Bruno Lopez

THE LAST GASPS OF VY CANIS MAJORIS: APERTURE SYNTHESIS AND ADAPTIVE OPTICS IMAGERY

We present new observations of the red supergiant VY CMa at 1.25, 1.65, 2.26, 3.08, and 4.8 km. Two complementary observational techniques were utilized: nonredundant aperture masking on the 10 m Keck I telescope, yielding images of the innermost regions at unprecedented resolution, and adaptive optics imaging on the ESO 3.6 m telescope at La Silla, attaining an extremely high (D105) peak-to-noise dynamic range over a wide —eld. For the —rst time the inner dust shell has been resolved in the near- infrared to reveal a one-sided extension of circumstellar emission within (D15 of the star. The 0A.1 R * ) line-of-sight optical depths of the circumstellar dust shell at 1.65, 2.26, and 3.08 km have been estimated to be 1.86 ^ 0.42, 0.85 ^ 0.20, and 0.44 ^ 0.11, respectively. These new results allow the bolometric lumi- nosity of VY CMa to be estimated independent of the dust shell geometry, yielding A L * B 2 ) 105 L _ . variety of dust condensations, including a large scattering plume and a bow-shaped dust feature, were observed in the faint, extended nebula up to 4A from the central source. While the origin of the nebulous plume remains uncertain, a geometrical model is developed assuming the plume is produced by radially driven dust grains forming at a rotating —ow insertion point with a rotational period between 1200 and 4200 yr, which is perhaps the stellar rotational period or the orbital period of an unseen companion. Subject headings: circumstellar matterdust, extinctionstars: individual (VY Canis Majoris) ¨ infrared: starstechniques: interferometric

Parsec-scale dust distributions in Seyfert galaxies - Results of the MIDI AGN snapshot survey

Aims. The emission of warm dust dominates the mid-infrared spectra of active galactic nuclei (AGN). Only interferometric observations provide the necessary angular resolution to resolve the nuclear dust and to study its distribution and properties. The investigation of dust in AGN cores is hence one of the main science goals for the MID-infrared Interferometric instrument MIDI at the VLTI. As the first step, the feasibility of AGN observations was verified and the most promising sources for detailed studies were identified. Methods. This was carried out in a “snapshot survey” with MIDI using Guaranteed Time Observations. In the survey, observations were attempted for 13 of the brightest AGN in the mid-infrared which are visible from Paranal. Results. The results of the three brightest, best studied sources have been published in separate papers. Here we present the interferometric observations for the remaining 10, fainter AGN. For 8 of these, interferometric measurements could be carried out. Size estimates or limits on the spatial extent of the AGN-heated dust were derived from the interferometric data of 7 AGN. These indicate that the dust distributions are compact, with sizes on the order of a few parsec. The derived sizes roughly scale with the square root of the luminosity in the mid-infrared, s ∝ √ LMIR, with no clear distinction between type 1 and type 2 objects. This is in agreement with a model of nearly optically thick dust structures heated to T ∼ 300 K. For three sources, the 10 μm feature due to silicates is tentatively detected either in emission or in absorption. Conclusions. The faint AGN of the snapshot survey are at the sensitivity limit of observations with MIDI. Thus, the data set presented here provides a good insight into the observational difficulties and their implications for the observing strategy and data analysis. Based on the results for all AGN studied with MIDI so far, we conclude that in the mid-infrared the differences between individual galactic nuclei are greater than the generic differences between type 1 and type 2 objects.

