arXiv subjects

Michael R. Line

Publications and source records attributed to Michael R. Line.

At least 127 records · Page 7Linked to original sources

arXiv2016

Transiting Exoplanet Studies and Community Targets for JWST's Early Release Science Program

The James Webb Space Telescope will revolutionize transiting exoplanet atmospheric science due to its capability for continuous, long-duration observations and its larger collecting area, spectral coverage, and spectral resolution compared to existing space-based facilities. However, it is unclear precisely how well JWST will perform and which of its myriad instruments and observing modes will be best suited for transiting exoplanet studies. In this article, we describe a prefatory JWST Early Release Science (ERS) program that focuses on testing specific observing modes to quickly give the community the data and experience it needs to plan more efficient and successful future transiting exoplanet characterization programs. We propose a multi-pronged approach wherein one aspect of the program focuses on observing transits of a single target with all of the recommended observing modes to identify and understand potential systematics, compare transmission spectra at overlapping and neighboring wavelength regions, confirm throughputs, and determine overall performances. In our search for transiting exoplanets that are well suited to achieving these goals, we identify 12 objects (dubbed "community targets") that meet our defined criteria. Currently, the most favorable target is WASP-62b because of its large predicted signal size, relatively bright host star, and location in JWST's continuous viewing zone. Since most of the community targets do not have well-characterized atmospheres, we recommend initiating preparatory observing programs to determine the presence of obscuring clouds/hazes within their atmospheres. Measurable spectroscopic features are needed to establish the optimal resolution and wavelength regions for exoplanet characterization. Other initiatives from our proposed ERS program include testing the instrument brightness limits and performing phase-curve observations.(Abridged)

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arXiv2016

A Characteristic Transmission Spectrum dominated by H$_{2}$O applies to the majority of HST/WFC3 exoplanet observations

Currently, 19 transiting exoplanets have published transmission spectra obtained with the Hubble/WFC3 G141 near-IR grism. Using this sample, we have undertaken a uniform analysis incorporating measurement-error debiasing of the spectral modulation due to H$_{2}$O, measured in terms of the estimated atmospheric scale height, ${H_s}$. For those planets with a reported H$_{2}$O detection (10 out of 19), the spectral modulation due to H$_{2}$O ranges from 0.9 to 2.9~${H_s}$ with a mean value of 1.8~$\pm$~0.5~${H_s}$. This spectral modulation is significantly less than predicted for clear atmospheres. For the group of planets in which H$_{2}$O has been detected, we find the individual spectra can be coherently averaged to produce a characteristic spectrum in which the shape, together with the spectral modulation of the sample, are consistent with a range of H$_{2}$O mixing ratios and cloud-top pressures, with a minimum H$_{2}$O mixing ratio of 17~$^{+12}_{-6}$~ppm corresponding to the cloud-free case. Using this lower limit, we show that clouds or aerosols must block at least half of the atmospheric column that would otherwise be sampled by transmission spectroscopy in the case of a cloud-free atmosphere. We conclude that terminator-region clouds, with sufficient opacity to be opaque in slant-viewing geometry, are common in hot Jupiters.

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arXiv2016

The Water Abundance of the Directly Imaged Substellar Companion κ And b Retrieved from a Near Infrared Spectrum

Spectral retrieval has proven to be a powerful tool for constraining the physical properties and atmospheric compositions of extrasolar planet atmospheres from observed spectra, primarily for transiting objects but also for directly imaged planets and brown dwarfs. Despite its strengths, this approach has been applied to only about a dozen targets. Determining the abundances of the main carbon and oxygen-bearing compounds in a planetary atmosphere can lead to the C/O ratio of the object, which is crucial in understanding its formation and migration history. We present a retrieval analysis on the published near-infrared spectrum of κ And b, a directly imaged substellar companion to a young B9 star. We fit the emission spectrum model utilizing a Markov Chain Monte Carlo algorithm. We estimate the abundance of water vapor, and its uncertainty, in the atmosphere of the object. In addition, we place an upper limit on the abundance of CH$_4$. We compare qualitatively our results to studies that have applied model retrieval on multiband photometry and emission spectroscopy of hot Jupiters (extrasolar giant planets with orbital periods of several days) and the directly imaged giant planet HR 8799b.

