2. Hipparcos catalogue data for pre-main sequence stars
The sample of young stars contained in the Herbig & Bell (1988) Catalogue of Orion population objects with emission lines (HBC) and observed by the Hipparcos satellite is fairly limited:13 Herbig Ae/Be stars (7 of which have significant parallax values); 16 CTTSs, 10 of which have significant parallaxes; 7 WTTSs or SU Aurigae stars, 4 of them are positively detected; and 9 stars with uncertain pre-main sequence status, 4 of which have significant parallaxes.
Table 1 displays the TTSs found both in the HBC and in the Hipparcos Catalogue (ESA 1997). Column 1 gives the star name, while Columns 2 and 3 indicate its HBC and HIP number, respectively. Entries are in order of increasing . Column 4 indicates the associated nebular region, and Column 5 indicates the object type according to the HBC nomenclature 1. A CTTS is denoted `tt', a WTTS is called `wt', a member of the Herbig Ae/Be (HAeBe) group is noted `ae', and a SU Aurigae-type star is marked `su'. A `?' symbol indicates an uncertain type, while a `*' symbol denotes uncertain pre-main sequence status. Column 6 gives the median Hipparcos magnitude, and Column 7 shows the Hipparcos parallax in milliarcsecond (mas), with standard error as indicated in Column 8. Column 9 indicates the derived distance in pc for stars with (marked `:') and for stars with , with error bars corresponding to . Column 10 gives the flag found in Field H59 (Double and Multiple Systems Annex flag) of the Hipparcos Catalogue, the meaning of which we now briefly describe.
For the majority of Hipparcos stars, the astrometric solutions were derived using a single star model, where the five astrometric parameters are the equatorial coordinates (), the proper motion components () and the parallax . However, detected non-single stars received different solutions, depending on the nature of their duplicity. Five different possibilities are noted in the H59 Field by different flags: either the system was resolved into several components with an assumed linear motion (component solutions, Flag C in Field H59), or an orbital solution could be computed (Flag O), or the duplicity was detected by a non-linear motion of the photocentre (acceleration solution, Flag G), or by the variability of one component, resulting in a specific motion of the photocentre (VIM solutions: Flag V), or by an excess scatter of the measurements possibly due to a short-period variation of the photocentre (stochastic solutions, Flag X). We emphasize here that the value of the derived parallax depends on the adopted astrometric model. It may happen, for example, that a single-star model was given in the Catalogue for a star which is now known to be a spectroscopic binary. One can then go back to the one-dimensional measurements archived in the Hipparcos Intermediate Astrometric Data, CD-ROM 5, in order to compute an orbital solution for this system, resulting in a presumably more accurate value of the parallax and other astrometric parameters. The reader is referred to ESA (1997) for a detailed explanation of astrometric solutions for non-single stars.
Cross-identification of the HBC and Hipparcos stars is in most cases obvious. Only 4 stars of Table 1 are not cross-identified in the SIMBAD database. They are the visual pair HIP 54738 and 54744, and the two single stars HIP 78053 and HIP 114995. The latter two stars are unambiguously identified as IM Lup and V628 Cas (MWC 1080) by inspection of the relevant sky regions. The situation is more confused for the HIP 54738 and 54744 pair, which is identified as CCDM J11125 -7644A/B in SIMBAD. Comparing the Hipparcos Input Catalogue (HIC, Turon et al. 1992) to the Hipparcos Catalogue, it seems that HIC 54738 was erroneously written down as HIP 54744. A careful examination of the sky atlas and arguments given in Sect. 4 lead us to propose the identification HIP 54738 = CV Cha and HIP 54744 = CW Cha.
In each star forming region (SFR), we searched for additional pre-main-sequence stars observed by Hipparcos and located in the vicinity of the stars given in Table 1, in order to improve the precision of the mean parallaxes. In addition to HBC stars, we thus considered HAeBe stars not found in the HBC but listed in the Thé et al. (1994) Catalogue along with young stars, mainly of WTTS type, which were discovered by the ROSAT satellite in the vicinity of star-forming regions or which form multiple systems with stars of Table 1. We restricted ourselves to those stars with confirmed pre-main sequence status discussed in the series of papers on ROSAT observations of SFRs (Neuhäuser & Brandner 1998, Krautter et al. 1997, Alcalá et al. 1995, Wichmann et al. 1996, Alcalá et al. 1997, Covino et al. 1997, Frink et al. 1998, Terranegra et al. 1999). Table 2 (with entries similar to Table 1) summarizes properties of these additional pre-main sequence stars.
Note that the Hipparcos data of a large sample of HAeBe stars, containing a number of likely members of the class in addition to those contained in HBC, were recently discussed by van den Ancker et al. (1997); we thus won't discuss them individually further here but will use them to compute mean parallaxes of YSO groups.
© European Southern Observatory (ESO) 1999
Online publication: December 2, 1999