The Type Ib SN 1999dn: One Year of Photometric and Spectroscopic Monitoring.?

Received................; accepted................ Extensive optical and near-infrared (NIR) observations of the type Ib supernova 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 (MV = −17.2 mag). From the bolometric light curve and ejecta expansion velocities, we estimate that about 0.11M of 56Ni were produced during the explosion and that the total ejecta mass was 4 − 6M with a kinetic energy of at least 5×1051 erg. The spectra of SN 1999dn at various epochs are similar to those of other Stripped Envelope (SE) SNe showing clear presence of H at early epochs. The high explosion energy and ejected mass, along with the small flux ratio [CaII]/[OI] measured in the nebular spectrum, together with the lack of signa-tures of dust formation and the moderate metallicity environment is not inconsistent with a single massive progenitor (MZAMS> 23 − 25 M) for SN 1999dn