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

TL;DR

This study investigates the chromospheric activity of low-activity early-type M dwarfs using time-resolved spectra of Ca II H&K and Hα lines, revealing correlations, variability, and implications for stellar magnetic activity and rotation periods.

Contribution

It introduces a new flux calibration and analysis scheme for HARPS-N spectra, providing detailed insights into chromospheric emission variability and its relation to stellar properties in early M dwarfs.

Findings

H and K flux excesses are strongly correlated.

Hα emission shows complex behavior, not monotonically increasing with H&K flux.

Estimated stellar rotation periods are on the order of tens of days.

Abstract

M dwarfs are prime targets for planet search programs, particularly of those focused on the detection and characterization of rocky planets in the habitable zone. Understanding their magnetic activity is important because it affects our ability to detect small planets, and it plays a key role in the characterization of the stellar environment. We analyze observations of the Ca II H&K and H{\alpha} lines as diagnostics of chromospheric activity for low-activity early-type M dwarfs. We analyze the time series of spectra of 71 early-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 for instrumental and atmospheric effects, and to provide flux-calibrated spectra in units of flux at the stellar surface. The H&K and H{\alpha} fluxes are compared with each other, and their variability is analyzed. We find that the H and K flux excesses are strongly correlated with each other, while the H{\alpha} flux excess is generally less correlated with the H&K doublet. We also find that H{\alpha} emission does not increase monotonically with the H&K line flux, showing some absorption before being filled in by chromospheric emission when H&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 few tens of days) for some of the program stars, and the typical lifetime of chromospheric active regions (a few stellar rotations). Our results are in good agreement with previous studies. In particular, we find evidence that the chromospheres of early-type M dwarfs could be characterized by different filaments coverage, affecting the formation mechanism of the H{\alpha} line. We also show that chromospheric structure is likely related to spectral type.