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Stars
Normal, 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 stars
Late-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 stars
Our 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 Binaries
A 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
Car, Corcoran
et al. 1995, ApJ 445, L121, and WR 140, Zhekov & Skinner, 2000, ApJ in
press). Our knowledge of
Car, perhaps the most massive star known, has benefited
particularly from studies using data from the HEASARC archive; these data
have helped reshape the previous conception of this star as a peculiar,
perhaps unique, single star to a new and more familiar (albeit rather
extreme) example of an interacting binary system. Ongoing research efforts
include a uniform reanalysis (using the same energy bands and background
subtractions) of all the archived X-ray data for Car to reconstruct its X-ray
variability cycle back into the pre-ROSAT era of the 1980s.
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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Last modified: Monday, 19-Jun-2006 11:24:57 EDT
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