A new, comprehensive set of low-temperature opacity data has been assembled. From this basic data set, Rosseland and Planck mean opacities have been computed for temperatures between 12,500 and 700 K. In addition to the usual continuous absorbers, atomic line absorption (with more than 8 million lines), molecular line absorption (with nearly 60 million lines), and grain absorption and scattering (by silicates, iron, carbon, and SiC) have been accounted for. The absorption due to lines is computed monochromatically and included in the mean with the opacity sampling technique. Grains are assumed to form in chemical equilibrium with the gas and to form into a continuous distribution of ellipsoids. Agreement of these opacities with other recent tabulations of opacities for temperatures above 5000 K is excellent. It is shown that opacities which neglect molecules become unreliable for temperatures below 5000 K. Triatomic molecules become important absorbers at 3200 K. Similarly, grains must be included in the computation for temperatures below 1700 K.

Low Temperature Rosseland Opacities.

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.

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Optical interferometry in astronomy

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.

/pdf/imaging-the-spotty-surface-of-betelgeuse-in-the-h-band-d3gwb9kehc.pdf

Imaging the spotty surface of Betelgeuse in the H band

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

The angular diameters of α Orionis and o Ceti were measured at a wavelength of 11.15 μm using the two-telescope Infrared Spatial Interferometer (ISI). Based on fitting the visibility data to uniform disk models, the diameter of α Orionis is 54.7 ± 0.3 mas and that of o Ceti at phase 0.90 is 47.8 ± 0.5 mas. These diameters are the most precise ever measured in the mid-infrared, due in part to the addition of a 56 m baseline to the ISI, which provided sufficient resolution to observe the first zero in the visibility function of the stellar disks. Moreover, the effects of limb darkening and stellar hot spots are small at these wavelengths. Theoretically estimated limb-darkening effects indicate the actual diameters are approximately 1% ± 0.5% larger than the uniform disk approximation, or 55.2 ± 0.5 and 48.2 ± 0.6 mas for α Orionis and o Ceti, respectively.

Precision Measurements of the Diameters of α Orionis and ο Ceti at 11 Microns

The variations in Mira's apparent size across the near- and mid-infrared are explained in terms of a single H2O shell surrounding the star. This simple model, consisting of a 2200 K H2O shell of column density 7 × 1019 cm-2 and radius 30.3 mas surrounding a 2700 K star of radius 12.8 mas, successfully reproduces a wide variety of interferometric and photometric observations that have been made near maximum phase.

Mira’s Apparent Size Variations due to a Surrounding Semiopaque H2O Layer

The size of the continuum photospheres of α Ori, α Her, R Leo, and χ Cyg have been measured at 11 μm using heterodyne interferometry to accuracies as high as 1%. An assortment of narrow wavelength bands near 11 μm (each having a width ~0.17 cm-1) were used to avoid spectral lines. The resulting apparent diameters for α Ori and α Her are ~30% larger than measured near-infrared sizes, whereas the Mira variables, R Leo and χ Cyg, have 11 μm apparent diameters roughly twice their reported near-infrared sizes. The complexities of interpretation are discussed.

https://iopscience.iop.org/article/10.1086/374779/pdf

Asymptotic giant branch and supergiant stellar diameters in the mid-infrared

The size and variability of the continuum photosphere of o Ceti have been measured with 11 μm heterodyne interferometry to an accuracy of about 1%. Narrow bandwidths (~0.17 cm-1) were used to avoid spectral lines and measure continuum only. The resulting 11 μm diameter of o Cet is larger than the previously measured visible and near-infrared sizes. In addition, variations in the diameter with phase and a possible elongation have been observed. Visibilities were also measured at wavelengths known to contain strong H2O spectral contamination, giving larger apparent stellar sizes and information on the distribution of hot H2O gas.

/pdf/interferometry-on-mira-in-the-mid-infrared-cyclic-54m2kx7ng1.pdf

Interferometry on Mira in the Mid-Infrared: Cyclic Variability of the Continuum Diameter and the Effect of Spectral Lines on Apparent Size

Mid-infrared observations of IK Tau have been made at 11.15 μm with the three-telescope Infrared Spatial Interferometer on Mount Wilson and also using individual segments of the Keck telescope for multiple-aperture interferometry on the Keck telescope at 10.7 μm. Both experiments provided closure phase and show temporal variations and asymmetries in the surrounding dust, with a difference of about 15% in intensity between two sides of the star. Asymmetries have been previously observed in the distribution of SiO masers closely surrounding the star. Comparison with earlier interferometric measurements shows substantial reduction in dust surrounding the star over the last decade. Several asymmetric dust models are investigated and simple images constructed.

J. Weiner

Papers

Precision Measurements of the Diameters of α Orionis and ο Ceti at 11 Microns

Mira’s Apparent Size Variations due to a Surrounding Semiopaque H2O Layer

Asymptotic giant branch and supergiant stellar diameters in the mid-infrared

Interferometry on Mira in the Mid-Infrared: Cyclic Variability of the Continuum Diameter and the Effect of Spectral Lines on Apparent Size

The Asymmetric dust environment of IK tauri