Chemical evolution along the circumnuclear ring of M83
  We report an astrochemical study on the evolution of interstellar molecular
clouds and consequent star formation in the center of the barred spiral galaxy
M83. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to image
molecular species indicative of shocks (SiO, CH$_3$OH), dense cores
(N$_2$H$^+$), and photodissociation regions (CN and CCH), as well as a radio
recombination line (H41$\alpha$) tracing active star-forming regions. M83 has a
circumnuclear gas ring that is joined at two areas by gas streams from the
leading-edge gas lanes on the bar. We found elevated abundances of the shock
and dense-core tracers in one of the orbit-intersecting areas, and found peaks
of CN and H41$\alpha$ downstream. At the other orbit-intersection area, we
found similar enhancement of the shock tracers, but less variation of other
tracers, and no sign of active star formation in the stream. We propose that
the observed chemical variation or lack of it is due to the presence or absence
of collision-induced evolution of molecular clouds and induced star formation.
This work presents the most clear case of the chemical evolution in the
circumnuclear rings of barred galaxies, thanks to the ALMA resolution and
sensitivity.
