Evidence for Asphericity in a Subluminous Type Ia Supernova: Spectropolarimetry of SN 1999by

By D. Howell, P. Hoeflich, Lifan Wang, J. C. Wheeler. 2001

139 citations recorded by the source corpus.

We present polarization spectra near maximum light for the strongly subluminous Type Ia supernova SN 1999by that show that the supernova is intrinsically polarized. SN 1999by has an observed, overall level of polarization of ≈0.3%-0.8%, a rise of the polarization P redward of 6500 Å, and a change in polarization across the Si II λ6150 feature of about 0.4%. The presentation of the polarization at different wavelengths in the Q-U plane is shown to be a powerful tool to determine the overall geometry and the interstellar component. The distribution of points with wavelength using this empirical Q-U plane method reveals that SN 1999by has a well-defined axis of symmetry and suggests an interstellar polarization (ISP) vector with PISP = 0.3% and position angle Θ = 150° with an error circle in the Q-U plane of radius about 0.1%. Synthetic non-LTE spectra for axisymmetric configurations based on delayed-detonation models have been computed assuming ellipsoidal geometry. The input ejecta structure and composition are based on a Chandrasekhar mass delayed-detonation model. The parameters of the explosion are chosen to reproduce the time evolution of IR spectra of SN 1999by without further adjustments. Spherical models are then mapped onto ellipsoidal geometries and the axis ratio, viewing angle, and ISP adjusted to provide the best agreement with the polarization spectra. Both flux and polarization spectra can be reasonably well reproduced by models with an asphericity of ≈20% observed equator-on. The general properties of the polarization can be understood as a consequence of the structure of subluminous models. Best fits are obtained for the theoretical models with PISP = 0.25% and Θ = 140°, consistent with the empirical method. We discuss our results for this subluminous Type Ia in the context of "normally bright" Type Ia supernovae. For normally bright Type Ia, the photosphere is near the inner iron-rich layers at maximum light and the ubiquitous iron lines give a rapid variation to the model polarization spectra. In subluminous models, the photosphere near maximum is in the silicon layers with fewer lines and a smoother overall polarization spectrum, as observed for SN 1999by. Though data are sparse, the low upper limits for polarization determined for many normal events in contrast to the high polarization in SN 1999by may suggest a relation between the asymmetry we observed and the mechanism that produces a subluminous Type Ia. Among various mechanisms, rapid rotation of the progenitor white dwarf and/or an explosion during a binary white dwarf merger process are likely candidates to explain the asphericity in SN 1999by.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/302faf4f9848ae2614d7e9da05b6aa6541b7d1e2

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Citations in the corpus

• Violent mergers of nearly equal-mass white dwarf as progenitors of subluminous Type Ia supernovae (2011, background): “Howell et al. (2001) find that this supernova shows continuum polarization of ≈ 0.3% − 0.8%.”
• On the Progenitors of Type Ia Supernovae (2018, unknown): “We should note though that spectropolarimetry studies of the subluminous SN 2005ke and SN 1999by did indicate deviations from spherical symmetry in these systems (e.g., Patat et al. 2012; Howell et al. 2001).”
• Infrared Spectra of the Subluminous Type Ia Supernova SN 1999by (2001, methodology): “Selection of the SN 1999byModel Detailed B and V light curves of SN 1999by have been collected and published by Toth & Szabo (2000), and optical spectra around maximum light are available from observations at McDonald Observatory (Howell et al. 2001).”
• The Luminosity of SN 1999by in NGC 2841 and the Nature of “Peculiar” Type Ia Supernovae (2001, background): “…1999by was a Type Ia event, and Garnavich et al. (1999) found line ratios and Ti ii absorption consistent with an SN 1991bg–like SN. SN 1999by is also one of the few SNe Ia to show significant intrinsic polarization (Howell et al. 2001; see also Wang et al. [2003] regarding the case of SN 2001el).”
• Observational Clues to the Progenitors of Type Ia Supernovae (2013, background): “At the other extreme of the same trend, Howell et al. (2001) and Patat et al. (2012) have used spectropolarimetry to deduce significant, co-aligned asphericity in both lines and continuum in the sub-luminous SNe Ia 199by and 2005ke, respectively, supporting fast rotation or a merger origin in these…”
• Spectropolarimetry of Supernovae (2008, background): “Another class of Type Ia that can be studied with spectropolarimetry are those in the low-luminosity category, the prototype of which is SN 1991bg. SN 1999by, observed by Howell et al. (2001), is a nice example of a sub-luminous Type Ia.”
• The Progenitors of Type Ia Supernovae (2012, background): “…in NGC 5128 at a distance of∼4 Mpc (Frogel et al., 1987). of SN Ia subclasses, e.g. sub-luminous SNe Ia with a continuum polarization about 0.3%−0.8% (Howell et al., 2001), and high-velocity (HV) SNe Ia with a high polarization about 2%, the spectra of which around maximum light are characterized…”
• On the evolution of rapidly rotating massive white dwarfs towards supernovae or collapses (2005, background): “The aspherical nature of rapidly rotating white dwarfs might, in principle, result in an observable polarization, as found in SN 1999by (Howell et al. 2001) and SN 2001el (Wang et al. 2003).”
• Time-dependent Monte Carlo Radiative Transfer Calculations for Three-dimensional Supernova Spectra, Light Curves, and Polarization (2006, unknown): “D SN (Höflich 1991; Wang et al. 1997; Howell et al. 2001).”
• Analysis of the Flux and Polarization Spectra of the Type Ia Supernova SN 2001el: Exploring the Geometry of the High-Velocity Ejecta (2003, background): “The only other indication of a clear non-zero polarization in a SN Ia was the subluminous and spectroscopically peculiar SN Ia 1999by, which showed an intrinsic continuum polarization of about 0.7% (Howell et al. 2001).”