3.1. Proper motions
Positions and proper motions of the stars in Tables 1 and 2 are plotted in Fig. 1. The two open star symbols denote the early type stars HD 97048 and HD 97300, the filled triangles classical TTS and the open triangles weak-line TTS known before the ROSAT mission. The filled circles represent confirmed low-mass PMS stars, while the open ones are objects classified as ZAMS stars or with dubious PMS nature by C97. One immediately notes that there is a trend for proper motion vectors pointing to the west, with some scatter especially for the ZAMS stars, as expected from their probably higher velocity dispersion. For a more detailed analysis, we also plot the data in galactic coordinates (Figs. 2 and 3), which are better suited for a rectangular illustration due to the position of the Chamaeleon association near to the southern equatorial pole. The motion of the stars is partly due to the reflex motion of the Sun, which is about () (-17,-6) mas yr-1 at the position of the Chamaeleon association and a distance of 170 pc. Note however that this value strongly depends on the adopted distance (for half the distance the value would be twice as high), whereas the variations caused by different positions on the sky are rather low within our study area.
3.1.1. Bona-fide PMS stars
From Fig. 3 we infer at least 2 or 3 different areas in the proper motion diagram where confirmed PMS stars tend to cluster (Table 4). It turns out that these subgroups are not only apparent in the proper motion diagram, but likewise they correspond to different regions in the position diagram, which independently confirms our subdivisions. The first subgroup consists of the 2 early type stars and 3 CTTS (Sz 6, Sz 19 and CV Cha), all located in the cloud core of Cha I . In the second subgroup there are 5 new PMS stars, which all have very similar proper motions and, with the exception of RXJ 0837.0-7856 1, are all located between the Cha I and Cha II clouds. Besides these ROSAT detected PMS stars one CTTS (VW Cha) and one WTTS (T Cha) also match the requirements of subgroup 2. Note that T Cha is also located between Cha I and Cha II , whereas the position of VW Cha is close to the core of the Cha I cloud.
Table 4. Subgroups derived from the proper motion diagram (Fig. 3) with their mean proper motions and dispersions. The number of stars in each subgroup is given in the last column.
The proper motions of subgroups 1 and 2 point into the same direction, but the absolute values are about twice as high for the second group. This finding is consistent with a scenario where the mean distance for the second subgroup is about half of the mean distance for the first subgroup. Then, both groups would have consistent space velocities. Indeed this picture is confirmed by the Hipparcos parallaxes: with our assumption for the mean distance of Cha I of 170 pc (the distances for 4 stars in subgroup 1 are 175 pc (HD 97048), 188 pc (HD 97300), 143 pc (Sz 6) and 210 pc (Sz 19) ) we would expect a mean distance of about 90 pc for stars in our subgroup 2. The parallaxes as observed by Hipparcos for 3 stars in subgroup 2 correspond to distances of 86 pc (RXJ 1158.5-7754a), 92 pc (RXJ 1159.7-7601) and 66 pc (T Cha), which gives very strong support to our interpretation.
The existence of subgroup 3 is not so obvious as for the other 2 subgroups. The 3 stars which we grouped together are CS Cha, CHXR 11 and RXJ 0850.1-7554. Nevertheless, if we assume its existence we could attribute 4 more stars with higher (BF Cha and CHXR 8) or lower distances (RXJ 0951.9-7901 and CHXR 32) to it.
Only 2 WTTS and 1 new PMS (Sz 41, CHXR 56 and RXJ 1150.4-7704) are left from the sample of the bona-fide PMS stars (Table 1 and upper part of Table 2) which do not fit in any of the above subgroups because of quite different proper motions. Sz 41 is at least a double system: besides a faint companion another star nearly as bright as the primary is located 11.4 away from Sz 41 (Brandner 1992; Reipurth & Zinnecker 1993). RXJ 1150.4-7704 is flagged as a possible spectroscopic binary by C97. Thus it is possible that the proper motions of these stars are not representing their space motions.
