arXiv subjects

J. Girard

Publications and source records attributed to J. Girard.

At least 37 records · Page 2Linked to original sources

arXiv2020

CS Cha B: A disc-obscured M-type star mimicking a polarised planetary companion

Context. Direct imaging provides a steady flow of newly discovered giant planets and brown dwarf companions. These multi-object systems can provide information about the formation of low-mass companions in wide orbits and/or help us to speculate about possible migration scenarios. Accurate classification of companions is crucial for testing formation pathways. Aims. In this work we further characterise the recently discovered candidate for a planetary-mass companion CS Cha b and determine if it is still accreting. Methods. MUSE is a four-laser-adaptive-optics-assisted medium-resolution integral-field spectrograph in the optical part of the spectrum. We observed the CS Cha system to obtain the first spectrum of CS Cha b. The companion is characterised by modelling both the spectrum from 6300 $\unicode{x212B}$ to 9300 $\unicode{x212B}$ and the photometry using archival data from the visible to the near-infrared (NIR). Results. We find evidence of accretion and outflow signatures in H$\mathrmα$ and OI emission. The atmospheric models with the highest likelihood indicate an effective temperature of $3450\pm50$ K with a $\log{g}$ of $3.6\pm0.5$ dex. Based on evolutionary models, we find that the majority of the object is obscured. We determine the mass of the faint companion with several methods to be between 0.07 $M_{\odot}$ and 0.71 $M_{\odot}$ with an accretion rate of $\dot{M} = 4 \times 10^{-11 \pm 0.4}$ Myr$^{-1}$. Conclusions. Our results show that CS Cha B is most likely a mid-M-type star that is obscured by a highly inclined disc, which has led to its previous classification using broadband NIR photometry as a planetary-mass companion. This shows that it is important and necessary to observe over a broad spectral range to constrain the nature of faint companions

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arXiv2020

VLT/SPHERE survey for exoplanets around young, early-type stars including systems with multi-belt architectures

Dusty debris disks around pre- and main-sequence stars are potential signposts for the existence of planetesimals and exoplanets. Giant planet formation is therefore expected to play a key role in the evolution of the disk. This is indirectly confirmed by extant sub-millimeter near-infrared images of young protoplanetary and cool dusty debris disks around main sequence stars usually showing substantial spatial structures. A majority of recent discoveries of imaged giant planets have been obtained around young, early-type stars hosting a circumstellar disk. In this context, we have carried out a direct imaging program designed to maximize our chances of giant planet discovery and targeting twenty-two young, early-type stars. About half of them show indication of multi-belt architectures. Using the IRDIS dual-band imager and the IFS integral field spectrograph of SPHERE to acquire high-constrast coronagraphic differential near-infrared images, we have conducted a systematic search in the close environment of these young, dusty and early-type stars. We confirmed that companions detected around HIP 34276, HIP 101800 and HIP 117452 are stationary background sources and binary companions. The companion candidates around HIP 8832, HIP 16095 and HIP 95619 are determined as background contamination. For stars for which we infer the presence of debris belts, a theoretical minimum mass for planets required to clear the debris gaps can be calculated . The dynamical mass limit is at least $0.1 M_J$ and can exceed $1 M_J$. Direct imaging data is typically sensitive to planets down to $\sim 3.6 M_J$ at 1 $''$, and $1.7 M_J$ in the best case. These two limits tightly constrain the possible planetary systems present around each target. These systems will be probably detectable with the next generation of planet imagers.

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arXiv2020

ISPY -- NACO Imaging Survey for Planets around Young stars: Survey description and results from the first 2.5 years of observations

