Delay Compensation for Real Time Disturbance Estimation at Extremely Large Telescopes

In ground-based astronomy, aberrations due to structural vibrations, such as piston, limit the achievable resolution and cannot be corrected using adaptive optics (AO) for large telescopes. We present a model-free strategy to estimate and compensate piston aberrations due to the vibrations of optical components using accelerometer disturbance feed forward, eventually allowing the use of fainter guide stars both for the fringe detector and in the AO loop. Because the correction performance is very sensitive to signal delays, we present a strategy to add a delay compensation to the developed disturbance estimator, which can, in principle, be applied to many other applications outside of astronomy that lack observer performance due to a measurement delay or need a prediction to compensate for input delays. The ability to estimate vibration disturbances in the critical frequency range of 8-60 Hz is demonstrated with on sky data from the Large Binocular Telescope (LBT) Interferometer, an interferometer at the LBT. The experimental results are promising, indicating the ability to suppress differential piston induced by telescope vibrations by a factor of about 3 (rms), which is significantly better than any currently commissioned system

HD139614: the interferometric case for a group-Ib pre-transitional young disk

The Herbig Ae star HD 139614 is a group-Ib object, which featureless SED indicates disk flaring and a possible pre-transitional evolutionary stage. We present mid- and near-IR interferometric results collected with MIDI, AMBER and PIONIER with the aim of constraining the spatial structure of the 0.1-10 AU disk region and assess its possible multi-component structure. A two-component disk model composed of an optically thin 2-AU wide inner disk and an outer temperature-gradient disk starting at 5.6 AU reproduces well the observations. This is an additional argument to the idea that group-I HAeBe inner disks could be already in the disk-clearing transient stage. HD 139614 will become a prime target for mid-IR interferometric imaging with the second-generation instrument MATISSE of the VLTI.Comment: SPIE Astronomical Telescopes and Instrumentation conference, June 2014, 11 pages, 7 Figure

Luminosity-variation independent location of the circum-nuclear, hot dust in NGC 4151

After recent sensitivity upgrades at the Keck Interferometer (KI), systematic interferometric 2um studies of the innermost dust in nearby Seyfert nuclei are within observational reach. Here, we present the analysis of new interferometric data of NGC 4151, discussed in context of the results from recent dust reverberation, spectro-photometric and interferometric campaigns. The complete data set gives a complex picture, in particular the measured visibilities from now three different nights appear to be rather insensitive to the variation of the nuclear luminosity. KI data alone indicate two scenarios: the K-band emission is either dominated to ~90% by size scales smaller than 30mpc, which falls short of any dust reverberation measurement in NGC 4151 and of theoretical models of circum-nuclear dust distributions. Or contrary, and more likely, the K-band continuum emission is dominated by hot dust (>= 1300K) at linear scales of about 50mpc. The linear size estimate varies by a few tens of percent depending on the exact morphology observed. Our interferometric, deprojected centro-nuclear dust radius estimate of 55+-5mpc is roughly consistent with the earlier published expectations from circum-nuclear, dusty radiative transfer models, and spectro-photometric modeling. However, our data do not support the notion that the dust emission size scale follows the nuclear variability of NGC 4151 as a R_dust \propto L_nuc^0.5 scaling relation. Instead variable nuclear activity, lagging, and variable dust response to illumination changes need to be combined to explain the observations.Comment: 19 pages, 3 figures, 3 tables, accepted for publication in Ap

Ruling out Stellar Companions and Resolving the Innermost Regions of Transitional Disks with the Keck Interferometer

With the Keck Interferometer, we have studied at 2 um the innermost regions of several nearby, young, dust depleted "transitional" disks. Our observations target five of the six clearest cases of transitional disks in the Taurus/Auriga star-forming region (DM Tau, GM Aur, LkCa 15, UX Tau A, and RY Tau) to explore the possibility that the depletion of optically thick dust from the inner disks is caused by stellar companions rather than the more typical planet-formation hypothesis. At the 99.7% confidence level, the observed visibilities exclude binaries with flux ratios of at least 0.05 and separations ranging from 2.5 to 30 mas (0.35 - 4 AU) over >= 94% of the area covered by our measurements. All targets but DM Tau show near-infrared excess in their SED higher than our companion flux ratio detection limits. While a companion has previously been detected in the candidate transitional disk system CoKu Tau/4, we can exclude similar mass companions as the typical origin for the clearing of inner dust in transitional disks and of the near-infrared excess emission. Unlike CoKu Tau/4, all our targets show some evidence of accretion. We find that all but one of the targets are clearly spatially resolved, and UX Tau A is marginally resolved. Our data is consistent with hot material on small scales (0.1 AU) inside of and separated from the cooler outer disk, consistent with the recent SED modeling. These observations support the notion that some transitional disks have radial gaps in their optically thick material, which could be an indication for planet formation in the habitable zone (~ a few AU) of a protoplanetary disk.Comment: 36 pages, 7 figures. Accepted for publication in Ap

