Thomas Bloecker

Astrophysical potential of the AMBER/VLTI instrument

AMBER is the near-infrared instrument of the Very Large Telescope Interferometer (VLTI). With a spectral resolution up to 10000 in the 1.2-2.4 micron wavelength range, AMBER will offer the possibility to combine 3 beams from the VLTI array either 8-m or 1.8m telescopes. The instrument has been designed to bring high precision measurement and high sensitivity and therefore opens the way to new domain of investigation in stellar physics and for the first time access to extragalactic sources. We show how the performance of the instrument can apply in these different astrophysical fields. We present the work of the Science Group and the AMBER consortium who defined precise astrophysical goals for the first years of operation.

Science opportunities with AMBER, the near-IR VLTI instrument

AMBER is the near-IR instrument for the VLTI, which will offer the possibility of combining two or three beams from either the 8 meter VLT main telescopes or the 1.8 meter auxiliary telescopes. With spectral dispersion up to 10,000 high visibility accuracy and the ability to obtain closure phases, AMBER will offer the means to perform high quality interferometric measurements in the 1 - 2.5 micron range initially, with later extensions to other portions of the spectrum. These design characteristics, coupled to the VLT interferometer potential, open up the access to investigation of several classes of objects, from stellar to extragalactic astronomy. We will review the projected performance in terms of sensitivity and angular resolution, and illustrate the potential applications in some key research areas. In particular, we will present the work of the AMBER Science Group, which is evaluating simulated data of source models and interferometric outputs for the purpose of defining the criteria for observations.

JHK-band spectro-interferometry of T Cep with the IOTA interferometer

Our new IOTA JHK-band beam combiner allows the simultaneous recording of spectrally dispersed J-, H- and K-band Michelson interferograms. In this paper we present our IOTA observations of the Mira star T Cep with this beam combiner (observations in June 2001; four baselines in the range of 14 m to 27 m). The beam combiner optics consists of an anamorphic cylindrical lens system and a prism. From the interferograms of T Cep we derive the visibilities and the J-, H-, and K-band uniform-disk diameters of 14.0 ± 0.6 mas, 13.7 ± 0.6 mas and 15.0 ± 0.6 mas, respectively. Angular stellar filter radii and Rosseland radii are derived from the measured visibilities by fitting theoretical center-to-limb intensity variations (CLVs) of different Mira star models. The available HIPPARCOS parallax (4.76 ± 0.75 mas) of T Cep allows us to determine linear radii. For example, from the K-band visibility we derive a Rosseland radius of 329-50/+70 solar radii if we use the CLVs of the M-models as fit functions. This radius is in good agreement with the theoretical M-model Rosseland radius of 315 solar radii. The comparison of measured stellar parameters (e.g. diameters, effective temperature, visibility shape) with theoretical parameters indicates whether any of the models is a fair representation of T Cep. The ratios of visibilities of different spectral channels can be measured with higher precision than absolute visibilities. Therefore, we use the visibility ratios V(λ1)/V(λ2) to investigate the wavelength dependence of the stellar diameter. We find that the 2.03 μm uniform-disk diameter of T Cep is about 1.26 times larger than the 2.26 μm uniform-disk diameter.

Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models

We present K-band observations of five Mira stars with the IOTA interferometer. The interferograms were obtained with the FLUOR fiber optics beam combiner which provides high- accuracy visibility measurements in spite of time-variable atmospheric conditions. For the Mira stars X Oph, R Aql, RU Her, R Ser, and V CrB we derived the uniform-disk diameters 11.7 mas, 10.9 mas, 8.4 mas, 8.1 mas, and 7.9 mas (+/- 0.3 mas), respectively. Simultaneous photometric observations yielded the bolometric fluxes. The derived angular Rosseland radii and the bolometric fluxes allowed the determination of effective temperatures. For instance, the effective temperature of R Aql was determined to be 3072 K +/- 161 K. A Rosseland radius for R Aql of 250 R. +/- 63 R. was derived from the angular Rosseland radius of 5.5 mas +/- 0.2 mas and the HIPPARCOS parallax of 4.73 mas +/- 1.19 mas. The observations were compared with theoretical Mira star models (D/P model Rosseland radius equals 255 R.; measured R Aql Rosseland radius equals 250 R. +/- 63 R.).

