Line Identifications of Type I Supernovae: On the Detection of Si II for these Hydrogen-poor Events

Here we revisit line identifications of type I supernovae and highlight trace amounts of unburned hydrogen as an important free parameter for the composition of the progenitor. Most 1-dimensional stripped-envelope models of supernovae indicate that observed features near 6000-6400 Ang in type I spectra are due to more than Si II 6355. However, while an interpretation of conspicuous Si II 6355 can approximate 6150 Ang absorption features for all type Ia supernovae during the first month of free expansion, similar identifications applied to 6250 Ang features of type Ib and Ic supernovae have not been as successful. When the corresponding synthetic spectra are compared to high quality time-series observations, the computed spectra are frequently too blue in wavelength. Some improvement can be achieved with Fe II lines that contribute red-ward of 6150 Ang, however the computed spectra either remain too blue, or the spectrum only reaches fair agreement when the rise-time to peak brightness of the model conflicts with observations by a factor of two. This degree of disagreement brings into question the proposed explosion scenario. Similarly, a detection of strong Si II 6355 in the spectra of broad-lined Ic and super-luminous events of type I/R is less convincing despite numerous model spectra used to show otherwise. Alternatively, we suggest 6000-6400 Ang features are possibly influenced by either trace amounts of hydrogen, or blue-shifted absorption and emission in Halpha, the latter being an effect which is frequently observed in the spectra of hydrogen-rich, type II supernovae.

The Broad-Lined Type Ic SN 2012ap and the Nature of Relativistic Supernovae Lacking a Gamma-ray Burst Detection

We present ultraviolet, optical, and near-infrared observations of SN 2012ap, a broad-lined Type Ic supernova in the galaxy NGC 1729 that produced a relativistic and rapidly decelerating outflow without a gamma-ray burst signature. Photometry and spectroscopy follow the flux evolution from -13 to +272 days past the B-band maximum of -17.4 +/- 0.5 mag. The spectra are dominated by Fe II, O I, and Ca II absorption lines at ejecta velocities of 20,000 km/s that change slowly over time. Other spectral absorption lines are consistent with contributions from photospheric He I, and hydrogen may also be present at higher velocities (> 27,000 km/s). We use these observations to estimate explosion properties and derive a total ejecta mass of 2.7 Msolar, a kinetic energy of 1.0x10^{52} erg, and a 56Ni mass of 0.1-0.2 Msolar. Nebular spectra (t > 200d) exhibit an asymmetric double-peaked [OI] 6300,6364 emission profile that we associate with absorption in the supernova interior, although toroidal ejecta geometry is an alternative explanation. SN 2012ap joins SN 2009bb as another exceptional supernova that shows evidence for a central engine (e.g., black-hole accretion or magnetar) capable of launching a non-negligible portion of ejecta to relativistic velocities without a coincident gamma-ray burst detection. Defining attributes of their progenitor systems may be related to notable properties including above-average environmental metallicities of Z > Zsolar, moderate to high levels of host-galaxy extinction (E(B-V) > 0.4 mag), detection of high-velocity helium at early epochs, and a high relative flux ratio of [Ca II]/[O I] > 1 at nebular epochs. These events support the notion that jet activity at various energy scales may be present in a wide range of supernovae.