Young stars and protostellar cores near NGC2023

摘要

Context. We investigate the young (proto)stellar population in NGC 2023 and the L 1630 molecular cloud bordering the h ii regionIC 434, using Spitzer IRAC and MIPS archive data, JCMT SCUBA imaging and spectroscopy as well as targeted BIMA observationsof one of the Class 0 protostars, NGC 2023 MM1.Aims. We study the distribution of gas, dust and young stars in this region to see where stars are forming, whether the expansion ofthe h ii region has triggered star formation, and whether dense cold cores have already formed stars.Methods. We have performed photometry of all IRAC and MIPS images, and used color–color diagrams to identify and classifyall young stars seen within a 22 × 26 field along the boundary between IC434 and L 1630. For some stars, which have sufficientoptical, IR, and/or sub-millimeter data we have also used the online SED fitting tool for a large 2D archive of axisymmetric radiativetransfer models to perform more detailed modeling of the observed SEDs.We identify 5 sub-millimeter cores in our 850 and 450 μmSCUBA images, two of which have embedded class 0 or I protostars. Observations with BIMA are used to refine the position andcharacteristics of the Class 0 source NGC2023MM1. These observations show that it is embedded in a very cold cloud core, whichis strongly enhanced in NH2D.Results. We find that HD37903 is the most massive member of a cluster with 20–30 PMS stars. We also find smaller groups ofPMS stars formed from the Horsehead nebula and another elephant trunk structure to the north of the Horsehead. Star formation isalso occurring in the dark lane seen in IRAC images and in the sub-millimeter continuum. We refine the spectral classification ofHD37903 to B2 Ve. We find that the star has a clear IR excess, and therefore it is a young Herbig Be star.Conclusions. Our study shows that the expansion of the IC434 h ii region has triggered star formation in some of the dense elephanttrunk structures and compressed gas inside the L 1630 molecular cloud. This pre-shock region is seen as a sub-millimeter ridge inwhich stars have already formed. The cluster associated with NGC2023 is very young, and has a large fraction of Class I sources.