arXiv subjects

Aaron M. Meisner

Publications and source records attributed to Aaron M. Meisner.

At least 19 recordsLinked to original sources

arXiv2026

JWST Observations of the Metal-poor T Dwarf WISEA J155349.96+693355.2: The First Brown Dwarf Associated with the Gaia-Enceladus Milky Way Substructure

We present JWST NIRSpec and MIRI observations of the metal-poor T dwarf WISEA J155349.96+693355.2. The combined NIRSpec/prism plus MIRI/LRS spectrum (R ~ 100) provides 0.6-12 $μ$m spectroscopic coverage that reveals the presence of H$_2$O, CH$_4$, and NH$_3$ absorption features, enhanced collision-induced H$_2$ absorption, and weak or absent CO and CO$_2$ absorption, all indicators of a metal-poor low-temperature atmosphere. Comparison of the spectral data to a suite of atmosphere models yields an effective temperature $T_{\rm eff}$ = 960$^{+28}_{-32}$ K and a solar-scaled metallicity of [M/H] = -1.16$^{+0.28}_{-0.25}$ dex. We find indications that the uncertain trigonometric parallax, likely subject to Lutz-Kelker bias, may be overestimated. Adopting the photometric distance yields a bolometric luminosity log$_{10}L_{bol}/L_{\odot}$ = -5.29 $\pm$ 0.10, from which we infer $T_{\rm eff}$ = 974 $\pm$ 56 K, consistent with the temperature derived via our atmospheric model fits. From the moderate-resolution (R ~ 3,000) NIRSpec/G395H spectrum, we measure a radial velocity of -163 $\pm$ 5 km/s, which combined with the source's measured proper motion and estimated distance indicates a highly eccentric (e $\geq$ 0.9) Galactic orbit consistent with an accreted halo object. The UVW kinematics of WISEA J155349.96+693355.2, its bulk metallicity, and its modest alpha enrichment ([$α$/Fe] = +0.15$^{+0.04}_{-0.06}$ dex) are all equivalent to stars identified in the Gaia-Enceladus Milky Way substructure, making it the first brown dwarf with kinematic and chemical evidence of association.

astro-ph.SR↗

arXiv2026

Metal-poor Brown Dwarf Kinematics from JWST NIRSpec Spectroscopy

Galactic archaeology relies on stellar kinematics and chemical abundances to identify various stellar populations and associations. With JWST, Galactic archaeology of ancient metal-poor brown dwarfs is now possible through high-sensitivity, medium-resolution spectroscopy. We present the first metal-poor brown dwarf radial velocity (RV) survey using JWST/NIRSpec G395H spectra ($λ/Δλ\sim$ 3,000) for a sample of 23 LTY dwarfs across a wide range of temperatures and metallicities. We introduce a forward-modeling framework to measure robust RVs for brown dwarfs by focusing on the CO fundamental band at $\sim$4.5 $μ$m. By comparing to sources with existing high-resolution RV measurements, we demonstrate that G395H RVs can achieve a systematic uncertainty of 5 km s$^{-1}$, sufficient to examine their Galactic space motions. We identify kinematic members of the Milky Way's thin disk, thick disk, and halo populations in this sample. We also confirm WISE J155349.98$+$693355.2 as likely associated with the Gaia-Enceladus merger remnant, and 2MASS J05325346$+$8246465 is likely associated with the Thamnos stream. Our study systematically demonstrates that JWST G395H spectroscopy can provide robust RVs in addition to detailed chemical abundances, necessary for Galactic archaeology studies. Future discoveries and characterization of brown dwarfs associated with Galactic halo substructures and streams will enable a stringent test of stellar/substellar formation and evolutionary models for diverse metallicities.

