The Walkaway Star HP Tau/G2: Evidence for a Stellar Merger

HP Tau/G2 is a luminous G-type weak-line T Tauri star with a 1.2 day low-amplitude variability and a rotational velocity v sin i ∼130 km s −1 , yielding a large r sin i ∼3.1 R ⊙ . It has star spots, an oblate shape with significant gravity-darkening, currently little circumstellar material, and is centered in a sharply outlined and slowly expanding cloud cavity. It is also an X-ray source and a variable nonthermal radio source. It forms, together with the late-type T Tauri star KPNO 15, a pair of oppositely directed walkaway stars launched when a multiple system broke apart ∼5600 yr ago. Momentum conservation indicates a mass of G2 of only ∼0.7 M ⊙ , much lower than ∼1.9 M ⊙ determined from evolutionary models. HP Tau/G2 is virtually a twin of FK Com, the prototype of a class of evolved stars resulting from coalescence of W UMa binaries. We suggest that HP Tau/G2 became a very close and highly eccentric binary during viscous evolution in the protostellar stage and formed a triple system with KPNO 15, which again was part of a larger tenuously bound group including the binary HP Tau/G3 and the single HP Tau/G1. Subsequent dynamical evolution led to the escape of KPNO 15 after ∼2 Myr, and as a result, the HP Tau/G2 binary recoiled and contracted about 5600 yr ago, became Darwin unstable and merged a few thousand years later. The nearby compact triple system HP Tau/G3 and HP Tau/G1 was also disturbed, and broke up about 4900 yr ago, forming another low-mass walkaway pair. HD 283572 is a second case with similar properties, indicating that HP Tau/G2 is not a pathological case. The current bloated state is transitory as the now more-massive star shrinks and fades toward a new stable configuration. We suggest that YSO mergers may be rather common and could explain some of the larger FUor eruptions.