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Stern, R. A.

Publications and source records attributed to Stern, R. A..

28 records · Page 2
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NASA NTRS1983

A giant X-ray flare in the Hyades

The Einstein Observatory has observed a giant stellar flare in the Hyades binary HD 27130 whose peak X-ray luminosity is greater than 10 to the 31st ergs/sec. This is several thousand times brighter than the most intense solar flares. HD 27130, first detected as an X-ray source in the central Hyades survey of Stern et al (1981), has recently been determined to be a double-lined eclipsing binary with a 5.6-d period. The primary and secondary are G and K dwarfs, respectively. The estimated 40 million K temperature of the flare decay suggests that it is solar-like, but its deduced size scales are much larger than those observed for typical solar flares. It is suggested that giant flares may be typical of young or rapidly rotating systems.

Stern, R. A.↗
NASA NTRS1983

Stellar X-ray activity in the Hyades

Preliminary results from monitoring the X-ray emission of about 20 Hyades stars both on short and long time scales are reported. All observations were made with the Imaging Proportional Counter of the Einstein Observatory. The preliminary conclusions are that flaring behavior is probably common in the Hyades, but that the influence of binarity on the level of flaring activity needs to be investigated further. The long-term observations suggest that intrinsic differences in the level of stellar X-ray luminosity may be required to account for the spread in the X-ray luminosity function for solar-type stars observed in the original survey by Stern et al. (1981).

Stern, R. A.↗
NASA NTRS1982

International ultraviolet explorer observations of Hyades stars

A description is presented of International Ultraviolet Explorer (IUE) satellite observations of transition region and chromospheric emission from a group of Hyades dwarfs which are strong X-ray emitters as seen in a survey conducted by Stern et al. (1981). Short-wavelength spectra (1175-2000 A) and long-wavelength spectra (1900-3200 A) have been obtained. Although the IUE sensitivity limit did not make it possible to detect emission lines in three stars, the presence of chromospheres and transition regions could be confirmed in BD +15 deg 640, 70 Tau, BD +14 deg 693, and BD +16 deg 592. The differential emission measure has been plotted as a function of temperature for the four considered stars.

Zolcinski, M.-C. S.↗
NASA NTRS1982

Einstein observations of cool stars

Observations of cool stars with the Einstein Observatory (HEAO-2) have brought about a fundamental change in our knowledge and understanding of stellar coronae. The existence of X-ray emission from stars throughout the H-R diagram, the wide range of X-ray luminosity within a given spectral and luminosity class, and the strong correlation of X-ray luminosity with stellar age and rotation are among the more significant Einstein results. These results are strong evidence for the influence of stellar dynamo action on the formation and heating of stellar coronae. A discussion of relevant consortium and guest observations is given. The Hyades cluster, in particular, serves as an example to demonstrate the usefulness of X-ray observations in the study of stellar activity and coronal evolution.

Stern, R. A.↗
NASA NTRS1981

X-ray and optical observations of HD 155638 - A remarkably active cool star

X-ray and optical observations of the 8.5-mag star HD 155638 are reported which reveal a remarkably active chromosphere and corona for the star. The object was first observed in soft X-rays with the HEAO 1 A2 low-energy detectors, and later reobserved with the HEAO 2 IPC. X-ray fluxes of about 2 x 10 to the -11th and 3 x 10 to the -12th ergs/sq cm per sec were obtained on the two occasions, indicative of source variability over the 2.5-year period between observations. SIT spectrographs taken in July-August 1980 show evidence of Ca II H and K core emission reversals and filled-in H alpha, while spectra taken in March and July 1981 show evidence of asymmetry and variability in the Ca II reversals. The spectral data are consistent with a G8 IV-V classification, from which is inferred a distance of 35-120 pc, and an X-ray luminosity from 4 x 10 to the 29th to 4 x 10 to the 31st ergs/sec. An absence of emission lines characteristic of mass transfer in binary systems indicates the X-rays to be coronal in origin. The properties of the star are noted to be similar to those of the RS CVn binaries.

Stern, R. A.↗
NASA NTRS1981

Stellar coronae in the Hyades - A soft X-ray survey with the Einstein Observatory

An X-ray survey of the central region of the Hyades cluster demonstrates that soft X-ray emission is a common property of the stars in the cluster. Half of the 85 stars surveyed are detected above a sensitivity threshold of 10 to the 28.5th ergs/s at the Hyades distance of 45 pc. The high incidence of X-ray emission and range of observed X-ray luminosities indicate that stellar coronas produce the observed X-ray emission, with a typical X-ray luminosity for solar-type Hyades of 10 to the 29th ergs/s. The use of coronal scaling laws is found to yield reasonable values of maximum coronal temperatures and the fraction of stellar surface covered for the Hyades coronas.

Stern, R. A.↗
NASA NTRS1981

The very soft X-ray transient H1853 + 60

The detection of a new soft X-ray transient, H1853 + 60, using data from the Low Energy Detectors (LEDs) on the HEAO 1 A-2 experiment, is reported. Although the solar flare hypothesis cannot be ruled out, the similarity between this X-ray transient's temporal and spectral characteristics and those of several other transients observed by HEAO 1 LEDs, suggest a common emission mechanism for these events. It is unlikely that more transients as strong as these will be discovered, except on an individual scan basis.

Stern, R. A.↗
NASA NTRS1981

Results from the central Hyades survey

Results of a soft X-ray survey of the central 5 deg of the Hyades star cluster made with the Einstein Observatory Imaging Proportional Counter are presented. Virtually all of the late F and early G stars in the cluster were detected at an X-ray luminosity of greater than 10 to the 28.5 erg/sec, although only about 50% of the stars in the cluster as a whole were detected. Plots of X-ray luminosity against B-V index indicate that the typical solar-type star in the Hyades is emitting soft X-rays at a level approximately 30 times that of the active sun. Histograms of the X-ray to bolometric luminosity ratio reveal a gradual distinction between late A-early F stars, which are expected to possess little or no convective envelope, and solar-type (F8-G5) stars, with convective outer atmospheres. Results confirm the dependence of stellar coronal activity on rotation, and establish the prevalence of stellar coronae throughout the main sequence and the giant regions of the H-R diagram.

Stern, R. A.↗
NASA NTRS1980

An extended soft X-ray source in Delphinus - H2027+19

A new extended soft X-ray source has been observed with the HEAO 1 A-2 experiment. The source, H2027+19, emits primarily in the 0.16-0.4 keV band with a total flux in this band of 2 x 10 to the -11th erg/sq cm s. It is found that both simple continuum and coronal plasma models provide good fits to the observed pulse-height spectrum. The most likely physical models are either that the source is an old supernova remnant or that it is a region of enhanced soft X-ray emission surrounding an H I cloud imbedded in a coronal plasma, as suggested by Hayakawa et al. (1979) for the Lupus Loop.

Stern, R. A.↗