* Astronomy

Title: WISE J080822.18-644357.3 - a 45 Myr-old accreting M dwarf hosting a pre-transitional disc
Author: Simon J. Murphy (University of New South Wales), Eric E. Mamajek (JPL, University of Rochester), Cameron P. M. Bell (AIP)

WISE J080822.18-644357.3 (WISE J0808-6443) 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_IR/L_*~ 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< EW <-65 Ang over 4 epochs) whose strength and broad width are consistent with accretion at a low level (~10^-10 solar masses yr^-1) from its disc. The spectral energy distribution of the star is consistent with a two-temperature disc structure, with a hot inner disc (T_in ~ 1100 K) at orbital radius ~0.006 au and a warm outer disc (T_out ~ 230 K) at radius ~0.12 au. The dust belts likely correspond to the sublimation temperatures of silicates and icy planetesimals, respectively. 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±9 pc. The spectroscopic and photometric data are consistent with WISE J0808-6443 being a ~0.1 solar masses Classical T-Tauri star and one of the oldest known accreting M-type stars. These results suggest that the upper limit on the lifetimes of gas-rich discs 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. Low-level accretion for tens of Myr may provide a mechanism for the migration, eccentricity dampening, and resonance locking of small planets, like those recently reported for the TRAPPIST-1 exoplanetary system.

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