Precise parameter determination of the open cluster NGC 1647 via asteroseismology of p-mode pulsators

Abstract

Asteroseismology of member pulsators provides a robust physical constraint on cluster parameters by linking internal stellar structures to the global properties of the host cluster. However, the parameters of NGC 1647 remains poorly constrained due to limited investigation, a situation that cluster asteroseismology can significantly refine. In this study, we identified 271 high confidential cluster members in NGC 1647, using HDBSCAN clustering with radial-velocity validation. Its initial age is determined in the range of 1250-280 Myr, derived from isochrone fitting based on multi-survey metallicities extinction-corrected Gaia photometry. Among the members, we found 96 periodic variables from TESS and K2 photometry, including nine p-mode pulsators (five δ Sct and four hybrid δ Sct-γ Dor stars). Assuming a common cluster age and initial chemical composition, joint asteroseismic modeling is performed based on measured large frequency separations and individual mode frequencies. This yields a metallicity of [Fe/H] = -0.08+0.04-0.01, well consistent with the spectroscopic determinations, and a seismic age of 178+11-9 Myr, more precise than isochrone-based estimates. This work shows the diagnostic potential of δ Sct asteroseismology in young open clusters and establishes a high-precision benchmark for future studies of NGC 1647 and other open clusters.

Figures (7)

• Figure 1: Membership selection for NGC 1647.Panel (a) shows the spatial sky distribution (RA vs. Dec), and panel (b) presents the proper motion diagram ($\mu_{\alpha} \cos\delta$ vs. $\mu_{\delta}$); Panel (c) displays the parallax distribution, while panel (d) shows the color--magnitude diagram (CMD, $G$ vs. $G_{BP}-G_{RP}$). In these panels, grey points (or bars) represent all potential sources identified by HDBSCAN, blue denotes candidate members with membership probabilities $P \geq 0.7$, and red highlights RV outliers. The black star denotes the red giant star in NGC 1647.
• Figure 2: Radial velocity (RV) distributions and Gaussian fitting for NGC 1647. The shaded red regions highlight stars located beyond the $5\sigma$ clipping threshold, which are identified as RV outliers and excluded from the member list.
• Figure 3: Metallicity as a function of magnitude for stars in NGC 1647. Blue dots represent data from the LAMOST MRS, the orange circle denotes the APOGEE measurement, and the green circle corresponds to the data from 2022Carrera. Error bars indicate the reported uncertainties. The black dashed line marks the weighted mean metallicity, and the pink shaded region represents the $\pm 3\sigma$ confidence interval around the mean.
• Figure 4: Isochrone fitting and variable stars in NGC 1647. (a) Isochrone fitting for NGC 1647. Orange points represent extinction-corrected member stars. The solid curves show the eight best-fitting MIST isochrones, where $\tau$ denotes the age of the isochrones. (b) CMD of the member stars in NGC 1647. Different symbols represent the various types of variable stars identified in this study. The blue dashed and dotted curves indicate the theoretical and empirical instability strips of $\gamma$ Dor and $\delta$ Sct stars, respectively. (c) Representative segments of the light curves for the six types of variable stars identified in NGC 1647. The top three panels display TESS data, while the bottom three panels present observations from the K2 mission.
• Figure 5: Amplitude spectra of the nine $p$-mode pulsators identified in NGC 1647 based on TESS light curves. The colored symbols denote pulsation modes with different spherical degrees: blue, red, green, and orange circles represent frequencies identified as $\ell = 0, 1, 2,$ and $3$ modes, respectively. For stars exhibiting regular frequency patterns, the measured $\Delta\nu$ values are indicated in the corresponding panels.