Star formation history of Canis Major OB1 - II. A bimodal X-ray
  population revealed by XMM-Newton
  The Canis Major OB1 Association has an intriguing scenario of star formation,
especially in the Canis Major R1 (CMa R1) region traditionally assigned to a
reflection nebula, but in reality an ionized region. This work is focused on
the young stellar population associated to CMa R1, for which our previous
results from ROSAT, optical and near-infrared data had revealed two stellar
groups with different ages, suggesting a possible mixing of populations
originated from distinct star-formation episodes. The X-ray data allow the
detected sources to be characterized according to hardness ratios, light curves
and spectra. Estimates of mass and age were obtained from the 2MASS catalogue,
and used to define a complete subsample of stellar counterparts, for
statistical purposes. A catalogue of 387 XMM-Newton sources is provided, 78%
being confirmed as members or probable members of the CMa R1 association.
Flares were observed for 13 sources, and the spectra of 21 bright sources could
be fitted by a thermal plasma model. Mean values of fits parameters were used
to estimate X-ray luminosities. We found a minimum value of log(L$_X$[erg/s]) =
29.43, indicating that our sample of low-mass stars (M$_\star$ $\leq$ 0.5
M$_\odot$), being faint X-ray emitters, is incomplete. Among the 250 objects
selected as our complete subsample (defining our best sample), 171 are found to
the East of the cloud, near Z CMa and dense molecular gas, 50% of them being
young (< 5 Myr) and 30% being older (> 10 Myr). The opposite happens to the
West, near GU CMa, in areas lacking molecular gas: among 79 objects, 30% are
young and 50% are older. These findings confirm that a first episode of
distributed star formation occurred in the whole studied region ~10 Myr ago and
dispersed the molecular gas, while a second, localized episode (< 5 Myr) took
place in the regions where molecular gas is still present.
