Imaging the Radio Photospheres of Mira Variables

We have used the VLA at 43 GHz to image the radio continuum emission from o Ceti, R Leo, and W Hya and to precisely locate their SiO maser emission with respect to the star. The radio continuum emission region for all three stars has a diameter close to 5.6 AU. These diameters are similar to those measured at infrared wavelengths in bands containing strong molecular opacity and about twice those measured in line-free regions of the infrared spectrum. Thus, the radio photosphere and the infrared molecular layer appear to be coextensive. The 43 GHz continuum emission is consistent with temperatures near 1600 K and opacity from H-minus free-free interactions. While the continuum image of o Ceti appears nearly circular, both R Leo and W Hya display significant elongations. The SiO masers for all three stars show partial rings with diameters close to 8 AU.Comment: 14 pages; 3 figure

On the Formation of Multiple-Shells Around Asymptotic Giant Branch Stars

Two types of models for the formation of semi-periodic concentric multiple shells (M-shells) around asymptotic giant branch (AGB) stars and in planetary nebulae are compared against observations. Models that attribute the M-shells to processes in an extended wind acceleration zone around AGB stars result in an optically thick acceleration zone, which reduces the acceleration efficiency in outer parts of the extended acceleration zone. This makes such models an unlikely explanation for the formation of M-shells. Models which attribute the M-shell to semi-periodic variation in one or more stellar properties are most compatible with observations. The only stellar variation models on time scales of 50-1500 years that have been suggested are based on an assumed solar-like magnetic cycle. Although ad-hoc, the magnetic cycle assumption fits naturally into the increasingly popular view that magnetic activity plays a role in shaping the wind from upper AGB stars.Comment: 8 pages, Submitted to Ap

Multi-wavelength visibility measurements of the red giant R Doradus

We present visibility measurements of the nearby Mira-like star R Doradus taken over a wide range of wavelengths (650--990 nm). The observations were made using MAPPIT (Masked APerture-Plane Interference Telescope), an interferometer operating at the 3.9-m Anglo-Australian Telescope. We used a slit to mask the telescope aperture and prism to disperse the interference pattern in wavelength. We observed in R Dor strong decreases in visibility within the TiO absorption bands. The results are in general agreement with theory but differ in detail, suggesting that further work is needed to refine the theoretical models.Comment: 8 pages; SPIE Conf. 4006 "Interferometry in Optical Astronomy

High fidelity imaging of geosynchronous satellites with the MROI

Interferometry currently provides the only practicable way to image satellites in Geosynchronous Earth Orbit (GEO) with sub-meter spatial resolution. The Magdalena Ridge Observatory Interferometer (MROI) is being funded by the US Air Force Research Laboratory to demonstrate the 9.5 magnitude sensitivity (at 2.2 µm wavelength) and baseline-bootstrapping capability that will be needed to realize a useful turn-key GEO imaging capability. This program will utilize the central three telescopes of the MROI and will aim to validate routine acquisition of fringe data on faint well-resolved targets. In parallel with this effort, the University of Cambridge are investigating the spatial resolution and imaging fidelity that can be achieved with different numbers of array elements. We present preliminary simulations of snapshot GEO satellite imaging with the MROI. Our results indicate that faithful imaging of the main satellite components can be obtained with as few as 7 unit telescopes, and that increasing the number of telescopes to 10 improves the effective spatial resolution from 0.75 meter to 0.5 meter and enables imaging of more complex targets.This is the author accepted manuscript. The final version is available from SPIE via http://dx.doi.org/10.1117/12.223247

Mid-infrared spectra of late-type stars: Long-term evolution

Recent ground-based mid-infrared spectra of 29 late-type stars, most with substantial dust shells, are compared to ground-based spectra of these stars from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra of about half the stars show no detectable changes, implying that their distributions of circumstellar material and associated dust grain properties have changed little over this time interval. However, many of the stars with strong silicate features showed marked changes. In nearly all cases the silicate peak has strengthened with respect to the underlying continuum, although there is one case (VY~CMa) in which the silicate feature has almost completely disappeared. This suggests that, in general, an oxygen-rich star experiences long periods of gradual silicate feature strengthening, punctuated by relatively rare periods when the feature weakens. We discuss various mechanisms for producing the changes, favoring the slow evolution of the intrinsic dust properties (i.e., the chemical composition or grain structure). Although most IRAS spectra agree well with ground-based spectra, there are a number of cases where they fall well outside the expected range of uncertainty. In almost all such cases the slopes of the red and blue LRS spectra do not match in their region of overlap.Comment: Accepted in ApJ, 20 pages, 5 figures, 1 tabl

