John D. Monnier

Imaging the Surface of Altair

Spatially resolving the surfaces of nearby stars promises to advance our knowledge of stellar physics. Using optical long-baseline interferometry, we constructed a near-infrared image of the rapidly rotating hot star Altair with a resolution of <1 milliarcsecond. The image clearly reveals the strong effect of gravity darkening on the highly distorted stellar photosphere. Standard models for a uniformly rotating star cannot explain our findings, which appear to result from differential rotation, alternative gravity-darkening laws, or both.

Optical interferometry in astronomy

Here I review the current state of the field of optical stellar interferometry, concentrating on ground-based work although a brief report of space interferometry missions is included. We pause both to reflect on decades of immense progress in the field as well as to prepare for a new generation of large interferometers just now being commissioned (most notably, the CHARA, Keck and VLT Interferometers). First, this review summarizes the basic principles behind stellar interferometry needed by the lay-physicist and general astronomer to understand the scientific potential as well as technical challenges of interferometry. Next, the basic design principles of practical interferometers are discussed, using the experience of past and existing facilities to illustrate important points. Here there is significant discussion of current trends in the field, including the new facilities under construction and advanced technologies being debuted. This decade has seen the influence of stellar interferometry extend beyond classical regimes of stellar diameters and binary orbits to new areas such as mapping the accretion discs around young stars, novel calibration of the cepheid period-luminosity relation, and imaging of stellar surfaces. The third section is devoted to the major scientific results from interferometry, grouped into natural categories reflecting these current developments. Lastly, I consider the future of interferometry, highlighting the kinds of new science promised by the interferometers coming on-line in the next few years. I also discuss the longer-term future of optical interferometry, including the prospects for space interferometry and the possibilities of large-scale ground-based projects. Critical technological developments are still needed to make these projects attractive and affordable.

A dusty pinwheel nebula around the massive star WR104

Wolf-Rayet (WR) stars are luminous, massive blue stars thought to be the immediate precursors to some supernovae. The existence of dust shells around such stars has been enigmatic since their discovery about 30 years ago, as the intense ultraviolet radiation from the star should be inimical to dust survival. Although dust creation models, including those involving interacting stellar winds, have been put forward to explain these dust shells, the high-resolution observations needed to distinguish between the models have hitherto been lacking. Here we present images of the dust outflow around WR104, obtained using a technique that allows us to resolve detail on scales of about 40 auat the distance of the star. Our images—taken at two epochs—show that the dust forms a spatially confined stream that follows precisely a linear (or archimedian) spiral trajectory with a rotation period of 220 ± 30 days. These results prove that, in this case, a binary companion is responsible for the creation of the circumstellar dust. Moreover, the spiral plume makes WR104 the prototype of a new class of circumstellar nebulae, which are unique to systems with interacting winds.

The Inner Regions of Protoplanetary Disks

To understand how planetary systems form in the dusty disks around premain-sequence stars, a detailed knowledge of the structure and evolution of these disks is required. Although this is reasonably well understood for the regions of the disk beyond about 1 AU, the structure of these disks inward of 1 AU remains a puzzle. This is partly because it is very difficult to spatially resolve these regions with current telescopes. But it is also because the physics of this region, where the disk becomes so hot that the dust starts to evaporate, is poorly understood. With infrared interferometry it has become possible in recent years to directly spatially resolve the inner 1 AU of protoplanetary disks, albeit in a somewhat limited way. These observations have partly confirmed current models of these regions, but also posed new questions and puzzles. Moreover, it has turned out that the numerical modeling of these regions is extremely challenging. In this review, we give a rough overview of the history and recent developments in this exciting field of astrophysics.

On the Interferometric Sizes of Young Stellar Objects

Long-baseline optical interferometers can now detect and resolve hot dust emission thought to arise at the inner edge of circumstellar disks around young stellar objects (YSOs). We argue that the near-infrared sizes being measured are closely related to the radius at which dust is sublimated by the stellar radiation field. We consider how realistic dust optical properties and gas opacity dramatically affect the predicted location of this dust destruction radius, an exercise routinely done in other contexts but so far neglected in the analysis of near-infrared sizes of YSOs. We also present the accumulated literature of near-infrared YSO sizes in the form of a size-luminosity diagram and compare with theoretical expectations. We find evidence that large (1.0 μm) dust grains predominate in the inner disks of T Tauri and Herbig Ae/Be stars, under the assumption that the innermost gaseous disks are optically thin at visible wavelengths.

