Astronomical News

Report on the ESO Workshop Astronomy at High Angular Resolution

held at ESO Headquarters, Garching, Germany, 24–28 November 2014

Henri Boffin1 Linda Schmidtobreick1 Gaitee Hussain1 Jean-Philippe Berger1

1 ESO

A workshop took place in Brussels in 2000 on astrotomography, a generic term for indirect mapping techniques that can be applied to a huge variety of astrophysical systems, ranging from planets, single stars and binaries to active galactic nuclei. It appeared to be timely to revisit the topic given the many past, recent and forthcoming improvements in telescopes and instru- mentation. We therefore decided to repeat the astrotomography workshop, Figure 1. Photograph of the participants at the work- but to put it into the much broader con- shop in the entrance hall at ESO Headquarters. text of high angular resolution astron- omy. Many techniques, from lucky and – single-dish tomographic reconstruction telescope in La Silla (Merkle et al., 1989). speckle imaging, adaptive optics to methods (astrotomography), with AO is now part of many systems at the interferometry, are now widely employed microarcsecond resolution, but appli- Paranal Observatory, as well as in all to achieve high angular resolution and cable only to certain classes of objects. major observatories, and has led to many they have led to an amazing number of the most important ESO discoveries: of new discoveries. A summary of the The workshop was structured to present the supermassive black hole at the centre workshop themes is presented. the various techniques, with extensive of the Milky Way, the first image of an reviews, and then to consider the various exoplanet and imaging of the young discs scientific fields that have profited from around stars. AO comes in many differ- The workshop brought together people these techniques: exoplanets and brown ent flavours and can be defined by the from different communities (see the con- dwarfs, stars, binaries, and large-scale total corrected field of view versus the ference photograph, Figure 1), who use phenomena. We give a very short sum- Strehl ratio which the system achieves. various techniques to construct images at mary of the material presented; the pro- These flavours include ground-layer AO very high angular resolution, with the aim gramme of the workshop is available1 and (GLAO), multi-conjugated AO (MCAO) and of reviewing these methods, the progress the presentations can be downloaded2. eXtreme AO (XAO). All major telescopes in the field, the new harvest of results that have second generation AO instruments have been collected, as well as to pre- currently in operation, which can reach pare the next generation of astronomers Beating the seeing: Reaching the typically 15–50 milliarcsecond (mas) res- to use these tools and techniques. The ­diffraction limit olution with high Strehl ratios and have various techniques used to beat the ten times more actuators than the first ­seeing and the telescope diffraction limit Atmospheric turbulence creates inhomo- generation AO systems. can be roughly distributed as follows: geneities in the refractive index of air that – single-dish imaging methods have affects the image quality — the infamous At the Very Large Telescope (VLT), the been developed to beat the seeing at “seeing”. To counter this, astronomers second generation AO system is SPHERE the focus of telescopes (e,g., adaptive have devised a series of techniques, such (an XAO system), which is beginning reg- optics, lucky/speckle/holographic as lucky imaging, speckle imaging and ular operations (see article on Science imaging, sparse aperture masking) adaptive optics. Verification by Leibundgut et al., p. 2) and aiming at sub-arcsecond spatial reso­ is based on a system with 1600 actuators lution up to the diffraction limit of single Adaptive optics (AO), which has a long (see the result in Figure 2, right). A GLAO telescopes; history at ESO, aims at neutralising the system (part of the VLT Adaptive Optics – techniques that allow the diffraction atmospheric turbulence by a closed-loop Facility [AOF]) is under development at limit to be beaten in order to achieve system. As Julien Milli reminded us in ESO to assist instruments such as MUSE milliarcsecond angular resolution using his presentation, the first astronomical and Hawk-I, combining diffraction-limit direct (optical interferometry) and system ever online was Come-On at the imaging and relatively wide-field imaging ­indirect (e.g., spectro-astrometry) tech- Observatoire de Haute-Provence, which spectroscopy. Not least, the whole niques; and later became operational at the 3.6-metre ­concept of the European Extremely Large

