arXiv subjects

Travis S. Barman

Publications and source records attributed to Travis S. Barman.

At least 37 records · Page 2Linked to original sources

arXiv2020

The PHOENIX Exoplanet Retrieval Algorithm and Using H$^{-}$ Opacity as a Probe in Ultra-hot Jupiters

Atmospheric retrievals are now a standard tool to analyze observations of exoplanet atmospheres. This data-driven approach quantitatively compares atmospheric models to observations in order to estimate atmospheric properties and their uncertainties. In this paper, we introduce a new retrieval package, the PHOENIX Exoplanet Retrieval Analysis (PETRA). PETRA places the PHOENIX atmosphere model in a retrieval framework, allowing us to combine the strengths of a well-tested and widely-used atmosphere model with the advantages of retrieval algorithms. We validate PETRA by retrieving on simulated data for which the true atmospheric state is known. We also show that PETRA can successfully reproduce results from previously published retrievals of WASP-43b and HD 209458b. For the WASP-43b results, we show the effect that different line lists and line profile treatments have on the retrieved atmospheric properties. Lastly, we describe a novel technique for retrieving the temperature structure and $e^{-}$ density in ultra-hot Jupiters using H$^{-}$ opacity, allowing us to probe atmospheres devoid of most molecular features with JWST.

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arXiv2020

UV Exoplanet Transmission Spectral Features as Probes of Metals and Rainout

The low-resolution transmission spectra of ultra-hot Jupiters observed shortward of 0.5 microns indicate strong absorption at short-wavelengths. Previous explanations have included scattering, photochemistry, escaping metals, and disequilibrium chemistry. In this Letter, we show that slopes and features shortward of 0.5 microns can be caused by opacity not commonly considered in atmosphere models of exoplanets but guaranteed to be present if conditions are near chemical equilibrium including Fe I, Fe II, Ti I, Ni I, Ca I, Ca II, and SiO. Even relatively trace species (e.g., Cr I and Mn I) can contribute through strong lines in the UV and blue-optical. Using the PHOENIX atmosphere model, we describe how the short-wavelength transit spectrum varies with equilibrium temperature between 1000 K and 4000 K, as well as the effect that the rainout of condensates has at these wavelengths. We define two spectral indices to quantify the strength of the NUV and blue absorption compared to that in the red-optical, finding that the NUV transit depth will significantly exceed the transit depth from Rayleigh scattering alone for all hot Jupiters down to around 1000 K. In the blue-optical, hot Jupiters warmer than 2000 K will have transit depths larger than that from Rayleigh scattering, but below 2000 K, Rayleigh scattering can dominate, if present. We further show that these spectral indices may be used to trace the effects of rainout. We then compare our simulated transit spectra to existing observations of WASP-12b, WASP-33b, WASP-76b, and WASP-121b.

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arXiv2019

Current Population Statistics Do Not Favor Photoevaporation over Core-Powered Mass Loss as the Dominant Cause of the Exoplanet Radius Gap

We search for evidence of the cause of the exoplanet radius gap, i.e. the dearth of planets with radii near $1.8\ R_\oplus$. If the cause was photoevaporation, the radius gap should trend with proxies for the early-life high-energy emission of planet-hosting stars. If, alternatively, the cause was core-powered mass loss, no such trends should exist. Critically, spurious trends between the radius gap and stellar properties arise from an underlying correlation with instellation. After accounting for this underlying correlation, we find no trends remain between the radius gap and stellar mass or present-day stellar activity as measured by near-UV emission. We dismiss the nondetection of a radius gap trend with near-UV emission because present-day near-UV emission is unlikely to trace early-life high-energy emission, but we provide a catalog of GALEX near-UV and far-UV emission measurements for general use. We interpret the nondetection of a radius gap trend with stellar mass by simulating photoevaporation with mass-dependent evolution of stellar high-energy emission. The simulation produces an undetectable trend between the radius gap and stellar mass under realistic sources of error. We conclude that no evidence, from this analysis or others in the literature, currently exists that clearly favors either photoevaporation or core powered mass loss as the primary cause of the exoplanet radius gap. However, repeating this analysis once the body of well-characterized $< 4\ R_\oplus$ planets has roughly doubled could confirm or rule out photoevaporation.