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arXiv2016

The Influence of Non-Uniform Cloud Cover on Transit Transmission Spectra

We model the impact of non-uniform cloud cover on transit transmission spectra. Patchy clouds exist in nearly every solar system atmosphere, brown dwarfs, and transiting exoplanets. Our major findings suggest that fractional cloud coverage can exactly mimic high mean molecular weight atmospheres and vice-versa over certain wavelength regions, in particular, over the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) bandpass (1.1-1.7 $μ$m). We also find that patchy cloud coverage exhibits a signature that is different from uniform global clouds. Furthermore, we explain analytically why the "patchy cloud-high mean molecular weight" degeneracy exists. We also explore the degeneracy of non-uniform cloud coverage in atmospheric retrievals on both synthetic and real planets. We find from retrievals on a synthetic solar composition hot Jupiter with patchy clouds and a cloud free high mean molecular weight warm Neptune, that both cloud free high mean molecular weight atmospheres and partially cloudy atmospheres can explain the data equally well. Another key find is that the HST WFC3 transit transmission spectra of two well observed objects, the hot Jupiter HD189733b and the warm Neptune HAT-P-11b, can be explained well by solar composition atmospheres with patchy clouds without the need to invoke high mean molecular weight or global clouds. The degeneracy between high molecular weight and solar composition partially cloudy atmospheres can be broken by observing the molecular Rayleigh scattering differences between the two. Furthermore, the signature of partially cloudy limbs also appears as a $\sim$100 ppm residual in the ingress and egress of the transit light curves, provided the transit timing is known to seconds.

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arXiv2015

A Search for Water in the Atmosphere of HAT-P-26b Using LDSS-3C

The characterization of a physically-diverse set of transiting exoplanets is an important and necessary step towards establishing the physical properties linked to the production of obscuring clouds or hazes. It is those planets with identifiable spectroscopic features that can most effectively enhance our understanding of atmospheric chemistry and metallicity. The newly-commissioned LDSS-3C instrument on Magellan provides enhanced sensitivity and suppressed fringing in the red optical, thus advancing the search for the spectroscopic signature of water in exoplanetary atmospheres from the ground. Using data acquired by LDSS-3C and the Spitzer Space Telescope, we search for evidence of water vapor in the transmission spectrum of the Neptune-mass planet HAT-P-26b. Our measured spectrum is best explained by the presence of water vapor, a lack of potassium, and either a high-metallicity, cloud-free atmosphere or a solar-metallicity atmosphere with a cloud deck at ~10 mbar. The emergence of multi-scale-height spectral features in our data suggests that future observations at higher precision could break this degeneracy and reveal the planet's atmospheric chemical abundances. We also update HAT-P-26b's transit ephemeris, t_0 = 2455304.65218(25) BJD_TDB, and orbital period, p = 4.2345023(7) days.

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arXiv2015

Characterizing transiting exoplanet atmospheres with JWST

We explore how well James Webb Space Telescope (JWST) spectra will likely constrain bulk atmospheric properties of transiting exoplanets. We start by modeling the atmospheres of archetypal hot Jupiter, warm Neptune, warm sub-Neptune, and cool super-Earth planets with clear, cloudy, or high mean molecular weight atmospheres. Next we simulate the $λ= 1 - 11$ $μ$m transmission and emission spectra of these systems for several JWST instrument modes for single transit and eclipse events. We then perform retrievals to determine how well temperatures and molecular mixing ratios (CH$_4$, CO, CO$_2$, H$_2$O, NH$_3$) can be constrained. We find that $λ= 1 - 2.5$ $μ$m transmission spectra will often constrain the major molecular constituents of clear solar composition atmospheres well. Cloudy or high mean molecular weight atmospheres will often require full $1 - 11$ $μ$m spectra for good constraints, and emission data may be more useful in cases of sufficiently high $F_p$ and high $F_p/F_*$. Strong temperature inversions in the solar composition hot Jupiter atmosphere should be detectable with $1 - 2.5+$ $μ$m emission spectra, and $1 - 5+$ $μ$m emission spectra will constrain the temperature-pressure profiles of warm planets. Transmission spectra over $1 - 5+$ $μ$m will constrain [Fe/H] values to better than 0.5 dex for the clear atmospheres of the hot and warm planets studied. Carbon-to-oxygen ratios can be constrained to better than a factor of 2 in some systems. We expect that these results will provide useful predictions of the scientific value of single event JWST spectra until its on-orbit performance is known.

