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(Abridged) The open cluster NGC 1893, illuminating the HII region IC410,contains a moderately large population of O-type stars and is one of theyoungest clusters observable in the optical range. We have probed the stellarpopulation of NGC 1893 in an attempt to determine its size and extent. Weclassify a large sample of cluster members with new intermediate resolutionspectroscopy. We use H-alpha slitless spectroscopy of the field to search foremission line objects, identifying 18 emission-line PMS stars. We then combineexisting optical photometry with 2MASS JHKs photometry to detect stars withinfrared excesses, finding close to 20 more PMS candidates. While almost allstars earlier than B2 indicate standard reddening, all later cluster membersshow strong deviations from a standard reddening law, which we interpret interms of infrared excess emission. Emission-line stars and IR-excess objectsshow the same spatial distribution, concentrating around two localised areas,the immediate vicinity of the pennant nebulae Sim 129 and Sim 130 and the areaclose to the cluster core where the rim of the molecular cloud associated withIC 410 is illuminated by the nearby O-type stars. In and around the emissionnebula Sim 130 we find three Herbig Be stars with spectral types in the B1-4range and several other fainter emission-line stars. We obtain a completecensus of B-type stars by combining Stroemgren, Johnson and 2MASS photometryand find a deficit of intermediate mass stars compared to massive stars. Weobserve a relatively extended halo of massive stars surrounding the clusterwithout an accompanying population of intermediate-mass stars. The overallpicture of star formation in NGC 1893 suggests a very complex process.

Pre-main-sequence stars in the young open cluster NGC 1893