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V838 Mon is a stellar merger remnant that erupted in 2002 in a luminous red novae event. Although it is well studied in the optical, near infrared and submillimeter regimes, its structure in the mid-infrared wavelengths remains elusive. We observed V838 Mon with the MATISSE (LMN bands) and GRAVITY (K band) instruments at the VLTI and also the MIRCX/MYSTIC (HK bands) instruments at the CHARA array. We geometrically modelled the squared visibilities and the closure phases in each of the bands to obtain constraints on physical parameters. Furthermore, we constructed high resolution images of V838 Mon in the HK bands, using the MIRA and SQUEEZE algorithms to study the immediate surroundings of the star. Lastly, we also modelled the spectral features seen in the K and M bands at various temperatures. The image reconstructions show a bipolar structure that surrounds the central star in the post merger remnant. In the K band, the super resolved images show an extended structure (uniform disk diameter $\sim 1.94$ mas) with a clumpy morphology that is aligned along a north-west position angle (PA) of $-40^\circ$. Whereas in the H band, the extended structure (uniform disk diameter $\sim 1.18$ mas) lies roughly along the same PA. However, the northern lobe is slightly misaligned with respect to the southern lobe, which results in the closure phase deviations. The VLTI and CHARA imaging results show that V838 Mon is surrounded by features that resemble jets that are intrinsically asymmetric. This is also confirmed by the closure phase modelling. Further observations with VLTI can help to determine whether this structure shows any variation over time, and also if such bipolar structures are commonly formed in other stellar merger remnants., Comment: 18 pages, 34 figures

With a possible angular resolution down to 0.1-0.2 millisecond of arc using the 330 m baselines and the access to the 600-900 nm spectral domain, the CHARA Array is ideally configured for focusing on precise and accurate fundamental parameters of stars. CHARA/SPICA (Stellar Parameters and Images with a Cophased Array) aims at performing a large survey of stars all over the Hertzsprung-Russell diagram. This survey will also study the effects of the different kinds of variability and surface structure on the reliability of the extracted fundamental parameters. New surface-brightness-colour relations will be extracted from this survey, for general purposes on distance determination and the characterization of faint stars. SPICA is made of a visible 6T fibered instrument and of a near-infrared fringe sensor. In this paper, we detail the science program and the main characteristics of SPICA-VIS. We present finally the initial performance obtained during the commissioning.

Red supergiants (RSGs) are evolved massive stars in a stage preceding core-collapse supernova. Understanding evolved-phases of these cool stars is key to understanding the cosmic matter cycle of our Universe, since they enrich the cosmos with newly formed elements. However, the physical processes that trigger mass loss in their atmospheres are still not fully understood, and remain one of the key questions in stellar astrophysics. We use a new method to study the extended atmospheres of these cold stars, exploring the effect of a stellar wind for both a simple radiative equilibrium model and a semi-empirical model that accounts for a chromospheric temperature structure. We then can compute the intensities, fluxes and visibilities matching the observations for the different instruments at the Very Large Telescope Interferometer (VLTI). Specifically, when comparing with the atmospheric structure of HD 95687 based on published VLTI/AMBER data, we find that our model can accurately match these observations in the Kband, showing the enormous potential of this methodology to reproduce extended atmospheres of RSGs., Comment: 6 pages, 5 figures, to appear at the Proceedings of the IAUS370: Winds of stars and exoplanets. Eds: A. Vidotto and M. Smith-Spanier. Based on a contributed talk on preliminary work from Gonz\'alez-Tor\`a et al., submitted to A&A

In 2002, V838 Monocerotis (V838 Mon) erupted in a red novae event which has been interpreted to be a stellar merger. Soon after reaching peak luminosity, it began to cool, and its spectrum evolved to later spectral types. Dust was also formed in the post-merger remnant, making it bright in the mid-infrared. Interferometric studies at these wavelengths have suggested the presence of a flattened, elongated structure. We investigate, for the first time, the structure and orientation of the dusty envelope surrounding V838 Mon in the $L$(2.8-4.2 $\mu$m) band using recent observations with the MATISSE instrument at the VLTI. We perform simple geometrical modelling of the interferometric observables using basic models (disks, Gaussians, point sources, along with their combinations). We also reconstructed an image and analyzed the corresponding $L$-band spectrum. This study indicates the presence of an elongated, disk-like structure near 3.5$\mu$m, similar to what has been observed in other wavelength regimes. In particular, the orientation at a position angle of -40 degrees agrees with prior measurements in other bands. The dusty elongated structure surrounding V838 Mon appears to be a stable and long lived feature that has been present in the system for over a decade. Its substructure and origin remain unclear, but may be related to mass loss phenomena that took place in the orbital plane of the merged binary., Comment: 9 pages, 11 figures

