The Cold and Dusty Circumstellar Matter around Fast-expanding Type Ia Supernovae

Abstract

Type Ia supernovae (SNe Ia) play key roles in revealing the accelerating expansion of the universe, but our knowledge about their progenitors is still very limited. Here we report the discovery of a rigid dichotomy in circumstellar (CS) environments around two subclasses of type Ia supernovae (SNe Ia) as defined by their distinct photospheric velocities. For the SNe Ia with high photospheric velocities (HV), we found a significant excess flux in blue light during 60-100 days past maximum, while this phenomenon is absent for SNe with normal photospheric velocity (Normal). This blue excess can be attributed to light echoes by circumstellar dust located at a distance of about 1-3×10 17 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 2×10 17 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 and Normal subclasses are systematically different, suggesting that they are likely from single and double degenerate progenitor systems, respectively.

An imaging polarimetry survey of Type Ia supernovae: are peculiar extinction and polarization properties produced by circumstellar or interstellar matter?

“…This is also consistent with the systematically observed blueshifted outflows in the Na I line (Sternberg et al 2011;Maguire et al 2013) in many SNe Ia. In contrast, SNe Ia that do not display HV features and possible evidence of CSM may have double-degenerate progenitor systems (Li et al 2011;González Hernández et al 2012;Schaefer & Pagnotta 2012;Olling et al 2015;Wang et al 2019).…”

“…Wang et al (2013) found that SNe Ia with high Si II velocity (HV SNe Ia) tend to explode at distances closer to the centre of the host galaxies than normal-velocity (NV) SNe Ia and are more likely to be associated with younger stellar environments and metal-rich progenitors. Furthermore, Wang et al (2019) found that HV SNe Ia display a significant excess of blue flux 60-100 days past peak brightness compared to SNe with normal photospheric velocities. This excess may be attributed to light echoes by dust in the CSM.…”

“…Therefore, Wang et al (2019) suggest that the HV SNe Ia may arise from single-degenerate progenitors. This is also consistent with the systematically observed blueshifted outflows in the Na I line (Sternberg et al 2011;Maguire et al 2013) in many SNe Ia.…”

A possible distance bias for type Ia supernovae with different ejecta velocities

“…The fractional depth of spectral features in SNe Ia with a larger overall flux levels should be smaller. Wang et al (2019) has suggested that a B-band offset in HV SNe at phases >+40 days could be due to a light echo from circumstellar dust. A light echo could dilute the specral features.…”

The Effects of Circumstellar Dust Scattering on the Light Curves and Polarizations of Type Ia Supernovae

“…Further evidence of the presence of a significant amount CSM around SNe Ia came from spectroscopic observations of the narrow Na I D lines. Some SNe Ia show blueshifted and time evolving narrow Na I D absorption lines (Patat et al 2007;Blondin et al 2009;Simon et al 2009;Sternberg et al 2011;Maguire et al 2013;Wang et al 2019). In particular, Wang et al (2009) divided the spectroscopic normal SNe Ia into two groups, the normal-velocity and high-velocity ones with Si II λ6355 velocity lower or higher than 11,800 km/s respectively.…”