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 Co-56 radioactive decay tail indicates that the mass of Ni-56 ejected during the explosion is much smaller (M-Ni approximate to 2-8 x 10(-3) M.) than the average (M-Ni approximate to 6-10 x 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 (approximate to100 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.