[Paper Review] Warm Gas Towards Young Stellar Objects In Corona Australis Herschel/PACS Observations From The Digit Key Programme

J. E. Lindberg|arXiv (Cornell University)|Nov 29, 2013

Astrophysics and Star Formation Studies53 references15 citations

TL;DR

This study uses Herschel/PACS far-infrared data to investigate warm gas and dust in low-mass protostars within the irradiated R Coronae Australis region, employing deconvolution to separate point-source and extended emission. It finds that external irradiation from the Herbig Be star R CrA does not increase molecular excitation temperatures but enhances photodissociation products like OH and O, indicating external heating drives extended emission without altering core excitation conditions.

ABSTRACT

The effects of external irradiation on the chemistry and physics in the protostellar envelope around low-mass young stellar objects are poorly understood. The Corona Australis star-forming region contains the R CrA dark cloud, comprising several low-mass protostellar cores irradiated by an intermediate-mass young star. We study the effects on the warm gas and dust in a group of low-mass young stellar objects from the irradiation by the young luminous Herbig Be star R CrA. Herschel/PACS far-infrared datacubes of two low-mass star-forming regions in the R CrA dark cloud are presented. The distribution of CO, OH, H2O, [C II], [O I], and continuum emission is investigated. We have developed a deconvolution algorithm which we use to deconvolve the maps, separating the point-source emission from the extended emission. We also construct rotational diagrams of the molecular species. By deconvolution of the Herschel data, we find large-scale (several thousand AU) dust continuum and spectral line emission not associated with the point sources. Similar rotational temperatures are found for the warm CO ($282\pm4$ K), hot CO ($890\pm84$ K), OH ($79\pm4$ K), and H2O ($197\pm7$ K) emission, respectively, in the point sources and the extended emission. The rotational temperatures are also similar to what is found in other more isolated cores. The extended dust continuum emission is found in two ridges similar in extent and temperature to molecular mm emission, indicative of external heating from the Herbig Be star R CrA. Our results show that a nearby luminous star does not increase the molecular excitation temperatures in the warm gas around a young stellar object (YSO). However, the emission from photodissociation products of H2O, such as OH and O, is enhanced in the warm gas associated with these protostars and their surroundings compared to similar objects not suffering from external irradiation.

Motivation & Objective

• To understand the effects of external irradiation from a luminous Herbig Be star on the chemistry and physical conditions in warm gas and dust around low-mass young stellar objects (YSOs).
• To investigate whether irradiation from R CrA increases molecular excitation temperatures in protostellar envelopes compared to isolated sources.
• To disentangle point-source emission from extended emission in far-infrared data using deconvolution techniques.
• To analyze rotational temperatures and line ratios of CO, OH, H2O, [C ii], [O i], and dust continuum to assess excitation and heating mechanisms.

Proposed method

• Acquisition of Herschel/PACS integral-field spectroscopy data covering two low-mass star-forming regions in the R CrA dark cloud, spanning 55–210 µm.
• Application of a custom deconvolution algorithm to separate point-source emission from extended emission in the datacubes.
• Construction of rotational diagrams for CO, OH, H2O, and other species to derive rotational temperatures and assess excitation conditions.
• Comparison of line fluxes and ratios (e.g., OH/H2O, [O i]/OH, [O i]/CO) between the IRS7 and IRS5 fields to assess irradiation effects.
• Correlation of FIR continuum and spectral line emission with millimeter-wave molecular tracers (H2CO, CH3OH) to identify externally heated material.
• Use of 13CO rotational diagrams to estimate optical depth and assess excitation in mid-J CO transitions.

Experimental results

Research questions

• RQ1 Does external irradiation from the Herbig Be star R CrA increase the rotational temperatures of warm and hot CO, OH, and H2O in low-mass YSOs compared to isolated sources?
• RQ2 To what extent does extended far-infrared emission in the R CrA region trace externally heated gas and dust, and how does it correlate with millimeter molecular emission?
• RQ3 Are the enhanced line ratios of photodissociation products like OH, O, and [O i] in the CrA region indicative of a photodissociation region (PDR) driven by external irradiation?
• RQ4 How do the excitation conditions in the extended emission compare to those in the central protostellar cores, and what does this imply about the origin of heating?
• RQ5 Is the large-scale molecular emission in the R CrA region consistent with radiative excitation of low-density gas, or does it require high-density tracers to be explained?

Key findings

• Extended dust continuum emission at 40–50 K, detected in two ridges north and south of IRS7, correlates with H2CO and CH3OH mm emission, indicating external irradiation from R CrA heats the extended material.
• Rotational temperatures for warm CO (282 ± 4 K), hot CO (890 ± 84 K), OH (79 ± 4 K), and H2O (197 ± 7 K) are similar in both point-source and extended emission, indicating no significant increase in excitation due to external irradiation.
• The [O i] 63.2 µm line is strongly enhanced in extended regions, supporting the presence of a photodissociation region (PDR) induced by external irradiation.
• Line ratios of OH/H2O, [O i]/H2O, [O i]/OH, and [O i]/CO are enhanced by factors of 1.5–4.0 in CrA sources compared to other embedded objects, consistent with PDR-like chemistry.
• The warm CO component in extended emission has a rotational temperature of 277 ± 7 K, and the hot CO component 829 ± 69 K, indicating that extended warm gas is not solely due to internal heating from YSOs.
• Despite stronger irradiation, the IRS7 field (closer to R CrA) shows only marginally higher rotational temperatures than IRS5, suggesting that excitation conditions in the dense gas are resilient to external heating.