Coordinated AMBER and MIDI observations of the Mira variable RR Aql

We have used near- and mid-infrared interferometry to investigate the pulsating atmosphere and the circumstellar environment of the Mira variable RR Aql. Observations were taken with the VLTI/AMBER (near infrared) and the VLTI/MIDI (mid infrared) instruments. We have obtained a total of 15 MIDI epochs between Apr 9, 2004 and Jul 28, 2007 covering 4 pulsation cycles and one AMBER epoch on Sep 9, 2006 at phase 2.82. This work is also part of an ongoing project of joint VLTI and VLBA observations to study the connection between stellar pulsation and the mass loss process. Here we present a comparison of the AMBER visibility data to a simple uniform disk model as well as to predictions by recent self-excited dynamic model atmospheres. The best fitting photospheric angular diameter of the model atmosphere at phase 2.82 is 9.9 +/- 2.4 mas.Comment: 4 pages, 3 figures, to appear in Proc. of Cool Stars 1

Radio and IR interferometry of SiO maser stars

Radio and infrared interferometry of SiO maser stars provide complementary information on the atmosphere and circumstellar environment at comparable spatial resolution. Here, we present the latest results on the atmospheric structure and the dust condensation region of AGB stars based on our recent infrared spectro-interferometric observations, which represent the environment of SiO masers. We discuss, as an example, new results from simultaneous VLTI and VLBA observations of the Mira variable AGB star R Cnc, including VLTI near- and mid-infrared interferometry, as well as VLBA observations of the SiO maser emission toward this source. We present preliminary results from a monitoring campaign of high-frequency SiO maser emission toward evolved stars obtained with the APEX telescope, which also serves as a precursor of ALMA images of the SiO emitting region. We speculate that large-scale long-period chaotic motion in the extended molecular atmosphere may be the physical reason for observed deviations from point symmetry of atmospheric molecular layers, and for the observed erratic variability of high-frequency SiO maser emissionComment: 8 pages, 4 figures, submitted to Proc. IAU Symp. 287 "Cosmic masers - from OH to H_0", R.S. Booth, E.M.L. Humphreys, W.H.T. Vlemmings (eds.), invited pape

Étude détaillée des pulsations d'étoiles évoluées "AGB" par interférométrie au V.L.T.I.

