Two Very Dissimilar Mass Stars in a Contact Binary

CCD photometric observations of V710 Monocerotis by L. Liu et al. (2014) indicate it is an extreme mass ratio, deep contact binary star system whose primary component (i.e. the more massive star) could be in an expanding phase as the star is entering its post-main-sequence stage of evolution. The primary and secondary components have 1.14 and 0.16 times the Sun’s mass, respectively. This large difference in mass between the primary and secondary components is what makes V710 Monocerotis an “extreme mass ratio binary”. Furthermore, being a contact binary system means that both stars are so close to each other, they actually touch. In fact, both stars are in deep, ~60 percent contact with each other, hence the term “deep contact binary”.

Figure 1: Artist’s impression of a planet circling a binary star system. Such a planet is known as a circumbinary planet.

Figure 2: Geometrical configuration of the deep contact binary V710 Monocerotis at phases 0.00 and 0.50. Only the primary component can be seen at phase 0.50. L. Liu et al. (2014).

V710 Monocerotis is a totally-eclipsing binary system. This is because during each orbit, the primary component completely blocks the secondary component. The primary and secondary components circle around each other every 0.4052 days. Observations reveal that the orbital period of the binary system is increasing at a rate of ~17 seconds per thousand years. For a contact binary system, an increase in orbital period is usually caused by the transfer of mass from the less massive component (i.e. secondary component) to the more massive component (i.e. primary component).

However, for V710 Monocerotis, the mass of the secondary component is so low that it wouldn’t be able to transfer mass to the primary component for much longer. After an estimated ~40,000 years, the secondary component’s mass would have gone down to ~0.08 times the Sun’s mass, placing it within the brown dwarf mass regime. A more plausible explanation for the increase in orbital period is that rather than the transfer of mass, the primary component is entering its post-main-sequence phase of evolution, causing the star’s entire envelope to expand. As it expands, its spin rate slows and the binary system settles into a new equilibrium with a longer orbital period.

Reference:

L. Liu et al., “A possible expanding component in the extreme mass ratio deep contact binary V710 Monocerotis”, New Astronomy, Volume 31, August 2014, Pages 60-64.