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At least 91 records · Page 5
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NASA NTRS2018

VLTI-GRAVITY Measurements of Cool Evolved Stars I. Variable Photosphere and Extended Atmosphere of the Mira Star R Peg

Context. Dynamic model atmospheres of Mira stars predict variabilities in the photospheric radius and in atmospheric molecular layers which are not yet strongly constrained by observations. Aim. Here we measure the variability of the oxygen-rich Mira star R Peg in near-continuum and molecular bands. Methods. We used near-infrared K-band spectro-interferometry with a spectral resolution of about 4000 obtained at four epochs between post-maximum and minimum visual phases employing the newly available GRAVITY beam combiner at the Very Large Telescope Interferometer (VLTI). Results. Our observations show a continuum radius that is anti-correlated with the visual lightcurve. Uniform disc (UD) angular diameters at a near-continuum wavelength of 2.24 m are steadily increasing with values of 8.7 0.1 mas, 9.4 0.1 mas, 9.8 0.1 mas, and 9.9 0.1 mas at visual phases of 0.15, 0.36, 0.45, 0.53, respectively. UD diameters at a bandpass around 2.05 m, dominated by water vapour, follow the near-continuum variability at larger UD diameters between 10.7 mas and 11.7 mas. UD diameters at the CO 2-0 bandhead, instead, are correlated with the visual lightcurve and anti-correlated with the near-continuum UD diameters, with values between 12.3 mas and 11.7 mas. Conclusions. The observed anti-correlation between continuum radius and visual lightcurve is consistent with an earlier study of the oxygen-rich Mira S Lac, and with recent 1D CODEX dynamic model atmosphere predictions. The amplitude of the variation is comparable to the earlier observations of S Lac, and smaller than predicted by CODEX models. The wavelength-dependent visibility variations at our epochs can be reproduced by a set of CODEX models at model phases between 0.3 and 0.6. The anti-correlation of water vapour and CO contributions at our epochs suggests that these molecules undergo different processes in the extended atmosphere along the stellar cycle. The newly available GRAVITY instrument is suited to conducting longer time series observations, which are needed to provide strong constraints on the model-predicted intra- and inter-cycle variability.

photospheric radius↗
NASA NTRS1993

Overview of the observations of symbiotic stars

The term Symbiotic stars commonly denotes variable stars whose optical spectra simultaneously present a cool absorption spectrum (typically TiO absorption bands) and emission lines of high ionization energy. This term is now used for the category of variable stars with composite spectrum. The main spectral features of these objects are: (1) the presence of the red continuum typical of a cool star, (2) the rich emission line spectrum, and (3) the UV excess, frequently with the Balmer continuum in emission. In addition to the peculiar spectrum, the very irregular photometric and spectroscopic variability is the major feature of the symbiotic stars. Moreover, the light curve is basic to identify the different phases of activity in a symbiotic star. The physical mechanisms that cause the symbiotic phenomenon and its variety are the focus of this paper. An astronomical phenomenon characterized by a composite stellar spectrum with two apparently conflicting features, and large variability has been observed. Our research set out to find the origin of this behavior and, in particular, to identify and measure the physical mechanism(s) responsible for the observed phenomena.

Viotti, Roberto↗
NASA NTRS1977

A comparison of lyman alpha and HeI lambda 10830 line structure and variations in early-type star atmospheres

Fabry-Perot interferometric profiles for fifty of the early-type stars including supergiants, eclipsing binaries, Bp and Ap stars, Be and shell stars, and variable stars have been obtained. Results for beta Persei (Algol) just before primary and secondary eclipses show strong emission profiles lasting about 0.1 phase. An absorption line was seen during secondary eclipse. Bright supergiant stars (O9-A2) show time-variable, complicated absorption/emission profiles similar to those obtained for the Be/shell stars.

Meisel, D. D.↗
NASA NTRS1980

The line profile variable B stars

The observational status of the line profile variable B stars is summarized. The following areas are discussed: recent history; evolutionary status; line profile variations; line strength variations; and photometric variations.

Smith, M. A.↗
NASA NTRS2011

Variability of Disk Emission in Pre-Main Sequence and Related Stars. II. Variability in the Gas and Dust Emission of the Herbig Fe Star SAO 206462

We present thirteen epochs of near-infrared (0.8-5 microns) spectroscopic observations of the pre-transitional, "gapped" disk system in SAO 206462 (=HD 135344B). In all, six gas emission lines (Br(alpha) , Br(gamma), Pa(beta), Pa(delta), Pa(epsilon), and the 0.8446 microns line of O I) along with continuum measurements made near the standard J, H, K, and L photometric bands were measured. A mass accretion rate of approximately 2 x 10(exp 8)Solar Mass/yr was derived from the Br(gamma) and Pa(beta) lines. However, the fluxes of these lines varied by a factor of over two during the course of a few months. The continuum also varied, but by only approx.30%, and even decreased at a time when the gas emission was increasing. The H I line at 1.083 microns was also found to vary in a manner inconsistent with that of either the hydrogen lines or the dust. Both the gas and dust variabilities indicate significant changes in the region of the inner gas and the inner dust belt that may be common to many young disk systems. If planets are responsible for defining the inner edge of the gap, they could interact with the material on time scales commensurate with what is observed for the variations in the dust, while other disk instabilities (thermal, magneto-rotational) would operate there on longer time scales than we observe for the inner dust belt. For SAO 206462, the orbital period would likely be 1-3 years. If the changes are being induced in the disk material closer to the star than the gap, a variety of mechanisms (disk instabilities, interactions via planets) might be responsible for the changes seen. The He I feature is most likely due to a wind whose orientation changes with respect to the observer on time scales of a day or less. To further constrain the origin of the gas and dust emission will require multiple spectroscopic and interferometric observations on both shorter and longer time scales that have been sampled so far.

