Compressed magnetized shells of atomic gas and the formation of the
  Corona Australis molecular cloud
  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 was never highlighted before. We use 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, 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 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 estimate the magnetic-field strength at
the intersection of the two shells via the Davis-Chandrasekhar-Fermi (DCF)
method. Albeit the many caveats that are behind the DCF method, we find a
magnetic-field strength of 27 +- 8 $\mu$G, 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 $\mu$G 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.
