4 Results and discussion

19 objects (including 17 early type stars and 2 Seyfert galaxies) were identified on the basis of the low-resolution spectroscopy and are presented in Table 1.

  

Table 1: Optical counterparts to X-ray sources excluded from the sample. Objects with sequential number of the sample, RASS source designation, position, type of stars or galaxies, and catalog identifier

  

Table 2: Optical data of the newly discovered candidate WTTS

 

Table 2: continued

  

Table 3: Optical data of the possible CTTS discovered and the two previously found WTTS candidates

  

Table 4: Optical data of the probable cloud members of Taurus-Auriga

  

Table 5: Optical data of the possible optical counterparts to the RASS sources

All spectra of the sub-sample of late type stars were examined and the following criteria were applied to confirm the optical counterparts' candidate nature as WTTS:

• apparent LiI absorption with equivalent width $W_{\lambda}\geq$ 0.1 Å, treated as an indicator of youth,
• with spectral type later than mid-F,
• equivalent width of H$_{\alpha}$ less than 10 Å.

H$_{\alpha}$ emission, an indicator of deep chromosphere or large surface coverage of plage, seems to be not so important in the identification of WTTS, because it might show up as significant emission, filled-in or even absorption due to different spectral types of stars. 75 optical counterparts to the X-ray sources could be identified as new candidate WTTS, two others, previously discovered by Neuhäuser et al. (1995c) and Alcala et al. (1996) respectively, were confirmed to be Li-rich stars in our program, and their optical data are provided in Table 3. Since ZAMS stars at the age of the Pleiades ($\sim$10⁸ yrs) still show significant Li with $W_{\lambda}$(Li) depending on spectral type (Soderblom et al. 1993), at a given spectral type WTTS should show a higher value of $W_{\lambda}$(Li) than the Pleiades. Most of the newly discovered WTTS candidates were found to have $W_{\lambda}$(Li) significantly higher than the Pleiades stars of the same spectral class (Soderblom et al. 1993), while the remaining ones have comparable $W_{\lambda}$(Li). Furthermore, a limited comparison conducted by Neuhäuser et al. (1997), between the detection of LiI absorption at high resolution ($\sim$0.25 Å) and relatively high resolution (0.7 $\sim$ 1.5 Å), conclude that $\sim$1 Å resolution (the resolution of our study is 1.2 Å/pixel) is sufficient to obtain accurate $W_{\lambda}$(Li) values despite of the possible overestimation due to blending. Therefore, the majority of the WTTS candidates found are probably PMS stars rather than ZAMS stars as those of the Pleiades. ROSAT source name, position, estimated V magnitude, equivalent width of H$_{\alpha}$and LiI absorption, spectral type and comments on pairs are given in Table 2, where positive values of equivalent width indicate line emission and negative means absorption. If available, the catalog identifier of a star is also provided. However, no radial velocity of these new candidate WTTS has been measured until now, thus membership to the T association could not be confirmed and subsequent analysis such as the distribution of WTTS all around the Taurus-Auriga region will be reported later (Li & Hu 1998). Notably, the object IRXS J052908.4+115207, observed on 1996 Dec. 27 was discovered to have rather strong H$_{\alpha}$ emission (with $W_{\lambda}$(H$_{\alpha}$) of at least 24.5 Å) and significant LiI absorption ($W_{\lambda}$(Li) = 0.45 Å), though the low-resolution spectrum obtained in the first run showed similar H$_{\alpha}$ but fairly weak H$_{\beta}$ emission. Additional intermediate-resolution spectroscopy (with dispersion of 50 Å/mm, 1.2 Å/pixel, centered at 3900 Å) carried out on 1997 Jan. 18 found no CaII H and K emission, which are regarded as characteristics of CTTS. Another low-resolution spectroscopic observation based on the same set-up as described in Sect. 3 has been undertaken 60 days later. The spectrum reduced with similar procedure shows significant increase both in H$_{\alpha}$ and the continuum emission, and the estimated V magnitude increased by about 0.4 mag. However, no other broad optical line emission was found on any of the nights. Although this needs to be confirmed, IRXS J052908.4+115207 was classified as a possible CTTS and optical information is presented in Table 3. Table 4 is a list of 19 optical counterparts having significant but weak LiI absorption, these were classified as probable cloud members of the Taurus-Auriga SFR which might have reached the ZAMS. The remaining 53 objects studied, showing no obvious LiI absorption in the spectra, were regarded as likely optical counterparts to the related X-ray sources, but not as WTTS, no statements could be made concerning the distance or their possible associations with the Taurus-Auriga region. These objects are listed in Table 5.

Normalized spectra of the newly discovered candidate WTTS are shown in Fig. 1, covering both H$_{\alpha}$ emission and LiI absorption. Finding charts for all new candidate WTTS are presented in Fig. 2, each having a coverage of $5'\times 5'$. Spectrum and finding chart of IRXS J052908.4+115207 are also provided at the end of Fig. 1 and Fig. 2, respectively.

With 75 new candidate WTTS, 5 thereof are pairs, 1 possible CTTS discovered and 2 previously identified Li-rich stars (Neuhäuser et al. 1995c; Alcalá et al. 1996) confirmed by our program out of the sample of 164 RASS sources, a discovery rate of about 47% was obtained as compared to 42% achieved by Wichmann et al. (1996). Thus, the selection criteria based on hardness ratios established by Neuhäuser et al. (1995a) turned out to be reliable, though possibly biased by our optical brightness limit.

Figure 1: Spectra of the 75 newly discovered candidate WTTS and one possible CTTS

Figure 2: Finding charts for the newly discovered candidate WTTS and the possible CTTS.

Acknowledgements

We are very grateful to the referee Dr. R. Wichmann for his constructive suggestions. The comments made by Dr. Chen Wenping helped greatly to improve and streamline our presentation. Thanks to Miss Cao Li and Mrs Xu Dawei for their kindly help in the data reduction of the low-resolution spectroscopy, and appreciation for Dr. Qiu Yulei and Dr. Wei Jianyan for their efforts in the preparation of the spectra and the finding-charts in PostScript format. This work has made use of the SIMBAD database.