Optical Observations of the Type Ia Supernova 2011fe in M101 for Nearly 500 Days

TL;DR

This paper presents extensive optical observations of Type Ia supernova 2011fe over nearly 500 days, revealing detailed light curve behavior, UV/NIR emission characteristics, and spectral features that inform progenitor and explosion models.

Contribution

It provides the first comprehensive time series of spectra and photometry for SN 2011fe, analyzing its luminosity evolution, UV/NIR emission, and spectral high-velocity features in detail.

Findings

SN 2011fe's luminosity follows a t^n law with n close to 2 in VRI bands

SN 2011fe exhibits stronger UV emission and earlier UV peak compared to similar SNe Ia

Early spectra show prominent high-velocity features of O I and Ca II, but weak Si II

Abstract

We present well-sampled optical observations of the bright Type Ia supernova (SN~Ia) SN 2011fe in M101. Our data, starting from $\sim16$ days before maximum light and extending to $\sim463$ days after maximum, provide an unprecedented time series of spectra and photometry for a normal SN~Ia. Fitting the early-time rising light curve, we find that the luminosity evolution of SN 2011fe follows a $t^n$ law, with the index $n$ being close to 2.0 in the $VRI$ bands but slightly larger in the $U$ and $B$ bands. Combining the published ultraviolet (UV) and near-infrared (NIR) photometry, we derive the contribution of UV/NIR emission relative to the optical. SN 2011fe is found to have stronger UV emission and reaches its UV peak a few days earlier than other SNe~Ia with similar $\Delta m_{15}(B)$, suggestive of less trapping of high-energy photons in the ejecta. Moreover, the $U$-band light curve shows a notably faster decline at late phases ($t\approx 100$--300 days), which also suggests that the ejecta may be relatively transparent to UV photons. These results favor the notion that SN 2011fe might have a progenitor system with relatively lower metallicity. On the other hand, the early-phase spectra exhibit prominent high-velocity features (HVFs) of O~I $\lambda$7773 and the Ca~II~NIR triplet, but only barely detectable in Si~II~6355. This difference can be caused either by an ionization/temperature effect or an abundance enhancement scenario for the formation of HVFs; it suggests that the photospheric temperature of SN 2011fe is intrinsically low, perhaps owing to incomplete burning during the explosion of the white dwarf.