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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The envelopes of RV Tauri stars. I - Linear polarimetric and spectroscopic observations

Simultaneous spectroscopic and polarimetric observations are presented for five RV Tauri stars and one RV Tauri-like semiregular variable. Time-dependent variations are found in the polarizations for four of the objects, as are wavelength-dependent variations in two and possibly three stars. In general, the largest polarizations and strongest wavelength dependences in the polarization occur immediately following primary light minima.

Nook, Mark A.↗
NASA NTRS1990

Studies of solar type stars

Satellite observations show a change of 0.1 percent in solar irradiance over the past decade. The question arises as to whether larger changes in irradiance may occur over century time scales. Researchers approached this question by looking at changes in surface magnetism, since the satellite observations also show that irradiance and surface magnetism are correlated. To obtain information on possible variations in surface magnetism over century-long intervals, researchers looked at records of surface magnetism spanning up to 20 years for a sample of 74 stars of solar type, i.e., of similar age and mass to the sun. The histories of these stars, observed in random phases of their long-term variability, give a snapshot at any one time of the behavior of a solar-type star over long periods of time. Evidence on Maunder Minimum phases and levels of magnetic activity in these solar-type stars are compared with information on solar magnetism. Implications for solar irradiance changes are discussed.

Baliunas, Sallie↗
NASA NTRS1972

Photometry and polarimetry of V 1057 Cygni.

Evaluation of extensive photometric and polarimetric data over a six-month interval for V 1057 Cyg. The polarization appears to be constant and of interstellar origin; the photometry, which extends nearly six octaves from the ultraviolet to 22 microns in the infrared, indicates that the total observed luminosity is decreasing - having now declined about 10% from its earlier level. A model characterized by two circumstellar shells which absorb and reradiate short-wavelength stellar flux from an underlying A1-type star provides the best fit to the observational data. The present variability, however, cannot be due to circumstellar activity, but appears to originate in the star itself.

Rieke, G.↗
NASA NTRS1995

Multiple rings around Wolf-Rayet evolution

We present optical narrow-band imaging of multiple rings existing around galactic Wolf-Rayet (WR) stars. The existence of multiple rings of material around Wolf-Rayet stars clearly illustrates the various phases of evolution that massive stars go through. The objects presented here show evidence of a three stage evolution. O stars produce an outer ring with the cavity being partially filled by ejecta from a red supergiant of luminous blue variable phase. A wind from the Wolf-Rayet star then passes into the ejecta materials. A simple model is presented for this three stage evolution. Using observations of the size and dynamics of the rings allows estimates of time scales for each stage of the massive star evolution. These are consistent with recent theoretical evolutionary models. Mass estimates for the ejecta, from the model presented, are consistent with previous ring nebula mass estimates from IRAS data, showing a number of ring nebulae to have large masses, most of which must in be in the form of neutral material. Finally, we illustrate how further observations will allow the determination of many of the parameters of the evolution of massive stars such as total mass loss, average mass loss rates, stellar abundances, and total time spent in each evolutionary phase.

Marston, A. P.↗
NASA NTRS2019

The Random Transiter – EPIC 249706694/HD 139139

We have identified a star, EPIC 249706694 (HD 139139), that was observed during K2 Campaign 15 with the Kepler extended mission that appears to exhibit 28 transit-like events over the course of the 87-day observation. The unusual aspect of these dips, all but two of which have depths of 200 ± 80 ppm, is that they exhibit no periodicity, and their arrival times could just as well have been produced by a random number generator. We show that no more than four of the events can be part of a periodic sequence. We have done a number of data quality tests to ascertain that these dips are of astrophysical origin, and while we cannot be absolutely certain that this is so, they have all the hallmarks of astrophysical variability on one of two possible host stars (a likely bound pair) in the photometric aperture. We explore a number of ideas for the origin of these dips, including actual planet transits due to multiple or dust emitting planets, anomalously large TTVs, S- and P-type transits in binary systems, a collection of dust-emitting asteroids, ‘dipper-star’ activity, and short-lived starspots. All transit scenarios that we have been able to conjure up appear to fail, while the intrinsic stellar variability hypothesis would be novel and untested.

