arXiv subjects

Adrian Price-Whelan

Publications and source records attributed to Adrian Price-Whelan.

At least 19 recordsLinked to original sources

arXiv2026

Inferring Stellar Densities with Flexible Models I: The Distribution of RR Lyrae in the Milky Way with $\textit{Gaia}$ DR3

Understanding the formation and evolutionary history of the Milky Way requires detailed mapping of its stellar components, which preserve fossil records of the Galaxy's assembly through cosmic time. RR Lyrae stars are particularly well-suited for this endeavor, as they are old, standard candle variables that probe the Galaxy's earliest formation epochs. In this work, we employ a hierarchical Bayesian Gaussian Mixture Model (GMM) to characterize the three-dimensional density distribution of RR Lyrae stars in the Milky Way over the galactocentric radius ($R$) of $\approx 0.2-120~\rm{kpc}$. This approach provides a flexible framework for modeling complex stellar distributions, particularly in the inner Galaxy where the bulge, disk, and halo components overlap. Our analysis reveals that the inner Galaxy ($R\lesssim10~\rm{kpc}$) is dominated by a distinct prolate stellar population with axis ratio $q$=1.31. Consistent with previous work, we find the halo follows a $r^{-4}$ power-law profile that flattens within 12 kpc of the Galactic center. We also confirm the outer halo ($R\gtrsim10~\rm{kpc}$) is oblate $q$=0.70 with a tilt angle of $18^{\circ}$. We report for the first time that this tilt aligns the halo major axis in the direction of the Sagittarius dwarf galaxy. These results establish GMMs as an effective and flexible tool for modeling Galactic structure and provide new constraints on the distribution of old stars in the inner Galaxy.

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arXiv2026

unxt: A Python package for unit-aware computing with JAX

unxt is a Python package for unit-aware computing with JAX. unxt is built on top of quax, which provides a framework for building array-like objects that can be used with JAX. unxt extends quax to provide support for unit-aware computing using the astropy.units package as a units backend. unxt provides seamless integration of physical units into high performance numerical computations, significantly enhancing the capabilities of JAX for scientific applications.

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arXiv2025

Dynamics of tidal spiral arms: Machine learning-assisted identification of equations and application to the Milky Way

Understanding the spiral arms of the Milky Way (MW) remains a key open question in galactic dynamics. Tidal perturbations, such as the recent passage of the Sagittarius dwarf galaxy (Sgr), could play a significant role in exciting them. We aim to analytically characterize the dynamics of tidally induced spiral arms, including their phase-space signatures. We ran idealized test-particle simulations resembling impulsive satellite impacts, and used the Sparse Identification of Non-linear Dynamics (SINDy) method to infer their governing Partial Differential Equations (PDEs). We validated the method with analytical derivations and a realistic $N$-body simulation of a MW-Sgr encounter analogue. For small perturbations, a linear system of equations was recovered with SINDy, consistent with predictions from linearised collisionless dynamics. In this case, two distinct waves wrapping at pattern speeds $Ω\pm κ/m$ emerge. For large impacts, we empirically discovered a non-linear system of equations, representing a novel formulation for the dynamics of tidally induced spiral arms. For both cases, these equations describe wave properties like amplitude and pattern speed, and their shape and temporal evolution in different phase-space projections. We fit the Gaia $L_Z-V_R$ waves with the linear model, providing a reasonable fit and plausible parameters for the Sgr passage. However, the predicted amplitude ratio of the two waves is inconsistent with observations, supporting a more complex origin for this feature (e.g. multiple passages, bar, spiral arms). We merge data-driven discovery with theory to create simple, accurate models of tidal spiral arms that match simulations and provide a simple tool to fit Gaia and external galaxy data. This methodology could be extended to model complex phenomena like self-gravity and dynamical friction. (ABR)

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arXiv2024

paired: A Statistical Framework for Detecting Stellar Binarity with Gaia RVs. I. Sensitivity to Unresolved Binaries

