Isochrone Fitting of Galactic Globular Clusters -- VII. NGC\,1904 (M79), NGC\,4372, and revision of NGC\,288, NGC\,362, NGC\,5904 (M5), NGC\,6205 (M13), and NGC\,6218 (M12)

TL;DR

This paper applies multi-filter isochrone fitting to seven Galactic globular clusters, including new analyses of NGC $1904$ and NGC $4372$, by combining BaSTI and DSED models with extensive Gaia DR3 and archival photometry to derive $[ ext{Fe/H}]$, ages, distances $R$, and reddening $E(B-V)$. It uses Gaia-based membership, cross-identification of diverse data sets, and empirical extinction laws to robustly fit hundreds of CMDs, while accounting for differential reddening and systematic uncertainties across data sets and models. The results reinforce metallicity as the primary parameter shaping HB morphology, with age as the secondary parameter, and highlight mass-loss efficiency $ ext{η}>0.3$ as a common third parameter driving blue HB tails; NGC $4372$ in particular shows substantial low-mass HB-star depletion related to its evolutionary history. The study also notes a BaSTI revision that lowers ages by about $1.2$ Gyr, and finds extinction laws close to CCM89 with $R_V$ values ranging around $3.0$–$3.9$, contributing precise distances, reddenings, and HB interpretations that improve the calibration of the Galactic globular cluster system.

Abstract

We estimate key parameters for the Galactic globular clusters NGC1904 and NGC4372 and update the parameters for NGC288, NGC362, NGC5904, NGC6205, and NGC6218, which were analysed in our previous papers. We fit various colour-magnitude diagrams (CMDs) of the clusters using isochrones from the DSED and BaSTI. The CMDs are constructed from data sets provided by the HST, Gaia, SMSS, a large compilation of Stetson, and other sources, using multiple filters for each cluster. Our cross-identification of almost all the data sets with those from Gaia or HST allows us to use their astrometry to precisely select cluster members in all the data sets. We obtain the following estimates, along with their total uncertainties, for NGC288, NGC362, NGC1904, NGC4372, NGC5904, NGC6205 and NGC6218, respectively: metallicities [Fe/H]$=-1.28$, $-1.26$, $-1.64$, $-2.28$, $-1.33$, $-1.56$, and $-1.27$ dex; ages $12.94$, $10.33$, $13.16$, $12.81$, $11.53$, $12.75$, and $13.03$ Gyr; distances $8.83$, $9.00$, $12.66$, $5.17$, $7.24$, $7.39$, and $4.92$ kpc; reddenings $E(B-V)=0.022$, $0.029$, $0.031$, $0.545$, $0.045$, $0.024$, and $0.210$ mag; extinctions $Av=0.09$, $0.09$, $0.11$, $1.58$, $0.13$, $0.09$, and $0.67$ mag; and extinction-to-reddening ratio $Rv=3.9$, $3.0$, $3.8$, $2.9$, $2.9$, $3.6$, and $3.2$. We confirm that the differences in horizontal branch morphology among the 16 Galactic globular clusters analysed in our studies can be explained by variations in their metallicity, age, mass-loss efficiency, and the loss of low-mass members during cluster evolution. Accordingly, most clusters indicate a relatively high mass-loss efficiency, consistent with the Reimers mass-loss law with $η>0.3$.

Figures (14)

• Figure 1: $G_\mathrm{BP}-G_\mathrm{RP}$ versus $G_\mathrm{RP}$ CMDs for cluster members from the Gaia DR3. The clusters are ordered by their [Fe/H]: those with [Fe/H]$\approx-1.3$ are in the left column, while NGC 1904 and NGC 6205 with [Fe/H]$\approx-1.6$ are in the top of the right column. The NGC 4372 CMDs before and after our DR correction are shown in the bottom of the right column. The isochrones for a primordial $Y\approx0.25$ from BaSTI (red), BaSTI ZAHB (purple), and DSED (green), isochrones for $Y=0.275$ from BaSTI (orange) and BaSTI ZAHB (blue), as well as isochrones for $Y=0.33$ from DSED (luminous green) are calculated with the best-fitting parameters from Table \ref{['cmds']}. Variable stars are shown by the magenta diamonds.
• Figure 2: $B-I$ versus $I$ CMDs for cluster members from the cross-identification of the Gaia DR3 and stetson2019 data sets. The clusters are ordered by their [Fe/H] as in Fig. \ref{['gaia']}. The isochrones for a primordial $Y\approx0.25$ from BaSTI (red), BaSTI ZAHB (purple), and DSED (green), DSED HB/AGB tracks (light green), isochrones for $Y=0.275$ from BaSTI (orange) and BaSTI ZAHB (blue), as well as isochrones for $Y=0.33$ from DSED (luminous green) are calculated with the best-fitting parameters from Table \ref{['cmds']}. Variable stars are shown by the magenta diamonds.
• Figure 3: The same as Fig. \ref{['stetson']} but for the $b-y$ versus $y$ CMDs for cluster members from the cross-identification of the Gaia DR3 and Lee data sets. For NGC 5904 the ordinate is the $V$ magnitude.
• Figure 4: DR maps derived from sfd98, gms2025, and various CMDs for the same NGC 4372 field. The DR maps derived from CMDs are converted from the initial adaptive angular resolution to the constant resolution of 1.5 arcmin for Gaia--unWISE and SMSS maps and 1 arcmin for the remaining CMD-based maps. sfd98 and gms2025 have the resolution of 6.1 arcmin. All the maps are converted into $\Delta E(B-V)$ using the ccm89 extinction law with $R_\mathrm{V}=3.1$. The white areas have no estimates. The cluster centre is the black cross, the position of bright star HD 107947 is marked by the magenta cross.
• Figure 5: The dependence of extinction $A_\mathrm{V}$ on distance $R$ for eight lines-of-sight within the NGC 4372 field from the 3D extinction map of gms2025 -- colour curves. Black line denotes the steepest increase of $A_\mathrm{V}$ in the Musca dark nebula at about 150 pc from the Sun.