Water in star-forming regions: Physics and chemistry from clouds to disks as probed by Herschel spectroscopy
Journal article, 2021
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 > 10Lpdbl) 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 (> 10cm-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 Jup > 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 102-10times 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 (Eup < 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.
Stars: formation
ISM: molecules
Infrared: ISM
ISM: jets and outflows
Protoplanetary disks
Astrochemistry
Author
E. F. van Dishoeck
Max Planck Society
Leiden University
L. E. Kristensen
Niels Bohr Institute
J. C. Mottram
Max Planck Society
A. O. Benz
Swiss Federal Institute of Technology in Zürich (ETH)
E. A. Bergin
University of Michigan
P. Caselli
Max Planck Society
F. Herpin
University of Bordeaux
M. Hogerheijde
University of Amsterdam
Leiden University
D. Johnstone
National Research Council Canada
University of Victoria
René Liseau
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
B. Nisini
Osservatorio Astronomico di Roma
M. Tafalla
Spanish National Observatory (OAN)
F. F. S. van der Tak
Netherlands Institute for Space Research (SRON)
University of Groningen
F. Wyrowski
Max Planck Society
A. Baudry
University of Bordeaux
M. Benedettini
Istituto nazionale di astrofisica (INAF)
Per Bjerkeli
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
G. A. Blake
California Institute of Technology (Caltech)
J. Braine
University of Bordeaux
S. Bruderer
Swiss Federal Institute of Technology in Zürich (ETH)
Max Planck Society
S. Cabrit
Paris Observatory
Jose Cernicharo
Spanish National Research Council (CSIC)
Yunhee Choi
Korea Astronomy and Space Science Institute
Netherlands Institute for Space Research (SRON)
A. Coutens
University of Bordeaux
T. de Graauw
Netherlands Institute for Space Research (SRON)
Leiden University
C. Dominik
University of Amsterdam
D. Fedele
Arcetri Astrophysical Observatory
M. Fich
University of Waterloo
A. Fuente
Spanish National Observatory (OAN)
Kenji Furuya
National Astronomical Observatory of Japan
J.R. Goicoechea
Spanish National Research Council (CSIC)
D. Harsono
Leiden University
F. Helmich
Netherlands Institute for Space Research (SRON)
University of Groningen
G. J. Herczeg
Beijing University of Technology
T. Jacq
University of Bordeaux
A. Karska
Nicolaus Copernicus University
M. Kaufman
San Jose State University
E. Keto
Harvard-Smithsonian Center for Astrophysics
T. Lamberts
Leiden Institute of Chemistry
B. Larsson
Stockholm University
S. Leurini
Max Planck Society
Istituto nazionale di astrofisica (INAF)
D. C. Lis
California Institute of Technology (Caltech)
G. J. Melnick
Harvard-Smithsonian Center for Astrophysics
D.A. Neufeld
Johns Hopkins University
L. Pagani
Paris Observatory
Magnus V. Persson
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
R. Shipman
Netherlands Institute for Space Research (SRON)
V. Taquet
Arcetri Astrophysical Observatory
T. A. van Kempen
Netherlands Institute for Space Research (SRON)
C. Walsh
University of Leeds
S. F. Wampfler
University of Bern
U. Ylldlz
California Institute of Technology (Caltech)
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 648 A24Subject Categories (SSIF 2011)
Water Engineering
Astronomy, Astrophysics and Cosmology
Oceanography, Hydrology, Water Resources
DOI
10.1051/0004-6361/202039084