Smoke signals from IRC+10216. I. Milliarcsecond proper motions of the dust

The results of a seven epoch interferometric imaging study, at wavelengths in the near-infrared K band, of the carbon star IRC+10216 are presented. The use of non- and partially redundant aperture masking techniques on the 10 m Keck I telescope has allowed us to produce images of the innermost regions of the circumstellar dust envelope with unprecedented detail. With roughly twice the resolving power of previous work, the complex asymmetric structures reported within the central 05 (~20 R*) have been imaged at the size scale of the stellar disk itself (~50 mas). A prominent dark lane at a position angle of approximately 120° is suggested to be an optically thick disk or torus of dust which could help to explain IRC +10216s well-known bipolarity at a position angle of ~20°. Observations spanning more than a pulsational cycle (~638 days) have revealed significant temporal evolution of the nebula, including the outward motion of bright knots and clumps. Registering these displacements against the compact bright core, which we tentatively identify as marking the location of the star, has allowed us to determine the apparent angular velocity at a number of points. The magnitudes of the proper motions were found to be in agreement with current estimates of the stellar distance and radial velocity. Higher outflow speeds were found for features with greater separation from the core. This is consistent with acceleration taking place over the region sampled by the measurements; however, alternate interpretations are also presented. Although a number of changes of morphology were found, none were clearly interpreted as the condensation of new dust over the pulsation cycle. Unfortunately, ambiguities associated with the true three-dimensional nature of the nebula weaken a number of our quantitative and qualitative conclusions.

Ten-micron instrument MIDI: getting ready for observations on the VLTI

The mid-infrared interferometric instrument MIDI is currently undergoing testing in preparation for commissioning on the Very Large Telescope Interferometer VLTI at the end of this year 2002. It will perform interferometric observations over the 8 μm - 13 μm wavelength range, with a spatial resolution of 20 milliarcsec, a spectral resolution of up to 250, and an anticipated point source sensitivity of N = 4 mag or 1 Jy for self-fringe tracking, which will be the only observing mode during the first months of operation. We describe the layout of the instrument and the performance during laboratory tests, both for broadband and spectrally resolved observing modes. We also briefly outline the planned guaranteed time observations.

Three-dimensional interferometric, spectrometric, and planetary views of Procyon

Context. Procyon is one of the brightest stars in the sky and one of our nearest neighbours. It is therefore an ideal benchmark object for stellar astrophysics studies using interferometric, spectroscopic, and asteroseismic techniques. Aims. We use a new realistic three-dimensional (3D) radiative-hydrodynamical (RHD) model atmosphere of Procyon generated with the Stagger Code and synthetic spectra computed with the radiative transfer code Optim3D to re-analyze interferometric and spectroscopic data from the optical to the infrared. We provide synthetic interferometric observables that can be validated using observations. Methods. We computed intensity maps from a RHD simulation in two optical filters centered on 500 and 800 nm (Mark III) and one infrared filter centered on 2.2 μm (Vinci). We constructed stellar disk images accounting for the center-to-limb variations and used them to derive visibility amplitudes and closure phases. We also computed the spatially and temporally averaged synthetic spectrum from the ultraviolet to the infrared. We compare these observables to Procyon data. Results. We study the impact of the granulation pattern on center-to-limb intensity profiles and provide limb-darkening coefficients in the optical as well as in the infrared. We show how the convection-related surface structures affect the visibility curves and closure phases with clear deviations from circular symmetry, from the 3rd lobe on. These deviations are detectable with current interferometers using closure phases. We derive new angular diameters at different wavelengths with two independent methods based on 3D simulations. We find that θ Vinci = 5.390 ± 0.03 mas, which we confirm by comparison with an independent asteroseismic estimation (θ seismic = 5.360 ± 0.07 mas. The resulting T eff is 6591 K (or 6556 K depending on the bolometric flux used), which is consistent with the value of T eff,IR = 6621 K found with the infrared flux method. We measure a surface gravity log g = 4.01 ± 0.03 [cm/s 2] that is higher by 0.05 dex than literature values. Spectrophotometric comparisons with observations provide very good agreement with the spectral energy distribution and photometric colors, allowing us to conclude that the thermal gradient in the simulation matches Procyon fairly well. Finally, we show that the granulation pattern of a planet-hosting Procyon-like star has a non-negligible impact on the detection of hot Jupiters in the infrared using interferometry closure phases. It is then crucial to have a comprehensive knowledge of the host star to directly detect and characterize hot Jupiters. In this respect, RHD simulations are very important to achieving this aim.

