Flare activity and photospheric analysis of Proxima Centauri^⋆

¹ Main Astronomical Observatory of the National Academy of Sciences, 27 Zabolotnoho, Kyiv, 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 Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
⁵ Consejo Superior de Investigaciones Científicas, CSIC, Spain
⁶ Observatoire Astronomique de l’Université de Genève, 1290 Versoix, Switzerland

Received: 6 March 2017
Accepted: 12 June 2017

Abstract

Context. We present the analysis of emission lines in high-resolution optical spectra of the planet-host star Proxima Centauri (Proxima) classified as a M5.5V.

Aims. We carry out a detailed analysis of the observed spectra to get a better understanding of the physical conditions of the atmosphere of this star.

Methods. We identify the emission lines in a series of 147 high-resolution optical spectra of the star at different levels of activity and compare them with the synthetic spectra computed over a wide spectral range.

Results. Our synthetic spectra computed with the PHOENIX 2900/5.0/0.0 model atmosphere fits the observed spectral energy distribution from optical to near-infrared quite well. However, modelling strong atomic lines in the blue spectrum (3900–4200 Å) requires implementing additional opacity. We show that high-temperature layers in Proxima Centauri consist of at least three emitting parts: a) a stellar chromosphere where numerous emission lines form; we suggest that some emission cores of strong absorption lines of metals form there; b) flare regions above the chromosphere, where hydrogen Balmer lines up to high transition levels (10–2) form; and c) a stellar wind component with V_r = −30 km s^-1 seen in some Balmer lines as blueshifted emission lines. We believe that the observed He line at 4026 Å in emission can be formed in that very hot region.

Conclusions. We show that the real structure of the atmosphere of Proxima is rather complicated. The photosphere of the star is best fit by a normal M5 dwarf spectrum. On the other hand, emission lines form in the chromosphere, flare regions, and extended hot envelope.