arXiv subjects

Adam Wheeler

Publications and source records attributed to Adam Wheeler.

11 recordsLinked to original sources

arXiv2026

A constrained linear model for continuum normalization of stellar spectra

Inferring stellar parameters and chemical abundances by forward modeling stellar spectra usually requires a spectral synthesis code, or an emulator constructed from a curated training set. In these situations continuum normalization is often implemented as a pre-processing step that is independent of stellar parameters. This leads to results that are biased, or inconsistent across signal-to-noise ratios. A more justified approach is to forward model spectra with all nuisances simultaneously, but in practice this can be an expensive or non-convex optimization procedure. Here we describe a constrained linear model that can fit stellar absorption, telluric transmission, the joint continuum-instrument response. Stellar absorption and telluric transmission are each modeled by factorizing a grid of rectified theoretical spectra into two non-negative matrices with a chosen number of basis components. This model characterizes all possible spectra in many fewer parameters than comparable data-driven models. The non-negativity constraint ensures basis vectors are strictly additive, which limits rectified flux to less than or equal to unity, such that we can distinguish normalized spectra from the joint instrument-continuum response. The model requires no initial guess, and the linearity ensures that inference is convex, stable, and fast. This model allows us to reliably fit nuisances (e.g., tellurics, continuum), and is readily extensible to radial velocity and rotational broadening, without any prior knowledge about the fundamental stellar properties. We demonstrate our method by fitting ESO/HARPS high-resolution echelle spectra of BAFGKM-type stars. With repeat observations of $\alpha$-Centauri A we present results that are best in class: consistent across time to 0.2% at S/N ~ 100, and to better than 0.5% at S/N ~ 30.

astro-ph.SR↗

arXiv2025

Spectroscopic Follow-up of Young High-$\alpha$ Dwarf Star Candidates: Still Likely Genuinely Young

The question of whether genuinely young high-$\alpha$ stars exist has been discussed for over a decade since their discovery from asteroseismology of giant stars as it is challenging to break the degeneracy between the binary interaction and the genuinely young scenarios. Young high-$\alpha$ stars are hard to explain with traditional chemical evolution model as the high-$\alpha$ disk is typically associated with the early epoch of star formation in the Milky Way. Combined with recent advances of gyrochronology, and that $^7$Li can serve as an unambiguous indicator for identifying merger products in dwarfs thanks to its low burning temperature, we identified young high-$\alpha$ dwarf candidate stars through their fast rotation in a previous study. In this paper, we performed high-resolution spectroscopic follow-up of these candidates using Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI), and confirm 3 additional stars that are most likely genuinely young. Together with the star from the earlier paper, we find three out of four of them center around [Fe/H]=-0.5 dex, are ~5 Gyr old, and have a similar amount of elevated Li (~0.5 dex) and Al (~0.1 dex) compared to stars with matching $\log g$, $T_{\rm eff}$, Mg, and Fe within observational uncertainties, hinting at their common formation pathway.

astro-ph.GA↗

arXiv2024

ACACIAS I: Element abundance labels for 192 stars in the dwarf galaxy NGC 6822

The element abundances of local group galaxies connect enrichment mechanisms to galactic properties and serve to contextualise the Milky Way's abundance distributions. Individual stellar spectra in nearby galaxies can be extracted from Integral Field Unit (IFU) data, and provide a means to take an abundance census of the local group. We introduce a program that leverages $R=1800$, $\mathrm{SNR}=15$, IFU resolved spectra from the Multi Unit Spectroscopic Explorer (MUSE). We deploy the data-driven modelling approach for labelling stellar spectra with stellar parameters and abundances, of The Cannon, on resolved stars in NGC 6822. We construct our model for The Cannon using $\approx$19,000 Milky Way LAMOST spectra with APOGEE labels. We report six inferred abundance labels (denoted $\ell_\mathrm{X}$), for 192 NGC 6822 disk stars, precise to $\approx$$0.15$ dex. We validate our generated spectral models provide a good fit the data, including at individual atomic line features. We infer mean abundances of $\ell_\mathrm{[Fe/H]} = -0.90 \pm 0.03$, $\ell_\mathrm{[Mg/Fe]} = -0.01 \pm 0.01$, $\ell_\mathrm{[Mn/Fe]} = -0.22 \pm 0.02$, $\ell_\mathrm{[Al/Fe]} = -0.33 \pm 0.03$, $\ell_\mathrm{[C/Fe]} =-0.43 \pm 0.03$, $\ell_\mathrm{[N/Fe]} =0.18 \pm 0.03$. These abundance labels are similar to dwarf galaxies observed by APOGEE, and the lower enhancements for NGC 6822 compared to the Milky Way are consistent with expectations. This approach supports a new era in extra-galactic archaeology of characterising the local group enrichment diversity using low-resolution, low-SNR IFU resolved spectra.

