arXiv subjects

Daniel P. Thorngren

Publications and source records attributed to Daniel P. Thorngren.

At least 37 records · Page 2Linked to original sources

arXiv2024

Giant Outer Transiting Exoplanet Mass (GOT 'EM) Survey. IV. Long-term Doppler Spectroscopy for 11 Stars Thought to Host Cool Giant Exoplanets

Discovering and characterizing exoplanets at the outer edge of the transit method's sensitivity has proven challenging owing to geometric biases and the practical difficulties associated with acquiring long observational baselines. Nonetheless, a sample of giant exoplanets on orbits longer than 100 days has been identified by transit hunting missions. We present long-term Doppler spectroscopy for 11 such systems with observation baselines spanning a few years to a decade. We model these radial velocity observations jointly with transit photometry to provide initial characterizations of these objects and the systems in which they exist. Specifically, we make new precise mass measurements for four long-period giant exoplanets (Kepler-111 c, Kepler-553 c, Kepler-849 b, and PH-2 b), we place new upper limits on mass for four others (Kepler-421 b, KOI-1431.01, Kepler-1513 b, and Kepler-952 b), and we show that several "confirmed" planets are in fact not planetary at all. We present these findings to complement similar efforts focused on closer-in short-period giant planets, and with the hope of inspiring future dedicated studies of cool giant exoplanets.

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arXiv2023

TESS Spots a Super-Puff: The Remarkably Low Density of TOI-1420b

We present the discovery of TOI-1420b, an exceptionally low-density ($ρ= 0.08\pm0.02$ g cm$^{-3}$) transiting planet in a $P = 6.96$ day orbit around a late G dwarf star. Using transit observations from TESS, LCOGT, OPM, Whitin, Wendelstein, OAUV, Ca l'Ou, and KeplerCam along with radial velocity observations from HARPS-N and NEID, we find that the planet has a radius of $R_p$ = 11.9 $\pm$ 0.3 $R_\Earth$ and a mass of $M_p$ = 25.1 $\pm$ 3.8 $M_\Earth$. TOI-1420b is the largest-known planet with a mass less than $50M_\Earth$, indicating that it contains a sizeable envelope of hydrogen and helium. We determine TOI-1420b's envelope mass fraction to be $f_{env} = 82^{+7}_{-6}\%$, suggesting that runaway gas accretion occurred when its core was at most $4-5\times$ the mass of the Earth. TOI-1420b is similar to the planet WASP-107b in mass, radius, density, and orbital period, so a comparison of these two systems may help reveal the origins of close-in low-density planets. With an atmospheric scale height of 1950 km, a transmission spectroscopy metric of 580, and a predicted Rossiter-McLaughlin amplitude of about $17$ m s$^{-1}$, TOI-1420b is an excellent target for future atmospheric and dynamical characterization.

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arXiv2023

uGMRT observations of the hot-Saturn WASP 69b: Radio-Loud Exoplanet-Exomoon Survey II (RLEES II)

Exomoons have so far eluded ongoing searches. Several studies have exploited transit and transit timing variations and high-resolution spectroscopy to identify potential exomoon candidates. One method of detecting and confirming these exomoons is to search for signals of planet-moon interactions. In this work, we present the first radio observations of the exomoon candidate system WASP 69b. Based on the detection of alkali metals in the transmission spectra of WASP-69b, it was deduced that the system might be hosting an exomoon. WASP 69b is also one of the exoplanet systems that will be observed as part of JWST cycle-1 GTO. This makes the system an excellent target to observe and follow up. We observed the system for 32 hrs at 150 MHz and 218 MHz using the upgraded Giant Metrewave Radio Telescope (uGMRT). Though we do not detect radio emission from the systems, we place strong $3σ$ upper limits of 3.3 mJy at 150 MHz and 0.9 mJy at 218 MHz. We then use these upper limits to estimate the maximum mass loss from the exomoon candidate.

