Late-time near-infrared observations of SN 2005df

Abstract

We present late-time ($200-400$ days) near-infrared spectral evolution for the Type Ia supernova SN 2005df. The spectra show numerous strong emission features of [CoII], [CoIII], and [FeII] throughout the $0.8-1.8$μm region. As the spectrum ages, the cobalt features fade as would be expected from the decay of ${}^{56}$Co to ${}^{56}$Fe. We show that the strong and isolated [FeII] emission line at $1.644$μm provides a unique tool to analyze near-infrared spectra of Type Ia supernovae. Normalization of spectra to this line allows separation of features produced by stable versus unstable isotopes of iron group elements. We develop a new method of determining the initial central density, ${\rho }_c$, and the magnetic field, $B$, of the white dwarf using the width of the $1.644$μm line. The line width is sensitive because of electron capture in the early stages of burning, which increases as a function of density. The sensitivity of the line width to $B$ increase with time and the effects of the magnetic field shift towards later times with decreasing ${\rho }_c$. The initial central density for SN 2005df is measured as ${\rho }_c=0.9(\pm 0.2)$ (in $10^9$g/cm${}^3$), which corresponds to a white dwarf close to the Chandrasekhar mass (${\mathrm{M}}_{\mathrm{Ch}}$) with ${\mathrm{M}}_{\mathrm{WD}}=1.313(\pm 0.034)$M${}_{\odot }$ and systematic error less than $0.04$M${}_{\odot }$. Within ${\mathrm{M}}_{\mathrm{Ch}}$ explosions, however, the central density found for SN 2005df is very low for a H-accretor, possibly suggesting a helium star companion or a tidally-disrupted white dwarf companion. As an alternative, we suggest mixing of the central region. We find some support for high initial magnetic fields of strength $10^6$G for SN 2005df, however, $0$G cannot be ruled out because of noise in the spectra combined with low ${\rho }_c$.

13 pages, 14 figures, submitted to ApJ

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