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arXiv2015

A Detection of Water in the Transmission Spectrum of the Hot Jupiter WASP-12b and Implications for its Atmospheric Composition

Detailed characterization of exoplanets has begun to yield measurements of their atmospheric properties that constrain the planets' origins and evolution. For example, past observations of the dayside emission spectrum of the hot Jupiter WASP-12b indicated that its atmosphere has a high carbon-to-oxygen ratio (C/O $>$ 1), suggesting it had a different formation pathway than is commonly assumed for giant planets. Here we report a precise near-infrared transmission spectrum for WASP-12b based on six transit observations with the Hubble Space Telescope/Wide Field Camera 3. We bin the data in 13 spectrophotometric light curves from 0.84 - 1.67 $μ$m and measure the transit depths to a median precision of 51 ppm. We retrieve the atmospheric properties using the transmission spectrum and find strong evidence for water absorption (7$σ$ confidence). This detection marks the first high-confidence, spectroscopic identification of a molecule in the atmosphere of WASP-12b. The retrieved 1$σ$ water volume mixing ratio is between $10^{-5}-10^{-2}$, which is consistent with C/O $>$ 1 to within 2$σ$. However, we also introduce a new retrieval parameterization that fits for C/O and metallicity under the assumption of chemical equilibrium. With this approach, we constrain C/O to $0.5^{+0.2}_{-0.3}$ at $1\,σ$ and rule out a carbon-rich atmosphere composition (C/O$>1$) at $>3σ$ confidence. Further observations and modeling of the planet's global thermal structure and dynamics would aid in resolving the tension between our inferred C/O and previous constraints. Our findings highlight the importance of obtaining high-precision data with multiple observing techniques in order to obtain robust constraints on the chemistry and physics of exoplanet atmospheres.

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arXiv2015

Spitzer Secondary Eclipse Observations of Five Cool Gas Giant Planets and Empirical Trends in Cool Planet Emission Spectra

In this work we present Spitzer 3.6 and 4.5 micron secondary eclipse observations of five new cool (<1200 K) transiting gas giant planets: HAT-P-19b, WASP-6b, WASP-10b, WASP-39b, and WASP-67b. We compare our measured eclipse depths to the predictions of a suite of atmosphere models and to eclipse depths for planets with previously published observations in order to constrain the temperature- and mass-dependent properties of gas giant planet atmospheres. We find that the dayside emission spectra of planets less massive than Jupiter require models with efficient circulation of energy to the night side and/or increased albedos, while those with masses greater than that of Jupiter are consistently best-matched by models with inefficient circulation and low albedos. At these relatively low temperatures we expect the atmospheric methane to CO ratio to vary as a function of metallicity, and we therefore use our observations of these planets to constrain their atmospheric metallicities. We find that the most massive planets have dayside emission spectra that are best-matched by solar metallicity atmosphere models, but we are not able to place strong constraints on metallicities of the smaller planets in our sample. Interestingly, we find that the ratio of the 3.6 and 4.5 micron brightness temperatures for these cool transiting planets is independent of planet temperature, and instead exhibits a tentative correlation with planet mass. If this trend can be confirmed, it would suggest that the shape of these planets' emission spectra depends primarily on their masses, consistent with the hypothesis that lower-mass planets are more likely to have metal-rich atmospheres.

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arXiv2015

Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs I: Characterizing Benchmarks, Gl570D and HD3651B

Interpreting the spectra of brown dwarfs is key to determining the fundamental physical and chemical processes occurring in their atmospheres. Powerful Bayesian atmospheric retrieval tools have recently been applied to both exoplanet and brown dwarf spectra to tease out the thermal structures and molecular abundances to understand those processes. In this manuscript we develop a significantly upgraded retrieval method and apply it to the SpeX spectral library data of two benchmark late T-dwarfs, Gl570D and HD3651B, to establish the validity of our upgraded forward model parameterization and Bayesian estimator. Our retrieved metallicities, gravities, and effective temperature are consistent with the metallicity and presumed ages of the systems. We add the carbon-to-oxygen ratio as a new dimension to benchmark systems and find good agreement between carbon-to-oxygens ratio derived in the brown dwarfs and the host stars. Furthermore, we have for the first time unambiguously determined the presence of ammonia in the low-resolution spectra of these two late T-dwarfs. We also show that the retrieved results are not significantly impacted by the possible presence of clouds, though some quantities are significantly impacted by uncertainties in photometry. This investigation represents a watershed study in establishing the utility of atmospheric retrieval approaches on brown dwarf spectra.

