GeminiFocus April 2013 | Page 5

A Near-infrared Shift Although SNe Ia remain the most proven technique for studying dark energy, we do not understand the nature of these explosions, and that ultimately limits their accuracy. Fortunately, shifting the observations to the near-infrared offers a way forward. In the near-infrared, SN Ia luminosities are less affected by dust and show much smaller intrinsic scatter than in the optical. A recent study, also using Gemini data, demonstrated an amazing distance accuracy of 6 percent using SN Ia peak luminosity in the near-infrared (Barone-Nugent et al., 2012). A key ingredient to realizing the full potential of near-infrared SN Ia cosmology is near-infrared spectroscopy, which allows us to convert the peak luminosities to the rest frame. With the limited size of the world’s current sample, the time evolution and the diversity of the near-infrared spectral features are poorly understood. These uncertainties directly affect the determination of the peak luminosity. To improve our knowledge of this relatively unexplored wavelength region, the Carnegie Supernova Project and the CfA Supernova Group have embarked on a joint program to obtain a statistically significant sample of near-infrared spectroscopic observations. On August 24, 2011, SN 2011fe was detected within hours of its explosion in M101 (Figure 1; Nugent et al., 2011). Its proximity and early detection provided a unique opportunity to make exquisitely detailed observations of a supernova. SN 2011fe appears to have been a typical SN Ia in all respects and serves as an ideal baseline to compare to other objects. Ten near-infrared spectra of SN 2011fe were obtained in the span of a month, including one SpeX spectrum and nine GNIRS spectra. We present two of the more intriguing findings from our recently published paper on these near-infrared spectra (Hsiao et al., 2013). April2013 Primordial Carbon in Type Ia Supernovae Figure 2. During a SN Ia explosion, the thermonuclear burning front rips through the carbon-oxygen white dwarf, converting carbon and oxygen into heavier elements. Since oxygen is also converted from carbon in this process, carbon provides the most direct probe of the primordial material from the progenitor carbon-oxygen white dwarf. Because conditions in the explosion models, such as the speed of the burning front, sensitively control the amount of carbon that remains, the detection of carbon in observed spectra serves as one of the most important discriminators between explosion models. The first convincing detection of carbon in a SN Ia was presented not long ago by Thomas et al., (2007). Since then, several studies using large and independent sets of optical spectra have reached the same conclusion that GeminiFocus SYNAPPS (Thomas et al. 2011) model spectrum fit to the GNIRS spectra of SN 2011fe around the near-infrared carbon line. The observed spectra are plotted as solid black curves. The best-fit model spectra are plotted as follows: with all ions, with carbon only, and with all ions except carbon. These are plotted as red dotted, green dashed, and blue dash-dotted curves, respectively. The vertical dotted lines mark the locations of the best-fit carbon velocity. The phases relative to maximum light are noted. 5