Delayed Circumstellar Interaction for Type Ia SN 2015cp Revealed by an HST Ultraviolet Imaging Survey

The nature and role of the binary companion of carbon–oxygen white dwarf stars that explode as Type Ia supernovae (SNe Ia) are not yet fully understood. Past detections of circumstellar material (CSM) that contain hydrogen for a small number of SN Ia progenitor systems suggest that at least some have a nondegenerate companion. In order to constrain the prevalence, location, and quantity of CSM in SN Ia systems, we performed a near-ultraviolet (NUV) survey with the Hubble Space Telescope (HST) to look for the high-energy signature of SN Ia ejecta interacting with the CSM. Our survey revealed that SN 2015cp, an SN 1991T-like overluminous SN Ia, was experiencing late-onset interaction between its ejecta and the surrounding CSM 664 days after its light-curve peak. We present ground- and space-based follow-up observations of SN 2015cp that reveal optical emission lines of H and Ca, typical signatures of ejecta–CSM interaction. We show how SN 2015cp was likely similar to the well-studied SN Ia–CSM event PTF11kx, making it the second case in which an unambiguously classified SN Ia was observed to interact with a distant shell of CSM that contains hydrogen (RCSM ≳ 1016 cm). The remainder of our HST NUV images of SNe Ia were nondetections that we use to constrain the occurrence rate of observable late-onset CSM interaction. We apply theoretical models for the emission from ejecta–CSM interaction to our NUV nondetections and place upper limits on the mass and radial extent of CSM in SN Ia progenitor systems.