The occurrence rate of long-period giant planets around young stars is highly uncertain since it is not only governed by the protoplanetary disc structure and planet formation process, but also reflects dynamical re-structuring processes after planet formation as well as possible capture of planets not formed in-situ. Direct imaging is currently the only feasible method to detect such wide-orbit planets and constrain their occurrence rate. We carry out a large L'-band high-contrast direct imaging survey for giant planets around young stars with protoplanetary or debris discs using the NACO instrument at the ESO Very Large Telescope on Cerro Paranal in Chile. We use very deep angular differential imaging observations with typically >60 deg field rotation, and employ a vector vortex coronagraph where feasible to achieve the best possible point source sensitivity down to an inner working angle of about 100mas. This paper introduces our NACO Imaging Survey for Planets around Young stars ("NACO-ISPY"), its goals and strategy, the target list, and data reduction scheme, and presents preliminary results from the first 2.5 survey years. We achieve a mean 5 sigma L' contrast of 6.4mag at 150mas and a background limit of 16.5mag at >1.5". Our detection probability is >50\% for companions with 8\,M$_{\rm Jup}$\ at semi-major axes 80-200au. It thus compares well to the detection space of other state-of-the-art high-contrast imaging surveys. We have contributed to the characterisation of two new planets originally discovered by VLT/SPHERE, but we have not yet independently discovered new planets around any of our target stars. We report the discovery of close-in low-mass stellar companions around four young stars and show L'-band scattered light images of the discs around eleven stars, six of which have never been imaged at L'-band before.

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arXiv2019

SPHERE: the exoplanet imager for the Very Large Telescope

Observations of circumstellar environments to look for the direct signal of exoplanets and the scattered light from disks has significant instrumental implications. In the past 15 years, major developments in adaptive optics, coronagraphy, optical manufacturing, wavefront sensing and data processing, together with a consistent global system analysis have enabled a new generation of high-contrast imagers and spectrographs on large ground-based telescopes with much better performance. One of the most productive is the Spectro-Polarimetic High contrast imager for Exoplanets REsearch (SPHERE) designed and built for the ESO Very Large Telescope (VLT) in Chile. SPHERE includes an extreme adaptive optics system, a highly stable common path interface, several types of coronagraphs and three science instruments. Two of them, the Integral Field Spectrograph (IFS) and the Infra-Red Dual-band Imager and Spectrograph (IRDIS), are designed to efficiently cover the near-infrared (NIR) range in a single observation for efficient young planet search. The third one, ZIMPOL, is designed for visible (VIR) polarimetric observation to look for the reflected light of exoplanets and the light scattered by debris disks. This suite of three science instruments enables to study circumstellar environments at unprecedented angular resolution both in the visible and the near-infrared. In this work, we present the complete instrument and its on-sky performance after 4 years of operations at the VLT.

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arXiv2019

The Potential of Exozodiacal Disks Observations with the WFIRST Coronagraph Instrument

The Wide Field Infrared Survey Telescope (WFIRST) Coronagraph Instrument (CGI) will be the first high-performance stellar coronagraph using active wavefront control for deep starlight suppression in space, providing unprecedented levels of contrast, spatial resolution, and sensitivity for astronomical observations in the optical. One science case enabled by the CGI will be taking images and(R~50)spectra of faint interplanetary dust structures present in the habitable zone of nearby sunlike stars (~10 pc) and within the snow-line of more distant ones(~20pc), down to dust density levels commensurate with that of the solar system zodiacal cloud. Reaching contrast levels below~10-7 for the first time, CGI will cross an important threshold in debris disks physics, accessing disks with low enough optical depths that their structure is dominated by transport phenomena than collisions. Hence, CGI results will be crucial for determining how exozodiacal dust grains are produced and transported in low-density disks around mature stars. Additionally, CGI will be able to measure the brightness level and constrain the degree of asymmetry of exozodiacal clouds around individual nearby sunlike stars in the optical, at the ~10x solar zodiacal emission level. This information will be extremely valuable for optimizing the observational strategy of possible future exo-Earth direct imaging missions, especially those planning to operate at optical wavelengths, such as Habitable Exoplanet Observatory (HabEx) and the Large Ultraviolet/Optical/Infrared Surveyor (LUVOIR).

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arXiv2019

Evidence for a circumplanetary disk around protoplanet PDS 70 b

We present the first observational evidence for a circumplanetary disk around the protoplanet PDS~70~b, based on a new spectrum in the $K$ band acquired with VLT/SINFONI. We tested three hypotheses to explain the spectrum: Atmospheric emission from the planet with either (1) a single value of extinction or (2) variable extinction, and (3) a combined atmospheric and circumplanetary disk model. Goodness-of-fit indicators favour the third option, suggesting circumplanetary material contributing excess thermal emission --- most prominent at $λ\gtrsim 2.3 μ$m. Inferred accretion rates ($\sim 10^{-7.8}$--$10^{-7.3} M_J$ yr$^{-1}$) are compatible with observational constraints based on the H$α$ and Br$γ$ lines. For the planet, we derive an effective temperature of 1500--1600 K, surface gravity $\log(g)\sim 4.0$, radius $\sim 1.6 R_J$, mass $\sim 10 M_J$, and possible thick clouds. Models with variable extinction lead to slightly worse fits. However, the amplitude ($ΔA_V \gtrsim 3$mag) and timescale of variation ($\lesssim$~years) required for the extinction would also suggest circumplanetary material.

