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

Abstract

Context. Water is an important chemical species in the process of star formation, and a sensitive tracer of physical conditions in star-forming regions because of its rich line spectrum and large abundance variations between hot and cold regions.
Aims: We use spectrally resolved observations of rotational lines of H₂O and its isotopologs to constrain the physical conditions of the water emitting region toward the high-mass protostar AFGL 2591.
Methods: Herschel/HIFI spectra from 552 up to 1669 GHz show emission and absorption in 14 lines of H ₂ O, H₂^18O, and H₂¹⁷O. We decompose the line profiles into contributions from the protostellar envelope, the bipolar outflow, and a foreground cloud. We use analytical estimates and rotation diagrams to estimate excitation temperatures and column densities of H₂O in these components. Furthermore, we use the non-local thermodynamic equilibrium (LTE) radiative transfer code RADEX to estimate the temperature and volume density of the H₂O emitting gas.
Results: Assuming LTE, we estimate an excitation temperature of ~42 K and a column density of ~2 × 10¹⁴ cm^-2 for the envelope and ~45 K and 4 × 10¹³ 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 × 10⁶-10⁸ cm^-3 for the envelope, and a kinetic temperature of ~70-90 K and a gas density of ~10⁷-10⁸ cm^-3 for the outflow. The ortho/para ratio of the narrow cold foreground absorption is lower than three (~1.9 ± 0.4), suggesting a low temperature. In contrast, the ortho/para ratio seen in absorption by the outflow is about 3.5 ± 1.0, as expected for warm gas.
Conclusions: The water abundance in the outer envelope of AFGL 2591 is ~10^-9 for a source size of 4″, similar to the low values found for other high-mass 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 for a source size of 30″, which is ~10× lower than in the envelope and in the outflows of high-mass 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 AFGL 2591 outflow is affected by dissociating UV radiation as a result of the low extinction in the outflow lobe.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.