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The GRAVITY instrument has been revolutionary for near-infrared interferometry by pushing sensitivity and precision to previously unknown limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) in GRAVITY+, these limits will be pushed even further, with vastly improved sky coverage, as well as faint-science and high-contrast capabilities. This upgrade includes the implementation of wide-field off-axis fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser guide stars in an upgraded facility. GRAVITY+ will open up the sky to the measurement of black hole masses across cosmic time in hundreds of active galactic nuclei, use the faint stars in the Galactic centre to probe General Relativity, and enable the characterisation of dozens of young exoplanets to study their formation, bearing the promise of another scientific revolution to come at the VLTI., Comment: Published in the ESO Messenger

Models of magnetically driven accretion and outflows reproduce many observational properties of T Tauri stars. For the more massive Herbig Ae/Be stars, the corresponding picture is not well established. Nonetheless, it is expected that accretion flows in pre-main-sequence stars are guided from the circumstellar disk to stellar regions of high latitude along the magnetic field lines inside a magnetosphere. Using near-infrared multi-epoch spectroscopic data obtained with ISAAC, CRIRES, and X-shooter on the VLT, we examined magnetospheric accretion in the two Herbig Ae stars HD101412 and HD104237. Spectroscopic signatures in He I 10830 and Pa_gamma, two near-infrared lines that are formed in a Herbig star's accretion region, show temporal modulation in both objects. For HD101412, this modulation is governed by its rotation period, which we could recover from the data. We could show that our spectroscopic observations can be explained within the magnetic geometry that we established earlier from magnetic field measurements. For HD104237, we struggled to clearly identify a rotation period. We intend to apply this method to a larger sample of Herbig Ae/Be stars to learn more about their rotation properties and the accretion mechanisms at work., Comment: 6 pages, 3 figures, to appear in "Radiative signatures from the Cosmos: A meeting in honor of Ivan Hubeny", 2018 October 23-26, Paris, France

The growing sample of magnetic stars shows a remarkable diversity in the properties of their magnetic fields. The overall goal of current studies is to understand the origin, evolution, and structure of stellar magnetic fields in stars of different mass at different evolutionary stages. In this chapter we discuss recent measurements together with the underlying assumptions in the interpretation of data and the requirements, both observational and theoretical, for obtaining a realistic overview of the role of magnetic fields in various types of stars., Comment: 23 pages, 3 figures, chapter 7 of "Astronomical Polarisation from the Infrared to Gamma Rays", published in Astrophysics and Space Science Library 460

Important diagnostics of the star/circumstellar interaction region of HerbigAe/Be stars are accessible by examining the temporal behavior of thenear-IR spectral lines He I λ10830 and Paschen gamma. We reduced andanalyzed 30 spectra of the strongly magnetic Herbig Ae star HD101412,acquired with the CRIRES and X-shooter spectrographs installed at theVLT (ESO, Chile). We found that the temporal behavior of these diagnosticlines in the near-infrared spectra can be explained by rotational modulationof line profiles generated by accreting gas with a period P = 20.53d.This period is about half of the magnetic rotation period P = 42.76dpreviously determined from measurements of the mean longitudinal magneticfield. Our finding indicates that the accreted matter falls onto the star in regions close to the magnetic poles intersecting the line-of-sight twiceduring the rotation cycle. The results of our study clearly demonstratethe power of such an analysis to diagnose the physical properties ofaccretion flows onto pre-main sequence stars. &nbsp

Herbig Ae/Be-type stars are analogs of T Tauri stars in the higher mass range. Spectropolarimetric studies involving sharp-lined Herbig Ae stars appear to be a promising approach for the detection of their magnetic fields. We report the results of our HARPS high-resolution spectropolarimetric analysis of four Herbig Ae stars with low v sin i values: HD 101412, HD 104237, HD 190073, and PDS 2. The role of contamination by the surrounding warm circumstellar matter for the appearance of Zeeman features is discussed for the first time.&nbsp

In this chapter, we give a brief introduction into the use of the Zeeman effect in astronomy and the general detection of magnetic fields in stars, concentrating on the use of FORS2 for longitudinal magnetic field measurements., Comment: 7 pages, 5 figures, chapter in "Determination of Atmospheric Parameters of B-, A-, F- and G-Type Stars", Springer (2014), eds. E. Niemczura, B. Smalley, W. Pych

