4. The PMS population found by X-ray surveys
The spectroscopic criteria discussed in the previous sections are useful for clarifying the nature of the stars found by the EINSTEIN and ROSAT satellites towards star-forming regions. For simplicity, the following convention will be used: class FG for F,G-type stars, which are either intermediate-mass PMS stars, or PTTSs, or else YMS stars; class KM for K,M-type PTTSs or YMSs; class PT for bona fide K,M-type PTTSs because they fall on the PTT-gap; class WT for WTTSs.
Starting with Taurus-Auriga, I have classified the NTTSs reported by Walter et al. (1988) using their spectral types and the values of Martín et al. (1994) as follows: 7 FG, 6 KM, 4 PT and 10 WT. The percentage of WTTSs over observed EINSTEIN X-ray sources in this sample is 25%. A lower limit to the PTTS/WTTS ratio is 0.4. From ROSAT observations, Wichmann et al. (1996) have identified many more possible WTTSs, but their low-resolution optical spectra did not allow them to measure . The classification of these stars awaits higher-resolution data. Neuhauser et al. (1995) and Magazzù et al. (1996) investigated with higher-resolution spectra 115 RASS sources in a region of 300 deg2 south of Taurus and reported 35 new PMS stars. Their stars are classified in this work as follows: 14 FG, 10 KM, 3 PT, 8 WT. The WTTSs represent only 7% of the X-ray sources they observed, which is a significantly lower fraction than for the Walter et al. (1988) sample. This indicates the number of WTTSs decreases with increasing distance to the clouds. The ratio of PTTS/WTTS is at least 0.4 and, surprisingly, does not seem to change as the surveys move far away from the clouds.
Walter et al. (1994) studied EINSTEIN images of the Upper Scorpius OB association. They found 28 low-mass PMS stars. Using the data given in their Table 7, the classification of their stars is: 2 FG, 3 KM, 8 PT and 15 WT. Hence, the WTTS represent 53% of their sample. The PTTS/WTTS ratio is at least 0.5. This result is not consistent with their conclusions, based on isochrone fitting, that their low-mass PMS stars are systematically younger than the massive B stars, and that they have a very small dispersion of ages. The apparent ages based on the HR diagramme could be in error due to observational uncertainties in distance, reddening or effective temperature, and theoretical uncertainties in the calculations of PMS low-mass isochrones.
In Chamaeleon and Orion, Alcalá et al. (1995, 1996) studied RASS sources and claimed to find 77 and 112 new WTTSs, respectively. They published values which should be taken with caution because the resolution of their spectra (3.5-8.1 Å) is too low for resolving the Li I 670.8 feature from nearby atomic and molecular lines. Using their , my tentative classification of their stars in Chamaeleon/Orion is the following: 23/30 FG, 13/18 KM, 13/13 PT, and 28/51 WT, respectively. It is particularly interesting that the ratio of X-ray discovered PTTSs versus WTTSs is similar in Chamaeleon, Taurus and Upper Scorpius, but lower in Orion. However, this result remains preliminary until higher-resolution spectra for improving the measurements of many stars become available.
To summarize, spectroscopic quantitative classification criteria applied to X-ray discovered stars allow to identify genuine WTTSs and PTTSs. The global fraction of WTTSs among the PMS candidates identified from previous qualitative criteria is 40%. The fraction of stars in the PTT-gap is 15%, which is a lower limit to the total number of PTTSs. However, it appears unlikely that the remaining 55% of X-ray emitting stars are all PTTSs outside the PTT-gap. Most of them are F and G-type stars with a detectable Li I feature. Briceño et al. (1997) have estimated that a significant number of solar-type young (but not PMS) stars could be contaminating the X-ray surveys. The results of this work seem consistent with their suggestion. There does not seem to be compelling evidence in the X-ray surveys for a large PTTS population. It is of great importance to obtain reliable measurements on the basis of high-resolution spectroscopy for all the K and M-type X-ray discovered stars, in order to improve the estimate of the fraction of PTTSs over WTTSs.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998