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Brianna Lacy

Publications and source records attributed to Brianna Lacy.

At least 19 recordsLinked to original sources

Technology and Science Advancing Observations with Roman Coronagraph Informed by Ground-Based High-Contrast Imaging

The Roman Coronagraph technology demonstration focuses on achieving $<$ 10$^{-7}$ contrasts within the instrument's dark hole and our ability to detect and characterize properties of faint companions around bright stars. Here, we describe results from a study of potential Roman Coronagraph technology demonstration phase observations focused on these goals, informed by the ongoing OASIS survey at the Subaru Telescope and precursor survey work. OASIS provides at least three compelling targets for the technology demonstration phase with imaged companions - the HIP 71618 B brown dwarf and superjovian planets HIP 54515 b and HIP 99770 b. HIP 71618 is well suited for demonstrating the Coronagraph's core performance requirement while all three targets are well suited for spectroscopic mode observations. Each target can be paired with a PSF reference star vetted for companions. While HIP 71618 and HIP 54515 are already planned for Technology Demonstration phase observations, we describe the programmatic and scientific value of adding spectroscopic mode observations of HIP 99770 as well.

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A Roman Coronagraph Spectroscopic Mode Demonstration

We propose 730 nm high-contrast spectroscopic observations of selected self-luminous directly-imaged planets as a key test of the Roman Coronagraph's planet characterization capabilities. The planet sample draws from ground-based IR discoveries with the NASA headquarters-supported Subaru/OASIS survey -- HIP 99770 b and HIP 54515 b -- and ``emblematic" planets $β$ Pic b and HR 8799 e. All of these planets are likely unsuitable for achieving the coronagraph's core TTR5 goal at 575 nm but are detectable at longer wavelength passbands. Their predicted contrasts at 730 $μm$ cover two orders of magnitude range; all companions reside within the dark hole region enabled by the shaped-pupil coronagraph at 730 nm. These observations will help to fulfill multiple Coronagraph Objectives, providing a first assessment of the wavelength dependence of speckle noise and the ability to extract accurate atmospheric information in the face of this noise. Additionally, they will provide a first experiment at extracting optical planet spectra in the face of signal contamination from a debris disk: prefiguring challenges that the Habitable Worlds Observatory may encounter with imaging Earths in exozodi-contaminated systems.

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The HIP 54515 b Superjovian Planet as an Early, Critical Look at the Roman Coronagraph's Performance in the Faint Target Star, Small IWA Limit

The Roman Coronagraph's capabilities in the faint star, small IWA limit has enormous scientific (programmatic) impacts. Testing its performance in this limit provides a first look at challenges that may be encountered with the Habitable Worlds Observatory in imaging rocky planets around the nearest K and M stars. We propose such a rigorous test with the HLC/575nm targeting a newly-discovered superjovian planet HIP 54515 b, whose predicted contrast is $\sim$4.7 $\times$10$^{-8}$--2.5 $\times$10$^{-7}$. The companion lies close to the coronagraph IWA (well interior to the TTR5 performance region) and orbits a V = 6.8 star, near the limit for which the coronagraph may yield deep contrasts. Multiple reference stars are available that will further test CGI's performance as a function of $Δ$ pitch angle to assess how the telescope's thermal environment impacts contrasts.

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Decisively Demonstrating Roman CGI's TTR5 Requirement by Reimaging a Newly-Discovered Brown Dwarf Orbiting a Bright Accelerating Star

We propose Roman Coronagraph project HLC/575 nm observations of a newly-discovered brown dwarf (HIP 71618 B) from the Subaru/OASIS survey of young accelerating stars, which is supported by NASA headquarters with the directive to identify targets for the Roman Coronagraph that could fulfill TTR5 requirements and be observed during the technology demonstration phase. The target and multiple bright PSF references are within/close to the Roman Continuous Viewing Zone. A high SNR detection of this companion would singlehandedly fulfill TTR5 and would be the first optical detection of a companion at $<$10$^{-6}$ contrast. Roman CPP reference star vetting prioritizing stars that can be paired with HIP 71618 would aid the execution of a successful technology demonstration. Additional similar targets may be discovered from OASIS over the next few years that could increase CGI scheduling flexibility and enhance its scientific and technical return. A close collaborative partnership with the CPP team could ensure that they are schedulable.

