Costas Papaliolios

Detection of Earth-like Planets Using Apodized Telescopes

The mission of NASAs Terrestrial Planet Finder (TPF) is to find Earth-like planets orbiting other stars and characterize the atmospheres of these planets using spectroscopy. Because of the enormous brightness ratio between the star and the reflected light from the planet, techniques must be found to reduce the brightness of the star. The current favorite approach to doing this is with interferometry: interfering the light from two or more separated telescopes with a π phase shift, nulling out the starlight. While this technique can, in principle, achieve the required dynamic range, building a space interferometer that has the necessary characteristics poses immense technical difficulties. In this paper, we suggest a much simpler approach to achieving the required dynamic range. By simply adjusting the transmissive shape of a telescope aperture, the intensity in large regions around the stellar image can be reduced nearly to zero. This approach could lead to construction of a TPF using conventional technologies, requiring space optics on a much smaller scale than the current TPF approach.

First Results with the IOTA3 Imaging Interferometer: The Spectroscopic Binaries λ Virginis and WR 140

We report the first spatially resolved observations of the spectroscopic binaries λ Vir and WR 140, including the debut of aperture-synthesis imaging with the upgraded three-telescope IOTA interferometer. Using IONIC-3, a new integrated optics beam combiner capable of a precise closure phase measurement, short observations were sufficient to extract the angular separation and orientation of each binary system and the component brightness ratio. Most notably, the underlying binary in the prototypical colliding-wind source WR 140 (WC7 + O4/O5) was found to have a separation of ~13 mas with a position angle of 152°, consistent with previous interpretations of the 2001 dust shell ejection only if the Wolf-Rayet star is fainter than the O star at 1.65 μm. We also highlight λ Vir, whose peculiar stellar properties of the Am star components will permit direct testing of current theories of tidal evolution when the full orbit is determined.

Asymmetries in the atmosphere of Mira

A two-dimensional high angular resolution study of ο Ceti (Mira), carried out at four epochs from 1983 November to 1988 November using speckle interferometry techniques, detected asymmetries in the extended atmosphere of this pulsating star. The reconstructed speckle images show that the strength and the shape of this asymmetry changes as a function of wavelength and time. The position angles of the major axes of the asymmetries at different epochs are determined and the axes are measured accurately as a function of wavelength. The origin of the observed asymmetries has not yet been identified. Plausible causes include instabilities in the pulsating atmosphere, nonspherical pulsation, or the interaction with the nearby companion

Speckle imaging with the PAPA detector

A new 2-D photon-counting camera, the PAPA (precision analog photon address) detector has been built, tested, and used successfully for the acquisition of speckle imaging data. The camera has 512 × 512 pixels and operates at count rates of at least 200,000/sec. In this paper we present technical details on the camera and include some of the laboratory and astronomical results which demonstrate the detectors capabilities.

THEORETICAL LIMB DARKENING FOR CLASSICAL CEPHEIDS. II. CORRECTIONS FOR THE GEOMETRIC BAADE-WESSELINK METHOD

The geometric Baade-Wesselink method is one of the most promising techniques for obtaining a better calibration of the Cepheid period-luminosity relation by means of interferometric measurements of accurate diameters. In this paper we present new wavelength- and phase-dependent limb-darkening (LD) corrections based on our time-dependent hydrodynamic models of the classical Cepheid ζ Gem. We show that a model simulation of a Cepheid atmosphere, taking into account the hydrodynamic effects associated with the pulsation, shows strong departures from the LD otherwise predicted by a static model. For most of its pulsational cycle the hydrodynamic model predicts a larger LD than the equivalent static model. The hydrodynamics affects the LD mainly at UV and optical wavelengths. Most of these effects evolve slowly as the star pulsates, but there are phases, associated with shocks propagating into the photosphere, in which significant changes in the LD take place on timescales of the order of less than a day. We assess the implication of our model LD corrections fitting the geometric Baade-Wesselink distance of ζ Gem for the available near-IR PTI data. We discuss the effects of our model LD on the best-fit result and analyze the requirements needed to test the time dependence of the LD with future interferometric measurements.