Data Release 3 (DR3) from the Gaia Mission includes radial velocity measurements of over 33 million targets. Among many scientific applications, the overlap of this stellar sample with targeted exoplanet transit survey stars presents an opportunity to understand planet occurrence in the context of stellar multiplicity on a large scale. Yet, any interpretation of occurrence relies upon an understanding of survey sensitivity. While the sensitivity to planets in transit surveys is well understood, a characterization of the sensitivity of Gaia to unresolved binaries is also critical. We describe here a statistical framework called paired, which we developed to enable the forward modeling of Gaia radial velocity observables for large samples of stars. The paired machinery links the reported radial velocity noise for a given star from Gaia to the probability of a spatially unresolved stellar companion. We demonstrate how this enables the user, given an observed distribution of individual binary ``probabilities" for a set of stars, to understand this distribution within the sensitivity limits of Gaia. For the subset of stars with the highest probability of excess radial velocity noise, we describe the ability of paired to constrain the semi-amplitude of the stellar binary. Where possible, we benchmark our inferred radial velocity semi-amplitudes against those from ground-based radial velocity surveys, and the subset published by Gaia DR3 itself. We aim for paired to be a community tool for the exploration of the effects of binarity on planets at a population level, but also for any user interested in stellar populations.

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arXiv2024

Many elements matter: Detailed abundance patterns reveal star-formation and enrichment differences among Milky Way structural components

Many nucleosynthetic channels create the elements, but two-parameter models characterized by $α$ and Fe nonetheless predict stellar abundances in the Galactic disk to accuracies of 0.02 to 0.05 dex for most measured elements, near the level of current abundance uncertainties. It is difficult to make individual measurements more precise than this to investigate lower-amplitude nucleosynthetic effects, but population studies of mean abundance patterns can reveal more subtle abundance differences. Here we look at the detailed abundances for 67315 stars from APOGEE DR17, but in the form of abundance residuals away from a best-fit two-parameter, data-driven nucleosynthetic model. We find that these residuals show complex structures with respect to age, guiding radius, and vertical action that are not random and are also not strongly correlated with sources of systematic error such as surface gravity, effective temperature, and radial velocity. The residual patterns, especially in Na, C+N, Ni, Mn, and Ce, trace kinematic structures in the Milky Way, such as the inner disk, thick disk, and flared outer disk. A principal component analysis suggests that most of the observed structure is low-dimensional and can be explained by a few eigenvectors. We find that some, but not all, of the effects in the low-$α$ disk can be explained by dilution with fresh gas, so that abundance ratios resemble those of stars with higher metallicity. The patterns and maps we provide could be combined with accurate forward models of nucleosynthesis, star formation, and gas infall to provide a more detailed picture of star and element formation in different Milky Way components.

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arXiv2024

Carbon enrichment in APOGEE disk stars as evidence of mass transfer in binaries

Carbon abundances in first-ascent giant stars are usually lower than those of their main-sequence counterparts. At moderate metallicities, stellar evolution of single stars cannot account for the existence of red-giant branch stars with enhanced carbon abundances. The phenomenon is usually interpreted as resulting from past mass transfer from an evolved binary companion now in the white dwarf evolutionary stage. Aims: We aim to confirm the links between [C/O] enhancement, s-process element enhancement and binary fraction using large-scale catalogues of stellar abundances and probable binary stars. Methods: We use a large data set from the 17 data release of the SDSS-IV/APOGEE~2 survey to identify carbon-enhanced stars in the Galactic disk. We identify a continuum of carbon enrichment throughout three different sub-populations of disk stars and explore links between the degree of carbon enrichment and binary frequency, metallicity and chemical compositions. Results: We verify a clear correlation between binary frequency and enhancement in the abundances of both carbon and cerium, lending support to the scenario whereby carbon-enhanced stars are the result of mass transfer by an evolved binary companion. In addition, we identify clustering in the carbon abundances of high-$α$ disk stars, suggesting that those on the high metallicity end are likely younger, in agreement with theoretical predictions for the presence of a starburst population following the gas-rich merger of the Gaia-Enceladus/Sausage system.

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arXiv2024

Iron Snails: non-equilibrium dynamics and spiral abundance patterns

Galaxies are not in a dynamical steady state. They continually undergo perturbations, e.g., from infalling dwarf galaxies and dark-matter substructure. After a dynamical perturbation, stars phase mix towards a new steady state; in so doing they generally form spiral structures, such as spiral density waves in galaxy disks and the Gaia Snail observed in the vertical phase-space density in the solar neighborhood. Structures in phase-space density can be hard to measure accurately, because spatially varying selection effects imprint their own patterns on the density. However, stellar labels such as metallicity, or other element abundances, or stellar masses and ages, can be measured even in the face of complex or unknown spatial selection functions. We show that if the equilibrium galaxy has phase-space gradients in these labels, any perturbation that could raise a spiral wave in the phase-space density will raise a spiral wave in the distribution of labels as well. We work out the relationship between the spiral patterns in the density and in the labels. As an example, we analyze the Gaia Snail and show that its amplitude and dynamical age as derived from elemental abundances (mainly [Mg/Fe]) follow similar patterns to those derived from the phase-space density. Our best model dates the Snail's perturbation to about 400 Myr ago although we find significant variations with angular momentum in the best-fit age. Conceptually, the ideas presented here are related to Orbital Torus Imaging, chemical tagging, and other methods that use stellar labels to trace dynamics.

