arXiv subjects

Alison Duck

Publications and source records attributed to Alison Duck.

17 recordsLinked to original sources

arXiv2026

Benchmark Brown Dwarf Systems I: Chemical Abundance Analysis of FGK Stars with Wide-Separation Brown Dwarf Companions Using PEPSI

We present results from a spectroscopic survey of 32 FGK stars hosting brown dwarfs, using high-resolution optical spectra (R = 130,000 and 50,000) obtained with the PEPSI spectrograph on the Large Binocular Telescope. The primary goal of this survey is to determine precise stellar parameters and abundances for 11 elements (C, O, Mg, Si, Ca, Al, Ti, Fe, Y, S, and N) in these systems. We employ spectral synthesis within the BACCHUS framework to derive precise stellar properties and elemental abundance ratios. For our average S/N $>$ 200 data, we achieve a typical error of 42 K in T$_\mathrm{eff}$ and $\sim$0.03 dex for [Fe/H]. We observe a significant dispersion from a solar C/O ratio among the sample of brown dwarf host stars that host primarily wide-orbit brown dwarfs. Using established theoretical chemical frameworks, we discuss the implications of the observed Mg/Si and Ca/Al ratios for cloud properties in the brown dwarf companions. Finally, we evaluate the applicability of the [Y/Mg] stellar clock for our sample and discuss the broader implications of our results. This work provides a timely and uniform abundance analysis of host stars, supporting extended wavelength brown dwarf observations in the era of JWST.

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arXiv2026

EBLM XVII - Tidal Synchronization and Circularization in Tight Stellar Binaries

Tidal interactions in close stellar binaries are central to their orbital and rotational evolution, making observational tests of theoretical predictions essential for our understanding of the evolution of these, as well as close exoplanetary systems. Such tests require precise measurements of the orbital eccentricity and stellar rotation. The EBLM (Eclipsing Binary Low Mass) survey delivers a homogeneous sample of eclipsing binaries, composed of F/G/K primaries and M-dwarf (or low-mass K-dwarf) secondaries. We analyze 68 unequal mass binaries ($0.1 \leq q \leq 0.6$, where $q$ is the mass ratio), with measurable primary star rotation rates from TESS, and over a decade of radial velocity observations. This sample probes the critical regime where tidal effects are expected to transition between being efficient and inefficient. We find that ~75% of our sample has circularized, with eccentric systems confined to $P_{\rm orb} \gtrsim 3$ days, with modest eccentricities (e < 0.25). Roughly ~78% of our sample is synchronized, with nearly all binaries within a 3-day orbital period residing in a well-defined "synchronization zone". Beyond this, a minority of asynchronous systems persist, which cannot be easily explained by our application of current tidal mechanisms or by differential rotation.

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arXiv2025

The EBLM Project XVIII. 3D Obliquities of Five Low-Mass Eclipsing Binaries

The formation of tight stellar binaries remains an unsolved problem. There is too much angular momentum in a collapsing and fragmenting protostellar cloud to form a stellar binary in situ with a separation less than an AU, yet thousands of these short-period binaries have been discovered. One indication of a binary's formation is the angle between the stellar spin and orbital axes --- its obliquity. The classical method for determining projected stellar obliquities is the Rossiter-McLaughlin effect. This has been applied to 132 hot Jupiters, but only a handful of stellar binaries. Of the binary systems with measured projected obliquities, even fewer have measured 3D obliquities. In this paper, we add five more short-period binary 3D obliquity measurements to the sample previously consisting of one system. We present Rossiter-McLaughlin measurements for EBLM J0239-20, EBLM J0941-31, EBLM J1037-25, EBLM J1141-37, and EBLM J2025-45. These systems consist of an M-dwarf eclipsing an F/G type primary. We combine CORALIE and HARPS spectroscopy with TESS photometry of primary and secondary eclipses. We show that even though the sky-projected obliquities seem to be aligned, there is modest but non-zero spin-orbit misalignment ($\psi$ between 5 and 20$^{\circ}$). Our primary stars straddle the Kraft break at $\sim 6250K$. We derive the M-dwarf masses and radii to precisions better than 3\%. With the exception of EBLM J0941-31, each system has an inflated radius, exceeding stellar model predictions by more than 5$\sigma$.

