arXiv subjects

Roxana Lupu

Publications and source records attributed to Roxana Lupu.

At least 19 recordsLinked to original sources

arXiv2026

Water Cloud and Chemical Modulations in the Coldest Brown Dwarf

We present high signal-to-noise, medium resolution (R ~ 1000), time-series JWST/NIRSpec spectra of WISE 0855 (265K), the coldest known brown dwarf. Medium resolution, time-series spectroscopy gives us the power to disentangle the effects of chemistry, temperature, and condensates on this cool world. Our observations span 11 hours with a 15 minute cadence covering 2.87 - 5.27 $μ$m. The strongest time variable spectroscopic feature is carbon monoxide gas absorption producing modulations with a peak-to-peak amplitude up to 10\% at some wavelengths. Using principal component analysis, we show that the variations in carbon monoxide and phosphine correlate with one another. By comparing our data to atmospheric and structure models we present evidence of patchy water clouds within the atmosphere of WISE 0855. We find that variations in CO and PH$_{3}$ abundances must originate from quenched atmospheric pressures while variations in water cloud thickness occurs at lower pressures.

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arXiv2026

The HST/WFC3 Transmission Spectrum of AU Mic b Part I: An Atmosphere Obscured by Contamination and Systematics

Young sub-Neptune progenitors around M dwarfs offer an excellent opportunity to probe the formation of their abundant, older cousins. At $\sim$20 Myr and only 9.7 pc away, AU Mic b is an ideal candidate for this effort, with its density and observations of escaping hydrogen pointing to a significant primordial atmosphere. Here we present the 0.8-1.6 $\micron$ transmission spectrum of AU Mic b observed with the Wide Field Camera 3 on the Hubble Space Telescope (HST). We find that HST experienced unstable scanning during its visits, resulting in a variable PSF that dramatically affects the orbit-to-orbit baseline of the observations. While we were able to somewhat mitigate this problem through spectral binning, the effects cannot be completely eliminated, limiting the precision of our results. Our data is further impacted by the intense magnetic activity of AU Mic, which introduced significant rotational variability along with spot crossings and the transit light source (TLS) effect into the light curves and spectrum, respectively. Through decomposition of the out-of-transit stellar SED, we are able to constrain AU Mic's photospheric and spot temperatures to 3891$\pm$37 and 3020$\pm$69 K, respectively, with a spot filling factor of $0.33\pm0.05$. Using Bayesian atmospheric retrievals, we show that the spectrum is dominated by the TLS effect with weak atmospheric constraints, with the data preferring a relatively small scale height of $<$185 km to 3$σ$. Extrapolation of our retrieved spectra shows that the TLS effect dominates over atmospheric features at optical and infrared wavelengths.

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arXiv2025

A Deep Search for Exomoons Around WISE 0855 With JWST

JWST is collecting time-series observations of many free-floating planets (FFPs) to study their weather, but these light curves are the ideal datasets to search for exomoons that transit the FFP during observations. In this paper, we present observations of the planetary-mass Y dwarf ($T=250-285K$, $M = 6.5\pm3.5 M_{Jup}$, d = 2.3$\,$pc) WISE J085510.83-071442.5 (WISE 0855), whose proximity and brightness make it ideal for a transiting exomoon search. We examine 11 hours of time-series spectra from the JWST Near-Infrared Spectrograph (NIRSpec) whose sensitivity, in combination with Gaussian process (GP) modeling, allows for the disentanglement of exomoon transits from WISE 0855's variability. We do not find statistically significant evidence of an exomoon transit in this dataset. Using injection and recovery tests of artificial transits for depths ranging between 0.1-1% (0.35-1.12 $R_{\oplus}$) we explore the exomoon parameter space where we could successfully detect transits. For transit depths $\geq 0.5\%$ (1.96$\,R_{\text{Titan}}$), our detection rate is 96%, which, for WISE 0855, corresponds to a moon with a companion-to-host mass ratio similar to that of Titan and Saturn. Given our sensitivity, transit probabilities, and our observational duration, we determine a $\sim$91% probability of detecting a Titan mass analog exomoon after 18 such observations if every observed system hosts a Titan mass analog exomoon in a Galilean-like system. This suggests that JWST observations of dozens of FFPs could yield meaningful constraints on the occurrence rate of exomoons. This paper is the first demonstration that JWST is sensitive to Galilean moon mass analogs around FFPs.

