arXiv subjects

Scott J. Kenyon

Publications and source records attributed to Scott J. Kenyon.

At least 19 recordsLinked to original sources

arXiv2026

An astrometric search for planets in debris disk systems

Debris disks are likely created and sculpted by planetary bodies in the orbital space they share. The properties of these disks, including mass, orbital extent, and morphology, can be indicators of their planetary shepherds. Recently, T. D. Pearce and collaborators placed limits on the masses and orbits of hypothetical planets around 178 stars with debris disks. Here, we consider 176 of these stars, all objects that have astrometric data in the Gaia Data Release 3 archive, to assess planet detection from astrometry. Our analysis begins with a set of stellar hosts of known exoplanets, selected to have parallax, apparent magnitude, and color similar to the 176 debris disk systems. We confirm that Gaia's ruwe parameter, a measure of the quality of astrometric fitting to a linear drift model, is sensitive to the presence of massive companions, even planetary ones. We also train a machine-learning model with a range of planet host properties from Gaia, considering more detailed astrometric data beyond ruwe that might indicate orbital motion. In addition, the model incorporates Gaia photometry to inform how brightness and stellar type affect detectability, with the risk that selection bias in the training set may affect reliability. Guided by ruwe and predictions of the machine-learning model, we identify stars with debris disks that may host as-yet-undiscovered planets. Overall, the model predictions, ruwe, and the mass limits from debris disk morphology do not show strong trends. Nonetheless, our top candidates with high ruwe and high planethood probability from the machine learning model will be compelling subjects for time-series analyses with Gaia Data Release 4.

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arXiv2026

SKYSURF-11: A New Zodiacal Light Model Optimized for Optical Wavelengths

We present an improved zodiacal light model, optimized for optical wavelengths, using archival Hubble Space Telescope (HST) imaging from the SKYSURF program. The Kelsall et al. 1998 model used infrared imaging from the Diffuse Infrared Background Experiment (DIRBE) on board the Cosmic Background Explorer to create a 3D structure of the interplanetary dust cloud. However, this model cannot accurately represent zodiacal light emission outside of DIRBE's nominal wavelength bandpasses, the bluest of which is 1.25 micron. We present a revision to this model (called ZodiSURF) that incorporates analytical forms of both the scattering phase function and albedo as a function of wavelength, which are empirically determined across optical wavelengths (0.3-1.6 micron) from over 5,000 HST sky surface brightness (sky-SB) measurements. This refined model results in significantly improved predictions of zodiacal light emission at these wavelengths and for Sun angles greater than 80 deg. Fits to HST data show an uncertainty in the model of ~4.5%. Remarkably, the HST sky-SB measurements show an excess of residual diffuse light (HST Sky - ZodiSURF - Diffuse Galactic Light) of 0.013 +/- 0.006 MJy/sr. We suggest that a very dim spherical dust cloud may need to be included in the zodiacal light model, which we present here as a toy model.

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arXiv2026

The Cold Debris Disk Surveys I. Host Star Properties

We describe the dynamical, photometric, and spectroscopic data available for stars targeted by Spitzer and Herschel to search for cold circumstellar dust emission from debris disks, a collection that we name the Cold Debris Disk Surveys (CDDS). These data include Hipparcos and Gaia parallaxes, 0.4-1250 micron photometry, spectral types, effective temperatures, gravities, bolometric luminosities, visual extinctions, metallicities, lithium abundances, rotational periods, projected rotational velocities, the Ca~II HK and IR triplet activity indicators, and X-ray luminosities for 3675 stars. Within this sample, we investigate the frequency of stellar and planetary companions (including potential new proper motion companions); use the data to assign CDDS stars to the field or one of many moving groups, open clusters, or stellar associations; and investigate correlations between stellar activity indicators. In future papers, we plan to explore the magnitude and frequency of infrared excess emission as a function of host star properties; to search for new companions with Gaia; and to examine the evolution of infrared excesses with the ages of stars in clusters and the field.