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arXiv2019

Lyman-$α$ Observations of High Radial Velocity Low-Mass Stars Ross 1044 and Ross 825

The discovery of habitable zone (HZ) planets around low-mass stars has highlighted the need for a comprehensive understanding of the radiation environments in which such planets reside. Of particular importance is knowledge of the far-ultraviolet (FUV) radiation, as low-mass stars are typically much more active than solar-type stars and the proximity of their HZs can be one tenth the distance. The vast majority of the flux emitted by low-mass stars at FUV wavelengths occurs in the Lyman-$α$ line at 1216 Angstroms. However, measuring a low-mass star's Lyman-$α$ emission directly is almost always impossible because of the contaminating effects of interstellar hydrogen and geocoronal airglow. We observed Ross 825 (K3) and Ross 1044 (M0), two stars with exceptional radial velocities, with the STIS spectrograph aboard the Hubble Space Telescope (HST). Their radial velocities resulted in significant line shifts, allowing for a more complete view of their Lyman-$α$ line profiles. We provide an updated relation between effective temperature and Lyman-$α$ flux using Gaia DR2 astrometry as well as updated, model-independent relationships between Lyman-$α$ flux and UV flux measurements from the Galaxy Evolution Explorer (GALEX) for low-mass stars. These new relations, in combination with GALEX's considerable spatial coverage, provide substantial predictive power for the Lyman-$α$ environments for thousands of nearby, low-mass stars.

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arXiv2019

High Spatial Resolution Thermal-Infrared Spectroscopy with ALES: Resolved Spectra of the Benchmark Brown Dwarf Binary HD 130948BC

We present 2.9-4.1 micron integral field spectroscopy of the L4+L4 brown dwarf binary HD 130948BC, obtained with the Arizona Lenslets for Exoplanet Spectroscopy (ALES) mode of the Large Binocular Telescope Interferometer (LBTI). The HD 130948 system is a hierarchical triple system, in which the G2V primary is joined by two co-orbiting brown dwarfs. By combining the age of the system with the dynamical masses and luminosities of the substellar companions, we can test evolutionary models of cool brown dwarfs and extra-solar giant planets. Previous near-infrared studies suggest a disagreement between HD 130948BC luminosities and those derived from evolutionary models. We obtained spatially-resolved, low-resolution (R~20) L-band spectra of HD 130948B and C to extend the wavelength coverage into the thermal infrared. Jointly using JHK photometry and ALES L-band spectra for HD 130948BC, we derive atmospheric parameters that are consistent with parameters derived from evolutionary models. We leverage the consistency of these atmospheric quantities to favor a younger age (0.50 \pm 0.07 Gyr) of the system compared to the older age (0.79 \pm 0.22 Gyr) determined with gyrochronology in order to address the luminosity discrepancy.

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arXiv2019

The Influence of Host Star Spectral Type on Ultra-Hot Jupiter Atmospheres

Ultra-hot Jupiters are the most highly irradiated gas giant planets, with equilibrium temperatures from 2000 to over 4000 K. Ultra-hot Jupiters are amenable to characterization due to their high temperatures, inflated radii, and short periods, but their atmospheres are atypical for planets in that the photosphere possesses large concentrations of atoms and ions relative to molecules. Here we evaluate how the atmospheres of these planets respond to irradiation by stars of different spectral type. We find that ultra-hot Jupiters exhibit temperature inversions that are sensitive to the spectral type of the host star. The slope and temperature range across the inversion both increase as the host star effective temperature increases due to enhanced absorption at short wavelengths and low pressures. The steep temperature inversions in ultra-hot Jupiters around hot stars result in increased thermal dissociation and ionization compared to similar planets around cooler stars. The resulting increase in H$^{-}$ opacity leads to a transit spectrum that has muted absorption features. The emission spectrum, however, exhibits a large contrast in brightness temperature, a signature that will be detectable with both secondary eclipse observations and high-dispersion spectroscopy. We also find that the departures from local thermodynamic equilibrium in the stellar atmosphere can affect the degree of heating caused by atomic metals in the planet's upper atmosphere. Additionally, we further quantify the significance of heating by different opacity sources in ultra-hot Jupiter atmospheres.