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SCExAO/CHARIS and Gaia Direct Imaging and Astrometric Discovery of a Superjovian Planet 3--4 lambda/D from the Accelerating Star HIP 54515

We present the discovery of a superjovian planet around the young A5 star HIP 54515, detected using precision astrometry from the Hipparcos Gaia Catalogue of Accelerations and high-contrast imaging with SCExAO/CHARIS from the recently-commenced OASIS program. SCExAO/CHARIS detects HIP 54515 b in five epochs 0\farcs{}145--0\farcs{}192 from the star ($\sim$3--4 $λ$/D at 1.65 $μm$), exhibiting clockwise orbital motion. HIP 54515 b lies near the M/L transition with a luminosity of log(L/L$_{\rm \odot}$) $\sim$ -3.52 $\pm$ 0.03. Dynamical modeling constrains its mass and mass ratio to be ${17.7}_{-4.9}^{+7.6}$ $M_{\rm Jup}$ and ${0.0090}_{-0.0024}^{+0.0036}$ and favors a $\sim$25 au semimajor axis. HIP 54515 b adds to a growing list of superjovian planets with moderate eccentricities (e $\approx$ 0.4). Now the third planet discovered from surveys combining high-contrast extreme adaptive optics imaging with precision astrometry, HIP 54515 b should help improve empirical constraints on the luminosity evolution and eccentricity distribution of the most massive planets. It may also provide a key technical test of the Roman Space Telescope Coronagraph Instrument's performance in the low stellar flux, small angular separation limit and a demonstration of its ability to yield constrainable planet spectral properties.

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OASIS Survey Direct Imaging and Astrometric Discovery of HIP 71618 B: A Substellar Companion Suitable for the Roman Coronagraph Technology Demonstration

We present the OASIS survey program discovery of a substellar companion orbiting the young A1V star HIP 71618, detected using precision astrometry from Gaia and Hipparcos and high-contrast imaging with SCExAO/CHARIS and Keck/NIRC2. Atmospheric modeling favors a spectral type of M5--M8 and a temperature of $\sim$2700 $\pm$ 100 $K$. Dynamical modeling constrains HIP 71618 B's mass to be ${60}_{-21}^{+27}$ $M_{\rm Jup}$ or ${65}_{-29}^{+54}$ $M_{\rm Jup}$, depending on the adopted companion mass prior. It has a nearly edge-on, 11 au-orbit with a high eccentricity. HIP 71618 B will be located within Roman Coronagraph's dark hole region during the instrument's technological demonstration phase. A high signal-to-noise ratio detection of HIP 71618 B at 575 nm would demonstrate a 5-$σ$ contrast of 10$^{-7}$ or better. The system is also located within or very close to Roman's Continuous Viewing Zone -- near multiple candidate reference stars for dark-hole digging -- and its primary is bright ($V$ $\approx$ 5). The suitability of HIP 71618 as one potential Roman Coronagraph target for demonstrating the instrument's core requirement (TTR5) should motivate the timely, deep vetting of candidate reference stars.

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The Diversity of Cold Worlds: Age and Characterization of the Exoplanet COCONUTS-2b

Studying cold brown dwarfs is key to understanding the diverse characteristics of cold giant exoplanets atmospheres. COCONUTS-2, is a wide binary system composed of a T9 brown dwarf and an M3 star, which presents a unique opportunity to characterize a cold benchmark brown dwarf. As part of a JWST program to study the range of physical and atmospheric properties of the coldest brown dwarfs, we obtained NIRSpec G395H spectra (R~2700, 2.87-5.13 um) and MIRI F1000W, F1280W, and F1800W photometry for COCONUTS-2b. In this work, we find a 99% probability of the system belonging to the Corona of Ursa Major (414+/-23 Myr) using BANYAN Sigma and its full kinematics. We also re-estimate the astrometry of COCONUTS-2b using the MIRI data. We support the membership with a comparison of rotation period, metallicity and C/O ratio of the group with those of the COCONUTS-2 system. We also calculate its bolometric luminosity, which combined with our age estimation, allows us to derive its mass, effective temperature, surface gravity, and radius with high precision. As a result of our analysis, we support the conclusion that COCONUTS-2b is a planetary mass object (7.5+/-0.4 MJup) which was likely formed via the same mechanism as stars. In addition we compare the JWST spectrum to another object in the sample, J082507.35+280548.5 (0825+2805), a Y0.5 brown dwarf, which is a candidate member of the same moving group, but has a lower mass (3.7+/-0.2 MJup). We identify absorption feature differences which could indicate that 0825+2805 has stronger vertical mixing.

