4. Distribution of spectral types
In contrast to other well-studied SFRs like Taurus-Auriga and Chamaeleon, the distribution of stellar masses for the Lupus CTTS is dominated by with very low-mass stars and thus very late spectral types (Krautter 1991 , Hughes et al. 1994 ).
For the study of the distribution of spectral types among the Lupus TTS, as well as for the discussion of other stellar parameters in the following sections, we have divided the data into three subsets, viz. (i) the CTTS, located in regions of high extinction, (ii) the WTTS found in regions of high extinction ('on-cloud' WTTS), and (iii) the WTTS found in regions of low extinction ('off-cloud' WTTS). (We distinguish between regions of high/low extinction qualitatively only, based on the optically visible dark nebulosities as described cf. in Schwartz 1977 and Krautter 1991 . Up to now there is no CO survey covering the whole area surveyed by Krautter et al. (1996 )).
In Fig. 1 we compare the spectral type distributions for these different subsets of Lupus TTS. It can be seen that the average spectral type of the 'off-cloud' WTTS is much earlier than that one of the TTS known prior to ROSAT, with the 'on-cloud' WTTS exhibiting an intermediate average spectral type. To evaluate the significance of the observed differences in the distribution of spectral types, we have performed two-sample Kolmogorov-Smirnov tests. Table 1 shows that the difference in spectral type distribution between the 'off-cloud' WTTS and both the CTTS and the 'on-cloud' WTTS is significant at more than 3 .
Table 1. Results for Kolmogorov-Smirnov tests on the distribution of spectral types. is the maximum discrepancy of both distributions, and the probability for both distributions being drawn from the same parent distribution.
Stars on the radiative part of the PMS evolutionary tracks evolve
towards higher values of (cf. D'Antona &
Mazzitelli 1994 ). Therefore, the observed differences in the
distribution of spectral types might indicate a systematic increase in
the mean ages from the previously known TTS to the 'off-cloud' WTTS.
However, as Neuhäuser et al. (1995a ) have shown, there is a
correlation between the optical and the X-ray luminosity. As the RASS
is flux-limited, the 'off-cloud' WTTS might be biased against stars at
the lower end of the mass range.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998