Helium lines in RR Lyrae spectra (Research Note)

Context. During a comparative study of atmospheric phenomena that occur during the pulsation cycles of 10 RRab stars I unexpectedly found helium lines in emission and/or absorption in all 10 stars during rising light. The progression of events in the time-evolution of the helium spectrum differs in detail from star to star, but 9 of the 10 stars share a number of general characteristics, illustrated approximately by the behavior of RV Oct described in this Note. Aims. My long-term aim is to provide a comprehensive empirical description of atmospheric phenomena that take place during the pulsation cycles of RRab stars. The short-term goal of this paper is to inform readers that measurements of helium lines place new constraints on shock-wave models for RRab stars. Methods. Results presented here are based on measurements and analysis of observations made with the echelle spectrograph of the du Pont 2.5-m telescope at Las Campanas Observatory. Results. A general pattern of behavior emerges albeit with significant star-to-star variation. He I lambda 5376 (D3) appears first as a pure emission feature, unlike the Balmer lines for which the emission profile is always overlain initially by strong redward-displaced absorption. The phase of maximum D3 emission coincides approximately with that of Ha, but the duration of conspicuous D3 emission ( in excess of the local continuum) is less than 15 mn, approximately half the duration of similarly defined Ha emission. The emission phase is followed by the appearance, first, of redward-displaced absorption, and shortly thereafter by a second violet-displaced absorption component that strengthens with advancing phase. Both absorption components gradually weaken and disappear after radial-velocity minimum, which lags luminosity maximum by no more than 0.01P as discussed in Sect. 3. However, bona fide He I lambda 5876 emission flux in excess of the local continuum flux reappears near maximum light for some stars in the sample and persists for about 45 mn. The radial velocities of the initial and final He I lambda 5876 emission features differ little from the time-average value of the metallic-line radial velocities for RV Octantis. However, the absorption features appear to distort the location of the emission centroid, when they are strong.