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NASA NTRS1986

Comment on the three-body theory for period changes in RS CVn systems

In the three-body theory for period variations in RS CVn systems, the timing residuals are interpreted as light-travel time differences as the eclipsing system moves about the barycenter of the triple. These residuals can require a larger orbit than Kepler's law allows, given the time scale of the period variations. For only two of eight systems investigated, SV Cam and V471 Tau, is the theory plausible in that the inferred barycentric motion of the binary is smaller than the orbit of the third body, and the inferred properties of the third body are both reasonable and consistent with its remaining hidden. The theory is thus not a general theory for period changes. Observational testing of the theory is straightforward and may lead to the detection of 'brown dwarfs' associated with eclipsing systems through their kinematic effects.

Van Buren, D.↗

NASA NTRS1986

The evolution of very low mass stars

The results of numerical evolutionary calculations for stars with masses in the range of 0.01-0.10 solar mass are presented. The stellar models by which these stars are followed from the early stages of contraction through deuterium burning to the very late stages of degenerate cooling at ages comparable to that of the Galaxy are described, and the methodology used to investigate the major uncertainties in the input physics are discussed. It is found that, for brown dwarfs with masses substantially below the end of the hydrogen-burning main sequence, the evolution of the effective temperature and bolometric luminosity are fairly well determined, despite the residual uncertainties in the input physics. In particular, the evolution is remarkably insensitive to the choice of the atmospheric opacity law at low temperatures and to the amount of mismatch in specific entropy across the stellar envelope. The minimum mass for a star to attain main-sequence status is moderately sensitive to the assumed input physics, particularly the surface boundary conditions.

Nelson, L. A.↗

NASA NTRS1987

Direct infrared imaging of VB 8

Near-infrared images of VB 8 taken at K (2.2 microns) and L-prime (3.75 microns) during a period of good seeing at the IRTF show no evidence of the brown dwarf companion reported by McCarthy et al. (1985). Any companion 1.0 arcsec away from the primary must be at least 4 mag fainter than the primary at K and 3 mag fainter at L-prime in order to escape detection. A faint nebula centered on the star with scattering properties similar to a nebula detected near another main-sequence M star, Lalande 21185 (reported by McCarthy in 1986), may account for the extended emission detected by speckle observations.

Skrutskie, M. F.↗

NASA NTRS1987

The IRAS view of the Galaxy and the solar system

Data from IRAS observations of the solar system and the Galaxy are compiled and analyzed in a general review. Sections are devoted to the zodiacal dust cloud, comets and asteroids, the search for Planet X, the formation of low-mass and high-mass stars, and the IR characteristics of different types of stars and of novae and supernovae. Consideration is given to protoplanetary disks and the Vega phenomenon, limits on brown dwarfs, and the diffuse Galactic emission.

Beichman, C. A.↗

NASA NTRS1987

Spectral differential imaging detection of planets about nearby stars

Direct ground-based optical imaging of planets in orbit about nearby stars may be accomplished by spectral differential imaging using multiple passband acoustooptic filters with a CCD. This technique provides two essential results. First, it provides a means to modulate the stellar flux reflected from a planet while leaving the flux from the star and other sources in the same field of view unmodulated. Second, spectral differential imaging enables the CCD detector to achieve a sufficiently high dynamic range to locate planets near a star in spite of an integrated brightness differential of 5 x 10 to the 8th. Spectral differential imaging at nearby diffraction limited imaging conditions with telescope apodization can reduce the time to conduct a sensitive planetary search to a few hours in some cases. The feasibility of this idea is discussed here and shown to provide, in principle, the discrimination and sensitivity to detect a Jovian-class planet about stars at distances of about 10 parsecs. The detection of brown dwarfs is shown to be feasible as well.

Smith, W. Hayden↗

NASA NTRS1988

SIRTF and star formation

Four problems in the field of star formation that can be attacked to advantage with SIRTF are discussed: (1) the patterns of star formation in spiral galaxies, (2) the physical mechanism for bimodal star formation, (3) the nature of bipolar outflows from young stellar objects, and (4) the birth of brown dwarfs. In each case, SIRTF can provide the crucial combination of high angular resolution with great sensitivity over a broad range of wavelengths that is needed to address the relevant issues.

Shu, Frank H.↗

NASA NTRS1989

Discovery of low mass objects in Taurus

In infrared (2.2 micron, K-band) search of small regions (25 in square) near 26 members of the Taurus star-forming association has revealed 20 dim (K = 13-16 mag) stellar objects near 13 of them. Of these 20 objects, 9 are exceptionally red. It is argued that these 9 are probably also Taurus members. From the luminosities (0.4 to 4 times 10 the -3 power luminosity) and ages (estimated at 10(exp 6) years), masses can be determined by reference to theoretical low-mass cooling curves. The masses are in the range 0.005 to 0.015 solar mass, i.e., low-mass brown dwarfs. Proper motion studies of 7 of the objects visible on the POSS plates conducted by Burton Jones establish that 4 are highly probable Taurus members while 1 is a possible member.

