$\mathrm{H}\alpha$ chromospheric activity of F-, G-, and K-type stars observed by the LAMOST Medium-Resolution Spectroscopic Survey

Abstract

The distribution of stellar $\mathrm{H}\alpha$ chromospheric activity with respect to stellar atmospheric parameters (effective temperature $T_{\mathrm{eff}}$, surface gravity $\log g$, and metallicity $[Fe/H]$) and main-sequence/giant categories is investigated for the F-, G-, and K-type stars observed by the LAMOST Medium-Resolution Spectroscopic Survey (MRS). A total of 329,294 MRS spectra from LAMOST DR8 are utilized in the analysis. The $\mathrm{H}\alpha$ activity index ($I_{\mathrm{H}\alpha }$) and the $\mathrm{H}\alpha$ $R$-index ($R_{\mathrm{H}\alpha }$) are evaluated for the MRS spectra. The $\mathrm{H}\alpha$ chromospheric activity distributions with individual stellar parameters as well as in the $T_{\mathrm{eff}}$ -- $\log g$ and $T_{\mathrm{eff}}$ -- $[Fe/H]$ parameter spaces are analyzed based on the $R_{\mathrm{H}\alpha }$ index data. It is found that: (1) for the main-sequence sample, the $R_{\mathrm{H}\alpha }$ distribution with $T_{\mathrm{eff}}$ has a bowl-shaped lower envelope with a minimum at about 6200 K, a hill-shaped middle envelope with a maximum at about 5600 K, and an upper envelope continuing to increase from hotter to cooler stars; (2) for the giant sample, the middle and upper envelopes of the $R_{\mathrm{H}\alpha }$ distribution first increase with a decrease of $T_{\mathrm{eff}}$ and then drop to a lower activity level at about 4300 K, revealing different activity characteristics at different stages of stellar evolution; (3) for both the main-sequence and giant samples, the upper envelope of the $R_{\mathrm{H}\alpha }$ distribution with metallicity is higher for stars with $[Fe/H]$ greater than about $-1.0$, and the lowest-metallicity stars hardly exhibit high $\mathrm{H}\alpha$ indices. A dataset of $\mathrm{H}\alpha$ activity indices for the LAMOST MRS spectra analyzed is provided with this paper.

32 pages, 12 figures, 1 table, accepted for publication in Astrophysics and Space Science

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