The knowledge of the structure and dynamics of AGB-star atmospheres is crucial to better understand the mass loss. The tomographic method, that relies on the design of spectral masks containing lines forming in given ranges of optical depths in the stellar atmosphere, is an ideal technique for this purpose. It is applied to high-resolution spectro-interferometric VLTI/AMBER observations of the Mira-type AGB star S Ori. First, the interferometric visibilities are extracted at wavelengths contributing to the tomographic masks and fitted to those computed from a uniform disk model. This allows the measurement of the geometrical extent of the atmospheric layer probed by the corresponding mask. Then, we compare the observed atmospheric extension with those measured from available 1D pulsation CODEX models and 3D radiative-hydrodynamics CO5BOLD simulations. We found that while the average optical depths probed by the tomographic masks in S Ori decrease (with $<\log \tau_0> = -0.45$, $-1.45$, and $-2.45$ from the innermost to the central and outermost layers), the angular diameters of these layers increase, from 10.59 $\pm$ 0.09 mas through 11.84 $\pm$ 0.17 mas, up to 14.08 $\pm$ 0.15 mas. A similar behavior is observed when the tomographic method is applied to 1D and 3D dynamical models. Thus, this study derives, for the first time, a quantitative relation between optical- and geometrical-depth scales when applied to the Mira star S Ori, or to 1D and 3D dynamical models. In the context of Mira-type stars, the knowledge of the link between the optical and geometrical depths opens the way to derive the shock-wave propagation velocity, which can not be directly observed in these stars., Comment: Accepted for publication in A&A

Molecules and dust produced by the atmospheres of cool evolved stars contribute to a significant amount of the total material found in the interstellar medium. To understand the mechanism behind the mass loss of these stars, it is of pivotal importance to investigate the structure and dynamics of their atmospheres. Our goal is to verify if the extended molecular and dust layers of the carbon-rich asymptotic giant branch (AGB) star V Oph, and their time variations, can be explained by dust-driven winds triggered by stellar pulsation alone, or if other mechanisms are operating. We model V Oph mid-infrared interferometric VLTI-MIDI data ($8$-$13~\mu$m), at phases $0.18$, $0.49$, $0.65$, together with literature photometric data, using the latest-generation self-consistent dynamic atmosphere models for carbon-rich stars: DARWIN. We determine the fundamental stellar parameters: $T_\text{eff} = 2600~$K, $L_\text{bol} = 3585~$L$_{\odot}$, $M = 1.5~$M$_{\odot}$, $C/O = 1.35$, $\dot{M} = 2.50\cdot10^{-6}$M$_{\odot}$/yr. We calculate the stellar photospheric radii at the three phases: $479$, $494$, $448$ R$_{\odot}$; and the dust radii: $780$, $853$, $787$ R$_{\odot}$. The dynamic models can fairly explain the observed $N$-band visibility and spectra, although there is some discrepancy between the data and the models, which is discussed in the text. We discuss the possible causes of the temporal variations of the outer atmosphere, deriving an estimate of the magnetic field strength, and computing upper limits for the Alfv\'{e}n waves velocity. In addition, using period-luminosity sequences, and interferometric modeling, we suggest V Oph as a candidate to be reclassified as a semi-regular star., Comment: Accepted for publication in ApJ. 21 pages, 11 Figures

