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arXiv · 2602.06465

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

Bo Reipurth·J. Bally·P. Friberg·D. M. Faes·C. Briceno·M. S. Connelley·C. Flores·A. M. Cody·H. Zinnecker
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Abstract

HP~Tau/G2 is a luminous, short-period, fast-rotating G-type weak-line T Tauri star with a large radius, an oblate shape with gravity-darkening, little circumstellar material, and centered in a slowly expanding cloud cavity. It is 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 Msun, much lower than the ~1.9 Msun determined from evolutionary models. 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 G2 became a very close and highly eccentric binary during viscous evolution in the protostellar stage and with KPNO 15 formed a triple system, which again was part of a larger unstable group including the binary G3 and the single G1. Dynamical evolution led to multiple bound ejections of KPNO 15 before it finally escaped after ~2 Myr. As a result the G2 binary recoiled and contracted 5600 yr ago, became Darwin unstable and merged in a major outburst ~2000 yr ago. The nearby compact triple system G1+G3 was also disturbed, and broke up 4900 yr ago, forming another walkaway pair. The G5 star HD 283572 has similar unusual properties, indicating that G2 is not a pathological case. G2 is now fading towards a new stable configuration. YSO mergers may be rather common and could explain some FUor eruptions.

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Bo Reipurth, J. Bally, P. Friberg, D. M. Faes, C. Briceno, M. S. Connelley, C. Flores, A. M. Cody, H. Zinnecker. 2026-02-06. The Walkaway Star HP Tau/G2: Evidence for a Stellar Merger. https://arxiv.org/abs/2602.06465

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