Christopher A. Haniff

Michelson Interferometry with the Keck I Telescope

We report the —rst use of Michelson interferometry on the Keck I telescope for diUraction- limited imaging in the near-infrared JHKL bands. By using an aperture mask located close to the f/25 secondary, the 10 m Keck primary mirror was transformed into a separate-element, multiple-aperture interferometer. This has allowed diUraction-limited imaging of a large number of bright astrophysical targets, including the geometrically complex dust envelopes around a number of evolved stars. The suc- cessful restoration of these images, with dynamic ranges in excess of 200:1, highlights the signi—cant capabil- ities of sparse aperture imaging as compared with more conventional —lled-pupil speckle imaging for the class of bright targets considered here. In particular, the enhancement of the signal-to-noise ratio of the Fourier data, precipitated by the reduction in atmospheric noise, allows high-—delity imaging of complex sources with small numbers of short-exposure images relative to speckle. Multiepoch measurements con—rm the reliability of this imaging technique, and our whole data set provides a powerful demonstration of the capabilities of aperture-masking methods when utilized with the current generation of large-aperture tele- scopes. The relationship between these new results and recent advances in interferometry and adaptive optics is brie—y discussed.

The starburst ring around the Seyfert nucleus in NGC 7469

High-resolution radio continuum, optical emission line, and optical continuum images of the luminous Seyfert plus circumnuclear starburst hybrid galaxy NGC 7469 are presented. The radio emission from the starburst is largely confined to a ring of diameter 3 arcsec, or 1.0 kpc. There is clear evidence for optical continuum emission and probably optical line emission from the ring. The disks of NGC 1068 and NGC 7469 exhibit a strong similarity in almost all observed properties. Current star formation models can account for the luminous far-infrared emission and the strength of the nonthermal radio emission in NGC 7469, with an expected supernova rate of about one per year. The starburst ring is probably a result of concentration of gas into a ring via resonances between orbital motion and a rotating barlike or ovally distorted potential.

THE LAST GASPS OF VY CANIS MAJORIS: APERTURE SYNTHESIS AND ADAPTIVE OPTICS IMAGERY

We present new observations of the red supergiant VY CMa at 1.25, 1.65, 2.26, 3.08, and 4.8 km. Two complementary observational techniques were utilized: nonredundant aperture masking on the 10 m Keck I telescope, yielding images of the innermost regions at unprecedented resolution, and adaptive optics imaging on the ESO 3.6 m telescope at La Silla, attaining an extremely high (D105) peak-to-noise dynamic range over a wide —eld. For the —rst time the inner dust shell has been resolved in the near- infrared to reveal a one-sided extension of circumstellar emission within (D15 of the star. The 0A.1 R * ) line-of-sight optical depths of the circumstellar dust shell at 1.65, 2.26, and 3.08 km have been estimated to be 1.86 ^ 0.42, 0.85 ^ 0.20, and 0.44 ^ 0.11, respectively. These new results allow the bolometric lumi- nosity of VY CMa to be estimated independent of the dust shell geometry, yielding A L * B 2 ) 105 L _ . variety of dust condensations, including a large scattering plume and a bow-shaped dust feature, were observed in the faint, extended nebula up to 4A from the central source. While the origin of the nebulous plume remains uncertain, a geometrical model is developed assuming the plume is produced by radially driven dust grains forming at a rotating —ow insertion point with a rotational period between 1200 and 4200 yr, which is perhaps the stellar rotational period or the orbital period of an unseen companion. Subject headings: circumstellar matterdust, extinctionstars: individual (VY Canis Majoris) ¨ infrared: starstechniques: interferometric

Diffraction limited infrared images of the binary star T Tauri

High-resolution images of T Tau and its infrared companion have been reconstructed from near- and mid-infrared data collected at the Hale 5 m telescope. The near-infrared ( 1-5 µm) results were obtained by two dimensional speckle imaging and the mid-infrared (10-20 µm) results were derived from shift and add procedures applied to slit scans. The spectral energy distributions of the separated components were constructed from 1 to 20 µm data collected in less than half a year (1990 September to 1991 January). The spectral energy distribution of the optical component (T Tau N) is interpreted as containing two distinct constituents, a photosphere and a surrounding disk of circumstellar material. Measurements at a number of infrared wavelengths over the period 1985 December to 1991 January show a 2 mag color-independent change in the brightness of the infrared component (T Tau S). We propose that this may have been caused by an increase in accretion onto T Tau S and model the spectral energy distribution of T Tau S as being dominated by an accretion disk.

