Isochrone fitting of Galactic globular clusters – I. NGC 5904

Abstract

We present new isochrone fits to colour-magnitude diagrams of the Galactic globular cluster NGC 5904 (M5). We utilise 29 photometric bands from the ultraviolet to mid-infrared by use of the data from the {\it Hubble Space Telescope}, {\it Gaia} DR2, {\it Wide-field Infrared Survey Explorer}, Sloan Digital Sky Survey (SDSS), and other photometric data. In our isochrone fitting we use the PAdova and TRieste Stellar Evolution Code, the MESA Isochrones and Stellar Tracks, the Dartmouth Stellar Evolution Program, and a Bag of Stellar Tracks and Isochrones both for the solar-scaled and enhanced He and $\alpha$ abundances with a metallicity about [Fe/H]$=-1.33$ adopted from the literature. All tools provide us with estimates of the distance, age, and extinction law to the cluster. The best-fit distance, true distance modulus, and age are $7.4\pm0.3$ kpc, $14.34\pm0.09$ mag, and $12.15\pm1.00$ Gyr, respectively. The derived distance agrees with the literature, including the {\it Gaia} DR2 parallax with its known global zero-point correction. All the data and models, except some UV and SDSS data, agree with the extinction law of Cardelli-Clayton-Mathis with $R_\mathrm{V}=3.60\pm0.05$ and $A_\mathrm{V}=0.20\pm0.02$ mag. This extinction is twice as high as generally accepted due to a rather high extinction between 625 and 2000 nm. An offset of the model colours instead of the high extinction in this range is a less likely, yet possible explanation of the discovered large deviations of the isochrones from the data.