Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990mnras.247...62e&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 247, NO. 1/NOV1, P. 62, 1990
Astronomy and Astrophysics
Astronomy
5
Scientific paper
UU Cnc is a 96-d binary system (K4 111+?) which has been interpreted as a contact binary, presumably representing the common-envelope stage of evolution. Since this is such a bizarre possibility, we have tested it by using the rotational velocity to determine the mass ratio and by obtaining lUE observations to search for the hot, accreting source that might be expected if the system were semi-detached. The rotational velocity near conjunction, V sin i=22±3 km s-1, based on two independent determinations, requires the optically undetected companion star to be more massive than the K4 giant; the measured mass ratio is 1.47±0.2. The lUE spectra covering the entire orbit show no variation in the ultraviolet continuum greater than 0.3 mag. The source of the ultraviolet continuum is still uncertain. The detected flux in the ultraviolet is lower than expected from a star as massive as the unseen component (M ≥ 1.6 Msun), unless it is evolved, swollen by accretion, or hidden in a disc. The continuum flux in the ultraviolet is roughly four times that predicted for a normal K4-5 giant or supergiant, and existing lUE spectra of K `giants indicate this excess is outside the range for the star's spectral type. We develop the model in which the system has reversed its mass ratio through a combination of mass loss and exchange. The luminosities of the two components would differ by two orders of magnitude, so most of the light in the optical would be from the K4 giant, as observed. The light-curve solution indicates the spectroscopically undetected star is surrounded by a dark disc which shields it in the optical. The ultraviolet excess probably comes from the central regions of this disc, which cannot be eclipsed. An inclination of about 70° is derived from this restriction, which is consistent with the light curve. Unless the solution is radically different than this, specifically, in the sense of being detached, the limb and gravity darkening of the K giant must be close to the levels expected for a convective star.
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