A Deep Look into the Intermediate-Age Open Cluster NGC 2506: What Binary Systems Reveal About Cluster Distance and Age

TL;DR

This work presents a self-consistent determination of the age and distance to the intermediate-age open cluster NGC 2506 by jointly modeling radial velocities and spectral energy distributions for five SB2 binary systems (including two eclipsing binaries). Using RVs from ground-based surveys, TESS light curves, Gaia DR3 astrometry, and multi-band photometry, the authors fit 18 parameters (10 stellar masses, 5 inclinations, and common cluster age, distance, and extinction) against MIST isochrones and Castelli atmospheres, yielding an age of $1.94\pm0.03$ Gyr, a photometric distance of $3189\pm53$ pc, and $A_V=0.21\pm0.04$ mag for [Fe/H] = $-0.30$. These results are independently corroborated by a Gaia-based distance to cluster members, $3105\pm75$ pc, demonstrating the power of binary systems for constraining cluster properties at this evolutionary stage. The methodology, which couples RVs, eclipsing and non-eclipsing SB2s, SEDs, and Gaia priors, provides one of the most precise characterizations of an intermediate-age open cluster to date and is readily applicable to other clusters with suitable binaries.

Abstract

Using high-precision observations from the space-based \textit{Gaia} and \textit{TESS} missions, complemented by ground-based spectroscopic data and multi-band photometric surveys, we perform a detailed investigation of the Galactic open cluster NGC~2506. We present a new analysis of the intermediate-age open cluster NGC~2506, using joint fits to the radial velocities (RVs) and spectral energy distributions (SEDs) of five double-lined binary systems, including two eclipsing binaries. The analysis yields self-consistent estimates of the cluster's age, distance, and extinction, based on 18 free parameters: 10 stellar masses, 5 orbital inclinations, and common values for age, distance, and $A_V$. The SED fitting incorporates stellar isochrones, and the resulting parameters are examined through HR diagrams (R--$T_{\rm eff}$, R--M, and M--$T_{\rm eff}$) to assess evolutionary consistency. The age we derive for the cluster is $1.94 \pm 0.03$ Gyr for an assumed [Fe/H] = -0.30, and a fitting formula is given for extrapolation to other metallicities. The distance we find from the SED fitting is $3189 \pm 53$ pc, and this is to be compared with our own inference from the Gaia data which is $3105 \pm 75$ pc, based on 919 stars identified as cluster members. Our results demonstrate the power of binary systems in tightly constraining cluster-wide age and distance at this evolutionary stage. This approach represents one of the most accurate characterizations of an intermediate-age open cluster using multiple binary systems.

Figures (7)

• Figure 1: Observed and modelled radial velocity variations for five binary systems analyzed, along with the residuals. Red symbols represent the primary components, and blue symbols represent the secondary components of the systems. See text for further details.
• Figure 2: Phase-folded TESS light curves (points) and best-fitting binary star models (solid lines) for WOCS 5002 and WOCS 17003. Panels (a) and (d) show the full light curves of the two systems folded with their orbital periods. Panels (b) and (c) show zoom-ins on the secondary and primary eclipses of WOCS 5002, respectively, while panels (e) and (f) show the same for WOCS 17003. The zoomed panels highlight the excellent agreement between the observed TESS photometry and the model fits near the eclipse minima.
• Figure 3: We performed a joint SED fit for five binary systems, assuming a common age, distance, and extinction ($A_V$). The models were computed for a metallicity of [Fe/H] = –0.30. Figure \ref{['fig:age_Fe']} illustrates how the derived age depends on metallicity. In the figure, blue and red lines represent the best-fit models for the individual components of each binary, while the black line corresponds to the combined (composite) SED. Observational data are shown as orange circles with associated error bars.
• Figure 4: Age of NGC 2506 vs. the assumed metallicity [Fe/H]. The same type of SED fits used to produce Table \ref{['tab:ngc2506_sedresults']} and Fig. \ref{['fig:seds']} was carried out for additional metallicites. We find a parabolic relation between the age of NGC 2506 and metallicity given in Sec. \ref{['sec:SEDs']}.
• Figure 5: Top panel: Proper motion density distribution of stars in the NGC 2506 field, based on Gaia DR3 data. Middle panel: Cumulative distribution of proper motion displacements $\Delta\mu$ from the cluster center ($\mu_\alpha = -2.55 \pm 0.10$; $\mu_\delta = +3.97 \pm 0.09$). The red dashed vertical line, at the first major inflection point of the green curve marks the threshold value of $\Delta\mu = 0.3$ mas yr$^{-1}$, used to distinguish cluster members from field stars. Bottom panel: Mean cumulative density (number of stars per $\Delta \mu^2$) as a function of $\Delta\mu$. This plot helps visualize how steeply the density of stars drops right around the selected membership threshold, reinforcing the distinction between cluster members and non-members.