astro-ph.SR↗

arXiv2026

SMDET-1: a Nearby Y Dwarf Candidate

We present the discovery of SMDET-1, a red, fast-moving object ($μ\approx 1.3$"/yr) identified in time-resolved unWISE coadds using a pixel-level deep learning methodology called SMDET. Despite being relatively bright at 4.5 microns compared to many other recent WISE-based brown dwarf discoveries ($m_{[4.5]} \approx 14.6$ mag Vega), SMDET-1 had remained overlooked due to its location in a very crowded Galactic plane field ($b \approx 2.25^{\circ}$) and contamination from brighter background objects. SMDET-1 is also serendipitously detected at 4.5 microns in late-2012 Spitzer Deep GLIMPSE survey imaging. SMDET-1 is undetected in UKIDSS and Palomar/WIRC near-infrared imaging, with the strongest constraint on its temperature ($T_{\rm eff}$ < 391 K) arising from its Deep GLIMPSE color limit of $m_{[3.6]} - m_{[4.5]} > 2.81$ mag, which also implies a very nearby photometric distance < 7.4 pc. The Spitzer color bound corresponds to a Y dwarf phototype. SMDET-1 illustrates the importance of continued searches for nearby brown dwarfs within archival datasets like WISE and Spitzer, as well as the potential of pixel-level deep learning to discover astronomical moving objects that challenge traditional data analysis approaches.

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arXiv2026

Development of a Retrieval-Augmented Generation Virtual Assistant for Enhanced Information Discovery at Rubin Observatory

The NSF-DOE Vera C. Rubin Observatory will generate petabytes of data through the Legacy Survey of Space and Time (LSST) over the next decade, enabling discoveries across a broad range of astrophysical fields. Alongside these data products, Rubin maintains a large but heterogeneous collection of supporting documentation, including operational guides, technical notes, and scientific papers. Because this material is distributed across multiple platforms and formats, staff and scientists often struggle to efficiently locate accurate, up-to-date information. Many resources also reside on internal systems, limiting the ability of general-purpose language models to provide reliable answers to Rubin-specific questions. To address these challenges, we explore the use of Retrieval Augmented Generation (RAG) to improve information discovery. We present a prototype RAG-based virtual assistant that delivers context-aware, factual, conversational access to Rubin's vast and heterogenous documentation ecosystem. The system integrates material from multiple sources and enables semantic search through a conversational interface, using Weaviate for embeddings, LangChain for query orchestration, and an OpenAI GPT model as the LLM backend. By grounding responses in domain-specific knowledge, the assistant reduces hallucinations, improves accuracy, and demonstrates the potential of RAG to enhance access to distributed knowledge, streamline workflows, and support effective use of LSST data products.

astro-ph.IM↗

arXiv2026

Disentangling the Distant Stellar Halo Using K-Giants in the DESI Year 3 Data

We present a sample of 88,959 K-giants from DESI Milky Way Survey Year 3 data, which we use to characterize the chemo-dynamical properties of the stellar halo at Galactocentric distances of 12 to ~100 kpc. Using HDBSCAN, we identify five prominent stellar halo substructures: Aleph, the Sagittarius stream, Gaia Sausage-Enceladus (GSE), Cetus-Palca and the Orphan-Chenab stream. We present the properties of each of these structures as they appear in our catalog, and examine how uncertainties on distance affect the characterization of substructure with this approach. We also examine regions associated with previously reported overdensities (such as the Virgo Overdensities and the Sagittarius spur) that we do not recover with HDBSCAN. The size and distance range of our catalog allows us to explore in detail the residual stellar halo, comprising stars that we do not associate with any substructure. We find that samples of ~2000 outer halo stars with both highly prograde and highly retrograde angular momenta have similar metallicity distribution functions (MDFs), which do not resemble the MDFs of either GSE or Sagittarius. Both the prograde and retrograde residual halo MDFs are bimodal, with a metal-poor peak at [Fe/H] ~ -2 and a metal-rich peak at [Fe/H] ~ -1.3 (prograde) or -1.5 (retrograde). The MDF for lower angular momentum residual halo K-giants does not show clear evidence for a metal-poor peak, and broadly resembles the MDF of GSE, even at much lower binding energies than GSE itself. We discuss possible interpretations of these findings for GSE accretion scenarios.

