We determined the pre-main sequence nature of a star by the strength of its 6708 Å lithium line. Since Li is strongly diminished in very early phases of stellar evolution, a high Li content is a good indication for the youth of a star (e.g. Herbig 1962, D'Antona & Mazzitelli 1994). However, Li depletion is not only a function of stellar age, but also of stellar mass and presumably even depends on additional factors like stellar rotation (cf. Soderblom 1996). Not only PMS stars, but also older stars which have already reached the main sequence, e.g. the G and K type stars in the years old Pleiades (cf. Soderblom et al. 1993a), can display quite strong Li lines. In order to classify stars as PMS, we thus have to define a spectral type dependent threshold for the Li line width.
4.1. Classification criteria
We are interested in late type (G - M) PMS stars, and according to PMS stellar evolution models (e.g. D'Antona & Mazzitelli 1994), the PMS phase for these stars last for at least Myrs. We therefore use the available data on Li line widths of stars in several well-known young clusters which have ages in the range Myrs. We have collected Li data from the young clusters IC 2601 ( Myrs; Randich et al. 1997), IC 4665 ( Myrs; Martin & Montes 1997), IC 2391 ( Myrs; Stauffer et al. 1989), and Per ( Myrs; Balachandran et al. 1996). Furthermore, we also have included Li data for the Pleiades from Soderblom et al. (1993a), Garcia Lopez et al. (1994), and Jones et al. (1996). The solid line in Fig. 3 shows the upper envelope for all Li measurements in these young clusters. Any star with a Li line width considerably above this threshold should be younger than Myrs and can therefore be classified as a PMS star. We note that similar classification schemes are now widely used in studies of young stars (e.g. Neuhäuser et al. 1997) and are thought to be very reliable for K and M type stars, while there might be some uncertainties for G type stars.
39 of our X-ray selected stars satisfy our Li criterion and thus are classified as new PMS stars. Most of the X-ray quiet proper motion candidates show no detectable or at best very weak Li lines and none of them can be classified as PMS.
For most of our new PMS stars the Li line widths are in the typical range found for bona-fide T Tauri stars (e.g. Basri et al. 1991, Magazzu et al. 1992). We therefore believe that most of our PMS stars are T Tauri stars with ages not exceeding a few Myrs, while others might be somewhat older. However, here we will not try to divide our PMS stars into T Tauri stars and "post T Tauri stars", since there is no general agreement on how to define theses classes of PMS stars (cf. discussion in Caillault 1998). We would like to note that of our new PMS stars show the H line in emission or completely filled in. Nearly all new PMS stars with H in absorption show considerably weaker H absorption lines than stars of similar spectral type classified as non-PMS.
Many of our non-PMS stars show weak but detectable Li lines. Since stars older than a few hundred Myrs generally show only very week Li lines (for example, the Myrs old stars in the Ursa Majori Group (Soderblom et al. 1993b) as well as the Myrs old Hyades stars (Thorburn et al. 1993) generally have Å), our non-PMS stars with Å probably are quite young with ages not exceeding a few 100 Myrs.
4.2. The new PMS stars
37 of the new PMS stars are the counterparts of RASS sources, 2 are the counterparts of sources from pointed ROSAT observations. One of these two sources is star 6770-655. Its count rate in the pointed observation is well above the RASS detection limit. Indeed, this source was also detected in the RASS, but due to its rather soft hardness ratios the discrimination probability is , slightly below our cutoff. The other source is star 6214-210. Its count rate in the pointed observation is only slightly above the RASS limit and it was not detected in the RASS.
In Fig. 5 we show the spatial distribution of the previously known and our new PMS stars in Upper Sco. One can see a clear concentration of PMS stars in the center of our area. Their spatial distribution is in good agreement with that of the early type probable members from de Geus et al. (1989), and also with that of the Hipparcos members (see Fig. 2 of de Bruijne et al. 1997). This means that the population of PMS stars is spatially coincident with that of the early type members.
For 37 of the observed X-ray selected candidates we know the proper motions from the STARNET catalogue. They are shown in Fig. 4 together with the proper motions of the other stars in the field. 8 of the new PMS stars fully satisfy our kinematic criteria for proper motion members. Another 12 of the new PMS stars have proper motions that miss our, very strict and conservative, kinematic selection criteria only very slightly. Thus, 20 of the 22 new PMS stars with known proper motions can be considered as probable kinematic members. In contrast to this, the non-PMS stars in our X-ray selected sample are evenly distributed in the proper motion space and most of them have proper motions typical for field stars.
Two of the PMS stars have rather large proper motions with mas/yr. It is interesting to compare their proper motions and positions. Star 6784-1219 seems to be coming from the direction to the Oph clouds. For star 6770-655 (located at the extreme south-western edge of our field) the proper motions suggest that it might have been located in the central cluster of PMS stars about 1 - 2 Myrs ago. This might indicate that this star has been ejected from there and perhaps is a run-away TTS (cf. Sterzik & Durisen 1995). However, this scenario has to be treated with some caution, since we do not know the actual age of this star. In fact, the rather weak Li line width close to our threshold and the rather strong H absorption line might indicate that this star is older than only a few Myrs.
4.3. The non-PMS stars
31 of the 69 RASS selected stars show Li lines too weak to be classified as PMS stars. It is well-known that many young active main sequence stars, e.g. Pleiades stars, have X-ray properties quite similar to those of PMS stars and can also meet our X-ray selection criteria for PMS candidates (Sterzik et al. 1995). Therefore, these 31 stars are probably active field stars. It is interesting to compare their number with the model for the stellar content of soft X-ray surveys of Guillout et al. (1996). For our source count rate limit of 0.02 counts/sec and galactic latitude this model predicts that we should detect active field stars per square-degree. This means that about 100 RASS sources in our square degree area in Upper Sco should be active field stars. This is consistent with our number of 29 field stars if we keep in mind that not all field stars will meet our X-ray selection criteria.
The model furthermore predicts that about half () of these RASS detected field stars are younger than 150 Myrs. Many of these young field stars should show Li lines with similar strength as the young cluster stars. This is consistent with our finding that 11 of the non-PMS RASS sources show Li line in that range.
Our sample of X-ray quiet proper motion candidates contains 19 stars which show Li lines comparable to those of the young cluster stars. These stars also probably are rather young main sequence stars. The other X-ray quiet proper motion candidates probably are older field stars.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998