5. Results

Our aim is to search for low-mass PMS stars among the RASS source counterparts observed in this work, by studying their LiI doublet. The presence of a strong LiI absorption is the most evident indicator of the PMS nature. Typical equivalent widths range from 0.7 Å, observed in cTTS (Magazzù et al. 1992) to 0.1 Å, observed in wTTS (Martín et al. 1994), while equivalent widths lower than about 0.3 Å have been measured in Pleiades objects (Soderblom et al.\ 1993; García López et al. 1994).

  
Figure 3: Equivalent widths of the LiI doublet vs. spectral type for the stars in our sample. The continuous line has been adapted from Fig. 2 (click here)d in Soderblom et al. (1993) and represents the upper envelope of Li equivalent width for Pleiades stars. Squares (circles) indicate two (three) points in the same position. Crosses are doubtful points (see text)

 

RXJ 0219.4-1321A 2 20 29 -13 20 25 13

RXJ 0219.4-1321C 2 19 25 -13 21 47 15.5

RXJ 0223.3-1615SW 2 23 25 -16 14 31 13.5

RXJ 0254.8-0709NW 2 54 53 -07 09 20 15.5

RXJ 0255.8-0750N 2 55 52 -07 50 29 15.5

RXJ 0309.1+0324S 3 09 10 03 23 42 11

RXJ 0312.8-0414SE 3 12 51 -04 14 19 10.5

RXJ 0330.7+0306S 3 30 43 03 05 18 15

RXJ 0347.2+0933NE 3 47 17 09 33 08 12

RXJ 0349.4+1255S 3 49 28 12 54 28 10 BD+12 511B

RXJ 0351.4+0953E 3 51 28 09 53 34 13.7

RXJ 0400.1+0818S 4 00 09 08 18 15 10 BD+07 582

RXJ 0407.2+0113S 4 07 16 01 13 12 12

RXJ 0426.4+0957E 4 27 30 09 57 00 11.5 also in GSC

RXJ 0525.7+1205NW 5 25 44 12 04 30 14.5

 

In Fig. 3 (click here) we show, for our objects in which Li has been detected, the LiI equivalent width vs. the spectral type. We plot also the upper envelope of LiI equivalent widths for stars in the Pleiades (adapted from Fig. 2 (click here)d in Soderblom et al. 1993).

In this paper we classify as low-mass PMS stars those objects, later than F7, satisfying the following criterion:

The LiI equivalent width must be greater than in Pleiades objects of the same spectral type.

In other words, a star is an PMS if it can be located above the continuous line in Fig. 3 (click here). Stars in which lithium has been detected, but below the Pleiades upper envelope, will be classified as possible PMS stars ("PMS?'' in Table 4 (click here)). Objects with no lithium will be classified as dKe or dMe stars, depending on their spectral type, if their is in emission or strongly filled in. The remaining objects will be classified as "non-PMS''. We are aware that this classification is somewhat arbitrary, for example among the non-PMS we could find some post-TTS, which in some cases show the Li resonance doublet weaker than 0.1 Å (Martín et al. 1992). However, we believe this is a conservative classification, which makes us quite confident that objects classified as "PMS'' are really PMS stars. In order to get accurate ages of our objects, we are planning further observations, aimed to locate our PMS candidates in the diagram and measure their lithium abundance.

Note that we do not impose any restriction on for PMS stars. In fact, TTS show a wide range of equivalent widths, from strong emission in cTTS to practically no emission in post-TTS. However, from Table 4 (click here) we can see that most of the objects classified as PMS or PMS? show weak emission or absorption shallower than in main sequence objects, as expected for wTTS.

Although the objects RXJ 0255.8-0750N, RXJ 0333.0 +0354, and RXJ 0422.9+0141 in Fig. 3 (click here) are located above the Pleiades upper envelope, in this paper we classify them as PMS? For the first two objects this is due to the quite high level of noise, which makes doubtful the detection of the lithium line. For RXJ 0422.9+0141 we note that this object is a spectroscopic binary (see remark 7 in Table 4 (click here)) and the determination of the spectral type of its components is rather uncertain.

  
Figure 4: Some finding charts (5 5). North is up, East on the left. The other PMS and PMS? stars can be easily identified (see text)

In total, there are 115 RASS sources selected for optical follow-up observations. As some RASS sources have several potential optical counterparts, we have performed spectroscopy for a total of 131 stars, all listed in Table 4 (click here). In Fig. 4 (click here) we show the finding charts for some object whose identification may be difficult; all the other new PMS can be easily identified, as they are GSC stars with no bright stars in the vicinity. In case of need, the charts for any object observed here are available on request.

There are 30 stars in our sample which can be classified as PMS stars, according to our criterion. These objects are counterparts to 28 RASS sources, i.e. for two RASS sources we have found two new PMS stars each (with separations of and ). In addition, there are 19 RASS counterparts classified as PMS? as well as 17 and 5 RASS counterparts classified as dKe and dMe, respectively. In several spectra, we clearly see double lines indicative of close binaries (identified as spectroscopic binaries - "SB'' - in Table 4 (click here)). Namely, we find seven (one) SB among the stars classified as dKe (dMe) stars, three among the PMS? stars, and four among the non-PMS stars.

The spectra of all 30 stars classified in Table 4 (click here) as new PMS stars are shown in Fig. 5 (click here). These and all the other spectra are available from the authors. In some cases one can see quite broad lines indicative of either close binaries and/or fast rotation. As some relatively fast rotators are expected to be present among TTS, we do not classify such PMS stars as SB.

As can be seen from Table 4 (click here), three of the 15 stars classified as wTTS in Neuhäuser et al. (1995c) may not be real young PMS stars according to our stricter criterion; here they are classified as dKe stars. Seven other stars classified as wTTS by Neuhäuser et al. are now classified as PMS? stars, while the other five wTTS are confirmed to be PMS stars also in this paper.

We note that one of the stars studied here (HD 23793 B) is the late-type secondary in the so-called Lindroos sample (Lindroos 1986), a set of double stars with the primary being an early-type star and the secondary being late-type. It has been suggested that many of the secondaries in this sample may be post-TTS (Lindroos 1986). However, both Pallavicini et al. (1992) and Martín et al.\ (1992) consistently found that only about one third of them may be genuine post-TTS. As far as HD 23793 B is concerned (spectral type F5), both Pallavicini et al. (1992) and ourselves find  absorption and very weak LiI. Pallavicini et al. (1992) did not detect Ca H and K emission and classified this Lindroos pair as an optical system. According to our criterion, we classify this star as non-PMS as well.