Multichromatic colour–magnitude diagrams of the globular cluster NGC 6366

Abstract

We present multichromatic isochrone fits to the colour–magnitude data of the globular cluster NGC 6366, based on Hubble Space Telescope Advanced Camera for Surveys/Wide Field Channel and Southern Astrophysical Research photometric data.We corrected the photometric data for differential reddening and calculated the mean ridge line of the colour–magnitude diagrams. We compared the isochrones of Dartmouth Stellar Evolution Database and PAdova and TRieste Stellar Evolution Code both with microscopic diffusion starting on the main sequence. Bracketing all previous determinations of this cluster, we tested metallicities from [Fe/H]=−1.00 to [Fe/H]=−0.50, and ages from 9 to 13 Gyr. After determining the total to selective extinction ratio only from stars belonging to this cluster, RV = 3.06 ± 0.14, we found the parameters for this cluster to be E(B − V) = 0.69 ± 0.02(int) ±0.04(ext), (m − M)V = 15.02 ± 0.07(int) ±0.13(ext), Age = 11 ± 1.15 Gyr. Evolutionary models fail to reproduce the low-Teff sequence in multiband colour–magnitude diagrams, indicating that they still have an incomplete physics. We found that the Dartmouth Stellar Evolution Database isochrones better fit the subgiant branch and low main sequence than the PAdova and TRieste Stellar Evolution Code.