Analysis of the Flux and Polarization Spectra of the Type Ia Supernova SN 2001el: Exploring the Geometry of the High-Velocity Ejecta

By D. Kasen, P. Nugent, Lifan Wang, D. Howell, J. C. Wheeler, P. Höflich, D. Baade, Eddie Baron, Peter Hauschildt. 2003

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SN 2001el is the first normal Type Ia supernova to show a strong, intrinsic polarization signal. In addition, during the epochs prior to maximum light, the Ca II IR triplet absorption is seen distinctly and separately at both normal photospheric velocities and at very high velocities. The high-velocity triplet absorption is highly polarized, with a different polarization angle than the rest of the spectrum. The unique observation allows us to construct a relatively detailed picture of the layered geometrical structure of the supernova ejecta: in our interpretation, the ejecta layers near the photosphere (v ≈ 10,000 km s-1) obey a nearly axial symmetry, while a detached, high-velocity structure (v ≈ 18,000-25,000 km s-1) with high Ca II line opacity deviates from the photospheric axisymmetry. By partially obscuring the underlying photosphere, the high-velocity structure causes a more incomplete cancellation of the polarization of the photospheric light and so gives rise to the polarization peak and rotated polarization angle of the high-velocity IR triplet feature. In an effort to constrain the ejecta geometry, we develop a technique for calculating three-dimensional synthetic polarization spectra and use it to generate polarization profiles for several parameterized configurations. In particular, we examine the case in which the inner ejecta layers are ellipsoidal and the outer, high-velocity structure is one of four possibilities: a spherical shell, an ellipsoidal shell, a clumped shell, or a toroid. The synthetic spectra rule out the spherical shell model, disfavor a toroid, and find a best fit with the clumped shell. We show further that different geometries can be more clearly discriminated if observations are obtained from several different lines of sight. Thus, assuming that the high-velocity structure observed for SN 2001el is a consistent feature of at least a known subset of Type Ia supernovae, future observations and analyses such as these may allow one to put strong constraints on the ejecta geometry and hence on supernova progenitors and explosion mechanisms.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/5a06734455ea584bd2c11a9bbf364f705b797cb5

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Citations in the corpus

Comparative Direct Analysis of Type Ia Supernova Spectra. I. SN 1994D (2005, background): “Krisciunas et al. (2007) termed SN 2004S a clone of SN 2001el, another CN with exceptionally strong high-velocity Ca II features (Wang et al. 2003; Kasen et al. 2003; Mattila et al. 2005; Paper II).”
Secondary Maximum in the Near-Infrared Light Curves of Type Ia Supernovae (2006, background): “In addition, polarization observations of SN 2001el indicate that the ejecta was not perfectly spherically symmetric (Wang et al. 2003; Kasen et al. 2003); therefore, aspherical ejecta structures may need to be considered to exactly account for all of the light-curve properties.”
THE SPECTROSCOPIC DIVERSITY OF TYPE Ia SUPERNOVAE (2000, background): “There is direct evidence for the multi-dimensional nature of HVFs in one SN Ia, SN 2001el, where spectro-polarimetric observations revealed the high-velocity component of the Ca ii IRT to be polarized at the ∼ 1% level (Wang et al. 2003; Kasen et al. 2003).”
SN 2003du: 480 days in the life of a normal type Ia supernova (2007, background): “Modeling of time sequences of flux and polarization spectra (e.g., Kasen et al., 2003; Thomas et al., 2004; Wang et al., 2007) may al- low us to recover this information and help to impose additional constraints on the SN Ia explosion and progenitor models.”
Early and late time VLT spectroscopy of SN 2001el - Progenitor constraints for a type Ia supernova (2005, background): “The bluemost minimum in the IR triplet profile has been identified to be due to the blend of theλ8542 andλ8498 components (∼1500 km s−1 separation) whereas the redmost minimum is produced by theλ8662 component (Kasen et al. 2003).”
Spectropolarimetry of Supernovae (2008, background): “Journal, 132, 189 Kasen et al.(2003)Kasen, D., et al. 2003, ApJ, 593, 788 Kasen et al.(2004)Kasen, D., Nugent, P., Thomas, R. C., & Wang, L. 2004, ApJ, 610, 876 Kasen et al.(2006)Kasen, D., Thomas, R. C., & Nugent, P. 2006, Ap.”
Observational Clues to the Progenitors of Type Ia Supernovae (2013, background): “The line polarizations have been interpreted in terms of clumpy distributions of the ejecta of particular elements (Hole, Kasen & Nordsieck 2010; Kasen et al. 2003; Kasen, Röpke & Woosley 2009).”