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

2. The stellar samples

Radial-velocity monitoring of several samples of chemically-peculiar red giants (PRG) has been performed by the team of McClure at the Dominion Astrophysical Observatory (DAO, Canada) and by the CORAVEL team on the Swiss 1-m telescope at Haute-Provence Observatory (France) and on the Danish 1.54-m telescope at the European Southern Observatory (La Silla, Chile), with the aim of deriving their binary frequencies. A detailed description of the CORAVEL data, along with the new orbits, is given in two companion papers (Udry et al. 1998a,b; see Baranne et al. 1979 for a description of the CORAVEL spectro-velocimeter).

A brief description of these samples, on which the present study relies, is given below, with special emphasis on their statistical significance.

2.1. Barium stars with strong anomalies

The CORAVEL and DAO samples taken together contain all 34 known barium stars with strong anomalies (i.e. Ba4 or Ba5 on the scale defined by Warner 1965) from the list of Lü et al. (1983). The binary frequency derived for this complete sample in Sect.  4thus allows us to address the question of whether binarity is a necessary condition to form a strong barium star. Three stars with a Ba3 index monitored by McClure were included as well in this sample of strong barium stars.

2.2. Barium stars with mild anomalies

The CORAVEL and DAO samples taken together include 40 stars with a mild barium anomaly (Ba 1, Ba1 and Ba2 on the scale of Warner 1965). The CORAVEL sample is a random selection of 33 Ba 1, Ba1 and a few Ba2 stars from the list of Lü et al. (1983). Although this sample is by no means complete, it provides a good comparison to the sample of strong barium stars described above, for investigating the correlation between the orbital elements and the intensity of the chemical anomaly.

Because orbital elements for barium stars are spread in the literature, Tables 1a and 2a collect all orbital elements available for mild and strong barium stars, respectively. The number in column `Ref.' of these tables refers to the following papers where the complete set of orbital elements for the considered star can be found: 0. This paper (see below); 1. Udry et al. (1998a); 2: Udry et al. (1998b); 5. Griffin (1996); 6. Griffin et al. (1996); 7. Jorissen et al. (1995); 11. Griffin & Keenan (1992); 12. Griffin (1991); 13. McClure & Woodsworth (1990); 20: Griffin & Griffin (1980). Fekel et al. (1993) report preliminary orbital elements for the mild barium star HD 165141; the lower limit on the orbital period quoted in Table 1a is derived from their more accurate KPNO data. For the sake of completeness, a note identifies stars with an orbital or acceleration solution in the Hipparcos Double and Multiple Systems Annex (ESA 1997). The comparison between the astrometric and spectroscopic elements is deferred to a future study.

Table 1a. Orbital elements for mild (Ba 1, Ba1 and Ba2) barium stars. Column 2 provides the spectral subclass ( if K type, if G type) and column 3 the Ba index, from Lü et al. (1983). The columns labeled and N give the average error on one measurement and the number of measurements, respectively. A dash in column Sb indicates that the spectral line width is smaller than the instrumental profile. For orbits obtained from instruments other than CORAVEL, the Sb parameter is not available (`na'). When an orbital solution is available, is the systemic radial velocity; otherwise, it is the average radial-velocity with its standard deviation. is a photometric index characterizing the strength of the Ba anomaly (see text). The numbers in column `Ref.' refer to Table 5, which gives the reference where the complete set of orbital parameters for the considered system may be found

Table 1b. Suspected binary mild barium stars. The numbers in column `Ref.' refer to the papers listed in Table 5

Table 1c. Mild barium stars with no evidence of binary motion. The numbers in column `Ref.' refer to the papers listed in Table 5

Table 1d. Supergiants misclassified as mild barium stars. The numbers in column `Ref.' refer to the papers listed in Table 5

For several barium stars monitored by McClure, a few CORAVEL measurements have been obtained to improve the DAO orbit, since these measurements significantly increase the time span of the monitoring. These updated orbits are listed in Table 1a and 2a under the reference number 0. A zero-point correction of -0.46  km s-1 has been applied to the DAO measurements, as derived from the average difference in systemic velocity for the 3 stars (HD 46407, HD 131670 and HD 223617) for which independent DAO and CORAVEL orbits are available.

Several barium stars have very long periods, exceeding the time span of the monitoring. In those cases, whenever possible, a preliminary orbit was nevertheless derived by fixing one of the orbital parameters (usually the period). Those cases can be readily identified in Tables 1a and 2a by the fact that there is no uncertainty given for the fixed parameter (see Udry et al. 1998a for more details).

Table 2a. Same as Table 1a for strong (Ba3, Ba4 and Ba5) barium stars. The numbers in column `Ref.' refer to the papers listed in Table 5

Table 2b. Strong barium stars with no evidence for binary motion. The numbers in column `Ref.' refer to the papers listed in Table 5

2.3. Non-variable S stars

Besides the orbit obtained for the S star HR 1105 (=HD 22649) by Griffin (1984), our CORAVEL monitoring of a sample of 56 S stars is the primary source for investigating the binary frequency among S stars. This sample includes 36 bright, northern S stars from the General Catalogue of Galactic S Stars (GCGSS; Stephenson 1984) with no variable star designation, neither in the General Catalogue of Variable Stars (Kholopov et al. 1985) nor in the New Catalogue of Suspected Variable Stars (Kukarkin et al. 1982). The criterion of photometric stability has been adopted to avoid the confusion introduced by the envelope pulsations masking the radial-velocity variations due to orbital motion. Such a selection criterion clearly introduces a strong bias against intrinsically bright S stars, which is of importance when deriving the binary frequency among S stars (see the discussion in Sect.  4.3).

Our samples include the border case HD 121447, sometimes classified as a Ba5 star and sometimes as an S star; in the analysis of the orbits presented in the next sections, this star has been included among both barium and S stars.

Table 3a presents all 25 orbits available for S stars, collected from the following papers, referred to in column `Ref.' of Table 3a: 0. This paper (see below); 1. Udry et al. (1998a); 3. Carquillat et al. (1998); 7: Jorissen et al. (1995); 10: Jorissen & Mayor (1992); 18: Griffin (1984). The orbits of Jorissen & Mayor (1992) have been updated with a few new measurements and listed in Table 3a with reference number 0 in column `Ref'.

2.4. Mira S stars

A sample of 13 Mira S stars has also been monitored with CORAVEL, in order not to restrict the search for binaries to low-luminosity S stars (see Sect.  2.3). However, the envelope pulsations of Mira stars will undoubtedly hamper that search (see Sect.  3and Udry et al. 1998a for a detailed discussion).

2.5. SC and Tc-poor carbon stars

A sample of 7 SC and CS stars has been monitored as well with CORAVEL, along with the 3 carbon stars lacking Tc from the list of Little et al. (1987).

2.6. CH stars

Orbits of CH stars are provided by McClure & Woodsworth (1990), and are not repeated here.

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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