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NASA NTRS1987

Two-color method for optical astrometry - Theory and preliminary measurements with the Mark III stellar interferometer

The two-color method for interferometric astrometry provides a means of reducing the error in a stellar position measurement attributable to atmospheric turbulence. The primary limitation of the method is shown to be turbulent water vapor fluctuations. Secondary atmospheric effects caused by diffraction from small refractive-index inhomogeneities and differential refraction for the observation of stars away from zenith are shown to introduce errors that behave as white noise and which should not be significant. Other potential error sources due to photon noise, systematic instrumental effects, and imperfect data reduction are also considered. The improvement in accuracy possible with the two-color method is estimated as a factor of 5-10 over the corresponding one-color measurement. Some preliminary two-color measurements with the Mark III stellar interferometer at Mt. Wilson are presented, which demonstrate a factor of about 5 reduction in the amplitude of the atmospheric fluctuations in a stellar position measurement.

Colavita, M. Mark↗

NASA NTRS1986

Stellar populations; Proceedings of the Meeting, Baltimore, MD, May 20-22, 1986

Various papers covering recent findings on stellar populations are presented. The topics addressed include: the bulges of the Galaxy and M31, kinematics of the local subdwarfs and the collapse of the Galaxy, the red stellar population of the local group dwarf galaxies, globular clusters in the local group galaxies, the initial mass function, and star formation histories of galactic disks. Also discussed are: yield and abundance constraints on galactic chemical evolution, population synthesis in early type galaxies, the evolution of stellar populations in galaxies, a theoretical perspective on spectral evolution of galaxies, and the structure and kinematics of galaxy populations.

Norman, Colin A.↗

NASA NTRS1988

Improved methods for the measurement and analysis of stellar magnetic fields

The paper presents several improved methods for the measurement of magnetic fields on cool stars which take into account simple radiative transfer effects and the exact Zeeman patterns. Using these methods, high-resolution, low-noise data can be fitted with theoretical line profiles to determine the mean magnetic field strength in stellar active regions and a model-dependent fraction of the stellar surface (filling factor) covered by these regions. Random errors in the derived field strength and filling factor are parameterized in terms of signal-to-noise ratio, wavelength, spectral resolution, stellar rotation rate, and the magnetic parameters themselves. Weak line blends, if left uncorrected, can have significant systematic effects on the derived magnetic parameters, and thus several methods are developed to compensate partially for them. The magnetic parameters determined by previous methods likely have systematic errors because of such line blends and because of line saturation effects. Other sources of systematic error are explored in detail. These sources of error currently make it difficult to determine the magnetic parameters of individual stars to better than about + or - 20 percent.

Saar, Steven H.↗

NASA NTRS1987

An investigation of stellar coronae with AXAF

Questions concerning the phenomenon of stellar coronae are discussed together with the physical parameters essential to model stellar coronae and the different AXAF instruments assigned for measurements of these parameters. As an illustration of what AXAF will be able to accomplish, three specific examples of studies by AXAF are described. These include coronal modeling for a bright X-ray source such as HR 1099, using spectra obtained by AXAF; time-resolved spectroscopy of AR Lacertae and YY Geminormum during their eclipses, to use the spectra for the identification and modeling of individual geometrical physical structures on these stars; and time-resolved spectroscopy during stellar flares to obtain high-resolution spectra which will make it possible to infer the changes in the flaring plasma temperatures as a function of time.

Linsky, Jeffrey L.↗

NASA NTRS1988

Near-infrared stellar-occultation predictions for Uranus and Neptune - 1987-1990

Stellar-occultation predictions are presented for Uranus and Neptune covering the period 1987-1990, based on automated scans of R and I plates taken with the UK Schmidt Telescope in Australia. These scans have revealed many late-type or highly reddened stars suitable for stellar-occultation observations at 2.2 microns that were missed in previous searches of yellow-sensitive astrographic plates (Mink and Klemola, 1985). Comparisons involving several hundred stars measured by Mink and Klemola (1985) indicate that the internal random errors in the stellar positions estimated here are less than about 0.2 arcsec, comparable to the precision reached using conventional astrographic methods. In addition, JHK photometry was obtained for many of the Neptune stars, which is used to establish an empirical (I-K) vs (R-I) calibration for the remaining occultation candidates, and thus provide estimates of their K magnitudes.

Nicholson, Philip D.↗

NASA NTRS1988

Cosmic-ray-modified stellar winds. II - A perturbation approach

A perturbation method is developed to describe the modification of a stellar wind flow with termination shock by the Galactic cosmic rays. The perturbation parameter epsilon used in the analysis is the ratio of the galactic cosmic-ray pressure to the thermal gas pressure at a large distance from the star. The positive galactic cosmic-ray pressure gradient serves to brake the outflowing stellar wind gas, particularly just upstream of the termination shock of the wind. A one-fluid polytropic model is used to describe the thermal stellar wind gas, with the coupling between the cosmic rays and the thermal gas being determined by an average hydrodynamical diffusion coefficient kappa describing the scattering of cosmic rays by magnetohydrodynamic waves traveling in the background flow. The analysis takes into account cosmic-ray modifications of the critical point of the wind, the thermal gas entropy constants on both sides of the shock, and the fluid velocity profile.

