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The Milky Way's oldest and most metal-poor brown dwarfs provide a unique and powerful way to study the dynamical and chemical evolution of our Galaxy and its environment. The Cool Star Lab is exploring ways of finding, characterizing, and analyzing samples of distant brown dwarfs through Substellar Galactic Archaeology.
The Milky Way's oldest and most metal-poor brown dwarfs provide a unique and powerful way to study the dynamical and chemical evolution of our Galaxy and its environment. The Cool Star Lab is exploring ways of finding, characterizing, and analyzing samples of distant brown dwarfs through Substellar Galactic Archaeology.
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Brown dwarfs are suited to this task because they don't fuse (preserving their natal chemical composition), are fully convective (observed atmosphere composition ≈ bulk composition), cool over time (a natural clock), and never die! However, the key challenges in this area of research are:
Detecting intrinsically cold and distant brown dwarfs: ≈10 billion-year-old brown dwarfs in the Milky Way's thick disk and halo populations have generally cooled off to low temperatures (T ≤ 1200 K). Moreover, while these sources can be found as rare sources near the Sun (≈ 0.5% of local stars), their are far more common at kiloparsec distances away from the Galactic plane. Fortunately, the sensitivity of HST. JWST, Euclid, and soon the Nancy Grace Roman Space Telescope allows us to identify and measure the spectra of ancient brown dwarfs at these large distances, and The Cool Star Lab is leading work with deep spectroscopic surveys such as UNCOVER, RUBIES, POPPIES, NEXUS, and others.
Inferring physical properties from photometric and spectral data: Atmosphere models for brown dwarfs have advanced considerably since their discovery, but the rarity of ancient, metal-poor brown dwarfs means such models are rarely computed or tested. Our team is addressing this issue by computing new forward model grids for subsolar metallicities and non-solar abundances; developing and applying new model-fitting approaches through retrieval methods, machine learning, and Monte Carlo techniques; and testing these on diverse populations of brown dwarfs, including metal-poor sources and age/metallicity benchmarks.
Placing populations of brown dwarfs in a Galactic context: While the evolution of brown dwarfs make them ideal for historical studies, it also muddles our ability to interpret the present-day observables of these sources due to the age-mass degeneracies. Our team has developed Galactic-scale simulations that incorporates metallicity-dependent brown dwarf evolution, formation histories, initial mass functions, multiplicity, kinematics, and spatial distributions to interpret both local and deep surveys of brown dwarfs, with the aim of extracting cluster- or population-scale properties.
Current Projects
Current Projects
JWST Arcana of the Ancients
JWST Arcana of the Ancients
This Cycle 3 JWST program led by Cool Star Lab PI Adam Burgasser is observing 32 L and T dwarfs of varying metallicities with NIRSpec and MIRI spectroscopy to analyze the spectral features that drive spectral shape and atmospheric chemistry in cold brown dwarfs.
Deep JWST Surveys
Deep JWST Surveys
Cool Star Lab members use deep JWST/NIRSpec spectra from programs such as UNCOVER, NEXUS, POPPIES, RUBIES, and others to identify distant brown dwarfs as "contaminants" of extragalactic surveys.
The Lowest-Mass Members of Globular Clusters
The Lowest-Mass Members of Globular Clusters
Globular clusters are massive conglomerations of gravitationally bound stars with a common age and (near) common composition, systems that are ideal for testing metal-poor brown dwarf models. Former Cool Star Lab graduate student Dr. Roman Gerasimov has led work in this area, generating metal-specific atmosphere and evolutionary grids and identifying the first globular cluster brown dwarfs.
Recent Findings
Recent Findings
Distant Brown Dwarfs Discovered as JWST RUBIES
Distant Brown Dwarfs Discovered as JWST RUBIES
(November 2025) Recent UCSD graduate Sara Morrissey has reported the findings of her Honors thesis, the discovery of 7 distant L and T dwarfs in deep JWST spectroscopy by the RUBIES survey. Sara used the 1-5 µm spectra to classify her discoveries and determine their temperatures, metallicities, and distances, the last reaching out to 3,000 pc from the Sun. Two of the sources show evidence of being metal-poor brown dwarfs. Congratulations on your first peer-reviewed article Sara! (see the preprint by Morrissey et al.)
