2017MNRAS.468.1198B

The 32 Orionis group was discovered almost a decade ago and despite the fact that it represents the first northern, young (age ∼ 25 Myr) stellar aggregate within 100 pc of the Sun (d ≃ 93 pc), a comprehensive survey for members and detailed characterization of the group has yet to be performed. We present the first large-scale spectroscopic survey for new (predominantly M type) members of the group after combining kinematic and photometric data to select candidates with Galactic space motion and positions in colour-magnitude space consistent with membership. We identify 30 new members, increasing the number of known 32 Ori group members by a factor of 3 and bringing the total number of identified members to 46, spanning spectral types B5 to L1. We also identify the lithium depletion boundary (LDB) of the group, i.e. the luminosity at which lithium remains unburnt in a coeval population. We estimate the age of the 32 Ori group independently using both isochronal fitting and LDB analyses and find it is essentially coeval with the β Pictoris moving group, with an age of 24 ± 4 Myr. Finally, we have also searched for circumstellar disc hosts utilizing the AllWISE catalogue. Although we find no evidence for warm, dusty discs, we identify several stars with excess emission in the WISEW4 band at 22 µm. Based on the limited number of W4 detections, we estimate a debris disc fraction of 32⁺¹²_–8 per cent for the 32 Ori group.