRS1987

Pre-main sequence stars

Observations of premain-sequence (PMS) stars obtained with the IUE satellite since its launch in 1978 are reviewed. The major types of PMS stars are listed and briefly characterized: T Tau stars, Herbig Ae stars, FU Ori objects, weak-emission PMS stars, and Herbig-Haro objects. The IUE data for a catalog of PMS objects are then presented in a table, graphs, and sample spectra and described in detail. Topics addressed include general spectral profiles, the Mg II h and k lines, the FUV emission lines, the UV continuum, and the impact of the IUE on astrophysical models of PMS stars. The need for observations with higher spectral resolution and much greater sensitivity is indicated, and the ability of the Hubble Space Telescope High-Resolution Spectrograph and the proposed NASA/ESA Lyman FUV Spectroscopic Explorer to meet these requirements is considered.

Imhoff, Catherine L.↗
NASA NTRS1987

The relative amounts of stars and interstellar matter in the local Milky Way

This paper considers the balance between star formation and mass loss from evolved stars in the region within 1 kpc of the sun. There is considerably more mass in stars than in the interstellar medium, and more material is being incorporated into new stars than is being returned by evolved stars. In the simplest interpretation of the data, it appears that unless there is some infall of new interstellar gas, the era of substantial star formation out of interstellar gas will be over in a few (perhaps 3) billion years.

Jura, M.↗
NASA NTRS1988

A search for radio emission from flare stars in the Pleiades

The VLA has been used to search for radio emission from flare stars in the Pleiades. Two observational strategies were employed. First, about 1/2 sq deg of cluster, containing about 40 known flare stars, was mapped at 1.4 GHz at two epochs. More than 120 sources with flux densities greater than 0.3 mJy exist on the maps. Detailed analysis shows that all but two of these sources are probably extragalactic. The two sources identified as stellar are probably not Pleiades members as judged by their proper motions; rather, based on their colors and magnitudes, they seem to be foreground G stars. One is a known X-ray source. The second observational strategy, where five rapidly rotating flare stars were observed at three frequencies, yielded no detections. The 0.3 mJy flux-density limit of this survey is such that only the most intense outbursts of flare stars in the solar neighborhood could have been detected if those stars were at the distance of the Pleiades.

Bastian, T. S.↗
NASA NTRS1988

The direction of Wolf-Rayet stars in a very powerful far-infrared galaxy - Direct evidence for a starburst

Spectra covering the wavelength range 4476-7610 A are presented for the powerful far-infrared galaxy IRAS 01003-2238. The broad emission band centered at a rest wavelength of roughly 4660 A, and other broad weaker features are interpreted, as arising from the combined effect of approximately 100,000 late Wolf-Rayet stars of the WN subtype. This represents perhaps the most direct evidence to date for the presence of a large number of hot massive stars in the nucleus of a very powerful far-infrared galaxy. The high number of Wolf-Rayet stars in relation to the number of O-type stars may be interpreted as arguing against continuous steady state star formation in 01003-2238, in favor of a recent burst of star formation occurring approximately 100 million yrs ago.

Armus, L.↗
NASA NTRS1988

Ultraviolet properties of IRAS-selected Be stars

New IUE observations were obtained of 35 Be stars from a list of stars which show excess infrared fluxes in IRAS data. The IRAS-selected Be stars show larger C IV and Si IV equivalent widths than other Be stars. Excess C IV and Si IV absorption seems to be independent of spectral type for IRAS-selected Be stars later than spectral type B4. This is interpreted as evidence for a possible second mechanism acting in conjunction with radiation pressure for producing the winds in Be stars. No clear correlation of IR excess of v sin i with C IV or Si IV equivalent widths is seen, although a threshold for the occurrence of excess C IV and Si IV absorption appears at a v sin i of 150 km/sec.

Bjorkman, Karen S.↗
NASA NTRS1988

Spectral class distribution of circumstellar material in main-sequence stars

A detailed statistical evaluation of IRAS survey data of main-sequence stars within 25 pc indicates that the presence of cool shells around A, F, and G main-sequence stars is the rule, rather than the exception. While luminosity bias favors A and F stars, the typical G star still appears to have almost three orders of magnitude more excess than the excess of the solar system due to known zodiacal dust and all planets combined. The Vega effect is thus not restricted to stars of particularly young age. The finding that the 'archetypical' solar system appears to have much less far-IR excess than the majority of G stars supports a speculation that the sun could have a cloud of cold particles beyond the outer planets, undetected by IRAS viewing from earth.

