WISE J080822.18-644357.3 - a 45 Myr-old accreting M dwarf hosting a primordial disc

Abstract

WISE J080822.18$-$644357.3 was recently identified as a new M dwarf debris disc system and a candidate member of the 45 Myr-old Carina association. Given that the strength of its infrared excess ($L_{\mathrm{IR}}/L_{\star }\simeq 0.1$) appears to be more consistent with a young protoplanetary disc, we present the first optical spectra of the star and reassess its evolutionary and membership status. We find WISE J0808-6443 to be a Li-rich M5 star with strong H$\alpha$ emission ($-125<\text{EW}<-65$ Å over 4 epochs) whose strength and broad width are consistent with accretion at a low level ($\sim$10${}^{-10}$ $M_{\odot }$ yr${}^{-1}$) from its disc. The spectral energy distribution of the star is consistent with a primordial disc and is well-fit using a two-temperature blackbody model with $T_{\mathrm{inner}}\simeq$ 1100 K and $T_{\mathrm{outer}}\simeq$ 240 K. AllWISE multi-epoch photometry shows the system exhibits significant variability in the 3.4 $\mu$m and 4.6 $\mu$m bands. We calculate an improved proper motion based on archival astrometry, and combined with a new radial velocity, the kinematics of the star are consistent with membership in Carina at a kinematic distance of $90\pm 9$ pc. The spectroscopic and photometric data are consistent with WISE J0808-6443 being a $\sim$0.1 $M_{\odot }$ Classical T-Tauri star and one of the oldest known accreting M-type stars. These results provide further evidence that the upper limit on the lifetimes of gas-rich discs - and hence the timescales to form and evolve protoplanetary systems - around the lowest mass stars may be longer than previously recognised, or some mechanism may be responsible for regenerating short-lived discs at later stages of pre-main sequence evolution.

MNRAS Accepted; 12 pages, 10 figures and 5 tables

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