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Multi-band Spectral and Astrometric Characterization of the HIP 99770 b Planet with SCExAO/CHARIS and Gaia

We present and analyze follow-up, higher resolution ($R$ $\sim$ 70) $H$ and $K$ band integral field spectroscopy of the superjovian exoplanet HIP 99770 b with SCExAO/CHARIS. Our new data recover the companion at a high signal-to-noise ratio in both bandpasses and more than double the astrometric baseline for its orbital motion. Jointly modeling HIP 99770 b's position and the star's astrometry from Hipparcos and Gaia yields orbital parameters consistent with those from the discovery paper, albeit with smaller errors, and a slight preference for a smaller semimajor axis ($\sim$15.7--15.8 au)and a larger eccentricity ($\sim$0.28--0.29), disfavoring a circular orbit. We revise its dynamical mass slightly downwards to 15.0$_{-4.4}^{+4.5}$ $M_{\rm Jup}$ for a flat prior and 13.1$_{-5.2}^{+4.8}$ $M_{\rm Jup}$ for a more standard log-uniform mass prior, where the inclusion of its relative radial-velocity measurement is primarily responsible for these changes. We find consistent results for HIP 99770 b's dynamical mass including recent VLTI/GRAVITY astrometry, albeit with a slightly smaller, better constrained eccentricity of $e$ $\sim$ 0.22$^{+0.10}_{-0.13}$. HIP 99770 b is a $\sim$ 1300 K object at the L/T transition with a gravity intermediate between that of the HR 8799 planets and older, more massive field brown dwarfs with similar temperatures but with hints of equilibrium chemistry. HIP 99770 b is particularly well suited for spectroscopic follow up with Roman CGI during the technology demonstration phase at 730 nm to further constrain its metallicity and chemistry; JWST thermal infrared observations could likewise explore the planet's carbon chemistry, metallicity, and clouds.

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Diversity of Cold Worlds: Predicted Near- to Mid-infrared Spectral Signatures of a Cold Brown Dwarf with Potential Auroral Heating

Recent JWST/NIRSpec observations have revealed strong methane emission at 3.326 microns in the $\approx$482 K brown dwarf CWISEP J193518.59$-$154620.3 (W1935). Atmospheric modeling suggests the presence of a $\approx$300 K thermal inversion in its upper atmosphere, potentially driven by auroral activity. We present an extension of the retrieved spectra of W1935 with and without inversion spanning 1--20 microns, to identify thermal inversion-sensitive spectral features and explore the origin of the object's peculiar characteristics. Our analysis indicates that atmospheric heating contributes approximately 15% to the bolometric luminosity. The model with inversion predicts an additional similar-strength methane emission feature at 7.7 microns and tentative ammonia emission features in the mid-infrared. Wavelengths beyond $\sim$2 microns are significantly influenced by the inversion, except for the 4.1--5.0 microns CO$_2$ and CO features that originate from atmospheric layers deeper than the region where the inversion occurs. W1935 appears as an outlier in Spitzer/IRAC mid-infrared color-magnitude diagrams (CMDs) based on the $m_{\rm Ch1}-m_{\rm Ch2}$ (IRAC 3.6 microns $-$ 4.5 microns) color, but exhibits average behavior in all other combinations that trace clear sequences. This anomaly is likely due to the Ch2 filter probing vertical mixing-sensitive CO$_2$ and CO features that do not correlate with temperature or spectral type. We find that the thermal inversion tends to produce bluer $m_{\rm Ch1}-m_{\rm Ch2}$ colors, so the overluminous and/or redder position of W1935 in diagrams involving this color cannot be explained by the thermal inversion. This analysis provides insights into the intriguing dispersion of cold brown dwarfs in mid-infrared CMDs and sheds light on their spectral diversity.

