Composite spectra - Paper 10: the equal-mass binary HR 2030 (K0 IIb+B8IV)

We separate the spectra of the individual components of HR 2030, a sixth-ma gnitude composite-spectrum binary system, and show that they have types clo se to K0 IIb and B8 IV, and masses that are equal to within the precision o f the measurements (mass ratio = 1.00 +/- 0.03). The orbit appears to have a very small eccentricity, although reasons are given for believing that su ch eccentricity is spurious; it has a period of 66 d and an inclination est imated at 30 degrees to the line of sight. Our photometric model of the sys tem confirms the luminosity types derived from the spectra and indicates an interstellar absorption of 0.4 mag, in accord with the observed strength o f the interstellar K line. We derive the physical parameters (T-eff, M-bol, R, L) of the components, and calculate that the mass of each star is close to 4.0 M.. We further show that the hot component (R = 5.9 +/- 0.6 R.) has already evolved to a position significantly above the zero-age main sequen ce (ZAMS), and we propose that the primary (R = 41 +/- 5 R.) is making its first ascent of the red-giant branch. From comparisons with evolutionary tr acks, we deduce that the age of the binary (since its arrival at the ZAMS) is in the range 1-2 x 10(8) yr. While we suspect that the components are sufficiently close for some tidal distortion to occur, the effects are not discernible in our data owing to t he rather low orbital inclination. The system shows Si I in emission as a r esult of irradiation of the primary by the hot secondary, but in the optica l spectrum we see little other clear evidence of interaction between the co mponents even though the object has a relatively short period and is a stro ng X-ray source. On the other hand, Hipparcos photometry suggests the exist ence of a major non-uniformity of the surface of the primary star.