Star formation and astrochemistry in peripheral regions of molecular clouds.

摘要

This dissertation presents results of a star formation study in the peripheral region of the L1630 (Orion B) molecular cloud and an astrochemistry study in the peripheral region of the L1204/S140 cloud. The peripheral regions of molecular clouds targeted in this dissertation have modest density {dollar}(10sp3{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar}) and column density (5-15 mag), and relatively low far-ultraviolet radiation. Such regions include most of the mass in the interstellar medium but have not been studied extensively. A multi-band near-infrared survey, covering 1320 arcmin{dollar}sp2{dollar} of the peripheral region of L1630 away from the known dense cores and young clusters, allowed a systematic search for distributed star formation in this cloud. Only 3%-8% sources in the peripheral region of L1630 display a near-infrared excess, in comparison to over 60% in the young clusters NGC 2023 and NGC 2024. The surface density of infrared-excess sources in the peripheral region of L1630 is also much lower than that of young clusters. Furthermore, a star counting study finds no enhancement of stellar surface density in the peripheral region of L1630, and field stars can adequately account for the observed star counts there. These results indicate that there is little evidence of distributed star formation in L1630, which is drastically different from the situation in the nearby cloud L1641. The results in L1630 are consistent with McKee's theory of photoionization-regulated star formation. The astrochemistry study in the peripheral region of S140 tests current understanding of photon-dominated regions under the condition of relatively low far-UV radiation and density. ISO observations found relatively smooth emission of the (C scII) 158 {dollar}mu{dollar}m and (O scI) 63 {dollar}mu{dollar}m lines. Chemical models fail to reproduce the (O scI) line and the (C scII) /(O scI) ratio unless the far-ultraviolet radiation field is about 5 times weaker than a traditional determination would give. Quantitative analysis of ambipolar diffusion and cloud collapse leads to predictions consistent with existing observations of star formation and McKee's theory of photoionization-regulated star formation.