Temporal changes of the flare activity of Proxima Centauri^★

¹ Main Astronomical Observatory of the National Academy of Sciences of Ukraine, 27 Zanolotnoho, Kyiv 03143, Ukraine
e-mail: yp@mao.kiev.ua
² Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
³ Center for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK
⁴ Observatoire Astronomique de l’Université de Genève, 1290 Versoix, Genève, Switzerland
⁵ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
⁷ Consejo Superior de Investigaciones Científicas, CSIC, Spain
⁸ Taras Shevchenko National University of Kyiv, Kyiv, Ukraine

Received: 17 September 2018
Accepted: 9 May 2019

Abstract

Context. We study temporal variations of the emission lines of H_α, H_ɛ, H and K CaII, D₁ and D₂ NaI, He₄₀₂₆, and He₅₈₇₆ in the HARPS spectra of Proxima Centauri across an extended time of 13.2 yr, from May 27, 2004 to September 30, 2017.

Aims. We analyse the common behaviour and differences in the intensities and profiles of different emission lines in flare and quiet modes of Proxima activity.

Methods. We compare the pseudo-equivalent widths (pEW) and profiles of the emission lines in the HARPS high-resolution (R ~ 115 000) spectra observed at the same epochs.

Results. All emission lines show variability with a timescale of at least 10 min. The strength of all lines except He₄₀₂₆ correlate with H_α. During strong flares the “red asymmetry” appears in the H_α emission line indicating the infall of hot condensed matter into the chromosphere with velocities greater than 100 km s⁻¹ disturbing chromospheric layers. As a result, the strength of the CaII lines anti-correlates with H_α during strong flares. The HeI lines at 4026 and 5876 Å appear in the strong flares. The cores of D₁ and D₂ NaI lines are also seen in emission. During the minimum activity of Proxima Centauri, CaII lines and H_ɛ almost disappear while the blue part of the NaI emission lines is affected by the absorption in the extending and condensing flows.

Conclusions. We see different behaviour of emission lines formed in the flare regions and chromosphere. Chromosphere layers of Proxima Cen are likely heated by the flare events; these layers are cooled in the “non-flare” mode. The self-absorption structures in cores of our emission lines vary with time due to the presence of a complicated system of inward and outward matter flows in the absorbing layers.