Water in Star-Forming Regions with the Herschel Space Observatory (WISH): Overview of key program and first results

Abstract: `Water In Star-forming regions with Herschel' (WISH) is a key program on the Herschel Space Observatory designed to probe the physical and chemical structure of young stellar objects using water and related molecules and to follow the water abundance from collapsing clouds to planet-forming disks. About 80 sources are targeted covering a wide range of luminosities and evolutionary stages, from cold pre-stellar cores to warm protostellar envelopes and outflows to disks around young stars. Both the HIFI and PACS instruments are used to observe a variety of lines of H2O, H218O and chemically related species. An overview of the scientific motivation and observational strategy of the program is given together with the modeling approach and analysis tools that have been developed. Initial science results are presented. These include a lack of water in cold gas at abundances that are lower than most predictions, strong water emission from shocks in protostellar environments, the importance of UV radiation in heating the gas along outflow walls across the full range of luminosities, and surprisingly widespread detection of the chemically related hydrides OH+ and H2O+ in outflows and foreground gas. Quantitative estimates of the energy budget indicate that H2O is generally not the dominant coolant in the warm dense gas associated with protostars. Very deep limits on the cold gaseous water reservoir in the outer regions of protoplanetary disks are obtained which have profound implications for our understanding of grain growth and mixing in disks.

• The paper details the comprehensive observational study of water's role using Herschel’s HIFI and PACS instruments.
• It reveals reduced water vapor in cold gas and enhanced shock-induced water emissions in protostellar environments.
• Findings challenge traditional cooling models and refine theoretical views on chemical evolution during star and planet formation.

Overview of the Herschel WISH Program

The paper "Water in Star-Forming Regions with the Herschel Space Observatory (WISH): Overview of Key Program and First Results" presents a comprehensive investigation of the role of water in star-forming regions using the Herschel Space Observatory. The study aims to probe the physical and chemical structure of young stellar objects across a range of evolutionary stages and luminosities. Approximately 80 sources are targeted, varying from low to high luminosities and from cold pre-stellar cores to warm protostellar environments. This program employs both HIFI and PACS instruments to capture a diverse range of H$_2$O, H$_2^{18}$O lines, and chemically related species.

Key Objectives and Methodology

The WISH program endeavors to follow the chemical evolution and distribution of water throughout the star and planet formation process. The study encompasses a diverse set of observational settings, including multi-line pointed observations using HIFI for insight into line kinematics and PACS spectral mapping for extensive coverage of warm water transitions. Ground-based observations complement the data, ensuring a self-consistent data set crucial for detailed analysis.

Initial Findings

One of the significant early findings includes the detection of reduced water vapor abundances in cold gas—observations that contrast earlier predictions. Additionally, the results highlight strong water emission predominantly emerging from shock regions in protostellar environments and the critical role of UV radiation in heating outflow cavity walls. Furthermore, WISH reveals a surprisingly widespread detection of hydrides such as OH$^+$ and H$_2$O$^+$ in outflows and foreground gas.

Astrophysical Implications

The implications of these observations are profound. They suggest that shocks and UV radiation significantly influence the water distribution and chemistry in star-forming regions. The data refines existing models of water's role as a gas coolant, indicating it is not as dominant in cooling warm dense gas as previously thought. Furthermore, stringent limits on cold gaseous water in the outer regions of protoplanetary disks indicate substantial implications for grain growth, ice mantle chemistry, and mixing in disks.

Theoretical and Future Directions

The analysis challenges traditional models and provides insights into high-temperature and solid-state chemical processes involved in water formation and destruction. The comprehensive dataset from WISH offers a legacy for future astronomical studies and can inform upcoming missions such as ALMA and future far-infrared space observatories. The lack of water vapor in certain regions also opens questions about the interaction of water formation with dust grain evolution.

In conclusion, the Herschel WISH program significantly advances our understanding of water's role in star-forming regions. It provides detailed observational constraints that challenge and refine theoretical models across the star and planet formation spectrum.