22 research outputs found
Applications of a Gaussian process framework for modelling of high-resolution exoplanet spectra
Observations of exoplanet atmospheres in high resolution have the potential to resolve individual planetary absorption lines, despite the issues associated with ground-based observations. The removal of contaminating stellar and telluric absorption features is one of the most sensitive steps required to reveal the planetary spectrum and, while many different detrending methods exist, it remains difficult to directly compare the performance and efficacy of these methods. Additionally, though the standard cross-correlation method enables robust detection of specific atmospheric species, it only probes for features that are expected a priori. Here we present a novel methodology using Gaussian process regression to directly model the components of high-resolution spectra, which partially addresses these issues. We use two archival CRIRES/VLT datasets as test cases, observations of the hot Jupiters HD 189733 b and 51 Pegasi b, recovering injected signals with average line contrast ratios of ∼4.37 × 10−3 and ∼1.39 × 10−3, and planet radial velocities ΔKp = 1.45 ± 1.53 km s−1 and ΔKp = 0.12 ± 0.12 km s−1 from the injection velocities respectively. In addition, we demonstrate an application of the GP method to assess the impact of the detrending process on the planetary spectrum, by implementing injection-recovery tests. We show that standard detrending methods used in the literature negatively affect the amplitudes of absorption features in particular, which has the potential to render retrieval analyses inaccurate. Finally, we discuss possible limiting factors for the non-detections using this method, likely to be remedied by higher signal-to-noise data
Prospects for Characterizing the Haziest Sub-Neptune Exoplanets with High Resolution Spectroscopy
Observations to characterize planets larger than Earth but smaller than
Neptune have led to largely inconclusive interpretations at low spectral
resolution due to hazes or clouds that obscure molecular features in their
spectra. However, here we show that high-resolution spectroscopy (R
25,000 to 100,000) enables one to probe the regions in these atmospheres above
the clouds where the cores of the strongest spectral lines are formed. We
present models of transmission spectra for a suite of GJ1214b-like planets with
thick photochemical hazes covering 1 - 5 m at a range of resolutions
relevant to current and future ground-based spectrographs. Furthermore, we
compare the utility of the cross-correlation function that is typically used
with a more formal likelihood-based approach, finding that only the likelihood
based method is sensitive to the presence of haze opacity. We calculate the
signal-to-noise of these spectra, including telluric contamination, required to
robustly detect a host of molecules such as CO, CO, HO, and
CH, and photochemical products like HCN, as a function of wavelength
range and spectral resolution. Spectra in M band require the lowest S/N
to detect multiple molecules simultaneously. CH is only observable for
the coolest models ( 412 K) and only in the L band. We
quantitatively assess how these requirements compare to what is achievable with
current and future instruments, demonstrating that characterization of small
cool worlds with ground-based high resolution spectroscopy is well within
reach.Comment: Submitted to AAS Journals, revised to reflect referee comments.
Posting of this manuscript on the arXiv was coordinated with S. Ghandi et a
Molecular cross sections for high resolution spectroscopy of super earths, warm Neptunes and hot Jupiters
High-resolution spectroscopy (HRS) has been used to detect a number of species in the atmospheres of hot Jupiters. Key to such detections is accurately and precisely modelled spectra for cross-correlation against the R ≳ 20 000 observations. There is a need for the latest generation of opacities which form the basis for high signal-to-noise detections using such spectra. In this study we present and make publicly available cross-sections for six molecular species, H2O, CO, HCN, CH4, NH3, and CO2 using the latest line lists most suitable for low- and high-resolution spectroscopy. We focus on the infrared (0.95–5 μm) and between 500 and 1500 K where these species have strong spectral signatures. We generate these cross-sections on a grid of pressures and temperatures typical for the photospheres of super-Earth, warm Neptunes, and hot Jupiters using the latest H2 and He pressure broadening. We highlight the most prominent infrared spectral features by modelling three representative exoplanets, GJ 1214 b, GJ 3470 b, and HD 189733 b, which encompass a wide range in temperature, mass, and radii. In addition, we verify the line lists for H2O, CO, and HCN with previous high-resolution observations of hot Jupiters. However, we are unable to detect CH4 with our new cross-sections from HRS observations of HD 102195 b. These high-accuracy opacities are critical for atmospheric detections with HRS and will be continually updated as new data become available
Applying a temporal systematics model to vector Apodizing Phase Plate coronagraphic data: TRAP4vAPP
The vector Apodizing Phase Plate (vAPP) is a pupil plane coronagraph that
suppresses starlight by forming a dark hole in its point spread function (PSF).
