Astron. Astrophys. 343, 273-280 (1999)

5. Conclusions

We have carried out spectroscopic observations and determined the orbital elements of seven binary systems whose brightest component is a chemically peculiar star. The results are summarized in Table 4.

Gerbaldi et al. (1985) have compared the period distribution of binary systems with B6-B9.5 primaries with the period distribution of systems with a CP star. These authors concluded that for normal stars the distribution peaks at days, whereas for cool stars the minimum orbital period is 3 days and the minimum orbital period for Si stars is 4.7 days. Our results for HD 11753, HD 15144 and HD 25267, which belong to the previous spectral range, confirm Gerbaldi and co-workers conclusions on cool CP and Si stars.

As to HD 25267 and HD 36485 whose stellar mass has been determined by North (1998) and Bohlender (1989) respectively, we have estimated the mass of the secondary star and confirmed Jaschek & Jaschek (1976) result that primary CP stars show a deficiency of massive companions.

Of the seven systems studied here, only the cool CP star HD 15144 presents a rotational period which is synchronised with the orbital period and a circular orbit. The lack of circular orbits for binaries with a CP star noted by Jaschek & Jaschek (1976) and Gerbaldi et al. (1985) is also here confirmed.

The helium-peculiar stars HD 37017, HD 36485, HD 142096 and HD 189178 have orbital periods longer than 12 days and large eccentricity values (0.26-0.40). It appears that the hot and cool CP stars show the same behavior with respect to normal-star binary systems.

From the previous results, it appears that when binary systems with a chemically peculiar star are compared with normal star systems, they present a lower incidence, a deficiency of short periods, rather eccentric orbits, and companions of low mass. We suggest that these characteristics of binary systems with a CP star are due to the fact that these stars can exist only in binary systems with long orbital periods. Short orbital periods, which are related to massive companions and small component separations, give origin to strong atmospheric mass motions preventing the element diffusion which is at the basis of the CP star phenomenon ¹. If CP stars cannot exist in binary systems with short orbital period, the incidence of binary systems with a CP star is expected to be lower than for normal-star systems. In binary systems with long orbital periods, the eccentricity-period distribution of CP stars is not different than for other early-type-star binaries. Giuricin et al. (1984) have shown that O-A normal stars present nearly circular orbits only for orbital periods shorter than two days.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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