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arXiv2018

Methane in Analogs of Young Directly Imaged Exoplanets

We present detections of methane in R of $\sim$1300, L band spectra of VHS 1256 b and PSO 318.5, two low gravity, red, late L dwarfs that share the same colors as the HR 8799 planets. These spectra reveal shallow methane features, which indicate VHS 1256 b and PSO 318.5 have photospheres that are out of chemical equilibrium. Directly imaged exoplanets usually have redder near infrared colors than the field-age population of brown dwarfs on a color magnitude diagram. These objects along the L to T transition show reduced methane absorption and evidence of photospheric clouds. Compared to the H and K bands, L band (3 micron - 4 micron) spectroscopy provides stronger constraints on the methane abundances of brown dwarfs and directly imaged exoplanets that have similar effective temperatures as L to T transition objects. When combined with near infrared spectra, the L band extends our conventional wavelength coverage, increasing our understanding of atmospheric cloud structure. Our model comparisons show relatively strong vertical mixing and photospheric clouds can explain the molecular absorption features and continua of VHS 1256 b and PSO 318.5. We also discuss the implications of this work for future exoplanet focused instruments and observations with the James Webb Space Telescope.

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arXiv2018

Ground- and Space-based Detection of the Thermal Emission Spectrum of the Transiting Hot Jupiter KELT-2Ab

We describe the detection of water vapor in the atmosphere of the transiting hot Jupiter KELT-2Ab by treating the star-planet system as a spectroscopic binary with high-resolution, ground-based spectroscopy. We resolve the signal of the planet's motion with deep combined flux observations of the star and the planet. In total, six epochs of Keck NIRSPEC $L$-band observations were obtained, and the full data set was subjected to a cross correlation analysis with a grid of self-consistent atmospheric models. We measure a radial projection of the Keplerian velocity, $K_P$, of 148 $\pm$ 7 km s$^{-1}$, consistent with transit measurements, and detect water vapor at 3.8$σ$. We combine NIRSPEC $L$-band data with $Spitzer$ IRAC secondary eclipse data to further probe the metallicity and carbon-to-oxygen ratio of KELT-2Ab's atmosphere. While the NIRSPEC analysis provides few extra constraints on the $Spitzer$ data, it does provide roughly the same constraints on metallicity and carbon-to-oxygen ratio. This bodes well for future investigations of the atmospheres of non-transiting hot Jupiters.

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arXiv2017

The Very Low Albedo of WASP-12b From Spectral Eclipse Observations with $\textit{Hubble}$

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 $A_g < 0.064$ (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 $\sim$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 $10^{-4}$ 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.

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arXiv2017

Detection of Water Vapor in the Thermal Spectrum of the Non-Transiting Hot Jupiter upsilon Andromedae b

The upsilon Andromedae system was the first multi-planet system discovered orbiting a main sequence star. We describe the detection of water vapor in the atmosphere of the innermost non-transiting gas giant ups~And~b by treating the star-planet system as a spectroscopic binary with high-resolution, ground-based spectroscopy. We resolve the signal of the planet's motion and break the mass-inclination degeneracy for this non-transiting planet via deep combined flux observations of the star and the planet. In total, seven epochs of Keck NIRSPEC $L$ band observations, three epochs of Keck NIRSPEC short wavelength $K$ band observations, and three epochs of Keck NIRSPEC long wavelength $K$ band observations of the ups~And~system were obtained. We perform a multi-epoch cross correlation of the full data set with an atmospheric model. We measure the radial projection of the Keplerian velocity ($K_P$ = 55 $\pm$ 9 km/s), true mass ($M_b$ = 1.7 $^{+0.33}_{-0.24}$ $M_J$), and orbital inclination \big($i_b$ = 24 $\pm$ 4$^{\circ}$\big), and determine that the planet's opacity structure is dominated by water vapor at the probed wavelengths. Dynamical simulations of the planets in the ups~And~system with these orbital elements for ups~And~b show that stable, long-term (100 Myr) orbital configurations exist. These measurements will inform future studies of the stability and evolution of the ups~And~system, as well as the atmospheric structure and composition of the hot Jupiter.

