1,265 research outputs found
Spectrophotometry with a transmission grating for detecting faint occultations
High-precision spectrophotometry is highly desirable in detecting and
characterizing close-in extrasolar planets to learn about their makeup and
temperature. For such a goal, a modest-size telescope with a simple
low-resolution spectroscopic instrument is potentially as good or better than a
complex general purpose spectrograph since calibration and removal of
systematic errors is expected to dominate. We use a transmission grating placed
in front of an imaging CCD camera on Steward Observatory's Kuiper 1.5 m
telescope to provide a high signal-to-noise, low dispersion visible spectrum of
the star HD 209458. We attempt to detect the reflected light signal from the
extra-solar planet HD 209458b by differencing the signal just before and after
secondary occultation. We present a simple data reduction method and explore
the limits of ground based low-dispersion spectrophotometry with a diffraction
grating. Reflected light detection levels of 0.1% are achievable for
5000-7000A, too coarse for useful limits on ESPs but potentially useful for
determining spectra of short-period binary systems with large (Delta m_vis=6)
brightness ratios. Limits on the precison are set by variations in atmospheric
seeing in the low-resolution spectrum. Calibration of this effect can be
carried out by measurement of atmospheric parameters from the observations
themselves, which may allow the precision to be limited by the noise due to
photon statistics and atmospheric scintillation effects.Comment: 34 pages and 17 figures. Accepted for publication in PAS
Feasibility of transit photometry of nearby debris discs
Dust in debris discs is constantly replenished by collisions between larger
objects. In this paper, we investigate a method to detect these collisions. We
generate models based on recent results on the Fomalhaut debris disc, where we
simulate a background star transiting behind the disc, due to the proper motion
of Fomalhaut. By simulating the expanding dust clouds caused by the collisions
in the debris disc, we investigate whether it is possible to observe changes in
the brightness of the background star. We conclude that in the case of the
Fomalhaut debris disc, changes in the optical depth can be observed, with
values of the optical depth ranging from for the densest dust
clouds to for the most diffuse clouds with respect to the background
optical depth of .Comment: 19 pages, 15 figures, accepted for publication in MNRA
Optimized Principal Component Analysis on Coronagraphic Images of the Fomalhaut System
We present the results of a study to optimize the principal component
analysis (PCA) algorithm for planet detection, a new algorithm complementing
ADI and LOCI for increasing the contrast achievable next to a bright star. The
stellar PSF is constructed by removing linear combinations of principal
components, allowing the flux from an extrasolar planet to shine through. The
number of principal components used determines how well the stellar PSF is
globally modelled. Using more principal components may decrease the number of
speckles in the final image, but also increases the background noise. We apply
PCA to Fomalhaut VLT NaCo images acquired at 4.05 micron with an apodized phase
plate. We do not detect any companions, with a model dependent upper mass limit
of 13-18 M_Jup from 4-10 AU. PCA achieves greater sensitivity than the LOCI
algorithm for the Fomalhaut coronagraphic data by up to 1 magnitude. We make
several adaptations to the PCA code and determine which of these prove the most
effective at maximizing the signal-to-noise from a planet very close to its
parent star. We demonstrate that optimizing the number of principal components
used in PCA proves most effective for pulling out a planet signal.Comment: Accepted for publication in ApJ, 7 pages, 9 figure
Characterising exo-ringsystems around fast-rotating stars using the Rossiter-McLaughlin effect
Planetary rings produce a distinct shape distortion in transit lightcurves.
However, to accurately model such lightcurves the observations need to cover
the entire transit, especially ingress and egress, as well as an out-of-transit
baseline. Such observations can be challenging for long period planets, where
the transits may last for over a day. Planetary rings will also impact the
shape of absorption lines in the stellar spectrum, as the planet and rings
cover different parts of the rotating star (the Rossiter-McLaughlin effect).
