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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Quasiperiodicity in cataclysmic variable stars caused by solar-type magnetic cycles

Cyclical variations of orbital periods, quiescent magnitudes and outburst intervals in the activity of cataclysmic variable binary stars are inter-related and are ascribed to variations in radii of the secondaries, caused by solar-type (sunspot) magnetic cycles. In the nova remnant DQ Herculis the observed variations in orbital period and quiescent magnitude are consistent with this mechanism. But accretion onto the white dwarf, from an accretion disk acquired from its companion, cannot explain the observed variation of the 71-second oscillations.

Warner, Brian↗
NASA NTRS1991

The variable stars of NGC 1866

A search has been conducted for new variables in the LMC cluster NGC 1866 using new multiepoch CCD photometry. Eight previously unknown Cepheid variables, most near the cluster core, are found. Of the new variables reported by Storm et al. (188), only six of 10 appear to be Cepheids and one of these is not a member. Periods and mean magnitudes and colors for sufficiently uncrowded variables are reported, as is one red giant variable of long period and one Cepheid which is a single-lined spectroscopic binary with a velocity semiamplitude greater than or equal to 10.5 km/s. The variation of light-curve amplitude with position in the instability strip is reported along with an apparently nonvariable star, which is a radial velocity member, in the strip. A true distance modulus of 18.57 + or - 0.01 mag is obtained for the cluster.

Welch, Douglas L.↗
NASA NTRS1984

X-ray observations of a large sample of cataclysmic variable stars using the Einstein Observatory

This paper presents the results of an X-ray survey of 31 known or suspected cataclysmic variables. Eighteen of these close binary systems are detected with inferred luminosities in the 0.1-4.0 keV band of between 10 to the 30th and 10 to the 32nd erg/sec. The majority have relatively hard X-ray spectra (kT greater than 2 keV) irrespective of luminosity state. Of seven dwarf novae observed during optical outbursts only U Gem exhibited enhanced ultrasoft X-ray emission (kT of about 10 eV) in addition to weak, hard X-ray emission. Variability of the X-ray flux is observed in many of these stars, on time-scales ranging from tens of seconds to hours. The contribution to the flux from extended X-ray emission is investigated for SU UMa and GK Per. Several possibilities for the origin of the hard X-rays are considered.

Cordova, F. A.↗
NASA NTRS2001

Characteristics of the Variable Star P Cygni Determined from Cluster Membership

Empirical information on the luminosity, reddening, age, and mass of the variable B2 Oe supergiant P Cygni is derived from its assumed membership in the sparse anonymous cluster on which it is projected, as well as its association with the spatially adjacent cluster IC 4996, which forms a double cluster with the P Cyg cluster. Evidence for the high luminosity of P Cyg is confirmed by its derived absolute magnitude of M(sub V)= -8.46 +/- 0.03, which translates to log (L/L(sun)) = 5.54 +/- 0.02 for an effective temperature consistent with the star's derived space reddening (E(sub B-V) = 0.53 +/- 0.02). More surprising is an age for the associated clusters of 6 (+/- 1.5) x 10(exp 6) years, corresponding to a turnoff point mass of 25.1 (+/- 5.5) M(sun). By inference, P Cygni, as a post main-sequence object, should have a mass of no more than approximately 23-35 M(sun).

Turner, David G.↗
NASA NTRS1981

54 Cam - A new variable star

Differential photometry of the RS CVn-type binary 54 Cam is presented, which shows that the light was variable with a period of 10.163 + or - 0.009 d, and with an amplitude increase from 0.03 to 0.06 m between 1979 and 1980. The photometric period of 10.163 d is 9% shorter than the orbital period of 11.0764 d, and is suggested as an explanation for the radio emission from 54 Cam due to a process of connection, disruption, and reconnection of magnetic field lines.

Hall, D. S.↗
NASA NTRS2012

Sonification of Kepler Field SU UMa Cataclysmic Variable Stars V344 Lyr and V1504 Cyg

Sonification is the conversion of quantitative data into sound. In this work we explain the methods used in the sonification of light curves provided by the Kepler instrument from Q2 through Q6 for the cataclysmic variable systems V344 Lyr and V1504 Cyg . Both systems are SU UMa stars showing dwarf nova outbursts and superoutbursts as well as positive and negative superhumps. Focused sonifications were done from average pulse shapes of each superhump, and separate sonifications of the full, residual light curves were done for both stars. The audio of these data reflected distinct patterns within the evolutions of supercycles and superhumps that matched pervious observations and proved to be effective aids in data analysis.

Tutchton, Roxanne M.↗
NASA NTRS2006

Image-Subtraction Photometry of Variable Stars in the Globular Clusters NGC 6388 and NGC 6441

We have applied Alard's image subtraction method (ISIS v2.1) to the observations of the globular clusters NGC 6388 and NGC 6441 previously analyzed using standard photometric techniques (DAOPHOT, ALLFRAME). In this reanalysis of observations obtained at CTIO, besides recovering the variables previously detected on the basis of our ground-based images, we have also been able to recover most of the RR Lyrae variables previously detected only in the analysis of Hubble Space Telescope WFPC2 observations of the inner region of NGC 6441. In addition, we report five possible new variables not found in the analysis of the EST observations of NGC 6441. This dramatically illustrates the capabilities of image subtraction techniques applied to ground-based data to recover variables in extremely crowded fields. We have also detected twelve new variables and six possible variables in NGC 6388 not found in our previous groundbased studies. Revised mean periods for RRab stars in NGC 6388 and NGC 6441 are 0.676 day and 0.756 day, respectively. These values are among the largest known for any galactic globular cluster. Additional probable type II Cepheids were identified in NGC 6388, confirming its status as a metal-rich globular cluster rich in Cepheids.