The unconventional and non-axisymmetrical PSF arising from the phase
modification applied by this coronagraph presents a special challenge to
post-processing techniques. We aim to implement a recently developed
post-processing algorithm, temporal reference analysis of planets (TRAP) on
vAPP coronagraphic data. The property of TRAP that uses non-local training
pixels, combined with the unconventional PSF of vAPP, allows for more
flexibility than previous spatial algorithms in selecting reference pixels to
model systematic noise. Datasets from two types of vAPPs are analysed: a double
grating-vAPP (dgvAPP360) that produces a single symmetric PSF and a
grating-vAPP (gvAPP180) that produces two D-shaped PSFs. We explore how to
choose reference pixels to build temporal systematic noise models in TRAP for
them. We then compare the performance of TRAP with previously implemented
algorithms that produced the best signal-to-noise ratio (S/N) in companion
detections in these datasets. We find that the systematic noise between the two
D-shaped PSFs is not as temporally associated as expected. Conversely, there is
still a significant number of systematic noise sources that are shared by the
dark hole and the bright side in the same PSF. We should choose reference
pixels from the same PSF when reducing the dgvAPP360 dataset or the gvAPP180
dataset with TRAP. In these datasets, TRAP achieves results consistent with
previous best detections, with an improved S/N for the gvAPP180 dataset.Comment: 15 pages, 10 figures, accepted to A&
The Roasting Marshmallows Program with IGRINS on Gemini South I: Composition and Climate of the Ultra Hot Jupiter WASP-18 b
We present high-resolution dayside thermal emission observations of the
exoplanet WASP-18b using IGRINS on Gemini South. We remove stellar and telluric
signatures using standard algorithms, and we extract the planet signal via
cross correlation with model spectra. We detect the atmosphere of WASP-18b at a
signal-to-noise ratio (SNR) of 5.9 using a full chemistry model, measure H2O
(SNR=3.3), CO (SNR=4.0), and OH (SNR=4.8) individually, and confirm previous
claims of a thermal inversion layer. The three species are confidently detected
(>4) with a Bayesian inference framework, which we also use to retrieve
abundance, temperature, and velocity information. For this ultra-hot Jupiter
(UHJ), thermal dissociation processes likely play an important role. Retrieving
abundances constant with altitude and allowing the temperature-pressure profile
to freely adjust results in a moderately super-stellar carbon to oxygen ratio
(C/O=0.75^{+0.14}_{-0.17}) and metallicity ([M/H]=1.03^{+0.65}_{-1.01}).
Accounting for undetectable oxygen produced by thermal dissociation leads to
C/O=0.45^{+0.08}_{-0.10} and [M/H]=1.17^{+0.66}_{-1.01}. A retrieval that
assumes radiative-convective-thermochemical-equilibrium and naturally accounts
for thermal dissociation constrains C/O<0.34 (2) and
[M/H]=0.48^{+0.33}_{-0.29}, in line with the chemistry of the parent star.
