A stellar census of the nearby, young 32 Orionis group

Abstract

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 characterisation 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 three 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 utilising the AllWISE catalogue. Although we find no evidence for warm, dusty discs, we identify several stars with excess emission in the WISE W 4-band at 22 µm. Based on the limited number of W 4 detections we estimate a debris disc fraction of 32 +12 −8 per cent for the 32 Ori group.

Pleiades or Not? Resolving the Status of the Lithium-rich M Dwarfs HHJ 339 and HHJ 430

“…Figure 6 shows another CMD, this time plotting probable members of the 32 Ori and Group 29 moving groups along with the two HHJ lithium-rich M dwarfs. The two moving groups appear to have quite similar isochronal ages; both moving groups have estimated ages of about 25 Myr (Bell et al 2017;David et al 2019). The two Oppenheimer stars have locations in this CMD consistent also with that age.…”

“…With the new evidence now available, we demonstrate that these two stars are indeed foreground to the Pleiades and that their properties are most consistent with membership in the 32 Ori moving group (Bell et al 2017). In Section 2, we discuss the new data that we utilize in this paper.…”

“…The radial velocities we use for the Pleiades come from Mermilliod et al (2009). The member list for Group 29 comes from Oh et al (2017) and the member list for the 32 Ori group comes from Bell et al (2017). The Taurus member list is based on Rebull et al (2020), which in turn heavily relies on the list from Luhman (2018) and Esplin & Luhman (2019), and the Taurus radial velocities are from Galli et al (2019).…”

“…Based on a lengthy analysis of the DR2 data and other published sources, both Luhman and Kraus et al concluded that the more spatially extended population most probably represents a previous generation of star formation, unconnected to the ∼3 Myr old Taurus SFR population. Group 29 (Oh et al 2017) and the 32 Ori Group (Bell et al 2017) have space motions, ages, and spatial distributions that make them likely contributors to the older-but-still-young spatially extended population of stars toward Taurus. We compare the properties of HHJ 339 and 430 to the members of Group 29, the 32 Ori Group, and Taurus in the following plots.…”

A kinematically unbiased, all-sky search for nearby, young, low-mass stars

“…The large Li EW suggests the star is < 30 Myr, consistent with all three groups. The sky position definitively rules out membership with EPS, and membership to THO is marginally ruled out as the target is ∼5 • further north than the most northerly THO members in Bell, Murphy & Mamajek (2017), whereas it is very close to the centroid of 118TAU. We therefore assign this object as a member of 118TAU, which agrees with the recent designation by Bowler et al (2019).…”