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arXiv2025

PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). IV. High-Resolution Phased-Resolved Spectroscopy of The Ultra Hot Jupiter KELT-20 b

We present five datasets of high-resolution optical emission spectra of the ultra-hot Jupiter KELT-20 b with the PEPSI spectrograph. Using a Bayesian retrieval framework, we constrain its dayside pressure-temperature profile and abundances of Fe, Ni, and Ca, providing the first measurements for Ni and Ca for KELT-20 b in emission. We retrieve the pre- and post-eclipse datasets separately (corresponding to the evening and morning sides, respectively), and compare the constraints on their thermal structures and chemical abundances. We constrain lower abundances in the pre-eclipse datasets compared to the post-eclipse datasets. We interpret these results with an equilibrium chemistry model which suggests ~10-30x supersolar refractory abundances. Due to the well-known degeneracy between absolute abundances and continuum opacities, the abundance ratios are more precise probes of the planetary abundances. Therefore we measure the abundance ratios [Ni/Fe] and [Ca/Fe] across these datasets and find they agree within 1-sigma. We constrain [Ni/Fe] to be consistent with solar within 2-sigma, and [Ca/Fe] to be 0.001-0.01x solar, not accounting for ionization. We compare these abundance ratios with literature results for KELT-20 b in transmission, and find they agree within 2-sigma, suggesting that even though the abundances vary significantly as a function of phase, the abundance ratios of these species remain relatively constant. We find a ~100 K difference in temperature at the top of the thermal inversion, suggesting a hotter evening side than morning side and underscoring the importance of considering 3D effects when studying ultra-hot Jupiters.

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arXiv2025

Are We There Yet? Challenges in Quantifying the Frequency of Earth Analogs in the Habitable Zone

Searching for life elsewhere in the universe is one of the most highly prioritized pursuits in astronomy today. However, the ability to observe evidence of Earth-like life through biosignatures is limited by the number of planets in the solar neighborhood with conditions similar to Earth. The occurrence rate of Earth-like planets in the habitable zones of Sun-like stars, $\eta_{\oplus}$, is therefore crucial for addressing the apparent lack of consensus on its value in the literature. Here we present a review of the current understanding of $\eta_{\oplus}$. We first provide definitions for parameters that contribute to $\eta_{\oplus}$. Then, we discuss the previous and current estimated parameter values and the context of the limitations on the analyses that produced these estimates. We compile an extensive list of the factors that go into any calculation of $\eta_{\oplus}$, and how detection techniques and surveys differ in their sensitivity and ability to accurately constrain $\eta_{\oplus}$. Understanding and refining the value of $\eta_{\oplus}$ is crucial for upcoming missions and telescopes, such as the planned Habitable Worlds Observatory and the Large Interferometer for Exoplanets, which aim to search for biosignatures on exoplanets in the solar neighborhood.

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arXiv2025

Technique-agnostic exoplanet demography for the Roman era -- I. Testing a demography retrieval framework using simulated Kepler-like transit datasets

The Nancy Grace Roman Space Telescope (Roman) will unveil for the first time the full architecture of planetary systems across Galactic distances through the discovery of up to 200,000 cool and hot exoplanets using microlensing and transit detection methods. Roman's huge exoplanet haul, and Galactic reach, will require new methods to leverage the full exoplanet demographic content of the combined microlensing and transit samples, given the different sensitivity bias of the techniques to planet and host properties and Galactic location. We present a framework for technique-agnostic exoplanet demography (TAED) that can allow large, multi-technique exoplanet samples distributed over Galactic distance scales to be combined for demographic studies. Our TAED forward modelling and retrieval framework uses parameterised model exoplanet demographic distributions to embed planetary systems within a stellar population synthesis model of the Galaxy, enabling internally consistent forecasts to be made for all detection methods that are based on spatio-kinematic system properties. In this paper, as a first test of the TAED framework, we apply it to simulated transit datasets based on the Kepler Data Release 25 to assess parameter recovery accuracy and method scalability for a single large homogeneous dataset. We find that optimisation using differential evolution provides a computationally scalable framework that gives a good balance between computational efficiency and accuracy of parameter recovery.

