Chemical and dynamical study towards the UC HII region Monoceros R2

Abstract

One of the first signposts of high-mass star formation is the presence of compact regions of ionized gas (called HII regions), surrounded by layers of atomic and molecular gas (photon-dominated regions or PDRs). The study of the properties (e. g., dynamics, chemistry, structure) of these objects is of special interest because they provide information on the dynamical and chemical evolution in the formation of high-mass stars. The main objective of this Thesis is to characterize the chemistry and dynamics of the ultracompact HII region and PDR system in Monoceros R2: the closest object of this type that stands out as an ideal case to study the transition from ionized to molecular gas. We have carried out large-scale (163.5 arcmin2) mapping of the Monoceros R2 cloud in the 13CO(1–0), C18O(1–0), HCN(1–0), and N2H+ (1–0) lines. These maps show that the cloud has a filamentary structure with the gas infalling along the filaments towards the central hub with accretion rates of 10−4–10−3 M⊙ yr−1. In order to explore the influence of the UV radiation on the chemistry, we carried out an unbiased spectral line survey towards the Mon R2 region covering a frequency range from 84 to 350 GHz. Our data show that a high UV PDR has been formed around the HII region with intense emission of HCN, DCN, CN, CO+, CF+ and C2H species. A pseudo-time-dependent gas-phase chemical model has been used to interpret the emission of the deuterated compounds. The deuteration fraction measured in the dense clumps around the HII region is consistent with a chemical age of a few 105 yr.