| 1. |
- Paladini, C, et al.
(author)
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Interferometric properties of pulsating C-rich AGB stars Intensity profiles and uniform disc diameters of dynamic model atmospheres
- 2009
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 501:3, s. 1073-1085
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Journal article (peer-reviewed)abstract
- Aims. On the basis of a set of dynamic model atmospheres of C-rich AGB stars, we present the first theoretical study of centre-to-limb variation (CLV) properties and relative radius interpretation on narrow and broad-band filters. We computed visibility profiles and the equivalent uniform disc (UD) radii to investigate the dependence of these quantities on the wavelength and pulsation phase. Methods. After an accurate morphological analysis of the visibility and intensity profiles determined in narrow and broad-band filters, we fitted our visibility profiles with a UD function simulating the observational approach. The UD-radii were computed using three different fitting-methods to investigate the influence of the visibility sampling profile: single point, two points and a least squares method. Results. The intensity and visibility profiles of models characterises by mass loss show a behaviour very different from a UD. We found that UD-radii are wavelength dependent and that this dependence is stronger if mass loss is present. Strong opacity contributions from C2H2 affect all radius measurements at 3 mu m and in the N-band, resulting in higher values for the UD-radii. In the case of models with mass loss the predicted behaviour of UD-radii versus phase is complicated, while the radial changes are almost sinusoidal for models without mass loss. Compared to the M-type stars, for the C-stars no windows are available for measuring the pure continuum.
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| 2. |
- Ramstedt, Sofia, et al.
(author)
-
The wonderful complexity of the Mira AB system
- 2014
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 570, s. Art. no. L14-
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Journal article (peer-reviewed)abstract
- We have mapped the (CO)-C-12(3-2) line emission around the Mira AB system at 0 ''.5 resolution using the Atacama Large Millimeter/submillimeter Array (ALMA). The CO map shows amazing complexity. The circumstellar gas has been shaped by different dynamical actors during the evolution of the system, and several morphological components can be identified. The companion is marginally resolved in continuum emission and is currently at 0 ''.487 +/- 0 ''.006 separation. In the main line component, centered on the stellar velocity, spiral arcs around Mira A are found. The spiral appears to be relatively flat and oriented in the orbital plane. An accretion wake behind the companion is clearly visible, and the projected arc separation is about 5 ''. In the blue wing of the line emission, offset from the main line, several large (similar to 5-10 '') opposing arcs are found. We tentatively suggest that this structure is created by the wind of Mira B blowing a bubble in the expanding envelope of Mira A.
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| 3. |
- Wittkowski, M., et al.
(author)
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CalVin 3 - A New Release of the ESO Calibrator Selection Tool for the VLT Interferometer
- 2011
-
In: The Messenger. - : ESO. ; 145, s. 7-9
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Journal article (peer-reviewed)abstract
- Interferometric observations require frequent measurements of calibration stars of known diameter to estimate the instrumental transfer function. ESO offers the preparation tool CalVin to select suitable calibrators from an underlying list of calibrators. The latest version 3, first released in January 2011, offers major improvements in the number of available calibrators, the functionality of the search tool, as well as in terms of performance and ease of use. It has been developed in a collaboration between ESO and the French Jean-Marie Mariotti Center (JMMC).
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| 4. |
- Chiavassa, A., et al.
(author)
-
Radiative hydrodynamic simulations of red supergiant stars III. Spectro-photocentric variability, photometric variability, and consequences on Gaia measurements
- 2011
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 528, s. A120-
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Journal article (peer-reviewed)abstract
- Context. It has been shown that convection in red supergiant stars (RSG) gives rise to large granules that cause surface inhomogeneities and shock waves in the photosphere. The resulting motion of the photocentre (on time scales ranging from months to years) could possibly have adverse effects on the parallax determination with Gaia. Aims. We explore the impact of the granulation on the photocentric and photometric variability. We quantify these effects in order to better characterise the error that could possibly alter the parallax. Methods. We use 3D radiative-hydrodynamics (RHD) simulations of convection with CO5BOLD and the post-processing radiative transfer code Optim3D to compute intensity maps and spectra in the Gaia G band [325-1030 nm]. Results. We provide astrometric and photometric predictions from 3D simulations of RSGs that are used to evaluate the possible degradation of the astrometric parameters of evolved stars derived by Gaia. We show in particular from RHD simulations that a supergiant like Betelgeuse exhibits a photocentric noise characterised by a standard deviation of the order of 0.1 AU. The number of bright giant and supergiant stars whose Gaia parallaxes will be altered by the photocentric noise ranges from a few tens to several thousands, depending on the poorly known relation between the size of the convective cells and the atmospheric pressure scale height of supergiants, and to a lower extent, on the adopted prescription for galactic extinction. In the worst situation, the degradation of the astrometric fit caused by this photocentric noise will be noticeable up to about 5 kpc for the brightest supergiants. Moreover, parallaxes of Betelgeuse-like supergiants are affected by an error of the order of a few percents. We also show that the photocentric noise, as predicted by the 3D simulation, does account for a substantial part of the supplementary "cosmic noise" that affects Hipparcos measurements of Betelgeuse and Antares.