Cool, evolved stars are the main source of chemical enrichment of the interstellar medium (ISM), and understanding their mass loss and structure offers a unique opportunity to study the cycle of matter in the Universe. Pulsation, convection, and other dynamic processes in cool evolved stars create an atmosphere where molecules and dust can form, including those necessary to the formation of life (e.g.~Carbon-bearing molecules). Understanding the structure and composition of these stars is thus vital to several aspects of stellar astrophysics, ranging from ISM studies to modeling young galaxies and exoplanet research. Recent modeling efforts and increasingly precise observations now reveal that our understanding of cool stars photospheric, chromospheric, and atmospheric structures is limited by inadequate knowledge of the dynamic and chemical processes at work. Here we outline promising scientific opportunities for the next decade. We identify and discuss the following main opportunities: (1) identify and model the physical processes that must be included in current 1D and 3D atmosphere models of cool, evolved stars; (2) refine our understanding of photospheric, chromospheric, and outer atmospheric regions of cool evolved stars, their properties and parameters, through high-resolution spectroscopic observations, and interferometric observations at high angular resolution; (3) include the neglected role of chromospheric activity in the mass loss process of red giant branch and red super giant stars, and understand the role played by their magnetic fields; (4) identify the important shaping mechanisms for planetary nebulae and their relation with the parent asymptotic giant branch stars., Comment: 10 pages, 2 figures, White Paper submitted to the Astronomy and Astrophysics Decadal Survey (Astro2020)

This work presents an interferometric study of the massive-binary fraction in the Orion Trapezium Cluster with the recently comissioned GRAVITY instrument. We observe a total of 16 stars of mainly OB spectral type. We find three previously unknown companions for $\theta ^1$ Ori B, $\theta ^2$ Ori B, and $\theta ^2$ Ori C. We determine a separation for the previously suspected companion of NU Ori. We confirm four companions for $\theta ^1$ Ori A, $\theta ^1$ Ori C, $\theta ^1$ Ori D, and $\theta ^2$ Ori A, all with substantially improved astrometry and photometric mass estimates. We refine the orbit of the eccentric high-mass binary $\theta ^1$ Ori C and we are able to derive a new orbit for $\theta ^1$ Ori D. We find a system mass of 21.7 $M_{\odot}$ and a period of $53$ days. Together with other previously detected companions seen in spectroscopy or direct imaging, eleven of the 16 high-mass stars are multiple systems. We obtain a total number of 22 companions with separations up to 600 AU. The companion fraction of the early B and O stars in our sample is about 2, significantly higher than in earlier studies of mostly OB associations. The separation distribution hints towards a bimodality. Such a bimodality has been previously found in A stars, but rarely in OB binaries, which up to this point have been assumed to be mostly compact with a tail of wider companions. We also do not find a substantial population of equal-mass binaries. The observed distribution of mass ratios declines steeply with mass, and like the direct star counts, indicates that our companions follow a standard power law initial mass function. Again, this is in contrast to earlier findings of flat mass ratio distributions in OB associations. We exclude collision as a dominant formation mechanism but find no clear preference for core accretion or competitive accretion., Comment: Accepted for publication in A&A

High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized on the 15th and 16th of January, 2015 in Nice with the support of the Action Specific in Haute Resolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la Cote d'Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41., Comment: White Paper prospective. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41. v2 includes some corrections to text

High-mass multiples might form via fragmentation of self-gravitational disks or alternative scenarios such as disk-assisted capture. However, only few observational constraints exist on the architecture and disk structure of high-mass protobinaries and their accretion properties. Here we report the discovery of a close ($57.9\pm0.2$mas=170au) high-mass protobinary, IRAS17216-3801, where our VLTI/GRAVITY+AMBER near-infrared interferometry allows us to image the circumstellar disks around the individual components with 3 milliarcsecond resolution. We estimate the component masses to $\sim20$ and $\sim18 M_{\odot}$ and find that the radial intensity profiles can be reproduced with an irradiated disk model, where the inner regions are excavated of dust, likely tracing the dust sublimation region in these disks. The circumstellar disks are strongly misaligned with respect to the binary separation vector, which indicates that the tidal forces did not have time to realign the disks, pointing towards a young dynamical age of the system. We constrain the distribution of the Br$\gamma$ and CO-emitting gas using VLTI/GRAVITY spectro-interferometry and VLT/CRIRES spectro-astrometry and find that the secondary is accreting at a higher rate than the primary. VLT/NACO imaging shows $L'$-band emission on 3-4 times larger scales than the binary separation, matching the expected dynamical truncation radius for the circumbinary disk. The IRAS17216-3801 system is $\sim3\times$ more massive and $\sim5\times$ more compact than other high-mass multiplies imaged at infrared wavelengths and the first high-mass protobinary system where circumstellar and circumbinary dust disks could be spatially resolved. This opens exciting new opportunities for studying star-disk interactions and the role of multiplicity in high-mass star formation., Comment: 9 pages, 4 figures, accepted for publication in The Astrophysical Journal Letters

