arXiv:1108.5682v1 [astro-ph.SR] 29 Aug 2011 2 naciaRsac tto,SuhPl AF Antarcti TAAF, Pole South France Station, Research 02, Antarctica Cedex (2) Nice Sophi 06108 Universit´e Nice Valrose, Cˆote d’Azur, Parc la de Observatoire (1) e pcr o erePetno h caino i birthd his of occasion Chadid Merieme the on Preston George Magnetic for and spectra detection few line A Helium waves, Shock Hypersonic Lyrae RR of Atmospheres Observatories) Carnegie (Pasadena: the McWilliam and A. Stars, ed. Metal-Poor 5: Stars, Vol. Lyrae RR Series, Astrophysics Observatories Carnegie hn3.TeHIln per rta neiso iead occurs and, line He emission th the with by an followed simultaneously period, as occurs pulsation greate which first the number phenomenon of appears doubling Mach % 2 absorption line a of interval reach HeI time a can The during and S emissio in strong 30. weak shock extremely a hypersonic than the as is with appears shock connected II strongly The He is origin The its Arae. and S of origin Blazhko the hypersonic line at and emission 2011). are (2009, helium shocks Preston the hypersonic by with detected the connected doubling and how formation th out line In point helium (1997). will amplificat modulation turbulence I Blazhko sta the the work, lyrae RR with and of 1996b) connected (Chadid atmosphere are hyper mechanism the they of in strongly 2008) existence how & and the 1996 found on (Chadid waves 1965) unexpectedly shock & I 1964 wor 1962, phenomena, started (1961, spheric I works st intensive/comparative spectroscopic When during like Preston’s and with results. would wo atmospheres, Lyrae Preston’s I spectro-polarimetric RR of on But, and ing line spectroscopic result the Pole. my follow photometric –South greatly consistently polar Charlie how Lyrae Dome RR ce show the to to to report expeditions Conference L not the RR Preston will of and George observations I of during birthday. results presented some (Chadid his the night report I will polar of that I the knowledge stars Here even our during 2010). awake improving al. up et in us keeping amazing and highly Universe remain they tury, ee ilpeetnwnurladsnl oie eimlnsin lines helium ionized single and neutral new present will I Here, cen a than more for studied been have stars Lyrae RR the Although 1 , 2 Abstract -nioi,UR6525, UMR a-Antipolis, ca fields ay lebrate atmo- udies sonic yrae ion rks k- rs e. is n e s r I - 30 M. Chadid

neutral metallic absorption line phenomenon during a time interval of about 8 % of the pulsation period. Despite the fact that several authors claimed to detect a magnetic field, Preston (1967) reported the absence of any strong magnetic field after two years of observations. This is consistent with my recent spectro-polarimetric measurements (2004), over four years, concluding that RR Lyr is a bona fide nonmagnetic star, leading to the falsifica- tion of all the models of the Blazhko effect requiring strong photospheric magnetic fields. Here, I will report on a new series of high precision circu- lar polarization spectroscopic observations of the Blazhko star RV UMa. The longitudinal field measurements of RVUMa show a mean longitu- dinal magnetic field value of Bℓ = −13 ± 35 G. None of measurements of RVUMa shows a significant detection over a whole pulsation period. Finally, all these spectroscopic results reveal a serious challenge for ex- plaining the complex behavior in the pulsation of hypersonic atmosphere of RR lyrae stars.

1. Introduction

Helium & Hydrogen emissions and double metallic absorption lines were first ob- served in W Virginis stars by Sanford in the 1940s. Schwarzschild (1952) invoked the propagation of shock waves across the atmosphere to explain the double absorption lines during rising light. The first quantitative model of shock waves explaining the Hydrogen emissions was developed by Whitey (1956) as reported by Wallerstein (1959) who rediscovered the emission lines of the He I. He I emission lines were used to confirm the shock model since they indicate that the high temperature behind the shock was sufficient to ionize Helium once, and then Hydrogen. In RR lyrae stars, the first detection of H–emissions and the double H–absorption lines were reported by Sanford (1949). Iroshnikov (1962) interpreted the observations of metallic and hydrogen line radial velocities, splitting of the spectral lines and hydrogen emission lines of RR Lyr made by Sanford (1949) with the help of a model in which the shock wave propagates through an exothermic gas in the atmosphere of RR Lyr. He concluded that the discontinuity in the curve, the emission and double absorption hydrogen lines, at the maximum of the light curve, would be the consequence of shock waves propagating through the atmosphere of RR Lyr. The best spectroscopic observations had been made half a century ago by George Preston and his collaborators (Preston 1961 & 1962; Preston and Paczynski 1964; and Preston et al. 1965). Using a spectrogram (10.7 Amm˚ −1) attached to the 3 m telescope of Lick Observatory, they rediscovered the H–emissions which occurs only during an in- terval of 4 % of the period, followed by the double H–absorption lines during rising light,

Electronic address: (1) [email protected] Atmosphere of RR Lyrae Stars 31 especially in RR Lyr and X Ari. They explained the phenomena as a consequence of the propagation of a strong shock wave through the atmosphere of RR Lyrae stars. Thanks to the improvements of CDD detectors compared to photographic plates, Chadid and Gillet (1996a) detected for the first time the doubling phenomenon over the ionized metallic absorption lines of RR Lyr by using the ELODIE spectrograph attached to the 2-m telescope at Observatoire de Haute Provence, France. They interpreted the phenomenon as the consequence of a “two–step” Schwarzschild mechanism. The shock is at first receding from the observer, becomes stationary and then advances. Later, they confirmed the metallic line doubling phenomenon in the one of largest amplitude RR ab stars, S Arae, and they detected for the first time a new mechanism, the neu- tral absorption line disappearance phenomenon during rising light, occurring during an interval of 8 % of pulsation (Chadid et al. 2008) using UVES at VLT-UT2. They interpreted the phenomenon as a consequence of an hypersonic shock existence in the photosphere of RR Lyrae stars. “... but no one applied Wallerstein’s ideas about WVirginis to RR lyrae stars during the next half century (Observers & theorists alike). Until, quite by accident... I detect emission or/and absorption lines of Helium in RRlyrae spectra...” George Preston said in a private communication. Thus, Preston (2009) detected for the first time the He I spectrum during rising light of RR Lyrae stars on a time scale of ∼ 2 h. This discovery adds new constraints on the rapidly evolving temperature structure of the atmosphere towards a better understanding of hypersonic shock waves in atmosphere of RR Lyrae stars (Chadid et al. 2008). More than a century after its discovery (Blazhko 1907), the Blazhko effect still remains a big challenge. It greatly complicates our accurate measurements of the atmospheric dynamics of RR Lyrae stars and introduces unavoidable complications in an understanding of stellar pulsation and evolution. George Preston had great adventures with the Blazhko effect, when he tried to understand the motions of his favorite star TYGruis (Preston et al 2006a and 2006b). “...Blazhko just makes it worse. I dislike Blazhko... So many Blazhko vectors aimed at me make me nervous...” George said. One of the models, trying to explain the Blazhko effect, predicts the dependence of the Blazhko amplitude upon the strength of a magnetic field of the order of 1.5 kG (the oblique–dipole rotator model). However, from the observational point of view,