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Brown, Alexander

Publications and source records attributed to Brown, Alexander.

64 records · Page 4
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NASA NTRS1990

Stringent limits on the ionized mass loss from A and F dwarfs

Following the suggestion of Willson et al. (1987) that A- and F-type main-sequence stars might undergo significant mass loss due to pulsationally driven winds, upper limits to the ionized mass loss from A and F dwarfs have been obtained using VLA observations. These stringent upper limits show that the level of ionized mass loss would have at most only a small effect on stellar evolution. Radiative-equilibrium atmospheric and wind models for early A dwarfs indicate that it is highly likely that a wind flowing from such stars would be significantly ionized. In addition, late A and early F dwarfs exhibit chromospheric emission indicative of significant nonradiative heating. The present mass-loss limits are thus representative of the total mass-loss rates for these stars. It is concluded that A and F dwarfs are not losing sufficient mass to cause A dwarfs to evolve into G dwarfs.

Brown, Alexander↗
NASA NTRS1990

New radio detections of early-type pre-main-sequence stars

Results of VLA radio continuum observations of 13 early-type pre-main-sequence stars selected from the 1984 catalog of Finkenzeller and Mundt are presented. The stars HD 259431 and MWC 1080 were detected at 3.6 cm, while HD 200775 and TY CrA were detected at both 3.6 and 6 cm. The flux density of HD 200775 has a frequency dependence consistent with the behavior expected for free-free emission originating in a fully ionized wind. However, an observation in A configuration suggests that the source geometry may not be spherically symmetric. In contrast, the spectral index of TY CrA is negative with a flux behavior implying nonthermal emission. The physical mechanism responsible for the nonthermal emission has not yet been identified, although gyrosynchrotron and synchrotron processes cannot be ruled out.

Skinner, Stephen L.↗
NASA NTRS1989

Mg II line profiles of the Mira S Carinae

A series of high-dispersion IUE observations obtained to investigate the evolution of the shock structure of the Mira variable S Carinae have produced (despite limited phase coverage) a set of five exceptionally interesting spectra of the Mg II h and k lines. The two primary findings of these observations are (1) there is significant emission from both the h and k lines at positions corresponding to velocities of -150 km/s relative to the stellar photosphere and (2) the k/h flux ratio of the Mg II doublet remains below the theoretically predicted values which lie in the range 2:1 to 1:1 and shows a smooth dependence on the optical phase. Archival studies of other Miras (e.g., R Car) indicate that S Car is not unique in possessing unusual and highly variable Mg II h and k line profiles. Several possible physical processes that may explain these results, including radiative transfer effects in a shocked atmosphere, circumstellar scattering, and circumstellar absorption, are discussed.

Bookbinder, Jay A.↗
NASA NTRS1988

The ultraviolet spectrum of noncoronal late-type stars - The Gamma Crucis (M3.4 III) reference spectrum

A guide is presented to the UV spectrum of M-type giants and supergiants whose outer atmospheres contain warm chromospheres but not coronae. The M3 giant Gamma Crucis is taken as the archetype of the cooler, oxygen-rich, noncoronal stars. Line identifications and integrated line flux measurements of the chromospheric emission features seen in the 1200-3200 A range of IUE high-resolution spectra are presented. The major fluorescence processes operating in the outer atmosphere of Gamma Crucis, including eight previously unknown pumping processes and 21 new fluorescent line products, are summarized, and the enhancements of selected line strengths by 'line leakage' is discussed. A set of absorption features toward the longer wavelength end of this range is identified which can be used to characterize the radial velocity of the stellar photospheres. The applicability of the results to the spectra of noncoronal stars with different effective temperatures and gravities is discussed.

Carpenter, Kenneth G.↗
NASA NTRS1988

Mg II flux and profile variability of hybrid-chromosphere stars

Results from an IUE investigation of the variability seen in the Mg II emission line fluxes and wind absorption components of hybrid-chromosphere stars are presented. Real Mg II flux variability is shown with most of the variation being long-term. This variability appears to be due to global changes in the Mg II surface activity. Major changes in the wind absorption component are not seen.

Veale, Anthony↗
NASA NTRS1988

Mg II line profiles of the Mira S Carinae

High-dispersion IUE observations obtained to investigate the evolution of the shock structure of the Mira S Carinae (S Car) produced, despite very limited phase coverage, a set of five spectra of the Mg II h and k lines. There is significant emission from both the h and k lines at velocities of -150 km/sec relative to the stellar photosphere. The h-to-k ratio of the Mg II doublet remains below the theoretically predicated values of 2:1 to 1:1, and shows a smooth dependence on the optical phase. Archival studies of other Miras (e.g., R Car) indicate that S Car is not unique in possessing unusual and highly variable Mg II h and k line profiles.

Brugel, Edward W.↗
NASA NTRS1987

Radio emission from pre-main-sequence stars in Corona Australis

The central region of the Corona Australis molecular cloud surrounding the stars R and TY CrA has been studied using the VLA at 6 cm. Eleven radio sources are detected including five associated with pre-main-sequence objects. The most striking is associated with the near-IR source IRS 7 and shows a complex structure comprising two strong pointlike sources positioned either side of the deeply embedded IR source and two extended lobes of radio emission. The IRS 7 radio source appears to be similar to that associated with Lynds 1551 IRS 5 but has a considerably larger angular size. The other detected sources include the massive pre-main-sequence star TY CrA, the near-IR sources IRS 1 and IRS 5, and the Herbig-Haro object HH 101. The stars R and T CrA were not detected.

Brown, Alexander↗
NASA NTRS1987

Variable radio emission from the 4 Draconis system

The 4 Dra system, which contains an M3 giant and a cataclysmic binary, has been detected as a 6 cm radio source with the VLA. This radio emission is variable on time scales of weeks to months. A number of possible sources of the radio emission within the 4 Dra system are considered, and the observed time variability probably rules out optically thick free-free emission from the wind of the M3 giant as the dominant source. The major source of the observed radio emission is likely to be either the cataclysmic binary or shocked regions due to the interaction between the cataclysmic binary and the red giant wind.

Brown, Alexander↗
NASA NTRS1986

Evolution of the chromospheric and coronal activity of intermediate mass stars

Recent ultraviolet and X-ray observations pertaining to the outer atmospheric structure of intermediate mass (4-6 solar masses) stars and the evolution of their structure are presented. A distance-limited (d equal to or less than 200 pc) IUE ultraviolet survey of early K bright giants shows that C IV emission commonly is present. These stars are almost evenly split between stars showing hybrid-chromospheric and coronal outer atmospheric structures. Exosat observations have been obtained for three hybrid stars, of which only Alpha TrA, the nearest, is detected. The temperature of the emitting plasma is likely to be about 10 to the 6th K. Observtions of six K II stars made with the Einstein satellite show no detections. The general conclusion from the available X-ray data is that early K bright giants are not strong X-ray sources.

Brown, Alexander↗