  Using the IRAM interferometer, we have observed the nearby starburst galaxy
M82 with a 4.2" resolution (~70 pc) in the 1->0 line of 13CO and in the 2.6-mm
continuum.
  The spatial distribution of the 13CO line shows the same gross features as
the 12CO(1->0) map of Shen & Lo (1995), namely two lobes and a compact central
source, though with different relative intensities. The lobes are more
conspicuous and the central source is fainter in 13CO than in 12CO.
  The velocity field observed around the nucleus shows a very steep gradient
(140 km/s over 75 pc), which is very probably caused by the stellar bar visible
in the near infrared. The dynamical centre coincides with the IR peak and is
shifted 6" north-east of the compact 13CO source. The two CO lobes appear to be
associated with the ends of the bar and not with a molecular ring, as usually
assumed. They are probably shaped by the strong UV radiation from the central
region. 13CO must be more photodissociated than the self-shielded 12CO
molecules in the central ~250 pc region, which may explain the relative
weakness of the 13CO central source.
  A 130 pc-wide bubble of molecular gas has been identified, which happens to
host the most luminous compact radio source in M82. It lies 120 pc west of the
IR peak between the central source and the western lobe and seems characterized
by warmer gas, strong UV radio free-free radiation, and an enhanced cosmic ray
production rate.
