Delayed circumstellar interaction for Type Ia SN 2015cp revealed by an HST ultraviolet imaging survey

Abstract

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 CSM. Our survey revealed that SN 2015cp, a SN 1991T-like overluminous SN Ia, was experiencing late-onset interaction between its ejecta and surrounding CSM at 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 Iaw as observed to interact with a distant shell of CSM that contains hydrogen (R_CSM ≳ 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

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Identifiers

ORCID for M. Sullivan: orcid.org/0000-0001-9053-4820

ORCID for M. Smith: orcid.org/0000-0002-3321-1432

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