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arXiv2015

The Atmospheric Circulation of the Hot Jupiter WASP-43b: Comparing Three-Dimensional Models to Spectrophotometric Data

The hot Jupiter WASP-43b has now joined the ranks of transiting hot Jupiters HD 189733b and HD 209458b as an exoplanet with a large array of observational constraints on its atmospheric properties. Because WASP-43b receives a similar stellar flux as HD 209458b but has a rotation rate 4 times faster and a much higher gravity, studying WASP-43b serves as a test of the effect of rotation rate and gravity on the circulation when stellar irradiation is held approximately constant. Here we present 3D atmospheric circulation models of WASP-43b using the SPARC/MITgcm, a coupled radiation and circulation model, exploring the effects of composition, metallicity, and frictional drag. We find that the circulation regime of WASP-43b is not unlike other hot Jupiters, with equatorial superrotation that yields an eastward-shifted hotspot and large day-night temperature variations (~600 K at photospheric pressures). We then compare our model results to observations from Stevenson et al. which utilize HST/WFC3 to collect spectrophotometric phase curve measurements of WASP-43b from 1.12-1.65 microns. Our results show the 5x solar model lightcurve provides a good match to the data, with a phase offset of peak flux and planet/star flux ratio that is similar to observations; however, the model nightside appears to be brighter. Nevertheless, our 5x solar model provides an excellent match to the WFC3 dayside emission spectrum. This is a major success, as the result is a natural outcome of the 3D dynamics with no model tuning, and differs significantly from 1D models that can generally only match observations when appropriately tuned. In sum, these results demonstrate that 3D circulation models can provide important insights in interpreting exoplanet atmospheric observations, even at high spectral resolution, and highlight the potential for future observations with HST, JWST and other next-generation telescopes.

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arXiv2014

A Precise Water Abundance Measurement for the Hot Jupiter WASP-43b

The water abundance in a planetary atmosphere provides a key constraint on the planet's primordial origins because water ice is expected to play an important role in the core accretion model of planet formation. However, the water content of the Solar System giant planets is not well known because water is sequestered in clouds deep in their atmospheres. By contrast, short-period exoplanets have such high temperatures that their atmospheres have water in the gas phase, making it possible to measure the water abundance for these objects. We present a precise determination of the water abundance in the atmosphere of the 2 $M_\mathrm{Jup}$ short-period exoplanet WASP-43b based on thermal emission and transmission spectroscopy measurements obtained with the Hubble Space Telescope. We find the water content is consistent with the value expected in a solar composition gas at planetary temperatures (0.4-3.5x solar at 1 $σ$ confidence). The metallicity of WASP-43b's atmosphere suggested by this result extends the trend observed in the Solar System of lower metal enrichment for higher planet masses.

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arXiv2014

Thermal structure of an exoplanet atmosphere from phase-resolved emission spectroscopy

Exoplanets that orbit close to their host stars are much more highly irradiated than their Solar System counterparts. Understanding the thermal structures and appearances of these planets requires investigating how their atmospheres respond to such extreme stellar forcing. We present spectroscopic thermal emission measurements as a function of orbital phase ("phase-curve observations") for the highly-irradiated exoplanet WASP-43b spanning three full planet rotations using the Hubble Space Telescope. With these data, we construct a map of the planet's atmospheric thermal structure, from which we find large day-night temperature variations at all measured altitudes and a monotonically decreasing temperature with pressure at all longitudes. We also derive a Bond albedo of 0.18 +0.07,-0.12 and an altitude dependence in the hot-spot offset relative to the substellar point.

