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Astron. Astrophys. 333, 619-628 (1998)

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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 [FORMULA] 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 [FORMULA] mag this corresponds to X-ray luminosities of about [FORMULA] erg/sec if we assume thermal plasma emission with [FORMULA] 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 [FORMULA] 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 [FORMULA] 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 [FORMULA] and all stars fainter than [FORMULA] 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 [FORMULA] 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 ([FORMULA]) 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 [FORMULA] as typical for late type PMS stars, we can expect that most RASS detected PMS stars have [FORMULA].

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 [FORMULA] 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 [FORMULA] 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 [FORMULA] 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 [FORMULA] 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 [FORMULA] 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 ([FORMULA]) 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.

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Online publication: April 20, 1998