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JWST Coronagraphic Images of 14 Her c: a Cold Giant Planet in a Dynamically Hot, Multi-planet System

Most observed multi-planet systems are coplanar, in a dynamically "cold" configuration of concentric orbits like our own Solar System. With the James Webb Space Telescope (JWST) we have detected 14 Her c, the first mature and cold exoplanet directly imaged in a dynamically "hot", multi-planet system. With large eccentricities and a nonzero mutual inclination, the present-day architecture of this system points to a turbulent past and ongoing angular momentum exchange between the planetary orbits of 14 Her b and c. The temperature of 14 Her c rivals both the coldest imaged exoplanet and the coldest known brown dwarf. Moreover, its photometry at 4.4 mu is consistent with the presence of carbon disequilibrium chemistry and water ice clouds in its atmosphere. 14 Her c presents a unique laboratory to study giant planet formation, dynamical evolution of multi-planet system architectures, and atmospheric composition and dynamics in extremely cold worlds.

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Diversity of Cold Worlds: A Near Complete Spectral Energy Distribution for 2MASS J04151954-0935066 using JWST

We present the a near complete spectral energy distribution (SED) for an extrasolar world: the T8 brown dwarf 2MASS~J04151954$-$0935066. Spanning from optical to mid-infrared (0.7--20.4 micron) wavelengths, the SED for this substellar atmosphere is constructed from new JWST NIRSpec G395H ($R\sim$2700) and Magellan FIRE echelle ($R\sim$8000) near-infrared spectra, along with MIRI mid-infrared photometry complemented by spectra from Keck I, IRTF, Magellan, AKARI, Spitzer and photometry from various surveys and missions. The NIRSpec G395H spectrum reveals strong molecular absorptions from NH$_{3}$, CH$_{4}$, H$_{2}$S, CO$_{2}$ and H$_{2}$O at approximately 3.00, 3.35, 3.95, 4.25, and 5.00 micron respectively, along with the presence of a CO absorption feature detected mainly at $\sim$ 4.6 micron. We detect no absorption of near-infrared K I doublets in the $R\sim8000$ FIRE spectra. In the mid-infrared IRS spectrum, we tentatively identify a new CO$_{2}$ feature at 14--16 micron. The comprehensive SED allows us to empirically constrain bolometric luminosity, effective temperature, mass and radius. Additionally, we demonstrate that the NIRSpec G395H resolution, the highest allowable by JWST, enables a precise radial velocity measurement of $47.1\pm1.8$ km s$^{-1}$ for the object, in agreement with previous measurements.

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Dynamical and Atmospheric Characterization of the Substellar Companion HD 33632 Ab from Direct Imaging, Astrometry, and Radial-Velocity Data

We present follow-up SCExAO/CHARIS $H$ and $K$-band (R $\sim$ 70) high-contrast integral field spectroscopy and Keck/NIRC2 photometry of directly-imaged brown dwarf companion HD 33632 Ab and new radial-velocity data for the system from the SOPHIE spectrograph, complemented by Hipparcos and Gaia astrometry. These data enable more robust spectral characterization compared to lower-resolution spectra from the discovery paper and more than double the available astrometric and radial-velocity baseline. HD 33632 Ab's spectrum is well reproduced by a field L8.5--L9.5 dwarf. Using the Exo-REM atmosphere models, we derive a best-fit temperature, surface gravity and radius of $T_{\rm eff}$ = 1250 $K$, log(g) = 5, and $R$ = 0.97 $R_{\rm J}$ and a solar C/O ratio. Adding the SOPHIE radial-velocity data enables far tighter constraints on the companion's orbital properties (e.g. $i$=${47.5}_{-4.7}^{+2.5}$$^{o}$) and dynamical mass (${52.8}_{-2.4}^{+2.6}$$M_{\rm J}$) than derived from imaging data and \textit{Gaia} eDR3 astrometry data alone. HD 33632 Ab should be a prime target for multi-band imaging and spectroscopy with the \textit{James Webb Space Telescope} and the \textit{Roman Space Telescope}'s Coronagraphic Instrument, shedding detailed light on HD 33632 Ab's clouds and chemistry and providing a key reference point for understanding young exoplanet atmospheres.