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arXiv2023

Removal of Hot Saturns in Mass-Radius Plane by Runaway Mass Loss

The hot Saturn population exhibits a boundary in mass-radius space, such that no planets are observed at a density less than $\sim$0.1 g cm$^{-3}$. Yet, planet interior structure models can readily construct such objects as the natural result of radius inflation. Here, we investigate the role XUV-driven mass-loss plays in sculpting the density boundary by constructing interior structure models that include radius inflation, photoevaporative mass loss and a simple prescription of Roche lobe overflow. We demonstrate that planets puffier than $\sim$0.1 g cm$^{-3}$ experience a runaway mass loss caused by adiabatic radius expansion as the gas layer is stripped away, providing a good explanation of the observed edge in mass-radius space. The process is also visible in the radius-period and mass-period spaces, though smaller, high-bulk-metallicity planets can still survive at short periods, preserving a partial record of the population distribution at formation.

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arXiv2022

The effect of interior heat flux on the atmospheric circulation of hot and ultra-hot Jupiters

Many hot and ultra-hot Jupiters have inflated radii, implying that their interiors retain significant entropy from formation. These hot interiors lead to an enhanced internal heat flux that impinges upon the atmosphere from below. In this work, we study the effect of this hot interior on the atmospheric circulation and thermal structure of hot and ultra-hot Jupiters. To do so, we incorporate the population-level predictions from evolutionary models of hot and ultra-hot Jupiters as input for a suite of General Circulation Models (GCMs) of their atmospheric circulation with varying semi-major axis and surface gravity. We conduct simulations with and without a hot interior, and find that there are significant local differences in temperature of up to hundreds of Kelvin and in wind speeds of hundreds of m s$^{-1}$ or more across the observable atmosphere. These differences persist throughout the parameter regime studied, and are dependent on surface gravity through the impact on photosphere pressure. These results imply that the internal evolution and atmospheric thermal structure and dynamics of hot and ultra-hot Jupiters are coupled. As a result, a joint approach including both evolutionary models and GCMs may be required to make robust predictions for the atmospheric circulation of hot and ultra-hot Jupiters.

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arXiv2022

Giant Outer Transiting Exoplanet Mass (GOT 'EM) Survey. II. Discovery of a Failed Hot Jupiter on a 2.7 Year, Highly Eccentric Orbit

Radial velocity (RV) surveys have discovered giant exoplanets on au-scale orbits with a broad distribution of eccentricities. Those with the most eccentric orbits are valuable laboratories for testing theories of high eccentricity migration. However, few such exoplanets transit their host stars thus removing the ability to apply constraints on formation from their bulk internal compositions. We report the discovery of Kepler-1704 b, a transiting 4.15 $M_{\rm J}$ giant planet on a 988.88 day orbit with the extreme eccentricity of $0.921^{+0.010}_{-0.015}$. Our decade-long RV baseline from the Keck I telescope allows us to measure the orbit and bulk heavy element composition of Kepler-1704 b and place limits on the existence of undiscovered companions. Kepler-1704 b is a failed hot Jupiter that was likely excited to high eccentricity by scattering events that possibly began during its gas accretion phase. Its final periastron distance was too large to allow for tidal circularization, so now it orbits it host from distances spanning 0.16 - 3.9 au. The maximum difference in planetary equilibrium temperature resulting from this elongated orbit is over 700 K. A simulation of the thermal phase curve of Kepler-1704 b during periastron passage demonstrates that it is a remarkable target for atmospheric characterization from the James Webb Space Telescope, which could potentially also measure the planet's rotational period as the hot spot from periastron rotates in and out of view. Continued characterization of the Kepler-1704 system promises to refine theories explaining the formation of hot Jupiters and cool giant planets like those in the solar system.