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arXiv2019

Geophysical signature of the transition zone between the sedimentary cover and the basement: an analogue approach to help de-risking geothermal prospects

Within the frame of the ANR-funded CANTARE-Alsace project, we have undertaken a multi-scale and multidisciplinary approach to increase our knowledge of the transition zone between the sedimentary cover and the basement and provide fundamental knowledge for the assessment of its geothermal potential. In this paper, we report out the results of a study performed on an exhumed transition zone in the Ringelbach area in the Vosges Mountains, on the flank of the Rhine graben. In this analogue of a deeply buried transition zone of the Rhine Graben, a thin layer of Triassic sandstones is still present on the top of the fractured and altered granitic basement providing the opportunity to study in-situ the physical properties of this transition zone. In this paper, we focused on electrical and acoustic properties of the transition zone as they are the main physical parameters usually assessed with the help of geophysical methods during the exploration phase of a geothermal project.

physics.geo-ph↗

arXiv2019

Separating extended disc features from the protoplanet in PDS 70 using VLT/SINFONI

Transition discs are prime targets to look for protoplanets and study planet-disc interactions. We present VLT/SINFONI observations of PDS~70, a transition disc with a recently claimed embedded protoplanet. We take advantage of the angular and spectral diversity present in our data for an optimal PSF modeling and subtraction using principal component analysis (PCA). We report the redetection of PDS 70 b, both the front and far side of the outer disc edge, and the detection of several extended features in the annular gap. We compare spectral differential imaging applied before (PCA-SADI), and after (PCA-ASDI) angular differential imaging. Our tests suggest that PCA-SADI better recovers extended features, while PCA-ASDI is more sensitive to point sources. We adapted the negative fake companion (NEGFC) technique to infer the astrometry of the companion, and derived $r = 193.5 \pm 4.9 \mathrm{mas}$ and PA = 158.7deg $\pm$ 3.0deg. We used both NEGFC and ANDROMEDA to infer the $K$-band spectro-photometry of the protoplanet, and found results consistent with recent VLT/SPHERE observations, except for their 2018/02 epoch measurement in the $K2$ filter. Finally, we derived an upper limit of $\dot{M_b} < 1.26 \times 10^{-7} \big[ \frac{5 M_{\rm Jup}}{M_b} \big] \big[ \frac{R_b}{R_{\rm Jup}}\big] M_{\rm Jup} $ yr$^{-1}$ for the accretion rate of the companion based on an adaptation of PCA-SADI/PCA-ASDI around the Br$γ$ line.

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arXiv2018

Spectral and orbital characterisation of the directly imaged giant planet HIP 65426 b

HIP 65426 b is a recently discovered exoplanet imaged during the course of the SPHERE-SHINE survey. Here we present new $L'$ and $M'$ observations of the planet from the NACO instrument at the VLT from the NACO-ISPY survey, as well as a new $Y-H$ spectrum and $K$-band photometry from SPHERE-SHINE. Using these data, we confirm the nature of the companion as a warm, dusty planet with a mid-L spectral type. From comparison of its SED with the BT-Settl atmospheric models, we derive a best-fit effective temperature of $T_{\text{eff}}=1618\pm7$ K, surface gravity $\log g=3.78^{+0.04}_{-0.03}$ and radius $R=1.17\pm0.04$ $R_{\text{J}}$ (statistical uncertainties only). Using the DUSTY and COND isochrones we estimate a mass of $8\pm1$ $M_{\text{J}}$. Combining the astrometric measurements from our new datasets and from the literature, we show the first indications of orbital motion of the companion (2.6$σ$ significance) and derive preliminary orbital constraints. We find a highly inclined orbit ($i=107^{+13}_{-10}$ deg) with an orbital period of $800^{+1200}_{-400}$ yr. We also report SPHERE sparse aperture masking observations that investigate the possibility that HIP 65426 b was scattered onto its current orbit by an additional companion at a smaller orbital separation. From this data we rule out the presence of brown dwarf companions with masses greater than 16 $M_{\text{J}}$ at separations larger than 3 AU, significantly narrowing the parameter space for such a companion.

