The Ara OB1a association: Stellar population and star formation history
  Context: The Ara OB1a association is a nearby complex in the fourth Galactic
quadrant where a number of young/embedded star clusters are projected close to
more evolved, intermediate age clusters. It is also rich in interstellar
matter, and contains evidence of the interplay between massive stars and their
surrounding medium, such as the rim HII region NGC 6188. Aims: We provide
robust estimates of the fundamental parameters (age and distance) of the two
most prominent stellar clusters, NGC 6167 and NGC 6193, that may be used as a
basis for studing the star formation history of the region. Methods: The study
is based on a photometric optical survey (UBVIHa) of NGC 6167 and NGC 6193 and
their nearby field, complemented with public data from 2MASS-VVV, UCAC3, and
IRAC-Spitzer in this region. Results: We produce a uniform photometric
catalogue and estimate more robustly the fundamental parameters of NGC 6167 and
NGC 6193, in addition to the IRAS 16375-4854 source. As a consequence, all of
them are located at approximately the same distance from the Sun in the
Sagittarius-Carina Galactic arm. However, the ages we estimate differ widely:
NGC 6167 is found to be an intermediate-age cluster (20-30 Myr), NGC 6193 a
very young one (1-5 Myr) with PMS, H? emitters and class II objects, and the
IRAS 16375-4854 source is the youngest of the three containing several YSOs.
Conclusions: These results support a picture in which Ara OB1a is a region
where star formation has proceeded for several tens of Myr until the present.
The difference in the ages of the different stellar groups can be interpreted
as a consequence of a triggered star formation process. In the specific case of
NGC 6193, we find evidence of possible non-coeval star formation.
