A METAL-POOR COMPACT H Ⅱ REGION WITH A SMALL, ISOLATED, AND DENSE YOUNG STELLAR CLUSTER

摘要

IRAS 04000+5052 appears to be a single, somewhat extended red source on optical CCD images. From our infrared imaging observations, an infrared cluster consisting of 15 objects is discovered in this region. In this paper, by discussing and analyzing the observational results of near-infrared imaging, optical spectrosco-py, and the ~(12)CO molecular line, together with IRAS and NRAO VLA Sky Survey (NVSS) data, we conclude that IRAS 04000+5052 is a compact H Ⅱ region with a very young dense stellar cluster. The spectrum of the compact H Ⅱ region shows that it is a metal-poor nebula, which has the lowest line ratio of [N Ⅱ]/Hα (～1/ 16.3) discovered in the H Ⅱ regions of our Galaxy until now. There are three possible explanations. The first is a Herbig Be star model, the second suggests that the material constituting this nebula could come from some nearby external galaxy from accretion of galaxies, and the third suggests that the material was produced in the primeval phase of our Galaxy and has not been chemically contaminated much. There exists an undetected gap between K = 13.6 and the detection limit of 16.2 mag. We give two possible interpretations for this undetected gap; one is based on the star formation itself, and the other is a possible observational effect. This infrared cluster is a very small (0.83 pc [north-south] x 1.55 pc [east-west]) and isolated high-mass protostel-lar cluster, in which there is a B0.5-type star exciting the compact H Ⅱ region. The derived heliocentric distance (～4.29 kpc) of IRAS 04000+5052 is twice as large as that of the Perseus arm. It is located far beyond the Perseus arm at the direction (l = 150.86).