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arXiv2018

Post conjunction detection of $β$ 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 $σ$), 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.

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arXiv2018

Mapping of shadows cast on a protoplanetary disk by a close binary system

For a comprehensive understanding of planetary formation and evolution, we need to investigate the environment in which planets form: circumstellar disks. Here we present high-contrast imaging observations of V4046 Sagittarii, a 20-Myr-old close binary known to host a circumbinary disk. We have discovered the presence of rotating shadows in the disk, caused by mutual occultations of the central binary. Shadow-like features are often observed in disks\cite{garufi,marino15}, but those found thus far have not been due to eclipsing phenomena. We have used the phase difference due to light travel time to measure the flaring of the disk and the geometrical distance of the system. We calculate a distance that is in very good agreement with the value obtained from the Gaia mission's Data Release 2 (DR2), and flaring angles of $α= 6.2 \pm 0.6 $ deg and $α= 8.5 \pm 1.0 $ deg for the inner and outer disk rings, respectively. Our technique opens up a path to explore other binary systems, providing an independent estimate of distance and the flaring angle, a crucial parameter for disk modelling.

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arXiv2018

Characterization of low-mass companion HD 142527 B

The circumstellar disk of the Herbig Fe star HD 142527 is host to several remarkable features including a warped inner disk, a 120 au-wide annular gap, a prominent dust trap and several spiral arms. A low-mass companion, HD 142527 B, was also found orbiting the primary star at $\sim$14 au. This study aims to better characterize this companion, which could help explain its impact on the peculiar geometry of the disk. We observed the source with VLT/SINFONI in $H$+$K$ band in pupil-tracking mode. Data were post-processed with several algorithms based on angular differential imaging (ADI). HD 142527 B is conspicuously re-detected in most spectral channels, which enables us to extract the first medium-resolution spectrum of a low-mass companion within 0.1'' from its central star. Fitting our spectrum with both template and synthetic spectra suggests that the companion is a young M2.5$\pm$1.0 star with an effective temperature of $3500\pm100$ K, possibly surrounded with a hot (1700 K) circum-secondary environment. Pre-main sequence evolutionary tracks provide a mass estimate of $0.34\pm0.06 M_{\odot}$, independent of the presence of a hot environment. However, the estimated stellar radius and age do depend on that assumption; we find a radius of $1.37 \pm 0.05 R_{\odot}$ (resp. $1.96 \pm 0.10 R_{\odot}$) and an age of $1.8^{+1.2}_{-0.5}$ Myr (resp. $0.75 \pm 0.25$ Myr) in the case of the presence (resp. absence) of a hot environment contributing in $H$+$K$. Our new values for the mass and radius yield a mass accretion rate of $\sim$5 $\times 10^{-9} M_{\odot}$ yr$^{-1}$ (2-3% that of the primary). Our results illustrate thus the potential for SINFONI+ADI to characterize faint close-in companions. The new spectral type makes HD 142527 B a twin of the well known TW Hya T-Tauri star, and the revision of its mass to higher values further supports its role in shaping the disk.

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arXiv2018

Discovery of a planetary-mass companion within the gap of the transition disk around PDS 70