This paper presents a summary of four invited and twelve contributed presentations on asymptotic giant branch stars and red supergiants, given over the course of two afternoon splinter sessions at the 19th Cool Stars Workshop. It highlights both recent observations and recent theory, with some emphasis on high spatial resolution, over a wide range of wavelengths. Topics covered include 3D models, convection, binary interactions, mass loss, dust formation and magnetic fields., Comment: To appear in the proceedings of the 19th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun; 19 pages and 20 figures

The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to about 100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the "Hill Sphere" of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs., Comment: 13 pages, 5 figures, Proceedings of SPIE 2016

We discuss and illustrate contributions that optical interferometry has made on our current understanding of cool evolved stars. We include red giant branch (RGB) stars, asymptotic giant branch (AGB) stars, and red supergiants (RSGs). Studies using optical interferometry from visual to mid-infrared wavelengths have greatly increased our knowledge of their atmospheres, extended molecular shells, dust formation, and winds. These processes and the morphology of the circumstellar environment are important for the further evolution of these stars toward planetary nebulae (PNe) and core-collapse supernovae (SNe), and for the return of material to the interstellar medium., Comment: To appear in the Book of the VLTI School 2013, held 9-21 Sep 2013 Barcelonnette (France), "What the highest angular resolution can bring to stellar astrophysics?", Ed. Millour, Chiavassa, Bigot, Chesneau, Meilland, Stee, EAS Publications Series (2015)

We present the latest update of the European Southern Observatory's Very Large Telescope interferometer (VLTI). The operations of VLTI have greatly improved in the past years: reduction of the execution time; better offering of telescopes configurations; improvements on AMBER limiting magnitudes; study of polarization effects and control for single mode fibres; fringe tracking real time data, etc. We present some of these improvements and also quantify the operational improvements using a performance metric. We take the opportunity of the first decade of operations to reflect on the VLTI community which is analyzed quantitatively and qualitatively. Finally, we present briefly the preparatory work for the arrival of the second generation instruments GRAVITY and MATISSE., Comment: 10 pages, 7 figures, Proceedings of the SPIE, 9146-19

The emission of warm dust dominates the mid-infrared spectra of active galactic nuclei (AGN). Only interferometric observations provide the necessary angular resolution to resolve the nuclear dust and to study its distribution and properties. The investigation of dust in AGN cores is hence one of the main science goals for the MID-infrared Interferometric instrument MIDI at the VLTI. As the first step, the feasibility of AGN observations was verified and the most promising sources for detailed studies were identified. This was carried out in a "snapshot survey" with MIDI using Guaranteed Time Observations. In the survey, observations were attempted for 13 of the brightest AGN in the mid-infrared which are visible from Paranal. The results of the three brightest, best studied sources have been published in separate papers. Here we present the interferometric observations for the remaining 10, fainter AGN. For 8 of these, interferometric measurements could be carried out. Size estimates or limits on the spatial extent of the AGN-heated dust were derived from the interferometric data of 7 AGN. These indicate that the dust distributions are compact, with sizes on the order of a few parsec. The derived sizes roughly scale with the square root of the luminosity in the mid-infrared, s ~ sqrt(L), with no clear distinction between type 1 and type 2 objects. This is in agreement with a model of nearly optically thick dust structures heated to T ~ 300 K. For three sources, the 10 micron feature due to silicates is tentatively detected either in emission or in absorption. Based on the results for all AGN studied with MIDI so far, we conclude that in the mid-infrared the differences between individual galactic nuclei are greater than the generic differences between type 1 and type 2 objects., Comment: 18 pages, 8 figures, updated to version published in A&A 502, 67-84

We have used near- and mid-infrared interferometry to investigate the pulsating atmosphere and the circumstellar environment of the Mira variable RR Aql. Observations were taken with the VLTI/AMBER (near infrared) and the VLTI/MIDI (mid infrared) instruments. We have obtained a total of 15 MIDI epochs between Apr 9, 2004 and Jul 28, 2007 covering 4 pulsation cycles and one AMBER epoch on Sep 9, 2006 at phase 2.82. This work is also part of an ongoing project of joint VLTI and VLBA observations to study the connection between stellar pulsation and the mass loss process. Here we present a comparison of the AMBER visibility data to a simple uniform disk model as well as to predictions by recent self-excited dynamic model atmospheres. The best fitting photospheric angular diameter of the model atmosphere at phase 2.82 is 9.9 +/- 2.4 mas., Comment: 4 pages, 3 figures, to appear in Proc. of Cool Stars 15

