Spectropolarimetry of SN 2011dh in M51: geometric insights on a Type IIb supernova progenitor and explosion

Abstract

<jats:title>Abstract</jats:title> <jats:p>We present seven epochs of spectropolarimetry of the Type IIb supernova (SN IIb) 2011dh in M51, spanning 86 d of its evolution. The first epoch was obtained 9 d after the explosion, when the photosphere was still in the depleted hydrogen layer of the stripped-envelope progenitor. Continuum polarization is securely detected at the level of P ≈ 0.5 per cent through day 14 and appears to diminish by day 30, which is different from the prevailing trends suggested by studies of other core-collapse SNe. Time-variable modulations in P and position angle are detected across P-Cygni line features. H α and He i polarization peak after 30 d and exhibit position angles roughly aligned with the earlier continuum, while O i and Ca ii appear to be geometrically distinct. We discuss several possibilities to explain the evolution of the continuum and line polarization, including the potential effects of a tidally deformed progenitor star, aspherical radioactive heating by fast-rising plumes of 56Ni from the core, oblique shock breakout, or scattering by circumstellar material. While these possibilities are plausible and guided by theoretical expectations, they are not unique solutions to the data. The construction of more detailed hydrodynamic and radiative-transfer models that incorporate complex aspherical geometries will be required to further elucidate the nature of the polarized radiation from SN 2011dh and other SNe IIb.</jats:p>

The shape of SN 1993J re-analysed

“…Although in a few instances the contribution of the ISP can be negligible (e.g. Patat et al 2011;Mauerhan et al 2015;, it is often an important contributor to the observed polarimetry signal. Because of the effects of vector addition, it can either decrease or increase p, and turn polarization peaks (troughs) into troughs (peaks).…”

“…Additionally, observations of stripped-envelope SNe have revealed that they exhibit high levels of continuum polarization at earlier phases compared to Type II SNe (e.g. Wang et al 2001;Chornock et al 2011;Mauerhan et al 2015;). This has led to a widely stated conclusion that the degree of polarization increases as we probe regions closer to an intrinsically asymmetrical explosion, i.e.…”

“…In addition, spectropolarimetric data analysis methods and science focuses have evolved as more high quality observations were obtained for CCSNe (e.g. Wang et al 2003;Leonard et al 2006;Tanaka et al 2009;Maund & Smartt 2009;Chornock et al 2011;Mauerhan et al 2015;Reilly et al 2016;Stevance et al 2017). Informed by the lessons learnt in the field since the original SN 1993J spectropolarimetric studies were published, we aim to provide an updated analysis and interpretation of the data.…”

Three-dimensional Explosion Geometry of Stripped-envelope Core-collapse Supernovae. II. Modeling of Polarization

“…Nonzero line polarization is common, and the polarization feature shows an inverted P-Cygni profile that peaks at the flux absorption minimum (e.g., Kawabata et al 2002;Leonard et al 2002;Wang et al 2003a;Maund et al 2007aMaund et al , 2007bMaund et al , 2009Tanaka et al 2008Tanaka et al , 2009aTanaka et al , 2012Mauerhan et al 2015Mauerhan et al , 2017Stevance et al 2016). 2.…”

“…2. When the polarization data across the line (middle panel of Figure 1) are plotted in the Stokes Q -U diagram (right panel), the observed data commonly show a loop (e.g., Maund et al 2007aMaund et al , 2007bMaund et al , 2009Tanaka et al 2012;Mauerhan et al 2015Mauerhan et al , 2017Stevance et al 2016). 8 3.…”