Early‐Time Infrared Spectra of the Type Ia Supernova 2000cx

Abstract

We present 0.8 to 2.5 μm spectra of the Type Ia supernova 2000cx from 8 and 7 days before maximum light. The spectra consist of a continuum that closely follows that of a hot blackbody (25,000 K) upon which is superposed a small number of absorption and emission features. The most prominent absorption is due to the Mg II multiplet at 1.0926 μm (rest frame); the strongest emission feature is at ~1.25 μm and is probably due to Fe III. The broad Si II feature at ~1.65 μm, if present, is very weak. There is no evidence of the He I singlet at 2.0581 μm. Velocities in the Mg II feature extend to beyond 20,000 km s -1, indicating that carbon burning reached the outermost layers of the progenitor, and providing support for the delayed detonation models. The blackbody shape of the continuum beyond 0.9 μm is used to provide a limit on the in situ reddening of E(B-V) ≤ 0.1 mag. Given the probable uniformity of the early-time spectra of Type Ia supernovae (SNe Ia), this technique should be applicable to other SNe Ia. Multicolor light curves reported by other observers indicate that SN 2000cx was overluminous, suggesting a higher than usual production of 56Ni. This can account for the hotter than normal continuum and the generally higher excitation of the line features.