Young circumstellar disks are of prime interest to understand the physical and chemical conditions under which planet formation takes place. Only very few detections of planet candidates within these disks exist, and most of them are currently suspected to be disk features. In this context, the transition disk around the young star PDS 70 is of particular interest, due to its large gap identified in previous observations, indicative of ongoing planet formation. We aim to search for the presence of planets and search for disk structures indicative for disk-planet interactions and other evolutionary processes. We analyse new and archival near-infrared (NIR) images of the transition disk PDS 70 obtained with the VLT/SPHERE, VLT/NaCo and Gemini/NICI instruments in polarimetric differential imaging (PDI) and angular differential imaging (ADI) modes. We detect a point source within the gap of the disk at about 195 mas (about 22 au) projected separation. The detection is confirmed at five different epochs, in three filter bands and using different instruments. The astrometry results in an object of bound nature, with high significance. The comparison of the measured magnitudes and colours to evolutionary tracks suggests that the detection is a companion of planetary mass. We confirm the detection of a large gap of about 54 au in size within the disk in our scattered light images, and detect a signal from an inner disk component. We find that its spatial extent is very likely smaller than about 17 au in radius. The images of the outer disk show evidence of a complex azimuthal brightness distribution which may in part be explained by Rayleigh scattering from very small grains. Future observations of this system at different wavelengths and continuing astrometry will allow us to test theoretical predictions regarding planet-disk interactions, planetary atmospheres and evolutionary models.

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arXiv2018

Orbital and atmospheric characterization of the planet within the gap of the PDS 70 transition disk

Aims: We aim to characterize the orbital and atmospheric properties of PDS 70 b, which was first identified on May 2015 in the course of the SHINE survey with SPHERE, the extreme adaptive-optics instrument at the VLT. Methods: We obtained new deep SPHERE/IRDIS imaging and SPHERE/IFS spectroscopic observations of PDS 70 b. The astrometric baseline now covers 6 years which allows us to perform an orbital analysis. For the first time, we present spectrophotometry of the young planet which covers almost the entire near-infrared range (0.96 to 3.8 micrometer). We use different atmospheric models covering a large parameter space in temperature, log(g), chemical composition, and cloud properties to characterize the properties of the atmosphere of PDS 70 b. Results: PDS 70 b is most likely orbiting the star on a circular and disk coplanar orbit at ~22 au inside the gap of the disk. We find a range of models that can describe the spectrophotometric data reasonably well in the temperature range between 1000-1600 K and log(g) no larger than 3.5 dex. The planet radius covers a relatively large range between 1.4 and 3.7 R_jupiter with the larger radii being higher than expected from planet evolution models for the age of the planet of 5.4 Myr. Conclusions: This study provides a comprehensive dataset on the orbital motion of PDS 70 b, indicating a circular orbit and a motion coplanar with the disk. The first detailed spectral energy distribution of PDS 70 b indicates a temperature typical for young giant planets. The detailed atmospheric analysis indicates that a circumplanetary disk may contribute to the total planet flux.

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arXiv2018

The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event

Proxima Centauri, our closest stellar neighbour, is a low-mass M5 dwarf orbiting in a triple system. An Earth-mass planet with an 11 day period has been discovered around this star. The star's mass has been estimated only indirectly using a mass-luminosity relation, meaning that large uncertainties affect our knowledge of its properties. To refine the mass estimate, an independent method has been proposed: gravitational microlensing. By taking advantage of the close passage of Proxima Cen in front of two background stars, it is possible to measure the astrometric shift caused by the microlensing effect due to these close encounters and estimate the gravitational mass of the lens (Proxima Cen). Microlensing events occurred in 2014 and 2016 with impact parameters, the closest approach of Proxima Cen to the background star, of 1\farcs6 $\pm$ 0\farcs1 and 0\farcs5 $\pm$ 0\farcs1, respectively. Accurate measurements of the positions of the background stars during the last two years have been obtained with HST/WFC3, and with VLT/SPHERE from the ground. The SPHERE campaign started on March 2015, and continued for more than two years, covering 9 epochs. The parameters of Proxima Centauri's motion on the sky, along with the pixel scale, true North, and centering of the instrument detector were readjusted for each epoch using the background stars visible in the IRDIS field of view. The experiment has been successful and the astrometric shift caused by the microlensing effect has been measured for the second event in 2016. We used this measurement to derive a mass of 0.150$^{\textrm{+}0.062}_{-0.051}$ (an error of $\sim$ 40\%) \MSun for Proxima Centauri acting as a lens. This is the first and the only currently possible measurement of the gravitational mass of Proxima Centauri.

