References of "Lagrange, A.-M"
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See detailSearching for companions down to 2 AU from β Pictoris using the L'-band AGPM coronagraph on VLT/NACO
Absil, Olivier ULg; Milli, J.; Mawet, D. et al

in Astronomy and Astrophysics (2013), 559

Context. The orbit of the giant planet discovered around β Pic is slightly inclined with respect to the outer parts of the debris disc, which creates a warp in the inner debris disc. This inclination ... [more ▼]

Context. The orbit of the giant planet discovered around β Pic is slightly inclined with respect to the outer parts of the debris disc, which creates a warp in the inner debris disc. This inclination might be explained by gravitational interactions with other planets. <BR /> Aims: We aim to search for additional giant planets located at smaller angular separations from the star. <BR /> Methods: We used the new L'-band AGPM coronagraph on VLT/NACO, which provides an exquisite inner working angle. A long observing sequence was obtained on β Pic in pupil-tracking mode. To derive sensitivity limits, the collected images were processed using a principal-component analysis technique specifically tailored to angular differential imaging. <BR /> Results: No additional planet is detected down to an angular separation of 0.''2with a sensitivity better than 5 M[SUB]Jup[/SUB]. Meaningful upper limits (<10 M[SUB]Jup[/SUB]) are derived down to an angular separation of 0.''1, which corresponds to 2 AU at the distance of β Pic. [less ▲]

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See detailAn interferometric study of the Fomalhaut inner debris disk. III. Detailed models of the exozodiacal disk and its origin
Lebreton, J; van Lieshout, R; Augereau, J-C et al

in Astronomy and Astrophysics (2013), 555

Context. Debris disks are thought to be extrasolar analogues to the Solar System planetesimal belts. The star Fomalhaut harbors a cold debris belt at 140AU comparable to the Edgeworth-Kuiper belt, as well ... [more ▼]

Context. Debris disks are thought to be extrasolar analogues to the Solar System planetesimal belts. The star Fomalhaut harbors a cold debris belt at 140AU comparable to the Edgeworth-Kuiper belt, as well as evidence for a warm dust component, unresolved by single-dish telescopes, that is suspected to be a bright analogue to the Solar System's zodiacal dust. Aims. Interferometric observations obtained with the VLTI/VINCI instrument and the Keck Interferometer Nuller have identi fied near- and mid-infrared excesses attributed respectively to hot and warm exozodiacal dust residing in the inner few AU of the Fomalhaut environment. We aim to characterize the properties of this double inner dust belt and to unveil its origin. Methods.We perform parametric modelling of the exozodiacal disk ("exozodi") using the GRaTer radiative transfer code in order to reproduce the interferometric data, complemented by mid- to far-infrared photometric measurements from Spitzer and Herschel. A detailed treatment of sublimation temperatures is introduced to explore the hot population at the size-dependent sublimation rim. We then use an analytical approach to test successively several source mechanisms for the dust and suspected parent bodies. Results. A good fi t to the multi-wavelength data is found by two distinct dust populations: (1) a population of very small (0.01 to 0.5 µ m) and therefore unbound, hot dust grains con ned in a narrow region ( ~0.1 - 0.3 AU) at the sublimation rim of carbonaceous material; (2) a population of bound grains at 2AU that is protected from sublimation and has a larger mass despite its fainter flux level. We propose that the hot dust is produced by the release of small carbon grains following the disruption of dust aggregates that originate from the warm component. A mechanism such as gas braking is required to further con ne the small grains for long enough. In situ dust production could hardly be ensured for the age of the star and we conclude that the observed amount of dust is triggered by intense dynamical activity. Conclusions. Fomalhaut may be representative of exozodis that are currently being surveyed at near and mid-infrared wavelengths worldwide. We propose a framework for reconciling the "hot exozodi phenomenon" with theoretical constraints: the hot component of Fomalhaut is likely the "tip of the iceberg" as it could derive from the more massive, but fainter, warm dust component residing near the ice line. This inner disk exhibits interesting morphology and can be considered a prime target for future exoplanet research. [less ▲]

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See detailHot circumstellar material resolved around β Pic with VLTI/PIONIER
Defrère, D.; Lebreton, J.; Le Bouquin, J.-B. et al

in Astronomy and Astrophysics (2012), 546

Aims: We aim at resolving the circumstellar environment around β Pic in the near-infrared in order to study the inner planetary system (<200 mas, i.e., ~4 AU). Methods: Precise interferometric fringe ... [more ▼]

Aims: We aim at resolving the circumstellar environment around β Pic in the near-infrared in order to study the inner planetary system (<200 mas, i.e., ~4 AU). Methods: Precise interferometric fringe visibility measurements were obtained over seven spectral channels dispersed across the H band with the four-telescope VLTI/PIONIER interferometer. Thorough analysis of interferometric data was performed to measure the stellar angular diameter and to search for circumstellar material. Results: We detected near-infrared circumstellar emission around β Pic that accounts for 1.37% ± 0.16% of the near-infrared stellar flux and that is located within the field-of-view of PIONIER (i.e., ~200 mas in radius). The flux ratio between this excess and the photosphere emission is shown to be stable over a period of 1 year and to vary only weakly across the H band, suggesting that the source is either very hot (≳1500 K) or dominated by the scattering of the stellar flux. In addition, we derive the limb-darkened angular diameter of β Pic with an unprecedented accuracy (θLD= 0.736 ± 0.019 mas). Conclusions: The presence of a small H-band excess originating in the vicinity of β Pic is revealed for the first time thanks to the high-precision visibilities enabled by VLTI/PIONIER. This excess emission is likely due to the scattering of stellar light by circumstellar dust and/or the thermal emission from a yet unknown population of hot dust, although hot gas emitting in the continuum cannot be firmly excluded. [less ▲]

