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.

/pdf/optical-interferometry-in-astronomy-290ps14qag.pdf

Optical interferometry in astronomy

We present high-resolution 4.6 mu m CO spectra of the circumstellar environments of two red supergiants (RSGs) that are potential supernova (SN) progenitors: Betelgeuse and VY Canis Majoris (VY CMa). Around Betelgeuse, (CO)-C-12 emission within +/-3 '' (+/-12 km s(-1)) follows a mildly clumpy but otherwise spherical shell, smaller than its similar to 55 '' shell in KI lambda 7699. In stark contrast, 4.6 mu m CO emission around VY CMa is coincident with bright KI in its clumpy asymmetric reflection nebula, within +/-5 '' (+/-40 km s(-1)) of the star. Our CO data reveal redshifted features not seen in KI spectra of VY CMa, indicating a more isotropic distribution of gas punctuated by randomly distributed asymmetric clumps. The relative CO and KI distribution in Betelgeuse arises from ionization effects within a steady wind, whereas in VY CMa, KI is emitted from skins of CO cloudlets resulting from episodic mass ejections 500-1000 yr ago. In both cases, CO and KI trace potential pre-SN circumstellar matter: we conclude that an extreme RSG like VY CMa might produce a Type IIn event like SN 1988Z if it were to explode in its current state, but Betelgeuse will not. VY CMa demonstrates that luminous blue variables are not necessarily the only progenitors of SNe IIn, but it underscores the requirement that SNe IIn suffer enhanced episodic mass loss shortly before exploding.

https://iopscience.iop.org/article/10.1088/0004-6256/137/3/3558/pdf

Red supergiants as potential Type IIn supernova progenitors: Spatially Resolved 4.6 Mum CO Emission Around VY CMa and Betelgeuse

We have observed Mira stars with the FLUOR beamcombiner on the IOTA interferometer in narrow bands around 2.2 μm wavelength. We find systematically larger diameters in bands contaminated by water vapor and CO. The visibility measurements can be interpreted with a model comprising a photosphere surrounded by a thin spherical molecular layer. The high quality of the fits we obtain demonstrates that this simple model accounts for most of the star's spatial structure. For each star and each period we were able to derive the radius and temperature of the star and of the molecular layer as well as the optical depth of the layer in absorption and continuum bands. The typical radius of the molecular layer is 2.2R * with a temperature ranging between 1500 and 2100 K. The photospheric temperatures we find are in agreement with spectral types of Mira stars. Our photospheric diameters are found smaller than in previous studies by several tens of percent. We believe previous diameters were biased by the use of unsuited geometrical models to explain visibilities. The conclusions of this work are various. First, we offer a consistent view of Mira stars over a wide range of wavelengths. Second, the parameters of the molecular layer we find are consistent with spectroscopic studies. Third, from our diameter measurements we deduce that all Mira stars are fundamental mode pulsators and that previous studies leading to the conclusion of the first-overtone mode were biased by too large diameter estimates.

/pdf/unveiling-mira-stars-behind-the-molecules-confirmation-of-1lw9qlqiaf.pdf

Unveiling Mira stars behind the molecules - Confirmation of the molecular layer model with narrow band near-infrared interferometry

We present first results of an experiment to combine data from Keck aperture masking and the Infrared-Optical Telescope Array to image the circumstellar environments of evolved stars with ~20 mas resolution. The unique combination of excellent Fourier coverage at short baselines and high-quality long-baseline fringe data allows us to determine the location and clumpiness of the innermost hot dust in the envelopes and to measure the diameters of the underlying stars themselves. We find evidence for large-scale inhomogeneities in some dust shells and also significant deviations from uniform brightness for the photospheres of the most evolved M stars. Deviations from spherically symmetric mass loss in the red supergiant NML Cyg could be related to recent evidence for dynamically important magnetic fields and/or stellar rotation. We point out that dust shell asymmetries, like those observed here, can qualitatively explain the difficulty recent workers have had in simultaneously fitting the broadband spectral energy distributions and high-resolution spatial information, without invoking unusual dust properties or multiple distinct shells (from hypothetical "superwinds"). This paper is the first to combine optical interferometry data from multiple facilities for imaging, and we discuss the challenges and potential for the future of this method, given current calibration and software limitations.