astro-ph.GA↗

arXiv2026

A SPHEREx Pipeline and Spectral Library for Ultracool Dwarfs

We present a Python spectrophotometry extraction tool tailored for fast-moving point sources detected in the SPHEREx mission, and use it to construct a set of 0.75-5.0 $μ$m low-resolution ($λ/Δλ\sim 50$) spectrophotometry data products based on the SPHEREx Quick Release 2 (QR2) for a set of 6003 L0-Y1 ultracool dwarfs: 2050 known ultracool dwarfs, 3008 known photometric ultracool dwarf candidates, and 947 newly identified ultracool dwarfs. This work more than doubles the number of ultracool dwarfs with spectroscopy, from 3449 to 7402. We provide SPHEREx templates for each spectral subtype and a set of tools to assign automated spectral types. The QR2 data release generates spectrophotometry with an average signal-to-noise per spectral channel above $\sim$10 for most objects with WISE W2 magnitudes of 14.0 mag and brighter. The compiled data set is made available publicly at https://mocadb.ca, where new spectral compilations from future data releases will also be made available as they are published. These new data provide a significant increase in the number of substellar objects for which the 2.4-5.0 $μ$m window is now accessible, making it possible to probe important molecular chemistry of key CNOS-bearing species for the coolest brown dwarfs. We flag 2668 ultracool dwarfs as candidate young brown dwarfs, 250 as candidate subdwarfs, and 865 as possibly otherwise peculiar for future investigation. The SPIFF library presented here opens the doors to efficient confirmation of candidate substellar objects and follow-up studies of population-level atmospheric properties of cold brown dwarfs.

astro-ph.SR↗

arXiv2026

Joint Multiband Photometry with crowdsource

We present a new multiband extension to the crowdsource photometric pipeline, enabling simultaneous fitting across multiple imaging bands in crowded fields. The core idea is that multiple images of the same part of the sky should have the same sources at the same locations; only the fluxes in the different images should be allowed to vary in fitting. The framework also allows us to use all images of a given region to detect faint sources, with configurable weighting among the different bandpasses as appropriate for different source spectra. Similar concepts are already present in other crowded field packages like DAOPHOT and DOLPHOT; we now include it in the crowdsource fitting approach. We describe the mathematical formulation of the multiband fit and demonstrate its performance using the Wide-field Infrared Survey Explorer (WISE) W1 and W2 imaging as a concrete application. The multiband algorithm improves flux consistency and reduces band-to-band positional scatter relative to independent-band fitting. We test the method on unWISE coadded tiles spanning both sparse and crowded regions and quantify improvements in photometric agreement and astrometric stability. This framework provides a general foundation for future multiband crowded-field catalogs.

astro-ph.IM↗

arXiv2026

Spectroscopic Characterization of WD J000801.25-350450 and its Two Co-Moving Companions

We present new spectroscopic data for Gaia DR3 2309499817384726016 (WD0008-350A) and its two wide, co-moving, low-mass companions. We confirm the white dwarf is a hydrogen-rich DA, with T$_{\rm eff}$=6200$\pm$90~K and a mass of 0.63$\pm$0.03~M${\odot}$, close to that of the average white dwarf. Near-infrared spectra of the two stellar companions to WD0008-350A reveal that the inner companion is an M dwarf, exhibiting a spectral type of M8. Furthermore, the outer companion is identified as a possible M6 + M9 binary. This paper examines the evidence which suggests the system may be quadruple.

astro-ph.SR↗

arXiv2026

Three Thousand Motion-Confirmed L and T Dwarf Candidates from the Backyard Worlds:~Planet 9 Citizen Science Project