Aumann, H. H.↗
NASA NTRS1988

The Magellanic irregular galaxy NGC 4214 - Star formation and the interstellar medium

Near- and far-IR data and mm-wave CO observations are used to study the star-forming properties of the Magellanic irregular galaxy NGC 4214 and its interstellar medium. The galaxy is forming stars at present at about 0.5-1 solar mass/yr in its central few arcmin, if the new stars follow the Salpeter initial mass function from 0.1 to 100 solar masses. This has been maintained for around a Hubble time, although some regions of the galaxy may undergo periods of enhanced and depressed star formation. Based on the mass of dust in emission at 160 microns, a plausible star formation efficiency of about 1.5 percent is estimated, if the H I gas does not take part in forming stars. Molecular masses derived using the H + 1 - 0 line and a conversion to M(H2) appropriate to spiral galaxies produces a mass estimate that is too low by a factor of five to ten.

Thronson, Harley A., Jr.↗
NASA NTRS1989

The Einstein X-ray Observatory Catalog of O-type stars

A catalog of X-ray count rates for all O-type stars surveyed by the Einstein Observatory is presented. The procedures by which the IPC and High Resolution Imager rates are converted to emitted X-ray luminosities are discussed in detail. The catalog contains 289 stars with 89 detections and 176 upper bounds for the observed count rates. Precise positions for 43 O-type stars are given whose previously known locations were unknown or erroneous. It is argued that most O-type stars are X-ray point sources, but some are possibly embedded in extended nebulae and/or bubbles. The X-ray spectra of O-type stars are discussed briefly, and it is argued that their mean spectrum can reasonably be taken as an 0.5 keV thermal bremsstrahlung model. It is argued that at least 30 percent of all O-type stars are variable in X-rays.

Chlebowski, T.↗
NASA NTRS1989

IRAS colors of carbon stars - An optical spectroscopic test

Optical spectra are obtained of 57 photographic counterparts to IRAS sources not previously studied spectroscopically, and expected on the basis of their IRAS colors to be M or C type stars. Confirmed carbon stars are found only in a restricted range of 12-25 index, and constitute a striking 'vertical' sequence in the 12-25-60 micron color-color diagram. This sequence is in accord with evolutionary models for AGB stars that convert M into C stars by dredge-up, and follow loops in the color-color plane. Optically visible and optically invisible carbon stars occupy different color-color locations consistent with their representations of different evolutionary states in the life of relatively low-mass stars.

Cohen, Martin↗
NASA NTRS1989

X-ray emission from O-type stars - Parameters which affect it

As a result of previous analyses of the X-ray fluxes of O-type stars, it has been established that, surprisingly, there is no correlation between X-ray flux and such basic parameters as M, R, v(rot), T(eff), mass accretion rate v(infinity), L(w) = mass accretion rate v(infinity), and wind energy. In this paper, other factors influencing the X-ray flux of the hottest stars are sought. As a result of the present analysis, it is found that stars embedded in dense clouds are often X-ray brighter, runaway stars are fainter in X-rays, massive binaries emit more X-rays than single stars, and radio-bright stars are also more active in X-rays. All these correlations lead to the conclusion that the stellar environment and the interaction of the stellar wind with circumstellar matter (possible remnants of protostellar clouds) play a dominant role in the production of the X-ray flux.

Chlebowski, T.↗
NASA NTRS1989

Newborn stars and stellar winds in Barnard 228

Imaging and spectroscopic observations of premain-sequence stars in the B228 molecular cloud have identified three sites of extended, shock-excited nebulosity. A highly collimated, blueshifted jet is associated with the T Tauri star Sz68 (CoD -33:10685 deg). A candidate Herbig-Haro object is found near Sz68 and the T Tauri star Sz69. A third region of shock-excited nebulosity is identified near the embedded newborn star IRAS 15398-3359. IR excesses observed toward these stellar sources may imply a relationship between outflow activity and the presence of a circumstellar disk. Only one of the two outflow sources with well-defined orientations is aligned with the large-scale, magnetic-field geometry of the cloud in contrast to the global alignment of outflows in the Taurus cloud complex. Data for three previously unidentified emission-line stars are presented. A faint 18th magnitude dMe star located 48 arcsecs west of Sz68 is identified.

Heyer, Mark H.↗
NASA NTRS1989

Flare stars at radio wavelengths

The radio emission from dMe flare stars is discussed using Very Large Array and Arecibo observations as examples. Active flare stars emit weak, unpolarized, quiescent radio radiation that may be always present. Although thermal bremsstrahlung and/or thermal gyroresonance radiation account for the slowly-varying, quiescent radio radiation of solar active regions, these processes cannot account for the long-wavelength quiescent radiation observed from nearby dMe flare stars. It has been attributed to nonthermal gyrosynchrotron radiation, but some as yet unexplained mechanism must be continually producing the energetic electrons. Long duration, narrow-band radiation is also emitted from some nearby dMe stars at 20 cm wavelength. Such radiation may be attributed to coherent plasma radiation or to coherent electron-cyclotron masers. Impulsive stellar flares exhibit rapid variations that require radio sources that are smaller than the star in size, and high brightness temperatures greater than 10(exp 15) K that are also explained by coherent radiation processes. Quasi-periodic temporal fluctuations suggest pulsations during some radio flares. Evidence for frequency structure and positive or negative frequency drifts during radio flares from dMe stars is also presented.