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arXiv2025

The MIRI Excesses around Degenerates (MEAD) Survey I: A candidate cold brown dwarf in orbit around the nearby white dwarf 2MASS J09424023-4637176

The MIRI Excesses Around Degenerates Survey is a Cycle 2 James Webb Space Telescope (JWST) Survey program designed to image nearby white dwarfs in the mid-IR with the MIRI imaging mode. Only a handful of white dwarfs have previously been observed beyond 8~\micron. This survey gathered observations for 56 white dwarfs within 25~pc at 10 and 15~\micron, probing each white dwarf for unresolved IR excesses, IR flux deficits indicative of collision induced absorption, or resolved substellar companions. We present in this paper observations of our first target, 2MASS J09424023-4637176, (also UCAC4 217-039132), henceforth called MEAD 62. It is a magnetic DA white dwarf with an estimated age of $7.6^{+1.7}_{-2.2}$\,Gyr. A red candidate companion, MEAD 62B, about 2 magnitudes fainter at 15\,$μ$m than the white dwarf is detected at an apparent separation of 1.95". If confirmed, MEAD 62B, would be a $0.014^{+0.002}_{-0.003}$\,\Msun\, brown dwarf with T$_{\rm eff} = 343^{+7}_{-11}$\,K, according to ATMO2020 evolutionary models. Although its red F1000W$-$F1500W color is similar to background galaxies, MEAD 62B, is consistent with being an unresolved point-source from empirical PSF fitting. A false positive analysis yields an expectation number of 0.66 red (F1000W$-$F1500$ \geq +0.80$\,mag) unresolved sources within the same separation (r$\leq2$ arcsec) for the entire MEAD survey. Thus, this candidate companion as likely to be an actual companion as a false-positive unresolved background galaxy. Additional observations to measure common proper motion or sample the SED are warranted to confirm the nature of MEAD 62B. A deep near-infrared imaging detection is achievable from the ground while JWST is needed at longer infrared wavelengths.

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arXiv2025

Symbiotic star candidates in Gaia Data Release 3

Symbiotic stars, binary pairs with a cool giant fueling accretion onto a hot compact companion, offer unique insights to our understanding of stellar evolution. Yet, only a few hundred symbiotic stars are confirmed. Here, we report on a new search for symbiotic star candidates in Gaia Data Release 3 (GDR3), based entirely on the archive's astrometric, photometric, and spectroscopic information. To begin our search, we identified known symbiotic stars in GDR3 and assessed their absolute magnitude and colors, which are dominated by the cool giant. We also considered measures of astrometric quality that might be affected by binary motion in these systems. Finally, from those sources with Gaia spectroscopic data, we built a low-resolution spectral template that characterizes the unique features of these systems, including H$α$ emission from interaction with the giant's wind and radiation from the hot star. We then queried the full GDR3 archive for sources with spectroscopic data that are bright (< 17 mag in G-band), have modest relative parallax uncertainties (< 20%), and fall within a region of color-magnitude space characteristic of red giants, keeping only sources with spectra that quantitatively match our template. A machine-learning algorithm, trained on known symbiotic stars, produced a new catalog of 1,674 sources. From cross-matches with infrared and X-ray surveys, we present 25 of these sources as particularly compelling candidates for new symbiotic stars.

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arXiv2025

Activity in White Dwarf Debris Disks I: Spitzer Legacy Reveals Variability Incompatible with the Canonical Model

