Spectroscopic characterization of X-ray emitting young stars associated with the Sh 2-296 nebula

We studied a sample of stars associated with the Sh 2-296 nebula, part of the reflection nebulae complex in the region of Canis Major (CMa R1). Our sample corresponds to optical counterparts of X-ray sources detected from observations with the XMM-Newton satellite, which revealed dozens of possible low-mass young stars not yet known in this region. A sample of 58 young star candidates were selected based on optical spectral features, mainly H{\alpha} and lithium lines, observed with multi-objects spectroscopy performed by the Gemini South telescope. Among the candidates, we find 41 confirmed T Tauri and 15 very likely young stars. Based on the H{\alpha} emission, the T Tauri stars were distinguished between classical (17%) and weak-lined (83%), but no significant difference was found in the age and mass distribution of these two classes. The characterization of the sample was complemented by near- and mid-infrared data, providing an estimate of ages and masses from the comparison with pre-main-sequence evolutionary models. While half of the young stars have an age of 1-2 Myrs or less, only a small fraction (~25%) shows evidence of IR excess revealing the presence of circumstellar discs. This low fraction is quite rare compared to most young star-forming regions, suggesting that some external factor has accelerated the disc dissipation

Excess gamma-rays in the direction of the rho Ophiuchi cloud: An exotic object?

The COS-B X-ray data in the direction of the rho Oph dark cloud show an extended structure; at the same time, the region of highest intensity has a spatial distribution compatible with a localized source; 2CG353+16 which is designated Oph gamma. The possibility of an excess gamma ray flux over what is expected on the basis of the interaction of average density cosmic rays with an estimated cloud mass of 2 to 4 000 M is still open, pending an extended CO survey matching the gamma ray data. Estimates for this excess factor are in the range 2 to 4. While the cloud mass may be underestimated, it should be noted that an excess of the same order appears to be present in the nearby Oph-Sag area, well surveyed in CO with the Columbia dish. Possible reasons for a gamma ray excess, in view of two recent observational developments: an Einstein X-ray survey and a VLA radio survey, both covering the approx 2 deg diameter Oph gamma error box. Current interpretations link the gamma ray excess to the cloud gas, in which some active agent is present: stellar winds, or interaction with the North Polar Spur

Shaping Disk Galaxy Stellar Populations via Internal and External Processes

In recent years, effects such as the radial migration of stars in disks have been recognized as important drivers of the properties of stellar populations. Radial migration arises due to perturbative effects of disk structures such as bars and spiral arms, and can deposit stars formed in disks to regions far from their birthplaces. Migrant stars can significantly affect the demographics of their new locales, especially in low-density regions such as in the outer disks. However, in the cosmological environment, other effects such as mergers and filamentary gas accretion also influence the disk formation process. Understanding the relative importance of these processes on the detailed evolution of stellar population signatures is crucial for reconstructing the history of the Milky Way and other nearby galaxies. In the Milky Way disk in particular, the formation of the thickened component has recently attracted much attention due to its potential to serve as a diagnostic of the galaxy's early history. Some recent work suggests, however, that the vertical structure of Milky Way stellar populations is consistent with models that build up the thickened component through migration. I discuss these developments in the context of cosmological galaxy formatio

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 ( 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.Comment: 38 pages, 21 figures, accepted for A&