THE EARLIEST NEAR-INFRARED TIME-SERIES SPECTROSCOPY OF A TYPE Ia SUPERNOVA

Abstract

We present ten medium-resolution, high signal-to-noise ratio near-infrared\n(NIR) spectra of SN 2011fe from SpeX on the NASA Infrared Telescope Facility\n(IRTF) and Gemini Near-Infrared Spectrograph (GNIRS) on Gemini North, obtained\nas part of the Carnegie Supernova Project. This data set constitutes the\nearliest time-series NIR spectroscopy of a Type Ia supernova (SN Ia), with the\nfirst spectrum obtained at 2.58 days past the explosion and covering -14.6 to\n+17.3 days relative to B-band maximum. C I {\\lambda}1.0693 {\\mu}m is detected\nin SN 2011fe with increasing strength up to maximum light. The delay in the\nonset of the NIR C I line demonstrates its potential to be an effective tracer\nof unprocessed material. For the first time in a SN Ia, the early rapid decline\nof the Mg II {\\lambda}1.0927 {\\mu}m velocity was observed, and the subsequent\nvelocity is remarkably constant. The Mg II velocity during this constant phase\nlocates the inner edge of carbon burning and probes the conditions under which\nthe transition from deflagration to detonation occurs. We show that the Mg II\nvelocity does not correlate with the optical light-curve decline rate\n{\\Delta}m15. The prominent break at ~1.5 {\\mu}m is the main source of concern\nfor NIR k-correction calculations. We demonstrate here that the feature has a\nuniform time evolution among SNe Ia, with the flux ratio across the break\nstrongly correlated with {\\Delta}m15. The predictability of the strength and\nthe onset of this feature suggests that the associated k-correction\nuncertainties can be minimized with improved spectral templates.\n