APEX-CHAMP+ high-J CO observations of low-mass young stellar objects: III. NGC 1333 IRAS 4A/4B envelope, outflow and UV heating

Abstract: NGC 1333 IRAS 4A and IRAS 4B sources are among the best studied Stage 0 low-mass protostars which are driving prominent bipolar outflows. Most studies have so far concentrated on the colder parts (T<30K) of these regions. The aim is to characterize the warmer parts of the protostellar envelope in order to quantify the feedback of the protostars on their surroundings in terms of shocks, UV heating, photodissociation and outflow dispersal. Fully sampled large scale maps of the region were obtained; APEX-CHAMP+ was used for 12CO 6-5, 13CO 6-5 and [CI] 2-1, and JCMT-HARP-B for 12CO 3-2 emissions. Complementary Herschel-HIFI and ground-based lines of CO and its isotopologs, from 1-0 upto 10-9 (Eu/k 300K), are collected at the source positions. Radiative-transfer models of the dust and lines are used to determine temperatures and masses of the outflowing and UV-heated gas and infer the CO abundance structure. Broad CO emission line profiles trace entrained shocked gas along the outflow walls, with typical temperatures of ~100K. At other positions surrounding the outflow and the protostar, the 6-5 line profiles are narrow indicating UV excitation. The narrow 13CO 6-5 data directly reveal the UV heated gas distribution for the first time. The amount of UV-heated and outflowing gas are found to be comparable from the 12CO and 13CO 6-5 maps, implying that UV photons can affect the gas as much as the outflows. Weak [CI] emission throughout the region indicates a lack of CO dissociating photons. Modeling of the C18O lines indicates the necessity of a "drop" abundance profile throughout the envelope where the CO freezes out and is reloaded back into the gas phase, thus providing quantitative evidence for the CO ice evaporation zone around the protostars. The inner abundances are less than the canonical value of CO/H_2=2.7x10^-4, indicating some processing of CO into other species on the grains.

An Analytical Overview of High-J CO Observations in Low-Mass Protostars: Focus on NGC 1333 IRAS 4A/4B

The study presented in the paper provides comprehensive spectrally-resolved insights into the NGC 1333 IRAS 4A/4B low-mass protostars using high-J CO observations. These observations were conducted with the APEX-CHAMP$^+$ instrument alongside complementary data from JCMT and Herschel-HIFI, probing molecular line transitions spanning various rotational levels.

Key Observational Insights and Numerical Results

1. Rotational Line Analysis: The study thoroughly investigates a series of $^{12}$CO, $^{13}$CO, and C$^{18}$O transitions, presenting a CO ladder that ranges up to the $J=10-9$ transition. Notably, rotational temperatures for $^{12}$CO lines were found to consistently support an excitation temperature around 69–83 K, while isotopologues exhibited marginally lower temperatures.

2. Morphology and Dynamics of Outflows: Detailed mapping identified well-collimated molecular outflows emanating from IRAS 4A, exhibiting distinct north-south orientations with prominent lobes. The dynamical ages of these outflows were calculated (5,900 years for the blue and 9,200 years for the red outflows in IRAS 4A) and verified against historical data, prompting discussions on episodic accretion processes.

3. UV Photon and Shock Heating: A particularly striking result was the differentiation of broad and narrow CO line profiles, indicative of shock-heated outflows versus UV photon-heated cavity walls, respectively. The narrow $^{13}$CO 6–5 emission, in particular, directly revealed the distribution of UV heated gas not previously documented to this extent.

4. Envelope Abundance Modelling: Through radiative transfer modeling, ‘drop’ and ‘jump’ abundance profiles were necessary to fit the C$^{18}$O lines, indicating the presence of CO freeze-out zones. Notably, the inner CO abundance was consistently found to be lower than canonical expectations, suggesting potential molecular processing during freeze-out.

Theoretical and Practical Implications

These results have substantial implications for the understanding of protostellar evolution. Quantitatively linking UV-heated gas and outflow processes offers new insights into the feedback mechanisms effective during early stellar development stages. The data distinctly highlight how $^{12}$CO covers outflow activities, while $^{13}$CO maps UV and ambient thermal environments. This dichotomy is essential for modeling the environment surrounding protostars, where shock and radiative processes are often interlinked.

Speculative Insights and Future Directions

The similarities between observed CO excitation in low-mass protostars and other astronomical contexts, such as the Orion Bar, suggest universal mechanisms at play. Moreover, the study’s mapping of UV photon-heated gases alongside existing CO levels can serve as a comparative framework for forthcoming ALMA observations, which are equipped for higher spatial resolutions. Such studies could refine our understanding of CO and water molecule interactions, given their co-existence in these environments.

In conclusion, the paper underlines the utility of high-J CO spectroscopic and imaging techniques in capturing the complex interplay between outflows and radiatively-heated components in protostellar environments. Additional multi-transitional analysis, possibly extending above the $J=10$ lines, could further unravel these complex processes, especially with next-generation observational tools like ALMA paving the way for unprecedented spatial resolutions.