513 research outputs found
Strict Upper Limits on the Carbon-to-Oxygen Ratios of Eight Hot Jupiters from Self-Consistent Atmospheric Retrieval
The elemental compositions of hot Jupiters are informative relics of planet
formation that can help us answer long-standing questions regarding the origin
and formation of giant planets. Here, I present the main conclusions from a
comprehensive atmospheric retrieval survey of eight hot Jupiters with
detectable molecular absorption in their near-infrared transmission spectra. I
analyze the eight transmission spectra using the newly-developed,
self-consistent atmospheric retrieval framework, SCARLET. Unlike previous
methods, SCARLET combines the physical and chemical consistency of complex
atmospheric models with the statistical treatment of observational
uncertainties known from atmospheric retrieval techniques. I find that all
eight hot Jupiters consistently require carbon-to-oxygen ratios (C/O) below
0.9. The finding of C/O<0.9 is highly robust for HD209458b, WASP-12b, WASP-19b,
HAT-P-1b, and XO-1b. For HD189733b, WASP-17b, and WASP-43b, I find that the
published WFC3 transmission spectra favor C/O<0.9 at greater than 95%
confidence. I further show that the water abundances on all eight hot Jupiters
are consistent with solar composition. The relatively small depth of the
detected water absorption features is due to the presence of clouds, not due to
a low water abundance as previously suggested for HD209458b. The presence of a
thick cloud deck is inferred for HD209458b and WASP-12b. HD189733b may host a
similar cloud deck, rather than the previously suggested Rayleigh hazes, if
star spots affect the observed spectrum. The approach taken in SCARLET can be
regarded as a new pathway to interpreting spectral observations of planetary
atmospheres. In this work, including our prior knowledge of H-C-N-O chemistry
enables me to constrain the C/O ratio without detecting a single carbon-bearing
molecule.Comment: under review at ApJ; updated to account for recently announced
observations of WASP-12b and HD 209458
Atmospheric Retrieval for Super-Earths: Uniquely Constraining the Atmospheric Composition with Transmission Spectroscopy
We present a retrieval method based on Bayesian analysis to infer the
atmospheric compositions and surface or cloud-top pressures from transmission
spectra of exoplanets with general compositions. In this study, we identify
what can unambiguously be determined about the atmospheres of exoplanets from
their transmission spectra by applying the retrieval method to synthetic
observations of the super-Earth GJ 1214b. Our approach to infer constraints on
atmospheric parameters is to compute their joint and marginal posterior
probability distributions using the MCMC technique in a parallel tempering
scheme. A new atmospheric parameterization is introduced that is applicable to
general atmospheres in which the main constituent is not known a priori and
clouds may be present. Our main finding is that a unique constraint of the
mixing ratios of the absorbers and up to two spectrally inactive gases (such as
N2 and primordial H2+He) is possible if the observations are sufficient to
quantify both (1) the broadband transit depths in at least one absorption
feature for each absorber and (2) the slope and strength of the molecular
Rayleigh scattering signature. The surface or cloud-top pressure can be
quantified if a surface or cloud deck is present. The mean molecular mass can
be constrained from the Rayleigh slope or the shapes of absorption features,
thus enabling to distinguish between cloudy hydrogen-rich atmospheres and high
mean molecular mass atmospheres. We conclude, however, that without the
signature of Rayleigh scattering--even with robustly detected infrared
absorption features--there is no reliable way to tell if the absorber is the
main constituent of the atmosphere or just a minor species with a mixing ratio
of <0.1%. The retrieval method leads us to a conceptual picture of which
details in transmission spectra are essential for unique characterizations of
well-mixed atmospheres.Comment: 23 pages, 13 figures, accepted at ApJ, submitted to ApJ on Nov 4,
201
Observing Exoplanets with High Dispersion Coronagraphy. I. The scientific potential of current and next-generation large ground and space telescopes
Direct imaging of exoplanets presents a formidable technical challenge owing
to the small angular separation and high contrast between exoplanets and their
host stars. High Dispersion Coronagraphy (HDC) is a pathway to achieve
unprecedented sensitivity to Earth-like planets in the habitable zone. Here, we
present a framework to simulate HDC observations and data analyses. The goal of
these simulations is to perform a detailed analysis of the trade-off between
raw star light suppression and spectral resolution for various instrument
configurations, target types, and science cases. We predict the performance of
an HDC instrument at Keck observatory for characterizing directly imaged
gas-giant planets in near infrared bands. We also simulate HDC observations of
an Earth-like planet using next-generation ground-based (TMT) and spaced-base
