Gaia DR2 view of the Lupus V-VI clouds: the candidate diskless young stellar objects are mainly background contaminants

TL;DR

Gaia DR2 data reveals that the previously identified diskless young stellar objects in Lupus V and VI are mostly background contaminants, clarifying the true disk fraction and membership in these star-forming regions.

Contribution

This study demonstrates that Gaia DR2 astrometry can effectively distinguish true young stellar objects from background contaminants in star-forming regions.

Findings

Most candidate YSOs in Lupus V and VI are background giants.

Only five YSOs are at the correct distance and proper motion, with four having disks.

The low disk fraction in these clouds is due to contamination, not actual absence of disks.

Abstract

Extensive surveys of star-forming regions with Spitzer have revealed populations of disk-bearing young stellar objects. These have provided crucial constraints, such as the timescale of dispersal of protoplanetary disks, obtained by carefully combining infrared data with spectroscopic or X-ray data. While observations in various regions agree with the general trend of decreasing disk fraction with age, the Lupus V and VI regions appeared to have been at odds, having an extremely low disk fraction. Here we show, using the recent Gaia data release 2 (DR2), that these extremely low disk fractions are actually due to a very high contamination by background giants. Out of the 83 candidate young stellar objects (YSOs) in these clouds observed by Gaia, only five have distances of 150 pc, similar to YSOs in the other Lupus clouds, and have similar proper motions to other members in this star-forming complex. Of these five targets, four have optically thick (Class II) disks. On the one hand, this result resolves the conundrum of the puzzling low disk fraction in these clouds, while, on the other hand, it further clarifies the need to confirm the Spitzer selected diskless population with other tracers, especially in regions at low galactic latitude like Lupus V and VI. The use of Gaia astrometry is now an independent and reliable way to further assess the membership of candidate YSOs in these, and potentially other, star-forming regions.