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The Diversity of Cold Worlds: a blended-light binary straddling the T/Y transition in brown dwarfs

We present the first brown dwarf spectral binary characterized with JWST: WISE J014656.66+423410.0, the coldest blended-light brown dwarf binary straddling the T/Y transition. We obtained a moderate resolution (R$\sim$2700) G395H spectrum of this unresolved binary with JWST/NIRSpec and we fit it to late-T and Y dwarf spectra from JWST/NIRSpec, and model spectra of comparable temperatures, both as individual spectra and pairs mimicking an unresolved binary system. We find that this tightly-separated binary is likely composed of two unequal-brightness sources with a magnitude difference of $0.50\pm0.08$ mag in IRAC [4.5] and a secondary $1.01\pm0.13$ mag redder than the primary in [3.6]-[4.5]. Despite the large color difference between the best fit primary and secondary, their temperature difference is only $92\pm23$\,K, a feature reminiscing of the L/T transition. Carbon disequilibrium chemistry strongly shapes the mid-infrared spectra of these sources, as a complex function of metallicity and surface gravity. While a larger library of JWST/NIRSpec spectra is needed to conclusively examine the peculiarities of blended-light sources, this spectral binary is a crucial pathfinder to both understand the spectral features of planetary-mass atmospheres and detect binarity in unresolved, moderate-resolution spectra of the coldest brown dwarfs.

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Direct-imaging Discovery of a Substellar Companion Orbiting the Accelerating Variable Star, HIP 39017

We present the direct-imaging discovery of a substellar companion (a massive planet or low-mass brown dwarf) to the young, $γ$ Doradus-type variable star, HIP 39017 (HD 65526). The companion's SCExAO/CHARIS JHK ($1.1-2.4μ$m) spectrum and Keck/NIRC2 L$^{\prime}$ photometry indicate that it is an L/T transition object. A comparison of the JHK+L$^{\prime}$ spectrum to several atmospheric model grids finds a significantly better fit to cloudy models than cloudless models. Orbit modeling with relative astrometry and precision stellar astrometry from Hipparcos and Gaia yields a semi-major axis of $23.8^{+8.7}_{-6.1}$ au, a dynamical companion mass of $30^{+31}_{-12}$~M$_J$, and a mass ratio of $\sim$1.9\%, properties most consistent with low-mass brown dwarfs. However, its mass estimated from luminosity models is a lower $\sim$13.8 $M_{\rm J}$ due to an estimated young age ($\lesssim$ 115 Myr); using a weighted posterior distribution informed by conservative mass constraints from luminosity evolutionary models yields a lower dynamical mass of $23.6_{-7.4}^{+9.1}$~M$_J$ and a mass ratio of $\sim$1.4\%. Analysis of the host star's multi-frequency $γ$ Dor-type pulsations, astrometric monitoring of HIP 39017b, and Gaia Data Release 4 astrometry of the star will clarify the system age and better constrain the mass and orbit of the companion. This discovery further reinforces the improved efficiency of targeted direct-imaging campaigns informed by long-baseline, precision stellar astrometry.

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Methane Emission From a Cool Brown Dwarf

Beyond our solar system, aurorae have been inferred from radio observations of isolated brown dwarfs (e.g. Hallinan et al. 2006; Kao et al. 2023). Within our solar system, giant planets have auroral emission with signatures across the electromagnetic spectrum including infrared emission of H3+ and methane. Isolated brown dwarfs with auroral signatures in the radio have been searched for corresponding infrared features but have only had null detections (e.g. Gibbs et al. 2022). CWISEP J193518.59-154620.3. (W1935 for short) is an isolated brown dwarf with a temperature of ~482 K. Here we report JWST observations of strong methane emission from W1935 at 3.326 microns. Atmospheric modeling leads us to conclude that a temperature inversion of ~300 K centered at 1-10 millibar replicates the feature. This represents an atmospheric temperature inversion for a Jupiter-like atmosphere without irradiation from a host star. A plausible explanation for the strong inversion is heating by auroral processes, although other internal and/or external dynamical processes cannot be ruled out. The best fit model rules out the contribution of H3+ emission which is prominent in solar system gas giants however this is consistent with rapid destruction of H3+ at the higher pressure where the W1935 emission originates (e.g. Helling et al. 2019).