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arXiv2017

Three's Company: An additional non-transiting super-Earth in the bright HD 3167 system, and masses for all three planets

HD 3167 is a bright (V = 8.9), nearby K0 star observed by the NASA K2 mission (EPIC 220383386), hosting two small, short-period transiting planets. Here we present the results of a multi-site, multi-instrument radial velocity campaign to characterize the HD 3167 system. The masses of the transiting planets are 5.02+/-0.38 MEarth for HD 3167 b, a hot super-Earth with a likely rocky composition (rho_b = 5.60+2.15-1.43 g/cm^3), and 9.80+1.30-1.24 MEarth for HD 3167 c, a warm sub-Neptune with a likely substantial volatile complement (rho_c = 1.97+0.94-0.59 g/cm^3). We explore the possibility of atmospheric composition analysis and determine that planet c is amenable to transmission spectroscopy measurements, and planet b is a potential thermal emission target. We detect a third, non-transiting planet, HD 3167 d, with a period of 8.509+/-0.045 d (between planets b and c) and a minimum mass of 6.90+/-0.71 MEarth. We are able to constrain the mutual inclination of planet d with planets b and c: we rule out mutual inclinations below 1.3 degrees as we do not observe transits of planet d. From 1.3-40 degrees, there are viewing geometries invoking special nodal configurations which result in planet d not transiting some fraction of the time. From 40-60 degrees, Kozai-Lidov oscillations increase the system's instability, but it can remain stable for up to 100Myr. Above 60 degrees, the system is unstable. HD 3167 promises to be a fruitful system for further study and a preview of the many exciting systems expected from the upcoming NASA TESS mission.

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arXiv2017

An Optical/near-infrared investigation of HD 100546 b with the Gemini Planet Imager and MagAO

We present H band spectroscopic and Halpha photometric observations of HD 100546 obtained with GPI and MagAO. We detect H band emission at the location of the protoplanet HD 100546b, but show that choice of data processing parameters strongly affects the morphology of this source. It appears point-like in some aggressive reductions, but rejoins an extended disk structure in the majority of the others. Furthermore, we demonstrate that this emission appears stationary on a timescale of 4.6 yrs, inconsistent at the 2sigma level with a Keplerian clockwise orbit at 59 au in the disk plane. The H band spectrum of the emission is inconsistent with any type of low effective temperature object or accreting protoplanetary disk. It strongly suggests a scattered light origin, as it is consistent with the spectrum of the star and the spectra extracted at other locations in the disk. A non detection at the 5sigma level of HD 100546b in differential Halpha imaging places an upper limit, assuming the protoplanet lies in a gap free of extinction, on the accretion luminosity and accretion rate of 1.7E-4 Lsun and MMdot<6.4E-7Mjup^2/yr for 1Rjup. These limits are comparable to the accretion luminosity and rate of TTauri-stars or LkCa 15b. Taken together, these lines of evidence suggest that the H band source at the location of HD 100546b is not emitted by a planetary photosphere or an accreting circumplanetary disk but is a disk feature enhanced by the PSF subtraction process. This non-detection is consistent with the non-detection in the K band reported in an earlier study but does not exclude the possibility that HD 100546b is deeply embedded.