astro-ph.GA↗

arXiv2024

Chemical Cartography with APOGEE: Two-process Parameters and Residual Abundances for 288,789 Stars from Data Release 17

Stellar abundance measurements are subject to systematic errors that induce extra scatter and artificial correlations in elemental abundance patterns. We derive empirical calibration offsets to remove systematic trends with surface gravity $\log(g)$ in 17 elemental abundances of 288,789 evolved stars from the SDSS APOGEE survey. We fit these corrected abundances as the sum of a prompt process tracing core-collapse supernovae and a delayed process tracing Type Ia supernovae, thus recasting each star's measurements into the amplitudes $A_{\text{cc}}$ and $A_{\text{Ia}}$ and the element-by-element residuals from this two-parameter fit. As a first application of this catalog, which is $8\times$ larger than that of previous analyses that used a restricted $\log(g)$ range, we examine the median residual abundances of 14 open clusters, nine globular clusters, and four dwarf satellite galaxies. Relative to field Milky Way disk stars, the open clusters younger than 2 Gyr show $\approx 0.1-0.2$ dex enhancements of the neutron-capture element Ce, and the two clusters younger than 0.5 Gyr also show elevated levels of C+N, Na, S, and Cu. Globular clusters show elevated median abundances of C+N, Na, Al, and Ce, and correlated abundance residuals that follow previously known trends. The four dwarf satellites show similar residual abundance patterns despite their different star formation histories, with $\approx 0.2-0.3$ dex depletions in C+N, Na, and Al and $\approx 0.1$ dex depletions in Ni, V, Mn, and Co. We provide our catalog of corrected APOGEE abundances, two-process amplitudes, and residual abundances, which will be valuable for future studies of abundance patterns in different stellar populations and of additional enrichment processes that affect galactic chemical evolution.

astro-ph.GA↗

arXiv2021

Chemodynamical signatures of bar resonances in the Galactic disk: current data and future prospects

The Galactic disk exhibits complex chemical and dynamical substructure thought to be induced by the bar, spiral arms, and satellites. Here, we explore the chemical signatures of bar resonances in action and velocity space and characterize the differences between the signatures of corotation and higher-order resonances using test particle simulations. Thanks to recent surveys, we now have large datasets containing metallicities and kinematics of stars outside the solar neighborhood. We compare the simulations to the observational data from Gaia EDR3 and LAMOST DR5 and find weak evidence for a slow bar with the "hat" moving group ($250~\text{km/s} \lesssim v_\phi \lesssim 270~\text{km/s}$) associated with its outer Lindblad resonance and "Hercules" ($170~\textrm{km/s} \lesssim v_\phi \lesssim 195~\text{km/s}$) with corotation. While constraints from current data are limited by their spatial footprint, stars closer in azimuth than the Sun to the bar's minor axis show much stronger signatures of the bar's outer Lindblad and corotation resonances in test particle simulations. Future datasets with greater azimuthal coverage, including the final Gaia data release, will allow reliable chemodynamical identification of bar resonances.

astro-ph.GA↗

arXiv2021

Disentangled Representation Learning for Astronomical Chemical Tagging

Modern astronomical surveys are observing spectral data for millions of stars. These spectra contain chemical information that can be used to trace the Galaxy's formation and chemical enrichment history. However, extracting the information from spectra, and making precise and accurate chemical abundance measurements are challenging. Here, we present a data-driven method for isolating the chemical factors of variation in stellar spectra from those of other parameters (i.e. \teff, \logg, \feh). This enables us to build a spectral projection for each star with these parameters removed. We do this with no ab initio knowledge of elemental abundances themselves, and hence bypass the uncertainties and systematics associated with modeling that rely on synthetic stellar spectra. To remove known non-chemical factors of variation, we develop and implement a neural network architecture that learns a disentangled spectral representation. We simulate our recovery of chemically identical stars using the disentangled spectra in a synthetic APOGEE-like dataset. We show that this recovery declines as a function of the signal to noise ratio, but that our neural network architecture outperforms simpler modeling choices. Our work demonstrates the feasibility of data-driven abundance-free chemical tagging.

astro-ph.IM↗

arXiv2020

The Swan: Data-Driven Inference of Stellar Surface Gravities for Cool Stars from Photometric Light Curves

