Optical emission from shocks. VI. Abundance gradient in M33 from supernova remnants.

The absolute abundance gradient in the spiral arms of the galaxy M 33 has been obtained using supernova remnants (SNRs) as tracers of gaseous phase abundance in the interstellar medium. No galactic enrichment theory is entirely successful in explaining the observations. The gradient in N/S abundance obtained from SNRs is identical to that given by H II region observations. Nitrogen seems to be produced first as a primary but later (at higher enrichment and star mass to gas mass ratio) as a secondary nucleosynthesis element.