Ko, C. M.↗

NASA NTRS1988

Induced stellar mass loss in AGN's

The effect of star-gas drag on stellar dynamics and stellar mass loss in active nuclei is discussed. It is assumed that red giants are much too rare in AGNs and that the stellar population consists mainly of late-type main-sequence stars. It is found that the star-gas drag is often the dominant mechanism triggering the mass loss from these stars at rates up to 100,000 times the usual rate for the main-sequence star. It is shown that the gas and radiation will impose a substantial turbulent stripping of or a Compton drag on the outer layers of stars.

Shlosman, Isaac↗

NASA NTRS1988

The Mark III stellar interferometer

The Mark III interferometer is an operational long baseline stellar interferometer on Mt. Wilson with four-possible baseline configurations from 9m NE-SW to 20m N-S. The interferometer was designed to be a highly automated astronomical instrument to measure stellar positions and diameters to a magnitude limit of seven. Initial fringe observations were made in September 1986 with a 12-m N-S baseline. In the following months, semi-automated astrometric and stellar diameter measurements were also made. This paper describes the hardware and software components of the instrument and its operational characteristics. The interferometer has several novel features. One is the use of optimal estimation and control algorithms (e.g. Kalman filters) in the control loops. Another is the ability to operate both as a closed-loop phased interferometer and eventually as an open-loop or absolute coherent interferometer. High thermal stability and mechanical accuracy should permit the instrument to point blind at an astronomical object and maintain optical path equality to within the limits set by the atmosphere. In this absolute interferometric mode of operation, it should be possible to observe faint astronomical objects that are too dim for phase tracking. In theory, measurements of amplitude, group delay, and closure phase will be possible to 14 mag.

Shao, M.↗

NASA NTRS1988

Neutral stellar winds that drive bipolar outflows in low-mass protostars

The Arecibo radio telescope at the 21-cm line of atomic hydrogen has been used to detect a neutral atomic wind in the bipolar flow source HH 7-11. An atomic mass of about 0.015 solar associated with the rapidly flowing gas is deduced. The stellar mass-loss rate is roughly 3 x 10 to the -6th solar mass/yr if the crossing time of the decelerating wind is 5000 yr. The excess emission in the H I line core gives a total duration of the outflow of about 70,000 yr. A detailed analysis of the H I line shape yields a reasonable deceleration rate for the atomic wind if the stellar wind continuously entrains ambient molecular gas as it propagates from the protostar. A stellar wind with the described characteristics and a terminal velocity of 170 km/s would be more than sufficient to drive the known extended CO bipolar outflow in HH 7-11.

Lizano, Susana↗

NASA NTRS1988

X-ray induced stellar mass loss near active galactic nuclei

The effects of UV and X-ray radiation on stars in active galactic nuclei (AGN) are critically evaluated. Mass loss rates in X-ray-induced winds are evaluated for realistic red giant models, and the effects of the ablation of stellar envelopes by radiation pressure are considered. The importance of X-ray-induced mass loss in the standard quasar model is evaluated and whether it can provide a source of accretion fuel or emission-line clouds is discussed. It is concluded that thermal winds driven by X-ray heating are a minor total supply of mass to AGN, but that thermal plus line-driven winds and stellar ablation may increase the mass loss and improve the chances for supplying a fraction of the necessary mass supply to the central object. It is speculated that when steady winds are inefficient, complex time-dependent processes due to X-ray energy injection deep into a stellar atmosphere could still release significant mass from stars.

Voit, G. Mark↗

NASA NTRS1988

Spectral synthesis in the ultraviolet. II - Stellar populations and star formation in blue compact galaxies

An initial attempt to apply optimizing spectral synthesis techniques to the far-UV spectra of blue compact galaxies (BCGs) is presented. The far-UV absorption-line spectra of the galaxies are clearly composite, with the signatures of the main-sequence types between O3 and mid-A. Most of the low-ionization absorption lines have a stellar origin. The Si IV and C IV features in several objects have P Cygni profiles. In Haro I the strength of Si IV indicates a significant blue supergiant population. The metal-poor blue compact dwarf Mrk 209 displays weak absorption lines, evidence that the stellar component has the same low metallicity as observed in the ionized gas. Good fits to the data are obtained the technique of optimizing population synthesis. The solutions yield stellar luminosity functions which display large discontinuities, indicative of discrete star formation episodes or bursts. The amount of UV extinction is low.

