The Type Ic SN 2007gr: a census of the ejecta from late-time optical–infrared spectra

Nebular spectra of supernovae (SNe) offer an unimpeded view of the inner region of the ejecta, where most nucleosynthesis takes place. Optical spectra cover most, but not all, of the emitting elements and therefore offer only a partial view of the products of the explosion. Simultaneous optical-infrared spectra, on the other hand, contain emission lines of all important elements, from C and O through to the intermediate mass elements (IME) Mg, Si, S, Ca and to Fe and Ni. In particular, Si and S are best seen in the IR. The availability of IR data makes it possible to explore in greater detail the results of the explosion. SN 2007 gr is the first Type Ic SN for which such data are available. Modelling the spectra with a non-local thermodynamic equilibrium (NLTE) code reveals that the inner ejecta contain ∼1 M ⊙ of material within a velocity of ≈4500 km s ―1 . The same mass of 56 Ni derived from the light-curve peak (0.076 M ⊙ ) was used to power the spectrum, yielding consistent results. Oxygen is the dominant element, contributing ∼0.8 M ⊙ . The C/O ratio is <0.2. IME account for ∼0.1 M ⊙ . This confirms that SN 2007gr was the explosion of a low-mass CO core, probably the result of a star of main-sequence mass ≈ 15 M ⊙ . The ratios of the Ca II lines, and those of Fe II , are sensitive to the assumed degree of clumping. In particular, the optical lines of [Fe II ] become stronger, relative to the IR lines, for higher degrees of clumping.