Water in star-forming regions (WISH): Physics and chemistry from clouds to disks as probed by Herschel spectroscopy

Abstract

Context. Water is a key molecule in the physics and chemistry of star and planet formation, but it is difficult to observe from Earth. The Herschel Space Observatory provided unprecedented sensitivity as well as spatial and spectral resolution to study water. The Water In Star-forming regions with Herschel (WISH) key program was designed to observe water in a wide range of environments and provide a legacy data set to address its physics and chemistry. Aims. The aim of WISH is to determine which physical components are traced by the gas-phase water lines observed with Herschel and to quantify the excitation conditions and water abundances in each of these components. This then provides insight into how and where the bulk of the water is formed in space and how it is transported from clouds to disks, and ultimately comets and planets. Methods. Data and results from WISH are summarized together with those from related open time programs. WISH targeted ∼80 sources along the two axes of luminosity and evolutionary stage: from low-to high-mass protostars (luminosities from <1 to > 10 5 L ) and from pre-stellar cores to protoplanetary disks. Lines of H 2 O and its isotopologs, HDO, OH, CO, and [O I], were observed with the HIFI and PACS instruments, complemented by other chemically-related molecules that are probes of ultraviolet, X-ray, or grain chemistry. The analysis consists of coupling the physical structure of the sources with simple chemical networks and using non-LTE radiative transfer calculations to directly compare models and observations. Results. Most of the far-infrared water emission observed with Herschel in star-forming regions originates from warm outflowing and shocked gas at a high density and temperature (> 10 5 cm −3 , 300-1000 K, v ∼ 25 km s −1 ), heated by kinetic energy dissipation. This gas is not probed by single-dish low-J CO lines, but only by CO lines with J up > 14. The emission is compact, with at least two different types of velocity components seen. Water is a significant, but not dominant, coolant of warm gas in the earliest protostellar stages. The warm gas water abundance is universally low: orders of magnitude below the H 2 O/H 2 abundance of 4 × 10 −4 expected if all volatile oxygen is locked in water. In cold pre-stellar cores and outer protostellar envelopes, the water abundance structure is uniquely probed on scales much smaller than the beam through velocity-resolved line profiles. The inferred gaseous water abundance decreases with depth into the cloud with an enhanced layer at the edge due to photodesorption of water ice. All of these conclusions hold irrespective of protostellar luminosity. For low-mass protostars, a constant gaseous HDO/H 2 O ratio of ∼0.025 with position into the cold envelope is found. This value is representative of the outermost photodesorbed ice layers and cold gas-phase chemistry, and much higher than that of bulk ice. In contrast, the gas-phase NH 3 abundance stays constant as a function of position in low-mass pre-and protostellar cores. Water abun...

X-ray induced chemistry of water and related molecules in low-mass protostellar envelopes

“…In diffuse and dense clouds, water gas and ice are important oxygen carriers (Melnick et al 2020;van Dishoeck et al 2021). In diffuse and cold gas (gas temperature T gas 100 K), water is mainly produced by ion-molecule reactions (Hollenbach et al 2009).…”

“…When such a cloud becomes opaque (extinction A V > 3 ⋆ RIKEN Special Postdoctoral Researcher (SPDR, Fellow) mag) and cool ( T gas 20 − 30 K) enough, water is also efficiently formed by hydrogenation of oxygen atoms sticking onto cold dust grain surfaces where it forms an icy mantle (e.g., Cuppen et al 2010). Water ice is a dominant oxygen carrier in dark-clouds and pre-stellar cores (e.g., Öberg et al 2011;Caselli et al 2012;Marboeuf et al 2014;Boogert et al 2015;Taquet et al 2016b;Melnick et al 2020;van Dishoeck et al 2021). In warm regions (T gas > 100 K), water ice sublimates from the dust-grain surfaces into the gas phase.…”

“…A&A proofs: manuscript no. 2021.4.14_accepted-ver_arXiv_Notsu2021_X-ray_chemistry velopes (T gas >100 K) of low-mass Class 0 protostars have been investigated using PdBI 1 (e.g., Jørgensen & van Dishoeck 2010;Persson et al 2012Persson et al , 2014Persson et al , 2016, ALMA 2 (e.g., Bjerkeli et al 2016;Jensen et al 2019), and Herschel 3 /HIFI (e.g., van Dishoeck et al 2011van Dishoeck et al , 2021Visser et al 2013). The interferometric observations using PdBI and ALMA targeted the para-H 2 18 O 203 GHz 3 13 − 2 20 line (upper state energy E up =203.7 K), which is also considered to be a tracer of water emission in the inner warm regions and the position of the water snowline in Class II disks (Notsu et al 2018(Notsu et al , 2019.…”

“…The interferometric observations using PdBI and ALMA targeted the para-H 2 18 O 203 GHz 3 13 − 2 20 line (upper state energy E up =203.7 K), which is also considered to be a tracer of water emission in the inner warm regions and the position of the water snowline in Class II disks (Notsu et al 2018(Notsu et al , 2019. The velocity-resolved observations using Herschel/HIFI targeted several water lines, including the 3 12 − 3 03 lines of ortho-H 2 16 O (1097 GHz, E up =249.4 K) and ortho-H 2 18 O (1096 GHz, E up =248.7 K), and were part of the key program "Water in star-forming regions with Herschel" (WISH; van Dishoeck et al 2011van Dishoeck et al , 2021, which aimed to study the physics and chemistry of water during star formation across a range of masses and evolutionary stages. The water abundances in the outer cold envelopes were also investigated, using e.g., the ground-state ortho-H 2 16 O 557 GHz 1 10 − 1 01 line (E up =61.0 K, e.g., Kristensen et al 2010Kristensen et al , 2012van Dishoeck et al 2011van Dishoeck et al , 2021Coutens et al 2012Coutens et al , 2013Mottram et al 2013;Schmalzl et al 2014).…”

“…While some of this decrease can be accounted for if the detailed small scale physical structure is considered, Persson et al (2016) even found such low water gas abundances when using thin disk+envelope models. Questions on how the water abundance is changed from dense clouds to protostellar envelopes and planet-forming disks and the nature of the main oxygen carrier instead thus arise (van Dishoeck et al 2021). Since ALMA has much higher sensitivity and higher spatial and spectral resolution compared with previous instruments, water line surveys toward more Class 0 (and also Class I) protostars are expected using ALMA.…”