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arXiv2025

The radiative effects of photochemical hazes on the atmospheric circulation and phase curves of sub-Neptunes

Measuring the atmospheric composition of hazy sub-Neptunes like GJ~1214b through transmission spectroscopy is difficult because of the degeneracy between mean molecular weight and haziness. It has been proposed that phase curve observations can break this degeneracy because of the relationship between mean molecular weight (MMW) and phase curve amplitude. However, photochemical hazes can strongly affect phase curve amplitudes as well. We present a large set of GCM simulations of the sub-Neptune GJ~1214b that include photochemical hazes with varying atmospheric composition, haze opacity and haze optical properties. In our simulations, photochemical hazes cause temperature changes of up to 200~K, producing thermal inversions and cooling deeper regions. This results in increased phase curve amplitudes and adds a considerable scatter to the phase curve amplitude--metallicity relationship. However, we find that if the haze production rate is high enough to significantly alter the phase curve, the secondary eclipse spectrum will exhibit either emission features or strongly muted absorption features. Thus, the combination of a white-light phase curve and a secondary eclipse spectrum can successfully distinguish between a hazy, lower MMW and a clear, high MMW scenario.

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arXiv2024

Protosolar D-to-H abundance and one part-per-billion PH$_{3}$ in the coldest brown dwarf

The coldest Y spectral type brown dwarfs are similar in mass and temperature to cool and warm ($\sim$200 -- 400 K) giant exoplanets. We can therefore use their atmospheres as proxies for planetary atmospheres, testing our understanding of physics and chemistry for these complex, cool worlds. At these cold temperatures, their atmospheres are cold enough for water clouds to form, and chemical timescales increase, increasing the likelihood of disequilibrium chemistry compared to warmer classes of planets. JWST observations are revolutionizing the characterization of these worlds with high signal-to-noise, moderate resolution near- and mid-infrared spectra. The spectra have been used to measure the abundances of prominent species like water, methane, and ammonia; species that trace chemical reactions like carbon monoxide; and even isotopologues of carbon monoxide and ammonia. Here, we present atmospheric retrieval results using both published fixed-slit (GTO program 1230) and new averaged time series observations (GO program 2327) of the coldest known Y dwarf, WISE 0855-0714 (using NIRSpec G395M spectra), which has an effective temperature of $\sim$ 264 K. We present a detection of deuterium in an atmosphere outside of the solar system via a relative measurement of deuterated methane (CH$_{3}$D) and standard methane. From this, we infer the D/H ratio of a substellar object outside the solar system for the first time. We also present a well-constrained part-per-billion abundance of phosphine (PH$_{3}$). We discuss our interpretation of these results and the implications for brown dwarf and giant exoplanet formation and evolution.

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arXiv2024

The Sonora Substellar Atmosphere Models. III. Diamondback: Atmospheric Properties, Spectra, and Evolution for Warm Cloudy Substellar Objects

We present a new grid of cloudy atmosphere and evolution models for substellar objects. These models include the effect of refractory cloud species, including silicate clouds, on the spectra and evolution. We include effective temperatures from 900 to 2400 K and surface gravities from log g=3.5-5.5, appropriate for a broad range of objects with masses between 1 and 84 Jupiter masses. Model pressure-temperature structures are calculated assuming radiative-convective and chemical equilibrium. We consider the effect of both clouds and metallicity on the atmospheric structure, resulting spectra, and thermal evolution of substellar worlds. We parameterize clouds using the Ackerman & Marley (2001) cloud model, including cloud parameter fsed values from 1-8; we include three metallicities (-0.5, 0.0, and +0.5). Refractory clouds and metallicity both alter the evolution of substellar objects, changing the inferred temperature at a given age by up to 100-200 K. We compare to the observed photometry of brown dwarfs, finding broad agreement with the measured photometry. We publish the spectra, evolution, and other data products online with open access.