This study presents all available, multi-epoch 3.6 and 4.5 $μ$m photometry from Spitzer Space Telescope observations of white dwarf debris disks, including weekly cadence observations of 16 relatively bright systems, and 5 h staring-mode observations for five of these. Significant variability is detected in 85 per cent of disks and across all timescales probed, from minutes to weeks to years, where the largest flux changes correlate with the longest time baselines, and the infrared excesses persist utterly. While each source is idiosyncratic, the overall results indicate the most variable disks correlate with those that are the brightest (dustiest), and also among those with detected gas, demonstrating both dust and gas are produced via ongoing collisions. There is a correlation between flux and colour changes, where disks tend to appear redder when dimmer and bluer when brighter, consistent with an excess of small dust grains produced in collisions, followed by a gradual return to equilibrium. The overall results are a drastic departure from the predictions of the canonical - geometrically thin, optically thick - disk in both flux and colour, but are broadly consistent with collisional evolution based on a simple model. The data presented herein constitute a legacy resource that can inform time-series studies of polluted and dusty white dwarfs, and importantly serve as a basis for future disk modelling, beyond the pioneering canonical framework.

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arXiv2025

A half-ring of ionized circumstellar material trapped in the magnetosphere of a white dwarf merger remnant

Many white dwarfs are observed in compact double white dwarf binaries and, through the emission of gravitational waves, a large fraction are destined to merge. The merger remnants that do not explode in a Type Ia supernova are expected to initially be rapidly rotating and highly magnetized. We here present our discovery of the variable white dwarf ZTF J200832.79+444939.67, hereafter ZTF J2008+4449, as a likely merger remnant showing signs of circumstellar material without a stellar or substellar companion. The nature of ZTF J2008+4449 as a merger remnant is supported by its physical properties: hot ($35,500\pm300$ K) and massive ($1.12\pm0.03$ M$_\odot$), the white dwarf is rapidly rotating with a period of $\approx$ 6.6 minutes and likely possesses exceptionally strong magnetic fields ($\sim$ 400-600 MG) at its surface. Remarkably, we detect a significant period derivative of $(1.80\pm0.09)\times10^{-12}$ s/s, indicating that the white dwarf is spinning down, and a soft X-ray emission that is inconsistent with photospheric emission. As the presence of a mass-transferring stellar or brown dwarf companion is excluded by infrared photometry, the detected spin down and X-ray emission could be tell-tale signs of a magnetically driven wind or of interaction with circumstellar material, possibly originating from the fallback of gravitationally bound merger ejecta or from the tidal disruption of a planetary object. We also detect Balmer emission, which requires the presence of ionized hydrogen in the vicinity of the white dwarf, showing Doppler shifts as high as $\approx$ 2000 km s$^{-1}$. The unusual variability of the Balmer emission on the spin period of the white dwarf is consistent with the trapping of a half ring of ionised gas in the magnetosphere of the white dwarf.

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arXiv2025

Discovery of two new polars evolved past the period bounce

We report the discovery of two new magnetic cataclysmic variables with brown dwarf companions and long orbital periods ($P_{\rm orb}=95\pm1$ and $104\pm2$ min). This discovery increases the sample of candidate magnetic period bouncers with confirmed sub-stellar donors from four to six. We also find their X-ray luminosity from archival XMM-Newton observations to be in the range $L_{\rm X}\approx10^{28}$$-$$10^{29} \mathrm{erg\,s^{-1}}$ in the 0.25$-$10 keV band. This low luminosity is comparable with the other candidates, and at least an order of magnitude lower than the X-ray luminosities typically measured in cataclysmic variables. The X-ray fluxes imply mass transfer rates that are much lower than predicted by evolutionary models, even if some of the discrepancy is due to the accretion energy being emitted in other bands, such as via cyclotron emission at infrared wavelengths. Although it is possible that some or all of these systems formed directly as binaries containing a brown dwarf, it is likely that the donor used to be a low-mass star and that the systems followed the evolutionary track for cataclysmic variables, evolving past the period bounce. The donor in long period systems is expected to be a low-mass, cold brown dwarf. This hypothesis is supported by near-infrared photometric observations that constrain the donors in the two systems to be brown dwarfs cooler than $\approx$1100 K (spectral types T5 or later), most likely losing mass via Roche Lobe overflow or winds. The serendipitous discovery of two magnetic period bouncers in the small footprint of the XMM-Newton source catalog implies a large space density of these type of systems, possibly compatible with the prediction of 40$-$70 per cent of magnetic cataclysmic variables to be period bouncers.

