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Astron. Astrophys. 361, 265-272 (2000)

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3. Our sample of stars with [FORMULA] 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 [FORMULA] value. This sample is represented in Fig. 5, where we plot the values of [FORMULA] as a function of the colour index [FORMULA]. The individual data will be published in a future paper.

[FIGURE] Fig. 5. Values of [FORMULA] derived from [FORMULA] values for our sample of stars as a function of the colour [FORMULA].

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 [FORMULA] 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 [FORMULA] 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 [FORMULA] 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 [FORMULA] [FORMULA] 0.9 and [FORMULA] [FORMULA] -4.7. Also, comparing to the similar diagram of Henry et al. (1996), we have a higher number of active dwarfs ([FORMULA] [FORMULA] -4.75) compared with non-active dwarfs ([FORMULA] [FORMULA] -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.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000