Isochrone fitting of Galactic globular clusters -- IV. NGC6362 and NGC6723

Abstract

We present new isochrone fits to the colour-magnitude diagrams of the Galactic globular clusters NGC\,6362 and NGC\,6723. We utilize 22 and 26 photometric filters for NGC\,6362 and NGC\,6723, respectively, from the ultraviolet to mid-infrared using data sets from {\it HST}, {\it Gaia}, unWISE, and other photometric sources. We use models and isochrones from the Dartmouth Stellar Evolution Database (DSED) and Bag of Stellar Tracks and Isochrones (BaSTI) for $\alpha$-enhanced [$\alpha$/Fe]$=+0.4$ and different helium abundances. The metallicities [Fe/H]$=-1.04\pm 0.07$ and $-1.09\pm 0.06$ are derived from the red giant branch slopes in our fitting for NGC\,6362 and NGC\,6723, respectively. They agree with spectroscopic estimates from the literature. We find a differential reddening up to $\Delta E(B-V)=0.13$ mag in the NGC\,6723 field due to the adjacent Corona Australis cloud complex. We derive the following for NGC\,6362 and NGC\,6723, respectively: distances $7.75\pm 0.03\pm 0.15$ (statistic and systematic error) and $8.15\pm 0.04\pm 0.15$ kpc; ages $12.0\pm 0.1\pm 0.8$ and $12.4\pm 0.1\pm 0.8$ Gyr; extinctions $A_{\mathrm{V}}=0.19\pm 0.04\pm 0.06$ and $0.24\pm 0.03\pm 0.06$ mag; reddenings $E(B-V)=0.056\pm 0.01\pm 0.02$ and $0.068\pm 0.01\pm 0.02$ mag. DSED provides systematically lower [Fe/H] and higher reddenings than BaSTI. However, the models agree in their relative estimates: NGC\,6723 is $0.44\pm 0.04$ kpc further, $0.5\pm 0.1$ Gyr older, $\Delta E(B-V)=0.007\pm 0.002$ more reddened, and with $0.05\pm 0.01$ dex lower [Fe/H] than NGC\,6362. The lower metallicity and greater age of NGC\,6723 with respect to NGC\,6362 explain their horizontal branch morphology difference. This confirms age as the second parameter for these clusters. We provide lists of the cluster members from the {\it Gaia} Data Release 3.

arXiv commentAccepted to be published in MNRAS, 20 pages, 3 online tables

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