Asteroseismology of the young open cluster NGC 2516 II. Constraining cluster age using gravity-mode pulsators

TL;DR

This study uses joint asteroseismic modelling of four gravity-mode pulsators in the young open cluster NGC 2516 to precisely constrain the cluster age and probe internal mixing processes. By computing rotating 1-D MESA models with exponential core overshoot and rotation-induced transport, and fitting their g-mode frequencies with GYRE, the authors derive a seismic cluster age of $132\pm8$ Myr, differing from the MIST isochrone age by about $2\sigma$. The analysis reveals a strong constraint on core overshooting but only weak sensitivity to radiative-envelope mixing, and it exposes a mass discrepancy for higher-mass pulsators when compared to MIST masses. Constructing asteroseismology-calibrated isochrones using identical input physics reduces the age discrepancy and yields an age around $129^{+28}_{-23}$ Myr, though a residual mass offset remains, highlighting remaining gaps between 1-D stellar models and the physics of fast-rotating stars. Overall, the work demonstrates that cluster asteroseismology can yield precise ages and provides crucial insights into mixing processes and the calibration of isochrones for young stellar populations. Key quantities include the asymptotic g-mode spacing $\Pi_0$, the convective boundary mixing parameter $f_{CBM}$, and the envelope mixing coefficient $\eta$, all of which influence age and mass estimates through their impact on near-core structure and period spacing patterns.

Abstract

Although asteroseismology is regarded as the most powerful tool for probing stellar interiors, seismic modelling remains dependent on global stellar parameters. Stellar clusters offer direct measurements of these parameters by fitting a CMD, making the application of asteroseismology in clusters a valuable approach to advancing stellar physics modelling. We aimed to develop seismic modelling for gravity-mode pulsators in the open cluster NGC 2516 to determine stellar ages. We computed 1D stellar models using MESA, incorporating rotation-induced transport processes. Exponential overshooting was included, as well as rotationally induced mixing in the radiative envelope. Grids of evolutionary models were computed covering isochrone-derived mass ranges. The models were evolved up to 300 Myr because of the cluster's young age (~100Myr). By fitting the frequencies of identified modes of four gravity-mode member pulsators simultaneously, we measure the seismic age of the cluster NGC 2516 as 132+-8Myr. This high-precision seismic age estimate deviates by 1sigma from the isochronal age derived from public MIST isochrones for rotating stars. Our findings show that seismic modelling strongly constrains core overshooting, but because the period spacing patterns are smooth, it provides weak constraints on mixing in the radiative envelopes. The two most massive gravity-mode pulsators have MIST masses ~2.0M_sun while their seismic masses are 1.75M_sun. We constructed new asteroseismology-calibrated isochrones using input physics identical to that of our seismic model grid. While this resolves the age discrepancy, the mass discrepancy is only partially addressed. The remaining small yet persisting mass discrepancy implies a mismatch between the physics in core to surface environments of 1D stellar models and the seismic observables probing those areas of fast-rotating stars.

Figures (30)

• Figure 1: Isochrone-derived luminosities and effective temperatures of the selected $\gamma$ Dor stars in NGC 2516. Error bars show the 1$\sigma$ level of the uncertainties. The solid curve is the best-fitting MIST isochrone.
• Figure 2: Posterior distributions of the model parameters for TIC 308307454 obtained using Approach 1. The diagonal panels show the marginalised one-dimensional distributions for each parameter, where the solid and dashed vertical lines indicate the median and the $\pm1\sigma$ range, respectively. The off-diagonal panels display the two-dimensional joint posterior distributions. We added random noise to the discrete samples to enhance their visual clarity.
• Figure 3: Observed and best-fitting period spacing pattern of TIC 308307454 from Approach 1. The vertical grey lines indicate the observed g-mode periods, and their period spacings are shown as red crosses. We randomly selected 30 period-spacing patterns from the MCMC result computed with GYRE, shown as circles, with the colour bar indicating the value of $f_\mathrm{CBM}$.
• Figure 4: Same as Fig. \ref{['fig:corner_TIC308307454']}, but for TIC 281582674.
• Figure 5: Same as Fig. \ref{['fig:DP_approach1_TIC308307454']}, but for TIC 281582674.