OPTICAL and ULTRAVIOLET OBSERVATIONS of the VERY YOUNG TYPE IIP SN 2014cx in NGC 337

Extensive photometric and spectroscopic observations are presented for SN 2014cx, a Type IIP supernova (SN) exploding in the nearby galaxy NGC 337. The observations are performed in optical and ultraviolet bands, covering from -20 to +400 days from the peak light. The stringent detection limit from prediscovery images suggests that this supernova was actually detected within about one day after explosion. Evolution of the very early time light curve of SN 2014cx is similar to that predicted from a shock breakout and post-shock cooling decline before reaching the optical peak. Our photometric observations show that SN 2014cx has a plateau duration of ∼100 days, an absolute V-band magnitude of ∼-16.5 mag at t ≈ 50 days, and a nickel mass of 0.056 ±0.008 M_⊙. The spectral evolution of SN 2014cx resembles that of normal SNe IIP like SN 1999em and SN 2004et, except that it has a slightly higher expansion velocity (∼4200 km s^-1 at 50 days). From the cooling curve of photospheric temperature, we derive that the progenitor has a pre-explosion radius of ∼640 R_⊙, consistent with those obtained from SuperNova Explosion Code modeling (∼620 R_⊙) and hydrodynamical modeling of the observables (∼570 R_⊙). Moreover, the hydrodynamical simulations yield a total explosion energy of ∼0.4 × 10⁵¹ erg, and an ejected mass of ∼8 M_⊙. These results indicate that the immediate progenitor of SN 2014cx is likely a red supergiant star with a mass of ∼10 M_⊙.