arXiv subjects

Sherelyn Alejandro Merchan

Publications and source records attributed to Sherelyn Alejandro Merchan.

12 recordsLinked to original sources

arXiv2026

Inspecting Cloudy Substellar Atmospheres with JWST MIRI Synthetic Magnitudes from Spitzer Mid-infrared Spectra

We examine the positions of substellar objects in mid-infrared color-magnitude and color-color diagrams to distinguish between cloudy and cloud-free atmospheres. Using Spitzer mid-infrared spectra of 113 M5-T9 ultracool dwarfs, we derive synthetic photometry for the JWST MIRI F560W, E'770W, F1000W, and F 1280W filters, which cover key absorption features including the ~9 um silicate signa-ture. We find that diagrams involving F770W and F1000W best separate L-type objects with silicate clouds in their photospheres. L dwarfs with mE77ow - mF1000w < 0.03 mag are seven times more likely to host cloudy atmospheres. Diagrams using F1000W and F1280W are less informative due to the lower signal of the spectra at long wavelengths. Current model predictions struggle to reproduce the positions of cloudy, warm brown dwarfs, likely because atmospheric models underestimate the ~9 um silicate feature. Cloudy Sonora Diamondback models better match the observed trends, although this may reflect improvements capturing indirect effects of clouds on the 6.25 um water absorption feature rather than accurately modeling the silicate feature itself. Our analysis indicates that JWST MIRI photometry can efficiently identify new cloudy extrasolar atmospheres for targeted spectroscopic follow-up, optimizing the use of telescope time.

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arXiv2026

Bridging the Gap: Using Brown Dwarfs to Examine Silicate Clouds in Giant Exoplanet Atmospheres

We present results from examining the silicate cloud modeling of four JWST-observed hot Jupiters in the context of brown dwarf theory to further explore signatures of formation in present-day atmospheres. We contextualize our understanding of protoplanetary disk refractory chemistry with empirical evidence from chondritic meteorites to show that giant planets forming and accreting in the outer disk adopt their stellar Mg/Si value. We show that current silicate cloud species determinations of WASP-17 b, WASP-107 b, WASP-39 b and HD 189733 b are in line with predictions laid out in Calamari et al. 2024 based on each system's host star Mg/Si ratio, further supporting this hypothesis. We discuss physical motivations for potential atmospheric scenarios where apparent silicate cloud species is not in agreement with that predicted by its host star chemistry. Additionally, we compare current transit spectroscopy for three of these four exoplanets against brown dwarf spectra to examine molecular absorption trends across the substellar mass temperature regime.

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arXiv2025

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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arXiv2025

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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arXiv2025

Silicate Precursor Silane detected in Cold Low-Metallicity Brown Dwarf

Within 20 pc of the Sun there are currently 29 known cold brown dwarfs, sources with measured distances and an estimated effective temperature between that of Jupiter (170K) and ~500K. These sources are almost all isolated and are the closest laboratories we have for detailed atmospheric studies of giant planets formed outside the solar system. Here we report JWST observations of one such source, WISEA J153429.75-104303.3 (W1534), which we confirm is a substellar mass member of the Galactic halo with a metallicity <0.01xsolar. Its spectrum reveals methane (CH4), water (H2O), and silane (SiH4) gas. Although SiH4 is expected to serve as a key reservoir for the cloud-forming element Si in gas giant worlds, it eluded detection until now because it is removed from observable atmospheres by the formation of silicate clouds at depth. These condensates are favored with increasing metallicity, explaining why SiH4 remains undetected on well studied, metal-rich solar system worlds like Jupiter and Saturn. On the metal-poor world W1534, we detect a clear signature of SiH4 centered at ~4.55 microns with an abundance of 19+/-2 parts per billion (ppb). Our chemical modelling suggests that this SiH4 abundance may be quenched at ~kilobar levels just above the silicate cloud layers, whereupon vertical atmospheric mixing can transport SiH4 to the observable photosphere. The formation and detection of SiH4 demonstrates key coupled relationships between composition, cloud formation, and atmospheric mixing in cold brown dwarf and planetary atmospheres.

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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 $μ$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.

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arXiv2025

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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arXiv2025

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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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

Retrieving Young Cloudy L-Dwarfs: A Nearby Planetary-Mass Companion BD+60 1417B and Its Isolated Red Twin W0047

We present an atmospheric retrieval analysis on a set of young, cloudy, red L-dwarfs -- CWISER J124332.12+600126.2 and WISEP J004701.06+680352.1 -- using the \textit{Brewster} retrieval framework. We also present the first elemental abundance measurements of the young K-dwarf (K0) host star, BD+60 1417 using high resolution~(R = 50,000) spectra taken with PEPSI/LBT. In the complex cloudy L-dwarf regime the emergence of condensate cloud species complicates retrieval analysis when only near-infrared data is available. We find that for both L dwarfs in this work, despite testing three different thermal profile parameterizations we are unable to constrain reliable abundance measurements and thus the C/O ratio. While we can not conclude what the abundances are, we can conclude that the data strongly favor a cloud model over a cloudless model. We note that the difficulty in retrieval constraints persists regardless of the signal to noise of the data examined (S/N $\sim$ 10 for CWISER J124332.12+600126.2 and~40 for WISEP J004701.06+680352.1). The results presented in this work provide valuable lessons about retrieving young, low-surface gravity, cloudy L-dwarfs. This work provides continued evidence of missing information in models and the crucial need for JWST to guide and inform retrieval analysis in this regime.

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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).

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arXiv2024

High-Precision Atmospheric Constraints for a Cool T Dwarf from JWST Spectroscopy

We present observations of the T8 dwarf 2MASS 0415-0935 with JWST's NIRSpec spectrograph using the G395H grating ($\sim$ 2.87 - 5.14 $μ$m). We perform the first atmospheric retrieval analysis at the maximum spectral resolution of NIRSpec (R$\sim$2700) and combine the spectrum with previous observations to study the 0.9-20 $μ$m spectral energy distribution. We obtain precise constraints on chemical abundances ($\sim$0.02 dex) for a number of species which complicate our understanding of disequilibrium chemistry, particularly for CO$_{2}$ and PH$_{3}$. Furthermore, we measure a $^{12}$CO/$^{13}$CO ratio of $\sim 97^{+9}_{-8}$, making 2MASS 0415-0935 the coldest ($\sim 760$ K) substellar object outside of our solar system with a measured $^{12}$CO/$^{13}$CO ratio. This work shows promise for similar observations with JWST to provide precise abundances of major chemical species as well as isotopologues, allowing for new tests of our understanding of the formation and atmospheres of substellar objects.

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