Attempting an accurate age estimate of the open cluster NGC 6633 using CoRoT and Gaia

TL;DR

This study combines CoRoT asteroseismic measurements of giant members with Gaia DR3 data to derive a precise, self-consistent age for the open cluster NGC 6633 while constraining internal mixing. By deriving luminosities from Gaia parallaxes and using four asteroseismic mass estimators (favoring the one using luminosity), the authors obtain consistent giant masses and radii that, together with CMD fits, point to a cluster age of $0.55_{-0.10}^{+0.05}$ Gyr. The Li abundances and [C/N] ratios indicate rotational mixing has played a very limited role in the giants’ evolution, validating non-rotating models for the best age determination. Four giants are confidently in the helium-core burning phase, with the evolutionary state of the brightest giant HD 170053 remaining inconclusive; the work also highlights biases in automated age estimates for helium-core-burning stars. Overall, the paper provides a more precise, rotation-insensitive age for NGC 6633 than previous estimates and demonstrates the value of combining asteroseismology with Gaia-based luminosities for young open clusters.

Abstract

Asteroseismology of solar-like oscillations in giant stars allow the derivation of their masses and radii. For members of open clusters, this provides an age of the cluster that should be identical to the one derived from the colour-magnitude diagram, but independent of the uncertainties that are present for that type of analysis. We aim to identify and measure the properties of giant members of the open cluster NGC6633, and combine these with asteroseismic measurements to derive a precise and self-consistent cluster age. Importantly, we wish to constrain the effects of rotational mixing on stellar evolution, since assumptions about internal mixing can have a significant impact on stellar age estimates. We identify five giant members of NGC6633 using photometry and Gaia data, supplemented by spectroscopic literature measurements. These are combined with asteroseismic measurements from CoRoT data and compared to stellar-model isochrones. This constrains the interior mixing to a low level and enables the most precise, accurate and self-consistent age estimate so far for this cluster. Asteroseismology of the giants and the cluster colour-magnitude diagram provide self-consistent masses of the giant members and their radii constrain the stellar interior mixing to a low level. The [C/N] ratios and Li abundances also suggest that rotation has had very little influence on the evolution of the stars in NGC 6633. This results in an age estimate of 0.55+0.05-0.10 Gyr for NGC6633. Four giant members appear to be in the helium-core burning evolutionary phase as also expected from evolutionary timescales. For the largest giant, the evidence remains inconclusive. A comparison to other age and mass estimates for the same stars in the literature uncovers biases for automated age estimates of helium-core burning stars.

Figures (5)

• Figure 1: Gaia CMDs of NGC 6633 and NGC 6866 proper motion and parallax members. Top panel: Blue diamonds mark NGC 6633 members that have been shifted to the absolute and unreddened scale using individual Gaia parallaxes and individual reddening estimates from the 3D reddening map of Green2019. Small red circles are the members of NGC 6866 shifted in the same way. All giant members of NGC 6633 are marked by a specific blue, green or yellow symbol with cross reference to Table \ref{['tab:data']}. The giants members of NGC 6866 are marked with red symbols as in Brogaard2023. Also shown are MESA isochrones with details in the legend and Sec. \ref{['sec:comparisons']}. Bottom panel: Zoom of the upper panel in the area of the giants.
• Figure 2: Mass-radius and $T_{\rm eff}$-radius diagrams for NGC 6633 giants compared to isochrones with details in the text and legends.
• Figure 3: A(Li) measurements of NGC 6633 giant stars as a function of $T_{\rm eff}$ compared to stellar evolutionary tracks Lagarde2012 with and without rotational effects for solar metallicity ($Z=0.014$) and relevant masses. The beginning of different evolutionary stages are marked with descriptions. The coloured symbols represent LTE A(Li) measurements for the giant stars of NGC 6633 from Morel2014Magrini2021Tsantaki2023 with 3D NLTE corrections from Wang2021, while the grey symbols mark the same measurements, but with NLTE corrections from Lind2009. The observations should be matched by the thick horizontal parts of the tracks, and the apparent match between models and observations for the green and red tracks is not real, as it assumes all giants belong to the fast-lived RGB phase.
• Figure 4: [C/N] measurements of HeCB stars as a function of $T_{\rm eff}$ compared to predictions from isochrones and stellar evolutionary tracks. Isochrones are from Syclist models of solar metallicity ($Z=0.014$) and an age of 0.5 Gyr unless otherwise stated. Evolutionary tracks of Solar metallicity are from Lagarde2012 with the exception of the 2.5 $M_{\odot},\rm{nonrot}, \beta=0$ track, which is from Vincenzo2021. The different evolutionary stages are located similarly to Fig. \ref{['fig:Li']}, but for clarity, many of the tracks and isochrones only show the part from the upper RGB to and including the AGB.
• Figure 5: Mass-radius diagrams for NGC 6633 giants compared to alternative isochrones with details in the text and legends.