S. Rappaport↗
NASA NTRS1982

A search for UV line variability in R136a

The brightest object in the central cluster of stars in the 30 Doradus nebula is R136a. Optical imaging and ultraviolet spectroscopy show that a luminosity originates in a volume less than 0.1 pc across. A very tight cluster of approximately 30 luminous O and WR stars produces this energy. An alternative interpretation is the possibility that a single supermassive object dominates the light. The visual spectrum is classified as WN 4.5. A common characteristic of WN spectra is variability of the emission lines, especially He II lambda 4686.

Ebbets, D. C.↗
NASA NTRS1983

The quiescent chromospheres and transition regions of active dwarf stars - What are we learning from recent observations and models?

Progress in understanding active dwarf stars based on recent IUE, Einstein, and ground-based observations is reviewed. The extent of magnetic field control over nonflare phenomena in active dwarf stars is considered, and the spatial homogeneity and time variability of active dwarf atmospheres is discussed. The possibility that solar like flux tubes can explain enhanced heating in active dwarf stars in examined, and the roles of systematic flows in active dwarf star atmospheres are considered. The relation between heating rates in different layers of active dwarf stars is summarized, and the mechanism of chromosphere and transition region heating in these stars are discussed. The results of one-component and two-component models of active dwarf stars are addressed.

Linsky, J. L.↗
NASA NTRS1986

The shapes of the circumstellar silicate features

Around oxygen-rich stars the spectra of most long-period variables (LPV) show an excess infrared emission which is attributed to circumstellar silicate dust grains. It is known that the spectral energy distribution of the 10 micron emissions shows variations from star to star. With the availability of many Infrared Astronomy Satellite (IRAS) Low Resolution Spectra (LRS) in the 8 to 22 micron region, the 10 micron feature can be studied to determine its uniformity (or lack thereof). The excess silicate emissions (10 micron emission), divided into three groups characterized by similar spectral shapes, are discussed.

Little-Marenin, I. R.↗
NASA NTRS1993

The circumstellar gas surrounding 51 Ophiuchi - A candidate proto-planetary system similar to Beta Pictoris

Combined archival and recent International Ultraviolet Explorer (IUE) observations of the star, 51 Oph, reveal the presence of variable, accreting gas with velocities as large as + 100 km/s relative to the system. The electron number density of the circumstellar gas is comparable to that observed around the candidate proto-planetary system, Beta Pic. In addition to the cooler gas, absorption from Al III, Si IV, and C IV is present over the velocity range of the accreting gas. The presence of Si IV and C IV in the spectrum of a B9.5 star provides evidence for collisional ionization of the circumstellar gas like that observed in Beta Pic. The combination of H-alpha profiles with double emission peaks to comparable strength, together with detection of transient mass ejection events similar to those observed in other Be stars, suggests that the inclination of the 51 Oph system is within 10-15 deg of the equatorial plane. Collectively these data imply that the 51 Oph system is similar to Beta Pic in both system constituents and orientation, and may be in a similar evolutionary state.

Grady, C. A.↗
NASA NTRS1993

Emission-line objects near R Coronae Australis

New CCD images show additional emission-line objects close to R Coronae Australis in the Corona Australis cloud. Two faint knots, strong in forbidden S II, are seen against the receding CO flow associated with R CrA. They are closely aligned with HH 104 A,B and R CrA and have thus been labeled HH 104 C,D. The position angle corresponds to that of the transient spike described by Hartigan and Graham (1987). HH 104 C,D lie on the edge of a diffuse patch of nebulosity, strongest in H-alpha. It is suggested that this is illuminated by light scattered from R CrA in the direction of the receding CO flow. The emission-line star 1-100 observed by Hartigan and Graham has a variable spectrum. There is a forbidden S II knot 17 arc sec W, 9 arcsec S of this star. A faint 19th magnitude emission-line star is identified. A change in the morphology of the HH 100 reflection nebula between 1973 and 1983 is noted.

