Astron. Astrophys. 361, 1143-1151 (2000)

4. Discussion

We present in Fig. 3 a general view of the spatial distribution and proper motions in galactic coordinates of the measured PMS stars, and in Fig. 4, Fig. 5, Fig. 6 and Fig. 7 - upper panels - zooms of the 4 main star-forming regions studied in this work. In all these regions, a dominant orientation of the proper motions towards smaller longitudes can be observed. This is in large part the effect of the reflex solar motion, as can be seen in the versus graphs of the same regions (Fig. 4, Fig. 5, Fig. 6 and Fig. 7 - lower panels). Since the effect of solar motion on the star proper motion depends on the distance of the stars, and the distances of PMS stars are usually poorly known, we present in these figures the reflex solar motion as a function of distance, from 50 to 200 pc. We assumed the basic solar motion, with components U = 9 km/s (directed towards the galactic center), V = 11 km/s, W = 6 km/s. The reflex solar motion depends also on the direction of the stars, and since some of the regions studied here have sizes of several degrees, we present it for two extreme directions in each field (except for Corona Australis, which is a small field).

Fig. 3. Positions and proper motions of PMS stars depicted in Figs. 4 to 7 - upper panels.

Fig. 4. Upper panel: Positions and proper motions of PMS stars in Chamaeleon. Lower panel: Components of proper motions of PMS stars in Chamaeleon, in galactic coordinates. The reflex solar motion is presented for two directions - (1): , and (2): , - for distances ranging from 50 pc to 200 pc, in steps of 25 pc, from left to right. The open symbols represent HAeBe stars, the filled ones T Tauri stars.

Fig. 5. Upper panel: Positions and proper motions of PMS stars in Lupus. Lower panel: Same as Fig. 4 - lower panel - for Lupus. (1): , and (2): ,

Fig. 6. Upper panel: Positions and proper motions of PMS stars in Upper Scorpius - Ophiuchus. Lower panel: Same as Fig. 4 - lower panel - for Upper Scorpius - Ophiuchus. (1): , and (2): ,

Fig. 7. Positions and proper motions of PMS stars in Corona Australis. Lower panel: Same as Fig. 4 - lower panel - for Corona Australis. ,

In all groups, a number of stars are found to have proper motions that lie well outside the average distribution. Notice that some of these stars that can be seen in the () maps (for Upper Sco, Chamaeleon and Corona Australis) are absent from the () graphs, due to the scale that we adopted. We consider that these stars have a large probability of being recently formed runaway stars and a possible explanation for them is the disruption of multiple systems. We cannot exclude the possibility of errors, like a wrong identification during the proper motion determination process; however, note that some of the stars with anomalous proper motions are bright and were found in several sources (eg. HD 137727, HD 140637), which reduces the probability of an erroneous identification.

Disregarding the runaway stars, the groups of PMS stars present proper motion dispersions of the order of 10 mas/yr typically. This means that in a period of about one million years, the stars would move apart several degrees in the sky, and would look very dispersed. We must bear in mind, however, that part of this apparent dispersion may not be intrinsic, but due to the fact that the sun is approaching the group. For instance, most of the PMS stars of Upper Scorpius present negative radial velocities, while most of the stars in Chamaeleon present positive radial ones (e.g. Gregorio-Hetem et al. 1992; Torres et al. 1995; Covino et al. 1997).

Let us now comment on the average proper motion of the stellar groups. We already noticed that the average values are largely explained by the reflex solar motion. However, they are not entirely due to this effect or, in other words, intrinsic average proper motion of the groups are detected. For instance, in the case of Lupus (our largest sample, Fig. 5), the center of mass of the points in the () graph is about (-30,-9), which suggests mean distance of about 85 pc, if we consider only the reflex solar motion. However, the average distance of 14 stars of this group that have parallaxes from HIPPARCOS is 138 pc. This suggests that the Lupus PMS stars have a mean intrinsic proper motion in the longitude direction of about = -10 mas/yr.

The Chamaeleon group is peculiar, in that it seems to present two distinct kinematic groups, even after excluding the runaway stars. A group with proper motions close to about = -40 mas/yr, = -15 mas/yr, seems to be well behaved. Its observed proper motion could be explained by the the reflex solar motion, if its distance is of the order of 70 pc. And indeed, some of the stars of this group have distances determined by HIPPARCOS, and are not too different from this value (T Cha, 66 pc, RX J1158.5-7754a, 86 pc, RX J1159.7-7601, 92 pc, HD 104237, 116 pc). This group was already discussed by Terranegra et al. (1999); notice, however, that Bertout et al. (1999) consider that the HIPPARCOS distance of T Cha is incorrect. Another group of stars presents positive values of , considerably different from the reflex solar motion. Among these, only HD97300 has parallax measured by HIPPARCOS (187 pc).

Finally, we remark that in the studied regions, no systematic differences between the proper motions of T Tauri stars and HAeBe stars can be observed. Our results favour the PMS nature of the candidates HAeBe stars included in our list.

A deeper analysis of the proper motion of the groups of PMS stars, in connection with the ages of the subgroups, will be presented in a separate paper, where our results are compared with the models proposed in the literature for the mechanisms that might have triggered the star formation.

© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000
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