[Paper Review] Water in star forming regions with Herschel (WISH) III. Far-infrared cooling lines in low-mass young stellar objects

A. Karska, Gregory J. Herczeg+23 more|arXiv (Cornell University)|Jan 21, 2013

Astrophysics and Star Formation StudiesPhysics and Astronomy80 references62 citations

TL;DR

This study analyzes far-infrared line emission from 18 low-mass protostars using Herschel-PACS spectroscopy within the WISH key program, revealing that water and CO dominate the gas cooling budget, with distinct excitation components linked to non-dissociative shocks along outflow walls. The results show a strong correlation between high-J CO and H2O line fluxes and bolometric luminosity, indicating shared physical origins, while [O i] and OH trace dissociative shocks in the inner envelope, with cooling efficiency decreasing from Class 0 to Class I sources.

ABSTRACT

(Abridged) Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied. We quantify their far-infrared line emission and the contribution of different atomic and molecular species to the gas cooling budget during protostellar evolution. We also determine the spatial extent of the emission and investigate the underlying excitation conditions. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 objects, including 5 Class I sources. The high-excitation H2O line at 63.3 micron is detected in 7 sources. CO transitions from J=14-13 up to 49-48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~350 K and ~700 K. H2O has typical excitation temperatures of ~150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern depending on the species and the transition. The H2O line fluxes correlate strongly with those of the high-J CO lines, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH and not with [OI] fluxes. The PACS data probe at least two physical components. The H2O and CO emission likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, likely resulting from H2O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24-23 line and the CO 14-13 line. [OI] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H2O and CO, with [OI] increasing for Class I sources.

Motivation & Objective

• To quantify far-infrared line emission from low-mass protostars and determine the contribution of atomic and molecular species to the gas cooling budget.
• To investigate the spatial extent and excitation conditions of emission lines in Class 0 and I sources.
• To understand how energetic processes and cooling mechanisms evolve during the embedded phase of protostellar evolution.
• To disentangle the roles of shocks and UV heating in exciting far-IR lines in young stellar objects.

Proposed method

• Herschel-PACS spectroscopy was used to observe 18 low-mass protostars across a range of luminosities and evolutionary stages.
• Rotational diagrams were constructed for CO and H2O to derive rotational temperatures and assess excitation conditions.
• Spatially resolved flux measurements were performed across multiple spaxels to distinguish compact vs. extended emission components.
• Correlations between line fluxes, bolometric luminosity (Lbol), and envelope mass (Menv) were analyzed to infer physical associations.
• Emission line profiles and spatial distributions were compared with lower-J CO outflow maps to link line emission to outflow structures.
• Shock and UV heating models were used to interpret the observed line ratios and spatial patterns.

Experimental results

Research questions

• RQ1 What is the relative contribution of H2O, CO, [O i], and OH to the far-infrared gas cooling budget in low-mass protostars?
• RQ2 How do the spatial distributions of emission from different species relate to outflow structures and physical components?
• RQ3 What physical conditions (temperature, density, shocks) excite the observed high-J CO and H2O lines?
• RQ4 How do the excitation and cooling properties of these lines evolve from Class 0 to Class I protostars?
• RQ5 What is the role of UV heating versus shock excitation in producing the observed far-IR line emission?

Key findings

• Water is detected in 17 out of 18 low-mass protostars, including 5 Class I sources, with the 212−101 line at 179.5 µm detected in all and the 818−707 line at 63.3 µm in 7 sources.
• CO lines were detected from J = 14−13 up to J = 48−47, with rotational diagrams revealing two distinct components at ~350 K and ~700 K.
• H2O and CO emission are spatially extended along outflows in half the sample, covering scales of at least 10,000 AU, while the other half show compact emission within 1,000 AU.
• H2O and high-J CO line fluxes strongly correlate with bolometric luminosity and envelope mass, indicating a common physical origin in shock-excited gas.
• The [O i] and OH lines peak at the central position and correlate with each other but not with H2O or CO fluxes, indicating they trace different, inner regions such as dissociative shocks.
• H2O contributes 25–50% and CO 5–50% to the total far-infrared cooling, while [O i] contributes 5–30% and increases in relative importance in Class I sources.