SED Analysis of the Old Open Cluster NGC 188

Abstract

In this study, we investigate the fundamental astrophysical parameters of the old open cluster NGC 188 using two complementary methods: isochron fitting and spectral energy distribution (SED) analysis. Using photometric, astrometric, and spectroscopic data from the {\it Gaia} Data Release 3 \citep[DR3,][]{Gaia_DR3}, we identified 868 most likely member stars with membership probabilities $P \geq 0.5$. The mean proper-motion components and trigonometric parallaxes of the cluster are derived as ($\mu_{\alpha}\cos \delta$, $\mu_{\delta}$) = (-$2.314 \pm 0.002$, -$1.022 \pm 0.002$) mas yr$^{-1}$ and $\varpi = 0.550 \pm 0.023$ mas, respectively. From this initial selection of high probable member stars, we proceed with the determination of astrophysical parameters using the isochron-fitting method. Simultaneously estimating the colour excess, distance and age of the cluster, we employe PARSEC isochrones to observational data on {\it Gaia} based colour-magnitude diagrams. These findings were obtained as $E(G_{\rm BP}-G_{\rm RP})=0.066\pm 0.012$ mag, $d=1806 \pm21$ pc, and $t=7.65 \pm 1.00$ Gyr, respectively. In addition, we identified and detected 19 previously confirmed blue straggler stars within NGC 188. Subsequently, we performed SED analyses for 412 of the 868 cluster members. We obtained colour excess, distance and age of the cluster as $E(B-V)=0.034\pm 0.030$ mag, $d=1854\pm 148$ pc, and $t=7.78\pm 0.23$ Gyr, respectively. The analysis of member stars revealed patterns of extinction in the $V$-band, with higher values of $A_{\rm V}$ observed in the lower right quadrant of the cluster. By comparing our SED analysis results with models of stellar evolution, particularly in terms of temperature and surface gravity, we confirm agreement with theoretical predictions. This comprehensive investigation sheds light on the astrophysical properties of NGC 188, contributing to our understanding of stellar evolution within open clusters.