Oliver P. Lay

Constraints on the HL Tauri Protostellar Disk from Millimeter- and Submillimeter-Wave Interferometry

Millimeter and submillimeter interferometry is used to probe the dusty accretion disks around young protostars. New 460 GHz (λ = 650 μm) data from the Caltech Submillimeter Observatory and the James Clerk Maxwell Telescope CSO-JCMT Interferometer are combined with previous 345 GHz (λ = 870 μm) data from CSO-JCMT, 220 GHz (λ = 1.4 mm) data from the Owens Valley Radio Observatory (OVRO) Millimeter Array, 110 GHz (λ = 2.7 mm) data from the Berkeley-Illinois-Maryland Association Array (BIMA), and 43 GHz (λ = 7 mm) data from the VLA, in order to constrain the nature of the protostellar disk around HL Tau on size scales of 50 AU and above. A power-law disk model is fitted directly to the measured visibility data, and probability distributions are derived for the parameters. The effects of instrumental uncertainties are included in a consistent way. The position angle of the major axis of the emission is determined to be 127° ± 5° and the inclination 42° ± 5° (where 0° is face-on), assuming the disk is thin, flat, and circular. A strongly flared disk that is close to edge-on cannot be ruled out, however. The CSO-JCMT and OVRO data favor centrally concentrated distributions of the surface density Σ, where Σ ∝ r-p, and p > 1. This is not compatible with the relatively large sizes measured at lower frequency by BIMA and the VLA. No simple power-law disk model can be found that reproduces all of the millimeter and submillimeter data well. Such a model, with radial power laws in the surface density and temperature, and a single dust component, is therefore unlikely to be a good representation of the actual disk structure. One possibility that would help to reconcile the model with the data is the existence of more than one dust component, i.e., a range of grain sizes or structures in the disk and no unique value of the emissivity index β. Future models should allow for this as well as for disk geometries that are not thin and flat.

Darwin - A Mission to Detect and Search for Life on Extrasolar Planets

The discovery of extrasolar planets is one of the greatest achievements of modern astronomy. The detection of planets that vary widely in mass demonstrates that extrasolar planets of low mass exist. In this paper, we describe a mission, called Darwin, whose primary goal is the search for, and characterization of, terrestrial extrasolar planets and the search for life. Accomplishing the mission objectives will require collaborative science across disciplines, including astrophysics, planetary sciences, chemistry, and microbiology. Darwin is designed to detect rocky planets similar to Earth and perform spectroscopic analysis at mid-infrared wavelengths (6-20 mum), where an advantageous contrast ratio between star and planet occurs. The baseline mission is projected to last 5 years and consists of approximately 200 individual target stars. Among these, 25-50 planetary systems can be studied spectroscopically, which will include the search for gases such as CO(2), H(2)O, CH(4), and O(3). Many of the key technologies required for the construction of Darwin have already been demonstrated, and the remainder are estimated to be mature in the near future. Darwin is a mission that will ignite intense interest in both the research community and the wider public.

MSTAR: a submicrometer absolute metrology system

The Modulation Sideband Technology for Absolute Ranging (MSTAR) sensor permits absolute distance measurement with subnanometer accuracy, an improvement of 4 orders of magnitude over current techniques. The system uses fast phase modulators to resolve the integer cycle ambiguity of standard interferometers. The concept is described and demonstrated over target distances up to 1 m. The design can be extended to kilometer-scale separations.

Systematic errors in nulling interferometers

Nulling interferometers combine on-axis suppression with high angular resolution, making them ideal instruments for the direct detection of faint planets close to their parent star. Analysis is developed to show that it is systematic errors, resulting from fluctuations in the null depth, that drive the instrument performance. A second-order combination of amplitude and phase errors is the dominant contributor. In the calculated example, the detection of an Earthlike planet around a Sunlike star at 15 pc requires that the arms of the interferometer must be phased to within approximately 1.5 nm and have their amplitudes matched to approximately 0.1%.

Interferometric Phase Correction Using 183 GHzGHz Water Vapor Monitors

The angular resolution that can be obtained by ground-based aperture synthesis telescopes at millimeter and submillimeter wavelengths is limited by phase fluctuations caused by water vapor in the Earths atmosphere. We describe here the successful correction of such fluctuations during observations at 0.85 mm wavelength with an interferometer consisting of the James Clark Maxwell Telescope and the Caltech Submillimeter Observatory. This was achieved by using two 183 GHz heterodyne radiometers to measure the water vapor content along the line of sight of each telescope. Further development of such techniques will enable future telescopes, such as the Submillimeter Array and the Atacama Large Millimeter Array, to reach their full capability, providing a resolution of up to 001.

