Robert A. Stern

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.

Evaluation of a virtual phase charged‐coupled device as an imaging x‐ray spectrometer

The x‐ray response of an 800×800 Texas Instruments virtual phase charge‐coupled device (CCD) has been measured in the range 1–8 keV. In the single‐photon counting mode, we find excellent energy resolution (∼250 eV FWHM) for single‐pixel Fe55 x‐ray events at a spatial resolution of 15 μm. The detector quantum efficiency for all events is 65% at 2.3 keV (S K line) and ∼34% at 5.9 keV (Mn K line from Fe55). The CCD response is linear in energy to a few percent over the 1–8 keV energy range. These results demonstrate that virtual phase CCDs are superior imaging x‐ray spectrometers with applications for x‐ray astronomy and laboratory plasma research.

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.

Einstein observations of cool stars

Abstract 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 will be given. The Hyades cluster, in particular, will serve as an example to demonstrate the usefulness of X-ray observations in the study of stellar activity and coronal evolution.

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.

X-ray activity in stellar clusters

Abstract Stellar cluster observations play a key role in understanding the evolution of coronal activity with rotation and time. Although the general picture of a decline in coronal X-ray emission from young to old clustess is well-established, many questions remain. These include the apparent absence of cyclic X-ray variability in active solar-type stars, the Hyades-Coma-Praesepe puzzle, and the recent evidence for “supersaturation” in very young clusters. I will review these issues in the context of both stellar and relevant solar observations.

Stellar X-Ray Activity in the Hyades

Observations of the Hyades cluster with the Einstein Observatory and with IUE have uncovered a high level of coronal x-ray emission (LX≃1029 era s-1 for solar-type stars) and similarly high fluxes of chromospheric and transition region line fluxes compared to the Sun (Stern et al.1981, Zolcinski et al. 1981, 1982). A giant x-ray flare from a spectroscopic binary system in the Hyades has also been reported (Stern, Antiochos and Underwood 1982).