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

TL;DR

The study investigates the peculiar HP Tau/G2 system, a luminous, rapidly rotating G-type pre-main-sequence star embedded in a structured reflection nebula. Through a comprehensive, multi-wavelength observational approach and dynamical analysis of a small multiple system, the authors argue that G2 is the product of a stellar merger triggered by the breakup of a prior multiple system roughly $5.6\times10^{3}$ years ago, producing a walkaway pair with KPNO 15 and leaving a Darwin-unstable configuration that merged in a major outburst about $2\times10^{3}$ years ago. The work further identifies a cluster of 12 nearby young stellar objects with substantial circumstellar material, suggesting that YSO mergers in compact groups may be more common than previously thought and potentially linked to FU Orionis-type eruptions. These results have implications for early stellar evolution, multiplicity statistics, and the understanding of dramatic accretion events in young stars.

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.

Figures (2)

• Figure 1: The large highly structured reflection nebula Magakian 77 surrounds the little group of young stars including HP Tau. The figure is almost 8 arcmin wide. This is a 13hr 20min exposure with a 1m telescope through B, G, R filters plus luminance. Courtesy Alexandr Zaytsev and Mark Hanson.
• Figure 2: (top) The YSOs within the Magakian 77 reflection nebula are identified in this 2MASS K-band image, and their properties are listed in Table 1. H6-28 is Haro 6-28 and 2M is 2MASS J04355760+2253574. FF Tau is outside the field to the west. North is up and east is left. (bottom) A J-H vs H-K color-color diagram of ten of the sources in the HP Tau multiple system. The dwarf and giant loci are from bessell1988 and the reddening lines from rieke1985. The abbreviations refer to the names in Table 1.