The near-infrared size-luminosity relations for Herbig Ae/Be disks

We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes using the 85 m baseline Keck Interferometer. Targets were chosen to span the maximum range of stellar properties to probe the disk size dependenceonluminosityandeffectivetemperature.Formosttargets,themeasurednear-infraredsizes(rangingfrom0.2to 4AU)supportasimple diskmodelpossessingacentralopticallythin(dust-free) cavity,ringedbyhotdustemitting at theexpected sublimation temperatures (Ts � 1000–1500 K).Furthermore, wefindatightcorrelation of disksizewith source luminosity R / L 1 =2 for Ae and late Be systems (valid over more than two decades in luminosity), confirming earlier suggestions based on lower quality data. Interestingly, the inferred dust-free inner cavities of the highest luminosity sources (Herbig B0–B3 stars) are undersized compared to predictions of the ‘‘optically thin cavity’’ model, likely because of optically thick gas within the inner AU. Subject headingg accretion, accretion disks — circumstellar matter — instrumentation: interferometers — radiative transfer — stars: formation — stars: pre–main-sequence

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.

Imaging the spotty surface of Betelgeuse in the H band

Aims. This paper reports on H-band interferometric observations of Betelgeuse made at the three-telescope interferometer IOTA. We image Betelgeuse and its asymmetries to understand the spatial variation of the photosphere, including its diameter, limb darkening, effective temperature, surrounding brightness, and bright (or dark) star spots. Methods. We used different theoretical simulations of the photosphere and dusty environment to model the visibility data. We made images with parametric modeling and two image reconstruction algorithms: MIRA and WISARD. Results. We measure an average limb-darkened diameter of 44.28 ± 0.15 mas with linear and quadratic models and a Rosseland diameter of 45.03 ± 0.12 mas with a MARCS model. These measurements lead us to derive an updated effective temperature of 3600 ± 66 K. We detect a fully-resolved environment to which the silicate dust shell is likely to contribute. By using two imaging reconstruction algorithms, we unveiled two bright spots on the surface of Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5% of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of the total flux. Conclusions. Resolved images of Betelgeuse in the H band are asymmetric at the level of a few percent. The MOLsphere is not detected in this wavelength range. The amount of measured limb-darkening is in good agreement with model predictions. The two spots imaged at the surface of the star are potential signatures of convective cells.

A Data Exchange Standard for Optical (Visible/IR) Interferometry

This paper describes the OI (Optical Interferometry) Exchange Format, a standard for exchanging calibrated data from optical (visible/infrared) stellar interferometers. The standard is based on the Flexible Image Transport System (FITS) and supports the storage of optical interferometric observables, including squared visibility and closure phase—data products not included in radio interferometry standards such as UV-FITS. The format has already gained the support of most currently operating optical interferometer projects, including COAST, NPOI, IOTA, CHARA, VLTI, PTI, and the Keck Interferometer, and is endorsed by the IAU Working Group on Optical Interferometry. Software is available for reading, writing, and the merging of OI Exchange Format files.

Imaging and Modeling Rapidly Rotating Stars: α Cephei and α Ophiuchi

We present submilliarcsecond resolution imaging and modeling of two nearby rapid rotators α Cephei and α Ophiuchi, obtained with the CHARA array—the largest optical/IR interferometer in the world. Incorporating a gravity-darkening model, we are able to determine the inclination, the polar and equatorial radius and temperature, as well as the fractional rotation speed of the two stars with unprecedented precision. The polar and equatorial regions of the two stars have ∼2000 K temperature gradient, causing their apparent temperatures and luminosities to be dependent on their viewing angles. Our modeling allow us to determine the true effective temperatures and luminosities of α Cep and α Oph, permitting us to investigate their true locations on the H-R diagram. These properties in turn give us estimates of the masses and ages of the two stars within a few percent of error using stellar evolution models. Also, based on our gravity-darkening modeling, we propose a new method to estimate the masses of single stars in a more direct way through V sin i measurements and precise geometrical constraint. Lastly, we investigate the degeneracy between the inclination and the gravity-darkening coefficient, which especially affects the modeling of α Oph. Although incorporating V sin i has lifted the degeneracy to some extent, higher-resolution observations are still needed to further constrain the parameters independently.