Theoretical Limb Darkening for Pulsating Cepheids

Ground-based interferometry has finally reached a stage in which accurate determination of Cepheid diameters using the Baade-Wesselink method is feasible. Determining these diameters is the basis for calibrating the period-luminosity relation for classical Cepheids, and thus the extragalactic distance scale, but requires accurate limb-darkened models. This work presents a new method to compute time- and wavelength-dependent center-to-limb brightness distributions for classical Cepheids. Our model atmospheres are based on second-order accurate one-dimensional hydrodynamic calculations performed in spherical geometry. The brightness intensity distributions and the resulting limb darkening are computed through the dynamic atmospheres by using a full set of atomic and molecular opacities. Our results confirm important differences with respect to equivalent hydrostatic models. The amount of limb darkening and the shape of the limb profiles show a strong dependence on the pulsational phase of the Cepheid, which cannot be reproduced by static models. Nonlinear effects in our hydrodynamic equations add a new level of complexity in the wavelength dependence of our limb profiles, which are affected by the presence of shock waves traveling through the atmosphere. These effects, already detectable by present-day interferometers, should be taken into consideration when deriving limb-darkened diameters for nearby Cepheids with the accuracy required to measure their radial pulsations.

Detection of a very bright source close to the LMC supernova SN 1987A

High angular resolution observations of the supernova in the Large Magellanic Cloud, SN 1987A, have revealed a bright source separated from the SN by approximately 60 mas with a magnitude difference of 2.7 at 656 nm (H-alpha). Speckle imaging techniques were applied to data recorded with the CfA two-dimensional photon counting detector on the CTIO 4 m telescope on March 25 and April 2 to allow measurements in H-alpha on both nights and at 533 nm and 450 nm on the second night. The nature of this object is as yet unknown, though it is almost certainly a phenomenon related to the SN.

Effects of photon noise on speckle image reconstruction with the Knox-Thompson algorithm

Abstract An analysis of the effects of photon noise on astronomical speckle image reconstruction using the Knox-Thompson algorithm is presented. It is shown that the quantities resulting from the speckle average are biased, but that the biases are easily estimated and compensated. Calculations are also made of the convergence rate for the speckle average as a function of the source brightness. An illustration of the effects of photon noise on the image recovery process is included.

Current status of the IOTA interferometer

The first two telescopes of the Infrared-Optical Telescope Array (IOTA) project are now in place and yielding data at the Smithsonian Institutions F. L. Whipple Observatory on Mt. Hopkins, near Tucson, Arizona. The IOTA collectors are 45 cm in diameter, and may be moved to various stations in an L-shaped configuration with a maximum baseline of 38 m. A third collector will be added as soon as funding permits. Each light-collector assembly consists of a siderostat feeding a stationary afocal Cassegrain telescope that produces a 10-X reduced parallel beam, which is in turn directed vertically downward by a piezo-driven active mirror that stabilizes the ultimate image position. The reduced beams enter an evacuated envelope and proceed to the corner of the array, where they are turned back along one arm for path compensation. The delay line, in one beam, consists of two parts: one dihedral reflector positioned in a slew-and-clamp mode to give the major part of the desired delay; and a second dihedral mounted on an air-bearing carriage to provide the variable delay that is needed. After delay, the beams exit from the vacuum and are directed by dichroic mirrors into the infrared beam-combination and detection system. The visible light passes on to another area, to the image-tracker detectors and the visible-light combination and detection system. The beams are combined in pupil-plane mode on beam splitters. The combined IR beams are conveyed to two cooled single-element InSb detectors. The combined visible-light beams are focussed by lenslet arrays onto multimode optical fibers that lead to the slit of a specially-designed prism spectrometer. For the visible mode, the delay line is run at several wavelengths on one side of the zero- path point, so that several cycles of interference occur across the spectrum. First results were obtained with the IR system, giving visibilities for several K and M stars, using 2.2 micrometers radiation on a N-S baseline of 21.2 m. From these measurements we obtained preliminary estimates of effective stellar diameters in the K band.

A Second Bright Source Detected near SN 1987A

Speckle interferometry observations, made just 30 and 38 days after the explosion of SN 1987A (which was first seen in 1987 February 23), showed evidence of a bright source separated from the SN by only 60 mas. The reprocessing of that data, using new image reconstruction algorithms, has resulted in much cleaner images that not only clearly show the bright spot reported in 1987 but also show a second spot on the opposite side of the SN with a larger spatial separation. If the spots were ejected from the SN, then the velocities of the spots are relativistic; the second spot appears to be superluminal and must be blueshifted. We explore the consequences of these results on the geometry of the SN 1987A system, and we conclude that our observations may well be evidence of relativistic jets emanating from the supernova.