All-sky Co-moving Recovery Of Nearby Young Members. (ACRONYM) II: The Beta Pictoris Moving Group

TL;DR

This study confirms 85 members of the Beta Pictoris Moving Group, including 41 new low-mass systems, refining the group's age, mass distribution, and membership census through spectroscopic and photometric analysis.

Contribution

It provides the most complete census of BPMG members, introduces new low-mass discoveries, and evaluates the group's age and initial mass function with improved methods.

Findings

Confirmed 66 low-mass members and 19 companions, 41 of which are new.

Estimated the group age at 22 +/- 6 Myr.

Revealed a deficit of 0.2-0.3 Msun stars in the IMF.

Abstract

We confirm 66 low-mass stellar and brown dwarf systems (K7-M9) plus 19 visual or spectroscopic companions of the beta Pictoris Moving Group (BPMG). Of these, 41 are new discoveries, increasing the known low-mass members by 45%. We also add four objects to the 14 known with masses predicted to be less than 0.07 Msun. Our efficient photometric+kinematic selection process identified 104 low-mass candidates which we observed with ground-based spectroscopy. We collected infrared observations of the latest spectral types (>M5) to search for low gravity objects. These and all <M5 candidates were observed with high-resolution optical spectrographs to measure the radial velocities and youth indicators, such as lithium absorption and H-alpha emission, needed to confirm BPMG membership, achieving a 63% confirmation rate. We also compiled the most complete census of the BPMG membership with which we tested the efficiency and false-membership assignments using our selection and confirmation criteria. We assess a group age of 22 +/- 6 Myr using the new census, consistent with past estimates. With the now densely sampled lithium depletion boundary, we resolve the broadening of the boundary by either an age spread or astrophysical influences on lithium burning rates. We find that 69% of the now known members with AFGKM primaries are M stars, nearing the expected value of 75%. However, the new IMF for the BPMG shows a deficit of 0.2-0.3 Msun stars by a factor of approximately 2. We expect that the AFGK census of the BPMG is also incomplete, probably due to biases of searches towards the nearest stars.