arXiv subjects

Melanie Rowland

Publications and source records attributed to Melanie Rowland.

7 recordsLinked to original sources

arXiv2026

Benchmark Brown Dwarfs as Chemical Laboratories: Linking System Bulk Properties to Atmospheric Retrievals

We present the first atmospheric retrieval analysis of two compositional benchmark mid-L dwarfs -- SDSSJ141659.78+500626.4 and GJ 499 C -- using the \textit{Brewster} retrieval framework, where the wide benchmark nature of these systems provides independent constraints on age and bulk composition from their stellar primaries. These targets were observed with JWST using NIRSpec Prism and MIRI LRS, providing low-resolution ($R$ $\sim$ 100) spectra between 0.6--14.0 $\mathrm{\mu}$m with high signal-to-noise ratios (SNR $\sim$100 -- 600). For SDSSJ141659.78+500626.4, the retrieved cloud combination of a high-altitude enstatite slab and low-altitude iron deck clouds matches phase-equilibrium predictions based on the primary star's Mg/Si ratio. We retrieve a super-solar C/O = 0.71$^{+0.01}_{-0.01}$ and slightly metal-rich [M/H] = 0.22$^{+0.03}_{-0.03}$. This C/O ratio can only be reconciled with the value inferred for the primary if additional oxygen sequestration beyond the retrieved cloud mass is present, or if there are uncertainties in the adopted opacities or other model deficiencies. The inferred [C/H] and [O/H] are 0.31$\pm$0.03 and 0.19$\pm$0.03, respectively, which are consistent within the relatively large uncertainties of the host star abundances. For GJ 499 C, the retrieved silicon-monoxide and forsterite slab clouds are difficult to explain with simple phase-equilibrium assumptions, yielding Mg/Si $\sim$ 1.9. We estimate C/O = 0.69$^{+0.02}_{-0.02}$ and [M/H] = 0.13$^{+0.04}_{-0.06}$. For both objects, the inferred radii of 0.85$^{+0.01}_{-0.01}$ $R_{\mathrm{Jup}}$ and 1.00$^{+0.02}_{-0.02}$ $R_{\mathrm{Jup}}$ and masses of 73.7$^{+4.9}_{-9.2}$ $M_{\mathrm{Jup}}$ and 68.0$^{+8.5}_{-12.7}$ $M_{\mathrm{Jup}}$ are consistent with evolutionary models and system ages, highlighting the plausibility of our results.

astro-ph.EP↗

arXiv2026

Connecting JWST Silicate Cloud Observations to Exoplanet Cloud Microphysics with Nimbus

The unprecedented accuracy of JWST has led to the detection of silicate clouds in exoplanet atmospheres, allowing for the first time to probe cloud formation in extreme environments. While parametrized cloud descriptions can fit these observations, the results do not fully agree with microphysical models. To bridge this gap, we developed Nimbus, a fast microphysical cloud model that can constrain cloud formation processes from observations and utilize Virga, an equilibrium condensation model balancing gravitational settling and diffusion. Using both models, we investigate WASP-107 b, WASP-17 b, VHS-1256 b, and YSES-1 c to determine their cloud structure and constrain cloud formation processes. Our results show that all four planets have cluster-sized silicate particles (r ~ 1 nm) at high altitudes. Within Nimbus and Virga, these particles can only be explained by highly inefficient cloud particle settling (fsed < 0.1) or by inefficient growth rates due to low sticking coefficients (s < 0.0001). Our results also show that the sticking coefficient is directly linked to the vertical extent of clouds and can therefore be constrained using the broad shape of the spectral energy distribution. The sticking coefficients found for VHS-1256 b and YSES-1 c are in agreement with expectations from laboratory experiments under Earth-like conditions (0.01 < s < 0.3). Panchromatic observations were crucial to achieve these constraints. Future cloud studies should therefore aim to combine observational data from 1 micron to 10 micron whenever possible.

astro-ph.EP↗

arXiv2025

Discovery of the Second Y+Y Dwarf Binary System: CWISEP J193518.59-154620.3

We present the discovery of a companion to the Y-dwarf, CWISEP J193518.59-154620.3, the second Y-Y dwarf binary detected to date. Y-dwarfs are the coldest known free-floating objects ($<$ 500 K) and on average represent the lowest mass objects directly formed through turbulent fragmentation of a molecular cloud. Studying their multiplicity allows us to place strong constraints on the ability to form multiple systems of planetary masses and approaching the opacity limit of fragmentation. Due to their physical properties, Y-dwarfs also serve as analogs to gas giant planets. CWISEP J193518.59-154620.3 has been shown to have a unique methane emission feature in its near infrared spectrum at 3.326 $\mu$m, potentially indicative of auroral processes without a clear origin. CWISEP J193518.59-154620.3 was observed with JWST's MIRI in the F1000W, F1280W, and F1800W filters. We applied a point-spread function (PSF) fitting algorithm using empirically derived PSF models and resolve a companion in the F1000W and F1280W filters separated by 172 milli-arcseconds, 2.48 au assuming the distance of 14.43 pc. Using the ATMO2020 evolutionary models, we estimate a mass of 12-39 $M_{\rm Jup}$ for the primary and 7-24 $M_{\rm Jup}$ for the companion assuming an age of 1-10 Gyr for a mass ratio of 0.55-0.62, resulting in an estimated period of 16-28 years. It is unknown which component of this binary exhibits the methane emission feature. We also resolve known companions WISE J014656.66+423410.0B and WISE J171104.60+350036.8B using MIRI data and present their F1000W and F1280W photometry.