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arXiv2024

The Sonora Substellar Atmosphere Models. IV. Elf Owl: Atmospheric Mixing and Chemical Disequilibrium with Varying Metallicity and C/O Ratios

Disequilibrium chemistry due to vertical mixing in the atmospheres of many brown dwarfs and giant exoplanets is well-established. Atmosphere models for these objects typically parameterize mixing with the highly uncertain $K_{\rm zz}$ diffusion parameter. The role of mixing in altering the abundances of C-N-O-bearing molecules has mostly been explored for solar composition atmospheres. However, atmospheric metallicity and the C/O ratio also impact atmospheric chemistry. Therefore, we present the \texttt{Sonora Elf Owl} grid of self-consistent cloud-free 1D radiative-convective equilibrium model atmospheres for JWST observations, which includes a variation of $K_{\rm zz}$ across several orders of magnitude and also encompasses sub-solar to super-solar metallicities and C/O ratios. We find that the impact of $K_{\rm zz}$ on the $T(P)$ profile and spectra is a strong function of both $T_{\rm eff}$ and metallicity. For metal-poor objects $K_{\rm zz}$ has large impacts on the atmosphere at significantly higher $T_{\rm eff}$ compared to metal-rich atmospheres where the impact of $K_{\rm zz}$ is seen to occur at lower $T_{\rm eff}$. We identify significant spectral degeneracies between varying $K_{\rm zz}$ and metallicity in multiple wavelength windows, in particular at 3-5 $μ$m. We use the \texttt{Sonora Elf Owl} atmospheric grid to fit the observed spectra of a sample of 9 early to late T- type objects from $T_{\rm eff}=550-1150$ K. We find evidence for very inefficient vertical mixing in these objects with inferred $K_{\rm zz}$ values lying in the range between $\sim$ 10$^1$-10$^4$ cm$^2$s$^{-1}$. Using self-consistent models, we find that this slow vertical mixing is due to the observations probing mixing in the deep detached radiative zone in these atmospheres.

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arXiv2024

Modeling the day-night temperature variations of ultra-hot Jupiters: confronting non-grey general circulation models and observations

Ultra-hot Jupiters (UHJs) are natural laboratories to study extreme physics in planetary atmospheres and their rich observational data sets are yet to be confronted with models with varying complexities at a population level. In this work, we update the general circulation model of Tan & Komacek (2019) to include a non-grey radiative transfer scheme and apply it to simulate the realistic thermal structures, phase-dependent spectra, and wavelength-dependent phase curves of UHJs. We performed grids of models over a large range of equilibrium temperatures and rotation periods for varying assumptions, showing that the fractional day-night brightness temperature differences remain almost constant or slightly increase with increasing equilibrium temperature from the visible to mid-infrared wavelengths. This differs from previous work primarily due to the increasing planetary rotation rate with increasing equilibrium temperature for fixed host star type. Radiative effects of varying atmospheric compositions become more significant in dayside brightness temperature in longer wavelengths. Data-model comparisons of dayside brightness temperatures and phase curve amplitudes as a function of equilibrium temperature are in broad agreement. Observations show a large scatter compared to models even with a range of different assumptions, indicating significantly varying intrinsic properties in the hot Jupiter population. Our cloud-free models generally struggle to match all observations for individual targets with a single set of parameter choices, indicating the need for extra processes for understanding the heat transport of UHJs.

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arXiv2023

Origin of Life Molecules in the Atmosphere After Big Impacts on the Early Earth

The origin of life on Earth would benefit from a prebiotic atmosphere that produced nitriles, like HCN, which enable ribonucleotide synthesis. However, geochemical evidence suggests that Hadean air was relatively oxidizing with negligible photochemical production of prebiotic molecules. These paradoxes are resolved by iron-rich asteroid impacts that transiently reduced the entire atmosphere, allowing nitriles to form in subsequent photochemistry. Here, we investigate impact-generated reducing atmospheres using new time-dependent, coupled atmospheric chemistry and climate models, which account for gas-phase reactions and surface-catalysis. The resulting H$_2$-, CH$_4$- and NH$_3$-rich atmospheres persist for millions of years, until hydrogen escapes to space. HCN and HCCCN production and rainout to the surface can reach $10^9$ molecules cm$^{-2}$ s$^{-1}$ in hazy atmospheres with a mole ratio of $\mathrm{CH_4} / \mathrm{CO_2} > 0.1$. Smaller $\mathrm{CH_4} / \mathrm{CO_2}$ ratios produce HCN rainout rates $< 10^5$ molecules cm$^{-2}$ s$^{-1}$, and negligible HCCCN. The minimum impactor mass that creates atmospheric $\mathrm{CH_4} / \mathrm{CO_2} > 0.1$ is $4 \times 10^{20}$ to $5 \times 10^{21}$ kg (570 to 1330 km diameter), depending on how efficiently iron reacts with a steam atmosphere, the extent of atmospheric equilibration with an impact-induced melt pond, and the surface area of nickel that catalyzes CH$_4$ production. Alternatively, if steam permeates and deeply oxidizes crust, impactors $\sim 10^{20}$ kg could be effective. Atmospheres with copious nitriles have $> 360$ K surface temperatures, perhaps posing a challenge for RNA longevity, although cloud albedo can produce cooler climates. Regardless, post-impact cyanide can be stockpiled and used in prebiotic schemes after hydrogen has escaped to space.

