Paper detail

Uniform detection of the pre-main sequence population in the 5 embedded clusters related to the H\,II region NGC\,2174 (Sh2-252)

We investigate 5 embedded clusters (ECs) and the extended stellar group itself of the prominent H\,II region NGC\,2174 (Sh2-252), which presents scarce and heterogeneous information, coming from the optical and infrared. Considering the discrepant values of distance and age, the clusters and the H\,II region appear to be physically unrelated. The analysis is based on field-star decontaminated 2MASS photometry, which allows sampling the pre-main sequence (PMS). We find that Sh2-252A, C, E, NGC\,2175s, and Teu\,136 are small ECs (radius within $1.0 - 2.3$\,pc) characterised by a similar age ($\sim5$\,Myr), reddening ($\aV\sim1$), distance from the Sun ($\ds\sim1.4$\,kpc), and low mass ($60-200\,\ms$). This age is consistent with the H\,II region, the presence of O and B stars still in the MS, and the dominance ($\ga95%$ in number) of PMS stars in colour-magnitude diagrams (CMDs). NGC\,2175 is not a star cluster, but an extended stellar group that encompasses the ECs Sh2-252\,A and C. It contains $\sim36%$ of the member stars (essentially PMS) in the area, with the remaining belonging to the 2 ECs. CMDs of the overall star-forming region and the ECs provide $\ds=1.4\pm0.4$\,kpc for the NGC\,2174 complex, consistent with the value estimated for the physically-related association Gem\,OB1. Our uniform approach shows that NGC\,2174 and its related ECs (except, perhaps, for Teu\,136) are part of a single star-forming complex. CMD similarities among the ECs and the overall region suggest a coeval (to within $\pm5$\,Myr) star-forming event extending for several Myr. At least 4 ECs originated in the event, together with the off-cluster star formation that probably gave rise to the scattered stars of NGC\,2175.