Contribution to the search of binaries among Am stars. II. HD 81976 and HD 98880, double-lined spectroscopic binaries

Abstract

We present the results of the observations of two Am stars of eighth magnitude, the double-lined spectroscopic binaries HD 81976 and HD 98880, carried out with the CORAVEL instrument at the Observatoire de Haute-Provence in order to determine their orbital elements. We found 1) for HD 81976: P = 5.655750 days, T = 2449785.941 HJD, omega = 341.4^deg, e = 0.061, K₁ = 61.68 km s^-1, K₂ = 63.84 km s^-1, V₀ = 19.85 km s^-1, a₁ sin i = 4.788 Gm, a₂ sin i = 4.956 Gm, M₁ sin ³ i = 0.5875 M_sun, M₂ sin ³ i = 0.5676 M_sun, and 2) for HD 98880: P = 14.20783 days, T₀ = 2448682.883 HJD (ascending node), e = 0., K₁ = 42.47 km s^-1, K₂ = 49.16 km s^-1, V₀ = 2.40 km s^-1, a₁ sin i = 8.298 Gm, a₂ sin i = 9.604 Gm, M₁ sin ³ i = 0.6091 M_sun, M₂ sin ³ i = 0.5262 M_sun. The first of these two systems, HD 81976, is formed by two quasi-identical stars, and the Hipparcos data (M_V, B-V) are consistent with late A stars in effective temperature; it is likely that the components rotate synchronised with the orbital motion. A third body may be present in this system since (i) the orbit has a significant eccentricity despite its short period and (ii) the systemic velocity V₀ shows a possible drift. For the second system, HD 98880, we give Delta m_B 1.25 and we propose a simple model based upon Strömgren photometric indices and the HR theoretical diagram of Schaller et al (1992) in addition to orbital parameters and Hipparcos data: T_eff = 7000 K, log ₁₀ g = 4.0, M₁ = 1.9 M_sun, M₂ = 1.6 M_sun, log ₁₀(age) = 9.12. The components do not rotate synchronously contrary to HD 81976. Both binaries appear to be detached systems without possibility of eclipses. Based on observations made at the Haute-Provence Observatory, France.