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Astron. Astrophys. 320, 185-195 (1997)

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6. Age and mass distribution of Lupus stars

The distributions of ages for our three subsets of Lupus TTS (CTTS, 'on-cloud' WTTS, and 'off-cloud' WTTS) are shown in Fig. 3, and in Table 2 we give mean ages and masses for the three subsets. We performed various statistical tests testing the null hypothesis that CTTS, 'on-cloud' WTTS and 'off-cloud' WTTS have the same age or mass distributions, with the results shown in Table 3. Independently of the statistical test used, this hypothesis can be rejected at a high confidence level for the distribution of the ages as well as for that of the masses of the stars. We conclude that the mean age of the 'off-cloud' WTTS is indeed higher than the mean age of CTTS and 'on-cloud' WTTS.

[FIGURE] Fig. 3. Distribution of stellar ages for CTTS (upper panel), 'on-cloud' WTTS (middle panel), and 'off-cloud' WTTS (lower panel).


Table 2. Mean ages and masses of Lupus TTS.


Table 3. Probabilities P for correctness of the hypotheses of equal distribution of ages and masses among different samples of Lupus TTS. WTTS1 are 'on-cloud' WTTS, WTTS2 are 'off-cloud' WTTS. For P the subscripts G, L, and PP refer to the Gehan, logrank and Peto-Peto tests, respectively, while the superscripts A and M refer to the age and mass distributions.

For undetected binaries the luminosities will be overestimated, and the ages determined from theoretical evolutionary tracks thus will be underestimated. However, while absolut ages might be quite uncertain, conclusions about age differences between Lupus WTTS and Lupus CTTS will not be affected much, provided that the multiplicity among both samples is about the same.

Reipurth & Zinnecker (1993 ) have found 8 binaries in the separation range of 1"-12" in their study of 60 TTS in the Lupus I-III clouds (and for a larger sample extrapolate a high multiplicity fraction of 80-90% for all separations, in line with other studies, cf. Leinert et al.  1993 , Ghez et al.  1993 , Brandner et al.  1996 ). The binaries found by Reipurth & Zinnecker (1993 ) have been taken into account in the study of the optically selected Lupus CTTS by Hughes et al. (1994 ). Therefore the mean age for this sample might be less underestimated than that of the new WTTS, for which no such binarity study exists. This would even strengthen our conclusion on the 'off-cloud' WTTS being older than the CTTS.

For the discussion of the distribution of masses among Lupus TTS it is important to keep in mind possible selection effects. As already discussed above, there might be a bias against the X-ray discovery of stars with very low masses, but probably not against the optical discovery of high-mass CTTS.

We therefore conclude that the 'present-day' IMF in Lupus, as represented by the youngest TTS in the Lupus SFR, i.e., the CTTS located in the vicinity of the dark cloud cores, shows a significant deficit at higher masses, while obviously in previous periods of star formation in Lupus large numbers of stars with masses of one solar mass or more have been formed. Unfortunately, the bias against X-ray discovery of very low-mass TTS prevents us from estimating the IMF for these earlier periods. In this context, it is very interesting that the Lupus SFR seems to be older than other well-known SFRs like Taurus-Auriga or Chamaeleon (Hughes et al.  1994 ), which do not show the deficit of high-mass CTTS as observed in Lupus. From our observations, it seems at least possible that the Lupus IMF was similar to the Taurus-Auriga IMF in the past, with the mean mass of newly formed stars shifting towards lower values in later periods.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998