We are developing an optical adaptive optics (AO) system for small telescopes. An AO instrument in optical wavelength mounted on a 1-2 m class telescope located at a good seeing site will make it possible to achieve high angular resolution of 0.1-0.2 arcsec. Such capability will enable us to perform unique astronomical programs, as well as to provide good opportunity in education for both astronomy and engineering. In order to examine the AO capability on small telescopes, we developed an experimental AO instrument, in which inexpensive commercial devices are extensively used to reduce cost for development. We designed the weight and the physical size so small that it is portable and easy to be mounted on a small telescope, which is a unique feature of our AO instrument. After the engineering observations performed in Japan, we mounted it on the 1-m telescope of the European Southern Observatory of La Silla in Chile in March 2018 to examine the performance. We found that there were approximately 4 times and 5 times improvements in the full-width-halfmaximum (FWHM) and Strehl ratio of the PSF from the natural seeing, respectively. The best AO-corrected PSF obtained during the observation achieved FWHM=0.18 arcsec and the Strehl ratio = 0.18. Based on the detailed analysis of the timeseries wavefront and deformable-mirror-operation data, further improvement in AO performance is expected by adjustment of the system parameters. We succeeded in demonstrating the feasibility of an inexpensive optical AO system for small telescopes.
Two interferometric instruments at ESO's Very Large Telescope Interferometer (VLTI) - MIDI and AMBER
operating in the mid-infrared (8-13 μm) and the near-infrared (JHK), respectively - have proven to be
very powerful to study the physical properties of the circumstellar material around evolved stars. With the
"spectro-interferometric" capability of MIDI and AMBER, we can disentangle spectral and spatial information
on the observed object. VLTI observations have confirmed our pictures on the circumstellar environment
of cool evolved stars in some cases but brought about entirely unexpected pictures in other cases. Here, we
present our recent results obtained with VLTI/MIDI.
The latest generation of infrared long-baseline interferometric instruments combines high spatial resolution with spectroscopic
capabilities, enabling fascinating new studies of the AU-scale circumstellar environment around young stellar
objects. Here, we present recent investigations, which we conducted using the VLTI instruments AMBER and MIDI and
which demonstrate these new observational possibilities.
In one study, we combine near- and mid-infrared interferometry
(H-/K-/N-band) to constrain the geometry and radial
temperature profile of the circumstellar accretion disk around the Herbig Be star MWC147. Using detailed radiative
transfer modeling, we find strong evidence for the presence of an optically-thick inner gaseous disk. In another investigation,
we used AMBER's medium spectral resolution mode (R = 1500) to study the spatial origin of the hydrogen Brγ
line for five Herbig Ae/Be stars, associating the line emission with different physical mechanisms, such as disk winds and
magnetospheric accretion. Finally, we present AMBER H- and K-band observations of the close binary star θ1OrionisC and illustrate the benefits of fitting wavelength-differential visibilities and closure phases. Besides yielding a high observing
efficiency, this approach is also insensitive to calibration errors, induced, for instance, by fast changing atmospheric
conditions.
We present the results of N-band spectro-interferometric observations of the silicate carbon star Hen 38 (IRAS08002-3803) with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). Our observations of IRAS08002-3803 with baseline lengths of 39-47 m have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N band: the uniform-disk diameter is found to be constant and ~36 mas (72 R*) between 8 and 10 μm, while it steeply increases longward of 10 μm to reach ~53 mas (106 R*) at 13 μm. Neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly - though not entirely satisfactorily - reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion.
