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The nature and role of the binary companion of carbon-oxygen white dwarfstars that explode as Type Ia supernovae (SNe Ia) are not yet fully understood.Past detections of circumstellar material (CSM) that contain hydrogen for asmall number of SN Ia progenitor systems suggest that at least some have anondegenerate companion. In order to constrain the prevalence, location, andquantity of CSM in SN Ia systems, we performed a near-ultraviolet (NUV) surveywith the Hubble Space Telescope (HST) to look for the high-energy signature ofSN Ia ejecta interacting with CSM. Our survey revealed that SN 2015cp, a SN1991T-like overluminous SN Ia, was experiencing late-onset interaction betweenits ejecta and surrounding CSM at $664$ days after its light-curve peak. Wepresent ground- and space-based follow-up observations of SN 2015cp that revealoptical emission lines of H and Ca, typical signatures of ejecta-CSMinteraction. We show how SN 2015cp was likely similar to the well-studied SNIa-CSM event PTF11kx, making it the second case in which an unambiguouslyclassified SN Ia was observed to interact with a distant shell of CSM thatcontains hydrogen ($R_{\rm CSM} \gtrsim 10^{16}\ {\rm cm}$). The remainder ofour HST NUV images of SNe Ia were nondetections that we use to constrain theoccurrence rate of observable late-onset CSM interaction. We apply theoreticalmodels 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 progenitorsystems.

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