56 Ni 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 IR spectroscopic evolution during the plateau phase of a typical type II 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 IR spectra were largely dominated by the continuum, but P Cygni Paschen lines and Brackett γ lines were also clearly seen. The other prominent IR features are confined to wavelengths blueward of 11 000 Å, and include Sr  II 10327, Fe  II 10547, C  I 10695 and He  I 10830 Å. 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 Å on days 69 and 85. The presence of this line at such late times implies reionization. A likely reionizing mechanism is γ‐ray deposition following the radioactive decay of 56 Ni. We examine this mechanism by constructing a spectral model for the He  I 10830‐Å line based on explosion model s15s7b2f of Weaver &38; Woosley. We find that this does not generate the observed line owing to the confinement of the 56 Ni 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 56 Ni, with ∼ 10 −5 of the total nickel mass reaching above the helium photosphere. In addition, we argue that 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‐Å emission during the photospheric phase of core‐collapse supernovae provides a promising tool for the constraint of initial mixing conditions in explosion models.