Shockingly low water abundances in Herschel / PACS observations of low-mass protostars in Perseus

Abstract

Protostars interact with their surroundings through jets and winds impacting on the envelope and creating shocks, but the nature of these shocks is still poorly understood. Our aim is to survey far-infrared molecular line emission from a uniform and significant sample of deeply-embedded low-mass young stellar objects in order to characterize shocks and the possible role of ultraviolet radiation in the immediate protostellar environment. Herschel/PACS spectral maps of 22 objects in the Perseus molecular cloud were obtained as part of the `William Herschel Line Legacy' survey. Line emission from H${}_2$O, CO, and OH is tested against shock models from the literature. Observed line ratios are remarkably similar and do not show variations with source physical parameters. Observations show good agreement with the shock models when line ratios of the same species are compared. Ratios of various H${}_2$O lines provide a particularly good diagnostic of pre-shock gas densities, $n_{\mathrm{H}}\sim 10^5$ cm${}^{-3}$, in agreement with typical densities obtained from observations of the post-shock gas. The corresponding shock velocities, obtained from comparison with CO line ratios, are above 20 km\,s${}^{-1}$. However, the observations consistently show one-to-two orders of magnitude lower H${}_2$O-to-CO and H${}_2$O-to-OH line ratios than predicted by the existing shock models. The overestimated model H${}_2$O fluxes are most likely caused by an overabundance of H${}_2$O in the models since the excitation is well-reproduced. Illumination of the shocked material by ultraviolet photons produced either in the star-disk system or, more locally, in the shock, would decrease the H${}_2$O abundances and reconcile the models with observations. Detections of hot H${}_2$O and strong OH lines support this scenario.

28 pages, 12 figures, accepted to Astronomy & Astrophysics

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• [OI]63micron jets in class 0 sources detected by Herschel
• OH mid-infrared emission as a diagnostic of H${}_2$O UV photodissociation. I. Model and application to the HH 211 shock
• Origin of warm and hot gas emission from low-mass protostars: Herschel-HIFI observations of CO J=16-15. I. Line profiles, physical conditions, and H2O abundance
• Herschel-PACS observations of far-IR lines in YSOs I: [OI] and H2O at 63 microns
• The Class 0 Protostar BHR71: Herschel Observations and Dust Continuum Models
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• Characterizing the Youngest Herschel-detected Protostars II. Molecular Outflows from the Millimeter and the Far-infrared
• The importance of radiative pumping on the emission of the H_2O submillimeter lines in galaxies
• Far infrared CO and H${}_2$O emission in intermediate-mass protostars
• Herschel observations of the circumstellar environment of the two Herbig Be stars R Mon and PDS27
• Signatures of UV radiation in low-mass protostars I. Origin of HCN and CN emission in the Serpens Main region
• Atomic Shocks in the Outflow of L1551 IRS 5 Identified with SOFIA-upGREAT Observations of [OI]
• UV-irradiated outflows from low-mass protostars in Ophiuchus with JWST/MIRI
• Rate Coefficients for Rotational State-to-State Transitions in H${}_2$O + H${}_2$ Collisions as Predicted by Mixed Quantum/Classical Theory (MQCT)
• SOFIA FIFI-LS spectroscopy of DR21 Main: energetics of the spatially-resolved outflow from a high-mass protostar
• Far-infrared line emission from the outer Galaxy cluster Gy 3-7 with SOFIA/FIFI-LS: Physical conditions and UV fields