The velocity dispersions in our subgroups are of the same order of magnitude as the errors of the proper motions, and so the intrinsic velocity dispersions must be much smaller (at a distance of 170 pc 1 mas yr-1 corresponds to 0.8 km s-1). To some extent this was expected, because we only grouped stars with similar proper motions together. On the other hand such low values for the intrinsic velocity dispersion agree with other determinations. Jones & Herbig (1979) derived a value of 1-2 km s-1 in one coordinate for the intrinsic velocity dispersions of subgroups in Taurus-Auriga and considered this as typical for associations. Dubath et al. (1996) calculated a value of 0.90.3 km s-1 based on the radial velocities of 10 stars in Cha I .
3.1.2. ZAMS stars and others
The stars of Table 2 classified as stars with dubious PMS nature or as ZAMS stars by C97 clearly show a very large range in proper motions, which independently confirms the conclusions from applying the lithium criterion by C97. This criterion is very conservative, as it rejects stars with lithium abundances similar to the Pleiades as weak-line TTS, although it may very well be the case that some truly pre-main sequence stars exhibit such low lithium strength. Note that all the stars with Hipparcos parallaxes in Table 2 fall well above the main sequence when comparing their positions in the HR diagram with various PMS evolutionary tracks (Neuhäuser & Brandner 1998).
There are a few other stars in Table 2 which - rated from their proper motions - probably fall into this category of unrecognized weak-line T Tauri stars. Judging from the proper motions alone one could assign RXJ 0928.5-7815, RXJ 0952.7-7933 and RXJ 1209.8-7344 to subgroup 1, RXJ 0917.2-7744 and RXJ 1125.8-8456 (its Hipparcos parallax corresponding to 83 pc also fits this interpretation) to subgroup 2, and RXJ 0849.2-7735 and RXJ 1223.5-7740 to subgroup 3. One must however bear in mind that the reflex motion of the sun is very similar to the typical proper motion of Chamaeleon member stars, making it difficult to distinguish between members and field stars on the basis of the proper motions alone.
3.2. Space velocities
We have calculated space velocities for all stars in subgroups 1 and 2 and the Hipparcos stars with radial velocities available in the literature (HD 97048 from Finkenzeller & Jankovics (1984), 4 CTTS from Dubath et al. (1996), and T Cha and stars in Table 2 from C97). For stars not observed by Hipparcos a distance of 170 pc for subgroup 1 and 90 pc for subgroup 2 was adopted.
We corrected the space velocities for the effect of differential galactic rotation, assuming the IAU standard values of 8.5 kpc for the distance to the galactic centre and 220 km s-1 for the velocity of the Local Standard of Rest. The value of this correction depends on the galactic azimuthal angle and therefore in general also on the distances of the stars. The mean corrections in the U-velocities for stars in subgroup 1 and 2 are 3.8 km s-1 and 2.0 km s-1, respectively (the corrections in the V-velocities are practically zero). Additionally, the motion of the Sun ( (U,V,W) = (9,12,7) km s-1, Delhaye 1965) has been added to the space velocites, although it does not change the relative velocities between the groups which are of interest here.
The coincidence of the mean values for the three space velocity components of Hipparcos PMS stars and the combined sample of subgroups 1 & 2 is artificial to some extent (see Table 5) as the 6 Hipparcos stars form a subset of subgroups 1 & 2. The mean values for subgroup 1 and 2 are also in quite good agreement.
Table 5. Mean space velocities and dispersions separately for stars of subgroups 1 & 2 and Hipparcos PMS stars as well as for the total samples, as shown in Fig. 4. U-velocities are positive in the direction of the galactic centre. The velocities have been corrected for the effects of differential galactic rotation and the reflex motion of the Sun. For the calculation of the space velocities we used either the Hipparcos parallaxes or, where not available, a distance of 170 pc for stars in subgroup 1 and 90 pc for stars in subgroup 2.
Our interpretation of different proper motions in terms of different distances is further confirmed when taking the additional information on the radial velocities and projection effects due to different positions in space into account.
Comparing the Hipparcos PMS stars with ZAMS stars and stars of dubious PMS nature in Fig. 4, one again notes the clear peak in the distribution of the PMS stars and the large scatter of the presumably older stars.
© European Southern Observatory (ESO) 1998
Online publication: September 14, 1998