A large moving group within the Lower Centaurus Crux association

Abstract

Scorpius-Centaurus is the nearest OB association and its hundreds of members were divided into sub-groups, including Lower Centaurus Crux. Here we study the dynamics of the Lower Centaurus Crux area stellar content. 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 to perform a kinematic and photometric selection. 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 135pc from the Sun. Our new members cover a mass range of 5M${}_{\odot }$ to 0.02M${}_{\odot }$, and add up to a total mass of about 700M${}_{\odot }$. The present-day mass function follows a log-normal law with m${}_c$ = 0.22 M${}_{\odot }$ and $\sigma$ = 0.64. We find more than 200 brown dwarfs in our sample. The star formation rate had its maximum of $8\times 10^{-5}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ at about 9Myr ago, We grouped the new members in four denser subgroups, which s have increasing age from 7 to 10Myr, 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 to 10Myr 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 sub-stellar objects for disk and exoplanet studies.

18 pages, 5 tables, accepted by ApJ. Modified the member description table, members.fits, to include uncertainties and covariance of the tangential velocity

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