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Spectral and Temporal Characteristics of X-Ray-Bright Stars in the PleiadesWe follow up our deep ROSAT imaging survey of the Pleiades (Stauffer et al. 1994) with an analysis of the spectral and temporal characteristics of the X-ray-bright stars in the Pleiades. Raymond & Smith (1977) one and two-temperature models have been used to fit the position-sensitive proportional counter (PSPC) pulse-height spectra of the dozen or so brightest sources associated with late-type Pleiades members. The best-fit temperatures suggest hot coronal temperatures for K, M, and rapidly rotating G stars, and cooler temperatures for F and slowly rotating G stars. In order to probe the many less X-ray-luminous stars, we have generated composite spectra by combining net counts from all Pleiades members according to spectral type and rotational velocity. Model fits to the composite spectra confirm the trend seen in the individual spectral fits. Particularly interesting is the apparent dependence of coronal temperature on L(sub x)/L(sub bol). A hardness-ratio analysis also confirms some of these trends. The PSPC data have also revealed a dozen or so strong X-ray flares with peak X-ray luminosities in excess of approx. 10(exp 30) ergs/sec. We have modeled the brightest of these flares with a simple quasi-static cooling loop model. The peak temperature and emission measure and the inferred electron density and plasma volume suggest a very large scale flaring event. The PSPC data were collected over a period of approx. 18 months, allowing us to search for source variability on timescales ranging from less than a day (in the case of flares) to more than a year between individual exposures. On approximately year-long timescales, roughly 25% of the late-type stars are variable. Since the Pleiades was also intensively monitored by the imaging instruments on the Einstein Observatory, we have examined X-ray luminosity variations on the 10 yr timescale between Einstein and ROSAT and find that up to 40% of the late-type stars are X-ray variable. Since there is only marginal evidence for increased variability on decade-long timescales, the variability observed on long and short timescales may have a common physical origin.
Document ID
19970023940
Acquisition Source
Headquarters
Document Type
Reprint (Version printed in journal)
Authors
Gagne, Marc
(Georgia Univ. Athens, GA United States)
Caillault, Jean-Pierre
(Georgia Univ. Athens, GA United States)
Stauffer, John R.
(Harvard-Smithsonian Center for Astrophysics Cambridge, MA United States)
Date Acquired
September 6, 2013
Publication Date
September 1, 1995
Publication Information
Publication: The Astronomical Journal
Publisher: The American Astronomical Society
Volume: 450
Subject Category
Astrophysics
Report/Patent Number
NASA-CR-204870
NAS 1.26:204870
Report Number: NASA-CR-204870
Report Number: NAS 1.26:204870
Accession Number
97N24159
Funding Number(s)
CONTRACT_GRANT: NAG5-1608
CONTRACT_GRANT: NAG5-1849
CONTRACT_GRANT: NAGw-2968
Distribution Limits
Public
Copyright
Public Use Permitted.
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