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arXiv2023

Stream Members Only: Data-Driven Characterization of Stellar Streams with Mixture Density Networks

Stellar streams are sensitive probes of the Milky Way's gravitational potential. The mean track of a stream constrains global properties of the potential, while its fine-grained surface density constrains galactic substructure. A precise characterization of streams from potentially noisy data marks a crucial step in inferring galactic structure, including the dark matter, across orders of magnitude in mass scales. Here we present a new method for constructing a smooth probability density model of stellar streams using all of the available astrometric and photometric data. To characterize a stream's morphology and kinematics, we utilize mixture density networks to represent its on-sky track, width, stellar number density, and kinematic distribution. We model the photometry for each stream as a single-stellar population, with a distance track that is simultaneously estimated from the stream's inferred distance modulus (using photometry) and parallax distribution (using astrometry). We use normalizing flows to characterize the distribution of background stars. We apply the method to the stream GD-1, and the tidal tails of Palomar 5. For both streams we obtain a catalog of stellar membership probabilities that are made publicly available. Importantly, our model is capable of handling data with incomplete phase-space observations, making our method applicable to the growing census of Milky Way stellar streams. When applied to a population of streams, the resulting membership probabilities from our model form the required input to infer the Milky Way's dark matter distribution from the scale of the stellar halo down to subhalos.

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arXiv2023

The Future of Astronomical Data Infrastructure: Meeting Report

The astronomical community is grappling with the increasing volume and complexity of data produced by modern telescopes, due to difficulties in reducing, accessing, analyzing, and combining archives of data. To address this challenge, we propose the establishment of a coordinating body, an "entity," with the specific mission of enhancing the interoperability, archiving, distribution, and production of both astronomical data and software. This report is the culmination of a workshop held in February 2023 on the Future of Astronomical Data Infrastructure. Attended by 70 scientists and software professionals from ground-based and space-based missions and archives spanning the entire spectrum of astronomical research, the group deliberated on the prevailing state of software and data infrastructure in astronomy, identified pressing issues, and explored potential solutions. In this report, we describe the ecosystem of astronomical data, its existing flaws, and the many gaps, duplication, inconsistencies, barriers to access, drags on productivity, missed opportunities, and risks to the long-term integrity of essential data sets. We also highlight the successes and failures in a set of deep dives into several different illustrative components of the ecosystem, included as an appendix.

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arXiv2023

Roman CCS White Paper: Adding Fields Hosting Globular Clusters To The Galactic Bulge Time Domain Survey

Despite multiple previous searches, no transiting planets have yet been identified within a globular cluster. This is believed to be due to a combination of factors: the low metallicities of most globular clusters suggests that there is significantly less planet-forming material per star in most globular clusters relative to the solar neighborhood, the high likelihood of dynamical interactions can also disrupt planetary orbits, and the data available for globular clusters is limited. However, transiting planets have been identified in open clusters, indicating that there may be planets in more massive clusters that have simply gone undetected, or that more massive clusters inhibit planet formation. Less than two degrees away from the nominal Galactic Bulge Time Domain Survey footprint, two globular clusters, NGC 6522 and NGC 6528, can be simultaneously observed by the Roman telescope during the Galactic Bulge Time Domain Survey. These clusters are comparable in mass (1-2 x 10$^5$ solar masses) and age (12 Gyr), but feature drastically different average metallicities: NGC 6522 has an average [Fe/H] $\sim$ -1.3, while NGC 6528 has an average [Fe/H] $\sim$ -0.1. If no transiting planets are detected in one season of time domain observations of these clusters, this would indicate a difference in planet occurrence among field stars and globular clusters at >3-$σ$ significance even after accounting for metallicity, which could be enhanced to >5-$σ$ significance with similar observations of another nearby field hosting a metal-rich globular cluster. Additionally, time domain observations of NGC 6522 and NGC 6528 will detect variable stars in both clusters, testing the connection between stellar variability and binary fraction to metallicity and cluster environment, as well as testing the dependence of exoplanet yields on stellar density and distance from the Galactic midplane.

