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A photometric-spectroscopic analysis and the evolutionary status of the Algol-type binary U Coronae Borealis

Yerli, Sinan Kaan
Sarna, MJ
Zola, S
Smith, RC
Tovmassian, G
The prime purposes of this study are to obtain reliable orbital parameters for the Algol-type binary U Coronae Borealis (U CrB) and to explain the evolutionary status of this system. All observations of the primary star's radial velocity are consistent with the value K(1) = 58.6 +/- 2.0 km s(-1). Measurements of the radial velocity of the secondary component give K(2) = 185.2 +/- 5.0 km s(-1). Using the photometrically determined inclination of 78.7 +/- 0.3, the masses of the two stars are therefore deduced to be 4.74 +/- 0.28 and 1.46 +/- 0.06 M. for the primary and secondary components, respectively. Using all available observations, we discuss the origin and evolution of the close binary system U CrB. We derive the restrictions concerning masses and period from a general network of calculations of medium mass close binary evolution. Detailed models are calculated within the derived ranges, giving the most likely initial system parameters as and P(i) = 1.4 d. It turns out that the interactive evolution up to the present stage has been non-conservative. During its evolution, U CrB has lost about 14 per cent of its initial total mass (DeltaM similar to 1 M.) and around 18 per cent of its initial total angular momentum. We also examine the possibility of probing dynamo action in the mass-losing component of U CrB. We point out that, in order to maintain the evolution of U CrB in its later stages, which is presumably driven by stellar 'magnetic braking', an efficient mechanism for producing large-scale surface magnetic fields in the donor star is required. We suggest that observed X-ray activity in U CrB may be a good indicator of its evolutionary status and the internal structure of the mass-losing component.