Analysis of Low Excitation HDO Transitions Toward the High-Mass Star-forming Regions G34.26+0.15, W51e$_{1}$/e$_{2}$, and W49N

Abstract

We present observations of the ground state 1$_{0,1}$-0$_{0,0}$ rotational transition of HDO at 464.925 GHz and the 1$_{1,0}$-1$_{0,1}$ transition at 509.292 GHz toward the three high-mass star forming regions: G34.26+0.15, W49N, and W51e$_{1}$/e$_{2}$, carried out with the Caltech Submillimeter Observatory. The latter transition is observed for the first time from the ground. The spectra are modeled, together with observations of higher-energy HDO transitions, as well as submillimeter dust continuum fluxes from the literature, using a spherically symmetric radiative transfer model to derive the radial distribution of the HDO abundance in the target sources. The abundance profile is divided into an inner hot core region, with kinetic temperatures higher than 100~K, and a cold outer envelope with lower kinetic temperatures. The derived HDO abundance with respect to H$_2$ is (0.3-3.7)$\times 10^{-8}$ in the hot inner region ($T > 100 \mathrm{K}$) and (7.0-10.0)$\times 10^{-11}$ in the cold outer envelope. We also used two H$_{2}^{18}$O fundamental transitions to constrain the H$_{2}$O abundances in the outer envelopes. The HDO/H$_{2}$O ratios in these cold regions are found to be (1.8-3.1)$\times 10^{-3}$ and are consequently higher than in the hot inner regions of these sources.