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| 5. |
- Cruzalebes, P., et al.
(author)
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Fundamental parameters of 16 late-type stars derived from their angular diameter measured with VLTI/AMBER(star)
- 2013
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 434:1, s. 437-450
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Journal article (peer-reviewed)abstract
- Thanks to their large angular dimension and brightness, red giants and supergiants are privileged targets for optical long-baseline interferometers. 16 red giants and supergiants have been observed with the VLTI/AMBER facility over a 2-year period, at medium spectral resolution (R = 1500) in the K band. The limb-darkened angular diameters are derived from fits of stellar atmospheric models on the visibility and the triple product data. The angular diameters do not show any significant temporal variation, except for one target: TX Psc, which shows a variation of 4 per cent using visibility data. For the eight targets previously measured by long-baseline interferometry (LBI) in the same spectral range, the difference between our diameters and the literature values is less than 5 per cent, except for TX Psc, which shows a difference of 11 per cent. For the eight other targets, the present angular diameters are the first measured from LBI. Angular diameters are then used to determine several fundamental stellar parameters, and to locate these targets in the Hertzsprung-Russell diagram (HRD). Except for the enigmatic Tc-poor low-mass carbon star W Ori, the location of Tc-rich stars in the HRD matches remarkably well the thermally-pulsating asymptotic giant branch, as it is predicted by the stellar evolution models. For pulsating stars with periods available, we compute the pulsation constant and locate the stars along the various sequences in the period-luminosity diagram. We confirm the increase in mass along the pulsation sequences, as predicted by theory, except for W Ori which, despite being less massive, appears to have a longer period than T Cet along the first-overtone sequence.
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| 6. |
- Cruzalebes, P., et al.
(author)
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SPIDAST : a new modular software to process spectrointerferometric measurements
- 2013
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 432:2, s. 1658-1671
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Journal article (peer-reviewed)abstract
- Extracting stellar fundamental parameters from Spectrointerferometric (SPI) data requires reliable estimates of observables and with robust uncertainties (visibility, triple product, phase closure). A number of fine calibration procedures are necessary throughout the reduction process. Testing departures from centrosymmetry of brightness distributions is a useful complement. Developing a set of automatic routines called spidast (made available to the community) to reduce, calibrate and interpret raw data sets of instantaneous spectrointerferograms at the spectral channel level, we complement (and in some respects improve) the ones contained in the amdlib Data Reduction Software. Our new software spidast is designed to work in an automatic mode, free from subjective choices, while being versatile enough to suit various processing strategies. spidast performs the following automated operations: weighting of non-aberrant SPI data (visibility, triple product), fine spectral calibration (subpixel level), accurate and robust determinations of stellar diameters for calibrator sources (and their uncertainties as well), correction for the degradations of the interferometer response in visibility and triple product, calculation of the centrosymmetry parameter from the calibrated triple product, fit of parametric chromatic models on SPI observables, to extract model parameters. spidast is currently applied to the scientific study of 18 cool giant and supergiant stars, observed with the VLTI/AMBER facility at medium resolution in the K band. Because part of their calibrators have no diameter in the current catalogues, spidast provides new determinations of the angular diameters of all calibrators. Comparison of spidast final calibrated observables with amdlib determinations shows good agreement, under good and poor seeing conditions.
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| 7. |
- Klotz, D., et al.
(author)
-
Geometrical model fitting for interferometric data : GEM-FIND
- 2012
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In: Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series. - : SPIE.
-
Conference paper (peer-reviewed)abstract
- We developed the tool GEM-FIND that allows to constrain the morphology and brightness distribution of ob- jects. The software fits geometrical models to spectrally dispersed interferometric visibility measurements in the N-band using the Levenberg-Marquardt minimization method. Each geometrical model describes the bright- ness distribution of the object in the Fourier space using a set of wavelength-independent and/or wavelength- dependent parameters. In this contribution we numerically analyze the stability of our nonlinear fitting approach by applying it to sets of synthetic visibilities with statistically applied errors, answering the following questions: How stable is the parameter determination with respect to (i) the number of uv-points, (ii) the distribution of points in the uv-plane, (iii) the noise level of the observations?
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| 8. |
- Olofsson, H., et al.
(author)
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A Possible Solution to the Mass-Loss Problem in M-type AGB Stars
- 2011
-
In: Why Galaxies Care about AGB Stars II.
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Conference paper (peer-reviewed)abstract
- Mass loss is a fundamental, observationally well-established feature of AGB stars, but many aspects of this process are not yet understood. To date, self-consistent dynamical models of dust-driven winds reproducing the observed mass-loss rates seem only possible for M-type stars if the grains in the close circumstellar environment grow to larger sizes than previously assumed. In order to study the grain-size distribution where the mass loss is initiated, high-spatial-resolution interferometry observations are necessary. We have observed two M-type stars using the VLTI/MIDI instrument to constrain the dust-grain sizes through modeling the 10 μm silicate feature. Here we present preliminary results.
