168 research outputs found
Reliable inference of exoplanet light curve parameters using deterministic and stochastic systematics models
Time-series photometry and spectroscopy of transiting exoplanets allow us to
study their atmospheres. Unfortunately, the required precision to extract
atmospheric information surpasses the design specifications of most general
purpose instrumentation, resulting in instrumental systematics in the light
curves that are typically larger than the target precision. Systematics must
therefore be modelled, leaving the inference of light curve parameters
conditioned on the subjective choice of models and model selection criteria.
This paper aims to test the reliability of the most commonly used systematics
models and model selection criteria. As we are primarily interested in
recovering light curve parameters rather than the favoured systematics model,
marginalisation over systematics models is introduced as a more robust
alternative than simple model selection. This can incorporate uncertainties in
the choice of systematics model into the error budget as well as the model
parameters. Its use is demonstrated using a series of simulated transit light
curves. Stochastic models, specifically Gaussian processes, are also discussed
in the context of marginalisation over systematics models, and are found to
reliably recover the transit parameters for a wide range of systematics
functions. None of the tested model selection criteria - including the BIC -
routinely recovered the correct model. This means that commonly used methods
that are based on simple model selection may underestimate the uncertainties
when extracting transmission and eclipse spectra from real data, and
low-significance claims using such techniques should be treated with caution.
In general, no systematics modelling techniques are perfect; however,
marginalisation over many systematics models helps to mitigate poor model
selection, and stochastic processes provide an even more flexible approach to
modelling instrumental systematics.Comment: 15 pages, 2 figures, published in MNRAS, typo in footnote eq
correcte
Time resolved spectroscopy of dust and gas from extrasolar planetesimals orbiting WD 1145+017
Multiple long and variable transits caused by dust from possibly
disintegrating asteroids were detected in light curves of WD 1145+017. We
present time-resolved spectroscopic observations of this target with QUCAM CCDs
mounted in the Intermediate dispersion Spectrograph and Imaging System at the
4.2-m William Herschel Telescope in two different spectral arms: the blue arm
covering 3800-4025 {\AA} and the red arm covering 7000-7430 {\AA}. When
comparing individual transits in both arms, our observations show with 20
{\sigma} significance an evident colour difference between the in- and
out-of-transit data of the order of 0.05-0.1 mag, where transits are deeper in
the red arm. We also show with > 6 {\sigma} significance that spectral lines in
the blue arm are shallower during transits than out-of-transit. For the
circumstellar lines it also appears that during transits the reduction in
absorption is larger on the red side of the spectral profiles. Our results
confirm previous findings showing the u'-band excess and a decrease in line
absorption during transits. Both can be explained by an opaque body blocking a
fraction of the gas disc causing the absorption, implying that the absorbing
gas is between the white dwarf and the transiting objects. Our results also
demonstrate the capability of EMCCDs to perform high-quality time resolved
spectroscopy of relatively faint targets.Comment: 9 pages, 5 figures. Accepted to MNRA
A ground-based NUV secondary eclipse observation of KELT-9b
KELT-9b is a recently discovered exoplanet with a 1.49 d orbit around a
B9.5/A0-type star. The unparalleled levels of UV irradiation it receives from
its host star put KELT-9b in its own unique class of ultra-hot Jupiters, with
an equilibrium temperature > 4000 K. The high quantities of dissociated
hydrogen and atomic metals present in the dayside atmosphere of KELT-9b bear
more resemblance to a K-type star than a gas giant. We present a single
observation of KELT-9b during its secondary eclipse, taken with the Wide Field
Camera on the Isaac Newton Telescope (INT). This observation was taken in the
U-band, a window particularly sensitive to Rayleigh scattering. We do not
detect a secondary eclipse signal, but our 3 upper limit of 181 ppm on
the depth allows us to constrain the dayside temperature of KELT-9b at
pressures of ~30 mbar to 4995 K (3). Although we can place an
observational constraint of 0.14, our models suggest that the actual
value is considerably lower than this due to H opacity. This places KELT-9b
squarely in the albedo regime populated by its cooler cousins, almost all of
which reflect very small components of the light incident on their daysides.
