5. Where is the young main-sequence population?
As remarked in the Sect. 1, the RASS-WTTS papers show a severe lack of normal, young active main-sequence stars in their samples. For example, Alcala et al. (1995) report (see their Table 4) to have investigated 112 X-ray sources (in an area of square deg) around the Chameleon SFR, and, among the 112 X-ray sources studied, to have found 75 new WTTS and only 10 active stars which are not classified as WTTS on the basis of their low-resolution spectra. It is suggestive, in the light of the results of Sect. 3and 3.1, that the non-WTTS active stars reported by Alcala et al. (1995) are all dMe stars, as it is at these spectral types that low-resolution spectra have some diagnostic value. In addition to this, they report another 13 non-WTT stellar sources which were previously known, although from their Table 6 it appears that some are early-type stars and thus non-coronal sources. Thus, 23 non-WTT stars, or 18, subtracting the early-type stars, in square deg. For the same sky area Alcala et al. (1997) report a limiting sensitivity of (the same limiting sensitivity can be derived from the RASS exposure time in the region of s reported by Alcala et al. 1995, leading to a limiting PSPC count rate of cts s-1 ). A computation based on the model of Favata et al. (1992) and Sciortino et al. (1995) predicts at this limiting flux level between 60 and 80 active non-PMS stars in that area of the sky, depending on the assumed value for the space density of RS CVn binaries. This is a factor of higher than the 23 active non-PMS stars reported by Alcala et al. (1995), showing that many of the putative WTTS in their sample are likely to be misclassified main-sequence stars. Note that the computation discussed here is in full agreement with the observed numbers in the EMSS, i.e. these large numbers of young main-sequence objects are not only expected, at the X-ray flux levels, but their presence has already been verified on fully identified samples. The Guillot et al. (1996) model, predicts an even larger number of main-sequence stellar sources, i.e. about one per square degree at these fluxes and latitudes, approximately two thirds of which are expected to be of age 1 Gyr or older (and with a significant fraction of the ones younger than 1 Gyr being on the main sequence). A similar prediction is made by Brice"no et al. (1997). The Guillot et al. (1996) model has been shown by Motch et al. (1997) to provide a good match to the RASS population of the Galactic plane, where they report 77 coronal sources in a sky area about half the size and at a limiting flux approximately twice as shallow of the one surveyed by Alcala et al. (1995) although at slightly lower galactic latitude.
Later works in the RASS-WTTS line find significantly higher fractions of non-PMS coronal sources. For example, Magazzu et al. (1997) find, in the Tau-Aur region (where the RASS exposure time is s), significant numbers of non-PMS coronal sources, even in a sample which had been optimized for searching for PMS sources. The difference in methodology is at least in part likely to be the cause of the difference in the detected source population, and in particular the larger spectral resolution employed ( Å), which allows better discrimination of true WTTS from main-sequence stars.
© European Southern Observatory (ESO) 1997
Online publication: October 15, 1997