Type Ia supernovae in globular clusters: observational upper limits

Aims. In the dense stellar environment of globular clusters it is expected that compact binaries can be produced dynamically. This in turn would mean that the fraction of type Ia supernovae that will explode in globular clusters will be higher than would be expected from the mass of stars present. Therefore we wish to determine observational constraints on the number of supernovae type Ia explosions in globular clusters as a means to constrain the number of dynamically formed binary systems. Methods. We searched for globular clusters at the positions of observed type Ia supernovae. We used archival HST images and literature data that cover the positions either before the supernovae exploded, or sufficiently long after the supernovae to have faded below the luminosities of globular clusters. Results. 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 another 17 type Ia supernovae, the brighter globular clusters would have been detected. Correcting for incompleteness, we derive a 90% upper limit of 0.09 for 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 ηco <∼ 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 ηco ∼ 1−10.