315 research outputs found
Detection of Close-In Extrasolar Giant Planets Using the Fourier-Kelvin Stellar Interferometer
We evaluate the direct detection of extrasolar giant planets with a
two-aperture nulling infrared interferometer, working at angles
, and using a new `ratio-of-two-wavelengths' technique.
Simple arguments suggest that interferometric detection and characterization
should be quite possible for planets much closer than the conventional inner
working angle, or angular resolution limit. We show that the peak signal from a
nulling infrared interferometer of baseline ( meters) will often
occur `inside the null', and that the signal variations from path-difference
fluctuations will cancel to first order in the ratio of two wavelengths. Using
a new interferometer simulation code, we evaluate the detectability of all the
known extrasolar planets as observed using this two-color method with the
proposed {\it Fourier Kelvin Stellar Interferometer (FKSI)}. In its minimum
configuration {\it FKSI} uses two 0.5-meter apertures on a 12.5-meter baseline,
and a field-of-regard. We predict that known
extrasolar planets are directly detectable using {\it FKSI}, with
low-resolution spectroscopy () being possible in the most favorable
cases. Spaceborne direct detection of extrasolar giant planets is possible with
meter baselines, and does not require the much longer baselines
provided by formation flying.Comment: Accepted for publication in ApJ Letter
Molecular line survey of Sagittarius B2(M) from 330 to 355 GHz and comparison with Sagittarius B2(N)
We have surveyed molecular line emission from Sgr B2 over the range from 330 to 355 GHz at the position designated Sgr B2(M). This position is prominent in millimeter continuum maps of the region and is associated with a compact H II region, a hot NH_3 core, and sources of H_2O and OH maser emission. We have also obtained observations contrasting the submillimeter molecular emission from Sgr B2(M) and Sgr B2(N), an additional center of activity thought to be a dense protostellar core.
The picture of the interstellar chemistry of these regions which we derive is substantially different from that
determined from previous observations at lower frequencies and with lower spatial resolution. In particular,
molecules such as SO_2 and CH_3OH dominate the submillimeter spectrum to a much greater extent than they do
the low-frequency observations. Much of this difference is due to the higher spatial resolution of the submillimeter
observations, which makes them much more sensitive to emission from compact, dense cores. The millimeter
data were most effective at sampling material in the surrounding lower density regions. The chemistry of the core
sources in Sgr B2 appears similar to that of other dense cores, such as the core of the Orion molecular cloud.
The spectral differences between Sgr B2(M) and Sgr B2(N) primarily relate to differences in excitation and
column density. For most molecular species the northern source (N) has a column density significantly higher
than that found in the middle source (M), often by a factor of about 5. The principal exceptions are the species SO
and SO_2 which seem to be substantially more abundant in the middle source. Generally excitation seems to be
higher in the northern source, suggesting a somewhat higher density core, although there are some departures
indicating that the excitation situation is more complicated. High optical depths in many of the submillimeter transitions systematically bias the interpretation of both column densities and excitation. Many of the millimeter lines may also have high optical depths, particularly those lines arising from the compact core sources
The Importance of Phase in Nulling Interferometry and a Three Telescope Closure-Phase Nulling Interferometer Concept
We discuss the theory of the Bracewell nulling interferometer and explicitly
demonstrate that the phase of the "white light" null fringe is the same as the
phase of the bright output from an ordinary stellar interferometer. As a
consequence a "closure phase" exists for a nulling interferometer with three or
more telescopes. We calculate the phase offset as a function of baseline length
for an Earth-like planet around the Sun at 10 pc, with a contrast ratio of
at 10 m. The magnitude of the phase due to the planet is radians, assuming the star is at the phase center of the array.
Although this is small, this phase may be observable in a three-telescope
nulling interferometer that measures the closure phase. We propose a simple
non-redundant three-telescope nulling interferometer that can perform this
measurement. This configuration is expected to have improved characteristics
compared to other nulling interferometer concepts, such as a relaxation of
pathlength tolerances, through the use of the "ratio of wavelengths" technique,
a closure phase, and better discrimination between exodiacal dust and planets
Inverse Ac Josephson Effect at Terahertz Frequencies
The inverse ac Josephson effect occurs when a Josephson junction driven by a microwave source of frequency f produces constant‐voltage steps at integer multiples of h f/2e. For low‐leakage current hysteretic junctions driven at microwave frequencies below about 100 GHz, some of these steps can cross the zero dc bias current axis. These zero‐crossing steps allow modern series array voltage standards to operate without individually biasing the junctions in the array. We reexamine the theory behind these steps and show that they can exist at frequencies much higher than thought previously. The Riedel singularity in the supercurrent response allows this effect to exist even up to terahertz frequencies. We describe a set of analytical calculations which provide limits on the amount of rounding of the Riedel peak which can be permitted while still allowing these zero‐crossing steps to occur. We also discuss practical considerations such as microwave power levels required and parameters for device fabrication. This analysis is supported by numerical frequency‐domain computations and time‐domain simulations for a number of realistic I‐V curves with rounded Riedel singularities and with quasiparticle subgap leakage currents
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