Lang, Kenneth R.↗
NASA NTRS1990

High-velocity interstellar gas in the lines of sight to the Wolf-Rayet stars HD 97152 and HD 96548

The interstellar medium was studied in the direction to the WR stars HD 96548 and HD 97152, and the results are reported. New observational data on the UV spectra of several field stars near both these WR stars are presented. The high-velocity gas seen in the spectra of these stars suggests that the detected expanding interstellar gas structure consists of two OB cluster supershells. The presence of high-velocity absorption components in one of five field star spectra in the direction of the more isolated WR star HD 96548 suggests that this expanding gas does not originate from the optical ring nebula RCW 58 surrounding HD 96548, as previously believed, but instead indicates the detection of a previously unknown expanding interstellar shell in this line of sight.

Nichols-Bohlin, Joy↗
NASA NTRS1990

RR Lyrae stars in local group galaxies. II - NGC 147

Deep CCD images of NGC 147 taken with the '4-shooter' on the Hale 5 m telescope have been processed to find and photometrically measure RR Lyrae stars. 36 variable stars have been found, of which 32 are surmised to be RR Lyrae stars. Finding charts, periods, and light curves are presented. The mean magnitude of the RR Lyraes is determined to be 25.25 mag. A distance modulus 23.92 is derived, based on the best currently available values of foreground extinction and mean absolute magnitudes of RR Lyrae stars. A wide range of periods is seen for the RR Lyrae stars, indicating a correspondingly wide range of metallicities for the stars in NGC 147. The distance modulus derived here places NGC 147 at a distance of 154 kpc from the center of M31, and in conjunction with the line sight velocities of these two galaxies, this implies a lower limit of 7.2 x 10 to the 11th solar masses for the mass of M31.

Saha, A.↗
NASA NTRS1990

Photometry of 50 suspected variable stars

Fifty stars have been chosen as suspected variable stars and analyzed for variability. A large portion of this sample are stars that are either proved active chromosphere stars or are candidates for such activity. The photometric data base consists of differential V measurements of the Vanderbilt 16 inch (41 cm) automatic photoelectric telescope and 25 observers at 26 observatories worldwide. Published photometric data have also been utilized, with proper adjustments made to ensure that all magnitudes are differential. Searches for photometric period, amplitudes, and times of minimum light showed 68 percent of the sample to be photometrically variable with periods found for 34. Two stars were deemed norvariable for the period of observation. Conclusive statements could not be made concerning the photometric variability of the 14 remaining stars.

Hooten, James T.↗
NASA NTRS1990

Small-scale star formation at low metallicity

Massive star formation in a low metallicity environment is investigated by studying the morphology of small HII regions in the Small Magellanic Cloud. A classification scheme based upon the symmetry of form in the light of H-alpha is proposed to make possible an examination of the properties of blister candidates with respect to nebulas embedded in a more uniform medium. A new diagnostic of size is developed to derive quantitative information about the ionized gas and ionizing stars. The asymmetrical surface-brightness distribution of many HII regions demonstrates that massive stars often form at the edge of dense neutral clouds. However, the existence of many symmetrical nebulas with similar sizes, luminosities, and surface brightnesses shows that massive star formation often occurs within these clouds. Nevertheless, the statistics of the two different forms indicate that the rate of massive star formation declines less steeply with radius across host clouds than in the Milky Way, suggesting that external triggering may play a larger role in initiating star formation.

Mccall, Marshall L.↗
NASA NTRS1990

The intrinsic H I Lyman-alpha line profiles of late-type stars

The Lyman-alpha line of neutral hydrogen is probably the most important cooling channel for chromospheric plasma in late-type stars, yet it is also the least studied major line in the far ultraviolet. The scattering of much of the stellar Lyman-alpha flux by interstellar hydrogen, coupled with the geocoronal emission foreground, seriously complicates the analysis of the Lyman-alpha spectra. The influence of the local interstellar medium on the observed profiles was circumvented by observing stars with radial velocities sufficiently high to Doppler shift the center of the stellar emission line out of the interstellar absorption core. There are several stars that have high radial velocities by virtue of their presence in close binary systems. High resolution IUE (International Ultraviolet Explorer) spectra of Ly alpha line of two such eclipsing binary stars, AR Lac and TY Pyx, are obtained, at each orbital quadrature phase, when the projected orbital velocity is a maximum. By combining the spectra from opposite quadratures it is possible to piece together the entire stellar emission profiles. The third star in this study, delta Lep, is a single star with a high space velocity.

Neff, J. E.↗