We present the first interferometric NIR observations of the LBV η Carinae with high spectral resolution. The observations were carried out with three 8.2 m VLTI Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1,500 (MR-K mode) and 12,000 (HR-K mode). The observations were performed in the wavelength range around both the He I 2.059 μm and the Brγ 2.166 μm emission lines. The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of η Car. In the K-band continuum, a diameter of 4.0±0.2 mas (Gaussian FWHM) was measured for η Car's optically thick wind region, whereas the Brγ and He I emission line regions are larger. If we fit Hillier et al. model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.3, 6.5 and 9.6 mas in the 2.17 μm continuum, the He I, and the Brγemission lines, respectively. In the continuum near the Brγ line, an elongation along a position angle of 128° ± 15° was found, consistent with previous VLTI/VINCI measurements. We find good agreement between the measured visibilities and the predictions of the radiative transfer model of Hillier et al. For the interpretation of the non-zero differential and closure phases measured within the Brγ line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.
We present K-band commissioning observations of the Mira star prototype o Cet obtained at the ESO Very Large Telescope Interferometer (VLTI) with the VINCI instrument and two siderostats.
The observations were carried out between 2001 October and December, in 2002 January and December, and in 2003 January. Rosseland angular radii are derived from the measured visibilities by fitting theoretical visibility functions obtained from center-to-limb intensity variations (CLVs) of Mira star models. Using the derived Rosseland angular radii and the spectral energy distributions (SEDs) reconstructed from available photometric and spectrophotometric data, we find effective temperatures ranging from T_eff=3192 +/- 200 K at phase 0.13 to 2918 +/- 183 K at phase 0.26. Comparison of these Rosseland radii, effective temperatures, and the shape of the observed visibility functions with model predictions suggests that o Cet is a fundamental mode pulsator. Furthermore, we investigated the variation of visibility function and diameter with phase. The Rosseland angular diameter of o Cet increased from 28.9 +/- 0.3 mas
(corresponding to a Rosseland radius of 332 +/- 38 Rsun for a distance of D=107 +/- 12 pc) at phase 0.13 to 34.9 +/- 0.4 mas (402 +/- 46 Rsun) at phase 0.4. The observational error of the Rosseland linear radius almost entirely results from the error of the parallax, since the error of the angular diameter is only approximately 1%.
AMBER (Astronomical Multiple BEam Recombiner) is a 3 aperture
interferometric recombiner operating between 1 and 2.5 um, for the Very Large Telescope Interferometer (VLTI). The control software of the instrument, based on the VLT Common Software, has been written to comply with specific features of the AMBER hardware, such as the Infrared detector read out modes or piezo stage drivers, as well as with the very specific operation modes of an interferomtric instrument.
In this respect, the AMBER control software was designed to insure that all operations, from the preparation of the observations to the control/command of the instrument during the observations, would be kept as simple as possible for the users and operators, opening the use of an interferometric instrument to the largest community of astronomers. Peculiar attention was given to internal checks and calibration procedures both to evaluate data quality in real time,
and improve the successes of long term UV plane coverage observations.
We report the first long-baseline interferometric observations of R CrB. The observations were carried out at the Infrared Optical Telescope Array (IOTA), using our new JHK beam combiner which enables us to record fringes simultaneously in the J-, H-, and K-bands. The circumstellar envelope of R CrB is resolved at a baseline of 21 m, and the K-band visibility is derived to be 0.61 ± 0.03 along a position angle of about 170 degrees. The visibility obtained with IOTA, as well as speckle visibilities with baselines up to 6 m and the spectral energy distribution (SED), are fitted with 2-component models consisting of the central star and an optically thin dust shell. The K-band visibilities predicted by the models are about 10% smaller than the visibility obtained with IOTA. However, given the simplifications adopted in our models and the complex nature of the object, this can be regarded as rough agreement. As a hypothesis to explain the small discrepancy, we propose that there might be a group of newly formed dust clouds, which might appear as a third visibility component.
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.
New beam combination techniques, using two and three telescopes, have been the focus of activity at IOTA during the past two years since our last update. In particular, we have added a third telescope, made closure-phase measurements, demonstrated two- and three-beam combination with integrated optics combiners, demonstrated two-beam combination with an asymmetric coupler, and made simultaneous JHK visibility measurements with an image-plane combiner.
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.
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