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Referred Publications:
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Astronomy & Astrophysics, 2011
The mid-infrared diameter of W Hydrae
R. Zhao-Geisler1,*, A. Quirrenbach1, R. Köhler1,3,
B. Lopez2, C. Leinert3
1 Zentrum für Astronomie der Universität Heidelberg,
Landessternwarte, Königstuhl 12, D-69117 Heidelberg, Germany
2 Lab Fizeau, UMR 6525, UNS-Observatoire de la côte d’azur, F-06304 Nice Cedex 4, France
3 Max-Planck-Institut f ur Astronomie, Konigstuhl 17, D-69120 Heidelberg
* Fellow of the International Max Planck Research School (IMPRS)
Abstract:
Aims. Asymptotic giant branch (AGB) stars are among the largest distributors of dust into the interstellar medium, and it is therefore important to understand the dust formation process and sequence in their strongly pulsating extended atmosphere. By monitoring the AGB star W Hya interferometrically over a few pulsations cycles, the upper atmospheric layers can be studied to obtain information on their chemical gas and dust composition and their intracycle and cycle-to-cycle behavior.
Methods. Mid-infrared (8 − 13 microns) interferometric data of W Hya were obtained with MIDI/VLTI between April 2007 and September 2009, covering nearly three pulsation cycles. The spectrally dispersed visibility data of all 75 observations were analyzed by fitting a circular fully limb-darkened disk (FDD) model to all data and individual pulsation phases. Asymmetries were studied with an elliptical FDD.
Results. Modeling results in an apparent angular FDD diameter of W Hya of about (80 +/- 1.2) mas (7.8 AU) between 8 and 10 microns, which corresponds to an about 1.9 times larger diameter than the photospheric one. The diameter gradually increases up to (105 +/- 1.2) mas (10.3 AU) at 12 microns. In contrast, the FDD relative flux fraction decreases from (0.85 +/- 0.02) to (0.77 +/- 0.02), reflecting the increased flux contribution from a fully resolved surrounding silicate dust shell. The asymmetric character of the extended structure could be confirmed. An elliptical FDD yields a position angle of (11 +/- 20) deg and an axis ratio of (0.87 +/- 0.07). A weak pulsation dependency is revealed with a diameter increase of (5.4 +/- 1.8) mas between visual minimum and maximum, while detected cycle-to-cycle variations are smaller.
Conclusions. W Hya’s diameter shows a behavior that is very similar to the Mira stars RR Sco and S Ori and can be described by an analogous model. The constant diameter part results from a partially resolved stellar disk, including a close molecular layer of H2O, while the increase beyond 10 μﾊm can most likely be attributed to the contribution of a spatially resolved nearby Al2O3 dust shell. Probably due to the low mass-loss rate, close Fe-free silicate dust could not be detected. The results suggest that the formation of amorphous Al2O3 occurs mainly at visual minimum. A possible close Al2O3 dust shell has now been revealed in a few objects calling for self-consistent dynamic atmospheric models including dust formation close to the star. The asymmetry might be explained by an enhanced dust concentration along an N-S axis.
download (english, 2.0 MB, pdf) or ADS
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Astronomy & Astrophysics, 2011, in submission process
The Dust and Molecular Shells in Asymptotic Giant Branch Stars:
Mid-Infrared Interferometric Observations of
R Aql, R Aqr, R Hya, W Hya and V Hya
R. Zhao-Geisler1,*, A. Quirrenbach1, R. Köhler1,3, B. Lopez2
1 Zentrum für Astronomie der Universität Heidelberg,
Landessternwarte, Königstuhl 12, D-69117 Heidelberg, Germany
2 Lab Fizeau, UMR 6525, UNS-Observatoire de la côte d’azur, F-06304 Nice Cedex 4, France
3 Max-Planck-Institut f ur Astronomie, Konigstuhl 17, D-69120 Heidelberg
* Fellow of the International Max Planck Research School (IMPRS)
Abstract:
in preperation
download (preliminary version, english, 3.8 MB, pdf)
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Astronomy & Astrophysics, 2011, in preperation
The ESPRI project: astrometric exoplanet search with PRIMA I.
Scientic goals and preparations
R. Launhardt1, Th. Henning2, D. Queloz3, A. Quirrenbach1
& ESPRI consortium1,2,3,4
1 Zentrum für Astronomie der Universität Heidelberg,
Landessternwarte, Königstuhl 12, D-69117 Heidelberg, Germany
2 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
3 Observatoire Astronomique de l’Universite de Geneve, CH-1290 Sauverny, Switzerland
4 European Southern Observatory (ESO), D-85748 Garching, Germany
Abstract:
Context. PRIMA, the instrument for Phase-Referenced Imaging and Microarcsecond Astrometry at the VLTI, is currently being integrated and tested at ESO Paranal in Chile. PRIMA will implement the dual-feed capability, at first for two ATs or UTs, to enable simultaneous interferometric observations of two objects that are separated by up to 1 arcmin. PRIMA is designed to perform narrow-angle astrometry in K-band with two ATs as well as phase-referenced aperture synthesis imaging with instruments like AMBER and MIDI.