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Direct Imaging and Astrometric Detection of a Gas Giant Planet Orbiting an Accelerating Star

Direct imaging of gas giant exoplanets provides key information on planetary atmospheres and the architectures of planetary systems. However, few planets have been detected in blind surveys used to achieve imaging detections. Using Gaia and Hipparcos astrometry we identified dynamical evidence for a gas giant planet around the nearby star HIP 99770 and then confirmed this planet by direct imaging with the Subaru Coronagraphic Extreme Adaptive Optics Project. HIP 99770 b orbits 17 astronomical units from its host star, with an insolation comparable to Jupiter's and a dynamical mass of 13.9--16.1 Jupiter masses. Its planet-to-star mass ratio (7--8$\times$10$^{-3}$) is comparable to that other directly-imaged planets. The planet's atmosphere resembles an older, less-cloudy analogue of the atmospheres of previously-imaged exoplanets around HR 8799.

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Self-consistent Models of Y Dwarf Atmospheres with Water Clouds and Disequilibrium Chemistry

Y dwarfs are the coolest spectral class of brown dwarf. They have effective temperatures less than 500 K, with the coolest detection as low as ~250 K. Their spectra are shaped predominantly by gaseous water, methane, and ammonia. At the warmer end of the Y dwarf temperature range, spectral signatures of disequilibrium carbon monoxide have been observed. Cooler Y dwarfs could host water clouds in their atmospheres. Since they make up the low-mass tail of the star formation process, and are a valuable analogue to the atmospheres of giant gaseous exoplanets in a temperature range that is difficult to observe, understanding Y dwarf atmospheric compositions and processes will both deepen our understanding of planet and star formation, and provide a stepping stone towards characterizing cool exoplanets. JWST spectral observations are anticipated to provide an unprecedented level of detail for these objects, and yet published self-consistent model grids do not accurately replicate even the existing HST and ground-based observations. In this work, we present a new suite of 1-d radiative-convective equilibrium models to aid in the characterization of Y dwarf atmospheres and spectra. We compute clear, cloudy, equilibrium-chemistry and disequilibrium-chemistry models, providing a comprehensive suite of models in support of the impending JWST era of panchromatic Y dwarf characterization. Comparing these models against current observations, we find that disequilibrium CH4-CO and NH3-N2 chemistry and the presence of water clouds can bring models and observations into better, though still not complete, agreement.

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JWST Transit Spectra II: Constraining Aerosol Species, Particle-size Distributions, Temperature, and Metallicity for Cloudy Exoplanets

JWST will provide moderate resolution transit spectra with continuous wavelength coverage from the optical to the mid-infrared for the first time. In this paper, we illustrate how different aerosol species, size-distributions, and spatial distributions encode information in JWST transit spectra. We use the transit spectral modeling code METIS, along with Mie theory and several flexible treatments of aerosol size and spatial distributions to perform parameter sensitivity studies, calculate transit contribution functions, compute Jacobians, and retrieve parameters with Markov Chain Monte Carlo. The broader wavelength coverage of JWST will likely encompass enough non-gray aerosol behavior to recover information about the species and size-distribution of particles, especially if distinct resonance features arising from the aerosols are present. Within the JWST wavelength range, the optical and mid-infrared typically provide information about 0.1-1 $μ$m sized aerosols, while the near-infrared to mid-infrared wavelengths usually provide information about gaseous absorption, even if aerosols are present. Strong gaseous absorption features in the infrared often remain visible, even when clouds and hazes are flattening the optical and NIR portion of the spectrum that is currently observable. For some combinations of aerosol properties, temperature, and surface gravity, one can make a precise measure of metallicity despite the presence of aerosols, but more often the retrieved metallicity of a cloudy or hazy atmosphere has significantly lower precision than for a clear atmosphere with otherwise similar properties. Future efforts to securely link aerosol properties to atmospheric metallicity and temperature in a physically motivated manner will ultimately enable a robust physical understanding of the processes at play in cloudy, hazy exoplanet atmospheres.

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