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arXiv2022

Radio-Loud Exoplanet-Exomoon Survey (RLEES): GMRT Search for Electron Cyclotron Maser Emission

We conducted the first dedicated search for signatures of exoplanet-exomoon interactions using the Giant Metrewave Radio Telescope (GMRT) as part of the radio-loud exoplanet-exomoon survey (RLEES). Due to stellar tidal heating, irradiation, and subsequent atmospheric escape, candidate `exo-Io' systems are expected to emit up to $10^6$ times more plasma flux than the Jupiter-Io DC circuit. This can induce detectable radio emission from the exoplanet-exomoon system. We analyze three `exo-Io' candidate stars: WASP-49, HAT-P 12, and HD 189733. We perform 12-hour phase-curve observations of WASP-49b at 400 MHz during primary $\&$ secondary transit, as well as first $\&$ third quadratures achieving a 3$σ$ upper-limit of 0.18 mJy/beam averaged over four days. HAT-P~12 was observed with GMRT at 150 and 325 MHz. We further analyzed the archival data of HD 189733 at 325 MHz. No emission was detected from the three systems. However, we place strong upper limits on radio flux density. Given that most exo-Io candidates orbit hot Saturns, we encourage more multiwavelength searches (in particular low frequencies) to span the lower range of exoplanet B-field strengths constrained here.

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arXiv2022

Creating the Radius Gap without Mass Loss

The observed exoplanet population features a gap in the radius distribution that separates the smaller super-Earths ($\lesssim$1.7 Earth radii) from the larger sub-Neptunes ($\sim$1.7--4 Earth radii). While mass loss theories can explain many of the observed features of this radius valley, it is difficult to reconcile them with potentially rising population of terrestrials beyond orbital periods of $\sim$30 days. We investigate the ability of gas accretion during the gas-poor phase of disk evolution to reproduce both the location of the observed radius gap and the existence of long-period terrestrial planets. Updating the analytic scaling relations of gas accretion rate accounting for the shrinking of the bound radius by hydrodynamic effects and deriving a more realistic disk temperature profile, we find that the late-stage gas accretion alone is able to carve out the observed radius gap, with slopes $R_{\rm gap} \propto P^{-0.096}$ and $R_{\rm gap} \propto M_\star^{0.15}$ for top-heavy; and $R_{\rm gap} \propto P^{-0.089}$ and $R_{\rm gap} \propto M_\star^{0.22}$ for bottom-heavy core mass distributions, in good agreement with observations. The general morphology of the primordial radius gap is stable against a range of disk gas density and disk accretion rate with the latter affecting mostly the population of large planets ($\gtrsim$3--4$R_\oplus$). The peaks and valleys in the radius distribution were likely set in place primordially while post-formation mass loss further tunes the exoplanetary population. We provide potential observational tests that may be possible with TESS, PLATO and Roman Space Telescope.

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arXiv2022

The TESS-Keck Survey. VIII. Confirmation of a Transiting Giant Planet on an Eccentric 261 day Orbit with the Automated Planet Finder Telescope

We report the discovery of TOI-2180 b, a 2.8 $M_{\rm J}$ giant planet orbiting a slightly evolved G5 host star. This planet transited only once in Cycle 2 of the primary Transiting Exoplanet Survey Satellite (TESS) mission. Citizen scientists identified the 24 hr single-transit event shortly after the data were released, allowing a Doppler monitoring campaign with the Automated Planet Finder telescope at Lick Observatory to begin promptly. The radial velocity observations refined the orbital period of TOI-2180 b to be 260.8$\pm$0.6 days, revealed an orbital eccentricity of 0.368$\pm$0.007, and discovered long-term acceleration from a more distant massive companion. We conducted ground-based photometry from 14 sites spread around the globe in an attempt to detect another transit. Although we did not make a clear transit detection, the nondetections improved the precision of the orbital period. We predict that TESS will likely detect another transit of TOI-2180 b in Sector 48 of its extended mission. We use giant planet structure models to retrieve the bulk heavy-element content of TOI-2180 b. When considered alongside other giant planets with orbital periods over 100 days, we find tentative evidence that the correlation between planet mass and metal enrichment relative to stellar is dependent on orbital properties. Single-transit discoveries like TOI-2180 b highlight the exciting potential of the TESS mission to find planets with long orbital periods and low irradiation fluxes despite the selection biases associated with the transit method.