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arXiv2023

Snowmass2021 Cosmic Frontier White Paper: Dark Matter Physics from Halo Measurements

The non-linear process of cosmic structure formation produces gravitationally bound overdensities of dark matter known as halos. The abundances, density profiles, ellipticities, and spins of these halos can be tied to the underlying fundamental particle physics that governs dark matter at microscopic scales. Thus, macroscopic measurements of dark matter halos offer a unique opportunity to determine the underlying properties of dark matter across the vast landscape of dark matter theories. This white paper summarizes the ongoing rapid development of theoretical and experimental methods, as well as new opportunities, to use dark matter halo measurements as a pillar of dark matter physics.

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arXiv2022

From Data to Software to Science with the Rubin Observatory LSST

The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) dataset will dramatically alter our understanding of the Universe, from the origins of the Solar System to the nature of dark matter and dark energy. Much of this research will depend on the existence of robust, tested, and scalable algorithms, software, and services. Identifying and developing such tools ahead of time has the potential to significantly accelerate the delivery of early science from LSST. Developing these collaboratively, and making them broadly available, can enable more inclusive and equitable collaboration on LSST science. To facilitate such opportunities, a community workshop entitled "From Data to Software to Science with the Rubin Observatory LSST" was organized by the LSST Interdisciplinary Network for Collaboration and Computing (LINCC) and partners, and held at the Flatiron Institute in New York, March 28-30th 2022. The workshop included over 50 in-person attendees invited from over 300 applications. It identified seven key software areas of need: (i) scalable cross-matching and distributed joining of catalogs, (ii) robust photometric redshift determination, (iii) software for determination of selection functions, (iv) frameworks for scalable time-series analyses, (v) services for image access and reprocessing at scale, (vi) object image access (cutouts) and analysis at scale, and (vii) scalable job execution systems. This white paper summarizes the discussions of this workshop. It considers the motivating science use cases, identified cross-cutting algorithms, software, and services, their high-level technical specifications, and the principles of inclusive collaborations needed to develop them. We provide it as a useful roadmap of needs, as well as to spur action and collaboration between groups and individuals looking to develop reusable software for early LSST science.

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arXiv2022

Constraining the Planet Occurrence Rate around Halo Stars of Potentially Extragalactic Origin

The search for planets orbiting other stars has recently expanded to include stars from galaxies outside the Milky Way. With the TESS and Gaia surveys, photometric and kinematic information can be combined to identify transiting planet candidates of extragalactic origin. Here, 1,080 low-luminosity red giant branch stars observed by Gaia and TESS with kinematics suggesting a high likelihood of extragalactic origin were searched for planet transits. Transit injection-recovery tests were performed to measure the sensitivity of the TESS data and completeness of the transit search. Injected signals of planets larger than Jupiter with orbital periods of 10 days or less were recovered in $\approx$44% of cases. Although no planet transits were detected in this sample, we find an upper limit on planet occurrence of 0.52% for hot Jupiters, consistent with previous studies of planet occurrence around similar host stars. As stars in the halo tend to be lower metallicity, and short period giant planet occurrence tends to be strongly correlated with stellar metallicity, we predict that relative to the Galactic disk population, a smaller fraction of halo stars will host planets detectable by transit surveys. Thus, applying the known planet occurrence trends to potential planet detection around halo stars, we predict $\gtrsim$7,000 stars must be searched with similar cadence and precision as the stars studied here before a detection of a planet of extragalactic origin is likely. This may be possible with future data releases from the TESS and Gaia missions.

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arXiv2022

Astrophysical and Cosmological Probes of Dark Matter

While astrophysical and cosmological probes provide a remarkably precise and consistent picture of the quantity and general properties of dark matter, its fundamental nature remains one of the most significant open questions in physics. Obtaining a more comprehensive understanding of dark matter within the next decade will require overcoming a number of theoretical challenges: the groundwork for these strides is being laid now, yet much remains to be done. Chief among the upcoming challenges is establishing the theoretical foundation needed to harness the full potential of new observables in the astrophysical and cosmological domains, spanning the early Universe to the inner portions of galaxies and the stars therein. Identifying the nature of dark matter will also entail repurposing and implementing a wide range of theoretical techniques from outside the typical toolkit of astrophysics, ranging from effective field theory to the dramatically evolving world of machine learning and artificial-intelligence-based statistical inference. Through this work, the theory frontier will be at the heart of dark matter discoveries in the upcoming decade.