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arXiv2025

Microlensing Constraints on the Stellar and Planetary Mass Functions

The mass function (MF) of isolated objects measured by microlensing consists of both a stellar and a planetary component. We compare the microlensing MFs of Gould et al (2022) and Sumi et al (2023) to other measurements of the MF. The abundance of brown dwarfs in the Sumi et al (2023) stellar MF is consistent with measurements from the local solar neighborhood (Kirkpatrick et al 2024). Microlensing free-floating planets ($μ$FFPs) may may be free-floating or orbit host stars with semimajor axes $a\gtrsim 10~\mathrm{au}$ and therefore can constrain the populations of both free-floating planetary-mass objects and wide-orbit planets. Comparisons to radial velocity and direct imaging planet populations suggest that either most of the $μ$FFP population with masses $>1~M_{\rm Jup}$ is bound to hosts more massive than M dwarfs or some fraction of the observed bound population actually comes from the low-mass tail of the stellar population. The $μ$FFP population also places strong constraints on planets inferred from debris disks and gaps in protoplanetary disks observed by ALMA.

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arXiv2025

The dearth of high-mass hydrogen-atmosphere metal-polluted white dwarfs within 40 pc

We present a population synthesis model which addresses the different mass distributions of the metal-polluted and non-metal-polluted hydrogen-atmosphere white dwarfs identified in volume-limited samples. Specifically, metal-pollution has been observed to be rare in white dwarfs more massive than $\approx$0.7 $M_{\odot}$. Our population synthesis model invokes episodic accretion of planetary debris onto a synthetic population of white dwarfs. We find that the observed difference can be explained in the regime where most debris disks last for $10^4$$-$$10^6$ years. This is broadly consistent with observational estimates that disk lifetimes are on the order 10$^5$$-$10$^7$ years. We also invoke an alternate model which explores an upper limit on planetary system formation and survival around the intermediate-mass progenitors of the more massive white dwarfs. In this scenario, we find an upper limit on the polluted white dwarf mass of $M_{\rm wd}<0.72^{+0.07}_{-0.03}$ M$_{\odot}$. This implies an empirical maximum progenitor mass of $M_{\rm ZAMS}^{\rm max}=2.9^{+0.7}_{-0.3}$ M$_{\odot}$. This value is consistent with the maximum reliable host star mass of currently known exoplanet systems. We conclude by imposing these two results on the sample of He-atmosphere white dwarfs within 40\,pc. We find that both scenarios are capable of providing a consistent solution to the full sample of H- and He-atmosphere white dwarfs.

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arXiv2025

A Pluto-Charon Sonata V. Long-term Stability of the HST State Vector

We analyze a new set of 275 n-body calculations designed to place limits on the masses of the small circumbinary satellites in the Pluto-Charon system. Together with calculations reported in previous papers, we repeat that a robust upper limit on the total mass of the four satellites is roughly $9.5 \times 10^{19}$ g. For satellite volumes derived from New Horizons, this mass limit implies a robust upper limit on the bulk densities of Nix and Hydra, $\lesssim 1.7$ g cm$^{-3}$, that are comparable to the bulk density of Charon. Additional calculations demonstrate that satellite systems with mass $\lesssim 8.25 \times 10^{19}$ g are robustly stable over the current age of the Sun. The bulk densities of Nix and Hydra in these lower mass systems are clearly smaller than the bulk density of Charon. These new n-body results enable accurate measurements of eccentricity and inclination for Nix, Kerberos, and Hydra that agree well with orbital elements derived from numerical calculations with new HST and New Horizons state vectors. With these new state vectors, Styx has a 37% larger eccentricity and an 85% smaller inclination, which makes it more prone to gravitational perturbations from Nix.

