2007A&A...471..485N

The open cluster NGC 1893 illuminating the HII region IC 410 contains a moderately large population of O-type stars and is one of the youngest clusters observable in the optical range. It is suspected of harbouring a large population of pre-main-sequence (PMS) stars. We have probed the stellar population of NGC 1893 in an attempt to determine its size and extent. In particular, we look for signs of sequential star formation. We classify a large sample of cluster members with new intermediate-resolution spectroscopy. We used Hα slitless spectroscopy of the field to search for emission-line objects, identifying 18 emission-line PMS stars. We then combined existing optical photometry with the 2MASS JHK_S photometry to detect stars with infrared excesses, finding close to 20 more PMS candidates. While almost all stars earlier than B2 indicate standard reddening, all later cluster members show strong deviations from a standard reddening law, which we interpret in terms of infrared excess emission. Emission-line stars and IR-excess objects show the same spatial distribution, concentrating around two localised areas, the immediate vicinity of the pennant nebulae Sim 129 and Sim 130 and the area close to the cluster core where the rim of the molecular cloud associated with IC 410 is illuminated by the nearby O-type stars. In and around the emission nebula Sim 130, we find three Herbig Be stars with spectral types in the B1-4 range and several other fainter emission-line stars. We obtain a complete census of B-type stars by combining Stroemgren, Johnson and 2MASS photometry and find a deficit of intermediate mass stars compared to massive stars. We observe a relatively extended halo of massive stars surrounding the cluster without an accompanying population of intermediate-mass stars. Stars in NGC 1893 show strong indications of being extremely young. The pennant nebula Sim 130 is an area of active massive star formation, displaying very good evidence of triggering by the presence of nearby massive stars. The overall picture of star formation in NGC 1893 suggests a very complex process.