251 research outputs found
The polarization of the planet-hosting WASP-18 system
We report observations of the linear polarization of the WASP-18 system,
which harbors a very massive ( approx 10 M_J) planet orbiting very close to its
star with an orbital period of 0.94 days. We find the WASP-18 system is
polarized at about 200 parts-per-million (ppm), likely from the interstellar
medium predominantly, with no strong evidence for phase dependent modulation
from reflected light from the planet. We set an upper limit of 40 ppm (99%
confidence level) on the amplitude of a reflected polarized light planetary
signal. We compare the results with models for a number of processes that may
produce polarized light in a planetary system to determine if we can rule out
any phenomena with this limit. Models of reflected light from thick clouds can
approach or exceed this limit, but such clouds are unlikely at the high
temperature of the WASP-18b atmosphere. Additionally, we model the expected
polarization resulting from the transit of the planet across the star and find
this has an amplitude of about 1.6 ppm, which is well below our detection
limits. We also model the polarization due to the tidal distortion of the star
by the massive planet and find this is also too small to be measured currently.Comment: 23 pages, 10 Figures, 6 Tables, Accepted to A
Observations of Intrahour Variable Quasars: Scattering in our Galactic Neighbourhood
Interstellar scintillation (ISS) has been established as the cause of the
random variations seen at centimetre wavelengths in many compact radio sources
on timescales of a day or less. Observations of ISS can be used to probe
structure both in the ionized insterstellar medium of the Galaxy, and in the
extragalactic sources themselves, down to microarcsecond scales. A few quasars
have been found to show large amplitude scintillations on unusually rapid,
intrahour timescales. This has been shown to be due to weak scattering in very
local Galactic ``screens'', within a few tens of parsec of the Sun. The short
variability timescales allow detailed study of the scintillation properties in
relatively short observing periods with compact interferometric arrays. The
three best-studied ``intrahour variable'' quasars, PKS 0405-385, J1819+3845 and
PKS 1257-326, have been instrumental in establishing ISS as the principal cause
of intraday variability at centimetre wavelengths. Here we review the relevant
results from observations of these three sources.Comment: 10 pages, 4 figures, to appear in Astronomical and Astrophysical
Transaction
Intraday variability of AGNs in the southern hemisphere
Understanding of the spectral and polarimetric characteristics of rapidly scintillating blazars is fundamental in order to describe both the innermost (sub-pc) regions of these compact objects and the interstellar medium responsible for the scintillation. A multi frequency analysis of the intraday variability in PMN J1326-5256, based on the combination of Australia Telescope Compact Array observations with the data from the monitoring projects at the University of Tasmania, will be described. Some implications concerning the structure of compact radio cores and the properties of the interstellar medium will be discussed
Why Do Compact Active Galactic Nuclei at High Redshift Scintillate Less?
The fraction of compact active galactic nuclei (AGNs) that exhibit
interstellar scintillation (ISS) at radio wavelengths, as well as their
scintillation amplitudes, have been found to decrease significantly for sources
at redshifts z > 2. This can be attributed to an increase in the angular sizes
of the \muas-scale cores or a decrease in the flux densities of the compact
\muas cores relative to that of the mas-scale components with increasing
redshift, possibly arising from (1) the space-time curvature of an expanding
Universe, (2) AGN evolution, (3) source selection biases, (4) scatter
broadening in the ionized intergalactic medium (IGM) and intervening galaxies,
or (5) gravitational lensing. We examine the frequency scaling of this redshift
dependence of ISS to determine its origin, using data from a dual-frequency
survey of ISS of 128 sources at 0 < z < 4. We present a novel method of
analysis which accounts for selection effects in the source sample. We
determine that the redshift dependence of ISS is partially linked to the
steepening of source spectral indices ({\alpha}^8.4_4.9) with redshift, caused
either by selection biases or AGN evolution, coupled with weaker ISS in the
{\alpha}^8.4_4.9 < -0.4 sources. Selecting only the -0.4 < {\alpha}^8.4_4.9 <
0.4 sources, we find that the redshift dependence of ISS is still significant,
but is not significantly steeper than the expected (1+z)^0.5 scaling of source
angular sizes due to cosmological expansion for a brightness temperature and
flux-limited sample of sources. We find no significant evidence for scatter
broadening in the IGM, ruling it out as the main cause of the redshift
dependence of ISS. We obtain an upper limit to IGM scatter broadening of <
110\muas at 4.9 GHz with 99% confidence for all lines of sight, and as low as <
8\muas for sight-lines to the most compact, \sim 10\muas sources.Comment: 38 pages, 13 figures, accepted for publication in The Astrophysical
Journa
The polarisation of HD 189733
Kimberly Bott; Jeremy Bailey; Lucyna Kedziora-Chudczer; Daniel V. Votton; P. W. Lucas; Jonathan P. Marshall; J. H. Hough, ‘The polarization of HD 189733’, Monthly Notices of the Royal Astronomical Society/Letters, Vol 459(1): L109-L113, published 20 March 2016, the version of record is available online at doi: https://doi.org/10.1093/mnras/slw046 © 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.We present linear polarization observations of the exoplanet system HD 189733 made with the HIgh Precision Polarimetric Instrument (HIPPI) on the Anglo-Australian Telescope (AAT). The observations have higher precision than any previously reported for this object. They do not show the large amplitude polarization variations reported by Berdyugina et al. 2008 and Berdyugina et al. 2011. Our results are consistent with polarization data presented by Wiktorowicz et al. 2015. A formal least squares fit of a Rayleigh-Lambert model yields a polarization amplitude of 29.4 +/- 15.6 parts-per-million. We observe a background constant level of polarization of ~ 55-70 ppm, which is a little higher than expected for interstellar polarization at the distance of HD 189733.Peer reviewedFinal Published versio
Multi-wavelength aperture polarimetry of debris disc host stars
Debris discs around main sequence stars have been extensively characterised
from infrared to millimetre wavelengths through imaging, spectroscopic, and
total intensity (scattered light and/or thermal emission) measurements.
Polarimetric observations have only been used sparingly to interpret the
composition, structure, and size of dust grains in these discs. Here we present
new multi-wavelength aperture polarisation observations with parts-per-million
sensitivity of a sample of twelve bright debris discs, spanning a broad range
of host star spectral types, and disc properties. These measurements were
mostly taken with the HIgh Precision Polarimetric Instrument on the
Anglo-Australian Telescope. We combine these polarisation observations with the
known disc architectures and geometries of the discs to interpret the
measurements. We detect significant polarisation attributable to circumstellar
dust from HD 377 and HD 39060, and find tentative evidence for HD 188228 and HD
202628.Comment: 25 pages, 8 figures, 9 tables, accepted for publication in MNRA
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