telescopes (HabEx and LUVOIR). We conclude that ground-based ELTs are more
suitable for HDC observations of an Earth-like planet than future space-based
missions owing to the considerable difference in collecting area. For
ground-based telescopes, HDC observations can detect an Earth-like planet in
the habitable zone around an M dwarf star at 10 starlight suppression
level. Compared to the 10 planet/star contrast, HDC relaxes the
starlight suppression requirement by a factor of 10. For space-based
telescopes, detector noise will be a major limitation at spectral resolutions
higher than 10. Considering detector noise and speckle chromatic noise,
R=400 (1600) is the optimal spectral resolutions for HabEx(LUVOIR). The
corresponding starlight suppression requirement to detect a planet with
planet/star contrast= is relaxed by a factor of 10 (100) for
HabEx (LUVOIR).Comment: 28 pages, 21 figures, 8 tables, accepted by A
Constraints on the Spin Evolution of Young Planetary-Mass Companions
Surveys of young star-forming regions have discovered a growing population of
planetary-mass (<13 M_Jup) companions around young stars. There is an ongoing
debate as to whether these companions formed like planets (that is, from the
circumstellar disk), or if they represent the low-mass tail of the star
formation process. In this study we utilize high-resolution spectroscopy to
measure rotation rates of three young (2-300 Myr) planetary-mass companions and
combine these measurements with published rotation rates for two additional
companions to provide a look at the spin distribution of these objects. We
compare this distribution to complementary rotation rate measurements for six
brown dwarfs with masses <20 M_Jup, and show that these distributions are
indistinguishable. This suggests that either that these two populations formed
via the same mechanism, or that processes regulating rotation rates are
independent of formation mechanism. We find that rotation rates for both
populations are well below their break-up velocities and do not evolve
significantly during the first few hundred million years after the end of
accretion. This suggests that rotation rates are set during late stages of
accretion, possibly by interactions with a circumplanetary disk. This result
has important implications for our understanding of the processes regulating
the angular momentum evolution of young planetary-mass objects, and of the
physics of gas accretion and disk coupling in the planetary-mass regime.Comment: 31 pages, 10 figures, published in Nature Astronomy,
DOI:10.1038/s41550-017-0325-
Detecting Water In the atmosphere of HR 8799 c with L-band High Dispersion Spectroscopy Aided By Adaptive Optics
High dispersion spectroscopy of brown dwarfs and exoplanets enables exciting
science cases, e.g., mapping surface inhomogeneity and measuring spin rate.
Here, we present band observations of HR 8799 c using Keck NIRSPEC
(R=15,000) in adaptive optics (AO) mode (NIRSPAO). We search for molecular
species (HO and CH) in the atmosphere of HR 8799 c with a template
matching method, which involves cross correlation between reduced spectrum and
a template spectrum. We detect HO but not CH, which suggests
disequilibrium chemistry in the atmosphere of HR 8799 c, and this is consistent
with previous findings. We conduct planet signal injection simulations to
estimate the sensitivity of our AO-aided high dispersion spectroscopy
observations. We conclude that contrast can be reached in band.
The sensitivity is mainly limited by the accuracy of line list used in modeling
spectra and detector noise. The latter will be alleviated by the NIRSPEC
upgrade.Comment: 14 pages, 5 figures, 5 tables, accepted for publication on AJ,
references update
High-contrast imaging and high-resolution spectroscopy observation of exoplanets
Detection and characterization of exoplanets faces challenges of smaller angular separation and high contrast between exoplanets and their host stars. High contrast imaging (HCI) instruments equipped with coronagraphs are built to meet these challenges, providing a way of spatially suppressing and separating stellar flux from that of a planet. Another way of separating stellar flux can be achieved by high-resolution spectroscopy (HRS), exploiting the fact that spectral features are different between a star and a planet. Observing exoplanets with HCI+HRS will achieve a higher contrast than the spatial or the spectroscopic method alone, improving the sensitivity to planet detection and enabling the study of the physical and chemical processes. Here, we simulate the performance of a HCI+HRS instrument (i.e., the upgrade Keck NIRSPEC and the fiber injection unit) to study its potential in detecting and characterizing currently known directly imaged planets. The simulation considers the spectral information content of an exoplanet, telescope and instrument specifications and realistic noise sources. The result of the simulation helps to set system requirement and informs designs at system-level. We also perform a trade study for a HCI+HRS instrument for a space mission to study an Earth-like planet orbiting a Sun-like star at 10 pc
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