Forrest, W. J.↗

NASA NTRS1990

The nature of unidentified 12 micron IRAS sources at high Galactic latitudes

A sample of 47 previously uncatalogued objects located above a Galactic latitude of 50 deg, and detected at 12 microns by the IRAS, is studied using near-infrared photometry. Ground-based observations show that the objects consist primarily of late-type M giant stars with long-wavelength excesses probably due to emission from dust associated with mass loss. The sample contains one oxygen-rich giant star undergoing rapid mass loss; an extremely cool (1230 K) carbon star 12560-1656 that may be as far as 10 kpc away; and a luminious quasar 13349-2438. The absence of nearby, low-luminosity infrared sources in this sample limits the space density of field brown dwarf stars. The fact that almost all the IRAS 12 micron sources have stellar counterparts visible on both the red and blue Palomar Observatory Sky Survey prints provides a tool for discriminating ordinary red stars.

Beichman, C. A.↗

NASA NTRS1990

A K band deep galaxy survey

The results of a K-band deep-imaging survey, with a 5 sigma limit of K = 21.4 for galaxy detection, are reported. Number counts from K = 16.5 to K = 21.25 are given. Comparison with optical survey data on the field shows objects as red as (I-K) = 5 which most probably correspond to high-redshift (z of about 2) normal galaxies. No evidence is seen for any population of high-z protogalaxies or brown dwarfs. The results are placed in the context of extragalactic background light measurements.

Cowie, L. L.↗

NASA NTRS1991

Gravitational microlensing of the Galactic bulge stars

The optical depth to microlensing on the ordinary disk stars is about 4 x 10 to the -7th for the Galactic bulge stars visible in Baade's window. This means that at any given time one out of about 2.5 x 10 to the 6th bulge stars must be brightened by at least 0.3 mag. The duration of events is proportional to the square root of mass of the lensing star, and it is in the range of 1 week to 1 month for the lensing stars of 0.1-1.0 solar mass. The average number of events with the amplitude exceeding 0.3 mag is expected to be about 4 per year per million bulge stars. If there are brown dwarfs in the Galactic disk with masses in the range 0.01-0.1 solar mass, with the total mass density equal to the total mass density of the ordinary stars, then the combined optical depth to microlensing of the bulge stars is about 8 x 10 to the -7th, the average number of events with the amplitude exceeding 0.3 mag is expected to be about 17 per year per million bulge stars, and the time scales of events are in the range of 3-20 days.

Paczynski, B.↗

NASA NTRS1991

The companion object to HD114762

We obtained 28 independent radial velocity measurements on the star HD114762, and are now able to derive orbit solution. We confirm the existence of a companion object in an orbit with a semimajor axis about 0.4 au. The new orbit solution is discussed. The result show that the companion is not a planet, but instead is a brown dwarf or a low mass star in a system viewed nearly pole-on.

Cochran, William D.↗

NASA NTRS1991

Scanning with charge-coupled devices

Results are presented of an investigation of six modes of using CCDs for searches of gamma-ray bursters, debris in geosynchronous space, satellites of asteroids, brown dwarfs, the tenth planet, comets, cometesimals, and various types of asteroids. A new discipline in astrophysics, 'scannerscopy', i.e., surveying with a CCD rather than with photography at a Schmidt telescope, is described. It uses the CCD in scanning rather than in sequencing of stare exposures as is done at most observatories, which can save telescope time and obviates the need for flat-fielding. Attention is given to the Tektronix 2048 x 2048 CCD, 38 arcmin wide, which is used to a limiting magnitude of 20.5 V. It is also shown to be successful for discovering rare and small near-earth asteroids. Possibilities with cameras on spacecraft that pass through the asteroid belt are described.

Gehrels, Tom↗

NASA NTRS1992

Excitation of pressure modes in common envelopes

The excitation of oscillatory modes by a low-mass star orbiting inside a common envelope with a more massive star is investigated, with emphasis on adiabatic high spherical harmonic degree (l is much greater than 1) modes propagating outward in the envelope. The dominant oscillatory modes are those for which the spherical harmonic order is large, and thus the amplitudes are large close to the equatorial plane and small closer to the poles. A secondary of mass about 1 percent of the primary mass excites modes with relative surface amplitudes of a few x 10 percent. Even a Jupiter-like brown dwarf, when it is very deep in the envelope of an asymptotic giant branch star, can cause perturbations of relative surface amplitudes of about 10 percent near the equatorial plane. If mass loss is influenced by oscillation, then this mechanism can lead to higher mass loss in the equatorial plane. In case of an asymptotic giant branch primary, this might lead to the formation of an elliptical planetary nebula.

