GeminiFocus April 2013 | Page 4

Eric Hsiao, Howie Marion, and Mark Phillips The Earliest Near-infrared Spectroscopy of a Type Ia Supernova Figure 1. Color image of SN 2011fe in M101. (Credit: B. J. Fulton/ LCOGT/PTF) Gemini Near-infrared Spectrograph (GNIRS) observations have lead to surprising results on the nature of Type Ia supernovae (SNe Ia). Time-series, near-infrared spectra of SN 2011fe reveal that more SNe Ia harbor unprocessed carbon than previously believed, and what we thought was the main driver of the luminositydecline rate relation may not be correct. Type Ia supernovae (SNe Ia) provide the most direct measure of the expansion history of the universe and have led to the discovery of the accelerated expansion, which was awarded the 2011 Nobel Prize in Physics. The unknown cause of the accelerated expansion is commonly referred to as  “dark energy.” SNe Ia are not perfectly homogenous, showing significant variation in the shapes and peak brightnesses of their light curves. Rather, their utility as cosmological distance indicators at optical wavelengths rests on the discovery of an empirical correlation between the SNe Ia’s peak absolute magnitude and the rate at which the brightness declines (luminosity-decline rate relation; Phillips 1993). Most astronomers agree that these explosions result from the total thermonuclear disruption of a carbon-oxygen white dwarf in a close binary system; however, the details of the explosion mechanism and the mass-donating companion star are still unclear. 4 GeminiFocus April2013