The low-mass pre-main-sequence population of Sco OB1

The low-mass members of OB associations, expected to be a major component of their total population, are in most cases poorly studied because of the difficulty of selecting these faint stars in crowded sky regions. Our knowledge of many OB associations relies only on a relatively small number of massive members. We study here the Sco OB1 association, with the aim of a better characterization of its properties such as global size and shape, member clusters and their morphology, age and formation history, and total mass. We use deep optical and NIR photometry from the VPHAS+ and VVV surveys, over a wide area (2.6 x 2.6 sq.deg.), complemented by Spitzer IR data, and Chandra and XMM-Newton X-ray data. A new technique is developed to find clusters of pre-main-sequence M-type stars using suitable color-color diagrams, which complements existing selection techniques using narrow-band H-alpha photometry or NIR and UV excesses, and X-ray data. We find a large population of approximately 4000 candidate low-mass Sco OB1 members, net of field-star contaminations, whose spatial properties correlate well with those of H-alpha-emission, NIR-excess, UV-excess, and X-ray detected members, and unresolved X-ray emission. The low-mass population is spread among several interconnected subgroups: they coincide with the HII regions G345.45+1.50 and IC4628, and the rich clusters NGC 6231 and Trumpler 24, with an additional subcluster intermediate between these two. The total mass of Sco OB1 is estimated to be 8500 Msun. Indication of a sequence of star-formation events is found, from South (NGC 6231) to North (G345.45+1.50). We suggest that the diluted appearance of Trumpler 24 indicates that the cluster is now dissolving into the field, and that tidal stripping by NGC 6231 nearby contributes to the process.