CS-22873-139 - A VERY METAL-POOR MAIN-SEQUENCE SPECTROSCOPIC BINARY WITH COLORS INDICATIVE OF INTERMEDIATE AGE

CS 22873-139, a metal-deficient ([Fe/H]=-3.1), high-velocity (Vrad=+243 km s-1) star with observed UBV colors (B-V=0.37, U-B=-0.22) that locate it near the main-sequence turnoff of an old metal-poor population [Preston et al. ApJS, 76, 1001 (1991); Beers et al. AJ, 103, 1987 (1992)], is, in fact, a double-lined spectroscopic binary with a period of 19.16 days, eccentricity e=0.26, and a mass ratio of approximately 0.88. The observed colors, corrected for reddening, can be matched by those for a family of artificial binaries constructed by use of colors and luminosities taken from isochrones for ages between 3 and 13 Gy, but the strengths of metal lines in the primary and secondary spectra require that the age of the system be less than ∼8 Gy. The inferred unreddened colors of the primary, (B-V)0,p=0.30, (U-B)0,P=-0.19, lie blueward of any globular cluster turnoff and near the low-abundance (upper) U-B boundary of BMP stars in the UB V two-color diagram, as discussed by Preston et al. [AJ, 108, 538 (1994)]. Were the primary product of binary merger in an initial triple star system, Harrington's [RMxA&A, 3, 139 (1977)] stability criterion requires for the initial close binary A1<5.2R⊙ and P<1.5 days. According to Vilhu [A&A, 109, 17 (1982)] such a system will merge in about 109 years, a time much shorter than the main-sequence age deduced for this system, so even if binary merger had taken place the conclusion that CS 22873-139 must be a relatively young metal-poor star is preserved.