Observations of water with Herschel/HIFI toward the high-mass protostar AFGL 2591

Abstract

Water is a sensitive tracer of physical conditions in star-forming regions because of its large abundance variations between hot and cold regions. We use spectrally resolved observations of rotational lines of H${}_2$O and its isotopologs with Herschel/HIFI to constrain the physical conditions of the water emitting region toward the high-mass protostar AFGL2591. We use analytical estimates and rotation diagrams to estimate T${}_{ex}$ and column densities of H${}_2$O of the envelope, the outflow, and a foreground cloud. Furthermore, we use the non-LTE radiative transfer code to estimate the temperature and volume density of the H${}_2$O emitting gas. Assuming LTE, we estimate an T${}_{ex}$ of 42 K and a column density of 2$\times$10${}^{14}$ cm${}^{-2}$ for the envelope and 45 K and 4$\times$10${}^{13}$ cm${}^{-2}$ for the outflow, in beams of 4" and 30", respectively. Non-LTE models indicate a kinetic temperature of 60-230 K and a volume density of 7$\times$10${}^6$-10${}^8$ cm${}^{-3}$ for the envelope, and a kinetic temperature of 70-90 K and a gas density of 10${}^7$-10${}^8$ cm${}^{-3}$ for the outflow. The o/p ratio of the foreground absorption is 1.9$\pm$0.4, suggesting a low temperature. In contrast, the o/p ratio seen in absorption by the outflow is 3.5$\pm$1.0, as expected for warm gas. The water abundance in the envelope is 10${}^{-9}$, similar to the low values found for other high- and low-mass protostars, suggesting that this abundance is constant during the embedded phase of high-mass star formation. The water abundance in the outflow is 10${}^{-10}$, which is 10$\times$ lower than in the envelope and in the outflows of high- and low-mass protostars. Since beam size effects can only increase this estimate by a factor of 2, we suggest that the water in the outflow is affected by dissociating UV radiation due to the low extinction in the outflow lobe.

arXiv commentAccepted for publication in A&A

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