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arXiv2021

Re-inflation of warm and hot Jupiters

Understanding the anomalous radii of many transiting hot gas giant planets is a fundamental problem of planetary science. Recent detections of re-inflated warm Jupiters orbiting post-main-sequence stars and the re-inflation of hot Jupiters while their host stars evolve on the main-sequence may help constrain models for the anomalous radii of hot Jupiters. In this work, we present evolution models studying the re-inflation of gas giants to determine how varying the depth and intensity of deposited heating affects both main-sequence re-inflation of hot Jupiters and post-main-sequence re-inflation of warm Jupiters. We find that deeper heating is required to re-inflate hot Jupiters than is needed to suppress their cooling, and that the timescale of re-inflation decreases with increasing heating rate and depth. We find a strong degeneracy between heating rate and depth, with either strong shallow heating or weak deep heating providing an explanation for main-sequence re-inflation of hot Jupiters. This degeneracy between heating rate and depth can be broken in the case of post-main-sequence re-inflation of warm Jupiters, as the inflation must be rapid to occur within post-main-sequence evolution timescales. We also show that the dependence of heating rate on incident stellar flux inferred from the sample of hot Jupiters can explain re-inflation of both warm and hot Jupiters. TESS will obtain a large sample of warm Jupiters orbiting post-main-sequence stars, which will help to constrain the mechanism(s) causing the anomalous radii of gas giant planets.

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arXiv2021

Slow Cooling and Fast Reinflation for Hot Jupiters

The unexpectedly large radii of hot Jupiters are a longstanding mystery whose solution will provide important insights into their interior physics. Many potential solutions have been suggested, which make diverse predictions about the details of inflation. In particular, although any valid model must allow for maintaining large planetary radii, only some allow for radii to increase with time. This reinflation process would potentially occur when the incident flux on the planet is increased. In this work, we examine the observed population of hot Jupiters to see if they grow as their parent stars brighten along the main sequence. We consider the relation between radius and other observables, including mass, incident flux, age, and fractional age (age over main sequence lifetime), and show that main sequence brightening is often sufficient to produce detectable reinflation. We further argue that these provide strong evidence for the relatively rapid reinflation of giant planets, and discuss the implications for proposed heating mechanisms. In our population analysis we also find evidence for a "delayed-cooling effect", wherein planets cool and contract far more slowly than expected. While not capable of explaining the observed radii alone, it may represent an important component of the effect. Finally, we identify a weak negative relationship between stellar metallicity and planet radius which is presumably the result of enhanced planetary bulk metallicity around metal-rich stars and has important implications for planet formation theory.

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arXiv2020

Beyond Equilibrium Temperature: How the Atmosphere/Interior Connection Affects the Onset of Methane, Ammonia, and Clouds in Warm Transiting Giant Planets

The atmospheric pressure-temperature profiles for transiting giant planets cross a range of chemical transitions. Here we show that the particular shape of these irradiated profiles for warm giant planets below 1300 K lead to striking differences in the behavior of non-equilibrium chemistry compared to brown dwarfs of similar temperatures. Our particular focus is H$_2$O, CO, CH$_4$, CO$_2$, and NH$_3$ in Jupiter- and Neptune-class planets. We show the cooling history of a planet, which depends most significantly on planetary mass and age, can have a dominant effect on abundances in the visible atmosphere, often swamping trends one might expect based on Teq alone. The onset of detectable CH$_4$ in spectra can be delayed to lower Teq for some planets compared to equilibrium, or pushed to higher Teq. The detectability of NH$_3$ is typically enhanced compared to equilibrium expectations, which is opposite to the brown dwarf case. We find that both CH$_4$ and NH$_3$ can become detectable at around the same Teq (at Teq values that vary with mass and metallicity) whereas these "onset" temperatures are widely spaced for brown dwarfs. We suggest observational strategies to search for atmospheric trends and stress that non-equilibrium chemistry and clouds can serve as probes of atmospheric physics. As examples of atmospheric complexity, we assess three Neptune-class planets GJ 436b, GJ 3470b, and WASP-107, all around Teq=700 K. Tidal heating due to eccentricity damping in all three planets heats the deep atmosphere by thousands of degrees, and may explain the absence of CH$_4$ in these cool atmospheres. Atmospheric abundances must be interpreted in the context of physical characteristics of the planet.