This work demonstrates the ability of ground-based 2m-class telescopes like the
INT to perform secondary eclipse studies in the NUV, which have previously only
been conducted from space-based facilities.Comment: Accepted in ApJL. 7 pages, 3 figure
High-resolution emission spectroscopy retrievals of MASCARA-1b with CRIRES+: Strong detections of CO, HO and Fe emission lines and a CO consistent with solar
The characterization of exoplanet atmospheres has proven to be successful
using high-resolution spectroscopy. Phase curve observations of hot/ultra-hot
Jupiters can reveal their compositions and thermal structures, thereby allowing
the detection of molecules and atoms in the planetary atmosphere using the
cross-correlation technique. We present pre-eclipse observations of the
ultra-hot Jupiter, MASCARA-1b, observed with the recently upgraded CRIRES+
high-resolution infrared spectrograph at the VLT. We report a detection of (8.3) in the K-band and confirm previous detections of
(>15) and (>10) in the day-side atmosphere
of MASCARA-1b. Using a Bayesian inference framework, we retrieve the abundances
of the detected species and constrain planetary orbital velocities, -
profiles, and the carbon-to-oxygen ratio (). A free retrieval results
in an elevated abundance (() =
), leading to a super-solar ratio. More
realistically, allowing for vertically-varying chemistry in the atmosphere by
incorporating a chemical-equilibrium model results in a of
and a metallicity of ,
both consistent with solar values. Finally, we also report a slight offset of
the feature in both K and v that could be a
signature of atmospheric dynamics. Due to the 3D structure of exoplanet
atmospheres and the exclusion of time/phase dependence in our 1D forward
models, further follow-up observations and analysis are required to confirm or
refute this result.Comment: 21 pages, 18 figures, 4 tables, accepted for publication in Monthly
Notices of the Royal Astronomical Societ
VLT FORS2 comparative transmission spectroscopy: Detection of Na in the atmosphere of WASP-39b from the ground
We present transmission spectroscopy of the warm Saturn-mass exoplanet
WASP-39b made with the Very Large Telescope (VLT) FOcal Reducer and
Spectrograph (FORS2) across the wavelength range 411-810nm. The transit depth
is measured with a typical precision of 240 parts per million (ppm) in
wavelength bins of 10nm on a V = 12.1 magnitude star. We detect the sodium
absorption feature (3.2-sigma) and find evidence for potassium. The
ground-based transmission spectrum is consistent with Hubble Space Telescope
(HST) optical spectroscopy, strengthening the interpretation of WASP-39b having
a largely clear atmosphere. Our results demonstrate the great potential of the
recently upgraded FORS2 spectrograph for optical transmission spectroscopy,
obtaining HST-quality light curves from the ground.Comment: 11 pages, 5 figures, accepted for publication in Ap
The Very Low Albedo of WASP-12b From Spectral Eclipse Observations with
We present an optical eclipse observation of the hot Jupiter WASP-12b using
the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope.
These spectra allow us to place an upper limit of (97.5%
confidence level) on the planet's white light geometric albedo across 290--570
nm. Using six wavelength bins across the same wavelength range also produces
stringent limits on the geometric albedo for all bins. However, our
uncertainties in eclipse depth are 40% greater than the Poisson limit and
may be limited by the intrinsic variability of the Sun-like host star --- the
solar luminosity is known to vary at the level on a timescale of
minutes. We use our eclipse depth limits to test two previously suggested
atmospheric models for this planet: Mie scattering from an aluminum-oxide haze
or cloud-free Rayleigh scattering. Our stringent nondetection rules out both
models and is consistent with thermal emission plus weak Rayleigh scattering
from atomic hydrogen and helium. Our results are in stark contrast with those
for the much cooler HD 189733b, the only other hot Jupiter with spectrally
resolved reflected light observations; those data showed an increase in albedo
with decreasing wavelength. The fact that the first two exoplanets with optical
albedo spectra exhibit significant differences demonstrates the importance of
spectrally resolved reflected light observations and highlights the great
diversity among hot Jupiters.Comment: 8 pages, 4 figures, 1 table, published in ApJL, in pres
Gemini/GMOS Optical Transmission Spectroscopy of WASP-121b: signs of variability in an ultra-hot Jupiter?
