SN 2005cs in M51 - I. The first month of evolution of a subluminous SNII plateau

Early-time optical observations of supernova (SN) 2005cs in the Whirlpool Galaxy (M51) are reported. Photometric data suggest that SN 2005cs is a moderately underluminous Type II plateau SN (SN IIP). The SN was unusually blue at early epochs (U-B approximate to -0.9 about three days after explosion) which indicates very high continuum temperatures. The spectra show relatively narrow P Cygni features, suggesting ejecta velocities lower than observed in more typical SNe IIP. The earliest spectra show weak absorption features in the blue wing of the He I 5876-angstrom absorption component and, less clearly, of H beta and H alpha. Based on spectral modelling, two different interpretations can be proposed: these features may either be due to high-velocity H and He I components, or ( more likely) be produced by different ions (N II, Si II). Analogies with the low-luminosity, Ni-56-poor, low-velocity SNe IIP are also discussed. While a more extended spectral coverage is necessary in order to determine accurately the properties of the progenitor star, published estimates of the progenitor mass seem not to be consistent with stellar evolution models.