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Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy

van Dishoeck, E. F. and Kristensen, L. E. and Mottram, J. C. and Benz, A. O. and Bergin, E. A. and Caselli, P. and Herpin, F. and Hogerheijde, M. R. and Johnstone, D. and Liseau, R. and Nisini, B. and Tafalla, M. and van der Tak, F. F. S. and Wyrowski, F. and Baudry, A. and Benedettini, M. and Bjerkeli, P. and Blake, G. A. and Braine, J. and Bruderer, S. and Cabrit, S. and Cernicharo, J. and Choi, Y. and Coutens, A. and de Graauw, Th. and Dominik, C. and Fedele, D. and Fich, M. and Fuente, A. and Furuya, K. and Goicoechea, J. R. and Harsono, D. and Helmich, F. P. and Herczeg, G. J. and Jacq, T. and Karska, A. and Kaufman, M. and Keto, E. and Lamberts, T. and Larsson, B. and Leurini, S. and Lis, D. C. and Melnick, G. and Neufeld, D. and Paganini, L. and Persson, M. and Shipman, R. and Taquet, V. and van Kempen, T. A. and Walsh, C. and Wampfler, S. F. and Yıldız, U. (2021) Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy. Astronomy and Astrophysics, 648 . Art. No. A24. ISSN 0004-6361. doi:10.1051/0004-6361/202039084.

