3. Our sample of stars with values
After about one-and-a-half years of measurements (from August 1998 to March 2000) we have about 400 stars (from the more than 1000 stars observed for radial velocities) with at least one derived value. This sample is represented in Fig. 5, where we plot the values of as a function of the colour index . The individual data will be published in a future paper.
Our original sample of stars (part of the CORALIE extra-solar planet survey) was chosen to be volume-limited (Udry et al. 2000), containing this way no sampling biases. This kind of choice will give us the possibility of studying the characteristics of extra-solar planetary systems and their occurrence in a statistical and unbiased way.
From the "activity" point of view, the determination of the chromospheric activities for the stars in such a sample can be particularly interesting: when completed, this survey will be the first large volume-limited survey for chromospheric activity in dwarfs. Unfortunately, the sub-sample of stars with presently derived values is not unbiased like the original sample. First, K dwarfs are intrinsically fainter than F or G dwarfs, and hence are more difficult for measuring activity (higher integration times are needed). Since K dwarfs have deeper lines, our radial-velocity technique is more accurate for these stars, and a lower flux is required to obtain high-precision radial velocity measurements than for a F or G dwarf. These facts make K dwarfs rather difficult targets for computing activity values with our spectra.
On the other hand, since active stars have more flux in the center of the CaII H line (compared to lower activity objects), we can easier find "acceptable" values (with high fluxes) of for active dwarfs than for their inactive counterparts. This way, we expect a bias favoring high-activity F dwarfs in our preliminary data (while we don't have values for all the CORALIE sample) and comparatively a small number of K dwarfs with low activity values.
These biases can be seen in the plot of Fig. 5 as the under-populated region with 0.9 and -4.7. Also, comparing to the similar diagram of Henry et al. (1996), we have a higher number of active dwarfs ( -4.75) compared with non-active dwarfs ( -4.75). This biases will be corrected as more measurements will be collected, and the sample covered. The distribution of stars in this diagram follows, however, a pattern very similar to the one found by Henry et al. The data gathered until now already permit us to do some interesting studies.
© European Southern Observatory (ESO) 2000
Online publication: September 5, 2000