astro-ph.EP↗

arXiv2025

Silicate clouds and a circumplanetary disk in the YSES-1 exoplanet system

Young exoplanets provide a critical link between understanding planet formation and atmospheric evolution. Direct imaging spectroscopy allows us to infer the properties of young, wide orbit, giant planets with high signal-to-noise. This allows us to compare this young population to exoplanets characterized with transmission spectroscopy, which has indirectly revealed the presence of clouds, photochemistry, and a diversity of atmospheric compositions. Direct detections have also been made for brown dwarfs, but direct studies of young giant planets in the mid-infrared were not possible prior to JWST. With two exoplanets around a solar type star, the YSES-1 system is an ideal laboratory for studying this early phase of exoplanet evolution. We report the first direct observations of silicate clouds in the atmosphere of the exoplanet YSES-1 c through its 9-11 micron absorption feature, and the first circumplanetary disk silicate emission around its sibling planet, YSES-1 b. The clouds of YSES-1 c are composed of either amorphous iron-enriched pyroxene or a combination of amorphous MgSiO3 and Mg2SiO4, with particle sizes of less than or equal to 0.1 micron at 1 millibar of pressure. We attribute the emission from the disk around YSES-1 b to be from submicron olivine dust grains, which may have formed through collisions of planet-forming bodies in the disk.

astro-ph.EP↗

arXiv2024

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).

astro-ph.SR↗

arXiv2023

The Complete CEERS Early Universe Galaxy Sample: A Surprisingly Slow Evolution of the Space Density of Bright Galaxies at z ~ 8.5-14.5

We present a sample of 88 candidate z~8.5-14.5 galaxies selected from the completed NIRCam imaging from the Cosmic Evolution Early Release Science (CEERS) survey. These data cover ~90 arcmin^2 (10 NIRCam pointings) in six broad-band and one medium-band imaging filter. With this sample we confirm at higher confidence early JWST conclusions that bright galaxies in this epoch are more abundant than predicted by most theoretical models. We construct the rest-frame ultraviolet luminosity functions at z~9, 11 and 14, and show that the space density of bright (M_UV=-20) galaxies changes only modestly from z~14 to z~9, compared to a steeper increase from z~8 to z~4. While our candidates are photometrically selected, spectroscopic followup has now confirmed 13 of them, with only one significant interloper, implying that the fidelity of this sample is high. Successfully explaining the evidence for a flatter evolution in the number densities of UV-bright z>10 galaxies may thus require changes to the dominant physical processes regulating star formation. While our results indicate that significant variations of dust attenuation with redshift are unlikely to be the dominant factor at these high redshifts, they are consistent with predictions from models which naturally have enhanced star-formation efficiency and/or stochasticity. An evolving stellar initial mass function could also bring model predictions into better agreement with our results. Deep spectroscopic followup of a large sample of early galaxies can distinguish between these competing scenarios.

astro-ph.GA↗

arXiv2023

Towards Robust Atmospheric Retrieval on Cloudy L Dwarfs: The Impact of Thermal and Abundance Profile Assumptions

Constraining L dwarf properties from their spectra is challenging. Near-infrared spectra probe a limited range of pressures, while many species condense within their photospheres. Condensation creates two complexities: gas-phase species "rain out" (decreasing in abundances by many orders of magnitude) and clouds form. We designed tests using synthetic data to determine the best approach for retrieving L dwarf spectra, isolating the challenges in the absence of cloud opacity. We conducted atmospheric retrievals on synthetic cloud-free L dwarf spectra derived from the Sonora Bobcat models at SpeX resolution using a variety of thermal and chemical abundance profile parameterizations. For objects hotter than L5 (T$_{eff}$ ~ 1700 K), the limited pressure layers probed in the near-IR are mostly convective; parameterized PT profiles bias results and free, unsmoothed profiles should be used. Only when many layers both above and below the radiative-convective boundary are probed can parameterized profiles provide accurate results. Furthermore, a nonuniform abundance profile for iron hydride (FeH) is needed to accurately retrieve bulk properties of early- to mid- L dwarfs. Nonuniform prescriptions for other gases in near-IR retrievals may also be warranted near the L/T transition (CH$_{4}$) and early Y dwarfs (Na and K). We demonstrate the utility of using realistic self-consistent models to benchmark retrievals and suggest how they can be used in the future.

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