Ages of Globular Clusters fromHIPPARCOSParallaxes of Local Subdwarfs

We report here initial but still strongly conclusive results on absolute agesof galactic globular clusters (GGCs). This study is based on high precisiontrigonometric parallaxes from the HIPPARCOS satellite coupled with accuratemetal abundances ([Fe/H], [O/Fe], and [\alpha/Fe]) from high resolutionspectroscopy for a sample of about thirty subdwarfs. Systematic effects due tostar selection (Lutz-Kelker corrections to parallaxes) and to the possiblepresence of undetected binaries in the sample of bona fide single stars areexamined, and appropriate corrections are estimated. They are found to be smallfor our sample. The new data allowed us to reliably define the absolutelocation of the main sequence (MS) as a function of metallicity. These resultsare then used to derive distances and ages for a carefully selected sample ofnine globular clusters having metallicities determined from high dispersionspectra of individual giants according to a procedure totally consistent withthat used for the field subdwarfs. Very precise and homogeneous reddeningvalues have also been independently determined for these clusters. We find thatHIPPARCOS parallaxes are smaller than the corresponding ground-basedmeasurements leading, in turn, to longer distance moduli ($\sim 0.2$ mag) andyounger ages ($\sim 2.8$ Gyr). The relation between the zero age HB (ZAHB)absolute magnitude and metallicity for the nine programme clusters turns out tobe: $M_V(ZAHB) = (0.22\pm 0.09)([Fe/H]+1.5) + (0.49\pm 0.04)$. Thecorresponding LMC distance modulus is $(m-M)_0=18.60\pm 0.07$. The age of thebona fide old globular clusters (Oosterhoff II and BHB) based on the absolutemagnitude of the turn-off, a theoretically robust indicator, is: Age ={11.8^{+2.1}_{-2.5}} Gyr.Comment: 82 pages, uses aasms4.sty, includes 10 eps figures. Revised version with detailed discussion of Lutz-Kelker correction and of the effect of binary contamination. Main results on ages are unchanged. Accepted on ApJ. Text and figures also available via anonymous ftp at boas5.bo.astro.it in pub/gisella/hfina