The type Ib SN 1999dn: one year of photometric and spectroscopic monitoring

Extensive optical and near-infrared observations of the Type Ib supernova (SNIb) 1999dn are presented, covering the first year after explosion. These new data turn this object, already considered a prototypical SNIb, into one of the best observed objects of its class. The light curve of SN 1999dn is mostly similar in shape to that of other SNeIb but with a moderately faint peak (M v = —17.2mag). From the bolometric light curve and ejecta expansion velocities, we estimate that about 0.11 M ⊙ of 56 Ni were produced during the explosion and that the total ejecta mass was 4―6 M ⊙ with a kinetic energy of at least 5 x 10 51 erg. The spectra of SN 1999dn at various epochs are similar to those of other stripped envelope SNe showing clear presence of H at early epochs. The high explosion energy and ejected mass, along with the small flux ratio [Ca II]/[I I] measured in the nebular spectrum, together with the lack of signatures of dust formation and the moderate metallicity environment is not inconsistent with a single massive progenitor (M ZAMS ≥ 23-25 M ⊙ ) for SN 1999dn.