52 The Messenger 159 – March 2015 Figure 2. Showing the NaCo L: sented in detail by Mike Ireland. In par- N progress achieved in ticular, he pointed out that there are AO: a comparative study of the disc around HR some moderate to high Strehl regimes, E 4796 A by NACO (left; where working in the Fourier plane using from Milli et al., 2014) a non-redundant mask offers improved and, more recently, with performance over standard pupil-plane SPHERE (right; from Milli et al. [2015] in prep.). analysis; the example of a putative form- ing planet in a young disc in LkCa 15 was discussed. As a complement, Makoto Uemura showed how sparse aperture N modelling can be applied, in a cross-­ disciplinary approach, to Very Long 1 arcsec 1 arcsec E Baseline Imaging (VLBI) images, Doppler –16 –11 –5.8–0.7 4.4 9.5 15 20 25 30 35 tomography and gamma-ray Compton imaging. In another vein, Jaeho Choi ­presented a non-interferometric phase- Telescope (E-ELT) requires the use of that, at the 5-metre Palomar telescope, differential imaging method that uses AO. The future lies also in the develop- AO and lucky imaging are combined, Foucault knife-edge filtering. ment of wide-field AO systems (WFAO), leading to the highest resolution image as shown by Benoît Neichel in his talk. ever taken in the visible on faint targets, By using multiple laser guide stars (LGS), with a resolution of 35 mas in the I-band Beating the diffraction limit: WFAO significantly increases the field and a Strehl ratio of 17 %. Gergely The milliarcsecond horizon of view of AO-corrected images. The first Csépány combined lucky imaging and AO such system in routine operation is GeMS, to study a sample of 38 T Tauri mul­tiple In order to reach higher spatial resolu- the Gemini Multi-Conjugate AO System, systems over a 20-year period, and was tions, different techniques, such as long- which uses five LGS and provides 87 by able to cover the orbits in several of them. baseline interferometry where two or 87 arcsecond AO-corrected images in In addition, Mackay presented a proposal more telescopes are combined to obtain the H-band with a resolution of 80 mas. for a mosaic of lucky imagers for the fringes, are required. The technique was Such wide-field images with uniform NTT. This project, called GravityCam, will reviewed by Jean-Baptiste LeBouquin. point-spread functions over the full field consist of no less than 100 CCDs with The contrast (“visibility”) and phase infor- of view are particularly useful for astro- 70 mas pixels and will mainly be used for mation (“closure” or “differential phases”) metric studies, for example in clusters or exoplanet microlensing surveys, down to of the interference fringe packets can at the Centre of the Milky Way. Moreover, below an Earth mass. be used to retrieve direct information on such developments are critical, as all the the observed object brightness distri­ ELTs rely on multi-LGS WFAO systems. Speckle imaging was introduced by bution. While the technique has been in ­Sridharan Rengaswamy, who showed the widespread use in the radio domain for Wolfgang Brandner reviewed lucky advantages of the method and its signifi- several decades, it is now also becoming ­imaging, which is often presented as a cant contribution to the imaging of binary a routine technique in the optical domain, poor man’s adaptive optics. It exploits stars. More details about this technique in particular at the VLT Interferometer moments of good seeing, that is, it takes is given in Rengaswamy et al. (2014). (VLTI) where the angular resolution in the many short exposures and then retains This technique was further developed, as near-infrared is of the order of a few milli- only those images that have the best speckle holography, by Rainer Schödel, arcseconds. quality (typically 10 % of the whole). The which he calls “lucky imaging on steroids” technique is still quite popular and new to reconstruct images of crowded and Two regimes of data analysis were pre- projects are being devised, especially “wide” fields of view of several tens of sented. In the first, the measurements since the availability of noise-free detec- arcseconds, while keeping the instrument are fitted with a parametric model of the tors such as L3 (Low Light Level, LLL) sensitivity. object. This is particularly well suited and extended multiplication (EM) charge when the number of measurements is coupled devices (CCDs), and the fact In sparse aperture masking (SAM; or limited, the object structure is well under- that these systems are quite affordable. non-redundant masking [NRM] depend- stood or the object is only marginally For example, the AstroLux Sur on the ing on which side of the Atlantic you resolved. This was, and still is, the bread ESO New Technology Telescope (NTT) work), a mask with a few holes is placed and butter of interferometry and is had a total hardware cost of only 50 000 in front of the AO system. This trans- applied to the measurement of precise euros. Lucky imaging is also quite useful forms the primary mirror into a separate- diameters, binary parameters or simple as it generally has fainter limiting magni- element, multiple-aperture interferometer morphological parameters; it is well tudes than most AO systems, and it can and allows diffraction-limited imaging ­illustrated by the spectacular case of the work in the visible where AO is not so of (bright) astrophysical targets to be expansion of a nova shell (Schaeffer et developed. As pioneer and expert, in his reconstructed, such as dust-enshrouded al., 2014) or the mass-loss in stars across presentation, Craig Mackay mentioned evolved stars. The technique was pre- the Hertzsprung–Russell diagram (e.g.,

