Compressed magnetized shells of atomic gas and the formation of the Corona Australis molecular cloud

Bracco, Andrea; Bresnahan, D.; Palmeirim, P.; Arzoumanian, D.; André, Ph.; Ward-Thompson, D.; Marchal, A. (2020) Compressed magnetized shells of atomic gas and the formation of the Corona Australis molecular cloud. Astronomy & Astrophysics, 644 . ISSN 0004-6361

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Abstract

We present the identification of the previously unnoticed physical association between the Corona Australis molecular cloud (CrA), traced by interstellar dust emission, and two shell-like structures observed with line emission of atomic hydrogen (HI) at 21 cm. Although the existence of the two shells had already been reported in the literature, the physical link between the HI emission and CrA had never been highlighted until now. We used both Planck and Herschel data to trace dust emission and the Galactic All Sky HI Survey (GASS) to trace HI. The physical association between CrA and the shells is assessed based both on spectroscopic observations of molecular and atomic gas and on dust extinction data with Gaia. The shells are located at a distance between ~140 and ~190 pc, which is comparable to the distance of CrA, which we derived as (150.5 ± 6.3) pc. We also employed dust polarization observations from Planck to trace the magnetic- field structure of the shells. Both of them show patterns of magnetic-field lines following the edge of the shells consistently with the magnetic-field morphology of CrA. We estimated the magnetic-field strength at the intersection of the two shells via the Davis-Chandrasekhar-Fermi (DCF) method. Despite the many caveats that are behind the DCF method, we find a magnetic-field strength of (27 ± 8) μG, which is at least a factor of two larger than the magnetic-field strength computed off of the HI shells. This value is also significantly larger compared to the typical values of a few μG found in the diffuse HI gas from Zeeman splitting. We interpret this as the result of magnetic-field compression caused by the shell expansion. This study supports a scenario of molecular-cloud formation triggered by supersonic compression of cold magnetized HI gas from expanding interstellar bubbles.

Item Type:	Article
Uncontrolled Keywords:	ISM: clouds ; ISM: bubbles ; ISM: magnetic fields ; ISM: structure ; evolution ; ISM: kinematics and dynamics
Subjects:	NATURAL SCIENCES > Physics
Divisions:	Division of Experimental Physics
Projects:	Project title Project leader Project code Project type Origin of Magnetic strUctureS in the ISM obscuring the Cosmic DAwn Jelić, Vibor; Braco, Andrea 843008 EK
Depositing User:	Vibor Jelić
Date Deposited:	16 Nov 2021 08:41
URI:	http://fulir.irb.hr/id/eprint/6732
DOI:	10.1051/0004-6361/202039282

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