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arXiv2021

APOGEE Chemical Abundance Patterns of the Massive Milky Way Satellites

The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf (Sgr), Fornax (Fnx), and the now fully disrupted \emph{Gaia} Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the [$α$/Fe]-[Fe/H] abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the MCs observed by Nidever et al. in the $α$-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3-4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier ($\sim$~5-7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.

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arXiv2020

Discovery of Extended Tidal Tails around the Globular Cluster Palomar 13

We use photometry from the DECam Legacy Survey to detect candidate tidal tails extending ~5 deg on either side of the Palomar 13 globular cluster. The tails are aligned with the proper motion of Palomar 13 and are consistent with its old, metal-poor stellar population. We identify three RR Lyrae stars that are plausibly associated with the tails, in addition to four previously known in the cluster. From these RR Lyrae stars, we find that the mean distance to the cluster and tails is $23.6 \pm 0.2$ kpc and estimate the total (initial) luminosity of the cluster to be $L_V=5.1^{+9.7}_{-3.4}\times 10^3 L_\odot$, consistent with previous claims that its initial luminosity was higher than its current luminosity. Combined with previously-determined proper motion and radial velocity measurements of the cluster, we find that Palomar 13 is on a highly eccentric orbit ($e\sim 0.8$) with a pericenter of ~9 kpc and an apocenter of ~69 kpc, and a recent pericentric passage of the cluster ~75 Myr ago. We note a prominent linear structure in the interstellar dust map that runs parallel to the candidate tidal features, but conclude that reddening due to dust is unlikely to account for the structure that we observe. If confirmed, the Palomar 13 stellar stream would be one of very few streams with a known progenitor system, making it uniquely powerful for studying the disruption of globular clusters, the formation of the stellar halo, and the distribution of matter within our Galaxy.

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arXiv2019

The Growing Importance of a Tech Savvy Astronomy and Astrophysics Workforce

Fundamental coding and software development skills are increasingly necessary for success in nearly every aspect of astronomical and astrophysical research as large surveys and high resolution simulations become the norm. However, professional training in these skills is inaccessible or impractical for many members of our community. Students and professionals alike have been expected to acquire these skills on their own, apart from formal classroom curriculum or on-the-job training. Despite the recognized importance of these skills, there is little opportunity to develop them - even for interested researchers. To ensure a workforce capable of taking advantage of the computational resources and the large volumes of data coming in the next decade, we must identify and support ways to make software development training widely accessible to community members, regardless of affiliation or career level. To develop and sustain a technology capable astronomical and astrophysical workforce, we recommend that agencies make funding and other resources available in order to encourage, support and, in some cases, require progress on necessary training, infrastructure and policies. In this white paper, we focus on recommendations for how funding agencies can lead in the promotion of activities to support the astronomy and astrophysical workforce in the 2020s.

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arXiv2019

Antlia2's role in driving the ripples in the outer gas disk of the Galaxy

We employ the earlier published proper motions of the newly discovered Antlia 2 dwarf galaxy derived from Gaia data to calculate its orbital distribution in the cosmologically recent past. Using these observationally motivated orbits, we calculate the effect of the Antlia 2 dwarf galaxy on the outer HI disk of the Milky Way, using both test particle and Smoothed Particle Hydrodynamics simulations. We find that orbits with low pericenters, $\sim$ 10 kpc, produce disturbances that match the observed outer HI disk perturbations. We have independently recalculated the proper motion of the Antlia 2 dwarf from Gaia data and found a proper motion of $(μ_αcosδ, μ_δ) = (-0.068,0.032) \pm (0.023,-0.031)~\rm mas/yr$, which agrees with results from Torrealba et al. (2019) within the errors, but gives lower mean pericenters, e.g., $\sim$ 15 kpc for our fiducial model of the Milky Way. We also show that the Sagittarius dwarf galaxy interaction does not match the observed perturbations in the outer gas disk. Thus, Antlia 2 may be the driver of the observed large perturbations in the outer gas disk of the Galaxy. The current location of the Antlia 2 dwarf galaxy closely matches that predicted by an earlier dynamical analysis (Chakrabarti \& Blitz 2009) of the dwarf that drove ripples in the outer Galaxy, and, in particular, its orbit is nearly coplanar to the Galactic disk. If the Antlia 2 dwarf galaxy is responsible for the perturbations in the outer Galactic disk, it would have a specific range of proper motions that we predict here; this can be tested soon with Gaia DR-3 and Gaia DR-4 data.

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