Evolution of Stellar Activity and Habitable Zone II: Ca H&K Emissions of Late-type Dwarfs

TL;DR

This work addresses the longstanding question of how chromospheric activity, as traced by Ca II H&K emissions via $R_{ m{HK}}^{'}$, decays with stellar age for late-type dwarfs. By combining open-cluster ages with a vast field-star sample from LAMOST DR12, the authors compute $S_{ m LAMOST}$ and convert it to $R_{ m{HK}}^{'}$ using established calibrations, then model $\log_{10}(R'_{HK})$ as a function of $\log_{10}({\rm Age})$ with first-, second-, and third-degree polynomials, adjudicating models with Bayesian evidence. They find that a simple linear (Skumanich-type) relation best describes the overall activity–age decline across F/G/K/M types, with the slope dependent on spectral type, and they detect metallicity-dependent offsets for F–K dwarfs but not for M dwarfs; hints of non-uniform decay rates and a possible knee near $1$–$2$ Gyr are discussed but require more data. The results refine age-dating through activity indicators, inform gyrochronology, and reveal mass- and metallicity-dependent nuances in chromospheric evolution, highlighting the value of large, heterogeneous samples that combine clusters with field stars.

Abstract

Stellar chromospheric activity serves as a valuable proxy for estimating stellar ages, though its applicable range and accurate functional form are still debated. In this study, utilizing the LAMOST spectra we compiled a catalog of open cluster members and field stars to investigate $R_{\rm{HK}}^{'}$--age relations across various spectral types. We find that a linear model, specifically a Skumanich-type relation, can best describe the overall decline of chromospheric activity with age, with the slope varying across different spectral types. However, we also identify variations in the decay rate along the main sequence, which call for more accurate follow-up investigation. Finally, we find that lower-metallicity stars exhibit enhanced activity for F-, G-, and K-type stars, whereas no clear metallicity dependence is observed for M dwarfs.

Figures (8)

• Figure 1: Histograms of distances corresponding to open cluster members and field stars across various stellar types. Peak positions are marked using black dashed lines.
• Figure 2: $R_{\rm{HK}}^{'}$--age relations of different kinds of stars. Panel (a), (b), (c), and (d) represents F-, G-, K-, and M-type dwarfs, respectively. Members of open clusters are presented in blue while fields stars are presented in red. Blue and red dots with errorbars are binned median points. Black lines are best-fit models corresponding to maximum a posteriori.
• Figure 3: Comparisons of $R_{\rm{HK}}^{'}-$age relations for G-type stars from different works. Gray dots with errorbars are our G-type star sample. Black lines represent results from our work with different models. Purple, dodgerblue, and orangered line shows the relation from 1999AA...348..897L, 2008ApJ...687.1264M, and 2018AA...619A..73L, respectively. The Sun was shown as black diamond with the $R_{\rm{HK, Sun}}^{'} = -4.91$ and its uncertainties adopted from 2008ApJ...687.1264M.
• Figure 4: Histograms of log$_{10}(R_{\rm{HK}}^{'})$ of solar twins. Different panels represent different age and distance ranges. Shaded areas mark the solar log$_{10}(R_{\rm{HK}}^{'})$ range adopted from 2008ApJ...687.1264M.
• Figure 5: Median curves of $R_{\rm{HK}}^{'}$--age plane corresponding to different stellar types for field stars. Different colors represent different metallicity ranges.