VLT Spectropolarimetry of the Type Ia SN 2005ke

Abstract

Aims. In this study we try to answer the question whether or not subluminous Type Ia supernovae have additional distinctive properties when examined from the point of view of the explosion geometry. Methods. We have performed optical spectropolarimetric observations of the Type Ia SN 2005ke at 3 epochs (days −8, −7, and +76). The explosion properties are derived by comparing the data to explosion and radiation transfer models. Results. The supernova shows polarimetric properties that are very similar to the only other subluminous event for which spectropolarimetry is available, i.e. SN 1999by. The data present a very marked dominant axis, which is shared by both the continuum and lines such as Si ii λ6355, suggesting that the relatively large, global asymmetry is common to the photosphere and the line-forming region. The maximum polarization degree observed in the Si ii λ6355 absorption reaches 0.39 ± 0.08%. At variance with what is seen in core-normal Type Ia, SN 2005ke displays significant continuum polarization, which grows from the blue to the red and peaks at about 7000 Å, reaching ∼0.7%. The properties of the polarization and flux spectra can be understood within the framework of a subluminous delayed-detonation (DD), or pulsating DD scenario, or white dwarf (WD) mergers. The difference in appearance with respect to core-normal SNe Ia is caused by low photospheric temperatures in combination with layers of unburned C, and more massive layers of the products of explosive C and O burning. The comparatively high level of continuum polarization is explained in terms of a significant global asymmetry (∼15%), which is well reproduced by an oblate ellipsoidal geometry within the general context of a DD explosion. Conclusions. Our results suggest that SN 2005ke arose either from a single-degenerate system in which the WD is especially rapidly rotating, close to the break-up velocity, or from a double-degenerate merger. Based on the current polarization data, we cannot distinguish between these two possibilities. Possible tests are discussed.

Light and Color Curve Properties of Type Ia Supernovae: Theory Versus Observations

“…The explosions will produce a wide variety of envelope masses ranging from well below M Ch to super-M Ch and very asymmetric density distributions in their ejecta. This class of objects will show a significant continuum polarization that has not be observed, and we must expect a directional dependence of the luminosity, which would result in a wide spread of observed ∆m 15 and ∆m 15,s (Höflich et al 2006;Patat et al 2012;Bulla et al 2016).…”

“…The comparison does not constitute spectral fits or analyzes. For dedicated fits and identification of features, we will refer to previous papers mentioned above (e.g., Hoeflich 1995;Wheeler et al 1998;Hoeflich et al 2002;Patat et al 2012). We will focus on spectral properties relevant for the LCs and the color evolution.…”

“…The suite of models involve a transition from deflagration to detonation burning parameterized by the transition density, ρ tr . While a wide variety of explosion mechanisms may be possible in nature, we choose the DDT scenario because it predicts some of the basic characteristics of SNe Ia: overall spherical density distributions as indicated by small continuum polarization (Howell et al 2001;Maund et al 2010;Patat et al 2012), overall spherical geometry of SN Ia remnants (SNR) with layered chemical structures (Rest et al 2005;Badenes et al 2006;Fesen et al 2007;Rest et al 2008), "broad" emission line profiles and stable…”

“…These models have been widely applied to analyze and fit individual SNe Ia, particularly flux in the optical and IR and polarization spectra (Hoeflich 1995;Hoeflich et al 1998;Wheeler et al 1998;Lentz et al 2001;Quimby et al 2006;Baron et al 2012;Patat et al 2012;Baron et al 2015;Diamond et al 2015;Telesco et al 2015). Note that very similar chemical abundances and velocity structures have been obtained by the analysis of observed spectral sequences using the method of "spectral tomog-…”

The Extremes of Thermonuclear Supernovae

“…SN 1991bg became the prototype of a whole class of objects, some of which have properties nearly identical to SN 1991bg, while others seem to bridge the gap to normal SNe Ia, and again others deviate in certain respects and are sometimes even more extreme than SN 1991bg itself. In the following we will first focus on SN 1991bg and its close twins, which include SNe 1997cn (Turatto et al, 1998;Jha et al, 2006), 1998de (Modjaz et al, 2001;Jha et al, 2006;Matheson et al, 2008), 1999by Vinkó et al, 2001;Höflich et al, 2002;Garnavich et al, 2004;Silverman et al, 2012), 2005bl (Taubenberger et al, 2008, 2005ke (Hicken et al, 2009;Contreras et al, 2010;Blondin et al, 2012;Silverman et al, 2012;Patat et al, 2012), 2006bz (Hicken et al, 2009), and several other examples contained in the SN Ia samples of Hicken et al (2009), Contreras et al (2010, Ganeshalingam et al (2010), Stritzinger et al (2011), Blondin et al (2012 and Silverman et al (2012). In Sections 3.6 and 3.7 we will then discuss transitional 86G-like objects and the very subluminous and peculiar transient PTF 09dav, respectively.…”