Stellar light curves are well known to encode physical stellar properties. Precise, automated and computationally inexpensive methods to derive physical parameters from light curves are needed to cope with the large influx of these data from space-based missions such as Kepler and TESS. Here we present a new methodology which we call The Swan, a fast, generalizable and effective approach for deriving stellar surface gravity ($\log g$) for main sequence, subgiant and red giant stars from Kepler light curves using local linear regression on the full frequency content of Kepler long cadence power spectra. With this inexpensive data-driven approach, we recover $\log g$ to a precision of $\sim$0.02 dex for 13,822 stars with seismic $\log g$ values between 0.2-4.4 dex, and $\sim$0.11 dex for 4,646 stars with Gaia derived $\log g$ values between 2.3-4.6 dex. We further develop a signal-to-noise metric and find that granulation is difficult to detect in many cool main sequence stars ($T_{\text{eff}}$ $\lesssim$ 5500 K), in particular K dwarfs. By combining our $\log g$ measurements with Gaia radii, we derive empirical masses for 4,646 subgiant and main sequence stars with a median precision of $\sim$7%. Finally, we demonstrate that our method can be used to recover $\log g$ to a similar mean absolute deviation precision for TESS-baseline of 27 days. Our methodology can be readily applied to photometric time-series observations to infer stellar surface gravities to high precision across evolutionary states.

astro-ph.SR↗

arXiv2020

Hundreds of new periodic signals detected in the first year of $\it{TESS}$ with the ${\tt weirddetector}$

We apply the ${\tt weirddetector}$, a nonparametric signal detection algorithm based on phase dispersion minimization, in a search for low duty-cycle periodic signals in the Transiting Exoplanet Survey Satellite (${\it TESS}$) photometry. Our approach, in contrast to commonly used model-based approaches specifically for flagging transits, eclipsing binaries, or other similarly periodic events, makes minimal assumptions about the shape of a periodic signal, with the goal of finding "weird" signals of unexpected or arbitrary shape. In total, 248,301 ${\it TESS}$ sources from the first-year Southern sky survey are run through the ${\tt weirddetector}$, of which we manually inspect the top 21,500 for periodicity. To minimize false-positives, we here only report on the upper decile in terms of signal score, a sample for which we obtain 97% recall of ${\it TESS}$ eclipsing binaries and 62% of the TOIs. In our sample, we find 377 previously unreported periodic signals, for which we make a first-pass assignment that 26 are ultra-short periods ($<0.3$ d), 313 are likely eclipsing binaries, 28 appear planet-like, and 10 are miscellaneous signals.

astro-ph.EP↗

arXiv2020

An unsupervised method for identifying $X$-enriched stars directly from spectra: Li in LAMOST

Stars with peculiar element abundances are important markers of chemical enrichment mechanisms. We present a simple method, tangent space projection (TSP), for the detection of $X$-enriched stars, for arbitrary elements $X$, even from blended lines. Our method does not require stellar labels, but instead directly estimates the counterfactual unrenriched spectrum from other unlabelled spectra. As a case study, we apply this method to the $6708~$\AA{} Li doublet in LAMOST DR5, identifying 8,428 Li-enriched stars seamlessly across evolutionary state. We comment on the explanation for Li-enrichement for different subpopulations, including planet accretion, nonstandard mixing, and youth.

astro-ph.SR↗

arXiv2020

Abundances in the Milky Way across five nucleosynthetic channels from 4 million LAMOST stars

Large stellar surveys are revealing the chemodynamical structure of the Galaxy across a vast spatial extent. However, the many millions of low-resolution spectra observed to date are yet to be fully exploited. We employ The Cannon, a data-driven approach to estimating abundances, to obtain detailed abundances from low-resolution (R = 1800) LAMOST spectra, using the GALAH survey as our reference. We deliver five (for dwarfs) or six (for giants) estimated abundances representing five different nucleosynthetic channels, for 3.9 million stars, to a precision of 0.05 - 0.23 dex. Using wide binary pairs, we demonstrate that our abundance estimates provide chemical discriminating power beyond metallicity alone. We show the coverage of our catalogue with radial, azimuthal and dynamical abundance maps, and examine the neutron capture abundances across the disk and halo, which indicate different origins for the in-situ and accreted halo populations. LAMOST has near-complete Gaia coverage and provides an unprecedented perspective on chemistry across the Milky Way.

astro-ph.SR↗

arXiv2019

The Weird Detector: Flagging periodic, coherent signals of arbitrary shape in time series photometry

By design, model-based approaches for flagging transiting exoplanets in light curves, such as boxed least squares, excel at detecting planets with low S/N at the expense of finding signals that are not well described by the assumed model, such as self-lensing binaries, disintegrating or evaporating planets, or planets with large rings. So far, such signals have typically been found through visual searches by professional or citizen scientists, or by inspection of the photometric power-spectra. We present a nonparametric detection algorithm, for short duty-cycle periodic signals in photometric time series based on phase dispersion minimization. We apply our code to 161,786 Kepler sources and detect 18 new periodic signals consistent with heartbeat binaries/planets, 4 new singly-transiting systems, and 2 new doubly-transiting systems. We show that our code is able to recover the majority of known Kepler objects of interest (KOIs) to high confidence, as well as more unusual events such as Boyajian's star and a comet passing through the Kepler field. Nonparametric signal-flagging techniques, such as the one presented here, will become increasingly valuable with the coming data from TESS and future transit surveys as the volume of data available to us exceeds that which can be feasibly examined manually.

astro-ph.EP↗