Studies by Mark Swain and a colleague at the Max Planck Institut für Astronomie, coupled with results from past and ongoing projects at Harvey Mudd College, strongly suggest that it may be possible to achieve imaging performance comparable to the Hubble Space Telescope at relatively low cost using available, commercial products. This is achievable by placing a 2.4 m telescope, with readily available adaptive optics, on a 30 m tower located at a highelevation geological “dome” in Antarctica. An initial project surveyed relevant tower design approaches, then generated and evaluated six concept designs for telescope towers. Using data for typical and extreme wind at Dome C to generate wind loads, finite element analysis yielded lateral deflections at the top of 0.3 mm for typical winds and 12.1 mm for extreme gusts, with the lowest resonant frequency at 0.7 Hz; some tower concepts are innovative and allow for easy shipment, setup, and relocation. A subsequent project analyzed a tower designed by Hammerschlag and found fundamental resonance frequencies at 4.3 Hz for bending and 5.9 Hz for torsion; this project also designed and simulated an active telescope control system that maintained 17 milliarcsecond pointing error for the telescope atop the tower during typical wind conditions.

Studies by Mark Swain and a colleague at the Max Planck Institut für Astronomie, coupled with results from past and ongoing projects at Harvey Mudd College, strongly suggest that it may be possible to achieve imaging performance comparable to the Hubble Space Telescope at relatively low cost using available, commercial products. This is achievable by placing a 2.4 m telescope, with readily available adaptive optics, on a 30 m tower located at a highelevation geological “dome” in Antarctica. An initial project surveyed relevant tower design approaches, then generated and evaluated six concept designs for telescope towers. Using data for typical and extreme wind at Dome C to generate wind loads, finite element analysis yielded lateral deflections at the top of 0.3 mm for typical winds and 12.1 mm for extreme gusts, with the lowest resonant frequency at 0.7 Hz; some tower concepts are innovative and allow for easy shipment, setup, and relocation. A subsequent project analyzed a tower designed by Hammerschlag and found fundamental resonance frequencies at 4.3 Hz for bending and 5.9 Hz for torsion; this project also designed and simulated an active telescope control system that maintained 17 milliarcsecond pointing error for the telescope atop the tower during typical wind conditions.

We present simultaneous optical and near-infrared high angular resolution observations of the close Herbig Ae/Be binary star NX Pup which is associated with cometary globule~1. The reconstructed images have a diffraction--limited resolution of 62 mas in V, 75 mas in R (speckle masking reconstruction), and 115 mas in H, 156 mas in K (adaptive optics + post--processing). Compared to previous results we were able to derive better estimates on spectral type and luminosity and hence put better constraints on the evolutionary status (mass & age) of NX Pup A and B: with NX Pup A of spectral type F0-F2 we estimate the spectral type of NX Pup B in the range F7-G4, masses of 2 M$_\odot$ and 1.6--1.9 M$_\odot$, respectively, and an age of 3--5~Myr for both stars. We discuss the implication of the new age determination on the physical relation between NX Pup and the cometary globule. The dynamical lifetime of $\approx$ 10$^6$ yr for cometary globule 1 suggests that cometary globule 1 and the nearby cometary globule 2 represent transient phenomena and are left overs of a larger molecular cloud which in turn was the parental cloud of NX~Pup A and B and finally got dispersed by photoevaporation. The IR excess of NX Pup A can be modeled by a viscous accretion disk, which is cut off at $\approx$ 20 AU from the star. NX Pup B has a smaller IR excess which indicates that there is less circumstellar material present than around the primary., 9 pages, 6 Postscript figures, fig2.ps, fig3a.ps and fig3b.ps available upon request, uses l-aa-ps.sty and psfig.sty Postscript File also available at http://www.astro.uni-wuerzburg.de/supplements.html; A&A 1996 in press

