2M1155-79 (= T chamaeleontis B): A Low-mass, wide-separation companion to the nearby, "old" T Tauri star T chamaeleontis

The early-K star T Cha, a member of the nearby (D 100pc) ε Cha Association, is a relatively "old" (age 7Myr) T Tauri star that is still sporadically accreting from an orbiting disk whose inner regions are now evidently being cleared by a close, substellar companion. We report the identification, via analysis of proper motions, serendipitous X-ray imaging spectroscopy, and follow-up optical spectroscopy, of a new member of the ε Cha Association that is very likely a low-mass companion to T Cha at a projected separation of 38kAU. The combined X-ray and optical spectroscopy data indicate that the companion, T Cha B (= 2M1155-79), is a weak-lined T Tauri star (wTTS) of spectral type M3 and age ≲ 10Myr. The serendipitous X-ray (XMM-Newton) observation of T Cha B, which targeted T Cha, also yields serendipitous detections of two background wTTS in the Chamaeleon cloud complex, including one newly discovered, low-mass member of the Cha cloud pre-main-sequence (pre-MS) population. T Cha becomes the third prominent example of a nearby, "old" yet still actively accreting, K-type pre-MS star/disk system (the others being TW Hya and V4046Sgr) to feature a low-mass companion at very large (12-40kAU) separation, suggesting that such wide-separation companions may affect the conditions and timescales for planet formation around solar-mass stars.