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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 > 10⁵ L⊙) and from pre-stellar cores to protoplanetary disks. Lines of H₂O 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 (> 10⁵ 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 H₂O/H₂ 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/H₂O 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 NH3 abundance stays constant as a function of position in low-mass pre- and protostellar cores. Water abundances in the inner hot cores are high, but with variations from 5 × 10⁻⁶ to a few × 10⁻⁴ for low- and high-mass sources. Water vapor emission from both young and mature disks is weak. Conclusions. The main chemical pathways of water at each of the star-formation stages have been identified and quantified. Low warm water abundances can be explained with shock models that include UV radiation to dissociate water and modify the shock structure. UV fields up to 10²−10³ times the general interstellar radiation field are inferred in the outflow cavity walls on scales of the Herschel beam from various hydrides. Both high temperature chemistry and ice sputtering contribute to the gaseous water abundance at low velocities, with only gas-phase (re-)formation producing water at high velocities. Combined analyses of water gas and ice show that up to 50% of the oxygen budget may be missing. In cold clouds, an elegant solution is that this apparently missing oxygen is locked up in larger μm-sized grains that do not contribute to infrared ice absorption. The fact that even warm outflows and hot cores do not show H₂O at full oxygen abundance points to an unidentified refractory component, which is also found in diffuse clouds. The weak water vapor emission from disks indicates that water ice is locked up in larger pebbles early on in the embedded Class I stage and that these pebbles have settled and drifted inward by the Class II stage. Water is transported from clouds to disks mostly as ice, with no evidence for strong accretion shocks. Even at abundances that are somewhat lower than expected, many oceans of water are likely present in planet-forming regions. Based on the lessons for galactic protostars, the low-J H₂O line emission (E_(up) < 300 K) observed in extragalactic sources is inferred to be predominantly collisionally excited and to originate mostly from compact regions of current star formation activity. Recommendations for future mid- to far-infrared missions are made.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
van Dishoeck, E. F.0000-0001-7591-1907
Bergin, E. A.0000-0003-4179-6394
Caselli, P.0000-0003-1481-7911
Johnstone, D.0000-0002-6773-459X
Nisini, B.0000-0002-9190-0113
Blake, G. A.0000-0003-0787-1610
Dominik, C.0000-0002-3393-2459
Fedele, D.0000-0001-6156-0034
Herczeg, G. J.0000-0002-7154-6065
Lis, D. C.0000-0002-0500-4700
Neufeld, D.0000-0001-8341-1646
Paganini, L.0000-0002-4639-5488
Taquet, V.0000-0003-0407-7489
Walsh, C.0000-0001-6078-786X
Additional Information:© ESO 2021. Article published by EDP Sciences. Received 1 August 2020; Accepted 14 December 2020; Published online 09 April 2021. The authors would like to thank all WISH team members over the years for their seminal contributions to this project,as well as the entire HIFI science consortium for two wonderful decades of working together to make this happen. They are particularly grateful to Malcolm Walmsley, who stimulated the WISH program from the very beginning and helped with numerous projects. He left a great legacy. Fruitful collaborations with the DIGIT and WILL teams, and with members of the HDO team, are also acknowledged. Detailed discussions with Kathrin Altwegg and Martin Rubin on the oxygen budget in comets and with Bruce Draine on interstellar clouds are appreciated. Constructive comments from the referee have helped to improve the paper. A big salute goes to the HIFI and PACS instrument teams and to ESA for designing, building and operating these two powerful instruments and the Herschel Space Observatory. Herschel was an ESA space observatory with science instruments provided by the European-led Principal Investigator consortia and with important participation from NASA. HIFI was designed and built by a consortium of institutes and university de-partments from across Europe, Canada and the US under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiolog"i"a (CSIC-INTA); Sweden: Chalmers University of Technology – MC2, RSS & GARD, Onsala Space Observatory, Swedish National Space Board, Stockholm University – Stockholm Observatory; Switzerland: ETH Zürich, FHNW; USA: Caltech, JPL, NHSC. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, OAMP (France); MPIA (Germany); IFSI, OAP/OAT, OAA/CAISMI, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy), and CICYT/MCYT (Spain) Astrochemistry in Leiden is supported by the Netherlands Research School for Astronomy (NOVA). J.R.G. thanks the Spanish MICIU for funding support under grants AYA2017-85111-P and PID2019-106110GB-I00. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. D.F. acknowledges financial support from the Italian Ministry of Education, Universities and Research, project SIR (RBSI14ZRHR) as well as project PRIN-INAF-MAIN-STREAM 2017. AK acknowledges support from the Polish National Science Center grant 2016/21/D/ST9/01098 and the First TEAM grant of the Foundation for Polish Science No. POIR.04.04.00-00-5D21/18-00.
Funding AgencyGrant Number
Bundesministerium für Verkehr, Innovation und Technologie (BMVIT)UNSPECIFIED
European Space Agency (ESA)UNSPECIFIED
Commissariat à l'énergie atomique (CEA)UNSPECIFIED
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Deutsches Zentrum für Luft- und Raumfahrt (DLR)UNSPECIFIED
Agenzia Spaziale Italiana (ASI)UNSPECIFIED
Comisión Interministerial de Ciencia y Tecnología (CICYT)UNSPECIFIED
Ministerio de Ciencia Y Tecnologia (MCYT)UNSPECIFIED
Nederlandse Onderzoekschool Voor Astronomie (NOVA)UNSPECIFIED
Ministerio de Ciencia, Innovación y Universidades (MICIU)AYA2017-85111-P
Ministerio de Ciencia, Innovación y Universidades (MICIU)PID2019-106110GB-I00
Ministero dell'Istruzione, dell'Universita e della Ricerca (MIUR)RBSI14ZRHR
National Science Centre (Poland)2016/21/D/ST9/01098
Foundation for Polish SciencePOIR.04.04.00-00-5D21/18-00
Subject Keywords:astrochemistry – infrared: ISM – stars: formation – ISM: jets and outflows – ISM: molecules – protoplanetary disks
Record Number:CaltechAUTHORS:20210409-101341225
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Official Citation:Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy. E. F. van Dishoeck, L. E. Kristensen, J. C. Mottram, A. O. Benz, E. A. Bergin, P. Caselli, F. Herpin, M. R. Hogerheijde, D. Johnstone, R. Liseau, B. Nisini, M. Tafalla, F. F. S. van der Tak, F. Wyrowski, A. Baudry, M. Benedettini, P. Bjerkeli, G. A. Blake, J. Braine, S. Bruderer, S. Cabrit, J. Cernicharo, Y. Choi, A. Coutens, Th. de Graauw, C. Dominik, D. Fedele, M. Fich, A. Fuente, K. Furuya, J. R. Goicoechea, D. Harsono, F. P. Helmich, G. J. Herczeg, T. Jacq, A. Karska, M. Kaufman, E. Keto, T. Lamberts, B. Larsson, S. Leurini, D. C. Lis, G. Melnick, D. Neufeld, L. Pagani, M. Persson, R. Shipman, V. Taquet, T. A. van Kempen, C. Walsh, S. F. Wampfler, U. Yıldız and the WISH team. A&A, 648 (2021) A24; DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108673
Deposited By: Tony Diaz
Deposited On:13 Apr 2021 20:27
Last Modified:23 Apr 2021 18:50

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