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arXiv2023

A reflective, metal-rich atmosphere for GJ 1214b from its JWST phase curve

There are no planets intermediate in size between Earth and Neptune in our Solar System, yet these objects are found around a substantial fraction of other stars. Population statistics show that close-in planets in this size range bifurcate into two classes based on their radii. It is hypothesized that the group with larger radii (referred to as "sub-Neptunes") is distinguished by having hydrogen-dominated atmospheres that are a few percent of the total mass of the planets. GJ 1214b is an archetype sub-Neptune that has been observed extensively using transmission spectroscopy to test this hypothesis. However, the measured spectra are featureless, and thus inconclusive, due to the presence of high-altitude aerosols in the planet's atmosphere. Here we report a spectroscopic thermal phase curve of GJ 1214b obtained with JWST in the mid-infrared. The dayside and nightside spectra (average brightness temperatures of 553 $\pm$ 9 and 437 $\pm$ 19 K, respectively) each show >3$σ$ evidence of absorption features, with H$_2$O as the most likely cause in both. The measured global thermal emission implies that GJ 1214b's Bond albedo is 0.51 $\pm$ 0.06. Comparison between the spectroscopic phase curve data and three-dimensional models of GJ 1214b reveal a planet with a high metallicity atmosphere blanketed by a thick and highly reflective layer of clouds or haze.

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arXiv2023

The puzzle of the formation of T8 dwarf Ross 458c

At the lowest masses, the distinction between brown dwarfs and giant exoplanets is often blurred and literature classifications rarely reflect the deuterium burning boundary. Atmospheric characterisation may reveal the extent to which planetary formation pathways contribute to the population of very-low mass brown dwarfs, by revealing if their abundance distributions differ from those of the local field population or, in the case of companions, their primary stars. The T8 dwarf Ross 458c is a possible planetary mass companion to a pair of M dwarfs, and previous work suggests that it is cloudy. We here present the results of the retrieval analysis of Ross 458c, using archival spectroscopic data in the 1.0 to 2.4 micron range. We test a cloud free model as well as a variety of cloudy models and find that the atmosphere of Ross 458c is best described by a cloudy model (strongly preferred). The CH4/H2O is higher than expected at 1.97 +0.13 -0.14. This value is challenging to understand in terms of equilibrium chemistry and plausible C/O ratios. Comparisons to thermochemical grid models suggest a C/O of ~ 1.35, if CH4 and H2O are quenched at 2000 K, requiring vigorous mixing. We find a [C/H] ratio of +0.18, which matches the metallicity of the primary system, suggesting that oxygen is missing from the atmosphere. Even with extreme mixing, the implied C/O is well beyond the typical stellar regime, suggesting a either non-stellar formation pathway, or the sequestration of substantial quantities of oxygen via hitherto unmodeled chemistry or condensation processes.

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arXiv2022

A 1.46-2.48 $μ$m Spectroscopic Atlas of a T6 Dwarf (1060 K) Atmosphere with IGRINS: First Detections of H$_2$S and H$_2$, and Verification of H$_2$O, CH$_4$, and NH$_3$ Line Lists

We present Gemini South/IGRINS observations of the 1060 K T6 dwarf 2MASS J08173001$-$6155158 with unprecedented resolution ($R\equivλ/Δλ=45\,000$) and signal-to-noise ratio (SNR > 200) for a late-type T dwarf. We use this benchmark observation to test the reliability of molecular line lists used up-to-date atmospheric models. We determine which spectroscopic regions should be used to estimate the parameters of cold brown dwarfs and, by extension, exoplanets. We present a detailed spectroscopic atlas with molecular identifications across the $H$ and $K$ bands of the near-infrared. We find that water (H$_2$O) line lists are overall reliable. We find the most discrepancies amongst older methane (CH$_4$) line lists, and that the most up-to-date CH$_4$ line lists correct many of these issues. We identify individual ammonia (NH$_3$) lines, a hydrogen sulfide (H$_2$S) feature at 1.5900 $μ$m, and a molecular hydrogen (H$_2$) feature at 2.1218 $μ$m. These are the first unambiguous detections of H$_2$S and H$_2$ absorption features in an extra-solar atmosphere. With the H$_2$ detection, we place an upper limit on the atmospheric dust concentration of this T6 dwarf: at least 500 times less than the interstellar value, implying that the atmosphere is effectively dust-free. We additionally identify several features that do not appear in the model spectra. Our assessment of the line lists is valuable for atmospheric model applications to high-dispersion, low-SNR, high-background spectra, such as an exoplanet around a star. We demonstrate a significant enhancement in the detection of the CH$_4$ absorption signal in this T6 dwarf with the most up-to-date line lists.

