3. Discussion and conclusions
Two of our three X-ray sources that have new wTTS candidates as optical counterparts are contained in the RASS Bright Source Catalogue (W. Voges et al. 1996), but they escaped the selection criterion of Webb et al. (1999). Their measured X-ray fluxes are typical for TW Hya association members (see Kastner et al. 1997).
The derived Lithium abundances for the three late type stars are comparable, or even higher than those found for similar types in young open clusters, like the 30 Myr old IC2602 (Randich et al. 1997). On the other hand they appear somewhat lower than those derived for TTS in regions of ongoing star formation. We do not attempt an absolute age determination based on rather uncertain Lithium depletion models, but stress that our "Lithium age" of 10-30 Myrs is fully consistent with the age determination of other TW Hya association members based on pre-main sequence evolutionary tracks (Webb et al. 1999). Also kinematics supports their membership in the TW Hya association: they have, within the errors, consistent radial velocities with TW Hya. From the Lithium abundance and radial velocities we conclude that the three M type stars are low-mass PMS stars and members of the TW Hya association.
The membership of RXJ1109.7-3907 is less clear: it has a considerable amount of Lithium, and therefore resembles wTTS properties. But Lithium depletion is much slower for a G3 type star as compared to M dwarfs. Its Lithium strength is just at the upper envelope, characteristic for Pleiades stars (Soderblom et al. 1993), and hence, only an upper limit of about 100Myrs for its age can be estimated. A lower limit on its distance is about 80-100pc, derived from placing the star on the zero-age main sequence. This is probably too far away to be associated with TW Hya. Also the radial velocity of this star is not consistent with that of the other stars (in case this object is not an unresolved spectroscopic binary corrupting the radial velocity). RXJ1109.7-3907 (PPM288568) has a proper motion of mas yr-1 and mas yr-1. These values are only marginally consistent with those of the other TW Hya members: the mean values derived for the members indicated by Webb et al. (1999) are mas yr-1 and mas yr-1 respectively. In summary we conclude that RXJ1109.7-3907 is probably a young (30-100Myrs old) field star located in the background of the TW Hya association. It resembles spectroscopic properties typical for very young, coronally and chromospherically active field stars (see e.g. Favata et al. 1993, Jeffries 1995, Sterzik & Schmitt 1997).
The spatial distribution together with all available proper motion data are shown in Fig. 2: Likely TW Hya association members are indicated either with diamonds (single stars) or with circles (binary and multiple systems). The length of the arrows corresponds to known proper motion vectors extrapolated over 200000 yrs (see Webb et al. 1999). Open symbols indicate those candidates already known, filled symbols mark our new identifications. RXJ1109.7-3907, an unlikely association member, has a different symbol. Our new candidate members double the number of TTS systems in the area roughly delimited between and , which could give the impression that the former cluster centre might have been located in this area. But due to the spatial selection bias, and the limited size of the fields investigated we cannot infer the true, large scale, spatial distribution of potential TTS potentially contained within the fainter RASS source sample.
Finally we note the presence of a highly significant excess of young stars around TW Hya over predictions from standard galactic models: the expectation on the number density of the X-ray emitting stellar population above the RASS completeness limit (0.03 cnts/sec) is about 0.5 X-ray sources/deg2 at the galactic latitude of TW Hya (see Guillout et al. 1996). They have shown that the highest fraction (about 40%) of these X-ray sources are low-mass stars younger than 150Myrs. Assuming a constant star formation rate over the last 150Myrs we expect in total about 2 RASS sources with ages less than 15Myrs within a 100 deg2 area. Contrarily, the observations reveal a much higher density: the 37.5 deg2 field defined above already contains 8 or 9 stars in 6 stellar systems with ages younger than a few 10Myrs!
In summary, there is compelling evidence that the TTS found around TW Hya are not unassociated, young, field stars, but rather generic members of a group that formed recently.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999