Type Ia supernovae in globular clusters: observational upper limits
  In the dense stellar environment of the globular clusters, compact binaries
are produced dynamically. Therefore the fraction of type Ia supernovae that
explode in globular clusters is expected to be higher than the fraction of mass
residing in these. We have searched for globular clusters at the positions of
observed type Ia supernovae. We used archival HST images and literature data,
covering the positions either before the supernovae exploded, or long enough
after that the supernovae have faded below the luminosities of globular
clusters. We did not find evidence for globular clusters at any of the
supernova positions. For 18 type Ia supernovae, the observations are sensitive
enough that any globular cluster would have been detected, and for further 17
type Ia supernovae, the brighter globular clusters would have been detected.
Correcting for incompleteness, we derive a 90% upper limit of 0.09 on the
fraction of type Ia supernovae that explode in globular clusters for the full
sample and 0.22 for the sample of supernovae in late-type galaxies. This allows
us to limit enhancements per unit stellar mass for a coeval population
eta_{co}<50 (100) with 90% (99%) confidence. We find that by observing the
positions of a sample of less than 100 type Ia supernovae in the outer parts of
early-type galaxies, it will be possible to probe the currently favoured range
of eta_{co}~1-10.
