Photometry and spectroscopy of the Type IIP SN 1999em from outburst to dust formation

We present photometry and spectra of the Type IIP supernova 1999em in NGC 1637 from several days after the outburst till day 642. The radioactive tail of the recovered bolometric light curve of SN 1999em indicates that the amount of the ejected ⁵⁶Ni is ≈0.02 M_⊙. The Hα and He I 10830-Å lines at the nebular epoch show that the distribution of the bulk of 56Ni can be represented approximately by a sphere of ⁵⁶Ni with a velocity of 1500 km s^-1, which is shifted towards the far hemisphere by about 400 km s^-1. The fine structure of the Ha at the photospheric epoch reminiscent of the 'Bochum event' in SN 1987A is analysed in terms of two plausible models: bipolar ⁵⁶Ni jets and non-monotonic behaviour of the Ha optical depth combined with the one-sided ⁵⁶Ni ejection. The late-time spectra show a dramatic transformation of the [O I] 6300-Å line profile between days 465 and 510, which we interpret as an effect of dust condensation during this period. Late-time photometry supports the dust formation scenario after day 465. The [O I] line profile suggests that the dust occupies a sphere with velocity ≈800 km s^-1 and optical depth ≫10. The latter exceeds the optical depth of the dusty zone in SN 1987A by more than 10 times. Use is made of the Expanding Photosphere Method to estimate the distance and the explosion time, D ≈ 7.83 Mpc and t₀ ≃ 1999 October 24.5 UT, in accord with observational constraints on the explosion time and with other results of detailed studies of the method. The plateau brightness and duration combined with the expansion velocity suggest a pre-supernova radius of 120-150 R_⊙, ejecta mass of 10-11 M _⊙ and explosion energy of (0.5-1) × 10⁵¹ erg. The ejecta mass combined with the neutron star and a conservative assumption about mass loss implies the main sequence progenitor of M_ms ≈ 12-14 M_⊙. The derived mass range is in agreement with the upper limit to the mass found using pre-supernova field images by Smartt et al. From the [O I] 6300, 6364 Å doublet luminosity we infer the oxygen mass to be a factor four lower than in SN 1987A which is consistent with the estimated SN 1999em progenitor mass according to nucleosynthesis and stellar evolution theory. We note a 'second-plateau' behaviour of the light curve after the main plateau at the beginning of the radioactive tail. This feature seems to be common to SNe IIP with low ⁵⁶Ni mass.