Aims. In order to speed up the full implementation of the astrometric mode of PRIMA and to carry out a large astrometric planet search program, a consortium consisting of Observatoire de Genéve, Max Planck Institute for Astronomy, and Landessternwarte Heidelberg, has built Differential Delay Lines for PRIMA and is currently developing the astrometric observationpreparation and data-reduction software. When the facility becomes fully operational in 2011, we will use PRIMA to carry out a systematic Exoplanet Search program, called ESPRI.
Methods. In this paper, we explain the astrometric measurement principle of PRIMA, (interferometric narrow-angle differential astrometry), give an overview of the ongoing hard and software developments, and describe the scientific goals, targets, preparations, and observing strategy of the ESPRI survey.
(no preliminary version)
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Conference Proceedings:
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SPIE: Astronomical Telescopes and Instrumentation, Proceedings of SPIE, upcoming conference in Marseille, France, June 23-28, 2008
Simulations of Imperfect PRIMA Fringe Sensing Units
and Calibration Strategies
Ronny Geisler1,2,*, Nicholas M. Elias II1,2, Andreas Quirrenbach1, Rainer Köhler1,2, Robert N. Tubbs2, Thomas Henning2 and Didier Queloz3
1 Zentrum für Astronomie der Universität Heidelberg,
Landessternwarte, Königstuhl 12, D-69117 Heidelberg, Germany
2 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany;
3 Observatoire Astronomique de l’Universite de Geneve, CH-1290 Sauverny, Switzerland
* Fellow of the International Max Planck Research School (IMPRS)
Abstract: The PRIMA (Phase-Referenced Imaging and Microarcsecond Astrometry) instrument at ESO/VLTI is scheduled for commissioning in late 2008. It is designed for phased-referenced imaging and narrow-angle astrometry. The latter, which is the focus of this paper, may be used for exoplanet detection.
A key PRIMA subsystem consists of two fringe sensing units. They employ polarized and dispersive optics to measure cross fluxes and differential phases in five narrow K band channels without the need of delay-line dithering. The differential phases are used to correct the differential delays, which are the primary observables used to determine relative proper motions, relative parallaxes, and planetary orbits. Real optical components are imperfect, which means that systematics will appear in the differential phases.
In this paper, we 1) present a closed mathematical form for the differential phase, including certain systematic offsets and random errors; 2) perform Monte Carlo simulations to understand how the systematic offsets and random errors affect the differential phases; and 3) show that delay-line stepping can be used to eliminate the effects of systematic offsets.
download (english, 0.5 MB, pdf) or ADS
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EXOPLANETS: Detection, Formation & Dynamics, Proceedings IAU Symposium No. 249, 2008, Y.-S. Sun, S. Ferraz-Mello & J.-L. Zhou, eds.:
Preparing the Exoplanet Search with PRIMA:
Searching for Reference Stars and Target Characterization
R. Geisler1,2,*, J. Setiawan2, Th. Henning2,D. Queloz3, A. Quirrenbach1,
R. Launhardt2, A. Müller2, S. Reffert1, P. Weise2 & ESPRI consortium1,2,3,4
1 Zentrum für Astronomie der Universität Heidelberg,
Landessternwarte, Königstuhl 12, D-69117 Heidelberg, Germany
2 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
3 Observatoire Astronomique de l’Universite de Geneve, CH-1290 Sauverny, Switzerland
4European Southern Observatory (ESO), D-85748 Garching, Germany
* Fellow of the International Max Planck Research School (IMPRS)
Abstract: The PRIMA (Phase-Referenced Imaging and Micro-arcsecond Astrometry) facility at ESO VLTI (Paranal observatory) is expected to be commissioned in mid 2008. The ESPRI (Exoplanet Search with PRIMA) consortium is currently preparing an astrometric survey to search for extrasolar planets. To achieve the scientific goal of this survey, a careful selection of target and reference stars is necessary. Apart from catalog search and modelling, extensive and dedicated preparatory observations are indispensable. Here we present two aspects of the preparatory observation programs: A high dynamic range near infrared (NIR) imaging survey to search for astrometric reference stars around the preselected target stars and characterization of the target stars by using high-resolution spectroscopy.
download (english, 0.7 MB, pdf) or ADS
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