Phosphine Appears in a Brown Dwarf
Phosphine Appears in a Brown Dwarf
(October 2025) Cool Star Lab PI Adam Burgasser reported the detection of abundant phosphine in the atmosphere of a cold brown dwarf named Wolf 1130C, based on JWST observations obtained as part of the Arcana of the Ancients program. The detection of phosphine, measured by co-author Dr. Eileen Gonzales to at the 100 ppb level, reverses the pattern of "missing phosphine" in other brown dwarf and exoplanet atmospheres, and raises new questions about our understanding of phosphorous chemistry. The result was the subject of a press release and reported widely in the New York Times, the Conversation, and other venues (see the UCSD press release, the published Science article, and the preprint by Burgasser et al.)
Silane Gas Appears in "The Accident"
Silane Gas Appears in "The Accident"
(September 2025) Cool Star Lab researchers have contributed to the first detection of silane (SiH4) in an extrasolar world with JWST. The world is named WISEA J153429.75-104303.3, or more colloquially "The Accident", and is a very metal-poor and low-temperature brown dwarf that is likely the lowest-mass member of the Galactic halo currently known. The detection of silane gas provides a unique window into silicon chemistry in low-temperature atmospheres, in particular the formation silicate condensates present in warmer L-type brown dwarfs. The study was led by Dr. Jackie Faherty, an alumna of the Cool Star Lab (see the Nature article by Faherty et al.)
NEXUS JWST Treasury Survey Releases First Dataset
NEXUS JWST Treasury Survey Releases First Dataset
(November 2024) The North ecliptic pole EXtragalactic Unified Survey (NEXUS) project, a Multi-Cycle JWST Treasuary program, released its first set of data, including NIRCam images and wide-field slitless spectroscopy over a 100 square arcminute area near the North Ecliptic Pole. This area aligns with the Euclid North Deep Field, promising multi-epoch deep imaging and spectroscopy over the next 4 years. The final data will encompass deep, multi-epoch NIRCam imaging and NIRSpec PRISM spectroscopy over 400 square arcminutes down to imaging depths of 28-29 mag in 6 infrared filters. In addition to thousands of high redshift galaxies, this survey is expected to uncover dozens of brown dwarfs at kpc scales (read the preprint by Zuang et al. and access the data at https://ariel.astro.illinois.edu/nexus/edr/).
A Comprehensive Study of Metal-poor T Subdwarfs
A Comprehensive Study of Metal-poor T Subdwarfs
(November 2024) CSL Director Adam Burgasser and members of the Backyard Worlds: Planet 9 team have conducted a comprehensive study of metal-poor T dwarfs, including sources discovered by citizen scientists from multi-epoch WISE data. Selecting sources based on reduced proper motion, the team identified dozens of metal-poor objects, including three "extreme" cases. They also identified three metal-rich sources with thick disk kinematics, likely ejected from the inner Milky Way. 3D kinematics enabled by Keck/NIRES observations reveal that two sources may be part of the Thamnos population, and one source part of the Helmi stream. They study also made the first metallicity classification system for T (sub)dwarfs, and defined a metallicity index for near-infrared spectra. This work helps ongoing studies that are searching for thick-disk and halo brown dwarfs in deep JWST and Euclid fields (read the preprint by Burgasser et al.)
The Faintest Members of Three Globular Clusters
The Faintest Members of Three Globular Clusters
(November 2024) Two new studies have probed the lowest-luminosity sources in three ancient globular clusters. Libralato et al. (2024) combined existing HST and new JWST observations of NGC 6121 and NGC 6397 to find that the lowest-mass members are more metal-rich and oxygen-poor than higher-mass stars, drawing on new evolutionary and atmosphere models generated by former CSL graduate student Roman Gerasimov. Scalco et al. (2024) explored the entirety of the white dwarf cooling sequence in Omega Centauri down to V ≈ 31 mag, finding the distribution is consistent with either a single-age population or one spread out over 5 billion years. Both studies capitalize on the incredible sensitivity, resolution, and multi-epoch astrometry from the two space telescopes (read the articles by Libralato et al. and Scalco et al. in Astronomy & Astrophysics).
A Speedy L Subdwarf
A Speedy L Subdwarf
(July 2024) Adam Burgasser led a study on a remarkably fast-moving, metal-poor L dwarf uncovered by citizen scientists associated with the Backyard Worlds: Planet 9 program. The source, CWISE J1249+3621, has speed of 456± 27 km/s in the Milky Way rest frame, placing it near the local Galactic escape velocity. The research team considered several possible origins for the source, including ejection from the center of the Milky Way or globular clusters after interaction with black holes, escape from an exploding Type Ia supernova, and infall from a Milky Way satellite. The result was highlighted in a press conference at AAS 244 (read the article by Burgasser et al. in Astrophysical Journal Letters)
Globular Cluster Work Makes the Cover
Globular Cluster Work Makes the Cover
(May 2024) Research led by Cool Star alum Roman Gerasimov was featured on the cover of the May 2024 edition of Astronomische Nachrichten! The cover shows the remarkable color-magnitude diagram of stars and white dwarfs in the globular cluster NGC 6752, based on HST data, and appears in Scalco et al. (2024). Roman computed the stellar models that accurately encompass the low mass stars in the upper right portion of the figure.