Graham, J. A.↗
NASA NTRS1981

Chromospheres and coronae in binary systems

Recent observations and spectral analysis of chromospheres and coronae are discussed for various types of binary systems containing cool stars. These include the W UMa-type systems, the RS CVn variables, and selected widely separated binary systems such as VV Cephei and the giant stars in the Hyades cluster. In many cases evidence for extreme examples of solar activity are found. Radiative losses in ultraviolet resonance lines can be at least one or two orders of magnitude larger than those found in the quiet sun. In lambda And, a RS CVn type system, there is evidence for inhomogeneous structure in the chromosphere and transition region. Rotation appears to be a determinant in the energy losses of these atmospheres although not the only one.

Dupree, A. K.↗
NASA NTRS1993

Cataclysmic variables and related objects

This volume begins with an introductory chapter on general properties of cataclysmic variables. Chapters 2 through 5 of Part 1 are devoted to observations and interpretation of dwarf novae and nova-like stars. Chapters 6 through 10, Part 2, discuss the general observational properties of classical and recurrent novae, the theoretical models, and the characteristics and models for some well observed classical novae and recurrent novae. Chapters 11 through 14 of Part 3 are devoted to an overview of the observations of symbiotic stars, to a description of the various models proposed for explaining the symbiotic phenomenon, and to a discussion of a few selected objects, respectively. Chapter 15 briefly examines the many unsolved problems posed by the observations of the different classes of cataclysmic variables and symbiotic stars.

Hack, Margherita↗
NASA NTRS2017

X-Ray Emissions from Accreting White Dwarfs: A Review

Interacting binaries in which a white dwarf accretes material from a companion-cataclysmic variables (CVs) in which the mass donor is a Roche-lobe filling star on or near the main sequence, and symbiotic stars in which the mass donor is a late type giant-are relatively commonplace. They display a wide range of behaviors in the optical, X-rays, and other wavelengths, which still often baffle observers and theorists alike. Here I review the existing body of research on X-ray emissions from these objects for the benefits of both experts and newcomers to the field. I provide introductions to the past and current X-ray observatories, the types of known X-ray emissions from these objects, and the data analysis techniques relevant to this field. I then summarize of our knowledge regarding the X-ray emissions from magnetic CVs, non-magnetic CVs and symbiotic stars, and novae in eruption. I also discuss space density and the X-ray luminosity functions of these binaries and their contribution to the integrated X-ray emission from the Galaxy. I then discuss open questions and future prospects.

cataclysmic variables – stars: dwarf novae –↗
NASA NTRS1982

Binary stars: Mass transfer and chemical composition

It is noted that mass exchange (and mass loss) within a binary system should produce observable changes in the surface chemical composition of both the mass losing and mass gaining stars as a stellar interior exposed to nucleosyntheses is uncovered. Three topics relating mass exchange and/or mass loss to nucleosynthesis are sketched: the chemical composition of Algol systems; the accretion disk of a cataclysmic variable fed by mass from a dwarf secondary star; and the hypothesis that classical Ba II giants result from mass transfer from a more evolved companion now present as a white dwarf.

Lambert, D. L.↗
NASA NTRS1985

Time-resolved UV spectroscopy of BE Ursae Majoris

Far-ultraviolet spectra have been obtained of the eclipsing, hot subdwarf binary, BE UMa, with phase coverage in the 2.3-day orbital period concentrated near minimum light. No clearly phase-related spectra changes were found, but the UV continuum appeared to be in a highly variable state, with changes of a factor of two occurring on a time scale of hours. The hot star in this system may be one of the few known PG1159-035 variables, which show nonradial pulsations. If so, this is the first such star known in a binary system and thus should be important for determining the physical properties of these stars.

Hutchings, J. B.↗