High precision Monte Carlo radiative transfer in dusty media

In this paper, we present a refined Monte Carlo method for the solution of multi-dimensional radiative transfer problems in dusty media, for instance in the circumstellar envelopes of AGB and post AGB stars. In order to push the performance of the Monte Carlo method, we have included several basic improvements of the standard procedure which avoid the usual increase of the noise level in extremely optically thin or optically thick situations. The improvements comprise an accurate temperature determination scheme, e.g. based on the calculation of mean intensities, and an inclusion of several deterministic elements for the treatment of the absorption during the photon propagation. The method is capable of accurately modelling discontinuous opacity structures, such as clumpy media, and is applicable in a broad range of optical depths. It can provide the gas and dust temperature structures required for a more complex physical modelling, e.g. hydrodynamics, chemistry or dust formation. It may be used to predict colours, spectral energy distributions, visibilities and monochromatic images. The paper is completed by an exploration of the influence of dust clouds and small holes in dust shells on the spectral energy distribution and their optical appearance in monochromatic images. The underlying temperature structures are discussed.

The molecular and dusty composition of Betelgeuse's inner circumstellar environment

Context. The study of the atmosphere of red supergiant stars in general and of Betelgeuse (α Orionis) in particular is of prime importance to understand dust formation and how mass is lost to the interstellar medium in evolved massive stars. Aims. A molecular shell, the MOLsphere (Tsuji, 2000a), in the atmosphere of Betelgeuse has been proposed to account for the nearand mid-infrared spectroscopic observations of Betelgeuse. The goal is to further test this hypothesis and to identify some of the molecules in this MOLsphere. Methods. We report on measurements taken with the mid-infrared two-telescope beam combiner of the VLTI, MIDI, operated between 7.5 and 13.5� m. The data are compared to a simple geometric model of a photosphere surrounded by a warm absorbing and emitting shell. Physical characteristics of the shell are derived: s ize, temperature and optical depth. The chemical constituents are determined with an analysis consistent with available infrared spectr a and interferometric data. Results. The MIDI data are well modeled with a geometrically thin shell whose radius varies from 1.31 to 1.43 R⋆ across the N band with a typical temperature of 1550 K. We are able to account for the measured optical depth of the shell in the N band, the ISO-SWS spectrum and K and L band interferometric data with a shell whose inner and outer radii are given by the above range and with the following species and densities: H2O (7.1± 4.7× 10 19 cm −2 ), SiO (4.0± 1.1× 10 20 cm −2 ), Al2O3 (2.4± 0.5× 10 15 cm −2 ). Conclusions. These results confirm the MOLsphere model. We bring evidence for more constituents and for the presence of species participating in the formation of dust grains in the atmosphere of the star, i.e. well below the distance at which the dust shell is detected. We believe these results bring key elements to the understanding of mass loss in Betelgeuse and red supergiants in general and bring support to the dust-driven scenario.

Can Life Develop in the Expanded Habitable Zones around Red Giant Stars

We present some new ideas about the possibility of life developing around subgiant and red giant stars. Our study concerns the temporal evolution of the habitable zone. The distance between the star and the habitable zone, as well as its width, increases with time as a consequence of stellar evolution. The habitable zone moves outward after the star leaves the main sequence, sweeping a wider range of distances from the star until the star reaches the tip of the asymptotic giant branch. Currently there is no clear evidence as to when life actually formed on the Earth, but recent isotopic data suggest life existed at least as early as 7 × 108 yr after the Earth was formed. Thus, if life could form and evolve over time intervals from 5 × 108 to 109 yr, then there could be habitable planets with life around red giant stars. For a 1 M☉ star at the first stages of its post-main-sequence evolution, the temporal transit of the habitable zone is estimated to be several times 109 yr at 2 AU and around 108 yr at 9 AU. Under these circumstances life could develop at distances in the range 2-9 AU in the environment of subgiant or giant stars, and in the far distant future in the environment of our own solar system. After a star completes its first ascent along the red giant branch and the He flash takes place, there is an additional stable period of quiescent He core burning during which there is another opportunity for life to develop. For a 1 M☉ star there is an additional 109 yr with a stable habitable zone in the region from 7 to 22 AU. Space astronomy missions, such as proposed for the Terrestrial Planet Finder (TPF) and Darwin, that focus on searches for signatures of life on extrasolar planets, should also consider the environments of subgiants and red giant stars as potentially interesting sites for understanding the development of life. We performed a preliminary evaluation of the difficulty of interferometric observations of planets around red giant stars compared to a main-sequence star environment. We show that pathfinder missions for TPF and Darwin, such as Eclipse and FKSI, have sufficient angular resolution and sensitivity to search for habitable planets around some of the closest evolved stars of the subgiant and red giant class.

