Abstract
Abstract SNe Ia play key roles in revealing the accelerating expansion of the universe, but our knowledge of their progenitors is still very limited. Here we report the discovery of a rigid dichotomy in circumstellar (CS) environments around two subclasses of SNe Ia as defined by their distinct photospheric velocities. For the SNe Ia with high photospheric velocities (HVs), we found a significant excess flux in blue light 60–100 days past maximum, while this phenomenon is absent for SNe with normal photospheric velocity. This blue excess can be attributed to light echoes by circumstellar dust located at a distance of about (1–2) × 1017 cm from the HV subclass. Moreover, we also found that the HV SNe Ia show systematically evolving Na i absorption line by performing a systematic search of variable Na i absorption lines in spectra of all SNe Ia, whereas this evolution is rarely seen in normal ones. The evolving Na i absorption can be modeled in terms of photoionization model, with the location of the gas clouds at a distance of about 2 × 1017 cm, in striking agreement with the location of CS dust inferred from B-band light-curve excess. These observations show clearly that the progenitors of HV subclass are likely from single-degenerate progenitor system (i.e., symbiotic binary), while the NV subclass may arise from double-degenerate system.
Highlights
It is conventionally accepted that type Ia supernovae (SNe Ia) result from thermonuclear explosion of a carbon-oxygen (CO) white dwarf (Nomoto et al 1997, Hillebrandt et al 2000, Maoz 2014)
Spectroscopically normal SNe Ia consists of ∼70% of all SNe Ia (Branch et al 1993, Li et al 2011b) and can be categorized into high-velocity (HV) and normal-velocity (NV) subclasses based on ejecta velocities inferred from the blueshifted Si II 6355 ̊A line, i.e., 12,000 km s−1 (Wang et al 2009)
Typical blueshift in the Na I lines of SNe Ia is ∼100 km s−1 according to a statistical study using high- and intermediate-resolution spectra of a large sample of SNe Ia (Sternberg et al 2011; Maguire et al 2013)
Summary
It is conventionally accepted that type Ia supernovae (SNe Ia) result from thermonuclear explosion of a carbon-oxygen (CO) white dwarf (Nomoto et al 1997, Hillebrandt et al 2000, Maoz 2014). The SD scenario is favored by the possible detections of circumstellar materials (CSM) around some SNe Ia through detections of strong ejecta-CSM interaction (Hamuy et al 2003, Wang et al 2004, Aldering et al 2006, Taddia et al 2012, Silverman et al 2013, Bochenek et al 2018) or evolving narrow absorption lines possibly due to CSM (Patat et al 2007, Blondin et al 2009, Sternberg et al 2011, Dilday et al 2012), while there are observational findings suggesting no companion signatures for some SNe Ia (Li et al 2011a, Gonzalez et al 2012, Schaefer et al 2012, Olling et al 2015).
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