Astron. Astrophys. 359, 181-190 (2000)

7. Conclusions

Based on high-resolution echelle spectroscopy, we have shown that a large fraction of the ROSAT LRSs found in and around the Taurus-Auriga SFR (Wichmann et al. 1996), if placed at the distance of Taurus-Auriga, have three-dimensional motions consistent with those of previously known, bona-fide T Tauri stars in Taurus-Auriga. Furthermore, for many of them their indicates an age significantly lower than that of the Pleiades.

In all the SFR studied with ROSAT so far, a large, but varying fraction (always at least two thirds) of the ROSAT LRSs could be confirmed as truly young stars, i.e. having in excess of Pleiades stars (this fraction is much lower for the sample of stars south of Taurus studied by Magazzù et al. 1997, which is, however, farther outside a SFR than any of the other). While some of these young stars might pertain to the respective SFRs - especially those found close to molecular clouds -, it has been argued that many of them are part of a more dispersed population of young, low-mass Gould Belt stars (cf. Wichmann et al. 1997b, 1999).

The latter proposition was based on the perception that most nearby SFRs are not isolated entities, but are part of the kpc-size, young, expanding star forming complex known as the Gould Belt, and thus should be regarded as the presently most active parts of this structure, presumably embedded in a more dispersed distribution of young Gould Belt stars. This picture is supported by several pieces of evidence:

i) By investigating the spatial distribution of Li-rich low-mass stars around the Lupus SFR perpendicular to the galactic plane, Wichmann et al. (1997b) found that this distribution is peaked on the galactic latitude of the Gould Belt in this region, and drops off sharply towards the galactic plane.

ii) By cross-correlating the ROSAT All-Sky Survey (RASS) with the Tycho catalogue, Guillot et al. (1998b) found that the population of active, and hence presumably young, low-mass stars in the RASS is centered on a belt, whose best-fit parameter (like angular width, inclination with respect to the galactic plane, longitude of ascending node) are in perfect agreement with the Gould Belt. They also found that the distribution of these stars can be modeled best by assuming that they are distributed in a disk-like structure between the outer radius of the Gould Belt and some inner radius located about 120 pc inwards.

iii) Based on high-resolution echelle spectroscopy, Wichmann et al. (1999) found that nearly all of the Li-rich stars found near the Lupus SFR have lithium equivalent widths in excess of Pleiades stars, and hence are presumably significants younger, while the contamination with Pleiades-aged stars in this sample is only about 10 per cent, i.e. lower than in any other SFR studied so far.

For evaluating these facts, and placing our observations of Taurus-Auriga stars into the proper context, one should take into account that first, the sun is located within the Gould Belt, about half-way between its centre and its outer edge, and second, the Lupus SFR is located at the near edge of the Gould Belt, while Taurus-Auriga is located near the centre.

Thus, in the framework of the Gould Belt hypothesis, Lupus is the natural candidate for a very `clean' sample of very young stars, as this is the direction where we look outwards from the Gould Belt's centre, towards its outermost and youngest parts. Moreover, due to the proximity of the outer edge in this direction, there is little chance to have much contamination.

On the other hand, Taurus-Auriga would appear as the natural candidate for a sample with a rather high degree of contamination with relatively old stars. As we are looking towards the centre of the Gould Belt, i.e. its oldest parts, we expect that even Gould Belt stars in this region are rather old, nearly as old as the Gould Belt itself (i.e. about 60-80 Myrs). The far edge of the Gould Belt is more than 300 pc away, and thus outside the detection horizon of the RASS for typical x-ray luminosities of low-mass PMS stars (see the discussion in Guillot et al. 1998b).

Of course this raises the question of the origin of those stars in our sample that show large lithium equivalent widths, significantly in excess of Pleiades stars, i.e. those stars in our sample we classified as PMS stars. We cannot rule out, but we find it unlikely that these stars are part of a more diffuse Gould Belt background population, because such a population should be rather old already in this particular region, as discussed above. There is simply no natural candidate for a young PMS population other than the Taurus-Auriga SFR in this region.

We therefore argue that those ROSAT LRSs in Taurus-Auriga that could be confirmed as likely PMS stars in this work, might be directly related to the present Taurus-Auriga SFR. In fact, also Neuhäuser & Brandner 1998report distances of 100-150 pc for the four ROSAT LRSs in Taurus observed by HIPPARCOS, in agreement with the distance of the Taurus-Auriga dark clouds.

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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