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| 9. |
- Paladini, C., et al.
(author)
-
To be or not to be asymmetric? VLTI and the mass loss geometry of red giants
- 2012
-
In: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9780819491466 ; 8445, s. Art. no. 84451R-
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Conference paper (peer-reviewed)abstract
- The mass-loss process is a key ingredient for our understanding in many fields of astrophysics, including stellar evolution and the enrichment of the interstellar medium (ISM) via stellar yields. We combined the capability of the VLTI/MIDI and VLT/VISIR instruments with very recent Herschel/PACS observations to characterize the geometry of mass loss from evolved red giants on the Asymptotic Giant Branch (AGB) at various scales. This paper describes the sample of objects, the observing strategy, the tool for the interpretation, and preliminary MIDI results for two targets: U Ant and θ Aps.
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| 10. |
- Paladini, C., et al.
(author)
-
VLTI/MIDI Large Program: AGB Stars at Different Spatial Scales
- 2015
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In: Conference on Why Galaxies Care About AGB Stars III: A Closer Look in Space and Time, Vienna, Austria, JUL 28-AUG 01, 2014. - 9781583818794 ; 497, s. 97-102
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Conference paper (peer-reviewed)abstract
- We have observed a sample of Asymptotic Giant Branch (AGB) stars from the Herschel Mass-loss of Evolved StarS (MESS) program with the VLTI MID infrared Interferometric instrument (MIDI). The program aims at providing insight to the atmospheres of those stars, to be able to understand the role of the mass-loss process at different spatial scales. We obtained visibilities and spectra of fourteen objects with different chemistries and variability classes. These observations, together with data we retrieved from the archive, allow us to characterize not only the geometry of the dust forming region, but in some cases also the time variability in the N band. As previously reported in the literature, we confirm the detection of spectroscopic but not interferometric variability. This result has implications on the size of the structures involved in the dust-formation process. We also report two cases of asymmetric structures; the nature of these structures will be clearly identified only with the second generation VLTI instrument MATISSE.
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| 11. |
- Sacuto, S., et al.
(author)
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Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution
- 2011
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 525, s. A42-
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Journal article (peer-reviewed)abstract
- Context. We study the circumstellar environment of the carbon-rich star R Sculptoris using the near- and mid-infrared high spatial resolution observations from the ESO-VLTI focal instruments VINCI and MIDI, respectively. Aims. These observations aim at increasing our knowledge of the dynamic processes in play within the very close circumstellar environment where the mass loss of AGB stars is initiated. Methods. We first compare the spectro-interferometric measurements of the star at different epochs to detect the dynamic signatures of the circumstellar structures at different spatial and spectral scales. We then interpret these data using a self-consistent dynamic model atmosphere to discuss the dynamic picture deduced from the observations. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort is first applied to determining these parameters. Results. Interferometric observations do not show any significant variability effect at the 16 m baseline between phases 0.17 and 0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97 and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find fairly good agreement between the dynamic model and the spectrophotometric data from 0.4 to 25 mu m. The model agrees well with the time-dependent flux data at 8.5 mu m, whereas it is too faint at 11.3 and 12.5 mu m. The VINCI visibility measurements are reproduced well, meaning that the extension of the model is suitable in the K-band. In the mid-infrared, the model has the proper extension to reveal molecular structures of C2H2 and HCN located above the stellar photosphere. However, the windless model used is not able to reproduce the more extended and dense dusty environment. Conclusions. Among the different explanations for the discrepancy between the model and the measurements, the strong nonequilibrium process of dust formation is one of the most probable. The transition from windless atmospheres to models with considerable mass-loss rates occurs in a very narrow range of stellar parameters, especially for the effective temperature, the C/O ratio, and the pulsation amplitude. A denser sampling of such critical regions of the parameter space with additional models might lead to a better representation of the extended structures of low mass-loss carbon stars like R Sculptoris. The complete dynamic coupling of gas and dust and the approximation of grain opacities with the small-particle limit in the dynamic calculation could also contribute to the difference between the model and the data.
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| 12. |
- Shulyak, D., et al.
(author)
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Analysis of surface structures of chemically peculiar stars with modern and future interferometers
- 2014
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In: OPTICAL AND INFRARED INTERFEROMETRY IV. - : SPIE. - 9780819496140
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Conference paper (peer-reviewed)abstract
- Interferometry is a very powerful observational technique known in astronomy for many decades. Its application to main-sequence stars, however, is still limited to only brightest objects. In this work we aim to explore the application of interferometry to a special class of main-sequence stars known as chemically peculiar (CP) stars. These stars demonstrate surface chemical abundance inhomogeneities (spots) that usually cover a considerable part of the stellar surface and induce a pronounced spectral and photometric variability. Interferometry thus has a potential to naturally resolve such spots in single stars, providing unique complementary information about spots sizes and contrasts. By means of numerical experiments we derive the actual interferometric requirements essential for the CP stars research that can be addressed in future instrument development. The first comparison between theoretical predictions and already available observations will also be discussed.
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| 13. |
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