The intermediate age open cluster NGC 2660

We present CCD UBVI photometry of the intermediate old open cluster NGC2660, covering from the red giants region to about seven magnitudes below the main sequence turn-off. Using the synthetic Colour - Magnitude Diagram method, we estimate in a self-consistent way values for distance modulus ((m-M)0 ~= 12.2), reddening (E(B-V) ~= 0.40), metallicity ([Fe/H] about solar), and age (age ~ 1 Gyr). A 30% population of binary stars turns out to be probably present.Comment: 12 pages, 8 (encapsulated) figures, to be published on MNRA

Old open clusters: UBGVRI photometry of NGC 2506

UBGVRI photometry for the open cluster NGC 2506 is presented. From comparison of the observed colour-magnitude diagrams with simulations based on stellar evolutionary models we derive in a self consistent way reddening, distance, and age of the cluster: E(B-V)=0-0.07, (m-M)o = 12.6, age = 1.5-2.2 Gyr. The cluster shows a well definite secondary sequence, suggesting that binary systems constitute about 20 % of the cluster members visible in the colour-magnitude diagram.Comment: 11 pages, 7 figures, MNRAS latex style, accepte

Many-body effective mass enhancement in a two-dimensional electron liquid

Motivated by a large number of recent magnetotransport studies we have revisited the problem of the microscopic calculation of the quasiparticle effective mass in a paramagnetic two-dimensional (2D) electron liquid (EL). Our systematic study is based on a generalized $GW$ approximation which makes use of the many-body local fields and takes advantage of the results of the most recent QMC calculations of the static charge- and spin-response of the 2D EL. We report extensive calculations for the many-body effective mass enhancement over a broad range of electron densities. In this respect we critically examine the relative merits of the on-shell approximation, commonly used in weak-coupling situations, {\it versus} the actual self-consistent solution of the Dyson equation. We show that already for $r_s \simeq 3$ and higher, a solution of the Dyson equation proves here necessary in order to obtain a well behaved effective mass. Finally we also show that our theoretical results for a quasi-2D EL, free of any adjustable fitting parameters, are in good qualitative agreement with some recent measurements in a GaAs/AlGaAs heterostructure.Comment: 12 pages, 3 figures, CMT28 Conference Proceedings, work related to cond-mat/041226

IORT with mobile linacs: the Italian experience

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