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We present the identification of the previously unnoticed physicalassociation between the Corona Australis molecular cloud (CrA), traced byinterstellar dust emission, and two shell-like structures observed with lineemission of atomic hydrogen (HI) at 21 cm. Although the existence of the twoshells had already been reported in the literature, the physical link betweenthe HI emission and CrA was never highlighted before. We use both Planck andHerschel 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 assessedbased both on spectroscopic observations of molecular and atomic gas and ondust extinction data with Gaia. The shells are located at a distance between140 and 190 pc, comparable to the distance of CrA, which we derive as 150.5 +-6.3 pc. We also employ dust polarization observations from Planck to trace themagnetic-field structure of the shells. Both of them show patterns ofmagnetic-field lines following the edge of the shells consistently with themagnetic-field morphology of CrA. We estimate the magnetic-field strength atthe intersection of the two shells via the Davis-Chandrasekhar-Fermi (DCF)method. Albeit the many caveats that are behind the DCF method, we find amagnetic-field strength of 27 +- 8 $\mu$G, at least a factor of two larger thanthe magnetic-field strength computed off of the HI shells. This value is alsosignificantly larger compared to the typical values of a few $\mu$G found inthe diffuse HI gas from Zeeman splitting. We interpret this as the result ofmagnetic-field compression caused by the shell expansion. This study supports ascenario of molecular-cloud formation triggered by supersonic compression ofcold magnetized HI gas from expanding interstellar bubbles.

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