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

L. E. Kristensen, E. F. van Dishoeck+23 more|arXiv (Cornell University)|Mar 30, 2012

Astrophysics and Star Formation StudiesPhysics and Astronomy111 references129 citations

TL;DR

This study presents the first systematic survey of spectrally resolved 557 GHz H₂O 1₁₀–1₀₁ emission in 29 low-mass protostars using Herschel/HIFI, revealing that water emission is dominated by outflow components in Class 0 sources and transitions to weaker, expanding-envelope-dominated emission in Class I sources. The key finding is a clear evolutionary trend: younger Class 0 sources show strong outflow-driven water emission with infall signatures, while Class I sources exhibit weaker, expanding envelopes and diminished outflow activity.

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.

Motivation & Objective

• To characterize the kinematic structure of water emission in low-mass protostars across different evolutionary stages.
• To distinguish between physical components (infall, outflow, expansion) traced by H₂O line profiles.
• To investigate how water abundance and excitation conditions evolve with protostellar evolution.
• To determine the role of shocks and envelope density in shaping H₂O emission in star-forming regions.
• To link H₂O line profiles to envelope dynamics and outflow activity, particularly in Class 0 vs. Class I sources.

Proposed method

• Single-point spectrophotometric observations of the 557 GHz H₂O 1₁₀–1₀₁ transition using the Heterodyne Instrument for the Far-Infrared (HIFI) on the Herschel space telescope.
• Complementary far-IR and sub-mm continuum data (from PACS and other instruments) were used to construct spectral energy distributions (SEDs) for each source.
• Line profile decomposition into kinematic components (broad outflow, narrow infall/expansion, absorption) was performed to identify physical regions.
• Inverse and regular P-Cygni profiles were modeled to infer infall and expansion velocities and mass accretion rates.
• H₂O/CO abundance ratios were derived as a function of velocity, assuming similar excitation conditions for H₂O and CO.
• Envelope density and outflow force were used as proxies to assess the influence of physical conditions on H₂O emission.

Experimental results

Research questions

• RQ1 How does the morphology of H₂O 557 GHz line emission vary across the evolutionary sequence from Class 0 to Class I protostars?
• RQ2 What physical components (infall, outflow, expansion) are traced by different features in the H₂O line profiles?
• RQ3 How does the H₂O/CO abundance ratio vary with velocity in the line wings, and does it differ between Class 0 and Class I sources?
• RQ4 What is the role of shocks in enhancing H₂O abundance in outflows, and how does this relate to envelope density?
• RQ5 How do infall and expansion velocities, inferred from P-Cygni profiles, evolve with protostellar age and luminosity?

Key findings

• H₂O emission was detected in all 29 low-mass protostars except TMC1A, with complex line profiles dominated by broad Gaussian components indicating outflow emission.
• Infall signatures (inverse P-Cygni profiles) were observed in six Class 0 and one Class I source, with inferred infall velocities of ~0.1–1.0 km s⁻¹ and mass accretion rates of ~10⁻⁶–10⁻⁴ M☉ yr⁻¹.
• Expansion signatures (regular P-Cygni profiles) were found primarily in Class I sources, with expansion velocities of ~0.5 km s⁻¹ and a short dispersion timescale (~10⁴ years), suggesting a transient phase.
• Molecular bullets moving at ≥50 km s⁻¹ were detected in four Class 0 sources, three of which were previously unknown to host such features.
• The H₂O/CO abundance ratio increases from ~10⁻³ at low velocities (<5 km s⁻¹) to ≥10⁻¹ at high velocities (>10 km s⁻¹), with no significant difference between Class 0 and Class I sources.
• Water abundance in the outer cold envelope is low, ~10⁻¹⁰, and absorption features are predominantly saturated in Class 0 sources, confirming low outer-envelope abundance and strong infall dominance in early stages.