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

6. Rotation and activity

It is generally believed that the activity of late-type stars, e.g. x-ray emission, is related to magnetic fields generated by a dynamo process, and thus should correlate somehow with stellar rotation. There is in fact strong evidence for such a correlation (cf. Bouvier 1990, Simon 1990, Stauffer et al. 1994).

The use of projected rotational velocities, as obtained by our observations, in the study of this correlation has two disadvantages. As one measures rather than the angular velocity (where P is the rotation period), on which the dynamo effect depends (cf. Hempelmann et al. 1995), the results are subject to uncertainties both due to the unknown projection factor , and due to the influence of the stellar radius .

In order to compare our stars with other young late-type stars, we have made use of the Open Cluster Database ¹ to collect a large sample of young stars for which rotational velocities are available. We used data for stars in Per, Hyades, IC 2602, IC 2391, IC 4665, Pleiades, and Preasepe. X-ray data were collected from Prosser et al. (1996), Stern et al. (1995), Randich et al. (1995), Giampapa et al. (1998), Stauffer et al. (1994), and Randich & Schmitt (1995). To reduce the influence of stellar radii, we divided by to obtain . For our sample, we use from Table 2, while for the open cluster stars, we computed the radii from and under the assumption of blackbody radiation.

In Fig. 7 we show, side by side, vs. (left) and vs. (right). The latter plot shows a notable improvement in the tightness of the correlation of stellar activity and rotation. Also, only in the right plot it becomes obvious that one of the stars (HE 577) is a very significant outlier. This has already been noted by Randich et al. (1996). The membership of the star to the Per cluster is doubtful (Prosser 1992).

Fig. 7. Activity vs. rotation for Li-rich Taurus-Auriga stars (PMS: filled circles, ZAMS: filled triangles), and three comparison samples (`old' ZAMS stars (Hyades): diamond; `young' ZAMS stars (IC 2602, Per, IC 4665, Pleiades): crosses; CTTS/WTTS: open circles).

As one can see from Fig. 7, the stars of our Taurus sample populate the same area of the diagrams as other active late-type stars. However, there are no ultra-fast rotators (like in the `young' ZAMS clusters) in our sample. This may be just an effect of the smaller sample size - even in the `young' ZAMS clusters like Per, only a small fraction of the stars are actually ultra-fast rotators.

The lower part of the diagram () is almost exclusively populated by stars of the Hyades and the Pleiades, which are the two oldest clusters among the plotted samples. However, this might be a bias due to x-ray selection effects, as cluster membership is often determination on the basis of x-ray activity.

If we divide our Taurus sample into those stars we regard as ZAMS, and those we regard as PMS, we do not find a significant difference with respect to . The mean values are for ZAMS stars and for PMS stars (the median is -3.34 for both). This is not very surprising, as many of the stars are apparently at - or close to - the saturation level.

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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