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arXiv2022

Observability of Evaporating Lava Worlds

Lava worlds belong to a class of short orbital period planets reaching dayside temperatures high enough to melt their silicate crust. Theory predicts that the resulting lava oceans outgas their volatile components, attaining equilibrium with the overlying vapour. This creates a tenuous, silicate-rich atmosphere that may be confined to the permanent dayside of the planet. The launch of JWST will provide the much needed sensitivity and spectral coverage to characterise these worlds. In this paper, we assess the observability of characterisable spectral features by self-consistently modelling silicate atmospheres for all the currently confirmed targets having sufficient substellar temperatures (> 1500 K). To achieve this we use outgassed equilibrium chemistry and radiative transfer methods to compute temperature-pressure profiles, atmospheric chemical compositions and emission spectra. We explore varying melt compositions, free of highly-volatile elements, accounting for possible atmospheric evolution. Our models include a large number of neutral and ionic species, as well as all up-to-date opacities. The results indicate that SiO and SiO2 infrared features are the best, unique identifiers of silicate atmospheres, detectable using MIRI instrument of JWST. Detection of these two species in emission would allow for strong constraints on atmospheric thermal structure and possibly the composition of the melt. We also propose that certain species, e.g., TiO, may be directly tied to different classes of melts, possibly revealing surface and interior dynamics. Currently, there are nearly a dozen confirmed lava planets ideal for characterisation of silicate atmospheres using JWST, with two of these already accepted for the initial General Observers program.

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arXiv2021

The Sonora Substellar Atmosphere Models. II. Cholla: A Grid of Cloud-free, Solar Metallicity Models in Chemical Disequilibrium for the JWST Era

Exoplanet and brown dwarf atmospheres commonly show signs of disequilibrium chemistry. In the James Webb Space Telescope era high resolution spectra of directly imaged exoplanets will allow the characterization of their atmospheres in more detail, and allow systematic tests for the presence of chemical species that deviate from thermochemical equilibrium in these atmospheres. Constraining the presence of disequilibrium chemistry in these atmospheres as a function of parameters such as their effective temperature and surface gravity will allow us to place better constrains in the physics governing these atmospheres. This paper is part of a series of works presenting the Sonora grid of atmosphere models (Marley et al 2021, Morley et al in prep.). In this paper we present a grid of cloud-free, solar metallicity atmospheres for brown dwarfs and wide separation giant planets with key molecular species such as CH4, H2O, CO and NH3 in disequilibrium. Our grid covers atmospheres with Teff~[500 K,1300 K], logg~[3.0,5.5] (cgs) and an eddy diffusion parameter of logKzz=2, 4 and 7 (cgs). We study the effect of different parameters within the grid on the temperature and composition profiles of our atmospheres. We discuss their effect on the near-infrared colors of our model atmospheres and the detectability of CH4, H2O, CO and NH3 using the JWST. We compare our models against existing MKO and Spitzer observations of brown dwarfs and verify the importance of disequilibrium chemistry for T dwarf atmospheres. Finally, we discuss how our models can help constrain the vertical structure and chemical composition of these atmospheres.

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arXiv2021

The first retrieval of a substellar subdwarf: A cloud-free SDSS J125637.13-022452.4

We present the first retrieval analysis of a substellar subdwarf, SDSS J125637.13-022452.4 (SDSS J1256-0224), using the Brewster retrieval code base. We find SDSS J1256-0224 is best fit by a cloud-free model with an ion (neutral H, H-, and electron) abundance corresponding to ion [Fe/H]=-1.5. However, this model is indistinguishable from a cloud-free model with ion [Fe/H]=-2.0 and a cloud-free model with ion Fe/H]=-1.5 assuming a subsolar carbon-to-oxygen ratio. We are able to constrain abundances for water, FeH, and CrH, with an inability to constrain any carbon-bearing species likely due to the low-metallicity of SDSS J1256-0224. We also present an updated spectral energy distribution (SED) and semi-empirical fundamental parameters. Our retrieval- and SED-based fundamental parameters agree with the Baraffe low-metallicity evolutionary models. From examining our "rejected" models (those with $Δ$BIC>45), we find that we are able to retrieve gas abundances consistent with those of our best-fitting model. We find the cloud in these poorer fitting "cloudy" models is either pushed to the bottom of the atmosphere or made optically thin.

