Period Changes of DF Hydrae: Evidence of Mass Transfer and the Presence of a Cool Tertiary Companion

Abstract

Abstract Combining our six new determined times of light minimum obtained from 2001 to 2007 with others compiled from the literature, we investigated the period changes of DF Hydrae. The general trend of the $O-C$ curve based on all available data reveals that the period of the binary star is increasing continuously at a rate of $dP/dt$$=$$+$1.11($\pm$0.02) $\times$ 10$^{-7}$dyr$^{-1}$. After the long-term period increase was removed from the $O-C$ diagram, it was discovered that the residuals of the CCD and photoelectric observations suggest a small-amplitude cyclic oscillation with a period of 21.5 years. The continuous period increase can be explained by mass transfer from the secondary to the primary, indicating that DF Hydrae is on the TRO-controlled stage of the evolutionary scenario proposed by Qian. The cyclic period variation suggests that DF Hydrae is a triple system containing a cool tertiary component with a mass of $M_3$sin${i^{\prime}}$$=$ 0.21$M_{\odot}$ in a 21.5-year orbit. By removing angular momentum from the central binary system via Kozai oscillation or a combination of the Kozai cycle and tidal friction, the tertiary companion may play an important role for the formation and evolution of the contact system, which makes the eclipsing pair to have a short initial orbital period (i.e., $P$$\lt$ 5$^{\rm d}$). In that case, the detached progenitor of DF Hydrae evolves into the present contact configuration via a combination of magnetic braking and a case A mass transfer.