Orbital period investigations of two short-period early-type overcontact binaries BH Cen and V701 Sco in two extremely young galactic clusters IC 2944 and NGC 6383

Abstract

Both V701 Sco and BH Cen are two early-type short-period overcontact systems ( P = 0. d 762 and P = 0. d 792, respectively). V701 Sco is a member of the young galactic cluster NGC 6383, while BH Cen is a component of a younger galactic cluster IC 2944 where star formation is in process. They provide good opportunity to understand the formation and evolution of binary stars. In the present paper, orbital period changes of the two binaries are investigated. It is discovered that the orbital period of BH Cen shows a long-term increase with a rate of d P/d t = +1.70(±0.39) × 10 −7 days/year while it undergoes a cyclic oscillation with a period of 44.6 years and an amplitude of A 3 = 0. d 0216. For V701 Sco, its O-C curve reveals a periodic change with a period of 41.2 years and amplitude of A 3 = 0. d 0158. The mass ratio of BH Cen is 0.84, but V701 Sco contains twin B1-1.5V type stars with a mass ratio of unit. The continuous period increase of BH Cen is caused by the mass transfer from the less massive component to the more massive one at a rate of d M 2/d t = 3.5 × 10 −6 days/year. The cyclic period changes of both systems can be plausibly explained as the results of light-travel time effects suggesting that they are triple systems. The astrophysical parameters of the unseen tertiary components in the two systems have been determined. We think that the invisible tertiary components in both binaries played an important role in the formations and evolutions of the overcontact configurations by bringing angular momentum out from the central systems. For BH Cen, this process created the initial short period and will support its evolution into an overcontact configuration via a Case A mass transfer within the life time of the extremely young cluster IC 2944. For V701 Sco, two identical zero-age main-sequence components in an overcontact configuration suggest that it may have been formed by fission, possibly by the fission of the third body. The fact that no long-term continuous period variations were found for V701 Sco may suggest that an overcontact binary with the mass ratio of unity can be in an equilibrium revealing that the original configuration of the binary was overcontact as is its present state. It has been reported that faint stars in the two extremely young clusters are relatively scare. From the present study, it is shown that faint stars in young clusters are usually formed as companions of OB stars (including binaries). It is very difficult to detect them because of their low luminosity when compared with the more luminous OB stars.