The Messenger 159 – March 2015 53 Astronomical News Boffin H. et al., Report on the ESO Workshop “Astronomy at High Angular Resolution”

Boffin et al. 2014). In the second case, 0 0.25 0.50 0.75 1.00 the object complexity precludes simple modelling. If an object is well resolved Relative intensity (e.g., about 10 mas or more) it is then 2014 Beauty Contes t Median Images possible to acquire many measurements 20 VY CMa 1.61 μm Median Image VY CMa 1.67 μm Median Image VY CMa 1.73 μm Median Image in the (u,v)-plane in order to carry out image reconstruction. Fabien Baron intro- 10 duced the art of aperture synthesis and presented some magic tips and tricks 0 for imaging stellar surfaces with interfer- ometry, introducing the SQUEEZE and –10 SIMTOI software. Clearly still an active area of research, it was shown that a –20 large range of priors and model selection methods can be used. It is critical to 20 R Car 1. 61 μm Median Image R Car 1.67 μm Median Image R Car 1. 73 μm Median Image test the methods and in this respect, the Milliarcseconds blind interferometric beauty contest plays 10 an important role (Baron et al., 2012; see Figure 3). In the same vein, Christian 0 Hummel presented a novel imaging ­algorithm that allows the combination of –10 multi-channel interferometric data in a single image. –20 20 10 0–10 –20 20 10 0–10 –20 20 10 0–10 –20 Interferometry is now coming of age and Milliarcseconds the VLTI has become a major piece of infrastructure with which it is possible to Figure 3. Results from the 2014 image reconstruc- carry out surveys of hundreds of objects, tion contest (see Monnier et al., 2014) based on data recorded with VLTI PIONIER. Upper: The VY CMa such as for young stellar objects as part supergiant. Lower: The Mira star R Car. of the Exozodi survey, presented by Steve Ertel (see Ertel et al., p. 24). Gains in sen- sitivity now allow several dozens of active 201 closure phases. The resulting image Often, it is necessary to use many galactic nuclei (AGN) to be observed is clearly aspherical, and shows that ­complementary techniques to gain the and their nuclei to be probed at unprece- the surface is partially obscured by a mix- necessary knowledge. This was most dented angular resolution. Sebastian ture of dust and molecular opacity, simi- evident in Stephan Gillesen’s review on Hoenig explained how the VLTI observa- larly to R Coronae Borealis. The case the already legendary observations of tions challenge the popular dusty torus was made that the combination of image the Galactic Centre: many years of AO model and reveal the true complexity of reconstruction and spectral resolution observations have resolved the stars of the inner kiloparsecs of AGN, such as its should, in the future, become a major the central cluster and revealed the orbits clumpiness and the inflow–outflow con- development at the VLTI, since it should of the inner stars around the Galactic nection. This idea was supported by vari- allow astronomers to peer through the Centre. These observations have proved ous other talks on interferometric obser- molecular layers in the complex stellar the central object to be a supermassive vations that all agreed that there is more atmosphere. Michel Hillen looked at even black hole. Gillesen reported on follow-up structure present in the mass around the more evolved stars using interferometry observations that were taken to address black hole than previously assumed to study the discs around post-AGB stars more details and a campaign to follow (Daniel Asmus, Leonhard Burtscher & and to constrain their vertical structure the periastron passage of the star S2 in Noel Lopez Gonzaga). and dust distribution. 2017 with several AO instruments. Future observations with GRAVITY in the Galac- Pierre Cruzalèbes showed how to use New instruments will soon be available, tic Centre will be able to test whether interferometry to relate deviations from such as GRAVITY (Eisenhauer et al., ­Einstein’s theory of general relativity holds centro-symmetry of evolved stars to the 2011) and MATISSE (Lopez et al., 2014) in the vicinity of a black hole. K giant–red supergiant–asymptotic giant at the VLTI. Jean-Philippe Berger showed branch (AGB) star sequence. Zooming in particular how GRAVITY will be useful Stefan Kraus discussed the long-term in on one object, Claudia Paladini showed for exoplanet detection. The increase plans to build a planet formation imager nice results from an imaging project of in sensitivity through the use of efficient with the goal of studying the formation the carbon-rich Mira R Fornicis, taking fringe tracking was discussed by Jorg- process and early dynamical evolution data with PIONIER for six half-nights over Uwe Pott, while Romain Petrov provided of exoplanetary systems on spatial scales two weeks with three different quadru- a detailed analysis of the possibility of corresponding to the Hill sphere of the plets, leading to 294 visibility points and more sensitivity gains. forming planets. This requires angular

