Isochrone fitting of Galactic globular clusters -- II. NGC6205 (M13)
  We present new isochrone fits to colour-magnitude diagrams of the Galactic
globular cluster NGC\,6205 (M13). We utilise 34 photometric bands from the
ultraviolet to mid-infrared by use of data from the {\it HST}, {\it Gaia} DR2,
SDSS, unWISE, Pan-STARRS DR1, and other photometric sources. In our isochrone
fitting we use the PARSEC, MIST, DSEP, BaSTI, and IAC-BaSTI theoretical models
and isochrones, both for the solar-scaled and He-$\alpha$-enhanced abundances,
with a metallicity of about [Fe/H]$=-1.58$ adopted from the literature. The
colour-magnitude diagrams, obtained with pairs of filters from different
datasets but of similar effective wavelengths, show some colour offsets up to
0.04 mag between the fiducial sequences and isochrones. We attribute these
offsets to systematic differences of the datasets. Some intrinsic systematic
differences of the models/isochrones remain in our results: the derived
distances and ages are different for the ultraviolet, optical and infrared
photometry used, while the derived ages are different for the different
models/isochrones, e.g. in the optical range from $12.3\pm0.7$ Gyr for
He-$\alpha$-enhanced DSEP to $14.4\pm0.7$ Gyr for MIST. Despite the presence of
multiple stellar populations, we obtain convergent estimates for the dominant
population: best-fitting distance $7.4\pm0.2$ kpc, true distance modulus
$14.35\pm0.06$ mag, parallax $0.135\pm0.004$ mas, extinction
$A_\mathrm{V}=0.12\pm0.02$, and reddening $E(B-V)=0.04\pm0.01$. These estimates
agree with other recent estimates, however, the extinction and reddening are
twice as high as generally accepted. The derived empirical extinction law
agrees with the Cardelli-Clayton-Mathis extinction law with the best-fitting
$R_\mathrm{V}=3.1^{+1.6}_{-1.1}$.
