The properties of Galactic globular clusters from Gaia EDR3 and other data compared with theoretical isochrones

We fit theoretical isochrones from different models of internal structure and evolution of stars to photometric data for thestars in globular clusters of our Galaxy. To select cluster members, determine cluster sizes and calculate systemic propermotions, we use parallaxes and proper motions from Gaia EDR3. To calculate the most probable distance, age, interstellarextinction in a variety of filters and differential reddening in cluster fields, we use photometry in more than 26 filters betweenthe ultraviolet and mid-infrared waverange from HST, Gaia EDR3, Pan-STARRS DR1, DES, SDSS, unWISE, SAGE andother datasets in combination with the PARSEC, MIST, DSEP, and BaSTI-IAC isochrones, as for the solar metallicityscale as for alpha- and helium-enriched scales. The metallicity and enrichment of the clusters is taken from spectroscopicobservations and tested for compliance with the photometric results. We carry out a thorough analysis of random andsystematic uncertainties of the obtained results. The derived extinctions in many filters allow us to estimate an empiricalextinction law for each cluster. A complete analysis has been performed for five clusters (NGC288, NGC362, NGC5904,NGC6205, and NGC6218), a preliminary analysis, based on the Gaia EDR3 astrometry and photometry only, has beendone for 38 more clusters. The main conclusions are as follows. First, unprecedentedly accurate astrometry of Gaia EDR3allows us to segregate the cluster members from fore- and background stars and to indicate that the size of many clustersis much larger than previously thought. Second, the distances, derived by us from the photometry-to-isochrone fitting, arestill more precise than distances from the Gaia EDR3 parallaxes. Third, contrary to the popular 2D reddening maps ofSchlegel-1998 and Planck, we found no clusters at high Galactic latitudes with an extinction A V < 0.1. Fourth, for thehorizontal branch second parameter quartet NGC 288–NGC 362–NGC 5904–NGC 6218, the age is undoubtedly the secondparameter.