Three JWST studies of the globular cluster NGC 6397
Three JWST studies of the globular cluster NGC 6397
(May 2024) Three new studies of the globular cluster NGC 6397 conducted with JWST have been released, including one led by former CLS graduate student Roman Gerasimov. Roman demonstrated the robust detection of brown dwarfs in this ancient stellar systems, and his new suite of SANDee evolutionary models successfully reproduced their location on the HR Diagram and demonstrated the determination of the brown dwarf cooling age of a globular cluster for the first time (read the articles by Bedin et al., Gerasimov et al., and Scalco et al.)
New atmosphere models for metal-poor brown dwarfs
New atmosphere models for metal-poor brown dwarfs
(May 2024) Cool Star Lab undergraduate researcher and UC LEADS Scholar Efrain Alvardo led the release of a new set of atmosphere models for metal-poor brown dwarfs. The Spectral ANalogs of Dwarfs (SAND) models were developed with form CSL graduate student Roman Gerasimov, and fills an important gap in current brown dwarf modeling suites (read the Research Note by Alvarado et al.)
The multiple stellar populations of NGC 6752
The multiple stellar populations of NGC 6752
(March 2024) Former Cool Star Lab graduate Roman Gerasimov contributed a new set of spectral and evolutionary models to accurately characterize the three distinct stellar populations of the NGC 6752 globular cluster imaged by HST. Roman's modeling framework spanning the entirety of the Main Sequence was able to show that these populations can be accurately modeled by varying sodium, carbon, oxygen, and aluminum abundances, and further show distinct luminosity functions that could be explained by dynamical scattering. This result made the cover of Astronomische Nachrichten. (read the article by Scalco et al. in Astronomische Nachrichten)
Oxygen in the Coldest Stars of 47 Tuc
Oxygen in the Coldest Stars of 47 Tuc
(Oct 2023) Recent Cool Star Lab graduate Roman Gerasimov has led a study investigating the lower Main Sequence of the globular cluster 47 Tucanae. Using a suite of low-temperature atmosphere models he computed, and a novel analysis method, Roman was able to explain the spread of the lower Main Sequence of the 47 Tucanae population in HST color-magnitude diagrams as arising in variations in Oxygen abundances, and was able to infer the distribution of Oxygen from photometry alone. He also inferred the luminosity and mass functions of the lowest mass stars in this ancient system (read the preprint by Gerasimov et al.)
An unusual and cold brown dwarf
An unusual and cold brown dwarf
(Oct 2023) Cool Star Lab members contributed to the discovery of a new Y dwarf identified by the Backyard Worlds Team. The source, CWISE J105512.11+544328.3, was confirmed and classified with Keck/NIRES near-infrared spectroscopy, and has an estimated temperature of 500 K. It's extremely blue mid-infrared color suggests it may have an unusual, possibly metal-poor atmosphere. The publication was led by U. Florida undergraduate Grady Robbins (see the preprint by Robbins et al.)
JWST Uncovers Distant Brown Dwarfs on the Outskirts of the Milky Way
JWST Uncovers Distant Brown Dwarfs on the Outskirts of the Milky Way
(Sep 2023) Adam Burgasser and Roman Gerasimov led an article analyzing JWST/NIRSpec data of three distant T dwarfs identified in the UNCOVER survey of the Abell 2744 lensing field. The NIRSpec prism data allowed full analysis of the 1-5 µm spectra, revealing all three to be T dwarfs at kiloparsec distances, two with evidence of subsolar metallicities. The coldest of the three, previously identified photometrically as GLASS-BD-1, shows evidence of phosphine in its infrared spectra, a potential new indicator of subsolar metallicity in cool brown dwarf spectra (read the preprint by Burgasser et al.).
Pop III Study highlighted in Nature Astronomy
Pop III Study highlighted in Nature Astronomy
(Jan 2023): The study of Population III star detectability led by former CSL undergraduate Mikaela Larkin was recently singled out as a Research Highlight by Nature Astronomy (read the highlight and the article).
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