Re-examining the membership and origin of the ϵ Cha association

Abstract

We present a comprehensive investigation of the ǫ Chamaeleontis association (ǫ Cha), one of several young moving groups spread across the southern sky. We re-assess the putative membership of ǫ Cha using the best-available proper motion and spectroscopic measurements, including new ANU 2.3-m/WiFeS observations. After applying a kinematic analysis our final membership comprises 35-41 stars from B9 to mid-M spectral types, with a mean distance of 110 ± 7 pc and a mean space motion of (U, V, W ) = (−10.9 ± 0.8, −20.4 ± 1.3, −9.9 ± 1.4) km s −1 . Theoretical evolutionary models suggest ǫ Cha is 3-5 Myr old, distinguishing it as the youngest moving group in the solar neighbourhood. Fifteen members show 3-22 µm spectral energy distributions attributable to circumstellar discs, including 11 stars which appear to be actively accreting. ǫ Cha's disc and accretion fractions (29 +8 −6 and 32 +9 −7 per cent, respectively) are both consistent with a typical 3-5 Myr-old population. Multi-epoch spectroscopy reveals three M-type members with broad and highly-variable Hα emission as well as several new spectroscopic binaries. We reject 11 stars proposed as members in the literature and suggest they may belong to the background Cha I and II clouds or other nearby young groups. Our analysis underscores the importance of a holistic and conservative approach to assigning young stars to kinematic groups, many of which have only subtly different properties and ill-defined memberships. We conclude with a brief discussion of ǫ Cha's connection to the young open cluster η Cha and the Scorpius-Centaurus OB association (Sco-Cen). Contrary to earlier studies which assumed η and ǫ Cha are coeval and were born in the same location, we find the groups were separated by ∼30 pc when η Cha formed 4-8 Myr ago in the outskirts of Sco-Cen, 1-3 Myr before the majority of ǫ Cha members.

LACEwING: A New Moving Group Analysis Code

“…5. Murphy et al (2013) analyzed the ò Cha moving group using techniques similar to those used for η Cha. We adopt their list of ò Cha members as bona fide members.…”

“…The ellipse-fitting process requires a minimum of four stars with full kinematic information. Five groups lacked sufficient numbers of stars with complete information, and their properties had to be taken instead from other sources: η Cha (Murphy et al 2010), ò Cha (Murphy et al 2013) (Röser et al 2011). They are thus represented by axis-locked ellipses, and as shown in Table 7, all rotation angles are set to 0.…”

“…For a time, it was assumed (e.g., Torres et al 2008, p. 757) to be co-eval with the open cluster and likely part of the same star-forming event; Murphy et al (2013) recently used evolutionary models that suggest it is younger than η Cha. The hottest (and likely most massive) member of ò Cha isò Cha itself, a B9V star.…”

New Binaries in the ϵ Cha Association*

“…We used for the target selection the list of proposed members of the ǫ Cha association from the Table 1 of Murphy et al (2013). Here in Table 1 we provide the characteristics of the 47 observed stars.…”

Fir View of Disks of Weak-Line T Tauri Stars

“…These sources have spectral types of F-M, and the ages of 0.1-70 Myr. Murphy et al (2013) have analyzed the membership of Chamaeleontis association, whose age is 3-5 Myr with distance of ∼100 pc. Since they concluded that 15 out of 77 ROSAT WTTSs are not the Chamaeleon members, we excluded them from the list, and our target contains 62 WTTSs.…”