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arXiv2025

The EBLM project XVI. Moderate spin-orbit misalignment of the low mass eclipsing binary EBLM J0021-16

Thousands of tight ($<1$ AU) main sequence binaries have been discovered, but it is uncertain how they formed. There is likely too much angular momentum in a collapsing, fragmenting protostellar cloud to form such binaries in situ, suggesting some post processing. One probe of a binary's dynamical history is the angle between the stellar spin and orbital axes -- its obliquity. The classical method for determining stellar obliquity is the Rossiter-McLaughlin effect. It has been applied to over 100 hot Jupiters, but less than a dozen stellar binaries. In this paper, we present the Rossiter-McLaughlin measurement of EBLM J0021-16, a $0.19M_\odot$ M-dwarf eclipsing a $1.05M_\odot$ G-dwarf on a 5.97 day, almost-circular orbit. We combine CORALIE spectroscopy with TESS photometry and a measured primary star rotation period of 7.04 days, according to star spot modulation. We show that the orbital axis is misaligned with the primary star's spin axis, with a true 3D obliquity of $\psi=28.9\pm2.1^{\circ}$. EBLM J0021-16, being neither spin-orbit aligned nor synchronized, yet with an almost circular orbit, is a curious case for tidal evolution in tight binaries. It becomes one of a handful of eclipsing binaries with true obliquity measurements. Finally, we derive the M-dwarf's mass and radius to a fractional precision better than 1\%. The radius of the M-dwarf is inflated by 6\% ($7.4\sigma$) with respect to stellar models, consistent with many other M-dwarfs in the literature.

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arXiv2025

PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). II. Phase-Resolved Cross-Correlation Transmission Spectroscopy of KELT-20b

KELT-20b is a well-studied ($T_{\text{eq}}=2262$ K) ultra hot Jupiter, but its multidimensional atmospheric structure remains unconstrained. We performed high-resolution cross-correlation transmission spectroscopy (HRCCTS) on a single transit time series of KELT-20b, observed with PEPSI on the LBT. Upon combining nineteen in-transit exposures, we detect Fe I $(11.9\sigma)$ and Fe II $(23.7\sigma)$ and tentatively detect Na I $(3.4\sigma)$ and Cr I $(3.3\sigma)$. The full-transit velocity offsets of the strongest absorbers are $\Delta V_{\text{Fe I}} = -1.0 \pm 0.7$ km s$^{-1}$ and $\Delta V_{\text{Fe II}}= 0.0\pm 0.5$ km s$^{-1}$, which are mostly inconsistent with previously published values for KELT-20b, although the previous measurements are mostly inconsistent with each other. By correcting for discrepant systemic velocity solutions of up to $1.7$ km s$^{-1}$ between studies, our Fe II offset becomes consistent with previous measurements ($\leq 1.7\sigma$), while Fe I remains significantly less blueshifted than in earlier studies ($ \geq 2.2-4.5\sigma$). We propose a set of detection criteria to improve future reproducibility in HRCCTS work. Phase-resolving the Fe I and Fe II absorption signatures into eight orbital phase bins reveals distinct dynamical regimes: Fe II exhibits a strong phase-dependent blueshift from ingress to egress along with significant limb asymmetry, while Fe I shows weaker signals and a more modest blueshift with phase. These patterns indicate day-to-night winds and suggest scale height differences are a significant driver of limb asymmetry in KELT-20b.

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arXiv2025

PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). III. Composition and winds in the atmosphere of TOI-1518 b

Ultra-hot Jupiters (UHJs) orbit close to their host stars and experience extreme conditions, making them important laboratories to explore atmospheric composition and dynamics. Transmission spectroscopy is a useful tool to reveal chemical species and their vertical and longitudinal distribution in the atmosphere. We use transmission spectra from the PEPSI spectrograph on the Large Binocular Telescope to search for species and measure their time-resolved wind velocities in the atmosphere of TOI-1518 b. We detect Fe I at 7.8$\sigma$ and Fe II at 8.9$\sigma$, and tentatively detect Cr I at 4.4$\sigma$ and Ni I at 4.0$\sigma$. The time-resolved wind velocities of Fe I show a velocity pattern that is consistent with the velocity pattern of Fe II. TOI-1518 b joins a small sample of UHJs for which time-resolved wind velocities have been measured.

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arXiv2025

The EBLM project -- XIV. TESS light curves for eclipsing binaries with very low mass companions

Accurate limb-darkening models are needed for accurate characterisation of eclipsing binary stars and transiting exoplanets from the analysis of their light curves. The limb-darkening observed in solar-type stars from the analysis of light curves for transiting hot-Jupiter exoplanets are systematically less steep than predicted by stellar model atmospheres that do not account for the stellar magnetic field. Hot-Jupiter host stars tend to be metal rich ([Fe/H] ~0.25) leading to a lack of low- and solar-metallicity targets in previous studies, so we have analysed the TESS light curves for a sample of 19 stars with transiting M-dwarf companions to extend the range of limb-darkening measurements to [Fe/H] values more typical for solar-type stars. We find that the systematic offset between the observed and predicted limb-darkening profiles observed in metal-rich hot-Jupiter systems is also observed for these solar-type stars at lower metallicity. These observations provide additional measurements to explore the impact of magnetic fields on the atmospheres of solar-type stars. We have also used the TESS light curves to make precise estimates of the radius and effective temperature of the M-dwarf companions in these 19 binary systems. We confirm the results from previous studies that find very low mass stars tend to be about 3 per cent larger than predicted by stellar models that use a mixing length prescription calibrated on the Sun.

