D. Gillet

The SOPHIE spectrograph: design and technical key-points for high throughput and high stability

SOPHIE is a new fiber-fed echelle spectrograph in operation since October 2006 at the 1.93-m telescope of Observatoire de Haute-Provence. Benefiting from experience acquired on HARPS (3.6-m ESO), SOPHIE was designed to obtain accurate radial velocities (~3 m/s over several months) with much higher optical throughput than ELODIE (by a factor of 10). These enhanced capabilities have actually been achieved and have proved invaluable in asteroseismology and exoplanetology. We present here the optical concept, a double-pass Schmidt echelle spectrograph associated with a high efficiency coupling fiber system, and including simultaneous wavelength calibration. Stability of the projected spectrum has been obtained by the encapsulation of the dispersive components in a constant pressure tank. The main characteristics of the instrument are described. We also give some technical details used in reaching this high level of performance.

Atmospheric dynamics in RR Lyrae stars - The Blazhko effect

Context. Discovered in 1907, the Blazhko effect is a modulation of the light variations of about half of the RR Lyr stars. It has remained unexplained for over 100 years, despite more than a dozen proposed explanations. Today it represents an ongoing challenge in variable-star research. Aims. We propose a new explanation. It is based on the observation that Blazhko stars seem to be located in the region of the instability strip where fundamental and first overtone modes are excited at the same time. Methods. An analysis of nonlinear and nonadiabatic pulsation models of RR Lyrae stars shows that a specific shock (called first overtone shock) may be generated by the perturbation of the fundamental mode by the transient first overtone. Results. The first overtone shock induces a sharp slowdown of the atmospheric layers during their infalling motion. This slowdown in turn affects the compression rate on the deep photospheric layers and the intensity of the κ-mechanism. After an amplification phase, the intensity of the main shock wave before the Blazhko maximum becomes high enough to provoke large radiative losses. These can be at least equal to 70% of the total energy flux of the shock, which induces a small decrease of the effective temperature at each pulsation cycle. In these conditions, when the intensity of the main shock reaches its highest critical value at the Blazhko maximum, it completely desynchronizes the motion of the phostospheric layers. At this point, the atmosphere relaxes and reaches a new synchronous state that occurs at the Blazhko minimum. Conclusions. The combined effects of these two shocks on the atmosphere cause the Blazhko effect. This effect can only exist if the first overtone mode is excited together with the fundamental mode. Because the involved physical mechanisms are essentially nonlinear (shocks, atmospheric dynamics, radiative losses, mode excitations), the Blazhko process is expected to be unstable and irregular. Consequently, the Blazhko process has a specific random nature that is in contrast with the pulsation of non-Blazhko stars.

The structure of radiative shock waves V. Hydrogen emission lines.

We considered the structure of steady-state plane-parallel radiative shock waves propagating through the partially ionized hydrogen gas of temperature T1 = 3000 K and density 10 −12 gc m −3 ≤ ρ1 ≤ 10 −9 gc m −3 . The upstream Mach numbers range within 6 ≤ M1 ≤ 14. In frequency intervals of hydrogen lines the radiation field was treated using the transfer equation in the frame of the observer for the moving medium, whereas the continuum radiation was calculated for the static medium. Doppler shifts in Balmer emission lines of the radiation flux emerging from the upstream boundary of the shock wave model were found to be roughly one-third of the shock wave velocity: −δV ≈ 1 U1. The gas emitting the Balmer line radiation is located at the rear of the shock wave in the hydrogen recombination zone where the gas flow velocity in the frame of the observer is approximately one-half of the shock wave velocity: −V ∗ ≈ 1 U1. The ratio of the Doppler shift to the gas flow velocity of δV/V ∗ ≈ 0.7 results both from the small optical thickness of the shock wave in line frequencies and the anisotropy of the radiation field typical for the slab geometry. In the ambient gas with density of ρ1 > 10 −11 gc m −3 the flux in the Hα frequency interval reveals the double structure of the profile. A weaker Hβ profile doubling was found for ρ1 10 −10 gc m −3 and U1 < 50 km s −1 . The unshifted redward component of the double profile is due to photodeexcitation accompanying the rapid growth of collisional ionization in the narrow layer in front of the discontinuous jump.

The structure of radiative shock waves - IV. Effects of electron thermal conduction

We consider the structure of steady–state radiative shock waves propagating in partially ionized hydrogen gas with density

Non-linear radiative models of post-AGB stars: Application to HD 56126

\rho_1 = 10^{-10}~{\rm gm}~{\rm cm}^{-3}

Hypersonic shock wave evidence in RR Lyrae stars - First detection of neutral line disappearance in S Arae

and temperature

The structure of radiative shock waves - III. The model grid for partially ionized hydrogen gas

3000~{\rm K}\le T_1\le 8000~{\rm K}

Emission lines and shock waves in RR Lyrae stars

. The radiative shock wave models with electron thermal conduction in the vicinity of the viscous jump are compared with pure radiative models. The threshold shock wave velocity above which effects of electron thermal conduction become perceptible is found to be

Shock-induced polarized hydrogen emission lines in the Mira star o Ceti

U_1^*\approx 70~{\rm km}~{\rm s}^{-1}

First detection of helium emissions in RR Lyrae

and corresponds to the upstream Mach numbers from