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arXiv2017

Integral Field Spectroscopy of the Low-Mass Companion HD984B with the Gemini Planet Imager

We present new observations of the low-mass companion to HD 984 taken with the Gemini Planet Imager as a part of the Gemini Planet Imager Exoplanet Survey campaign. Images of HD 984 B were obtained in the J (1.12--1.3 micron) and H (1.50--1.80 micron) bands. Combined with archival epochs from 2012 and 2014, we fit the first orbit to the companion to find an 18 AU (70 year) orbit with a 68% confidence interval between 14 and 28 AU, an eccentricity of 0.18 with a 68% confidence interval between 0.05 and 0.47, and an inclination of 119 degrees with a 68% confidence interval between 114 degrees and 125 degrees. To address considerable spectral covariance in both spectra, we present a method of splitting the spectra into low and high frequencies to analyze the spectral structure at different spatial frequencies with the proper spectral noise correlation. Using the split spectra, we compare to known spectral types using field brown dwarf and low-mass star spectra and find a best fit match of a field gravity M6.5+/-1.5 spectral type with a corresponding temperature of 2730+120 K. Photometry of the companion yields a luminosity of log(L_bol/L_sun) = -2.88+/-0.07 dex, using DUSTY models. Mass estimates, again from DUSTY models, find an age-dependent mass of 34+/-1 to 95+/-4 M_Jup. These results are consistent with previous measurements of the object.

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arXiv2016

Dynamical Mass Measurement of the Young Spectroscopic Binary V343 Normae AaAb Resolved With the Gemini Planet Imager

We present new spatially resolved astrometry and photometry from the Gemini Planet Imager of the inner binary of the young multiple star system V343 Normae, which is a member of the beta Pictoris moving group. V343 Normae comprises a K0 and mid-M star in a ~4.5 year orbit (AaAb) and a wide 10" M5 companion (B). By combining these data with archival astrometry and radial velocities we fit the orbit and measure individual masses for both components of M_Aa = 1.10 +/- 0.10 M_sun and M_Ab = 0.290 +/- 0.018 M_sun. Comparing to theoretical isochrones, we find good agreement for the measured masses and JHK band magnitudes of the two components consistent with the age of the beta Pic moving group. We derive a model-dependent age for the beta Pic moving group of 26 +/- 3 Myr by combining our results for V343 Normae with literature measurements for GJ 3305, which is another group member with resolved binary components and dynamical masses.

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arXiv2016

Evidence for the Direct Detection of the Thermal Spectrum of the Non-Transiting Hot Gas Giant HD 88133 b

We target the thermal emission spectrum of the non-transiting gas giant HD 88133 b with high-resolution near-infrared spectroscopy, by treating the planet and its host star as a spectroscopic binary. For sufficiently deep summed flux observations of the star and planet across multiple epochs, it is possible to resolve the signal of the hot gas giant's atmosphere compared to the brighter stellar spectrum, at a level consistent with the aggregate shot noise of the full data set. To do this, we first perform a principal component analysis to remove the contribution of the Earth's atmosphere to the observed spectra. Then, we use a cross-correlation analysis to tease out the spectra of the host star and HD 88133 b to determine its orbit and identify key sources of atmospheric opacity. In total, six epochs of Keck NIRSPEC L band observations and three epochs of Keck NIRSPEC K band observations of the HD 88133 system were obtained. Based on an analysis of the maximum likelihood curves calculated from the multi-epoch cross correlation of the full data set with two atmospheric models, we report the direct detection of the emission spectrum of the non-transiting exoplanet HD 88133 b and measure a radial projection of the Keplerian orbital velocity of 40 $\pm$ 15 km/s, a true mass of 1.02$^{+0.61}_{-0.28}M_J$, a nearly face-on orbital inclination of 15${^{+6}_{-5}}^{\circ}$, and an atmosphere opacity structure at high dispersion dominated by water vapor. This, combined with eleven years of radial velocity measurements of the system, provides the most up-to-date ephemeris for HD 88133.

