The SOL $\textit{(Solar Origin and Life)}$ Project: Detailed characterization of candidates for the ZAMS and Subgiant stages

TL;DR

This study targets Sun-like stars that trace the solar evolutionary path, aiming to identify proxies at the ZAMS and subgiant stages. It combines high-resolution spectroscopy with Fe I/Fe II excitation–ionization analyses, isochrone-based evolutionary inferences (MIST), and Gaia-based kinematics to derive atmospheric, chemical, and stellar-structural parameters. Age diagnostics from Ca II H&K, Hα, and TESS-derived rotation corroborate isochronal ages, enabling robust stage classifications. The results pinpoint HD 13531 and HD 61033 as young Sun analogs near 0.3–0.5 Gyr and HD 148577 as a compelling SG Sun proxy, supporting the broader goal of contextualizing habitability and exoplanet studies along the Sun’s life story, with future Gaia-driven expansion on the horizon.

Abstract

The context of the Sun in the galactic neighborhood is not well understood, especially when we compare its physical properties to those of nearby stars. Thereby, we still cannot fully comprehend whether or not the Sun is a typical star. This work aims to identify and characterize stars aligned with the solar evolutionary track that could represent it at the ZAMS and subgiant stages. We performed a spectroscopic analysis of 18 photometrically selected candidates using high-resolution and high-SNR spectra as well as the classical spectroscopic method, based on the excitation and ionization equilibria of Fe I and Fe II lines. Additionally, we derived evolutionary parameters using isochrones, and kinematic parameters. We also estimated chromospheric activity levels and performed age estimates through 3 additional independent methods: activity-age relations using the Ca II H $\&$ K and H$α$ lines, and rotation periods estimated from TESS light curves. We identified three candidates that provide a good match to the Sun at $\approx$ 0.5 Gyr (HD 13531 and HD 61033) and subgiant (HD 148577) stages. Moreover, HD 197210 could be of interest when studying the Sun at $\approx$ 2 Gyr, when the Earth's atmosphere started having a significant amount of oxygen. Our selection method was successful and we were able to identify stars similar to the Sun at different evolutionary stages, which is essential for future research in the search of exoplanets and understand habitability, especially with the advent of the next generation of exoplanet-hunting instruments.

Figures (9)

• Figure 1: Final result of the atmospheric parameters iteration for the ZAMS candidate HD 13531, using the EW measurements from a ESPaDOnS spectrum (SNR = 310). Upper panel: No trend between the determined A(Fe I) abundances and the excitation potential. Middle panel: No trend between the A(Fe I) abundances and the reduced equivalent width log(EW/$\lambda$) of each line. Lower panel: determined abundances as a function of the wavelength of each spectral line. The blue dots represent the Fe I abundances, while the red dots represent the Fe II abundances. The solid lines show the mean A(Fe) abundance. The dashed lines (which overlaps with the solid line because convergence was achieved) show the linear fits performed. The dotted lines show 2$\sigma$ limits around the mean.
• Figure 2: Effective temperatures $\bm{T}_{eff}$ and surface gravities $\text{log}$$g$ for all candidates. We present the ZAMS candidates in the left panel and the SG candidates in the right panel. Each unique color represents one of the candidates of the sample. The different values for same colored points represent the results obtained for each spectrum of the same star. The error bars show the respective uncertainties of each atmospheric parameter. The black stars represent the reference atmospheric parameters of the Sun at ZAMS and SG stages respectively.
• Figure 3: Chemical abundances for HD 13531 (ZAMS candidate; top panel), HD 61033 (ZAMS candidate; middle panel) and HD 148577 (SG candidate; lower panel). The dotted-dashed line marks the solar reference ([X/H] = 0.00).
• Figure 4: The output of the isoclassify code for the ZAMS candidate HD 61033. From the upper to lower panels we have the fits for the radius, mass, luminosity and age, respectively. The blue line represents the cumulative distribution function, while the solid red line represents the best value fitted. The dashed red lines represent uncertainties for the best value.
• Figure 5: Distribution of stellar parameters for the entire sample. The panels show histograms of stellar mass, radius, and two age estimates (from isochrone fitting, $\mathrm{Age}_{\mathrm{Iso}}$, and from chromospheric activity, $\mathrm{Age}_{\mathrm{HK}}$). The y-axis in all panels represents the number of stars in each bin. A kernel density estimate (KDE) is overplotted to highlight the underlying distribution trends.