Silicate carbon stars show the 10 micron silicate emission, despite their carbon-rich photospheres. They are considered to have circumbinary or circum-companion disks, which serve as a reservoir of oxygen-rich material shed by mass loss in the past. We present N-band spectro-interferometric observations of the silicate carbon star BM Gem using MIDI at the Very Large Telescope Interferometer (VLTI). Our aim is to probe the spatial distribution of oxygen-rich dust with high spatial resolution. BM Gem was observed with VLTI/MIDI at 44--62 m baselines using the UT2-UT3 and UT3-UT4 baseline configurations. The N-band visibilities observed for BM Gem show a steep decrease from 8 to ~10 micron and a gradual increase longward of ~10 micron, reflecting the optically thin silicate emission feature emanating from sub-micron-sized amorphous silicate grains. The differential phases obtained at baselines of ~44--46 m show significant non-zero values (~ -70 degrees) in the central part of the silicate emission feature between ~9 and 11 micron, revealing a photocenter shift and the asymmetric nature of the silicate emitting region. The observed N-band visibilities and differential phases can be fairly explained by a simple geometrical model in which the unresolved star is surrounded by a ring with azimuthal brightness modulation. The best-fit model is characterized by a broad ring (~70 mas across at 10 micron) with a bright region which is offset from the unresolved star by ~20 mas at a position angle of ~280 degrees. This model can be interpreted as a system with a circum-companion disk and is consistent with the spectroscopic signatures of an accretion disk around an unseen companion recently discovered in the violet spectrum of BM Gem., Comment: 7 pages, 3 figures, accepted for publication in A&A

We present N-band spectro-interferometric observations of the red supergiant WOH G64 in the Large Magellanic Cloud (LMC) using MIDI at the Very Large Telescope Interferometer (VLTI). The location of WOH G64 on the H-R diagram based on the previously estimated luminosities is in serious disagreement with the current stellar evolution theory. The dust envelope around WOH G64 has been spatially resolved with a baseline of ~60 m--the first MIDI observations to resolve an individual stellar source in an extragalactic system. The observed N-band visibilities show a slight decrease from 8 to 10 micron and a gradual increase longward of 10 micron, reflecting the 10 micron silicate feature in self-absorption. The visibilities measured at four position angles differing by ~60 degrees but at approximately the same baseline length (~60 m) do not show a noticeable difference, suggesting that the object appears nearly centrosymmetric. The observed N-band visibilities and spectral energy distribution can be reproduced by an optically and geometrically thick silicate torus model viewed close to pole-on. The luminosity of the central star is derived to be 2.8 x 10^5 Lsun, which is by a factor of 2 lower than the previous estimates based on spherical models. The lower luminosity newly derived from our MIDI observations and two-dimensional modeling brings the location of WOH G64 on the H-R diagram in much better agreement with theoretical evolutionary tracks for a 25 Msun star. We also identify the H2O absorption features at 2.7 and 6 micron in the spectra obtained with the Infrared Space Observatory and the Spitzer Space Telescope. The 2.7 micron feature originates in the photosphere and/or the extended molecular layers, while the 6 micron feature is likely to be of circumstellar origin., Comment: 9 pages, 6 figures, accepted for publication in A&A

Dusty tori have been suggested to play a crucial role in determining the physical characteristics of active galactic nuclei (AGN), but investigation of their properties has stalled for lack of high resolution mid-IR imaging. Recently, a long-awaited breakthrough in this field was achieved: NGC 1068, a nearby AGN, was the first extragalactic object to be observed with a mid-IR interferometer, thereby obtaining the needed angular resolution to study the alleged torus. In this proceeding, first the field of AGN research is briefly reviewed, with an emphasis on models of dusty tori. Second, the general properties of the key object NGC 1068 are discussed. Third, the MIDI mid-IR interferometric data set is presented together with a first attempt to interpret this data in the context of tori models. Fourth, preliminary MIDI interferometric spectra of the nucleus of the nearby starbursting galaxy Circinus are presented. Finally, we briefly discuss the prospects of ESA's Darwin mission for observing nearby and distant AGN. This mission will allow detailed mapping of tori of low luminosity AGN such as NGC 1068 up to redshifts of 1 - 2 and more luminous AGN up to redshift of 10 and beyond (abridged)., Comment: 10 pages, 3 figures, published in Proceedings of SPIE Volume 5491 "New Frontiers in Stellar Interferometry"

