The bright Type IIP SN 2009bw, showing signs of interaction

We present photometry and spectroscopy of the type IIP supernova 2009bw in UGC 2890 from few days after the outburst to 241 days. The light curve of SN 2009bw during the photospheric phase is similar to that of normal SNe IIP but with brighter peak and plateau (Mmax R = -17.82 mag, Mplateau R = -17.37 mag). The luminosity drop from the photospheric to the nebular phase is one of the fastest ever observed, ~2.2 mag in about 13 days. The radioactive tail of the bolometric light curve indicates that the amount of ejected 56 Ni is \approx 0.022 M\odot. The photospheric spectra reveal high velocity lines of H{\alpha} and H{\beta} until about 105 days after the shock breakout, suggesting a possible early interaction between the SN ejecta and pre-existent circumstellar material, and the presence of CNO elements. By modeling the bolometric light curve, ejecta expansion velocity and photospheric temperature, we estimate a total ejected mass of 8-12M\odot, a kinetic energy of ~0.3 foe and an initial radius of ~ 3.6 - 7 \times 10^13 cm.