We present the work developed within the science team of the Very Large Telescope Interferometer Spectro-Imager (VSI) during the Phase A studies. VSI aims at delivering ~ 1 milliarcsecond resolution data cubes in the near-infrared, with several spectral resolutions up to 12 000, by combining up to 8 VLTI telescopes. In the design of an instrument, the science case plays a central role by supporting the instrument construction decision, defining the top-level requirements and balancing design options. The overall science philosophy of VSI was that of a general user instrument serving a broad community. The science team addressed themes which included several areas of astrophysics and illustrated specific modes of operation of the instrument: a) YSO disks and winds; b) Multiplicity of young stars; c) Exoplanets; d) Debris disks; e) Stellar surface imaging; f) The environments of evolved stars; g) AGN tori; h) AGN's Broad Line Region; i) Supermassive black-holes; and j) Microlensing. The main conclusions can be summarized as follows: a) The accessible targets and related science are extremely sensitive to the instrument limiting magnitude; the instrument should be optimized for sensitivity and have its own fringe tracker. b) Most of the science cases are readily achievable with on-axis fringe tracking, off-axis fringe tracking enabling extra science. c) In most targets (YSOs, evolved stars and AGNs), the interpretation and analysis of circumstellar/nuclear dust morphology requires direct access to the gas via spectral resolved studies of emission lines, requiring at least a spectral resolution of 2 500. d) To routinely deliver images at the required sensitivity, the number of telescopes in determinant, with 6 telescopes being favored. e) The factorial increase in the number of closure phases and visibilities, gained in a single observation, makes massive surveys of parameters and related science for the first time possible. f) High dynamic range imaging and very high dynamic range differential closure phase are possible allowing the study of debris disks and characterization of pegasides. g) Spectro-imaging in the near-infrared is highly complementary to ALMA, adaptive optics and interferometric imaging in the thermal infrared.

A rotating nulling coronagraph has been built for use on ground-based telescopes. The system is based on the concept of sub-aperturing the pupil of the telescope with two elliptical apertures and combining the resulting two input beams on a single-mode fiber. By a relative π phase shift of the beams, the starlight can be nulled and a relatively faint companion star can be detected. Rotation of the aperture mask on the telescope pupil results in a signal similar to that expected from a space-borne telescope system such as the proposed TPF/Darwin interferometer. The design of the nulling coronagraph and the ancillary systems that are needed, such as the fringe tracker, are described and the potential for observations on telescopes such as the Palomar 200" is discussed. Results of a nulling experiment using a single mode fiber as a beam combiner for broadband light between 1.50 μm and 1.80 μm are shown.

Using the sub-milli-arcsecond resolution of the CHARA interferometer array and combining light with the 2-telescope combiner CHARA Classic, we have detected strong near-infrared (NIR) emission interior to the dust-sublimation radius of Herbig Ae stars MWC275 and AB Aur. The large contribution of this emission component, which we argue to be hot gas, to the total NIR spectral energy distribution (SED) is not predicted by current models of the dust evaporation front, indicating that the NIR disk is more complicated than expected. Furthermore, we demonstrate that the structure of the evaporation front in MWC275 is time variable, making single epoch, large uv coverage observations critical to decoding front geometry. With the commissioning of CHARA Michigan Phase Tracker in the summer of 2008, the Michigan Infrared Combiner (a 6 telescope combiner at CHARA) will become an ideal instrument for studying the evaporation front, achieving the required sensitivities to begin the first "true" interferometric imaging of the gas-dust transition region in young stellar objects (YSOs). Here, we summarize results on the evaporation front structure obtained with CHARA Classic and describe future prospects with CHARA MIRC in elucidating morphology of the gas-dust transition region.

This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer (TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like planets in the habitable zones around nearby stars. The overall technology roadmap is presented and progress with each of the testbeds is summarized. The current interferometer architecture, design trades, and the viability of possible reduced-scope mission concepts are also presented.

We report observations of the nova RS Ophiuchi using the Keck Interferometer Nuller (KIN) taken approximately 3.8 days following the most recent outburst that occurred on 2006 February 12. The KIN operates in N-band from 8 to 12.5 μm in a nulling mode. In this mode the stellar light is suppressed by a destructive fringe, effectively enhancing the contrast of the circumstellar material located near the star. In a second, constructive-fringe mode, the instrument detects primarily the light from the central, bright source. These are the outer and inner spatial regimes, respectively. We will describe the capabilities of the KIN, including these unique modes, and outline how they were key in our discovery that dust was created between nova events. We also show how these first results from the KIN are consistent with Spitzer data. The KIN data show evidence of enhanced neutral atomic hydrogen emission and atomic metals including silicon located in the inner spatial regime (< 4 AU from theWD) relative to the outer regime. There are also nebular emission lines and evidence of hot silicate dust in the outer spatial region, centered at approximately ~ 17 AU from the WD, that are not found in the inner regime. The KIN and Spitzer data suggest that these emissions were excited in the outer spatial regime before the blast wave reached these regions. We describe the present results in terms of a new model for dust creation in recurrent novae that includes an increase in density in the plane of the orbit of the two stars created by a spiral shock wave caused by the motion of the stars through the cool wind of the red giant star. These data show the power and potential of the nulling technique which has been developed for the detection of Earth-like planets around nearby stars for the Terrestrial Planet Finder Mission and Darwin missions.

