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Publications and source records attributed to Basri, G..
Hubble Space Telescope near infrared camera and multiobject spectrometer (NICMOS) imaging observations of two nearby young brown dwarfs, DENIS-P J1228.2-1547 and Kelu 1, show that the DENIS object is resolved into two components of nearly equal brightness with a projected separation of 0.275 arcsec.
A large sample of late-type binaries with orbital periods from 1 to 100 days has been examined for relationships between stellar activity and rotation period (assumed - with a few exceptions - to be synchronized with the stellar orbital period). Most of the systems are in the RS Canum Venaticorum class of close binaries; none are contact binaries. The activity diagnostics were observed with the IUE and the Einstein Observatory and range from the mid-chromosphere to the corona. There is a dependence of activity on rotation in the synchronized binaries; surface flux is the preferable measure of activity and is best when steps are taken to remove variations in activity filling factors.
The first simultaneous, calibrated observations of the Ca II K and Mg II k resonance lines in T Tauri stars are presented. It is found that for T Tauri stars with mass greater than 1.5 solar mass, which have radiative cores and tend to be fast rotators, the k line seems to arise in an extended region (probably also responsible for the H-alpha emission), whereas the K line apparently originates closer to the highly inhomogeneous stellar surface. The lower mass stars, which are fully convective and tend to be slow rotators, are more easily described by a largely chromospheric model, consistent with main-sequence activity structures but at greater values of the nonradiative flux. The strongest emission-line stars in the low-mass group, however, are also likely to have extended k line regions.
An atlas of IUE short-wavelength spectra for a set of 14 bright G0 V-G5 V stars are presented, and it is shown that these manifest a range of qualitatively different chromospheric and transition region spectra and significant differences in radiative flux originating at the temperature minimum level. A comprehensive survey of observational data and physical parameters of these stars has been performed, and tabular summaries of the data are presented. It is concluded that the UV continuum longward of about 1600 A is a diagnostic of the temperature minimum. A considerable range of minimum temperature for stars of similar effective temperature and spectral type is found. The temperature minimum of the sun is highly structured on spatial scales of about one arcsec.
A detailed numerical simulation of light scattering from the low-resolution grating in the short wavelength spectrograph of the IUE Observatory was developed, in order to quantitatively analyze the effects of scattering on both continuum and line emission spectra. It is found that: (1) the redistribution of light by grating scattering did not appreciably alter either the shape or the absolute flux level of continuum spectra for A-F stars; (2) late-type stellar continua showed a tendency to flatten when observed in scattered light toward the shorter wavelengths; and (3) the effect of grating scattering on emission lines is to decrease measured line intensities by an increasing percentage toward the shorter wavelengths. The spectra obtained from scattering experiments for solar-type and late type stars are reproduced in graphic form.
The question as to whether the amount of activity observed in stars with the same fundamental stellar parameters depends on rotational velocity is considered in view of close binary systems, making use of their orbital rather than their rotational period. It is noted that the main sequence G star binaries lie systematically lower than the K subgiants for the same rotational periods, so that relations including a variety of spectral classes cannot be used to directly deduce correlations between magnetic fields and activity without suitable correction. The behavior of the present sample suggests that activity increases with later spectral type and with evolution off the main sequence. The chromosphere shows a minor reaction to rotational velocity increase.
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The soft X-ray spectrum of Sirius obtained by the HEAO 1 A-2 experiment is used to investigate the origin of the X-radiation from that system. It is shown that, if the emission is coronal, then the coronal temperature must be less than 150,000 K, and the coronal luminosity must be greater than 10 to the 33rd ergs/s. Such a corona conflicts severely with EUV observations and is physically untenable. White dwarf atmosphere models are fitted to the spectrum and it is shown that a photospheric temperature of 28,000 K is much more consistent with the data. The relative consistency of the soft X-ray, EUV, and UV observations of Sirius B is discussed and it is concluded that the evidence favors the white dwarf photosphere as the origin of the soft X-rays.
Both IUE short and long wavelength exposures of the 56 Peg system are discussed. This mild barium star has an X-ray luminosity of 3 x 10 to the 31st power ergs/1, comparable to the rapidly rotating RS CVn binary systems, yet lies in a region of the HR diagram where stellar X-rays are generally not observed. This cool, bright giant is not a rapid rotator and the key to understanding its emission lies in the recent discovery of its white dwarf companion. Accretion onto the white dwarf of approximately 0.1% of the stellar wind of the primary is sufficient to power an X-ray source of the observed luminosity. Reprocessing of the X-rays in the cool dense stellar wind explains the origin of the UV emission line spectrum, and may explain the time varying asymmetry of the Mg 2 kappa line profile that is observed. Graphs which show observed fluxes and wavelengths are discussed.
Stellar chromospheric and coronal activity appears ubiquitous among late type stars to the left of the TR-wind boundary line (Linksy and Haisch 1979). The level of activity as measured by the X-ray surface flux is linearly proportional to the stellar angular velocity, with the exception of slowly rotating dwarfs (Walter 1981, 1982; Walter and Bowyer 1981). The peculiar rapidly rotating G giant FK Comae (Merrill 1948) appears to fit into this pattern. Line widths indicate V sin i = 120 + or - 20 km s(-1) (Bopp and Stencel 1981). FK Comae has strong Ca II H and K and H alpha emission, strong transition region UV lines (Bopp and Stencel 1981), and an X-ray surface flux in good agreement with its rapid rotation (Walter 1981). Yet, FK Comae is an enigmatic star. It is a rapid rotator, but it is not clear why it is a rapid rotator. There is no direct evidence for duplicity; indeed, the upper limit of 20 km s(-1) on the K velocity puts tight constraints on any binary configuration, especially if sin i approx 1, as indicated by the large V sin i. Bopp and Stencel (1981) have suggested that FK Comae is an example of a coalesced W UMa system (Webbink 1976), wherein the orbital angular momentum has become rotational angular momentum of the coalesced star.