/pdf/high-resolution-imaging-of-dust-shells-by-using-keck-la8ns3ho0r.pdf

High-resolution imaging of dust shells by using Keck aperture masking and the IOTA interferometer

Supergiant stars such as Betelgeuse have very extended atmospheres, the properties of which are poorly understood. Alfven waves1,2,3,4, acoustic waves1,2,5,6,7 and radial pulsations8 have all been suggested as likely mechanisms for elevating these atmospheres and driving the massive outflows of gas seen in these stars: such mechanisms would heat the atmosphere from below, and there are indeed observations showing that Betelgeuse's extended atmosphere is hotter than the underlying photosphere9,10. Here we report radio observations of Betelgeuse that reveal the temperature structure of the extended atmosphere from two to seven times the photospheric radius. Close to the star, we find that the atmosphere has an irregular structure, and a temperature (3,450 ± 850 K) consistent with the photospheric temperature but much lower than that of gas in the same region probed by optical and ultraviolet observations10. This cooler gas decreases steadily in temperature with radius, reaching 1,370 ± 330 K by seven stellar radii. The cool gas coexists with the hot chromospheric gas, but must be much more abundant as it dominates the radio emission. Our results suggest that a few inhomogeneously distributed large convective cells (which are widely believed11,12,13,14,15,16 to be present in such stars) are responsible for lifting the cooler photospheric gas into the atmosphere; radiation pressure on dust grains that condense from this gas may then drive Betelgeuse's outflow.

Large convection cells as the source of Betelgeuse's extended atmosphere

Visibility observations at 11 km of o Ceti have been made with the University of California (Berkeley) Infrared Spatial Interferometer during the time period 1988E1995. The observed visibilities change dra- matically from one epoch to another and are not consistent with simple heating or cooling of the dust with change in luminosity as a function of stellar phase. Instead, large temporal variations in the density of dust within a few stellar radii of the photosphere of o Ceti have occurred. Spherically symmetric models of the dust distribution with two dust shells, one within three stellar radii of the photosphere of the star, the other approximately 10 stellar radii from the star, can account reasonably well for the observed changes. Four types of axially symmetric radiative transfer models were also compared with the dataEa spherical shell with an ellipsoidal inner cavity, a disk, a spherical shell with one or two inhomogeneities or clumps, and a set of thin partial shells with a -xed distance between them. Of the four models, only the one with the ellipsoidal inner cavity is excluded. The data were best--tted with the last two models, which emphasize inhomogeneities or clumps. To -t the observed temporal changes in the visibility data, all models must include a change in the densityEincreasing and decreasingEof dust close to the photosphere of the star. The axially symmetric models had clumps placed at distances from the star in agreement with distances of the spherical models. Good -ts to the observed broadband spec- trum of the star were also obtained with these models. Subject headings: circumstellar matter E infrared: stars E stars: individual (o Ceti) E stars: mass loss E stars: variables: long-period variables E techniques: interferometric

https://web.physics.ucsb.edu/~lipman/pubs/berkeley/lopez97.pdf

NONSPHERICAL STRUCTURES AND TEMPORAL VARIATIONS IN THE DUST SHELL OF o CETI OBSERVED WITH A LONG BASELINE INTERFEROMETER AT 11 MICRONS

Measurement at 11 Micron Wavelengths of the Diameters of alpha Orionis and alpha Scorpii: Changes in Effective Temperature of alpha Orionis and Very Recent Dust Emission

Radiative transfer in axisymmetric circumstellar dust shells.

First high spatial resolution observations of the carbon star IRAS 15194-5115 are presented Visibility curves are obtained in K (22 μm) and L (36 μm) spectral bands They mainly sample the dust shell component of the source These measurements are combined with photometric and spectroscopic data resulting both from ground-based and from the Infrared Astronomical Satellite (IRAS) observations A spherical dust shell modelling based on numerical simulation is proposed The optical depth at λ = 1 μm is estimated to be 8 and the inner boundary of the envelope to be at about 6 stellar radii A temperature profile for amorphous carbon dust is displayed The temperature of the hottest grains is about 900K The importance of long baseline interferometry observations is emphasized

D. Mekarnia

Papers

NONSPHERICAL STRUCTURES AND TEMPORAL VARIATIONS IN THE DUST SHELL OF o CETI OBSERVED WITH A LONG BASELINE INTERFEROMETER AT 11 MICRONS

Measurement at 11 Micron Wavelengths of the Diameters of alpha Orionis and alpha Scorpii: Changes in Effective Temperature of alpha Orionis and Very Recent Dust Emission

Radiative transfer in axisymmetric circumstellar dust shells.

Dust shell modelling of the carbon star IRAS 15194-5115