Characterization of mid-infrared single mode fibers as modal filters

We present a technique for measuring the modal filtering ability of single mode fibers. The ideal modal filter rejects all input field components that have no overlap with the fundamental mode of the filter and does not attenuate the fundamental mode. We define the quality of a nonideal modal filter Q(f) as the ratio of transmittance for the fundamental mode to the transmittance for an input field that has no overlap with the fundamental mode. We demonstrate the technique on a 20 cm long mid-infrared fiber that was produced by the U.S. Naval Research Laboratory. The filter quality Q(f) for this fiber at 10.5 microm wavelength is 1000+/-300. The absorption and scattering losses in the fundamental mode are approximately 8 dB/m. The total transmittance for the fundamental mode, including Fresnel reflections, is 0.428+/-0.002. The application of interest is the search for extrasolar Earthlike planets using nulling interferometry. It requires high rejection ratios to suppress the light of a bright star, so that the faint planet becomes visible. The use of modal filters increases the rejection ratio (or, equivalently, relaxes requirements on the wavefront quality) by reducing the sensitivity to small wavefront errors. We show theoretically that, exclusive of coupling losses, the use of a modal filter leads to the improvement of the rejection ratio in a two-beam interferometer by a factor of Q(f).

Terrestrial Planet Finder Interferometer: 2007-2008 Progress and Plans

This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer (TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like planets in the habitable zones around nearby stars. The overall technology roadmap is presented and progress with each of the testbeds is summarized.

StarLight mission: a formation-flying stellar interferometer

The StarLight mission is designed to validate the technologies of formation flying and stellar interferometry in space. The mission consists of two spacecraft in an earth-trailing orbit that formation-fly over relative ranges of 40 to 600m to an accuracy of 10 cm. The relative range and bearing of the spacecraft is sensed by a novel RF sensor, the Autonomous Formation Flyer sensor, which provides 2cm and 1mrad range and bearing knowledge between the spacecraft. The spacecraft each host instrument payloads for a Michelson interferometer that exploit the moving spacecraft to generate variable observing baselines between 30 and 125m. The StarLight preliminary design has shown that a formation-flying interferometer involves significant coupling between the major system elements - spacecraft, formation-flying control, formation-flying sensor, and the interferometer instrument. Mission requirements drive innovative approaches for long-range heterodyne metrology, optical design, glint suppression, formation estimation and control, spacecraft design, and mission operation. Experimental results are described for new technology development areas.

Experimental evaluation of achromatic phase shifters for mid-infrared starlight suppression.

Phase shifters are a key component of nulling interferometry, one of the potential routes to enabling the measurement of faint exoplanet spectra. Here, three different achromatic phase shifters are evaluated experimentally in the mid-infrared, where such nulling interferometers may someday operate. The methods evaluated include the use of dispersive glasses, a through-focus field inversion, and field reversals on reflection from antisymmetric flat-mirror periscopes. All three approaches yielded deep, broadband, mid-infrared nulls, but the deepest broadband nulls were obtained with the periscope architecture. In the periscope system, average null depths of 4x10(-5) were obtained with a 25% bandwidth, and 2x10(-5) with a 20% bandwidth, at a central wavelength of 9.5 mum. The best short term nulls at 20% bandwidth were approximately 9x10(-6), in line with error budget predictions and the limits of the current generation of hardware.

Modal filtering for midinfrared nulling interferometry using single mode silver halide fibers

We demonstrate the modal filtering properties of newly developed single mode silver halide fibers for use at midinfrared wavelengths, centered at 10.5 microm. The goal was to achieve a suppression of nonfundamental modes greater than a factor of 300 to enable the detection and characterization of Earthlike exoplanets with a space-based nulling interferometer. Fiber segments of 4.5 cm, 10.5 cm, 15 cm, and 20 cm lengths were tested. We find that the performance of the fiber was limited not by the modal filtering properties of the core but by the unsuppressed cladding modes present at the output of the fiber. In 10.5 cm and longer sections, this effect can be alleviated by properly aperturing the output. Exclusive of coupling losses, the fiber segments of 10.5-20 cm length can provide power suppression of undesirable components of the input field by a factor of 15,000 at least. The demonstrated performance thus far surpasses our requirements, such that even very short sections of fiber provide adequate modal filtering for exoplanet characterization.