SN 2006bt: A Perplexing, Troublesome, and Possibly Misleading Type Ia Supernova

摘要

SN 2006bt displays characteristics unlike those of any other known Type Iasupernova (SN Ia). We present optical light curves and spectra of SN 2006btwhich demonstrate the peculiar nature of this object. SN 2006bt has broad,slowly declining light curves indicative of a hot, high-luminosity SN, butlacks a prominent second maximum in the i band as do low-luminosity SNe Ia. Itsspectra are similar to those of low-luminosity SNe Ia, containing features thatare only present in cool SN photospheres. Light-curve fitting methods suggestthat SN 2006bt is reddened by a significant amount of dust; however, itoccurred in the outskirts of its early-type host galaxy and has no strong Na Dabsorption in any of its spectra, suggesting a negligible amount of host-galaxydust absorption. C II is possibly detected in our pre-maximum spectra, but at amuch lower velocity than other elements. The progenitor was likely very old,being a member of the halo population of a galaxy that shows no signs of recentstar formation. SNe Ia have been very successfully modeled as a one-parameterfamily, and this is fundamental to their use as cosmological distanceindicators. SN 2006bt is a challenge to that picture, yet its relatively normallight curves allowed SN 2006bt to be included in cosmological analyses. Wegenerate mock SN Ia datasets which indicate that contamination by similarobjects will both increase the scatter of a SN Ia Hubble diagram andsystematically bias measurements of cosmological parameters. However, spectraand rest-frame i-band light curves should provide a definitive way to identifyand eliminate such objects.