A. Gemmo

High-resolution imaging of the Pluto-Charon system with the Faint Object Camera of the Hubble Space Telescope

Images of the Pluto-Charon system were obtained with the Faint Object Camera (FOC) of the Hubble Space Telescope (HST) after the refurbishment of the telescope. The images are of superb quality, allowing the determination of radii, fluxes, and albedos. Attempts were made to improve the resolution of the already diffraction limited images by image restoration. These yielded indications of surface albedo distributions qualitatively consistent with models derived from observations of Pluto-Charon mutual eclipses.

The Luminosity Function of White Dwarfs: A Study of the Selected Area 94

A search for white dwarfs has been carried out in the field of the SA 94, using multicolor techniques. 12 DA, 1 DB, 1 DC, plus a magnetic DQ white dwarfs have been confirmed with slit spectroscopy. The sample, complete up to B=18.5 and for M V < 12.5, has been used for a determination of the Luminosity Function of White Dwarfs. A new estimation of the relative frequency of WD+MS systems has also been inferred.

A Search for Infrared Companions to White Dwarfs

The initial mass function for star formation at the bottom of the main sequence can be estimated by measuring the number of low-luminosity companions to white dwarfs. Infrared (J, H and K) photometry of a statistically complete sample of WDs, which are hot and very small objects, is a unique tool to detect very low-mass and cool companions. The result of this search has potential implications for the local missing mass, galactic chemical evolution and the nature of a massive halo around the Galaxy. In cases where the presence of a cool companion can already be inferred from optical data (e. g. composite spectra), infrared photometry is necessary to obtain direct information on its nature. Here I describe a project aimed at searching for infrared companions to twenty complete samples of WDs identified by the ESO Key Programme: A Homogeneous Bright QSO Survey.

A Homogeneous Bright Quasar Survey

In undertaking a large shallow survey for quasars, we considered especially two issues: (I) On the one hand, the shape of the Luminosity Function and the determination of the form of evolution as L ∝ (1 + z) k or L ∝ e T/τ are uncertain. Trends of this form, potentially telling about the mechanism to fuel the central engine (in principle they can reveal if the QSO phenomenon is driven by the surrounding environment or is determined by its nuclear conditions only), can be spuriously favoured by fits that overlook the observational biases. To probe the real trend, not only the database at faint magnitudes and higher redshifts has to be enlarged, but also the incompleteness at bright magnitudes should be bound or removed with better samples allowing an adequately sophisticated analysis.