Stefan Hippler

Lucky Imaging survey for southern M dwarf binaries

Context. While M dwarfs are the most abundant stars in the Milky Way, there is still large uncertainty about their basic physical properties (mass, luminosity, radius, etc.) as well as their formation environment. Precise knowledge of multiplicity characteristics and how they change in this transitional mass region, between Sun-like stars on the one side and very low mass stars and brown dwarfs on the other, provide constraints on low mass star and brown dwarf formation. Aims. In the largest M dwarf binary survey to date, we search for companions to active, and thus preferentially young, M dwarfs in the solar neighbourhood. We study their binary/multiple properties, such as the multiplicity frequency and distributions of mass-ratio and separation, and identify short period visual binaries, for which orbital parameters and hence dynamical mass estimates can be derived in the near future. �

Stellar companions to exoplanet host stars: Lucky Imaging of transiting planet hosts

Observed properties of stars and planets in binary/multiple star systems provide clues to planet formation and evolution. We extended our survey for visual stellar companions to the hosts of transiting exoplanets by 21 stars, using the Lucky Imaging technique with the two AstraLux instruments: AstraLux Norte at the Calar Alto 2.2-m telescope, and AstraLux Sur at the ESO 3.5-m New Technology Telescope at La Silla. We present observations of two previously unknown binary candidate companions, to the transiting planet host stars HAT-P-8 and WASP-12, and derive photometric and astrometric properties of the companion candidates. The common proper motions of the previously discovered candidate companions with the exoplanet host stars TrES-4 and WASP-2 are confirmed from follow-up observations. A Bayesian statistical analysis of 31 transiting exoplanet host stars observed with AstraLux suggests that the companion star fraction of planet hosts is not significantly different from that of solar-type field stars, but that the binary separation is on average larger for planet host stars.

MAD the ESO multi-conjugate adaptive optics demonstrator

Multi-Conjugate Adaptive Optics (MCAO) is working on the principle to perform wide field of view atmospheric turbulence correction using many Guide Stars located in and/or surrounding the observed target. The vertical distribution of the atmospheric turbulence is reconstructed by observing several guide stars and the correction is applied by some deformable mirrors optically conjugated at different altitudes above the telescope. The European Southern Observatory together with external research institutions is going to build a Multi-Conjugate Adaptive Optics Demonstrator (MAD) to perform wide field of view adaptive optics correction. The aim of MAD is to demonstrate on the sky the feasibility of the MCAO technique and to evaluate all the critical aspects in building such kind of instrument in the framework of both the 2nd generation VLT instrumentation and the 100-m telescope OWL. In this paper we present the conceptual design of the MAD module that will be installed at one of the VLT unit telescope in Paranal to perform on-sky observations. MAD is based on a two deformable mirrors correction system and on two multi-reference wavefront sensors capable to observe simultaneously some pre-selected configurations of Natural Guide Stars. MAD is expected to correct up to 2 arcmin field of view in K band.

GRAVITY: getting to the event horizon of Sgr A*

We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase. GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches the scientific needs; in particular that 10µas astrometry is feasible for a source with a magnitude of K=15 like Sgr A*, given the availability of suitable phase reference sources.

The Very Red Afterglow of GRB 000418: Further Evidence for Dust Extinction in a Gamma-Ray Burst Host Galaxy*

We report near-infrared and optical follow-up observations of the afterglow of the GRB 000418 start- ing 2.5 days after the occurrence of the burst and extending over nearly 7 weeks. GRB 000418 represents the second case for which the afterglow was initially identi—ed by observations in the near-infrared. During the —rst 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times the temporal evolution of the afterglow —attens with respect to a simple power-law decay. Attributing this to an underlying host galaxy, we —nd its magnitude to be R \ 23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R(K B 4 mag. The observations can be explained by an adiabatic, spherical —reball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions. Subject heading: gamma rays: bursts