54 The Messenger 159 – March 2015 a) b)

Figure 4. Large-scale magnetic field maps obtained field maps have been used to test magnetospheric using Zeeman Doppler imaging are key inputs for accretion scenarios (Romanova et al., 2011). b) In numerical simulations of their coronae and extended main sequence stars they can be used to model stel- environments. a) In pre-main sequence stars that are lar winds, and the conditions in interplanetary envi- still accreting from their circumstellar discs, magnetic­ ronments (from Alvarado-Gomez et al., in prep.). resolution of 0.03 AU, or 0.2 mas, which looked at the cores of planetary nebulae, ets in time series of high-resolution clearly points to a whole new facility. while on another scale, Jonathan Stern ­spectra could be used to recover details used the same technique to spatially of the system’s spin–orbit alignment While interferometry provides direct resolve the kinematics of 0.1 mas wide through the Rossiter–McLaughlin effect. ­information with milliarcsecond resolution, quasar broad-line regions. Measuring these spin–orbit alignments Klaus Pontoppidan presented spectro- is important for understanding the origins astrometry, an inexpensive method Luca Pasquini presented a new method, of the system. In rapidly rotating stars, for measuring spatial properties of line- tachoas­trometry, which aims at doing similar studies are important for exo- emitting gas far beyond the diffraction astrometry with radial velocities, and is planet validation and for breaking degen- limit, and particularly powerful in deter- particularly adapted for double-lined eracy between v sin i and spin–orbit mining the position angles and sizes of spectroscopic binaries, leading to a pre- ­misalignment for low impact parameters. rotating and jet-like compact objects on cision of better than two mas. Tilted and retrograde orbits have been scales as small as 0.1 mas. The rather recovered for a number of systems, point- simple idea is to measure the centroid on ing to a number of different migration a spectrum as a function of wavelength, Microarcsecond resolution pathways. Studies of time series of near- giving two-dimensional information. The infrared spectra have been employed to technique has been applied to a wide The last set of techniques, allowing detect the transmission spectra of exo- range of astronomical objects, from astronomers to reach microarcsecond planets themselves, enabling the masses proto­planetary discs, binary stars, discs spatial resolutions, are known together of both the planet and star to be com- around massive stars to AGN. Although as astrotomography, and consist of puted. The recent detection of the dayside it can be carried out with most spectro- ­indirect imaging techniques from obser- spectrum of the hot Jupiters, τ Boo b, graphs, AO-fed high dispersion echelle vations of projections, that are obtained 51 Peg b and HD 189733b show that spectrographs allow much higher spatial through eclipses, Doppler shifts, and/or non-transiting planetary atmospheres can resolution to be reached and this is why time delays (Boffin et al., 2001). These be detected; with β Pic b found to spin instruments such as the CRyogenic high- techniques have been used to probe the significantly faster than any planet in the resolution InfraRedEchelle Spectrogrph orbits of exoplanets, map the surfaces Solar System. These are key pathfinder (CRIRES) or NIRSPEC with AO are ideal of a variety of stars and the structure of studies for the ELT. for this technique. Miwa Goto described accretion discs in binary systems. spectro-astrometry with CRIRES at the Stellar surface-imaging techniques, VLT to study the anomalies in discs Andrew Cameron showed how the dis- ­Doppler imaging, Zeeman Doppler imag- around stars, Mónica Blanco Cárdenas tinctive signature from transiting exoplan- ing and Roche tomography exploit the

