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

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10. Conclusions

In this work, we have performed a photometric study of the population of Lupus WTTS recently discovered by Krautter et al. (1996 ).

One of our most important results is that the new WTTS represent an older population than the hitherto known CTTS in Lupus. This finding is in agreement with the spatial distribution of these stars: while the CTTS are located in the vicinity of the dark clouds, as one would expect for young stars, the newly found WTTS are spread over a large area of more than 200 square degrees. We presume that these WTTS also formed in the Lupus dark clouds, and have spread over a large region due to their velocity dispersion.

For most of the Lupus WTTS, a velocity dispersion of [FORMULA] as determined by Jones & Herbig (1979 ) for Taurus-Auriga and by Dubath et al. (1995) for Chamaeleon would be sufficient to explain their spatial distribution, given the ages determined by us. However, some WTTS, which are relatively young, but are located at large distances to the dark clouds, might also be ejected with high velocities by three-body interactions in the way proposed by Sterzik & Durison (1995 ).

Furthermore, comparing the masses of the newly found WTTS in Lupus with those of the CTTS, there seems to be some evidence that the IMF of the CTTS, (i.e. the youngest TTS in Lupus), shows some deficit at high masses with respect to the (older) WTTS. Thus possibly, the IMF has become steeper during the history of the Lupus SFR. A deficit of young high-mass TTS like that observed for the Lupus SFR has not been found in other SFR's, but this might be due to the fact that the Lupus SFR is significantly older than other well-studied SFR's (Hughes et al.  1994 ).

Our study of the correlations between the X-ray emission of Lupus TTS and other stellar parameters indicates that the X-ray emission of TTS is similar to that of other active late-type stars, and possibly is caused by the same mechanism, i.e. coronal heating by magnetic activity caused by a dynamo mechanism. Obviously, the X-ray emission of TTS is limited by the same saturation limit than that one of other active late-type stars. Moreover, we can show that the X-ray surface flux increases with stellar age. An increase with stellar age has also been observed for the rotational velocities of TTS (cf. Bouvier et al.  1993, Edwards et al.  1993 ). Thus, the observed increase of the X-ray surface flux with age is in qualitative agreement with the idea that the magnetic activity in TTS is correlated with rotation via a dynamo mechanism. We do not have direct evidence for such a correlation, because only few measurements of the rotational velocities of Lupus TTS have been done yet, but for Taurus TTS such a correlation has indeed been found (cf. Bouvier  1990 , Neuhäuser et al.  1995a ).

As stellar parameters like bolometric luminosity, radius, mass, and (in the special case of our sample) also age correlate with each other, it is difficult to determine which parameters are the most important for the X-ray emission. In current theories, which link the X-ray emission to coronal heating caused by magnetic activity, basic parameters are the stellar rotation (which enters in our data via age and stellar rotational evolution), and, for stars near the saturation limit for [FORMULA], the stellar surface area (which enters via the radius). From our analysis we cannot rule out possible additional parameters. However, it seems that, if the evolution of stellar rotation is taken into account, our observations can be explained without the introduction of additional parameters.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998