Least-squares Fourier phase estimation from the modulo 2π bispectrum phase

The recovery of Fourier phases from measurements of the bispectrum occupies a vital role in many astronomical speckle imaging schemes. In arecent paper [J. Opt. Soc. Am. A 7, 14 (1990)] it was suggested that a least-squares solution to this problem must fail if the bispectrum phase is known only modulo 2π. Here an alternative nonlinear least-squares algorithm is presented that differs from the linear method discussed in the aforementioned paper and that permits the fitting of Fourier phases directly to modulo 2π measurements of the bispectrum phase, thus eliminating any need for phase unwrapping. Numerical simulations of this method confirm that it is reliable and robust in the presence of noise and verify its enhanced performance when compared with a linear least-squares method that includes the unwrapping of the bispectral phase before Fourier phase retrieval.

The application of interferometry to optical astronomical imaging

In the first part of this review we survey the role optical/infrared interferometry now plays in ground–based astronomy. We discuss in turn the origins of astronomical interferometry, the motivation for its development, the techniques of its implementation, examples of its astronomical significance, and the limitations of the current generation of interferometric arrays. The second part focuses on the prospects for ground–based astronomical imaging interferometry over the near to mid–term (i.e. 10 years) at optical and near–infrared wavelengths. An assessment is made of the astronomical and technical factors which determine the optimal designs for imaging arrays. An analysis based on scientific capability, technical feasibility and cost argues for an array of large numbers of moderate–sized (2 m class) telescopes rather than one comprising a small number of much larger collectors.

Resolving the hot dust around HD69830 and η Corvi with MIDI and VISIR

Aims. Most of the known debris discs exhibit cool dust in regions analogous to the Edgeworth-Kuiper Belt. However, a rare subset show hot excess from within a few AU, which moreover is often inferred to be transient from models for planetesimal belt evolution. In this paper we examine 2 such sources to place limits on their location to help distinguish between different interpretations for their origin. Methods. We use MIDI on the VLTI to observe the debris discs around η Corvi and HD69830 using baseline lengths from 44–130 m. New VISIR observations of HD69830 at 18.7 μm are also presented. These observations are compared with disc models to place limits on disc size. Results. The visibility functions measured with MIDI for both sources show significant variation with wavelength across 8–13 μm in a manner consistent with the disc flux being well resolved, notably with a dip at 10–11.5 μm due to the silicate emission feature. The average ratio of visibilities measured between 10–11.5 μm and 8–9 μ mi s 0.934 ± 0.015 for HD69830 and 0.880 ± 0.013 for η Corvi over the four baselines for each source, a departure of 4 and 9σ from that expected if the discs were unresolved. HD69830 is unresolved by VISIR at 18.7 μm. The combined limits from MIDI and 8 m imaging constrain the warm dust to lie within 0.05–2.4 AU for HD69830 and 0.16–2.98 AU for η Corvi. Conclusions. These results represent the first resolution of dust around main sequence stars using mid-infrared interferometry. The constraints placed on the location of the dust are consistent with radii predicted by SED modelling (1.0 AU for HD69830 and 1.7 AU for η Corvi). Tentative evidence for a common position angle for the dust at 1.7 AU with that at 150 AU around η Corvi, which might be expected if the hot dust is fed from the outer disc, demonstrates the potential of this technique for constraining the origin of the dust and more generally for the study of dust in the terrestrial regions of main sequence stars.

Optical aperture synthetic images of the photosphere and molecular atmosphere of Mira

We have used aperture synthetic imaging methods to obtain diffraction-limited images of the photosphere and molecular atmosphere of the long period variable star Mira (o Ceti). These maps, obtained close to the peak of Miras variability cycle, clearly indicate substantial distortions from circular symmetry. Our image of the emission due to resonant scattering of TiO shows a significant contribution due to a cool atmosphere with a radius one and a half times as great as that of the photosphere. We suggest that the photospheric asymmetry is most likely associated with the intrinsic pulsation mode of the star and that stable nonradial pulsation modes can coexist with the radial modes that are responsible for the photometric variability. Further aperture synthetic mapping, which should be possible for a major fraction of the photometric cycle, is capable of verifying this hypothesis.

Nulling Data Reduction and On-sky Performance of the Large Binocular Telescope Interferometer

National Aeronautics and Space Administration, Exoplanet Exploration Program; National Aeronautics and Space Administration; European Union through ERC [279973]

Design and performance of COAST

The Cambridge Optical Aperture Synthesis Telescope, COAST, is a four-telescope array for high resolution imaging using measurements of complex visibilities and closure phases. This paper describes what its component parts are and why.