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arXiv2020

Tentative Evidence for Water Vapor in the Atmosphere of the Neptune-Size Exoplanet HD 106315 c

We present a transmission spectrum for the Neptune-size exoplanet HD 106315 c from optical to infrared wavelengths based on transit observations from the Hubble Space Telescope/Wide Field Camera 3, K2, and Spitzer. The spectrum shows tentative evidence for a water absorption feature in the $1.1 - 1.7μ$m wavelength range with a small amplitude of 30 ppm (corresponding to just $0.8 \pm 0.04$ atmospheric scale heights). Based on an atmospheric retrieval analysis, the presence of water vapor is tentatively favored with a Bayes factor of 1.7 - 2.6 (depending on prior assumptions). The spectrum is most consistent with either enhanced metallicity, high altitude condensates, or both. Cloud-free solar composition atmospheres are ruled out at $>5σ$ confidence. We compare the spectrum to grids of cloudy and hazy forward models and find that the spectrum is fit well by models with moderate cloud lofting or haze formation efficiency, over a wide range of metallicities ($1 - 100\times$ solar). We combine the constraints on the envelope composition with an interior structure model and estimate that the core mass fraction is $\gtrsim0.3$. With a bulk composition reminiscent of that of Neptune and an orbital distance of 0.15 AU, HD 106315 c hints that planets may form out of broadly similar material and arrive at vastly different orbits later in their evolution.

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arXiv2020

Aerosol Composition of Hot Giant Exoplanets Dominated by Silicates and Hydrocarbon Hazes

Aerosols are common in the atmospheres of exoplanets across a wide swath of temperatures, masses, and ages. These aerosols strongly impact observations of transmitted, reflected, and emitted light from exoplanets, obfuscating our understanding of exoplanet thermal structure and composition. Knowing the dominant aerosol composition would facilitate interpretations of exoplanet observations and theoretical understanding of their atmospheres. A variety of compositions have been proposed, including metal oxides and sulphides, iron, chromium, sulphur, and hydrocarbons. However, the relative contributions of these species to exoplanet aerosol opacity is unknown. Here we show that the aerosol composition of giant exoplanets observed in transmission is dominated by silicates and hydrocarbons. By constraining an aerosol microphysics model with trends in giant exoplanet transmission spectra, we find that silicates dominate aerosol opacity above planetary equilibrium temperatures of 950 K due to low nucleation energy barriers and high elemental abundances, while hydrocarbon aerosols dominate below 950 K due to an increase in methane abundance. Our results are robust to variations in planet gravity and atmospheric metallicity within the range of most giant transiting exoplanets. We predict that spectral signatures of condensed silicates in the mid-infrared are most prominent for hot (>1600 K), low-gravity (<10 m s$^{-2}$) objects.

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arXiv2020

The Intrinsic Temperature and Radiative-Convective Boundary Depth in the Atmospheres of Hot Jupiters

In giant planet atmosphere modelling, the intrinsic temperature $T_\mathrm{int}$ and radiative-convective boundary (RCB) are important lower boundary conditions. Often in one-dimensional radiative-convective models and in three-dimensional general circulation models it is assumed that $T_\mathrm{int}$ is similar to that of Jupiter itself, around 100 K, which yields a RCB around 1 kbar for hot Jupiters. In this work, we show that the inflated radii, and hence high specific entropy interiors, of hot Jupiters suggest much higher $T_\mathrm{int}$ values. Assuming the effect is primarily due to current heating (rather than delayed cooling), we derive an equilibrium relation between $T_\mathrm{eq}$ and $T_\mathrm{int}$, showing that the latter can take values as high as 700 K. In response, the RCB moves upward in the atmosphere. Using one-dimensional radiative-convective atmosphere models, we find RCBs of only a few bars, rather than the kilobar typically supposed. This much shallower RCB has important implications for the atmospheric structure, vertical and horizontal circulation, interpretations of phase curves, and the effect of deep cold traps on cloud formation.

