Envelope tomography of long-period variable stars: I. The Schwarzschild mechanism and the Balmer emission lines

This paper is the first one in a series devoted to the study of the dynamics of the atmospheres of long-period variable stars. Results from a two-month-long monitoring of the Mira variables RT Cyg and X Oph around maximum light with the ELODIE spectrograph at the Haute-Provence Observatory are presented. The monitoring covers phases 0.80 to 1.16 for RT Cyg and phases 0.83 to 1.04 for X Oph. The cross-correlation profile of the spectrum of RT Cyg with a K0 III mask confirms that the absorption lines of RT Cyg in the optical domain appear double around maximum light. No line doubling was found in the optical spectrum of X Oph around maximum light, indicating that this feature is not common to all long-period variables. This paper also presents the application to RT Cyg of a new tomographic technique deriving the velocity field across the atmosphere by cross-correlating the optical spectrum with numerical masks constructed from synthetic spectra and probing layers of increasing depths. This technique reveals that both the temporal evolution of the line doubling, and its variation with depth in the atmosphere of RT Cyg, are consistent with the ``Schwarzschild scenario''. This scenario relates the temporal evolution of the red and blue peaks of the double absorption lines to the progression of a shock wave in the atmosphere. The temporal evolution of the Balmer Halpha, H beta, Hgamma and Hdelta emission lines around maximum light is also presented for RT Cyg and X Oph. The velocity variations of Halpha and of the absorption lines are discussed in the framework of two competing models for the formation of Balmer emission lines in long-period variable stars.Comment: 11 pages, 8 figures, Latex, accepted for publication in Astronomy and Astrophysics main journal. Also available at http://www-astro.ulb.ac.be/Html/ps.htm

Crystal nucleation and cluster-growth kinetics in a model glass under shear

Crystal nucleation and growth processes induced by an externally applied shear strain in a model metallic glass are studied by means of nonequilibrium molecular dynamics simulations, in a range of temperatures. We observe that the nucleation-growth process takes place after a transient, induction regime. The critical cluster size and the lag-time associated with this induction period are determined from a mean first-passage time analysis. The laws that describe the cluster growth process are studied as a function of temperature and strain rate. A theoretical model for crystallization kinetics that includes the time dependence for nucleation and cluster growth is developed within the framework of the Kolmogorov-Johnson-Mehl-Avrami scenario and is compared with the molecular dynamics data. Scalings for the cluster growth laws and for the crystallization kinetics are also proposed and tested. The observed nucleation rates are found to display a nonmonotonic strain rate dependency

Understanding the dynamical structure of pulsating stars: The Baade-Wesselink projection factor of the delta Scuti stars AI Vel and beta Cas

Aims. The Baade-Wesselink method of distance determination is based on the oscillations of pulsating stars. The key parameter of this method is the projection factor used to convert the radial velocity into the pulsation velocity. Our analysis was aimed at deriving for the first time the projection factor of delta Scuti stars, using high-resolution spectra of the high-amplitude pulsator AI Vel and of the fast rotator beta Cas. Methods. The geometric component of the projection factor (i.e. p0) was calculated using a limb-darkening model of the intensity distribution for AI Vel, and a fast-rotator model for beta Cas. Then, using SOPHIE/OHP data for beta Cas and HARPS/ESO data for AI Vel, we compared the radial velocity curves of several spectral lines forming at different levels in the atmosphere and derived the velocity gradient associated to the spectral-line-forming regions in the atmosphere of the star. This velocity gradient was used to derive a dynamical projection factor p. Results. We find a flat velocity gradient for both stars and finally p = p0 = 1.44 for AI Vel and p = p0 = 1.41 for beta Cas. By comparing Cepheids and delta Scuti stars, these results bring valuable insights into the dynamical structure of pulsating star atmospheres. They suggest that the period-projection factor relation derived for Cepheids is also applicable to delta Scuti stars pulsating in a dominant radial mode

Exact solutions of basic variational inverse boundary value problem of aerohydrodynamics

The problems of determining the maximum lift attainable on wing profile and the corresponding profile form are considered, in particular, for the ideal incompressible fluid model the construction of exact solutions different from a circle (with non-circle extremal) is reported

Design and optimization of airfoils in non-stalling incompressible flow with a prescribed range of the angle of attack

A simple and reliable method of aerodynamic design of airfoils is proposed for a given range of the angle of attack. The method is based on the solution of a new inverse boundary-value problem of aerohydrodynamics. In the problem the initial velocity distributions are given along the contour of the required airfoil as a function of the arc co-ordinate for extreme values of the angle of attack from a fixed range. We justify a method of choice of these distributions which guarantees a non-stalling flow around the airfoil. The problem of airfoil optimization is also considered. Results of numerical calculations are presented

Upper estimates of airfoil aerodynamic characteristics for a viscous incompressible flow

In the present work the method of the inverse boundary-value problems is used to obtain upper estimates for two main integral airfoil characteristics: lift coefficient Cl and the aerodynamic ratio K in a certain class of airfoils flown by an incompressible flow of viscous fluid at a fixed angle of attack at high Reynolds number (106-107) with the presence of a thin non-separating turbulent boundary layer

Self consistent modelling of the projection factor for interferometric distance determination

Astronomy and Astrophysics, v. 428, p. 131-137, 2004. http://dx.doi.org/10.1051/0004-6361:20041419International audienc

New Metallicities of RR Lyrae Stars in omega Centauri: Evidence for a Non He-Enhanced Metal-Intermediate Population

We present new spectroscopic metal abundances for 74 RR Lyrae stars in omega Cen obtained with FLAMES. The well-known metallicity spread is visible among the RR Lyrae variables. The metal-intermediate (MInt) RR Lyrae stars ([Fe/H] ~ -1.2) are fainter than the bulk of the dominant metal-poor population ([Fe/H] ~ -1.7), in good agreement with the corresponding zero-age horizontal branch models with cosmological helium abundance Y = 0.246. This result conflicts with the hypothesis that the progenitors of the MInt RR Lyrae stars correspond to the anomalous blue main-sequence stars, which share a similar metallicity but whose properties are currently explained by assuming for them a large helium enhancement. Therefore, in this scenario, the coexistence within the cluster of two different populations with similar metallicities ([Fe/H] ~ -1.2) and different helium abundances has to be considered.Comment: 9 pages, 4 figures, accepted for publication by ApJ

A Double-Mode RR Lyrae Star with a Strong Fundamental Mode Component

NSVS 5222076, a thirteenth magnitude star in the Northern Sky Variability Survey, was identified by Oaster as a possible new double-mode RR Lyrae star. We confirm the double-mode nature of NSVS 5222076, supplementing the survey data with new V band photometry. NSVS 5222076 has a fundamental mode period of 0.4940 day and a first overtone period of 0.3668 day. Its fundamental mode light curve has an amplitude twice as large as that of the first overtone mode, a ratio very rarely seen. Data from the literature are used to discuss the location in the Petersen diagram of double-mode RR Lyrae stars having strong fundamental mode pulsation. Such stars tend to occur toward the short period end of the Petersen diagram, and NSVS 5222976 is no exception to this rule.Comment: 14 pages, 4 figures, To be published in the March, 2006, issue of PAS