HADES RV Programme with HARPS-N at TNG

IV. Time resolved analysis of the Ca ii H&K and Hα chromospheric emission of low-activity early-type M dwarfs

¹ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
e-mail: gas@oact.inaf.it
² INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ INAF–Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
⁴ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breaña Baja, TF, Spain
⁵ INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁶ Dipartimento di Fisica e Chimica, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁷ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁸ Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
⁹ Institut de Ciències de l’Espai (IEEC-CSIC), Campus UAB, C/ Can Magrans, s/n, 08193 Bellaterra, Spain
¹⁰ Dip. di Fisica e Astronomia Galileo Galilei – Università di Padova, vicolo dell’Osservatorio 2, 35122 Padova, Italy
¹¹ Dept. d’Astronomia i Meteorologia, Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí Franquès 1, 08028 Barcelona, Spain
¹² Reial Acadèmia de Ciències i Arts de Barcelona (RACAB), 08002 Barcelona, Spain

Received: 23 July 2016
Accepted: 18 October 2016

Abstract

Context. M dwarfs are prime targets for current and future planet search programs, particularly those focused on the detection and characterization of rocky planets in the habitable zone. In this context, understanding their magnetic activity is important for two main reasons: it affects our ability to detect small planets and it plays a key role in the characterization of the stellar environment.

Aims. We analyze observations of the Ca ii H&K and Hα lines as diagnostics of chromospheric activity for low-activity early-type M dwarfs.

Methods. We analyze the time series of spectra of 71 early-type M dwarfs collected in the framework of the HADES project for planet search purposes. The HARPS-N spectra simultaneously provide the Ca ii H&K doublet and the Hα line. We develop a reduction scheme able to correct the HARPS-N spectra for instrumental and atmospheric effects, and also to provide flux-calibrated spectra in units of flux at the stellar surface. The Ca ii H&K and Hα fluxes are then compared with each other, and their time variability is analyzed.

Results. We find that the Ca ii H and K flux excesses are strongly correlated with each other, while the Hα flux excess is generally less correlated with the Ca ii H&K doublet. We also find that Hα emission does not increase monotonically with the Ca ii H&K line flux, showing some absorption before being filled in by chromospheric emission when Ca ii H&K activity increases. Analyzing the time variability of the emission fluxes, we derive a tentative estimate of the rotation period (on the order of a few tens of days) for some of the program stars, and the typical lifetime of chromospheric active regions (on the order of a few stellar rotations).

Conclusions. Our results are in good agreement with similar previous studies. In particular, we find evidence that the chromospheres of early-type M dwarfs could be characterized by different filament coverage, affecting the formation mechanism of the Hα line. We also show that chromospheric structure is likely related to spectral type.