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.