An extremely low mass ratio contact binary NW Aps with a potential compact companion star

Abstract

The photometric solutions of NW Aps reveal that it is a low mass ratio ($q = 0.086$) contact binary system. Investigation of orbital period shows that its orbital period is increasing continuously at a rate of $dP/dt=+1.117(\pm0.005)\times{10^{-6}}day\cdot year^{-1}$, which may be caused by mass transfer from the less massive component to the more massive one at a rate of $\frac{dM_{2}}{dt}=-3.36(\pm0.02)\times{10^{-8}}M_\odot/year$. A cyclic variation of $P_3 = 22.9(\pm0.1)$ is also found in the O - C curve. There may be a potential compact object orbiting around NW Aps, with its minimum mass to be $M_3 = 0.436(\pm0.007)M_\odot$. However, the magnetic activity of the primary star may also account for the cyclic change. NW Aps is a stellar merger candidate with the longest orbital period among all stellar merge candidates with mass ratio $q < 0.1$. It is still in a stable state since the ratio of orbital angular momentum ($J_{orb}$) to spin angular momentum ($J_{spin}$) is $\frac{J_{orb}}{J_{spin}}$ = 3.257. Both of its primary and secondary star are oversized to main sequence stars, and the surface gravity of the primary and secondary stars are significant lower than main sequence stars. The P - log g relationship is fitted with parabola for low mass ratio contact binary systems. More targets laid in the gap are needed to confirm the P - log g relationship and reveal the final evolutionary state of low mass ratio contact binary system.