Cette thèse exploite des mesures interférométriques multi-époques d étoiles évoluées de la branche horizontale asymptotique (AGB) du diagramme HR. Il s agit d étoiles de masses petites à intermédiaires (0.6 10 Mo) en phase finale de leur évolution. Ces étoiles pulsantes subissent une perte de masse très importante via un vent stellaire dense et poussiéreux qui enrichit l environnement stellaire de gaz et de poussières. Ces processus de perte de masse sont encore mal compris et font l objet de beaucoup d investigations. Grâce à leur luminosité et diamètre très importants, ces étoiles sont des cibles privilégiées d observations à Haute Résolution Angulaire. Les étoiles AGB étudiées ici ont été mesurées pendant plusieurs cycles d oscillation au moyen des deux interféromètres AMBER et MIDI du VLTI, donc en proche et moyen infrarouge. Le but de cette investigation est la connexion entre le mécanisme de pulsation, la condensation et la chimie des poussières dans le but de mieux comprendre leur perte de masse. J ai étudié un échantillon de quatre étoiles évoluées, les variables de type Mira riches en oxygène RR Aql, S Ori, Gx Mon et R Cnc, au moyen de donnée MIDI multi-époque et de données AMBER individuelles. Ces données sont modélisées par des modèles standards de disques uniformes ou gaussiens. Les données AMBER sont modélisées par une atmosphère auto-excitée libre de poussière incluant les couches moléculaires juste supérieures responsables du continuum spectral (modèles de série P et M, Ireland et al. 2004a,b). Les données MIDI permettent de rajouter à ce modèle l enveloppe gazeuse radiative. Le transfert radiatif de l enveloppe utilise le code mcsim_mpi (Ohnaka et al., 2006) et prolonge le travail de Wittkowski et al. (2007). Deux types de poussière sont envisagés, silicate et Al O . Les modèles ont été simulés à différentes phases et avec différents paramètres de l enveloppe de poussière pour étudier la variabilité de la photométrie et de la dimension interférométrique en infrarouge moyen. Les visibilité spectrales n ont pas permis, pour aucune de nos étoiles, de mettre en évidence de variation significative intra-cycle ou d un cycle à l autre, dans nos barres d erreur de 5 à 20%. Notre étude montre que les spectres de visibilité et de photométrie des quatre étoiles sont bien décrits par le modèle d enveloppe radiative au-dessus d une source centrale décrite par un modèle d atmosphère dynamique. L enveloppe de poussière optiquement mince de RR Aql est bien modélisée par des grains de silicate. L addition de grains de Al O n améliore pas l ajustement du modèle, mais n exclut pas la présence d une enveloppe interne optiquement plus mince que celle de silicate. GX Mon est bien modélisé par une combinaison de A Al O et de silicates, alors que les enveloppes d nS Ori et R Cncn sont modélisables par des grains de Al O seul sans contribution de silicate. Les rayons inférieurs des enveloppes sont de l ordre de 2 à 2.5 rayons photosphériques pour Al O et de 4 à 5 pour le silicate. Les modélisations effectuées dans ce travail de thèse confirment que les variations inter et intra-cycle attendues ne sont pas détectables dans la précision de nos mesures. Cette étude est la première comparaison entre des mesures interférométriques couvrant plusieurs cycles d oscillation et des modèles couvrant une large gamme de phases des oscillations.This thesis presents multi-epoch interferometric observations of evolved stars on the Asymptotic Giant Branch (AGB). The Asymptotic Giant Branch is populated by low to intermediate mass stars (0.6 10 Mo) in the final stage of their stellar evolution. Pulsating AGB stars suffer from strong mass loss via a dense and dusty outflow from an extended stellar atmosphere returning gas and dust to the interstellar medium. The processes leading to the violent mass loss are not well understood and are therefore under intense investigations. Due to the AGB stars large diameters and high luminosities, evolved stars are ideal targets for high angular resolution observations. The AGB stars presented in this thesis were observed over several pulsation cycles using the near-infrared AMBER and mid-infrared MIDI instruments at the Very Large Telescope Interferometer (VLTI). The goal of this study is to investigate the connection between the pulsation mechanism, the dust chemistry of AGB stars, and the condensation sequence in order to better understand the mass loss process. The thesis studies a sample of four evolved stars, the oxygen-rich Mira variables RR Aql, S Ori, GX Mon, and R Cnc. We obtained multi-epoch MIDI observations and individual AMBER observations. The presented data are modelled using basic models of uniform disks and Gaussian profiles. The AMBER data, along with the basic models, are modelled using dust-free self-excited dynamic model atmospheres (P and M series). These modeles represent the stellar atrmosphere including the continuum forming atmosphere and overlying molecular layers (Ireland et al., 2004b,a). In addition to the basic models, the MIDI data are fitted by a radioactive transfer model of the circumstellar dust shell where the central stellar intensity profile is described by the previously mentioned series of dust-free dynamic model atmospheres, which are based on self-excited pulsation model. The radiative transfer model is computed using the radiative transfer code mcsim_mpi (Ohnaka et al., 2006). The modelling approach follows the work presented by Wittkowski et al. (2007). For all stars, two dust species of silicate and Al O grains were examined. In order to investigate the expected variability of our mid-infrared photometric and interferometric data, model simulations using variations of model phase and dust shell parameters were performed. The observed visibility spectra for all studied stars do not show indications of variations as a function of pulsation phase and cycle within our investigated phases and uncertainties (5%-20%). The observed photometry spectra may indicate intra-cycle and cycle-to-cycle variations with a significance of 1-5 . However, the photometry variations cannot be confirmed within our uncertainties. Our study shows that the photometric and visibility spectra for all stars can be well described by the radiative transfer model of the dust shell that uses a dynamic model atmosphere describing the central source. For all epochs, we found the best fitting models consisting of a dynamic model atmosphere, and dust shells parameters including the optical depth of the dust shell, the inner radii, the power-law index of the density distribution, and the photospheric aangular diameter. The optically thin dust shell did not improve the model fit. However, the model simulations indicate that the presence of an inner Al O dust shell with low optical depth compared to the silicate dust shell can not be excluded. GX Mon can be modelled with a combination of Al O and silicate grains. The dust shell of S Ori and R Cnc can be well modelled using only Al O dust grains without a contribution of silicates. The inner boundary radii of Al O appears around 2-2.5 photospheric radii and the inner moundary radii of silicate appears around 4-5 photospheric radii. The modelling simulations performed in this thesis confirmed that significant intra-cycle and cycle-to-cycle visibility variations are not expected at mid-infrared flux and visibility values are expected to be less that 25% and 20% respectively, and are too low to be detected within the measurement uncertainties. The study represents the first comparison between interferometric observation and a combination of a radiative transfer modelling with dynamic atmosphere models over and extended range of pulsation phases covering several cycles.NICE-BU Sciences (060882101) / SudocSudocFranceF