GRAVITY: a four-telescope beam combiner instrument for the VLTI

GRAVITY is an adaptive optics assisted Beam Combiner for the second generation VLTI instrumentation. The instrument will provide high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the astronomical K-band for faint objects. We describe the wide range of science that will be tackled with this instrument, highlighting the unique capabilities of the VLTI in combination with GRAVITY. The most prominent goal is to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky Way. We present the preliminary design that fulfils the requirements that follow from the key science drivers: It includes an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near-infrared wavefrontsensing adaptive optics; fringe-tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that 10 μas astrometry within few minutes is feasible for a source with a magnitude of mK = 15 like Sgr A*, given the availability of suitable phase reference sources (mK = 10). Using the same setup, imaging of mK = 18 stellar sources in the interferometric field of view is possible, assuming a full night of observations and the corresponding UV coverage of the VLTI.

Adaptive Optics on Large Telescopes

Observations withground based telescopes suffer from atmospheric turbulence.Independent of the telescope size the angular resolution inthe visible is equivalent to that of a telescope with adiameter of 10–20 cm. This effect is caused by the turbulentmixing of air with different temperatures in the atmosphere.Thus, the perfect plane wave from a star at infinity isaberrated before it enters the telescope. In the following,we will discuss the physical background of imaging throughturbulence, using Kolmogorov statistics, and the differenttechniques to sense and to correct the wave-front aberrationswith adaptive optics. The requirements for the control loop ofan adaptive optics system are discussed including formulas forthe limiting magnitude of the guide star as a function of thewave-front sensing method, of the quality of the wave-frontsensor camera, and of the degree of correction. Finally, ashort introduction to deformable mirror technology will begiven followed by the presentation of a new method to measureand to distinguish individual turbulent layers in order toincrease the isoplanatic angle.

CHEOPS/ZIMPOL: a VLT instrument study for the polarimetric search of scattered light from extrasolar planets

We present results from a phase A study supported by ESO for a VLT instrument for the search and investigation of extrasolar planets. The envisaged CHEOPS (CHaracterizing Extrasolar planets by Opto-infrared Polarization and Spectroscopy) instrument consists of an extreme AO system, a spectroscopic integral field unit and an imaging polarimeter. This paper describes the conceptual design of the imaging polarimeter which is based on the ZIMPOL (Zurich IMaging POLarimeter) technique using a fast polarization modulator combined with a demodulating CCD camera. ZIMPOL is capable of detecting polarization signals on the order of p=0.001% as demonstrated in solar applications. We discuss the planned implementation of ZIMPOL within the CHEOPS instrument, in particular the design of the polarization modulator. Further we describe strategies to minimize the instrumental effects and to enhance the overall measuring efficiency in order to achieve the very demanding science goals.

MAGIC: a new near-infrared camera for Calar Alto

This paper describes the basic design, operation, and initial performance of MAGIC, the new MPI fur Astronomie General-purpose Infrared Camera. MAGIC uses a 256 X 256 NICMOS3 HgCdTe detector array and has flexible optics and drive electronics that permit a variety of observing configurations. The camera was designed and built to MPIA specifications by Infrared Laboratories of Tucson, Arizona. MAGIC is based at the 3.5 meter telescope on Calar Alto, although it may be used at a number of other sites, including the 2.2 and 1.2 meter telescopes on Calar Alto and the 2.2 meter MPIA/ESO telescope at La Silla. The design of MAGIC places particular emphasis on wide field, deep imaging at the f/10 focus of the 3.5 m telescope and on providing some spectroscopic and speckle interferometric capability.

Optimal compensation and implementation for adaptive optics systems

This paper develops a compensation algorithm based on Linear–Quadratic–Gaussian (LQG) control system design whose parameters are determined (in part) by a model of the atmosphere. The model for the atmosphere is based on the open-loop statistics of the atmosphere as observed by the wavefront sensor, and is identified from these using an auto-regressive, moving average (ARMA) model. The (LQG) control design is compared with an existing compensation algorithm for a simulation developed at ESO that represents the operation of MACAO adaptive optics system on the 8.2 m telescopes at Paranal, Chile.