StarsNormal, i.e, non X-ray binary, stars are typically fairly cool (Te ~106-7K) and relatively faint log Lx ~1026-32 erg s-1) X-ray sources. Their X-ray spectra appear to be due to fairly complex, multi-temperature thermal plasmas, some of whose specific properties, such as the peculiar non-cosmic elemental abundances that are sometimes inferred, are still not well understood. X-ray emission is also intrinsically quite variable for most stellar classes, with observed time-scales from minutes or less to as long as years, and observed amplitudes of orders of magnitude in some cases, e.g., dMe flare and pre-main sequence stars. Because of these interesting properties, the soft X-ray missions in the 1990s such as ROSAT and ASCA have typically dedicated between 10% and 20% of their observing programs to stellar sources, and thus a substantial number of targeted observations are available in the HEASARC archive. Moreover, because of their ubiquity and proximity, stars comprise a fair fraction (e.g., about one-third of the ROSAT All-Sky Survey discrete sources) of the serendipitous sources detected in imaging X-ray observations by ROSAT and ASCA. Thus, the HEASARC archive is a rich resource that can be exploited for the study of the high-energy properties of literally thousands of stars of various types. We cite a few sample studies which have relied in whole or in large part on such archival data, to give a flavor for the type of research that can be pursued.
Late-type starsLate-type, coronal stars comprise the vast majority of stellar X-ray sources, with active binary stars and pre-main sequence stars such as T Tauri stars and protostars the most luminous (Lx ~1030-32 erg s-1) and M dwarf stars the most numerous. Only with the advent of ROSAT has there been the capability to detect a significant number of stars with coronae similar to the solar corona in luminosity (Lx ~1026-28 erg s-1). Out of the vast literature on coronal stars, some examples that have made use of a large number of archival datasets and/or multi-mission datasets include the work of: 1) Osten and Brown (1999, ApJ, 515, 746) who studied over 30 EUVE observations of the RS CVn class of active binaries and discovered that these systems are flaring at least one-third of the time, 2) Singh et al. (1996, AJ 111, 2415) who searched the ROSAT archive for observations of detached (RS CVn) and semi-detached (Algol) binaries and concluded that, despite previous claims to the contrary, the Algol systems were actually 3--4 times less X-ray luminous than the RS CVns, and 3) Marino et al. (2000, A&A 353, 177) who searched the ROSAT archive for pointed observations of M dwarf stars in order to characterize the time-scales and amplitudes of variability of their coronal emission.
Massive starsOur understanding of the X-ray properties of the O and early B stars with massive stellar winds has benefited greatly from the availability of the archived ROSAT and ASCA observations. Berghofer et al. (1997, A&A 322, 167), for example, have searched the ROSAT All-Sky Survey data for known positions of 1838 OB stars and found 237 X-ray sources, further noting that an important difference between these stars and the lower-mass, coronal-type stars is that the X-ray emission of the former is remarkably steady, with detectable variability being very rare.
Massive BinariesA small number of anomalously hard and bright X-ray sources have been identified with early-type stars and, in many cases, detailed studies using archival data have shown that the X-ray emission is probably produced by wind-wind collisions in binary systems (for example
Without the uniform datasets and calibrations available through the HEASARC, such work utilizing data from multiple missions would have been much more difficult, if not impossible. Many of the X-ray point sources associated with stars in the HEASARC archive still have not been studied in a systematic way, but it is quite possible that "mining the archives" for the various classes in the stellar "zoo" will revolutionize our understanding of the X-ray emission processes of both high- and low-mass stars, of coronally active stars, of the activity cycles in solar-type and active single and binary stars, and of the star-formation process. Comparison of these archival data with newer data being obtained by Chandra and XMM-Newton will allow a better characterization of the variability time-scales and source properties. The HEASARC archive will be a "gold-mine" for normal star research for many years to come.
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