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arXiv2014

New Analysis Indicates No Thermal Inversion in the Atmosphere of HD 209458b

An important focus of exoplanet research is the determination of the atmospheric temperature structure of strongly irradiated gas giant planets, or hot Jupiters. HD 209458b is the prototypical exoplanet for atmospheric thermal inversions, but this assertion does not take into account recently obtained data or newer data reduction techniques. We re-examine this claim by investigating all publicly available Spitzer Space Telescope secondary-eclipse photometric data of HD 209458b and performing a self-consistent analysis. We employ data reduction techniques that minimize stellar centroid variations, apply sophisticated models to known Spitzer systematics, and account for time-correlated noise in the data. We derive new secondary-eclipse depths of 0.119 +/- 0.007%, 0.123 +/- 0.006%, 0.134 +/- 0.035%, and 0.215 +/- 0.008% in the 3.6, 4.5, 5.8, and 8.0 micron bandpasses, respectively. We feed these results into a Bayesian atmospheric retrieval analysis and determine that it is unnecessary to invoke a thermal inversion to explain our secondary-eclipse depths. The data are well-fitted by a temperature model that decreases monotonically between pressure levels of 1 and 0.01 bars. We conclude that there is no evidence for a thermal inversion in the atmosphere of HD 209458b.

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arXiv2014

An Approach for Retrieving Temperatures and Abundances in Brown Dwarf Atmospheres

Brown dwarf spectra contain a wealth of information about their molecular abundances, temperature structure, and gravity. We present a new data driven retrieval approach, previously used in planetary atmosphere studies, to extract the molecular abundances and temperature structure from brown dwarf spectra. The feasibility of the approach is first demonstrated on a synthetic brown dwarf spectrum. Given typical spectral resolutions, wavelength coverage, and noise properties precisions of tens of percent can be obtained for the molecular abundances and 10s-100s K on the temperature profile. The technique is then applied to the well studied brown dwarf, Gl 570D. From this spectral retrieval the spectroscopic radius is constrained to be 0.75 - 0.83 $R_{\mathrm{J}}$, $\log(g)$ to be 5.13 - 5.46 and $T_{\mathrm{eff}}$ to be between 804 and 849 K. Estimates for the range of abundances and allowed temperature profiles are also derived. The results from our retrieval approach are in agreement with the self-consistent grid modeling results of Saumon et al (2006). This new approach will allow us to address issues of compositional differences between brown dwarfs and possibly their formation environments, disequilibrium chemistry, missing physics in current grid modeling approaches as well as a many other issues.

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arXiv2014

Mid-Infrared Spectroscopy of Uranus from the Spitzer Infrared Spectrometer: 1. Determination of the Mean Temperature Structure of the Upper Troposphere and Stratosphere

On 2007 December 16-17, spectra were acquired of the disk of Uranus by the Spitzer Infrared Spectrometer (IRS) when its equator was close to the sub-earth point. This spectrum provides the highest-resolution broad-band spectrum ever obtained for Uranus from space, allowing a determination of the disk-averaged temperature and molecule composition to a greater degree of accuracy than ever before. The temperature profiles derived from the Voyager radio occultation experiments that match these data best are those that assume a high abundance of methane in the deep atmosphere, but none of these models provides a satisfactory fit over the full spectral range. This be the result of spatial differences between global and low-latitudinal regions, changes in time, missing continuum opacity sources such as stratospheric hazes or unknown tropospheric constituents, or undiagnosed systematic problems with either the radio-occultation or the Spitzer IRS data sets. The spectrum is compatible with the stratospheric temperatures derived from the Voyager ultraviolet occultations measurements. Thermospheric temperatures determined from the analysis of the observed H2 quadrupole emission features are colder than those derived by Herbert et al. at pressures less than ~1 microbar. Extrapolation of the nominal model spectrum to far-infrared through millimeter wavelengths shows that the spectrum arising solely from H2 collision-induced absorption is too warm to reproduce observations between wavelengths of 0.8 and 3.3 mm. Adding an additional absorber such as H2S provides a reasonable match to the spectrum, although a unique identification of the responsible absorber is not yet possible with available data. An immediate practical use for the spectrum resulting from this model is to establish a high-precision continuum flux model for use as an absolute radiometric standard for future astronomical observations.