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See detailSearching for faint companions with VLTI/PIONIER. I. Method and first results
Absil, Olivier ULg; Le Bouquin, J.-B.; Berger, J.-P. et al

in Astronomy and Astrophysics (2011), 535

Context. A new four-telescope interferometric instrument called PIONIER has recently been installed at VLTI. It provides improved imaging capabilities together with high precision. <BR /> Aims: We search ... [more ▼]

Context. A new four-telescope interferometric instrument called PIONIER has recently been installed at VLTI. It provides improved imaging capabilities together with high precision. <BR /> Aims: We search for low-mass companions around a few bright stars using different strategies, and determine the dynamic range currently reachable with PIONIER. <BR /> Methods: Our method is based on the closure phase, which is the most robust interferometric quantity when searching for faint companions. We computed the χ[SUP]2[/SUP] goodness of fit for a series of binary star models at different positions and with various flux ratios. The resulting χ[SUP]2[/SUP] cube was used to identify the best-fit binary model and evaluate its significance, or to determine upper limits on the companion flux in case of non-detections. <BR /> Results: No companion is found around <ASTROBJ>Fomalhaut</ASTROBJ>, <ASTROBJ>tau Cet</ASTROBJ> and <ASTROBJ>Regulus</ASTROBJ>. The median upper limits at 3σ on the companion flux ratio are respectively of 2.3 × 10[SUP]-3[/SUP] (in 4 h), 3.5 × 10[SUP]-3[/SUP] (in 3 h) and 5.4 × 10[SUP]-3[/SUP] (in 1.5 h) on the search region extending from 5 to 100 mas. Our observations confirm that the previously detected near-infrared excess emissions around Fomalhaut and tau Cet are not related to a low-mass companion, and instead come from an extended source such as an exozodiacal disk. In the case of <ASTROBJ>del Aqr</ASTROBJ>, in 30 min of observation, we obtain the first direct detection of a previously known companion, at an angular distance of about 40 mas and with a flux ratio of 2.05 × 10[SUP]-2[/SUP] ± 0.16 × 10[SUP]-2[/SUP]. Due to the limited u,v plane coverage, its position can, however, not be unambiguously determined. <BR /> Conclusions: After only a few months of operation, PIONIER has already achieved one of the best dynamic ranges world-wide for multi-aperture interferometers. A dynamic range up to about 1:500 is demonstrated on unresolved targets, but significant improvements are still required to reach the ultimate goal of directly detecting hot giant extrasolar planets. Based on observations obtained at the European Southern Observatory (ESO) Very Large Telescope Interferometer (VLTI), Paranal, Chile. [less ▲]

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See detailA near-infrared interferometric survey of debris disc stars. II. CHARA/FLUOR observations of six early-type dwarfs
Absil, Olivier ULg; di Folco, E.; Mérand, A. et al

in Astronomy and Astrophysics (2008), 487

Aims. We aim at directly detecting the presence of optically thin circumstellar dust emission within the terrestrial planetary zone around main sequence stars known to harbour cold debris discs. The ... [more ▼]

Aims. We aim at directly detecting the presence of optically thin circumstellar dust emission within the terrestrial planetary zone around main sequence stars known to harbour cold debris discs. The present study focuses on a sample of six bright A- and early F-type stars. Methods: High-precision interferometric observations have been obtained in the near-infrared K band with the FLUOR instrument installed on the CHARA Array. The measured squared visibilities are compared to the expected visibility of the stellar photospheres based on theoretical photospheric models taking into account rotational distortion. We search for potential visibility reduction at short baselines, a direct piece of evidence for resolved circumstellar emission. Results: Our observations bring to light the presence of resolved circumstellar emission around one of the six target stars (zeta Aql) at the 5sigma level. The morphology of the emission source cannot be directly constrained because of the sparse spatial frequency sampling of our interferometric data. Using complementary adaptive optics observations and radial velocity measurements, we find that the presence of a low-mass companion is a likely origin for the excess emission. The potential companion is characterised by a K-band contrast of four magnitudes. It has a most probable mass of about 0.6~Msun and is expected to orbit between about 5.5 AU and 8 AU from its host star assuming a purely circular orbit. Nevertheless, by adjusting a physical debris disc model to the observed Spectral Energy Distribution of the zeta Aql system, we also show that the presence of hot dust within 10 AU from zeta Aql, producing a total thermal emission equal to 1.69 ± 0.31% of the photospheric flux in the K band, is another viable explanation for the observed near-infrared excess. Our re-interpretation of archival near- to far-infrared photometric measurements shows however that cold dust is not present around zeta Aql at the sensitivity limit of the IRS and MIPS instruments onboard Spitzer, and urges us to remove zeta Aql from the category of bona fide debris disc stars. Conclusions: The hot debris disc around Vega (Absil et al. 2006) currently remains our only secure resolved detection within the context of this survey, with six genuine early-type debris disc stars observed so far. Further observations will be needed to assess whether zeta Aql also belongs to this hot debris disc category. Partly based on observations collected at the European Southern Observatory, La Silla, Chile, under program IDs 073.C-0733, 077.C-0295 and 080.C-0712. [less ▲]

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