[Paper Review] Origin of the hot gas in low-mass protostars: Herschel-PACS spectroscopy of HH 46

T. A. van Kempen, L. E. Kristensen+63 more|UvA-DARE (University of Amsterdam)|May 12, 2010

Astrophysics and Star Formation Studies25 references62 citations

TL;DR

This study uses Herschel-PACS spectroscopy to investigate the origin of hot gas in the low-mass protostar HH 46, finding that shock heating—particularly from J- and C-type shocks—dominates far-IR line cooling, accounting for at least 60% of total cooling, with UV-heated cavity walls and passive envelope heating contributing the remainder. The data rule out passive heating alone and support shock-driven excitation of H₂O, OH, and [O i] emission.

ABSTRACT

'Water in Star-forming regions with Herschel' (WISH) is a Herschel Key Programme aimed at understanding the physical and chemical structure of young stellar objects (YSOs) with a focus on water and related species. The low-mass protostar HH 46 was observed with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory to measure emission in H2O, CO, OH, [OI], and [CII] lines located between 63 and 186 um. The excitation and spatial distribution of emission can disentangle the different heating mechanisms of YSOs, with better spatial resolution and sensitivity than previously possible. Far-IR line emission is detected at the position of the protostar and along the outflow axis. The OH emission is concentrated at the central position, CO emission is bright at the central position and along the outflow, and H2O emission is concentrated in the outflow. In addition, [OI] emission is seen in low-velocity gas, assumed to be related to the envelope, and is also seen shifted up to 170 km/s in both the red- and blue-shifted jets. Envelope models are constructed based on previous observational constraints. They indicate that passive heating of a spherical envelope by the protostellar luminosity cannot explain the high-excitation molecular gas detected with PACS, including CO lines with upper levels at >2500 K above the ground state. Instead, warm CO and H2O emission is probably produced in the walls of an outflow-carved cavity in the envelope, which are heated by UV photons and non-dissociative C-type shocks. The bright OH and [OI] emission is attributed to J-type shocks in dense gas close to the protostar. In the scenario described here, the combined cooling by far-IR lines within the central spatial pixel is estimated to be 2 imes 10-2 L_sun, with 60-80% attributed to J- and C-type shocks produced by interactions between the jet and the envelope.

Motivation & Objective

• To determine the dominant heating mechanism responsible for the observed far-IR line emission in the low-mass protostar HH 46.
• To disentangle contributions from passive envelope heating, UV-heated cavity walls, and shock processes in powering line emission.
• To quantify the relative cooling contributions of CO, H₂O, OH, and [O i] in the central protostellar region.
• To test whether shock models or passive envelope models can reproduce the observed line ratios and intensities.

Proposed method

• Far-IR spectroscopy of HH 46 using the Herschel-PACS instrument to detect emission lines of H₂O, CO, OH, [O i], and [C ii] between 63 and 186 μm.
• Spatially resolved line emission analysis using spaxels to distinguish emission from the central protostar, outflow, and envelope.
• Modeling of line ratios and intensities using a single-component slab model with an escape probability code including dust continuum absorption and emission.
• Comparison of observed line ratios (e.g., H₂O 183 μm / 119 μm, [O i] 63 μm / 146 μm) with predictions from shock models (J- and C-type) and passive envelope models.
• Cooling rate estimation for each species by integrating line luminosities and comparing to protostellar luminosity.
• Use of shock models (Neufeld & Dalgarno, 1989; Snell et al., 2005) to constrain physical conditions such as density (n_H ~ 10⁷ cm⁻³) and temperature (T_gas > 800 K) in the emitting regions.

Experimental results

Research questions

• RQ1 What is the dominant heating mechanism responsible for the observed high-excitation molecular gas in HH 46: passive envelope heating or shock processes?
• RQ2 Can the observed line ratios of H₂O, OH, and [O i] be reproduced by shock models, and what physical conditions (density, temperature) do they imply?
• RQ3 How do the contributions of UV-heated cavity walls, C-type shocks, and J-type shocks compare in powering the total far-IR line cooling?
• RQ4 Is the observed OH emission consistent with origin in the cavity walls or in the jet-impingement region, and what does this imply about shock morphology?
• RQ5 To what extent does the observed line emission in the central spaxel account for the total far-IR cooling, and how does this compare to previous ISO-LWS measurements?

Key findings

• The [O i] 63 μm / 146 μm line ratio of ~16 in the central spaxel is consistent with fast, dissociative shocks at low densities (~10⁴ cm⁻³), ruling out passive envelope heating.
• OH emission originates in a high-density (n_H ~ 10⁷ cm⁻³) and high-temperature (T_gas > 800 K) region, with a physical size of ~0.5″ (~250 AU), inconsistent with passive heating or C-type shocks along cavity walls.
• Shock models (J- and C-type) account for at least 60% of the total far-IR line cooling in the central spaxel, with a total cooling rate of 1.5 × 10⁻² L☉ from shocks alone.
• CO cooling is distributed among three components: passive envelope (0.1 × 10⁻³ L☉), UV-heated cavity walls (3.8 × 10⁻³ L☉), and C-type shocks (2.8 × 10⁻³ L☉), indicating multiple heating mechanisms.
• H₂O emission is best explained by UV heating or C-type shocks, with total cooling from these components reaching 5.0 × 10⁻³ L☉, and cannot be reproduced by passive heating alone.
• The total far-IR line cooling in the central spaxel exceeds 23.8 × 10⁻³ L☉, consistent with ISO-LWS measurements and indicating that shocks are the dominant energy source for line emission.