Low-luminosity Type II supernovae: Spectroscopic and photometric evolution

In this paper we present spectroscopic and photometric observations for four core-collapsed supernovae (SNe), namely SNe 1994N, 1999br, 1999eu and 2001dc. Together with SN 1997D, we show that they form a group of exceptionally low-luminosity events. These SNe have narrow spectral lines (indicating low expansion velocities) and low luminosities at every phase (significantly lower than those of typical core-collapsed supernovae). The very-low luminosity during the ⁵⁶Co radioactive decay tail indicates that the mass of ⁵⁶Ni ejected during the explosion is much smaller (M_Ni ≈ 2-8 × 10^-3 M_⊙) than the average (M _Ni ≈ 6-10 × 10^-2 M_⊙). Two supernovae of this group (SN 1999br and SN 2001dc) were discovered very close to the explosion epoch, allowing us to determine the lengths of their plateaux (≈100 d) as well as establishing the explosion epochs of the other, less completely observed SNe. It is likely that this group of SNe represent the extreme low-luminosity tail of a single continuous distribution of Type II plateau Supernovae events. Their kinetic energy is also exceptionally low. Although an origin from low-mass progenitors has also been proposed for low-luminosity core-collapsed SNe, recent work provides evidence in favour of the high-mass progenitor scenario. The incidence of these low-luminosity SNe could be as high as 4-5 per cent of all Type II SNe.