Corwin, Michael T.↗
NASA NTRS1994

The Sun as a Variable Star: Solar and Stellar Irradiance Variations; Colloquium of the International Astronomical Union, 143rd, Boulder, CO, Jun. 20-25, 1993

Variations in solar and stellar irradiances have long been of interest. An International Astronomical Union (IAU) colloquium reviewed such relevant subjects as observations, theoretical interpretations, and empirical and physical models, with a special emphasis on climatic impact of solar irradiance variability. Specific topics discussed included: (1) General Reviews on Observations of Solar and Stellar Irradiance Variability; (2) Observational Programs for Solar and Stellar Irradiance Variability; (3) Variability of Solar and Stellar Irradiance Related to the Network, Active Regions (Sunspots and Plages), and Large-Scale Magnetic Structures; (4) Empirical Models of Solar Total and Spectral Irradiance Variability; (5) Solar and Stellar Oscillations, Irradiance Variations and their Interpretations; and (6) The Response of the Earth's Atmosphere to Solar Irradiance Variations and Sun-Climate Connections.

Pap, Judit M.↗
NASA NTRS1975

Combined optical and X-ray observations of variable stars

Questions concerning the optical identification of X-ray sources are considered. There are now a total of eight optically identified galactic X-ray sources. Of these eight, five are definitely established as binaries. The nature of the other three sources remains unknown. Studies of U Geminorum conducted on the basis of optical and X-ray observations are also discussed. From the upper limit to the accretion rate for U Gem obtained with the aid of soft X-ray data, it is seen that most of the mass flow in U Gem is lost from the system.

Bowyer, C. S.↗
NASA NTRS1979

The HEAO-A2 soft X-ray survey of cataclysmic variable stars - EX Hydrae during optical quiescence

Results are reported for HEAO A2 soft X-ray (below 2 keV) scanning observations of the southern dwarf nova EX Hya. An X-ray light curve is presented which shows no apparent orbital modulation. The best-fitting spectral parameters are derived for the source, and the observations are compared with the spectral behavior of the dwarf nova SS Cyg during optical quiescence. The results are discussed in terms of models for X-ray production by accreting white dwarfs.

Cordova, F. A.↗
NASA NTRS1991

Spectral transformation of the unusual variable star MWC560 to resemble a nova

A dramatic change has occurred in the ultraviolet spectrum of the emission-line star MWC560, so that it now closely resembles the spectrum of a nova shortly after outburst. This event may signal a major mass-ejection episode such as presumably occurred in past centuries in the symbiotic star R Aquarii to produce the well-known bipolar nebula, and it may herald the emergence of a standard symbiotic-star emission-line spectrum in MWC560, corresponding to a change in evolutionary state.

Maran, Stephen P.↗
NASA NTRS1990

Radiative transfer in the dynamic atmospheres of long period variable stars

An iterative procedure is presented for determining the thermal structure and dynamics of Mira-type stellar atmospheres, where the non-LTE radiative transfer code PANDORA is used in conjunction with the Bowen hydrodynamics code of Iowa State University. Preliminary results are reported for an atmospheric model of a pulsating AGB star of 1 solar mass, 240 solar radii, Teff = 3000 K, and a period of 320 days. At the present time, H, H(-), Mg I, and Mg II radiative transfer calculations have been completed and synthetic spectra are shown for H-alpha. The radiative transfer calculations demonstrate that cooling in the innermost shock of the original Bowen model is underestimated due to the omission of various hydrogen transitions. These initial results suggest that the main shock of the Bowen models are too hot and/or too deep.

Luttermoser, Donald G.↗
NASA NTRS1992

Computation of ephemerides for long-period variable stars for the Hipparcos mission

In order to predict the luminosities of LPVs observed by Hipparcos, many visual light curves, characterized by irregularly spaced data, were analyzed using original and complementary methods. The obtained power spectra, including amplitudes and phases, are utilized in the computation of the ephemerides. The predictions are regularly checked with observations. In this paper we present the methods and suggest some physical implications that are evoked.

Mennessier, M. O.↗
NASA NTRS1997

The Chromosphere/Shock Dilemma of Non-Mira, Late-Type Variable Stars

An investigation of the atmospheric structure of non-Mira, asymptotic giant branch stars through NLTE radiative transfer modeling applied to hydrodynamic models is discussed. Synthetic spectra resulting from these calculations were compared with IUE observations of these stars to test the validity of the models. The development of the hydrodynamic models is detailed.

Willson, Lee Anne↗
NASA NTRS2023

Understanding Sun-as-a-Star Variability of Solar Balmer Lines

Precise, high-cadence, long-term records of stellar spectral variability at different temporal scales lead to better understanding of a wide variety of phenomena including stellar atmospheres and dynamos, convective motions, and rotational periods.

solar activity, solar cycle↗