The Backyard Worlds: Planet 9 citizen science project uses data from the Wide-field Infrared Survey Explorer to detect infrared objects with significant motion. In this work, we present the majority of the L and T dwarf candidates discovered through this effort. For each candidate, we provide proper motion measurements as well as optical, near-infrared, and mid-infrared photometry (when available), photometric spectral types and distance estimates. Three thousand and six new motion-confirmed discoveries are presented in this work, 2,357 with L-type photometric spectral types and 649 with T-type photometric spectral types. We also present an additional 80 objects as likely L or T dwarfs based on available photometry, but for which a significant motion measurement could not be obtained. We identify 28 objects in this sample as new comoving companions to higher-mass stars, and an additional 9 sources that are candidate binary systems made up of two ultracool dwarfs of L-type or later. Follow-up spectroscopic observations will be necessary to confirm spectral types and further characterize the sources discovered through this project. This work presents the largest single sample of motion-confirmed L and T dwarf discoveries to date, which would more than double the number of known L and T dwarfs, if confirmed. We wish to sincerely thank our citizen scientist collaborators for their monumental efforts that have directly impacted this project's success.

astro-ph.SR↗

arXiv2026

Spitzer + HST parallaxes of 13 late T and Y dwarfs

We present astrometric measurements for 13 cold brown dwarfs in the solar neighborhood (d < 20pc). By combining archival Spitzer data with our own Hubble Space Telescope (HST) observations, we achieve parallax uncertainties typically around 10%. Using Spitzer and HST photometry we compare our targets with other known late T and Y dwarfs in the Solar neighborhood, confirming that there is large intrinsic scatter in the near- and mid-infrared absolute magnitudes and colors of this population, further highlighting the diversity observed spectroscopically by several James Webb Space Telescope (JWST) programs. This scatter makes photometric distance estimates highly unreliable and, therefore, makes astrometric parallax measurements fundamental for a meaningful characterization of even the nearest cold brown dwarfs.

astro-ph.SR↗

arXiv2026

Disappearance of a massive star in the Andromeda Galaxy due to formation of a black hole

When a massive star reaches the end of its lifetime, its core collapses and releases neutrinos that drive a shock into the outer layers (stellar envelope). A sufficiently strong shock ejects the envelope, producing a supernova. If the shock fails to eject it, the envelope is predicted to fall back onto the collapsing core, producing a stellar-mass black hole (BH) and causing the star to disappear. We report observations of M31-2014-DS1, a hydrogen-depleted supergiant in the Andromeda Galaxy. In 2014 it brightened in the mid-infrared. From 2017 to 2022 it faded by factors of $\gtrsim10^4$ in optical light, becoming undetectable, and $\gtrsim10$ in total light. We interpret these observations, and those of a previous event in NGC 6946, as evidence for failed supernovae forming stellar-mass BHs.

astro-ph.HE↗

arXiv2025

A search for late-type brown dwarfs in the Euclid Quick Data Release 1

We present the identification and characterization of 15 mid-to-late T dwarf candidates in the Euclid Quick Release 1 (Q1) dataset, based on a combined photometric and spectroscopic analysis. Candidates were initially selected via color-based cuts in the Euclid $Y_E - J_E$ and $J_E - H_E$ color-color space, targeting the region occupied by ultracool dwarfs in synthetic photometry from the Sanghi et al. (2024) sample. From an initial pool of 38,845 sources, we extracted low-resolution near-infrared spectra from the Euclid NISP instrument and applied a two-stage validation procedure that included spectral template fitting followed by visual inspection. Eight of the 15 validated candidates are newly identified objects with no prior literature association. We examined their morphological and photometric properties and compared them with established spectral standards. Photometric distances were derived using band-averaged distance modulus estimates. We discuss the limitations and promise of the Euclid survey for ultracool dwarf studies, and demonstrate the potential for discovering substellar populations beyond the reach of current wide-field surveys.