We present the first detection of the envelope which surrounds the post-AGB binary source HR 4049. VINCI-VLTI K-band interferometric observations of this source imply the existence of a large structure with a Gaussian angular FWHM 22.4 +/- 1.4 mas or uniform disk diameter of 34.9 +/- 1.9 mas. With the Hipparcos parallax of 1.50 +/- 0.64 mas these values correspond to a physical size of 14.9 (+11.1,-4.4) AU and 23.3 (+17.3,-7.0) AU, respectively. Our measurements, covering an azimuth range of about 60 degrees, for the sky-projected baseline, provide information on the geometry of the emitting region and show that there is only a slight variation of the measured angular values along the different directions sampled. Thus, our results are consistent with a spherical geometry of the envelope. However, we cannot completely rule out the existence of an asymmetric envelope (like the circumbinary disk envisaged by some recent models) because of the limited spatial frequency and azimuth range covered by the observations., Comment: 4 pages, 4 postscript figures, accepted letter for A&A

We present preliminary results on a recently started project to perform coordinated observations of Mira stars using near-infrared and radio long-baseline interferometry. We concentrate on recent observations of the Mira star S Ori. Observations with the ESO Very Large Telescope Interferometer (VLTI) were performed to measure the near-infrared diameter of the stellar surface. Concurrent VLBA observations of SiO maser emission towards this Mira star were performed to probe the structure and dynamics of the circumstellar atmosphere. Our near-infrared measurements suggest a diameter change consistent with the photometric period. The SiO maser emission appears to be at a distance of $\sim$ 2 stellar radii ($\Theta_{\rm UD} \sim$ 10.7 mas, $\Theta_{\rm SiO} \sim$ 20 mas, both at stellar phase $\sim$ 0.7), and to show a clumpy distribution within a ring-like structure., Comment: 5 pages, to appear in Proc. of "Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA", J.D Romney, M.J. Reid (eds.), ASP Conf. Ser

Long-baseline interferometry at optical and near-infrared wavelengths is an emerging technology which is quickly becoming a useful tool to investigate stellar atmospheres and to compare observations with models. Stellar atmosphere models have so far mainly been constrained by comparisons with stellar spectra which are integrated over the stellar disks. Interferometric observations provide spatially and spectrally resolved information and can thus provide important complementary observational information which can be compared to model predictions. Here, I summarize the different aspects on this topic which were discussed at a round table on Thursday, June 20, 2002, during IAU Symposium 210. This summary gives an overview on discussed interferometric facilities and techniques, concepts to study atmospheres by optical interferometry, and particular classes of objects. We conclude that more frequent interactions between the efforts of atmosphere modelling and interferometric observations promise to lead to increased confidence in stellar model atmospheres and to further advancement of the field in the next years., Comment: To appear in proceedings of IAU Symp. 210 "Modeling of stellar atmospheres". Invited Round Table Summary

So far, high resolution techniques on the one hand provide morphological information on bright nearby objects. On the other hand, telescopes with large collecting areas allow us to detect very faint and distant objects, but not to obtain a spatial resolution which is sufficient for detailed morphological studies. Currently, the construction of large optical and infrared interferometers like the Keck Interferometer, the Very Large Telescope Interferometer (VLTI) and the Large Binocular Telescope Interferometer (LBTI) is in progress. These instruments will simultaneously provide larger collecting areas and higher spatial resolutions than current instruments. Thus, they might enable for the first time near-infrared studies of galaxies in the young universe with an absolute spatial resolution as available today only for the closest galaxies. Using recent results in the field of high resolution studies of nearby galaxies, a rough idea of what might be expected to be observed is given. The concepts of the forthcoming interferometers is reviewed and technical aspects that are essential for observations of distant galactic centers are discussed. An outlook is given on which observational tasks can be addressed to these instruments and how the results will increase our knowledge in the field of the evolution of galaxies and the universe., Comment: 9 pages, to appear in proceedings of JSPS/DFG Japan-German Seminar 2001 "Studies of Galaxies in the Young Universe with New Generation Telescopes", July 24-28, Sendai, Japan