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arXiv2016

Discovery of a Substellar Companion to the Nearby Debris Disk Host HR 2562

We present the discovery of a brown dwarf companion to the debris disk host star HR 2562. This object, discovered with the Gemini Planet Imager (GPI), has a projected separation of 20.3$\pm$0.3 au (0.618$\pm$0.004") from the star. With the high astrometric precision afforded by GPI, we have confirmed common proper motion of HR 2562B with the star with only a month time baseline between observations to more than $5σ$. Spectral data in $J$, $H$, and $K$ bands show morphological similarity to L/T transition objects. We assign a spectral type of L7$\pm$3 to HR 2562B, and derive a luminosity of $\log$(L$_{\rm bol}$/L$_{\odot}$)=-4.62$\pm$0.12, corresponding to a mass of 30$\pm$15 M$_{\rm Jup}$ from evolutionary models at an estimated age of the system of 300-900 Myr. Although the uncertainty in the age of the host star is significant, the spectra and photometry exhibit several indications of youth for HR 2562B. The source has a position angle consistent with an orbit in the same plane as the debris disk recently resolved with Herschel. Additionally, it appears to be interior to the debris disk. Though the extent of the inner hole is currently too uncertain to place limits on the mass of HR 2562B, future observations of the disk with higher spatial resolution may be able to provide mass constraints. This is the first brown dwarf-mass object found to reside in the inner hole of a debris disk, offering the opportunity to search for evidence of formation above the deuterium burning limit in a circumstellar disk.

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arXiv2016

The Orbit and Transit Prospects for $β$ Pictoris b constrained with One Milliarcsecond Astrometry

A principal scientific goal of the Gemini Planet Imager (GPI) is obtaining milliarcsecond astrometry to constrain exoplanet orbits. However, astrometry of directly imaged exoplanets is subject to biases, systematic errors, and speckle noise. Here we describe an analytical procedure to forward model the signal of an exoplanet that accounts for both the observing strategy (angular and spectral differential imaging) and the data reduction method (Karhunen-Loève Image Projection algorithm). We use this forward model to measure the position of an exoplanet in a Bayesian framework employing Gaussian processes and Markov chain Monte Carlo (MCMC) to account for correlated noise. In the case of GPI data on $β$ Pic b, this technique, which we call Bayesian KLIP-FM Astrometry (BKA), outperforms previous techniques and yields 1$σ$-errors at or below the one milliarcsecond level. We validate BKA by fitting a Keplerian orbit to twelve GPI observations along with previous astrometry from other instruments. The statistical properties of the residuals confirm that BKA is accurate and correctly estimates astrometric errors. Our constraints on the orbit of $β$ Pic b firmly rule out the possibility of a transit of the planet at 10-$σ$ significance. However, we confirm that the Hill sphere of $β$ Pic b will transit, giving us a rare chance to probe the circumplanetary environment of a young, evolving exoplanet. We provide an ephemeris for photometric monitoring of the Hill sphere transit event, which will begin at the start of April in 2017 and finish at the end of January in 2018.

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arXiv2015

Characterizing the Atmospheres of the HR8799 Planets with HST/WFC3

We present results from a Hubble Space Telescope (HST) program characterizing the atmospheres of the outer two planets, in the HR8799 system. The images were taken over 15 orbits in three near-infrared medium-band filters - F098M, F127M and F139M - using the Wide Field Camera 3. One of the three filters is sensitive to water absorption band inaccessible from ground-based observations, providing a unique probe of the thermal emission from the atmospheres of these young giant planets. The observations were taken at 30 different spacecraft rolls to enable angular differential imaging, and the full data set was analyzed with the Karhunen-Loeve Image Projection (KLIP) routine, an advanced image processing algorithm adapted to work with HST data. To achieve the required high contrast at sub arcsecond resolution, we utilized the pointing accuracy of HST in combination with an improved pipeline designed to combine the dithered, angular differential imaging data with an algorithm designed to both improve the image resolution and accurately measure the photometry. The results include F127M ($J$) detections of the outer planets, HR8799 b and c and the first detection of HR8799 b in the water-band (F139M) filter. The F127M photometry for HR8799 c agrees well with fitted atmospheric models resolving a long standing difficulty to model the near-IR flux for the planet consistently

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