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arXiv2021

Impacts of Water Latent Heat on the Thermal Structure of Ultra-Cool Objects: Brown Dwarfs and Free-Floating Planets

Brown dwarfs are essential targets for understanding planetary and sub-stellar atmospheres across a wide range of thermal and chemical conditions. As surveys continue to probe ever deeper, and as observing capabilities continue to improve, the number of known Y dwarfs -- the coldest class of sub-stellar objects, with effective temperatures below about 600 K -- is rapidly growing. Critically, this class of ultra-cool objects has atmospheric conditions that overlap with Solar System worlds and, as a result, tools and ideas developed from studying Earth, Jupiter, Saturn and other nearby worlds are well-suited for application to sub-stellar atmospheres. To that end, we developed a one-dimensional (vertical) atmospheric structure model for ultra-cool objects that includes moist adiabatic convection, as this is an important process for many Solar System planets. Application of this model across a range of effective temperatures (350, 300, 250, 200 K), metallicities ([M/H] of 0.0, 0.5, 0.7, 1.5), and gravities (log $g$ of 4.0, 4.5, 4.7, 5.0) demonstrates strong impacts of water latent heat release on simulated temperature-pressure profiles. At the highest metallicities, water vapor mixing ratios reach an Earth-like 3%, with associated major alterations to the thermal structure in the atmospheric regions where water condenses. Spectroscopic and photometric signatures of metallicity and moist convection should be readily detectable at near- and mid-infrared wavelengths, especially with James Webb Space Telescope observations, and can help indicate the formation history of an object.

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arXiv2021

The Sonora Brown Dwarf Atmosphere and Evolution Models I. Model Description and Application to Cloudless Atmospheres in Rainout Chemical Equilibrium

We present a new generation of substellar atmosphere and evolution models, appropriate for application to studies of L, T, and Y-type brown dwarfs and self-luminous extrasolar planets. The atmosphere models describe the expected temperature-pressure profiles and emergent spectra of atmospheres in radiative-convective equilibrium with effective temperatures and gravities within the ranges $200\le T_{\rm eff}\le2400\,\rm K$ and $2.5\le \log g \le 5.5$. These ranges encompass masses from about 0.5 to 85 Jupiter masses for a set of metallicities ($[{\rm M/H}] = -0.5$ to $+0.5$), C/O ratios (from 0.5 to 1.5 times that of solar), and ages. The evolution tables describe the cooling of these substellar objects through time. These models expand the diversity of model atmospheres currently available, notably to cooler effective temperatures and greater ranges in C/O. Notable improvements from past such models include updated opacities and atmospheric chemistry. Here we describe our modeling approach and present our initial tranche of models for cloudless, chemical equilibrium atmospheres. We compare the modeled spectra, photometry, and evolution to various datasets.

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arXiv2021

Detection and Bulk Properties of the HR 8799 Planets with High Resolution Spectroscopy

Using the Keck Planet Imager and Characterizer (KPIC), we obtained high-resolution (R$\sim$35,000) $K$-band spectra of the four planets orbiting HR 8799. We clearly detected \water{} and CO in the atmospheres of HR 8799 c, d, and e, and tentatively detected a combination of CO and \water{} in b. These are the most challenging directly imaged exoplanets that have been observed at high spectral resolution to date when considering both their angular separations and flux ratios. We developed a forward modeling framework that allows us to jointly fit the spectra of the planets and the diffracted starlight simultaneously in a likelihood-based approach and obtained posterior probabilities on their effective temperatures, surface gravities, radial velocities, and spins. We measured $v\sin(i)$ values of $10.1^{+2.8}_{-2.7}$~km/s for HR 8799 d and $15.0^{+2.3}_{-2.6}$~km/s for HR 8799 e, and placed an upper limit of $< 14$~km/s of HR 8799 c. Under two different assumptions of their obliquities, we found tentative evidence that rotation velocity is anti-correlated with companion mass, which could indicate that magnetic braking with a circumplanetary disk at early times is less efficient at spinning down lower mass planets.

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