Optical spectroscopy of X-ray sources in the Taurus molecular cloud: discovery of ten new pre-main sequence stars

We have analyzed optical spectra of 25 X-ray sources identified as potential new members of the Taurus molecular cloud (TMC), in order to confirm their membership in this SFR. Fifty-seven candidates were previously selected among the X-ray sources in the XEST survey, having a 2MASS counterpart compatible with a PMS star based on color-magnitude and color-color diagrams. We obtained high-resolution optical spectra for 7 of these candidates with the SARG spectrograph at the TNG telescope, which were used to search for Li absorption and to measure the Ha line and the radial and rotational velocities; 18 low-resolution optical spectra obtained with DOLORES for other candidate members were used for spectral classification, for Ha measurements, and to assess membership together with IR color-color and color-magnitude diagrams and additional information from the X-ray data. We found that 3 sources show Li absorption, with equivalent widths of ~500 mA, broad spectral line profiles, indicating v sin i ~20-40 km/s, radial velocities consistent with those for known members, and Ha emission. Two of them are classified as new WTTSs, while the EW (~ -9 Ang) of the Ha line and its broad asymmetric profile clearly indicate that the third star (XEST-26-062) is a CTTS. Fourteen sources observed with DOLORES are M-type stars. Fifteen sources show Ha emission; 6 of them have spectra that indicate surface gravity lower than in MS stars, and their de-reddened positions in IR color-magnitude diagrams are consistent with their derived spectral type and with PMS models at the distance of the TMC. The K-type star XEST-11-078 is confirmed as a new member from the strength of its Ha emission line. Overall, we confirm membership to the TMC for 10 out of 25 X-ray sources observed in the optical. Three sources remain uncertain.
Comment: 15 pages, 7 figures, accepted by Astronomy & Astrophysics