We present ground-based, spectroscopic observations of two transits of the
ultra-hot Jupiter WASP-121b covering the wavelength range 500 - 950 nm
using Gemini/GMOS. We use a Gaussian process framework to model instrumental
systematics in the light curves, and also demonstrate the use of the more
generalised Student's-T process to verify our results. We find that our
measured transmission spectrum, whilst showing overall agreement, is slightly
discrepant with results obtained using HST/STIS, particularly for wavelengths
shortward of 650 nm. In contrast to the STIS results, we find evidence
for an increasing blueward slope and little evidence for absorption from either
TiO or VO in our retrieval, in agreement with a number of recent studies
performed at high-resolution. We suggest that this might point to some other
absorbers, particularly some combination of recently detected atomic metals, in
addition to scattering by hazes, being responsible for the excess optical
absorption and observed vertical thermal inversion. Our results are also
broadly consistent with previous ground-based photometry and 3D GCM
predictions, however, these assumed different chemistry to our retrievals. In
addition, we show that the GMOS observations are repeatable over short periods
(days), similarly to the HST/STIS observations. Their difference over longer
periods (months) could well be the result of temporal variability in the
atmospheric properties (i.e. weather) as predicted by theoretical models of
ultra-hot Jupiters; however, more mundane explanations such as instrumental
systematics and stellar activity cannot be fully ruled out, and we encourage
future observations to explore this possibility.Comment: 17 pages, 10 Figures. Accepted for publication in MNRA
Retrieval survey of metals in six ultra-hot Jupiters: Trends in chemistry, rain-out, ionisation and atmospheric dynamics
Ground-based high-resolution spectroscopy (HRS) has detected numerous
chemical species and atmospheric dynamics in exoplanets, most notably ultra-hot
Jupiters (UHJs). However, quantitative estimates on abundances have been
challenging but are essential for accurate comparative characterisation and to
determine formation scenarios. In this work we retrieve the atmospheres of six
UHJs (WASP-76~b, MASCARA-4~b, MASCARA-2~b, WASP-121~b, HAT-P-70~b and
WASP-189~b) with ESPRESSO and HARPS-N/HARPS observations, exploring trends in
eleven neutral species and dynamics. While Fe abundances agree well with
stellar values, Mg, Ni, Cr, Mn and V show more variation, highlighting the
difficulty in using a single species as a proxy for metallicity. We find that
Ca, Na, Ti and TiO are under-abundant, potentially due to ionisation and/or
night-side rain-out. Our retrievals also show that relative abundances between
species are more robust, consistent with previous works. We perform spatially-
and phase-resolved retrievals for WASP-76~b and WASP-121~b given their high
signal-to-noise observations, and find the chemical abundances in each of the
terminator regions are broadly consistent. We additionally constrain dynamics
for our sample through Doppler shifts and broadening of the planetary signals
during the primary eclipse, with median blue shifts between 0.9-9.0~km/s
due to day-night winds. Furthermore, we constrain spectroscopic masses for
MASCARA-2~b and HAT-P-70~b consistent with their known upper limits, but we
note that these may be biased due to degeneracies. This work highlights the
importance of future HRS studies to further probe differences and trends
between exoplanets.Comment: 26 pages, 11 figures, 5 tables, published in A
The Deep Blue Color of HD 189733b: Albedo Measurements with Hubble Space Telescope/Space Telescope Imaging Spectrograph at Visible Wavelengths
We present a secondary eclipse observation for the hot Jupiter HD 189733b across the wavelength range 290-570 nm made using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We measure geometric albedos of A_g = 0.40 ± 0.12 across 290-450 nm and A_g < 0.12 across 450-570 nm at 1σ confidence. The albedo decrease toward longer wavelengths is also apparent when using six wavelength bins over the same wavelength range. This can be interpreted as evidence for optically thick reflective clouds on the dayside hemisphere with sodium absorption suppressing the scattered light signal beyond ~450 nm. Our best-fit albedo values imply that HD 189733b would appear a deep blue color at visible wavelengths
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