Michelson Interferometry with the Keck I Telescope

We report the first use of Michelson interferometry on the Keck I telescope for diffraction-limited imaging in the near infrared JHK and L 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 diffraction-limited imaging of a large number of bright astrophysical targets, including the geometrically complex dust envelopes around a number of evolved stars. The successful restoration of these images, with dynamic ranges in excess of 200:1, highlights the significant capabilities of sparse aperture imaging as compared with more conventional filled-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 fidelity imaging of complex sources with small numbers of short-exposure images relative to speckle. Multi-epoch measurements confirm the reliability of this imaging technique and our whole dataset provides a powerful demonstration of the capabilities of aperture masking methods when utilized with the current generation of large-aperture telescopes. The relationship between these new results and recent advances in interferometry and adaptive optics is briefly discussed.Comment: Accepted into Publications of the Astronomical Society of the Pacific. To appear in vol. 112. Paper contains 10 pages, 8 figure

The last gasps of VY CMa: Aperture synthesis and adaptive optics imagery

We present new observations of the red supergiant VY CMa at 1.25 micron, 1.65 micron, 2.26 micron, 3.08 micron and 4.8 micron. Two complementary observational techniques were utilized: non-redundant 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 extremely high (~10^5) peak-to-noise dynamic range over a wide field. For the first time the inner dust shell has been resolved in the near-infrared to reveal a one-sided extension of circumstellar emission within 0.1" (~15 R_star) of the star. The line-of-sight optical depths of the circumstellar dust shell at 1.65 micron, 2.26 micron, and 3.08 micron have been estimated to be 1.86 +/- 0.42, 0.85 +/- 0.20, and 0.44 +/- 0.11. These new results allow the bolometric luminosity of VY~CMa to be estimated independent of the dust shell geometry, yielding L_star ~ 2x10^5 L_sun. A variety of dust condensations, including a large scattering plume and a bow-shaped dust feature, were observed in the faint, extended nebula up to 4" 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 flow insertion point with a rotational period between 1200-4200 years, which is perhaps the stellar rotational period or the orbital period of an unseen companion.Comment: 25 pages total with 1 table and 5 figures. Accepted by Astrophysical Journal (to appear in February 1999

Diffraction-limited near-IR imaging at Keck reveals asymmetric, time-variable nebula around carbon star CIT 6

We present multi-epoch, diffraction-limited images of the nebula around the carbon star CIT 6 at 2.2 microns and 3.1 microns from aperture masking on the Keck-I telescope. The near-IR nebula is resolved into two main components, an elongated, bright feature showing time-variable asymmetry and a fainter component about 60 milliarcseconds away with a cooler color temperature. These images were precisely registered (~35 milliarcseconds) with respect to recent visible images from the Hubble Space Telescope (Trammell et al. 2000), which showed a bipolar structure in scattered light. The dominant near-IR feature is associated with the northern lobe of this scattering nebula, and the multi-wavelength dataset can be understood in terms of a bipolar dust shell around CIT 6. Variability of the near-IR morphology is qualitatively consistent with previously observed changes in red polarization, caused by varying illumination geometry due to non-uniform dust production. The blue emission morphology and polarization properties can not be explained by the above model alone, but require the presence of a wide binary companion in the vicinity of the southern polar lobe. The physical mechanisms responsible for the breaking of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC+10216, remain uncertain.Comment: 18 pages, 5 figures (one in color), to appear in the Astrophysical Journa

Planet Formation Imager (PFI): Introduction and Technical Considerations

Complex non-linear and dynamic processes lie at the heart of the planet formation process. Through numerical simulation and basic observational constraints, the basics of planet formation are now coming into focus. High resolution imaging at a range of wavelengths will give us a glimpse into the past of our own solar system and enable a robust theoretical framework for predicting planetary system architectures around a range of stars surrounded by disks with a diversity of initial conditions. Only long-baseline interferometry can provide the needed angular resolution and wavelength coverage to reach these goals and from here we launch our planning efforts. The aim of the "Planet Formation Imager" (PFI) project is to develop the roadmap for the construction of a new near-/mid-infrared interferometric facility that will be optimized to unmask all the major stages of planet formation, from initial dust coagulation, gap formation, evolution of transition disks, mass accretion onto planetary embryos, and eventual disk dispersal. PFI will be able to detect the emission of the cooling, newly-formed planets themselves over the first 100 Myrs, opening up both spectral investigations and also providing a vibrant look into the early dynamical histories of planetary architectures. Here we introduce the Planet Formation Imager (PFI) Project (www.planetformationimager.org) and give initial thoughts on possible facility architectures and technical advances that will be needed to meet the challenging top-level science requirements.Comment: SPIE Astronomical Telescopes and Instrumentation conference, June 2014, Paper ID 9146-35, 10 pages, 2 Figure