Astron. Astrophys. 333, 619-628 (1998)
2. Selection of PMS candidates
2.1. X-ray selected candidates
The ROSAT X-ray satellite (see Trümper 1983) performed an All
Sky Survey (RASS) in the 0.1 - 2.4 keV soft X-ray band. The mean
limiting flux of the survey was about and more
than 60 000 X-ray sources were detected. The data of this survey
provide a spatially complete, flux-limited sample of X-ray sources and
have led to the detection of hundreds of new PMS stars in star forming
regions all over the sky (for a review see Krautter 1996 or
Neuhäuser 1997).
In Upper Sco the typical RASS exposure time was about 400 sec, the
minimum detectable source count rate was about 0.02 counts/sec. For a
distance of 145 pc and a typical extinction of
mag this corresponds to X-ray luminosities of about
erg/sec if we assume thermal plasma emission
with keV as typical for PMS stars
(cf. Montmerle 1996). In our field in Upper Sco, 606 X-ray
sources were detected in the RASS data. Of course, not all of these
X-ray sources are PMS stars: active field stars, RS CVn binaries,
galaxies and quasars build up a large fraction of the RASS sources. In
order to keep the observational effort in the search for PMS stars
within reasonable limits, a selection method for finding promising PMS
candidates in the RASS data was introduced by Sterzik et
al. (1995). This method compares several properties of each RASS
source to those of a "training-set", consisting from known PMS stars
and non-PMS stars. These properties are the hardness ratios, the
magnitude of the closest optical counterpart in the GSC within
around the X-ray source position, and the X-ray
to optical flux ratio. For each RASS source a discrimination
probability P is computed. High values of P indicate
that the source properties are similar to those within the sample of
known PMS stars in the training-set. All details of this method are
described in Sterzik et al. (1995). We used
as a cutoff value for the X-ray selected PMS
candidates. This resulted in a total of 180 PMS candidates in our
field. For observational reasons (see Sect. 3), we excluded all
stars brighter than and all stars fainter than
from this sample, reducing the sample to 130
objects. 31 of these RASS candidates are known PMS stars from the
studies of W94 and K98. This left us with 99 unidentified RASS
candidates.
We are fully aware that our RASS selected sample is flux-limited
and not complete. Many known PMS stars in other star forming regions
have X-ray luminosities considerably below
erg/sec; a rough estimate shows that only about 30% of the PMS stars
exceed this limit (cf. Neuhäuser et al. 1995; Preibisch
et al. 1996; Preibisch 1997). Some regions in Upper Sco have also
been observed in deep ROSAT pointed observations with exposure times
up to 30 000 sec and many additional X-ray sources can be found in
these deep data. We did not attempt to systematically include these
faint X-ray sources into our sample of X-ray selected PMS candidates.
The first reason is that these pointed observations cover only a small
fraction of our area and with very inhomogeneous sensitivity. The
second reason is of technical nature: We could take spectra of rather
bright stars ( ) only, and most of the faint
X-ray sources have fainter counterparts. However, we have included the
rather bright optical counterparts of 9 X-ray sources detected in 7
individual pointed observations in order to get some insight in the
completeness of the RASS selected sample of PMS stars.
Finally, we would like to note that the flux limit of the RASS
agrees quite well with the magnitude limit of our spectroscopy.
Assuming a ratio of as typical for late type
PMS stars, we can expect that most RASS detected PMS stars have
.
2.2. X-ray quiet proper motion candidates
In order to search for possible X-ray quiet PMS stars in Upper Sco,
we have observed a large sample of stars with magnitudes similar to
those of the RASS candidates. X-ray quiet candidates were selected by
the criterion that their proper motion is similar to that of the known
early-type members in Upper Sco.
In our field we found 2014 stars listed in the PPM catalogue.
Nearly all of the PPM stars are rather bright with
and thus much brighter than our RASS
candidates. In order to get a comparable sample we needed fainter
stars. An ideal source for proper motions of rather faint stars is the
STARNET catalogue (cf. Röser 1996). This catalogue is based
on a comparison between the Astrographic Catalogue and the Guide Star
Catalogue (GSC 1.2, Röser et al. 1996), with an epoch
difference of about 80 yrs. It contains 4.3 million stars with
magnitudes down to about 14, and provides proper motions in the
Hipparcos system with an accuracy of mas/yr.
For 11 463 stars in our area proper motions are listed in the STARNET
catalogue.
Since our field is rather large, we have to take projection effects
into account in the kinematic candidate selection. In other words, a
constant space motion of possible members of an association transforms
into a proper motion depending on position. Therefore we performed the
kinematic candidate selection separately for each of the 6 UKST survey
fields in our area. For each UKST field we took
all the B stars from the PPM Catalogue and averaged the proper motions
of those B stars which clustered near the mean overall proper motion
for the Upper Sco association of about mas/yr
(de Bruijne et al. 1997). We regarded every star in the PPM or STARNET
Catalogue with a proper motion difference to the mean of the
corresponding field of less than 7.5 mas/yr as possible kinematic
member. For observational reasons stars brighter than
were excluded. After removing all stars which
were detected in the RASS, we ended up with a sample of 648 X-ray
quiet proper motion candidates.
We also investigated whether there are proper motion members among
the RASS sources rejected as PMS candidates according to their X-ray
properties. There are 418 rejected RASS sources but only 109 of them
have an optical counterpart. For 71 of these stars we know the proper
motions, and 17 stars meet our kinematic member criteria. With only
one exception, all of these 17 objects are bright
( ) stars. Most of them are bright HD stars of
spectral type A and F and therefore do not belong to the class of late
type PMS stars we are looking for. We conclude that our RASS candidate
selection procedure did not exclude a significant number of potential
late type PMS members.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998
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