Fanelli, Michael N.↗

NASA NTRS1989

A study of the stellar population in the Lynds 1641 dark cloud - A possible dense cluster associated with IRAS 05338-0624

Sensitive images obtained at 1.6 and 2.2 microns reveal a possible dense cluster of stars associated with IRAS 05338-0624 and located within an optically thick molecular core; the visual extinction suffered by cluster members ranges from 7 to 40 mag. A circular region of diameter 70 arcsec (0.16 pc at a distance of 480 pc) contains 20 stellar objects or semistellar knots. Assuming that the depth of the region is comparable to its projected size leads to an estimated stellar density of 6800/cu pc. The ratio of stellar to molecular material contained within the core must lie between 4 percent and 43 percent. It is likely that this cluster will emerge from its parent core as a bound group.

Strom, Karen M.↗

NASA NTRS1990

X-rays from colliding stellar winds

A stellar wind from a massive OB or Wolf-Rayet star in a binary system will strike the surface or stellar wind of its companion, forming shocked gas that can radiate X-rays. The X-ray spectrum from the shocked winds will vary in a predictable way with orbital phase, owing to photoelectric absorption by the stellar winds. Detailed models are calculated for the hydrodynamics and X-ray emission from two such systems. In one of these systems (HD 165052), the winds are nearly identical in strength. In the other (V444 Cygni), the wind of the Wolf-Rayet star overwhelms and crushes that of its companion. The calculated X-ray luminosities agree fairly well with the observed values for HD 165052 and for V444 Cygni. These results can be scaled to other such systems.

Luo, Ding↗

NASA NTRS1992

Photometric calibration of NGS/POSS and ESO/SRC plates using the NOAO PDS measuring engine. I - Stellar photometry

The PDS/Monet measuring engine at the National Optical Astronomy Observatory was used to obtain photometry of nearly 10,000 stars on the NGS/POSS and 2000 stars on the ESO/SRC Survey glass plates. These measurements have been used to show that global transformation functions exist that allow calibration of stellar photometry from any blue or red plate to equivalent Johnson B and Cousins R photoelectric magnitudes. The four transformation functions appropriate for the POSS O and E and ESO/SRC J and R plates were characterized, and it was found that, within the measurement uncertainties, they vary from plate to plate only by photometric zero-point offsets. A method is described to correct for the zero-point shifts and to obtain calibrated B and R photometry of stellar sources to an average accuracy of 0.3-0.4 mag within the range R between values of 8 and 19.5 for red plates in both surveys, B between values of 9 and 20.5 on POSS blue plates, and B between values of 10 and 20.5 on ESO/SRC blue plates. This calibration procedure makes it possible to obtain rapid photometry of very large numbers of stellar sources.

Cutri, Roc M.↗

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

Detecting faint echoes in stellar-flare light curves

Observational considerations are discussed for detecting echoes from flare-star photospheres and from stellar or planetary companions. Synthetic spectra are used to determine optimal conditions for the recovery of echoes in flare light curves. The most detectable echoes are expected to appear in broadband observations of the UV continuum. Short-lived flares are ideal for resolving echoes from the flare-star photosphere and may provide constraints for stellar-flare models. Strong outbursts may be used to detect stellar or planetary companions of a flare star. However, the possible planetary configurations that may be probed by this method are limited to Jupiter-size objects in tight orbits about the parent star.

Bromley, Benjamin C.↗

NASA NTRS1993

Stellar models and the brightening of P Cygni

P Cygni is currently in the transition phase between core hydrogen and helium burning. The behavior of the stellar envelope in this phase is strongly affected by the intermittant dominance of hydrogen shell burning. We perform detailed stellar evolution calculations of this rapid transition phase using the most recent Livermore opacities. We find that observations and stellar evolution models agree very well if the initial main-sequence mass of P Cygni was about 60 +/- 5 solar masses. Depending on the mass-loss rate, P Cygni's current mass could be as low as 50 solar masses, which is still about a factor of 2 higher than that predicted by the radiation driven wind theory. Our calculations seem to favor the Schwarzschild criterion for convection over the Ledoux criterion in conjunction with semiconvection.

El Eid, Mounib F.↗

NASA NTRS1992

Radio observations of solar and stellar coronae

Recent results of radio observations of the solar and stellar coronae are reviewed. Attention is given to the results obtained on quiet-sun fine structures; the active region observations, with particular consideration given to the soft X-ray and radio comparisons of observations made during the Coronal Magnetic Structures Observing Campaign and the strength of the magnetic fields in the corona; the radio observations of solar flares; and the radio observations of stellar coronae of the RS Canum Venaticorum binaries, M dwarf stars, pre-main-sequence stars, and chemically-peculiar B stars. Also discussed are current issues in stellar radiophysics and perspectives in solar radio physics.

Kundu, M. R.↗