Astrometry with the Keck-Interferometer: the ASTRA project and its science

The sensitivity and astrometry upgrade ASTRA of the Keck Interferometer is introduced. After a brief overview of the underlying interferometric principles, the technology and concepts of the upgrade are presented. The interferometric dual-field technology of ASTRA will provide the KI with the means to observe two objects simultaneously, and measure the distance between them with a precision eventually better than 100 uas. This astrometric functionality of ASTRA will add a unique observing tool to fields of astrophysical research as diverse as exo-planetary kinematics, binary astrometry, and the investigation of stars accelerated by the massive black hole in the center of the Milky Way as discussed in this contribution.Comment: 22 pages, 10 figures (low resolution), contribution to the summerschool "Astrometry and Imaging with the Very Large Telescope Interferometer", 2 - 13 June, 2008, Keszthely, Hungary, corrected authorlis

First faint dual-field phase-referenced observations on the Keck interferometer

Ground-based long baseline interferometers have long been limited in sensitivity by the short integration periods imposed by atmospheric turbulence. The first observation fainter than this limit was performed on January 22, 2011 when the Keck Interferometer observed a K=11.5 target, about one magnitude fainter than its K=10.3 limit. This observation was made possible by the Dual Field Phase Referencing instrument of the ASTRA project: simultaneously measuring the real-time effects of the atmosphere on a nearby bright guide star, and correcting for it on the faint target, integration time longer than the turbulence time scale are made possible. As a prelude to this demonstration, we first present the implementation of Dual Field Phase Referencing on the interferometer. We then detail its on-sky performance focusing on the accuracy of the turbulence correction, and on the resulting fringe contrast stability. We conclude with a presentation of early results obtained with Laser Guide Star AO and the interferometer.Comment: 10 pages, 12 figures, Proc. SPIE 201

SOWAT: High-resolution imaging with only partial AO correction

Star Clusters: From the Milky Way to the Early Universe. Proceedings of the International Astronomical Union, Volume 351, pp. 185-188Observations of dense stellar systems such as globular clusters (GCs) are limited in resolution by the optical aberrations induced by atmospheric turbulence (atmospheric seeing). At the example of holographic speckle imaging, we now study, to which degree image reconstruction algorithms are able to remove residual aberrations from a partial adaptive optics (AO) correction, such as delivered from ground-layer AO (GLAO) systems. Simultaneously, we study, how such algorithms benefit from being applied to pre-corrected instead of natural point-spread functions (PSFs). We find that using partial AO corrections already lowers the demands on the holography reference star by ∼3 mag, what makes more fields accessible for this technique, and also that the discrete integration times may be chosen about 2-3× longer, since the effective wavefront evolution is slowed down by removing the perturbation power. © International Astronomical Union 2020

Thermal imaging of dust hiding the black hole in the Active Galaxy NGC 1068

In the widely accepted 'Unified Model' solution of the classification puzzle of Active Galactic Nuclei, the orientation of a dusty accretion torus around the central black hole dominates their appearance. In 'type-1' systems, the bright nucleus is visible at the centre of a face-on torus. In 'type-2' systems the thick, nearly edge-on torus hides the central engine. Later studies suggested evolutionary effects and added dusty clumps and polar winds but left the basic picture intact. However, recent high-resolution images of the archetypal type-2 galaxy NGC 1068 suggested a more radical revision. They displayed a ring-like emission feature which the authors advocated to be hot dust surrounding the black hole at the radius where the radiation from the central engine evaporates the dust. That ring is too thin and too far tilted from edge-on to hide the central engine, and ad hoc foreground extinction is needed to explain the type-2 classification. These images quickly generated reinterpretations of the type 1-2 dichotomy. Here we present new multi-band mid-infrared images of NGC1068 that detail the dust temperature distribution and reaffirm the original model. Combined with radio data, our maps locate the central engine below the previously reported ring and obscured by a thick, nearly edge-on disk, as predicted by the Unified Model. We also identify emission from polar flows and absorbing dust that is mineralogically distinct from that towards the Milky Way centre