The Precision Astronomical Visible Observations (PAVO) beam combiner is a new concept in visible beam combination, recently commissioned at the CHARA array. By creating spatially-modulated fringes in a pupil plane and then dispersing with an integral field unit, PAVO utilizes the full multi-r0 aperture of the CHARA array over a standard 50% (630-950nm) bandwidth. In addition, minimal optimized spatial filtering ensures calibration that is in principle as good as using single-mode fibers. We describe the design of and initial results from the PAVO instrument.

The Palomar Testbed Interferometer (PTI) is a long-baseline, near-infrared interferometer located on Palomar Mountain, California. PTI has been conducting science operations since 1997 and continues to function both as a productive scientific instrument and as a technical testbed. Here I will review the current status and near-term plans for PTI, highlight some of the science results of the last two years, and describe the recently released PTI data archive.

We present the design for a near-infrared (JHK) fringe tracker to be used at the CHARA Array, a long baseline optical interferometer located at Mount Wilson Observatory. The CHARA Michigan Phase-tracker (CHAMP) is being fabricated and tested at the University of Michigan and will be transported to the CHARA Array for general use. CHAMP is separate from the science combiners and can therefore be optimized for fringe tracking. It will modulate around fringe center by 1-2λ at up to 500 Hz and calculate phase offsets in real-time using a modified 'ABCD' method . Six pair-wise Mach-Zehnder combiners will phase the entire Array. We give an overview of the optical layout and discuss our design strategy. Components such as the path-length modulators, low-OH fiber transport system, 1024x1024 HAWAII-1 detector, and control computer are discussed.

This paper reports on efforts to control a tethered formation flight spacecraft array for NASA's SPECS mission using the SPHERES test-bed developed by the MIT Space Systems Laboratory. Specifically, advances in methodology and experimental results realized since the 2005 SPIE paper are emphasized. These include a new test-bed setup with a reaction wheel assembly, a novel relative attitude measurement system using force torque sensors, and modeling of non-ideal tethers to account for tether vibration modes. The nonlinear equations of motion of multi-vehicle tethered spacecraft with elastic flexible tethers are derived from Lagrange's equations. The controllability analysis indicates that both array resizing and spin-up are fully controllable by the reaction wheels and the tether motor, thereby saving thruster fuel consumption. Based upon this analysis, linear and nonlinear controllers have been successfully implemented on the tethered SPHERES testbed, and tested at the NASA MSFC's flat floor facility using two and three SPHERES configurations.

The nulling mode of the Keck Interferometer is being commissioned at the Mauna Kea summit. The nuller combines the two Keck telescope apertures in a split-pupil mode to both cancel the on-axis starlight and to coherently detect the residual signal. The nuller, working at 10 um, is tightly integrated with the other interferometer subsystems including the fringe and angle trackers, the delay lines and laser metrology, and the real-time control system. Since first 10 um light in August 2004, the system integration is proceeding with increasing functionality and performance, leading to demonstration of a 100:1 on-sky null in 2005. That level of performance has now been extended to observations with longer coherent integration times. An overview of the overall system is presented, with emphasis on the observing sequence, phasing system, and differences with respect to the V2 system, along with a presentation of some recent engineering data.

We present a brief review of recent scientific and technical advances at the Infrared Optical Telescope Array (IOTA). IOTA is a long-baseline interferometer located atop Mount Hopkins, Arizona. Recent work has emphasized the use of the three-telescope interferometer completed in 2002. We report on results obtained on a range of scientific targets, including AGB stars, Herbig AeBe Stars, binary stars, and the recent outburst of the recurrent nova RS Oph. We report the completion of a new spectrometer which allows visibility measurements at several high spectral resolution channels simultaneously. Finally, it is our sad duty to report that IOTA will be closed this year.

The Keck Interferometer (KI) combines the two 10m diameter Keck telescopes providing milliarcsecond angular resolution. KI has unique observing capabilities such as sensitive K-band V^2, L-band V^2 and N-band nulling modes. The instrument improvements and status of the Keck Interferometer since the 2010 SPIE meeting are summarized. We discuss the current capabilities of the KI, operational improvements, and the science from the KI during the past two years. We will conclude with a brief note on the closure of the KI facility. Details of dual field phase referencing developments and nulling science results are presented elsewhere at this conference.