A New Look at the HS Hydrae System

Abstract

Abstract A growing group of stellar triple systems contains an eclipsing binary for which the depth of eclipses has been proven to change in time, a sign of evolving orbital inclination. The recent analysis of historical observations of HS Hya significantly extended the timespan over which the inclination changes are known for this system. Here we add a few more observations and reanalyze the whole data set with a single methodology. We also improve our own analytical approach to enable describing the secular evolution of the orbital architecture of hierarchical triple systems applicable to the HS Hya case. Analyzing the available photometric and spectroscopic data we obtain two main results. First, the dynamical evolution itself allows to constrain the masses of the stars in the inner binary to 1.31 ± 0.03 M ⊙ and 1.27 ± 0.03 M ⊙, and the mass of the unseen third component to 0.56 − 0.09 + 0.12 M ⊙ (all 95% confidence level results). This makes it an M- or K-type dwarf accompanying the binary. Second, the orbital planes of the inner binary and the third component are significantly noncoplanar, allowing two solutions for their mutual angle J. Either the motion of the third component is prograde, and J is most likely in the 50°–65° range, or the motion is retrograde, and J is most likely in the 120°–150° range. The precession period of both orbital planes about the total angular angular momentum is ∼700 yr (with about two centuries of uncertainty). This implies HS Hya will become eclipsing again around the year 2200.