Soker, Noam↗

NASA NTRS1991

Frontiers of stellar evolution

The present conference discusses theoretical and observational views of star formation, spectroscopic constraints on the evolution of massive stars, very low mass stars and brown dwarfs, asteroseismology, globular clusters as tests of stellar evolution, observational tests of stellar evolution, and mass loss from cool evolved giant stars. Also discussed are white dwarfs and hot subdwarfs, neutron stars and black holes, supernovae from single stars, close binaries with evolved components, accretion disks in interacting binaries, supernovae in binary systems, stellar evolution and galactic chemical evolution, and interacting binaries containing compact components.

Lambert, David L.↗

NASA NTRS1992

The use of robotic telescopes for detecting planetary systems

It is argued that increases in the precision of photometric measurements and the development of fully automatic photometric telescopes (APTs) now make possible the detection of Jupiter-size objects around other stars. The two most promising approaches are to monitor binary stars that have their orbital plane nearly in our line of sight and to measure the rotation periods of stars that are known to have dark companions and have very accurate values for Vsini. Differential photometry with APTs, based on a rapid comparison among a group of stars, provides a precision of 2 parts in 1000. Transits of solar-type stars by planets or brown dwarfs the size of Jupiter or Saturn will produce brightness variations of about 10 parts per 1000.

Borucki, William J.↗

NASA NTRS1993

The initial stellar population

Current theories for the origin of stellar masses and the initial mass function are surveyed, with emphasis on the formation of stars in clusters, and the possibility that several different formation mechanisms are at work in the Galaxy. Models are presented for the evolution of the 2.2-micron premain-sequence luminosity function of a young coeval star cluster, and the implications the results have for the conversion of an observed luminosity function into the underlying mass function are discussed. A description of theories which allow spectral energy distributions of young stellar objects to be used as diagnostics of their evolutionary states is given. The question of the existence of substellar objects (brown dwarfs) in nearby young clusters and regions of star formation is addressed.

Zinnecker, Hans↗

NASA NTRS1993

The faintest stars - The luminosity and mass functions at the bottom of the main sequence

We present IR K-band photometry of complete samples of VLM candidates constructed from IIIaF and IVN plates in 10 fields taken as part of the POSSII and UKSRC surveys. Using the I-K colors constructed for these stars we estimate a bolometric luminosity function which extends to M(Bol) = 13.75. We find significant evidence for a luminosity function decreasing toward these luminosities. We also find that our results are consistent with those of studies based on the Nearby Star sample, when those data are presented as a bolometric luminosity function. We convert our observed luminosity function into a mass function, which extends with reasonable statistics to 0.08 solar masses - the H-burning minimum mass. We find significant evidence for features in the mass function at these masses. Specifically, the mass function 'turns over' at 0.25 solar mass, goes through a local minimum at about 0.15 solar mass, and may increase again below 0.1 solar mass - none of these features are predicted by any of the current theories of star formation. Lastly, the mass density we observe just above the H-burning minimum mass makes it difficult to envisage brown dwarfs contributing significant quantities of missing mass without invoking either a mass function in this region significantly steeper than that seen for main-sequence stars, or an extremely low cutoff mass to the mass function.

Tinney, Christopher G.↗

NASA NTRS1993

Photometry and spectroscopy in the open cluster Alpha Persei, 2

Results from a combination of new spectroscopic and photometric observations in the lower main-sequence and pre-main sequence of the open cluster alpha Persei are presented. New echelle spectroscopy has provided radial and rotational velocity information for thirteen candidate members, three of which are nonmembers based on radial velocity, absence of a Li 6707A feature, and absence of H-alpha emission. A set of revised rotational velocity estimates for several slowly rotating candidates identified earlier is given, yielding rotational velocities as low as 7 km/s for two apparent cluster members. VRI photometry for several pre-main sequence members is given; the new (V,V-I(sub K)) photometry yields a more clearly defined pre-main sequence. A list of approximately 43 new faint candidate members based on the (V,V-I(sub K)) CCD photometry is presented in an effort to identify additional cluster members at very low masses. Low-dispersion spectra obtained for several of these candidates provide in some cases supporting evidence for cluster membership. The single brown dwarf candidate in this cluster is for the first time placed in a color-magnitude diagram with other cluster members, providing a better means for establishing its true status. Stars from among the list of new photometric candidates may provide the means for establishing a sequence of cluster members down to very faint magnitudes (V approximately 21) and consequently very low masses. New coordinate determinations for previous candidate members and finding charts for the new photometric candidates are provided in appendices.

Prosser, Charles F.↗