Prehospital Care: A Search for Indication?

Gillet, Jean Bernard

doi:10.1007/978-3-642-83737-1_52

Cited by 2 publications

(2 citation statements)

References 24 publications

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“…A secondary UV-excess may be present around ϕ = 0.7. This was clearly observed in non-Blazhko stars X Ari, RX Eri, and RR Cet (Gillet & Crowe 1988;Gillet et al 1989;Burki & Meylan 1986). These excesses occur during the hump (ϕ = 0.9) and the bump (ϕ = 0.7) of the light curve.…”

Section: Uv-excessesmentioning

confidence: 77%

Atmospheric dynamics in RR Lyrae stars

Gillet

2013

A&A

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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.

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Section: Uv-excessesmentioning

confidence: 77%

Atmospheric dynamics in RR Lyrae stars

Gillet

2013

A&A

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“…The anomalous features of hydrogen absorption lines (emission, line doubling) in the spectra of RRab variables have been known for a long time (Struve 1947;Sanford 1949), and were attributed to the existence of shock waves at certain phases during the pulsation cycle, while there is no evidence to date of shock waves in the atmosphere of RRc stars. Studying this phenomenon requires spectroscopic observations with both high spectral and time resolution as well as high S/N, and until quite recently it could only be done on a small number of the nearest RRab variables with photographic spectra (Preston 1964;Preston & Paczynski 1964;Preston et al 1965;Oke 1966), and later with electronic light detectors (Gillet & Crowe 1988;Gillet et al 1989). The most accurate and detailed studies, however, were obviously done during the last decade thanks to the use of more advanced observation technology (Chadid et al 2008;Kolenberg et al 2010;For et al 2011;Preston 2011).…”

Section: Use Of Static Model Atmospheresmentioning

confidence: 99%

Chemical abundances of solar neighbourhood RR Lyrae stars★

Pancino

Britavskiy

Romano

et al. 2015

Monthly Notices of the Royal Astronomical Society

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We have analysed a sample of 18 RR Lyrae stars (17 fundamental-mode -RRaband one first overtone -RRc) and three Population II Cepheids (two BL Her stars and one W Vir star), for which high-resolution (R 30 000), high signal-to-noise (S/N 30) spectra were obtained with either SARG at the Telescopio Nazionale Galileo (La Palma, Spain) or UVES at the ESO Very Large Telescope (Paranal, Chile). Archival data were also analyzed for a few stars, sampling 3 phases for each star. We obtained atmospheric parameters (T eff , logg, v t , and [M/H]) and abundances of several iron-peak and α-elements (Fe, Cr, Ni, Mg, Ca, Si, and Ti) for different pulsational phases, obtaining [α/Fe] =+0.31±0.19 dex over the entire sample covering -2.2<[Fe/H]<-1.1 dex. We find that silicon is indeed extremely sensitive to the phase, as reported by previous authors, and cannot be reliably determined. Apart from this, metallicities and abundance ratios are consistently determined, regardless of the phase, within 0.10-0.15 dex, although caution should be used in the range 0 φ 0.15. Our results agree with literature determinations for both variable and non-variable field stars, obtained with very different methods, including low and high-resolution spectroscopy. W Vir and BL Her stars, at least in the sampled phases, appear indistinguishable from RRab from the spectroscopic analysis point of view. Our large sample, covering all pulsation phases, confirms that chemical abundances can be obtained for RR Lyrae with the classical EW-based technique and static model atmospheres, even rather close to the shock phases.

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Prehospital Care: A Search for Indication?

Cited by 2 publications

References 24 publications

Atmospheric dynamics in RR Lyrae stars

Atmospheric dynamics in RR Lyrae stars

Chemical abundances of solar neighbourhood RR Lyrae stars★

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