The Messenger 159 – March 2015 55 Astronomical News Boffin H. et al., Report on the ESO Workshop “Astronomy at High Angular Resolution”

information in time series of intensity and phenomena, including pulsations, and and future instrumentation. Suzanne polarised spectra to reconstruct detailed can be applied to CVs. Jan Cechura Ramsay presented the current and future maps of stellar photospheres. In the described how hydrodynamic simula- ESO instruments that may be used for brightest, most rapidly rotating systems, tions can be used to construct synthetic high angular resolution, and the list is these maps can achieve a latitudinal ­Doppler tomograms of the high-mass quite long: MATISSE, GRAVITY, the future ­spatial resolution close to two degrees. X-ray binary, Cyg X-2, in both hard and AO Facility, ERIS, as well as many instru- Oleg Kochukhov, Julien Morin and Chris soft states. These can be directly com- ments to come with the E-ELT. Ulli Kaeufl Watson reviewed the application of inver- pared with observations and aid the presented the recent VISIR upgrade (see sion techniques to reconstruct surface ­conversion of tomograms, which are in Käufl et al., p. 15). brightness maps, inhomogeneities in velocity space, into spatial coordinates. chemical elements and the large-scale The workshop was purposely limited to magnetic fields on stars in systems Raymundo Baptista reviewed how the the optical and infrared domain, but of including: brown dwarfs; G–M-type main detailed structure in accretion discs can course high angular resolution astronomy sequence and pre-main sequence stars; be probed using eclipse mapping tech- can also be done in other wavelength Ap and Bp stars; B-type HgMn stars; and niques including the white dwarf, mass ranges, and the recent, amazing images late-type stars in cataclysmic variable donor, disc and outflowing gas. He also coming from ALMA (Atacama Large binaries (CVs) and X-ray binaries (exam- demonstrated how the time evolution of ­Millimeter/submillimeter Array) are a per- ples in Figure 4). These maps probe discs can be used to follow changes fect demonstration. If the workshop has ­fundamental processes including surface ­during outburst events and changes in shown how much progress is evident on differential rotation, timescales of flux disc viscosity during flickering (using light many fronts, it has also highlighted how emergence and diffusion processes, and curves of HT Cas spanning two years). very bright the future is! in CVs may explain the properties and Stephen Potter showed how photo-­ timescales of accretion outbursts. polarisation light curves could be inverted to create images of accretion shocks Acknowledgements Julian Alvarado Gomez presented a and detail in the structure near the white It is a pleasure to thank the other members of the robust criterion to prevent over/under-­ dwarf photosphere in magnetic CVs. Scientific Organising Committee who helped to fitting in maps obtained withDoppler ­ Genetic optimisation is found to be effec- set up a very nice scientific programme for a very imaging techniques. He demonstrated tive in finding robust fits to the Stokes stimulating meeting: Keith Horne, Danny Steeghs, how the large-scale magnetic fields parameters. The models used for these Klaus Strassmeier and Claus Tappert. Thanks go as well to the members of the Local Organising