A Large Moving Group within the Lower Centaurus Crux Association

Abstract

Scorpius–Centaurus is the nearest OB association, and its hundreds of members are divided into subgroups, including the Lower Centaurus Crux (LCC). Here we study the dynamics of the LCC area. We report the revelation of a large moving group containing more than 1800 intermediate- and low-mass young stellar objects and brown dwarfs that escaped identification until Gaia DR2 allowed a kinematic and photometric selection to be performed. We investigate the stellar and substellar content of this moving group using the Gaia DR2 astrometric and photometric measurements. The median distance of the members is 114.5 pc, and 80% lie between 102 and 135 pc from the Sun. Our new members cover a mass range of 0.02–5 M ⊙ and add up to a total mass of about 700 M ⊙ . The present-day mass function follows a log-normal law with m c  = 0.22 M ⊙ and σ  = 0.64. We find more than 200 brown dwarfs in our sample. The star formation rate had its maximum of about 9 Myr ago. We grouped the new members into four denser subgroups, which have increasing age from 7 to 10 Myr, surrounded by “free-floating” young stars with mixed ages. Our isochronal ages, now based on accurate parallaxes, are compatible with several earlier studies of the region. The whole complex is presently expanding, and the expansion started between 8 and 10 Myr ago. Two hundred members show infrared excess compatible with circumstellar disks from full to debris disks. This discovery provides a large sample of nearby young stellar and substellar objects for disk and exoplanet studies.