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arXiv2024

PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). I. The Ubiquity of Fe I Emission and Inversions in Ultra Hot Jupiter Atmospheres

We present high-resolution optical emission spectroscopy observations of the ultra hot Jupiters (UHJs) TOI-1431 b and TOI-1518 b using the PEPSI spectrograph on the LBT. We detect emission lines from Fe I with a significance of 5.68 $\sigma$ and 7.68 $\sigma$ for TOI 1431 b and TOI-1518 b, respectively. We also tentatively detect Cr I emission from TOI-1431 b at $4.32\sigma$. For TOI-1518 b, we tentatively detect Ni I, Fe II, and Mg I at significance levels ranging from $3-4\sigma$. Detection of emission lines indicates that both planets possess temperature inversions in their atmospheres, providing further evidence of the ubiquity of stratospheres among UHJs. By analyzing the population of hot Jupiters, we compare models that predict the distribution of planets in the temperature-gravity space, and find a recent global circulation model suite from Roth et al. (2024) provides a reasonable match to the observed onset of inversions at $T_{\mathrm{eq}}\sim2000$ K. The ubiquity of strong Fe I emission lines among UHJs, together with the paucity of detections of TiO, suggest that atomic iron is the dominant optical opacity source in their atmospheres and can be responsible for the inversions.

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arXiv2024

BEBOP V. Homogeneous Stellar Analysis of Potential Circumbinary Planet Hosts

Planets orbiting binary systems are relatively unexplored compared to those around single stars. Detections of circumbinary planets and planetary systems offer a first detailed view into our understanding of circumbinary planet formation and dynamical evolution. The BEBOP (Binaries Escorted by Orbiting Planets) radial velocity survey plays a special role in this adventure as it focuses on eclipsing single-lined binaries with an FGK dwarf primary and M dwarf secondary allowing for the highest-radial velocity precision using the HARPS and SOPHIE spectrographs. We obtained 4512 high-resolution spectra for the 179 targets in the BEBOP survey which we used to derive the stellar atmospheric parameters using both equivalent widths and spectral synthesis. We furthermore derive stellar masses, radii, and ages for all targets. With this work, we present the first homogeneous catalogue of precise stellar parameters for these eclipsing single-lined binaries.

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arXiv2023

The EBLM Project XII. An eccentric, long-period eclipsing binary with a companion near the hydrogen-burning limit

In the hunt for Earth-like exoplanets it is crucial to have reliable host star parameters, as they have a direct impact on the accuracy and precision of the inferred parameters for any discovered exoplanet. For stars with masses between 0.35 and 0.5 ${\rm M_{\odot}}$ an unexplained radius inflation is observed relative to typical stellar models. However, for fully convective objects with a mass below 0.35 ${\rm M_{\odot}}$ it is not known whether this radius inflation is present as there are fewer objects with accurate measurements in this regime. Low-mass eclipsing binaries present a unique opportunity to determine empirical masses and radii for these low-mass stars. Here we report on such a star, EBLM J2114-39\,B. We have used HARPS and FEROS radial-velocities and \textit{TESS} photometry to perform a joint fit of the data, and produce one of the most precise estimates of a very low mass star's parameters. Using a precise and accurate radius for the primary star using {\it Gaia} DR3 data, we determine J2114-39 to be a $M_1 = 0.998 \pm 0.052$~${\rm M_{\odot}}$ primary star hosting a fully convective secondary with mass $M_2~=~0.0986~\pm 0.0038~\,\mathrm{M_{\odot}}$, which lies in a poorly populated region of parameter space. With a radius $R_2 =~0.1275~\pm0.0020~\,\mathrm{R_{\odot}}$, similar to TRAPPIST-1, we see no significant evidence of radius inflation in this system when compared to stellar evolution models. We speculate that stellar models in the regime where radius inflation is observed might be affected by how convective overshooting is treated.

