Chandra X-ray observation of the H ii region Gum 31 in the Carina nebula complex

Context. Gum 31 is a prominent, but still rather poorly studied H ii region around the stellar cluster NGC 3324 at the northwestern periphery of the Carina nebula complex.
Aims: Our aim was to reveal and characterize the young stellar population in Gum 31. An X-ray survey is the only efficient way to identify young stars in this region with extremely high galactic field-star contamination that can avoid the strong biases of infrared excess selected samples of disk-bearing young stars.
Methods: We used the Chandra observatory to perform a deep (70 ks) X-ray observation of the Gum 31 region and detected 679 X-ray point sources. This extends and complements the X-ray survey of the central Carina nebula regions performed in the Chandra Carina Complex Project (CCCP). Using deep near-infrared images from our recent VISTA survey of the Carina nebula complex, our comprehensive Spitzer point-source catalog, and optical archive data, we identify counterparts for 75% of these X-ray sources.
Results: The spatial distribution of the X-ray selected young stars shows two major concentrations, the central cluster NGC 3324 and a partly embedded cluster in the southern rim of the H ii region. However, these two prominent clusters contain only about 30% of the X-ray selected population, whereas the majority (~70%) of X-ray sources constitute a rather homogeneously distributed population of young stars. Our color-magnitude diagram analysis suggests ages of ~1-2 Myr for the two clusters, whereas the distributed population shows a wider age range up to ~10 Myr. We also identify previously unknown companions to two of the three O-type members of NGC 3324 and detect diffuse X-ray emission in two parts of the region.
Conclusions: An extrapolation based on the observed X-ray luminosity function suggests that the observed region contains about 4000 young stars in total (down to 0.1 M_⊙). This shows that the Gum 31 area contains a substantial fraction of the total stellar population in the CNC. The distributed population of young stars in the Gum 31 region is probably a part or extension of the widely distributed population of ~1-10 Myr old stars, that was identified in the CCCP area. This implies that the global stellar configuration of the Carina nebula complex is a very extended stellar association, in which the (optically prominent) clusters contain only a minority of the stellar population.

The Chandra data described in this paper have been obtained in the open time project with Sequence Number 200767 and ObsID 13613 (PI: T. Preibisch).Tables 1 and 3 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/564/A120