Photometric and spectroscopic investigation of 2867 Steins, target of the Rosetta mission Ground-based results prior to the Rosetta fly-by

Context. Rosetta is the cornerstone mission of ESA devoted to the study of minor bodies of the Solar System. During its journey to the comet 67P Churyumov-Gerasimenko, the main target, Rosetta also investigates two main belt asteroids, 2867 Steins (fly-by in September 2008) and 21 Lutetia (fly-by in July 2010). Aims. In Spring 2008, we performed a broad observational campaign in order to complete the ground-based photometric and spectroscopic investigation of Steins. Before the Rosetta fly-by, this was the last opportunity to perform ground-based observations useful for calibrating properly the imaging and spectroscopic data obtained by the instruments on board the Rosetta spacecraft. Methods. Visible photometry was carried out at a wide range of phase angles, and visible spectra were acquired at different rotational phases to retrieve information about the absolute magnitude and surface properties. Results. The lightcurve was completely sampled in V and R bands. A rotational period of 6.057 +/- 0.003 h and color index V - R = 0.51 +/- 0.03 mag were computed. We investigate Steins' phase relation over the range between 3.3 and 42 degrees in solar phase angle. The opposition effect is not evident down to the phase angle of 3 degrees, as is typical of other E-type asteroids. Assuming for Steins an opposition surge similar to that of other E-type asteroids, we calculated an absolute magnitude H-R = 12.81 +/- 0.03, and slope parameter GR = 0.42 +/- 0.02. Eight visible spectra, obtained at different rotational phases, exhibit similar behavior, confirming a homogeneous composition of the asteroid surface and the E[II] classification. All spectra display a spectral feature centered on about 0.49 mu m that is typical of E-type asteroids and usually attributed to the presence of sulfides (e.g. oldhamite). A tentative model of the surface composition is presented.