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arXiv2022

Exploring Systematic Errors in the Inferred Parameters of the Transiting Planet KELT-15b and its Host Star

Transiting planet systems offer a unique opportunity to measure the masses and radii of many planets and their host stars. Yet, relative photometry and radial velocity measurements alone only constrain the density of the host star. In remedy, the community uses theoretical and semi-empirical methods to break this one-parameter degeneracy and measure the mass and radius of the host star and its planet(s). We investigate the differences in the inferred system parameters due to modeling a host star with the Torres mass-radius relations, YY evolutionary tracks, MIST evolutionary tracks, and a stellar radius estimate from the spectral energy distribution (SED). We consider the effects of different priors on the stellar effective temperature, limb darkening, and eccentricity of the planet. Using the publicly available software package EXOFASTv2, we globally model TESS photometry and radial velocity observations of KELT-15, which hosts a fairly representative hot Jupiter. In total, we explore the impact of 28 different choices of priors on the inferred parameters of KELT-15b. We find broad agreement in the inferred system parameters across methodologies at the level of ~1.1 sigma between the MIST and SED constraints. This gives some confidence that systematic errors are not ubiquitous in transiting planets systems. We also find a ~2 sigma difference in the stellar radius estimated by the MIST models when we adopt differing literature spectroscopic effective temperature estimates. Similar studies of a large number of systems are needed to definitely assess systematic uncertainties the exoplanet population as a whole.

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arXiv2022

Revised Temperatures For Two Benchmark M-dwarfs -- Outliers No More

Well-characterised M-dwarfs are rare, particularly with respect to effective temperature. In this letter we re-analyse two benchmark M-dwarfs in eclipsing binaries from Kepler/K2: KIC 1571511AB and HD 24465AB. Both have temperatures reported to be hotter or colder by approximately 1000 K in comparison with both models and the majority of the literature. By modelling the secondary eclipses with both the original data and new data from TESS we derive significantly different temperatures which are not outliers. Removing this discrepancy allows these M-dwarfs to be truly benchmarks. Our work also provides relief to stellar modellers. We encourage more measurements of M-dwarf effective temperatures with robust methods.

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arXiv2022

The EBLM project X. Benchmark masses, radii and temperatures for two fully convective M-dwarfs using K2

M-dwarfs are the most abundant stars in the galaxy and popular targets for exoplanet searches. However, their intrinsic faintness and complex spectra inhibit precise characterisation. We only know of dozens of M-dwarfs with fundamental parameters of mass, radius and effective temperature characterised to better than a few per cent. Eclipsing binaries remain the most robust means of stellar characterisation. Here we present two targets from the Eclipsing Binary Low Mass (EBLM) survey that were observed with K2: EBLM J0055-00 and EBLM J2217-04. Combined with HARPS and CORALIE spectroscopy, we measure M-dwarf masses with precisions better than 5%, radii better than 3% and effective temperatures on order 1%. However, our fits require invoking a model to derive parameters for the primary star. By investigating three popular models, we determine that the model uncertainty is of similar magnitude to the statistical uncertainty in the model fits. Therefore, whilst these can be considered benchmark M-dwarfs, we caution the community to consider model uncertainty when pushing the limits of precise stellar characterisation.

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arXiv2021

K2, Spitzer, and TESS Transits of Four Sub-Neptune Exoplanets

We present new Spitzer transit observations of four K2 transiting sub-Neptunes: K2-36c, K2-79b, K2-167b, and K2-212b. We derive updated orbital ephemerides and radii for these planets based on a joint analysis of the Spitzer, TESS, and K2 photometry. We use the EVEREST pipeline to provide improved K2 photometry, by detrending instrumental noise and K2's pointing jitter. We used a pixel level decorrelation method on the Spitzer observations to reduce instrumental systematic effects. We modeled the effect of possible blended eclipsing binaries, seeking to validate these planets via the achromaticity of the transits (K2 versus Spitzer). However, we find that Spitzer's signal-to-noise ratio for these small planets is insufficient to validate them via achromaticity. Nevertheless, by jointly fitting radii between K2 and Spitzer observations, we were able to independently confirm the K2 radius measurements. Due to the long time baseline between the K2 and Spitzer observations, we were also able to increase the precision of the orbital periods compared to K2 observations alone. The improvement is a factor of 3 for K2-36c, and more than an order of magnitude for the remaining planets. Considering possible JWST observations in 1/2023, previous 1 sigma uncertainties in transit times for these planets range from 74 to 434 minutes, but we have reduced them to the range of 8 to 23 minutes.

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