2,135 research outputs found
Differential phase technique with the Keck Interferometer
We present the motivation and development of the novel `differential phase' technique being developed for the Keck Interferometer with the goal of detecting faint companions near a bright source. The differential phase technique uses simultaneous phase measurements at several infrared wavelengths to detect the astrophysical signature produced by a chromatic, asymmetric brightness distribution. We discuss the origin of the differential phase signature and present results of test observations taken at the Palomar Testbed Interferometer. One important test result is the larger than expected effect of water vapor turbulence on these multi-wavelength observations due to the infrared dispersion of water. In order to reach the design goal of 0.1 milliradians, the phase noise caused by both temperature and water vapor fluctuations in the atmosphere must be corrected, and we discuss several ways to achieve this
A Multiple Scattering Polarized Radiative Transfer Model: Application to HD 189733b
We present a multiple scattering vector radiative transfer model which
produces disk integrated, full phase polarized light curves for reflected light
from an exoplanetary atmosphere. We validate our model against results from
published analytical and computational models and discuss a small number of
cases relevant to the existing and possible near-future observations of the
exoplanet HD 189733b. HD 189733b is arguably the most well observed exoplanet
to date and the only exoplanet to be observed in polarized light, yet it is
debated if the planet's atmosphere is cloudy or clear. We model reflected light
from clear atmospheres with Rayleigh scattering, and cloudy or hazy atmospheres
with Mie and fractal aggregate particles. We show that clear and cloudy
atmospheres have large differences in polarized light as compared to simple
flux measurements, though existing observations are insufficient to make this
distinction. Futhermore, we show that atmospheres that are spatially
inhomogeneous, such as being partially covered by clouds or hazes, exhibit
larger contrasts in polarized light when compared to clear atmospheres. This
effect can potentially be used to identify patchy clouds in exoplanets. Given a
set of full phase polarimetric measurements, this model can constrain the
geometric albedo, properties of scattering particles in the atmosphere and the
longitude of the ascending node of the orbit. The model is used to interpret
new polarimetric observations of HD 189733b in a companion paper.Comment: 13 pages, 13 figures. Accepted for publication in Ap
A Ground-Based Albedo Upper Limit for HD 189733b from Polarimetry
We present 50 nights of polarimetric observations of HD 189733 in band
using the POLISH2 aperture-integrated polarimeter at the Lick Observatory Shane
3-m telescope. This instrument, commissioned in 2011, is designed to search for
Rayleigh scattering from short-period exoplanets due to the polarized nature of
scattered light. Since these planets are spatially unresolvable from their host
stars, the relative contribution of the planet-to-total system polarization is
expected to vary with an amplitude of order 10 parts per million (ppm) over the
course of the orbit. Non-zero and also variable at the 10 ppm level, the
inherent polarization of the Lick 3-m telescope limits the accuracy of our
measurements and currently inhibits conclusive detection of scattered light
from this exoplanet. However, the amplitude of observed variability
conservatively sets a upper limit to the planet-induced polarization
of the system of 58 ppm in band, which is consistent with a previous upper
limit from the POLISH instrument at the Palomar Observatory 5-m telescope
(Wiktorowicz 2009). A physically-motivated Rayleigh scattering model, which
includes the depolarizing effects of multiple scattering, is used to
conservatively set a upper limit to the geometric albedo of HD
189733b of . This value is consistent with the value derived from occultation observations with HST STIS (Evans et al.
2013), but it is inconsistent with the large albedo
reported by (Berdyugina et al. 2011).Comment: 10 pages, 9 figures, submitted to Ap
SPIFI: a Direct-Detection Imaging Spectrometer for Submillimeter Wavelengths
The South Pole Imaging Fabry-Perot Interferometer (SPIFI) is the first instrument of its kind -a direct-detection imaging spectrometer for astronomy in the submillimeter band. SPIFI ’s focal plane is a square array of 25 silicon bolometers cooled to 60 mK; the spectrometer consists of two cryogenic scanning Fabry-Perot interferometers in series with a 60-mK bandpass filter. The instrument operates in the short submillimeter windows (350 and 450 μm) available from the ground, with spectral resolving power selectable between 500 and 10,000. At present, SPIFI’s sensitivity is within a factor of 1.5-3 of the photon background limit, comparable with the best heterodyne spectrometers. The instrument ’s large bandwidth and mapping capability provide substantial advantages for specific astrophysical projects, including deep extragalactic observations. We present the motivation for and design of SPIFI and its operational characteristics on the telescope
Differential phase technique with the Keck Interferometer
We present the motivation and development of the novel `differential phase' technique being developed for the Keck Interferometer with the goal of detecting faint companions near a bright source. The differential phase technique uses simultaneous phase measurements at several infrared wavelengths to detect the astrophysical signature produced by a chromatic, asymmetric brightness distribution. We discuss the origin of the differential phase signature and present results of test observations taken at the Palomar Testbed Interferometer. One important test result is the larger than expected effect of water vapor turbulence on these multi-wavelength observations due to the infrared dispersion of water. In order to reach the design goal of 0.1 milliradians, the phase noise caused by both temperature and water vapor fluctuations in the atmosphere must be corrected, and we discuss several ways to achieve this
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