astro-ph.SR↗

arXiv2025

ODIN: The LAE Lyα Luminosity Function over Cosmic Time and Environmental Density

The ubiquity and relative ease of discovery make $2\lesssim z\lesssim 5$ Ly$α$ emitting galaxies (LAEs) ideal tracers for cosmology. In addition, because Ly$α$ is a resonance line, but frequently observed at large equivalent width, it is potentially a probe of galaxy evolution. The LAE Ly$α$ luminosity function (LF) is an essential measurement for making progress on both of these aspects. Although several studies have computed the LAE LF, very few have delved into how the function varies with environment. The large area and depth of the One-hundred-deg$^2$ DECam Imaging in Narrowbands (ODIN) survey makes such measurements possible at the cosmic noon redshifts of z~2.4, ~3.1, and ~4.5. In this initial work, we present algorithms to rigorously compute the LAE LF and test our methods on the ~16,000 ODIN LAEs found in the extended COSMOS field. Using these limited samples, we find slight evidence that protocluster environments either suppress the numbers of very faint and very bright LAEs or enhance medium-bright LAEs in comparison to the field. We also find that the LF decreases in number density and evolves towards a steeper faint-end slope over cosmic time from z~4.5 to z~2.4.

astro-ph.GA↗

arXiv2025

High Proper Motion Discoveries from the UKIRT Hemisphere Survey

We used the third data release of the UKIRT Hemisphere Survey to locate previously unrecognized high proper motion objects. We identify a total of 127 new discoveries with total proper motions $\gtrsim$300 mas yr$^{-1}$. A significant fraction of these sources with counterparts in the Gaia DR3 catalog are found to be distant ($>$100 pc) low-mass stars, where their large tangential velocities and placement on color-magnitude diagrams suggest that they are likely low-metallicity M-type subdwarfs. Optical spectroscopy is used to confirm the low-mass and low-metallicity for two such sources. Using available optical and infrared photometry, we estimate the spectral type for all non-Gaia sources and find 10 likely late-M dwarfs, 15 objects with colors most consistent with L-type dwarfs, and 9 possible T-type dwarfs. Follow-up spectroscopy is needed to confirm spectral types and further characterize these new discoveries.

astro-ph.SR↗

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.

astro-ph.EP↗

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.

astro-ph.SR↗

arXiv2025

Unveiling the Infrared Excess of SIPS J2045-6332: Evidence for a Young Stellar Object with Potential Low-Mass Companion

The Disk Detective project, a citizen science initiative, aims to identify circumstellar discs around stars by detecting objects with infrared (IR) excess using data from the Wide-field Infrared Survey Explorer (WISE). In this study, we investigate SIPS J2045-6332, a potential brown dwarf with significant IR excess in WISE and 2MASS bands, initially identified by project volunteers. Despite early indicators of a circumstellar disc, discrepancies between observed brightness and expected Spectral Energy Distribution (SED) models suggested unusual properties. To explore potential explanations, we created SED templates for spectral types M9 to L4 and compared them with SIPS J2045-6332's photometric data, revealing an excess brightness that points to either an unresolved low-mass companion or a young, inflated primary star. Further analysis of infrared spectral features and surface gravity indicators supports a youthful classification, estimating the object's age at 26-200 million years. Observations also suggest the presence of a mid L-type companion at a projected distance of 6.7 AU. This study highlights SIPS J2045-6332 as an intriguing system with unique IR characteristics and recommends follow-up observations with high-resolution telescopes to confirm the companion hypothesis and further characterize the system.

astro-ph.SR↗

arXiv2024

New Ultracool Companions to Nearby White Dwarfs

We conducted a search for new ultracool companions to nearby white dwarfs using multiple methods, including the analysis of colors and examination of images in both the optical and the infrared. Through this process, we identified fifty-one previously unrecognized systems with candidate ultracool companions. Thirty-one of these systems are resolved in at least one catalog, and all but six are confirmed as co-moving companions via common proper motion and consistent parallax measurements (when available). We have followed up four co-moving companions with near-infrared spectroscopy and confirm their ultracool nature. The remaining twenty candidates are unresolved, but show clear signs of infrared excess which is most likely due to the presence of a cold, low-mass companion or a dusty circumstellar disk. Three of these unresolved systems have existing optical spectra that clearly show the presence of a cool stellar companion to the white dwarf primary via spectral decomposition. These new discoveries, along with our age estimates for the primary white dwarfs, will serve as valuable benchmark systems for future characterization of ultracool dwarfs.

astro-ph.SR↗