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The evolutionary properties of the old metal-rich Galactic open clusterNGC6791 are assessed, based on deep UB photometry and 2Mass JK data. For 4739stars in the cluster, bolometric luminosity and effective temperature have beenderived from theoretical (U-B) and (J-K) color fitting. The derived H-R diagramhas been matched with the UVBLUE grid of synthetic stellar spectra to obtainthe integrated SED of the system, together with a full set UV (Fanelli) andoptical (Lick) narrow-band indices. The cluster appears to be a fairly goodproxy of standard elliptical galaxies, although with significantly bluerinfrared colors, a shallower 4000A Balmer break, and a lower Mg2 index. Theconfirmed presence of a dozen hot stars, along their EHB evolution, leads thecluster SED to consistently match the properties of the most active UV-upturngalaxies, with 1.7+/-0.4% of the total bolometric luminosity emitted shortwardof 2500A.  The cluster Helium abundance results Y=0.30 +/-0.04, while the Post-MSimplied stellar lifetime from star number counts fairly agrees with thetheoretical expectations from both the Padova and BASTI stellar tracks. APost-MS fuel consumption of 0.43 +/- 0.01 M_sun is found for NGC6791 stars, inclose agreement with the estimated mass of cluster He-rich white dwarfs. Such atight figure may lead to suspect that a fraction of the cluster stellarpopulation does actually not reach the minimum mass required to effectivelyignite He in the stellar core.

astrophysical journal/the astrophysical journal

Stellar lifetime and ultraviolet properties of the old metal-rich  Galactic open cluster NGC6791: a pathway to understand the UV upturn of  elliptical galaxies