56Ni dredge-up in the type IIp supernova 1995V

We present contemporary infrared (IR) and optical spectra of the plateau type II SN 1995V in NGC 1087 covering four epochs, approximately 22 to 84 d after shock break-out. The data show, for the first time, the TR spectroscopic evolution during the plateau phase of a typical type Il event. In the optical region P Cygni lines of the Balmer series and of metals such as Sc II, Fe II, Sr II, Ca II and Ba II lines were identified. The TR spectra were largely dominated by the continuum, but P Cygni Paschen lines and Brackett gamma lines were also clearly seen. The other prominent IR features are confined to wavelengths blueward of 11000 Angstrom, and include Sr II 10327, Fe II 10547, C I 10695 and He I 10830 Angstrom. Helium has never before been unambiguously identified in a type IIp supernova spectrum during the plateau phase, We demonstrate the presence of He I 10830 Angstrom on days 69 and 85. The presence of this line at such late times implies reionization, A likely reionizing mechanism is gamma-ray deposition following the radioactive decay of Ni-56. We examine this mechanism by constructing a spectral model for the He I 10830-Angstrom Line based on explosion model s15s7b2f of Weaver & Woosley. We find that this does not generate the observed line owing to the confinement of the Ni-56 to the central zones of the ejecta. In order to reproduce the He I line, it was necessary to introduce additional upward mixing or 'dredge-up' of the Ni-56, with similar to 10(-5) of the total nickel mass reaching above the helium photosphere, In addition, we argue char the He I line formation region is likely to have been in the form of pure helium clumps in the hydrogen envelope. The study of He I 10830-Angstrom emission during the photospheric phase of core-collapse supernovae provides a promising tool for the constraint of initial mixing conditions in explosion models.