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arXiv2019

An Empirical Mass-Radius Relation for Cool Giant Planets

Probabilistic relationships between the mass and radius of planets are useful for a variety of purposes, including estimating the yields of planet discovery efforts and the radii of discovered planets given their masses. Previous work on giant planets often do not make the important distinction between cool and (inflated) hot giants. This can lead to needlessly large uncertainties and overestimated radii for the cooler objects. Using hierarchical bayesian modeling and the AIC model selection criterion, we identify an empirical mass-radius relationship for cool giant planets above 15 Earth masses. It takes the form of a quadratic function in log mass, with a power-law dispersion. Parametric uncertainties from MCMC are provided, and we compare our results with the outputs of interior structure models and the radii of the solar system giant planets, which fit comfortably within our relation.

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arXiv2019

Investigating Trends in Atmospheric Compositions of Cool Gas Giant Planets Using Spitzer Secondary Eclipses

We present new 3.6 and 4.5 micron secondary eclipse measurements for five cool (less than approximately 1000 K) transiting gas giant planets: HAT-P-15b, HAT-P-17b, HAT-P-18b, HAT-P-26b, and WASP-69b. We detect eclipses in at least one bandpass for all planets except HAT-P-15b. We confirm and refine the orbital eccentricity of HAT-P-17b, which is also the only planet in our sample with a known outer companion. We compare our measured eclipse depths in these two bands, which are sensitive to the relative abundances of methane versus carbon monoxide and carbon dioxide, respectively, to predictions from 1D atmosphere models for each planet. For planets with hydrogen-dominated atmospheres and equilibrium temperatures cooler than approximately 1000 K, this ratio should vary as a function of both atmospheric metallicity and the carbon-to-oxygen ratio. For HAT-P-26b, our observations are in good agreement with the low atmospheric metallicity inferred from transmission spectroscopy. We find that all four of the planets with detected eclipses are best matched by models with relatively efficient circulation of energy to the nightside. We see no evidence for a solar-system-like correlation between planet mass and atmospheric metallicity, but instead identify a potential (1.9 sigma) correlation between the inferred methane/(carbon monoxide + carbon dioxide) ratio and stellar metallicity. Our ability to characterize this potential trend is limited by the relatively large uncertainties in the stellar metallicity values. Our observations provide a first look at the brightness of these planets at wavelengths accessible to the James Webb Space Telescope, which will be able to resolve individual methane, carbon monoxide, and carbon dioxide bands and provide much stronger constraints on their atmospheric compositions.

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arXiv2019

Connecting Giant Planet Atmosphere and Interior Modeling: Constraints on Atmospheric Metal Enrichment

Atmospheric characterization through spectroscopic analysis, an essential tool of modern exoplanet science, can benefit significantly from the context provided by interior structure models. In particular, the planet's bulk metallicity $Z_p$ places an upper limit on potential atmospheric metallicity. Here we construct interior structure models to derive $Z_p$ and atmospheric metallicity upper limits for 403 known transiting giant exoplanets. These limits are low enough that they can usefully inform atmosphere models. Additionally, we argue that comparing $Z_p$ to the observed atmospheric metallicity gives a useful measure of how well-mixed metals are within the planet. This represents a new avenue for learning about planetary interiors. To aid in the future characterization of new planet discoveries we derive analytic prior predictions of atmosphere metallicity as a function of planet mass, and evaluate the effectiveness of our approach on Jupiter and Saturn. We we include log-linear fits for approximating the metallicities of planets not in our catalog.

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