These line-profile distortions depend on the size, structure, opacity,
obliquity and sky projected angle of the ring system. For slow rotating stars,
this mainly impacts the amplitude of the induced velocity shift, however, for
fast rotating stars the large velocity gradient across the star allows the line
distortion to be resolved, enabling direct determination of the ring
parameters. We demonstrate that by modeling these distortions we can recover
ring system parameters (sky-projected angle, obliquity and size) using only a
small part of the transit. Substructure in the rings, e.g. gaps, can be
recovered if the width of the features () relative to the size of the
star is similar to the intrinsic velocity resolution (set by the width of the
local stellar profile, ) relative to the stellar rotation velocity (
sin, i.e. sin/). This opens up a new
way to study the ring systems around planets with long orbital periods, where
observations of the full transit, covering the ingress and egress, are not
always feasible.Comment: Accepted for publication in MNRA
All NIRspec needs is HST/WFC3 pre-imaging? The use of Milky Way Stars in WFC3 Imaging to Register NIRspec MSA Observations
The James Webb Space Telescope (JWST) will be an exquisite new near-infrared
observatory with imaging and multi-object spectroscopy through ESA's NIRspec
instrument with its unique Micro-Shutter Array (MSA), allowing for slits to be
positioned on astronomical targets by opening specific 0.002"-wide micro
shutter doors.
To ensure proper target acquisition, the on-sky position of the MSA needs to
be verified before spectroscopic observations start. An onboard centroiding
program registers the position of pre-identified guide stars in a Target
Acquisition (TA) image, a short pre-spectroscopy exposure without dispersion
(image mode) through the MSA with all shutters open.
The outstanding issue is the availability of Galactic stars in the right
luminosity range for TA relative to typical high redshift targets. We explore
this here using the stars and candidate galaxies identified in the
source extractor catalogs of Brightest of Reionizing Galaxies survey
(BoRG[z8]), a pure-parallel program with Hubble Space Telescope Wide-Field
Camera 3.
We find that (a) a single WFC3 field contains enough Galactic stars to
satisfy the NIRspec astrometry requirement (20 milli-arcseconds), provided its
and the NIRspec TA's are AB in WFC3 F125W, (b) a single WFC3
image can therefore serve as the pre-image if need be, (c) a WFC3 mosaic and
accompanying TA image satisfy the astrometry requirement at AB mag in
WFC3 F125W, (d) no specific Galactic latitude requires deeper TA imaging due to
a lack of Galactic stars, and (e) a depth of AB mag in WFC3 F125W is
needed if a guide star in the same MSA quadrant as a target is required.
We take the example of a BoRG identified candidate galaxy and
require a Galactic star within 20" of it. In this case, a depth of 25.5 AB in
F125W is required (with 97% confidence).Comment: 17 pages, 15 figures, to appear in the Journal of Astronomical
Instrumentatio
Minimizing Strong Telluric Absorption in Near Infra-red Stellar Spectra
We have obtained high resolution spectra (R = 25000) of an A star over
varying airmass to determine the effectiveness of telluric removal in the limit
of high signal to noise. The near infra-red line HeI at 2.058 microns, which is
a sensitive indicator of physical conditions in massive stars, supergiants, HII
regions and YSOs, resides among pressure broadened telluric absorption from
carbon dioxide and water vapor that varies both in time and with observed
airmass.
Our study shows that in the limit of bright stars at high resolution,
accuracies of 5% are typical for high airmass observations (greater than 1.9),
improving to a photon-limited accuracy of 2% at smaller airmasses (less than
1.15). We find that by using the continuum between telluric absorption lines of
a ro-vibrational fan a photon-limited 1% accuracy is achievable.Comment: 14 pages, 7 figures. Accepted for publication in PAS
SPIRAL Phase A: A Prototype Integral Field Spectrograph for the AAT
We present details of a prototype fiber feed for use on the Anglo-Australian
Telescope (AAT) that uses a dedicated fiber-fed medium/high resolution (R >
10000) visible-band spectrograph to give integral field spectroscopy (IFS) of
an extended object. A focal reducer couples light from the telescope to the
close-packed lenslet array and fiber feed, allowing the spectrograph be used on
other telescopes with the change of a single lens. By considering the
properties of the fibers in the design of the spectrograph, an efficient design
can be realised, and we present the first scientific results of a prototype
spectrograph using a fiber feed with 37 spatial elements, namely the detection
of Lithium confirming a brown dwarf candidate and IFS of the supernova remnant
SN1987A.Comment: 41 pages, 15 figures, 3 tables; accepted by PAS
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