Com- reconstructed for cool stars can be used reconstructions employ realistic stratifica- mittee who made for a very smooth and enjoyable to model the extended environments tions of the accretion column structure workshop: Julian Alvarado Gómez, Stella Chasiotis-­ and mass loss properties using three- based on radiation hydrdodynamic simu- Klingner, Gergely Csépány and Jason Grunhut. dimensional magnetohydrodynamic mod- lations. The latest results of tomographic ­Special thanks should also go to Keith Horne for presenting the summary talk of the workshop and to els that have been developed for the Sun. studies of polars, covering consecutive Stella Chasiotis-Klingner for her efficient help in all Colin Hill reported the first measurement half-orbits for two systems and revealing the practical aspects of the organisation. of differential rotation at the surface of the variability in the structure of the mag- the K4-type secondary of the CV, AE Aqr netic accretion flow in V834 Cen and References in contrast to predictions of tidal locking HU Aqr, were presented by Enrico Kotze. causing solid-body rotation. The time- Baron, F. et al. 2012, SPIE, 8445, 84451E scales of the variability of activity implied Misty Bentz introduced the technique Boffin, H. M. J., Steeghs, D. & Cuypers, J. 2001, by the measured differential rotation rates of AGN reverberation mapping (or echo Astrotomography, Indirect Imaging Methods in Observational Astronomy, LNP Series 573, may contribute to the observed mass- mapping), where time delays between Springer transfer variations in the CV. signals in the continuum and in various Boffin, H. M. J. et al. 2014, The Messenger, 156, 35 broad lines are transformed into spatial Eisenhauer, F. et al. 2011, The Messenger, 143, 16 Tom Marsh introduced the technique information on the broad-line region Lopez, B. et al. 2014, The Messenger, 157, 5 Merkle, F. et al. 1989, The Messenger, 58, 1 of Doppler tomography, reviewing how (BLR). This allows the measurement of Milli, J. et al. 2014, arXiv:1407.2539 structure within accretion discs and flows the mass of the black hole and, together Monnier, J. D. et al. 2014, Proc. SPIE, 9146, 91461Q can be recovered from well-sampled with velocity delays of the lines, also Rengaswamy, S. et al. 2014, The Messenger, 155, 12 spectroscopic time series. Results were gives a detailed picture of the BLR itself. Romanova, M. M. et al. 2011, MNRAS, 411, 915 Schaefer, G. H. et al. 2014, Nature, 515, 234 shown for a range of accreting systems,­ As shown by Suvendu Rakshit, com­ showing how different diagnostics can bining the angular sizes of the interfero- allow more detail to be reconstructed. metric measurements with the rever­ Links Axel Schwope showed how Doppler beration maps yields geometric distances 1 Workshop programme: http://www.eso.org/sci/ tomography can be applied to polars to the AGNs, which is necessary for the meetings/2014/hires2014/program.html (cataclysmic binary stars) to map the calibration of black-hole masses as well 2 Workshop presentations available: detail of magnetic accretion spots as for general cosmology. http://bit.ly/1y4WR4W on white dwarfs. Petr Hadrava showed how Fourier disentangling of the spectra As is most fitting for an ESO workshop, of binaries can be used to study various the final session was devoted to current

56 The Messenger 159 – March 2015