The very young type Ia supernova 2013dy: Discovery, and strong carbon absorption in early-time spectra

The Type Ia supernova (SN Ia) 2013dy in NGC 7250 (d 48 13.7 Mpc) was discovered by the Lick Observatory Supernova Search. Combined with a prediscovery detection by the Italian Supernova Search Project, we are able to constrain the first-light time of SN 2013dy to be only 0.10 \ub1 0.05 days (2.4 \ub1 1.2 hr) before the first detection. This makes SN 2013dy the earliest known detection of an SN Ia. We infer an upper limit on the radius of the progenitor star of R 0 72 0.25 R 99, consistent with that of a white dwarf. The light curve exhibits a broken power law with exponents of 0.88 and then 1.80. A spectrum taken 1.63 days after first light reveals a C II absorption line comparable in strength to Si II. This is the strongest C II feature ever detected in a normal SN Ia, suggesting that the progenitor star had significant unburned material. The C II line in SN 2013dy weakens rapidly and is undetected in a spectrum 7 days later, indicating that C II is detectable for only a very short time in some SNe Ia. SN 2013dy reached a B-band maximum of MB = -18.72 \ub1 0.03 mag 3c17.7 days after first light. \ua9 2013. The American Astronomical Society. All rights reserved.Peer reviewed: YesNRC publication: Ye