Water in star-forming regions with Herschel (WISH): II. Evolution of 557 GHz 110-101 emission in low-mass protostars

Abstract: (Abridged) Water is a key tracer of dynamics and chemistry in low-mass protostars, but spectrally resolved observations have so far been limited in sensitivity and angular resolution. In this first systematic survey of spectrally resolved water emission in low-mass protostellar objects, H2O was observed in the ground-state transition at 557 GHz with HIFI on Herschel in 29 embedded Class 0 and I protostars. Complementary far-IR and sub-mm continuum data (including PACS data from our program) are used to constrain the spectral energy distribution of each source. H2O intensities are compared to inferred envelope and outflow properties and CO 3-2 emission. H2O emission is detected in all objects except one. The line profiles are complex and consist of several kinematic components. The profiles are typically dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows, not the quiescent envelope. Several sources show multiple shock components in either emission or absorption, thus constraining the internal geometry of the system. Furthermore, the components include inverse P-Cygni profiles in 7 sources (6 Class 0, 1 Class I) indicative of infalling envelopes, and regular P-Cygni profiles in 4 sources (3 Class I, 1 Class 0) indicative of expanding envelopes. "Bullets" moving at >50 km/s are seen in 4 Class 0 sources; 3 of these are new detections. In the outflow, the H2O/CO abundance ratio as a function of velocity is nearly the same for all sources, increasing from 10^-3 at <5 km/s to >10^-1 at >10 km/s. The H2O abundance in the outer envelope is low, ~10^-10. The different H2O profile components show a clear evolutionary trend: in the Class 0 sources, emission is dominated by outflow components originating inside an infalling envelope. When the infall diminishes during the Class I phase, the outflow weakens and H2O emission disappears.

Comprehensive Study of Water Emission in Low-Mass Protostars: Insights from Herschel Observations

The paper titled "Water in star-forming regions with Herschel (WISH): Evolution of 557 GHz 1${10}$--1${01}$ emission in low-mass protostars" offers an in-depth analysis of water emission in low-mass star-forming regions. Utilizing data from the Heterodyne Instrument for the Far-Infrared (HIFI) on the Herschel Space Observatory, the study examines 29 low-mass protostellar objects to understand the evolution of water emission profiles and their correlation with the protostar's evolutionary state.

Key Findings and Methodology

Water, a crucial tracer of both dynamics and chemical processes in star-forming regions, exhibits complex interactions depicted through its emission profiles. The study distinguishes between various kinematic components within the observed systems: broad, medium, and narrow line components, as well as potentially identifying molecular "bullets"—fast-moving molecular gas indicative of high-velocity outflow phenomena. These components trace different physical areas across the protostellar systems providing insights into the underlying processes at play during star formation.

1. Profile Components and Kinematics: Emission profiles in Class 0 sources are dominated by broad and medium components, linked to dynamic outflows, while Class I sources generally display weaker emission profiles, indicating a decrease in outflow activity. The broad components suggest active outflows, possibly linked to the protostar's wind interacting with the surrounding material. Bullet-like features detected in some sources are indicative of highly dynamic, potentially episodic outflow events not previously associated with some of these systems.

2. Inverse and Regular P-Cygni Profiles: An intriguing finding is the presence of inverse P-Cygni profiles in several sources. These profiles indicate infall motions, showing gas moving towards the protostar, while regular P-Cygni profiles, observed in less dense Class I sources, suggest expanding motions potentially driven by protostellar winds overcoming infall. These kinematic features provide direct evidence of the accretion processes at various evolutionary stages.

3. Evolutionary Trends and Water Abundance: The study reveals clear evolutionary trends in water emission, with Class 0 sources displaying more dynamic profiles and higher emission levels compared to Class I sources. This can be attributed to denser envelopes and stronger outflows in Class 0 protostars which facilitate higher water emission, whereas in Class I sources, both envelope mass and outflow force diminish, leading to less pronounced water signatures.

4. Empirical Correlations: Water emission intensity correlates strongly with envelope mass, inferred H$_2$ density, and outflow velocity width, emphasizing its dependency on both physical and kinetic conditions surrounding the protostar. Surprisingly, despite the evolutionary changes, the water-to-CO ratio remains relatively stable across velocity ranges, indicating robust chemical processes governing their relative abundance.

Implications and Future Directions

This study underscores the pivotal role of water as a tracer of physical and dynamical conditions in star-forming regions. It enhances understanding of how star formation processes evolve, illustrated through nuances in water emission as a protostar matures. The consistency of the H$_2$O/CO ratio with velocity suggests a finely tuned balance in chemical destruction and formation processes that merits further investigation.

Future research directions could include high-resolution observations to dissect the spatial distribution of these kinematic components further, potentially uncovering the precise locations of infall and outflow interfaces. Moreover, continuing explorations into the chemical environment could elucidate mechanisms behind the evidently stable water abundance ratios despite varied physical conditions.

This comprehensive survey contributes significantly to the broader understanding of star formation by elucidating how water emission, as observed from the Herschel Space Observatory, serves as an insightful probe into the complex interplay of dynamical and chemical components within low-mass protostellar environments.