Adaptive Optics Feedback Control

This book is a collection of 19 articles which reflect the courses given at the Collège de France/Summer school “Reconstruction d'images − Applications astrophysiques“ held in Nice and Fréjus, France, from June 18 to 22, 2012. The articles presented in this volume address emerging concepts and methods that are useful in the complex process of improving our knowledge of the celestial objects, including Earth

Microstructural Controls on the Uniaxial Compressive Strength of Porous Rocks Through the Granular to Non‐Granular Transition

Under uniaxial compression, a porous rock fails by coalescence of stress‐induced microcracks. The micromechanical models developed to analyze uniaxial compressive strength data consider a single mechanism for the initiation and propagation of microcracks and a fixed starting microstructure. Because the microstructure of clastic porous rock transitions from granular to non‐granular as porosity decreases during diagenesis, their strength cannot be captured by a single model. Using synthetic samples with independently controlled porosity and initial grain radius we show that high‐porosity granular samples, where microcracks grow at grain‐to‐grain contacts, are best described by a grain‐based model. Low‐porosity non‐granular samples, where microcracks grow from pores, are best described by a pore‐based model. The switch from one model to the other depends on porosity and grain radius. We propose a regime plot that indicates which micromechanical model may be more suitable to predict strength for a given porosity and grain radius

The Influence of Grain Size Distribution on Mechanical Compaction and Compaction Localization in Porous Rocks

The modes of formation of clastic rocks result in a wide variety of microstructures, from poorly-sorted heterogeneous rocks to well-sorted and nominally homogeneous rocks. The mechanical behavior and failure mode of clastic rocks is known to vary with microstructural attributes such as porosity and grain size. However, the influence of the grain size distribution, in particular the degree of polydispersivity or modality of the distribution, is not yet fully understood, because it is difficult to study experimentally using natural rocks. To better understand the influence of grain size distribution on the mechanical behavior of porous rocks, we prepared suites of synthetic samples consisting of sintered glass beads with polydisperse grain size distributions. We performed hydrostatic compression experiments and found that, all else being equal, the onset of grain crushing occurs much more progressively and at lower pressure in polydisperse synthetic samples than in monodisperse samples. We conducted triaxial experiments in the regime of shear-enhanced compaction and found that the stress required to reach inelastic compaction was lower in polydisperse samples compared to monodisperse samples. Further, our microstructural observations show that compaction bands developed in monomodal polydisperse samples while delocalized cataclasis developed in bimodal polydisperse samples, where small grains were systematically crushed while largest grains remained intact. In detail, as the polydispersivity increases, microstructural deformation features appear to transition from localized to delocalized through a hybrid stage where a compaction front with diffuse bands propagates from both ends of the sample toward its center with increasing bulk strain

Shadows cast on the transition disk of HD 135344B. Multiwavelength VLT/SPHERE polarimetric differential imaging

The protoplanetary disk around the F-type star HD 135344B (SAO 206462) is in a transition stage and shows many intriguing structures both in scattered light and thermal (sub-)millimeter emission which are possibly related to planet formation processes. We study the morphology and surface brightness of the disk in scattered light to gain insight into the innermost disk regions, the formation of protoplanets, planet-disk interactions traced in the surface and midplane layers, and the dust grain properties of the disk surface. We have carried out high-contrast polarimetric differential imaging (PDI) observations with VLT/SPHERE and obtained polarized scattered light images with ZIMPOL in R- and I-band and with IRDIS in Y- and J-band. The scattered light images reveal with unprecedented angular resolution and sensitivity the spiral arms as well as the 25 au cavity of the disk. Multiple shadow features are discovered on the outer disk with one shadow only being present during the second observation epoch. A positive surface brightness gradient is observed in the stellar irradiation corrected images in southwest direction possibly due to an azimuthally asymmetric perturbation of the temperature and/or surface density by the passing spiral arms. The disk integrated polarized flux, normalized to the stellar flux, shows a positive trend towards longer wavelengths which we attribute to large aggregate dust grains in the disk surface. Part of the the non-azimuthal polarization signal in the Uphi image of the J-band observation could be the result of multiple scattering in the disk. The detected shadow features and their possible variability have the potential to provide insight into the structure of and processes occurring in the innermost disk regions.Comment: Accepted for publication in A&A, 20 pages, 15 figure

VLT/SPHERE deep insight of NGC 3603's core: Segregation or confusion?

We present new near-infrared photometric measurements of the core of the young massive cluster NGC 3603 obtained with extreme adaptive optics. The data were obtained with the SPHERE instrument mounted on ESO Very Large Telescope, and cover three fields in the core of this cluster. We applied a correction for the effect of extinction to our data obtained in the J and K broadband filters and estimated the mass of detected sources inside the field of view of SPHERE/IRDIS, which is 13.5"x13.5". We derived the mass function (MF) slope for each spectral band and field. The MF slope in the core is unusual compared to previous results based on Hubble space telescope (HST) and very large telescope (VLT) observations. The average slope in the core is estimated as -1.06^{+0.26}_{-0.26} for the main sequence stars with 3.5 Msun < M < 120 Msun.Thanks to the SPHERE extreme adaptive optics, 814 low-mass stars were detected to estimate the MF slope for the pre-main sequence stars with 0.6 Msun< M < 3.5 Msun , Gamma = -0.54^{+0.11}_{-0.11} in the K-band images in two fields in the core of the cluster. For the first time, we derive the mass function of the very core of the NGC 3603 young cluster for masses in the range 0.6 - 120 Msun. Previous studies were either limited by crowding, lack of dynamic range, or a combination of both

Post conjunction detection of β\beta Pictoris b with VLT/SPHERE

With an orbital distance comparable to that of Saturn in the solar system, \bpic b is the closest (semi-major axis $\simeq$\,9\,au) exoplanet that has been imaged to orbit a star. Thus it offers unique opportunities for detailed studies of its orbital, physical, and atmospheric properties, and of disk-planet interactions. With the exception of the discovery observations in 2003 with NaCo at the Very Large Telescope (VLT), all following astrometric measurements relative to \bpic have been obtained in the southwestern part of the orbit, which severely limits the determination of the planet's orbital parameters. We aimed at further constraining \bpic b orbital properties using more data, and, in particular, data taken in the northeastern part of the orbit. We used SPHERE at the VLT to precisely monitor the orbital motion of beta \bpic b since first light of the instrument in 2014. We were able to monitor the planet until November 2016, when its angular separation became too small (125 mas, i.e., 1.6\,au) and prevented further detection. We redetected \bpic b on the northeast side of the disk at a separation of 139\,mas and a PA of 30$^{\circ}$ in September 2018. The planetary orbit is now well constrained. With a semi-major axis (sma) of $a = 9.0 \pm 0.5$ au (1 $\sigma$), it definitely excludes previously reported possible long orbital periods, and excludes \bpic b as the origin of photometric variations that took place in 1981. We also refine the eccentricity and inclination of the planet. From an instrumental point of view, these data demonstrate that it is possible to detect, if they exist, young massive Jupiters that orbit at less than 2 au from a star that is 20 pc away.Comment: accepted by A&