First observation of H$\alpha$ redshifted emission in RR Lyr

• Main Authors: Gillet, D, Mauclaire, B, Garrel, T, Lemoult, T, Mathias, Ph, de France, T, Devaux, J-S, Boussier, H, Verilhac, D, Brabant, G, Desbordes, J, Garde, O, Collaboration, the GRRR
• Format: Journal Article
• Language: English
• Published: 11-09-2017
• Subjects: Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.1709.03556

A&A 607, A51 (2017) The so-called H$\alpha$ third emission occurs around pulsation phase
$\varphi$=0.30. It has been observed for the first time in 2011 in some RR
Lyrae stars. The emission intensity is very weak, and its profile is a tiny
persistent hump in the red side-line profile. We report the first observation
of the H$\alpha$ third emission in RR Lyr itself (HD 182989), the brightest RR
Lyrae star in the sky. New spectra were collected in 2013-2014 with the
Aurelie}spectrograph (resolving power R=22$\,$700, T152, Observatoire de
Haute-Provence, France) and in 2016-2017 with the eShel spectrograph
(R=11$\,$000, T035, Observatoire de Chelles, France). In addition, observations
obtained in 1997 with the Elodie spectrograph (R=42$\,$000, T193, Observatoire
de Haute-Provence, France) were reanalyzed. The H$\alpha$ third emission is
clearly detected in the pulsation phase interval $\varphi$=0.188-0.407, that
is, during about 20% of the period. Its maximum flux with respect to the
continuum is about 13%. The presence of this third emission and its strength
both seem to depend only marginally on the Blazhko phase. The physical origin
of the emission is probably due to the infalling motion of the highest
atmospheric layers, which compresses and heats the gas that is located
immediately above the rising shock wave. The infalling velocity of the hot
compressed region is supersonic, almost 50 km$\cdot$s$^{-1}$, while the shock
velocity may be much lower in these pulsation phases. When the H$\alpha$ third
emission appears, the shock is certainly no longer radiative because its
intensity is not sufficient to produce a blueshifted emission component within
the H$\alpha$ profile. At phase $\varphi$=0.40, the shock wave is certainly
close to its complete dissipation in the atmosphere.