Absolute Ca II H & K and H-alpha flux measurements of low-mass stars: Extending $R_{\mathrm{HK}}^{\prime }$ to M dwarfs

Abstract

Context: With the recent surge of planetary surveys focusing on detecting Earth-mass planets around M dwarfs, it is becoming more important to understand chromospheric activity in M dwarfs. Stellar chromospheric calcium emission is typically measured using the $R_{\mathrm{HK}}^{\prime }$ calibrations of Noyes et al. (1984), which are only valid for $0.44\le B-V\le 0.82$. Measurements of calcium emission for cooler dwarfs $B-V\ge 0.82$ are difficult because of their intrinsic dimness in the blue end of the visible spectrum. Aims: We measure the absolute Ca II HK and H$\hat{I}\pm$ flux of a sample of 110 HARPS M dwarfs and also extend the calibration of $R_{\mathrm{HK}}^{\prime }$ to the M dwarf regime using PHOENIX stellar atmosphere models. Methods: We normalized a template spectrum with a high signal-to-noise ratio that was obtained by coadding multiple spectra of the same star to a PHOENIX stellar atmosphere model to measure the chromospheric Ca II HK and H$\hat{I}\pm$ flux in physical units. We used three different $T_{\mathrm{eff}}$ calibrations and investigated their effect on Ca II HK and H$\hat{I}\pm$ activity measurements. We performed conversions of the Mount Wilson S index to $R_{\mathrm{HK}}^{\prime }$ as a function of effective temperature for the range 2300 K $\le T_{\mathrm{eff}}\le$ 7200 K. Last, we calculated continuum luminosity $\ddot{I}‡$ values for Ca II HK and H$\hat{I}\pm$ in the same manner as West & Hawley (2008) for $-1.0\le Fe/H\le +1.0$ in steps of $\hat{I}”Fe/H=0.5$. Results: We compare different $T_{\mathrm{eff}}$ calibrations and find $\hat{I}”T_{\mathrm{eff}}\sim$ several 100 K for mid- to late-M dwarfs. Using these different $T_{\mathrm{eff}}$ calibrations, we establish a catalog of $\log R_{\mathrm{HK}}^{\prime }$ and ${\mathcal{F}}_{\mathrm{H}\hat{\mathrm{I}}\pm }^{\prime }/{\mathcal{F}}_{\mathrm{bol}}$ measurements for 110 HARPS M dwarfs. [abridged]

26 pages, 17 figures, accepted for publication in Astronomy & Astrophysics. Abstract abridged for arXiv submission