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arXiv2018

Dynamical masses of M-dwarf binaries in young moving groups: I - The case of TWA 22 and GJ 2060

Evolutionary models are widely used to infer the mass of stars, brown dwarfs, and giant planets. Their predictions are thought to be less reliable at young ages ($<$ 200 Myr) and in the low-mass regime ($\mathrm{<1~M_{\odot}}$). GJ 2060 AB and TWA 22 AB are two rare astrometric M-dwarf binaries respectively members of the AB Doradus and Beta Pictoris moving groups. As their dynamical mass can be measured within a few years, they can be used to calibrate the evolutionary tracks and set new constraints on the age of young moving groups. We find a total mass of $\mathrm{0.18\pm 0.02~M_\odot}$ for TWA 22. That mass is in good agreement with model predictions at the age of the Beta Pic moving group. We obtain a total mass of $\mathrm{1.09 \pm 0.10~M_{\odot}}$ for GJ 2060. We estimate a spectral type of M$1\pm0.5$, $\mathrm{L/L_{\odot}=-1.20\pm0.05}$ dex, and $\mathrm{T_{eff}=3700\pm100}$ K for GJ 2060 A. The B component is a M$3\pm0.5$ dwarf with $\mathrm{L/L_{\odot}=-1.63\pm0.05}$ dex and $\mathrm{T_{eff}=3400\pm100}$ K. The dynamical mass of GJ 2060 AB is inconsistent with the most recent models predictions (BCAH15, PARSEC) for an ABDor age in the range 50-150 Myr. It is 10 to 20\% (1-2 sigma, depending on the assumed age) above the models predictions, corresponding to an underestimation of $0.10$ to $0.20~\mathrm{M_\odot}$. Coevality suggests a young age for the system ($\sim$ 50 Myr) according to most evolutionary models. TWA 22 validates the predictions of recent evolutionary tracks at $\sim$20 Myr. On the other hand, we evidence a 1-2 sigma mismatch between the predicted and observed mass of GJ 2060 AB. This slight departure may indicate that one of the star hosts a tight companion. Alternatively, this would confirm the models tendency to underestimate the mass of young low-mass stars.

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arXiv2018

French SKA White Book - The French Community towards the Square Kilometre Array

The "Square Kilometre Array" (SKA) is a large international radio telescope project characterised, as suggested by its name, by a total collecting area of approximately one square kilometre, and consisting of several interferometric arrays to observe at metric and centimetric wavelengths. The deployment of the SKA will take place in two sites, in South Africa and Australia, and in two successive phases. From its Phase 1, the SKA will be one of the most formidable scientific machines ever deployed by mankind, and by far the most impressive in terms of data throughput and required computing power. With the participation of almost 200 authors from forty research institutes and six private companies, the publication of this French SKA white paper illustrates the strong involvement in the SKA project of the French astronomical community and of a rapidly growing number of major scientific and technological players in the fields of Big Data, high performance computing, energy production and storage, as well as system integration.

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arXiv2018

Observations of fast-moving features in the debris disk of AU Mic on a three-year timescale: Confirmation and new discoveries

The nearby and young M star AU Mic is surrounded by a debris disk in which we previously identified a series of large-scale arch-like structures that have never been seen before in any other debris disk and that move outward at high velocities. We initiated a monitoring program with the following objectives: 1) track the location of the structures and better constrain their projected speeds, 2) search for new features emerging closer in, and ultimately 3) understand the mechanism responsible for the motion and production of the disk features. AU Mic was observed at 11 different epochs between August 2014 and October 2017 with the IR camera and spectrograph of SPHERE. These high-contrast imaging data were processed with a variety of angular, spectral, and polarimetric differential imaging techniques to reveal the faintest structures in the disk. We measured the projected separations of the features in a systematic way for all epochs. We also applied the very same measurements to older observations from the Hubble Space Telescope (HST) with the visible cameras STIS and ACS. The main outcomes of this work are 1) the recovery of the five southeastern broad arch-like structures we identified in our first study, and confirmation of their fast motion (projected speed in the range 4-12 km/s); 2) the confirmation that the very first structures observed in 2004 with ACS are indeed connected to those observed later with STIS and now SPHERE; 3) the discovery of two new very compact structures at the northwest side of the disk (at 0.40" and 0.55" in May 2015) that move to the southeast at low speed; and 4) the identification of a new arch-like structure that might be emerging at the southeast side at about 0.4" from the star (as of May 2016). Abridged.

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