Looking at the velocity information, we see a tantalising signature of
different Doppler shifts at the level of a few km/s for different molecules,
which might probe dynamics as a function of altitude and location on the planet
disk. Our results demonstrate that ground-based, high-resolution spectroscopy
at infrared wavelengths can provide meaningful constraints on the compositions
and climate of highly irradiated planets. This work also elucidates potential
pitfalls with commonly employed retrieval assumptions when applied to UHJ
spectra.Comment: 27 pages, 18 figures, submitted to AAS Journals. Community feedback
welcom
Measuring the variability of directly imaged exoplanets using vector Apodizing Phase Plates combined with ground-based differential spectrophotometry
Clouds and other features in exoplanet and brown dwarf atmospheres cause variations in brightness as they rotate in and out of view. Ground-based instruments reach the high contrasts and small inner working angles needed to monitor these faint companions, but their small fields of view lack simultaneous photometric references to correct for non-astrophysical variations. We present a novel approach for making ground-based light curves of directly imaged companions using high-cadence differential spectrophotometric monitoring, where the simultaneous reference is provided by a double-grating 360○ vector Apodizing Phase Plate (dgvAPP360) coronagraph. The dgvAPP360 enables high-contrast companion detections without blocking the host star, allowing it to be used as a simultaneous reference. To further reduce systematic noise, we emulate exoplanet transmission spectroscopy, where the light is spectrally dispersed and then recombined into white-light flux. We do this by combining the dgvAPP360 with the infrared Arizona Lenslets for Exoplanet Spectroscopy integral field spectrograph on the Large Binocular Telescope Interferometer. To demonstrate, we observed the red companion HD 1160 B (separation ∼780 mas) for one night, and detec
A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization
A key legacy of the recently launched TESS mission will be to provide the
astronomical community with many of the best transiting exoplanet targets for
atmospheric characterization. However, time is of the essence to take full
advantage of this opportunity. JWST, although delayed, will still complete its
nominal five year mission on a timeline that motivates rapid identification,
confirmation, and mass measurement of the top atmospheric characterization
targets from TESS. Beyond JWST, future dedicated missions for atmospheric
studies such as ARIEL require the discovery and confirmation of several hundred
additional sub-Jovian size planets (R_p < 10 R_Earth) orbiting bright stars,
beyond those known today, to ensure a successful statistical census of
exoplanet atmospheres. Ground-based ELTs will also contribute to surveying the
atmospheres of the transiting planets discovered by TESS. Here we present a set
of two straightforward analytic metrics, quantifying the expected
signal-to-noise in transmission and thermal emission spectroscopy for a given
planet, that will allow the top atmospheric characterization targets to be
readily identified among the TESS planet candidates. Targets that meet our
proposed threshold values for these metrics would be encouraged for rapid
follow-up and confirmation via radial velocity mass measurements. Based on the
catalog of simulated TESS detections by Sullivan et al. (2015), we determine
appropriate cutoff values of the metrics, such that the TESS mission will
ultimately yield a sample of high-quality atmospheric
characterization targets across a range of planet size bins, extending down to
Earth-size, potentially habitable worlds.Comment: accepted to PAS
Water observed in the atmosphere of τ Boötis Ab with CARMENES/CAHA
Characterising the atmospheres of hot Jupiters is important in understanding the formation and migration of these exotic planets. However, there are still many open questions about the chemical and physical properties of these atmospheres. Here, we confirm the detection of water vapour in thermal emission from the non-transiting hot Jupiter τ Boötis Ab with the high resolution NIR CARMENES spectrograph. Combining over 17 h of observations (560 spectra) and using a Bayesian cross-correlation to log-likelihood approach, we measure a systemic velocity of km s−1 and a radial velocity semi-amplitude of km s−1 for the planet, which results in an absolute mass of and an orbital inclination of degrees. Our retrieved Vsys shows a significant shift (+5 km s−1 ) from the literature value, which could be caused by an inaccurate time of periastron. Within the explored model grid, we measure a preference for solar water abundance (VMR = 10−3) and no evidence for additional minor species in the atmosphere. Given the extensive orbital coverage of the data, we searched for a phase dependency in the water signal but found no strong evidence of variation with orbital phase. This detection is at odds with recent observations from SPIRou/CFHT and their tight upper limit on water abundance. We recommend further observations of the atmosphere τ Boötis Ab to try and resolve these discrepancies
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