AMBER: the near-infrared focal instrument for the Very Large Telescope Interferometer

AMBER is a focal instrument for the Very Large Telescope Interferometer working in the near infrared from 1.1 to 2.4 micrometers . It has been designed having in mind the General User of interferometric observations and the full range of his possible astrophysical programs. However the three programs used to define the key specifications have been the study of Young Stellar Objects, the study of Active Galactic Nuclei dust tori and broad line regions and the measure of masses and spectra of hot Extra Solar Planets. AMBER combines up to three beams produced by the VLTI 8 m Unit Telescopes equipped with Adaptive Optics and/or by the 1.8 m Auxiliary Telescopes. The fringes are dispersed with resolutions ranging from 35 to 10000. It is optimized for high accuracy single mode measurements of the absolute visibility, of the variation of the visibility and phase with wavelength (differential interferometry) and of phase closure relations with three telescopes. The instrument and its software are designed to allow a highly automated user friendly operation and an easy maintenance.

Dust and molecular shells in asymptotic giant branch stars - Mid-infrared interferometric observations of R Aquilae, R Aquarii, R Hydrae, W Hydrae, and V Hydrae

Context. Asymptotic giant branch (AGB) stars are one of the largest distributors of dust into the interstellar medium. However, the wind formation mechanism and dust condensation sequence leading to the observed high mass-loss rates have not yet been constrained well observationally, in particular for oxygen-rich AGB stars. Aims. The immediate objective in this work is to identify molecules and dust species which are present in the layers above the photosphere, and which have emission and absorption features in the mid-infrared (IR), causing the diameter to vary across the Nband, and are potentially relevant for the wind formation. Methods. Mid-IR (8 13 m) interferometric data of four oxygen-rich AGB stars (R Aql, R Aqr, R Hya, and W Hya) and one carbon-rich AGB star (V Hya) were obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally dispersed visibility data are analyzed by fitting a circular fully limb-darkened disk (FDD). Results. The FDD diameter as function of wavelength is similar for all oxygen-rich stars. The apparent size is almost constant between 8 and 10 m and gradually increases at wavelengths longer than 10 m. The apparent FDD diameter in the carbon-rich star V Hya essentially decreases from 8 to 12 m. The FDD diameters are about 2.2 times larger than the photospheric diameters estimated from K-band observations found in the literature. The silicate dust shells of R Aql, R Hya and W Hya are located fairly far away from the star, while the silicate dust shell of R Aqr and the amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to the star at around 8 photospheric radii. Phase-to-phase variations of the diameters of the oxygen-rich stars could be measured and are on the order of 15% but with large uncertainties. Conclusions. From a comparison of the diameter trend with the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars the overall larger diameter originates from a warm molecular layer of H 2 O, and the gradual increase longward of 10 m can be most likely attributed to the contribution of a close Al2O 3 dust shell. The chromatic trend of the Gaussian FWHM in V Hya can be explained with the presence of AMC and SiC dust. The observations suggest that the formation of amorphous Al2O 3 in oxygenrich stars occurs mainly around or after visual minimum. However, no firm conclusions can be drawn concerning the mass-loss mechanism. Future modeling with hydrostatic and self-consistent dynamical stellar atmospheric models will be required for a more certain understanding.