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arXiv2014

Mid-Infrared Spectroscopy of Uranus from the Spitzer Infrared Spectrometer: 2. Determination of the Mean Composition of the Upper Troposphere and Stratosphere

Mid-infrared spectral observations Uranus acquired with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope are used to determine the abundances of C2H2, C2H6, CH3C2H, C4H2, CO2, and tentatively CH3 on Uranus at the time of the 2007 equinox. For vertically uniform eddy diffusion coefficients in the range 2200-2600 cm2 s-1, photochemical models that reproduce the observed methane emission also predict C2H6 profiles that compare well with emission in the 11.6-12.5 micron wavelength region, where the nu9 band of C2H6 is prominent. Our nominal model with a uniform eddy diffusion coefficient Kzz = 2430 cm2 sec-1 and a CH4 tropopause mole fraction of 1.6x10-5 provides a good fit to other hydrocarbon emission features, such as those of C2H2 and C4H2, but the model profile for CH3C2H must be scaled by a factor of 0.43, suggesting that improvements are needed in the chemical reaction mechanism for C3Hx species. The nominal model is consistent with a CH3D/CH4 ratio of 3.0+-0.2x10-4. From the best-fit scaling of these photochemical-model profiles, we derive column abundances above the 10-mbar level of 4.5+01.1/-0.8 x 10+19 molecule-cm-2 for CH4, 6.2 +- 1.0 x 10+16 molecule-cm-2 for C2H2 (with a value 24% higher from a different longitudinal sampling), 3.1 +- 0.3 x 10+16 molecule-cm-2 for C2H6, 8.6 +- 2.6 x 10+13 molecule-cm-2 for CH3C2H, 1.8 +- 0.3 x 10+13 molecule-cm-2 for C4H2, and 1.7 +- 0.4 x 10+13 molecule-cm-2 for CO2 on Uranus. Our results have implications with respect to the influx rate of exogenic oxygen species and the production rate of stratospheric hazes on Uranus, as well as the C4H2 vapor pressure over C4H2 ice at low temperatures.

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arXiv2013

A Near-Infrared Transmission Spectrum for the Warm Saturn HAT-P-12b

We present a HST WFC3 transmission spectrum for the transiting exoplanet HAT-P-12b. This warm (1000 K) sub-Saturn-mass planet has a smaller mass and a lower temperature than the hot-Jupiters that have been studied so far. We find that the planet's measured transmission spectrum lacks the expected water absorption feature for a hydrogen-dominated atmosphere, and is instead best-described by a model with high-altitude clouds. Using a frequentist hypothesis testing procedure, we can rule out a hydrogen-dominated cloud free atmosphere to 4.9$σ$. When combined with other recent WFC3 studies, our observations suggest that clouds may be common in exo-planetary atmospheres.

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arXiv2013

A Systematic Retrieval Analysis of Secondary Eclipse Spectra I: A Comparison of Atmospheric Retrieval Techniques

Spectra of exoplanet atmospheres provide us the opportunity to improve our understanding of these objects just as remote sensing in our own solar system has increased our understanding of the solar system bodies. The challenge is to quantitatively determine the range of temperatures and species abundances allowed by the data. This challenge is often difficult given the low information content of most exoplanet spectra which commonly leads to degeneracies in the interpretation. A variety of temperature and abundance retrieval approaches have been applied to exoplanet spectra, but no previous investigations have sought to compare these approaches. In this investigation we compare three different retrieval methods: Optimal Estimation, Differential Evolution Markov Chain Monte Carlo, and Bootstrap Monte Carlo. We call our suite of retrieval algorithms the Caltech Inverse Modeling and Retrieval Algorithms (CHIMERA). We discuss what we can expect in terms of uncertainties in abundances and temperatures given current observations as well as potential future observations and what conclusions can be drawn given those uncertainties. In general we find that the three approaches agree for high quality spectra expected to come from potential future spaceborne missions, but disagree for low quality spectra representative of current observations. We also show that the Gaussian posterior probability distribution assumption made in the Optimal Estimation approach is valid for high quality spectral data. We also discuss the implications of our models for the inferred C to O ratios of exoplanetary atmospheres, which of course are important for understanding formation environments. More specifically we show that in the observational limit of a few photometric points, the retrieved C/O is biased towards values near solar and near one simply due to the assumption of uninformative priors.

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