THE VERY YOUNG TYPE IA SUPERNOVA 2013DY: DISCOVERY, AND STRONG CARBON ABSORPTION

The Type Ia supernova (SN Ia) 2013dy in NGC 7250 (d � 13.7Mpc) 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 ± 0.05d (2.4 ± 1.2hr) 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 R0 . 0.25R⊙, 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 1.63d 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± 0.03mag �17.7 d after first light. Subject headings: supernovae: general — supernovae: individual (SN 2013dy)