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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. [ABSTRACT FROM AUTHOR]

Despite their cosmological utility, the progenitors of Type Ia supernovae (SNe Ia) are still unknown, with many efforts focused on whether accretion from a nondegenerate companion can grow a carbon–oxygen white dwarf to near the Chandrasekhar mass. The association of SNe Ia resembling SN 1991T (“91T-like”) with circumstellar interaction may be evidence for this “single-degenerate” channel. However, the observed circumstellar medium (CSM) in these interacting systems is unlike a stellar wind—of particular interest, it is sometimes detached from the stellar surface, residing at ∼1016 cm. A Hubble Space Telescope (HST) program to discover detached CSM around 91T-like SNe Ia successfully discovered interaction nearly two years after explosion in SN 2015cp (Graham et al. 2018). In this work, we present radio and X-ray follow-up observations of SN 2015cp and analyze them in the framework of Harris et al. (2016) to limit the properties of a constant-density CSM shell in this system. Assuming the HST detection took place shortly after the shock crossed the CSM, we constrain the total CSM mass in this system to be <0.5 . This limit is comparable to the CSM mass of supernova PTF11kx, but does not rule out lower masses predicted for recurrent novae. From lessons learned modeling PTF11kx and SN 2015cp, we suggest a strategy for future observations of these events to increase the sample of known interacting SNe Ia. [ABSTRACT FROM AUTHOR]

The optical transient PTF11kx exhibited both the characteristic spectral features of Type Ia supernovae (SNe Ia) and the signature of ejecta interacting with circumstellar material (CSM) containing hydrogen, indicating the presence of a nondegenerate companion. We present an optical spectrum at 1342 days after peak from Keck Observatory, in which the broad component of Hα emission persists with a similar profile as in early-time observations. We also present Spitzer IRAC detections obtained 1237 and 1818 days after peak, and an upper limit from Hubble Space Telescope ultraviolet imaging at 2133 days. We interpret our late-time observations in the context of published results—and reinterpret the early-time observations—in order to constrain the CSM’s physical parameters and to compare to theoretical predictions for recurrent-nova systems. We find that the CSM’s radial extent may be several times the distance between the star and the CSM’s inner edge, and that the CSM column density may be two orders of magnitude lower than previous estimates. We show that the Hα luminosity decline is similar to other SNe with CSM interaction and demonstrate how our infrared photometry is evidence for newly formed, collisionally heated dust. We create a model for PTF11kx’s late-time CSM interaction and find that X-ray reprocessing by photoionization and recombination cannot reproduce the observed Hα luminosity, suggesting that the X-rays are thermalized and that Hα radiates from collisional excitation. Finally, we discuss the implications of our results regarding the progenitor scenario and the geometric properties of the CSM for the PTF11kx system. [ABSTRACT FROM AUTHOR]