Sitko, Michael L.↗
NASA NTRS1973

Photometric systems

Stroemgren's classification of photometric systems is used to survey wideband, medium, and narrow-band systems for studying variable stars. Multicolor photometry of variable stars is used to obtain curves of brightness change and to study the physical properties of the variables as they change during alteration of brightness.

Straizius, V. L.↗
NASA NTRS1990

RR Lyrae stars in local group galaxies. II - NGC 147

Deep CCD images of NGC 147 taken with the '4-shooter' on the Hale 5 m telescope have been processed to find and photometrically measure RR Lyrae stars. 36 variable stars have been found, of which 32 are surmised to be RR Lyrae stars. Finding charts, periods, and light curves are presented. The mean magnitude of the RR Lyraes is determined to be 25.25 mag. A distance modulus 23.92 is derived, based on the best currently available values of foreground extinction and mean absolute magnitudes of RR Lyrae stars. A wide range of periods is seen for the RR Lyrae stars, indicating a correspondingly wide range of metallicities for the stars in NGC 147. The distance modulus derived here places NGC 147 at a distance of 154 kpc from the center of M31, and in conjunction with the line sight velocities of these two galaxies, this implies a lower limit of 7.2 x 10 to the 11th solar masses for the mass of M31.

Saha, A.↗
NASA NTRS1990

RR Lyrae stars in local group galaxies. I - NGC 185

Deep CCD images of NGC 185 taken with the '4-shooter' on the Hale 5-m telescope have been processed to find and photometrically measure RR Lyrae stars. 176 variable stars have been found, of which 151 are surmised to be RR Lyrae stars. Finding charts, periods, and light curves are presented. The RR Lyrae stars in this galaxy have a very wide distribution of periods indicating a wide range of metallicity. The mean magnitudes of the RR Lyraes is determined to be 25.20 mag. A distance modulus of 23.79 is derived, based on the best currently available values of foreground extinction and mean absolute magnitudes of RR Lyrae stars.

Saha, A.↗
NASA NTRS2009

The Kepler Project: Mission Update

Kepler is a Discovery-class mission designed to determine the frequency of Earth-size planets in and near the habitable zone of solar-like stars. The instrument consists of a 0.95 m aperture photometer designed to obtain high precision photometric measurement of > 100,000 stars to search for patterns of transits. The focal plane of the Schmidt-telescope contains 42 CCDs with at total of 95 mega pixels that cover 116 square degrees of sky. The photometer was launched into an Earth-trailing heliocentric orbit on March 6, 2009, finished its commissioning on May 12, and is now in the science operations mode. During the commissioning of the Kepler photometer, data were obtained at a 30 minute cadence for 53,000 stars for 9.7 days. Although the data have not yet been corrected for the presence of systematic errors and artifacts, the data show the presence of hundreds of eclipsing binary stars and variable stars of amazing variety. To provide some estimate of the capability of the photometer, a quick analysis of the photometric precision was made. Analysis of the commissioning data also show transits, occultations and light emitted from the known exoplanet HAT-P7b. The data show a smooth rise and fall of light: from the planet as it orbits its star, punctuated by a drop of 130 +/- 11 ppm in flux when the planet passes behind its star. We interpret this as the phase variation of the dayside thermal emission plus reflected light from the planet as it orbits its star and is occulted. The depth of the occultation is similar in amplitude to that expected from a transiting Earth-size planet and demonstrates that the Mission has the precision necessary to detect such planets.

Borucki, William J.↗
NASA NTRS1994

A general catalogue of close binary systems

A comprehensive catalog of close binary stars to be used for the study of the fundamental properties of stars and for the exploration and elucidation of evolutionary processes in those systems, is presented. Spectroscopic binaries, variable stars, suspected variable stars, and interacting binaries are included in the scope of the catalog.

Webbink, Ronald F.↗
NASA NTRS1993

One-milliarsecond precision parallax studies in the regions of Delta Cephei and EV Lacertae

Trigonometric parallaxes for stars in the regions of the variable stars delta Cephei and EV Lacertae are derived from data collected with the Multichannel Astrometric Photometer (MAP) and the Thaw Refractor of the University of Pittsburgh's Allegheny Observatory. The weighted mean parallax of all trigonometric studies of delta Cephei is now + 0.0030 sec + or - 0.00093 sec, corresponding to a distance modulus of 7.61 + or - 0.67 mag. This indicates that this luminosity standard star is approximately one standard deviation more distance than has been generally accepted. The weighted mean trigonometric parallax of all studies of the variable star EV Lacertae (BD + 43 deg 4305) is + 0.1993 sec + or - 0.00093 sec, implying a distance modulus of - 1.498 + or - 0.0010 mag. The calculated absolute magnitude of this star is almost exactly that predicted by its (R-I)(sub Kron) magnitude and by the Gliese (R-I) main-sequence value for stars in the solar neighborhood. We also find a parallax of 0.0189 sec + or - 0.0008 sec for the FO IVn star, HR 8666 (BD + 43 sec 4300). The derived luminosity of this star is midway between that expected for luminosity class IV and V stars at the indicated temperature.

Gatewood, George↗
NASA NTRS1980

Velocity fields and spectrum peculiarities in Beta Cephei stars

The acquisition of short wavelength spectra of Beta Cephei variable stars from the International Ultraviolet Explorer is reported. A total of 122 images of 10 variable stars and 3 comparison stars were obtained. All of the images were observed in the high dispersion mode through a small aperture. The development of image processing methods is also briefly discussed.

Lesh, J. R.↗