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M dwarfs are prime targets for planet search programs, particularly of thosefocused on the detection and characterization of rocky planets in the habitablezone. Understanding their magnetic activity is important because it affects ourability to detect small planets, and it plays a key role in thecharacterization of the stellar environment. We analyze observations of the CaII H&K and H{\alpha} lines as diagnostics of chromospheric activity forlow-activity early-type M dwarfs. We analyze the time series of spectra of 71early-type M dwarfs collected for the HADES project for planet search purposes.The HARPS-N spectra provide simultaneously the H&K doublet and the H{\alpha}line. We develop a reduction scheme able to correct the HARPS-N spectra forinstrumental and atmospheric effects, and to provide flux-calibrated spectra inunits of flux at the stellar surface. The H&K and H{\alpha} fluxes are comparedwith each other, and their variability is analyzed. We find that the H and Kflux excesses are strongly correlated with each other, while the H{\alpha} fluxexcess is generally less correlated with the H&K doublet. We also find thatH{\alpha} emission does not increase monotonically with the H&K line flux,showing some absorption before being filled in by chromospheric emission whenH&K activity increases. Analyzing the time variability of the emission fluxes,we derive a tentative estimate of the rotation period (of the order of a fewtens of days) for some of the program stars, and the typical lifetime ofchromospheric active regions (a few stellar rotations). Our results are in goodagreement with previous studies. In particular, we find evidence that thechromospheres of early-type M dwarfs could be characterized by differentfilaments coverage, affecting the formation mechanism of the H{\alpha} line. Wealso show that chromospheric structure is likely related to spectral type.

astronomy and astrophysics

The HADES RV Programme with HARPS-N@TNG IV. Time resolved analysis of  the Ca ii H&K and H{\alpha} chromospheric emission of low-activity early-type  M dwarfs