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2003ApJ...595.1206S - Astrophys. J., 595, 1206-1221 (2003/October-1)
ROSAT view of Hipparcos F stars.
SUCHKOV A.A., MAKAROV V.V. and VOGES W.
Abstract (from CDS):
The group of strongest X-ray emitters, logLX>30.4, which constitutes only a small fraction of the sample stars, ∼2.8%, has been found to include a variety of stellar types. It is dominated by very young objects, followed by RS CVn-type stars and other active binaries. As to the sample as a whole, it seems to consist of at least three distinct groups of F stars that differ in their X-ray luminosity and evolution of their X-ray emission. The first one is represented by normal zero-age main-sequence (ZAMS) and post-ZAMS stars whose X-ray emission declines with age, just as expected in the conventional picture of stellar X-ray evolution (roughly half of the sample). The second group consists of strong X-ray emitters that appear to be very young, possibly pre-main-sequence stars. These stars are characterized by high metallicity, high temperature, and high overluminosity (the latter indicating that the star's optical luminosity is too high for the star's temperature and surface gravity). Many of them are located in the distance range ∼130-200 pc, near the known regions of ongoing star formation or OB associations. The third group also consists of strong X-ray emitters. However, those are very old rather than very young stars, and they happen to be extremely overluminous as well. Their strong X-ray emission implies that many F stars are capable of increasing rather than decreasing their X-ray luminosity as they get older, thus challenging the idea that X-ray emission continuously declines with age. Most of these old X-ray emitters have significantly evolved away from the ZAMS. If they increase their outer convective zone as they evolve, they would probably be able to maintain a high level of coronal activity, and hence enhanced X-ray emission, despite decaying rotation.
Other relationships found for the ROSAT F stars include the correlation of X-ray luminosity with hardness ratio, optical absolute magnitude, effective temperature, overluminosity, metallicity, and reddening. There is also a difference between binary and single stars, with X-ray luminosity being on average slightly higher for unresolved binaries. There is a distinct difference between the groups of soft and hard X-ray emitters in our sample: while the soft sources show a significant correlation between the X-ray luminosity and hardness ratio, no such correlation is seen for hard sources. The tendency for X-ray luminosity to be higher for harder sources implies that stars more luminous in the X-rays have a hotter corona. This tendency breaks up at large hardness values, which may be due to coronal saturation.
Abstract Copyright: ∼
Journal keyword(s): Stars: Binaries: General - Stars: Activity - Stars: Evolution - Stars: Kinematics - Stars: Pre-Main-Sequence - X-Rays: Stars
VizieR on-line data: <Available at CDS (J/ApJ/595/1206): table1.dat table4.dat table5.dat>
Status at CDS : All or part of tables of objects will not be ingested in SIMBAD.
CDS comments: Table 5 : CX Cru is a misprint for CX Gru (= HIP 111718)
Simbad objects: 4
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