Near-infrared IOTA interferometry of the symbiotic star CH Cyg

We present observations of the symbiotic star CH Cyg with a new JHK-band beam combiner mounted to the IOTA interferometer. The new beam combiner consists of an anamorphic cylindrical lens system and a grism, and allows the simultaneous recording of spectrally dispersed J-, H- and K-band Michelson interferograms. The observations of CH Cyg were conducted on 5, 6, 8 and 11 June 2001 using baselines of 17m to 25m. From the interferograms of CH Cyg, J-, H-, and K-band visibility functions can be determined. Uniform-disk fits to the visibilities give, e.g., stellar diameters of (7.8 ± 0.6) mas and (8.7 ± 0.8) mas in H and K, respectively. Angular stellar filter radii and Rosseland radii are derived from the measured visibilities by fitting theoretical center-to-limb intensity variations (CLVs) of Mira star models. The available HIPPARCOS parallax of CH Cyg allows us to determine linear radii. For example, on the basis of the K-band visibility, Rosseland radii in the range of 214 to 243 solar radii can be derived utilizing CLVs of different fundamental mode Mira models as fit functions. These radii agree well within the error bars with the corresponding theoretical model Rosseland radii of 230 to 282 solar radii. Models of first overtone pulsators are not in good agreement with the observations. The wavelength dependence of the stellar diameter can be well studied by using visibility ratios V(λ1)/V(λ2) since ratios of visibilities of different spectral channels can be measured with higher precision than absolute visibilities. We found that the 2.03 μm uniform disk diameter of CH Cyg is approximately 1.1 times larger than the 2.15 μm and 2.26 μm uniform-disk diameter.

Bispectrum speckle interferometry and future long-baseline interferometry of the carbon star IRC+10216

We present near-infrared (JHK) bispectrum speckle-interferometry monitoring of IRC+10216 obtained with the SAO 6m telescope. The present speckle observations covering baselines up to 6m provide important complementary informations for future long-baseline interferometry. To disentangle the apparent motions of the various IRC+10216 components and to reveal the location of the central star, future high-resolution observations are of utmost value for the interpretation of this astrophysical key object. The J-, H-, and K-band resolutions of our speckle observations are 50 mas, 56 mas, and 73 mas, resp. The K-band observations cover 8 different epochs from 1995 to 2001 and show the dynamical evolution of the dust shell which consists of several compact components within a 200 milli-arcsecond radius. Our recent two-dimensional radiative transfer modelling has shown that the central star is probably not located at the brightest dust-shell component A but at the position of the northern component B. The bright and compact component A is the southern lobe of a bipolar structure. The changes of the dust-shell structure can be related to corresponding changes of the optical depth caused, for instance, by mass-loss variations. The present observations are consistent with the predictions of hydrodynamical models that enhanced dust formation takes place on a timescale of several pulsational cycles.

Computer simulations of interferometric imaging with the VLT interferometer and the AMBER instrument

We present computer simulations of interferometric imaging with the VLT interferometer and the AMBER instrument. These simulations include both the astrophysical modeling of a stellar object by radiative transfer calculations and the simulation of light propagation from the object to the detector (through atmosphere, telescopes, and the AMBER instrument), simulation of photon noise and detector read- out noise, and finally data processing of the interferograms. The results show the dependence of the visibility error bars on the following observational parameters: different seeing during the observation of object and reference star (Fried parameters r0,object equals 2.4 m, r0,ref. equals 2.5 m), different residual tip- tilt error ((delta) tt,object equals 2% of the Airy disk diameter, (delta) tt,ref. equals 0.1%), and object brightness (Kobject equals 3.5 mag and 11 mag, Kref. equals 3.5 mag). Exemplarily, we focus on stars in late stages of stellar evolution and study one of its key objects, the dusty supergiant IRC + 10420 that is rapidly evolving on human timescales. We show computer simulations of VLTI interferometry of IRC + 10420 with two ATs (wide-field mode, i.e. without fiber optics spatial filters) and discuss whether the visibility accuracy is sufficient to distinguish between different theoretical model predictions.