Astron. Astrophys. 332, 877-903 (1998)

3. The radial-velocity jitter: a new diagnostic

The standard deviation of the residuals for some of the orbits computed by Udry et al. (1998a b) is clearly larger than expected from the error on one measurement (Tables 1a, 2a and 3a). Fig. 1 shows that there is a tendency for the largest residuals to be found in the systems with the broadest spectral lines, as measured by the CORAVEL line broadening index Sb. The significance of this correlation is discussed in this section.

Fig. 1. The jitter (where is the average uncertainty on one measurement, and is the standard deviation of the radial-velocity measurements for non-binary stars, and of the residuals around the computed orbit for binary stars) as a function of the CORAVEL line broadening index Sb (see text). Stars with a cc-dip narrower than the instrumental profile have been assigned . Similarly, the jitter has been set to 0 if . Data are from Tables 1-3. Typical error bars on Sb are displayed in the upper left corner. The suspectedly misclassified S stars HD 262427 and BD+22 O4385 (Table 3b) have not been plotted. The inset is a zoom of the lower left corner

The CORAVEL spectrovelocimeter (Baranne et al. 1979) measures the stellar radial velocity by cross-correlating the stellar spectrum with a mask reproducing about 1500 lines of neutral and ionized iron-group species from the spectrum of Arcturus (K1III). Consequently, the width of the cross-correlation dip (cc-dip) is an indicator of line broadening. More precisely, the cc-dip of minor planets (reflecting the sun light), corrected for the solar rotational velocity and photospheric turbulence, allows the determination of an `instrumental profile' . That parameter corresponds to the sigma of a gaussian function fitted to the cc-dip of a hypothetical star without rotation and turbulence. An estimator of the total broadening of stellar spectral lines can then be derived from the observed width of the stellar cc-dip as .

In cool red giants where macroturbulence is the main line-broadening factor, the Sb parameter is expected to increase with luminosity, as does macroturbulence (e.g. Gray 1988). This prediction is confirmed from the luminosities derived by Van Eck et al. (1998) from HIPPARCOS parallaxes for 23 S stars in common with the present sample. A least-square fit to these data yields the relation

valid for   km s-1 . Since bright giants also exhibit large velocity jitters probably associated with envelope pulsations (e.g. Mayor et al. 1984), a correlation between Sb and the radial velocity jitter must indeed be expected, as observed on Fig. 1. Orbits with a large jitter tend to be associated with giants having large Sb indices. This trend is especially clear among binary S stars, and continues in fact among non-binary S stars (the jitter being then simply the standard deviation of the radial-velocity measurements). Binary (`extrinsic') and non-binary (`intrinsic') S stars actually form a continuous sequence in the (Sb, jitter) diagram of Fig. 1, the transition between extrinsic and intrinsic S stars occurring around   km s-1 . Intrinsic S stars, with their larger Sb indices (   km s-1), may thus be expected to be more luminous than extrinsic S stars [ (km s-1) ] ¹. This conclusion is confirmed by the luminosities derived from the HIPPARCOS parallaxes (Van Eck et al. 1998). Extrinsic S stars are in turn more luminous than barium stars, with the border case HD 121447 (K7IIIBa5 or S0; Keenan 1950, Ake 1979) having   km s-1 , intermediate between Ba and S stars. HD 60197 (K3Ba5) and BD (K0Ba1.5) are two other barium stars with especially large Sb indices, suggestive of a luminosity larger than average for barium stars, though there is no information available in the literature to confirm that suggestion.

Not represented (because accurate values are lacking) are the two remarkable mild barium stars HD 77247 (   km s-1) and HD 204075 (   km s-1) observed at DAO. The latter is indeed known to be a bright giant, with Mv (Bergeat & Knapik 1997), thus confirming the fact that Sb is a good luminosity indicator for red giants.

The large velocity jitter observed in Mira S stars is a consequence of their complex and variable cc-dips (Barbier et al. 1988; see also Udry et al. 1998a). In some cases however (like AA Cyg and R Hya), the cc-dips are featureless, broad and very stable. These stars with a comparatively smaller jitter are located on the lower boundary of the region occupied by intrinsic S stars in Fig. 1.

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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