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arXiv2024

SKYSURF VI: The Impact of Thermal Variations of HST on Background Light Estimates

The SKYSURF project constrained extragalactic background light (EBL) and diffuse light with the vast archive of Hubble Space Telescope (HST) images. Thermal emission from HST itself introduces an additional uncertain background and hinders accurate measurement of the diffuse light level. Here, we use archival WFC3/IR engineering data to investigate and model changes in the temperature of various components in HST's optical path as a function of time (solar cycle) and time of the year (Earth-Sun distance). We also specifically investigate changes in temperature with HST's orbital phase and time since Earth occultation. We investigate possible correlations between HST component temperature and year, and temperature and month. The thermal background changes by less than one Kelvin in the WFC3 pick-off mirror, one of the most important contributors to the thermal background. We model these data to describe the impact that orbital phase, year, and time of year have on the HST and WFC3 component temperatures, and use this to derive the impact on the thermal dark signal and the resulting diffuse light measurements. Based on this improved modeling, we provide new upper limits on the level of diffuse light of 21 nW m-2 sr-1, 32 nW m-2 sr-1, and 25 nW m-2 sr-1 for F125W, F140W, and F160W. Additionally, by accounting for all known sources of measurement uncertainty, we report lower limits on the level of diffuse light of 12 nW m-2 sr-1, 20 nW m-2 sr-1, and 2 nW m-2 sr-1 for F125W, F140W, and F160W.

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arXiv2024

Expansion properties of the young supernova type Iax remnant Pa 30 revealed

The recently discovered Pa 30 nebula, the putative type Iax supernova remnant associated with the historical supernova of 1181 AD, shows puzzling characteristics that make it unique among known supernova remnants. In particular, Pa 30 exhibits a complex morphology, with a unique radial and filamentary structure, and it hosts a hot stellar remnant at its center, which displays oxygen-dominated, ultra-fast winds. Because of the surviving stellar remnant and the lack of hydrogen and helium in its filaments, it has been suggested that Pa 30 is the product of a failed thermonuclear explosion in a near- or super-Chandrasekhar white dwarf, which created a sub-luminous transient, a rare sub-type of the Ia class of supernovae called type Iax. We here present a detailed study of the 3D structure and velocities of a full radial section of the remnant. The Integral Field Unit (IFU) observations, obtained with the new red channel of the Keck Cosmic Web Imager spectrograph, reveal that the ejecta are consistent with being ballistic, with velocities close to the free-expansion velocity. Additionally, we detect a large cavity inside the supernova remnant and a sharp inner edge to the filamentary structure, which coincides with the outer edge of a bright ring detected in infrared images. Finally, we detect a strong asymmetry in the amount of ejecta along the line of sight, which might hint to an asymmetric explosion. Our analysis provides strong confirmation that the explosion originated from SN 1181.

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arXiv2024

Polarization of circumstellar debris disk light echoes

Light echoes of debris disks around active stars can reveal disk structure and composition even when disks are not spatially resolved. Unfortunately, distinguishing reflected light from quiescent starlight and unexpected post-peak flare structure is challenging, especially for edge-on geometries where the time delay between observed flare photons and light scattered from the near side of the disk is short. Here, we take advantage of the fact that scattered light from a dusty disk is polarized, depending on the location of the scattering site and the orientation of the disk relative to a distant observer. Filtering reflected light into its polarized components allows echoes to stand out in predictable ways. We test this idea with a simple model for a disk around an active M dwarf. Our results demonstrate that the use of polarimetric data of flaring stars can significantly enhance echo signals relative to starlight and yield more robust and accurate fits to disk parameters compared to analyses based on the total intensity alone.

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arXiv2024

The splashback radius and the radial velocity profile of galaxy clusters in IllustrisTNG

We use 1697 clusters of galaxies from the Illustris TNG300-1 simulation (mass $M_{200c}>10^{14}$M$_\odot$ and redshift range $0.01\leq z \leq 1.04$) to explore the physics of the cluster infall region. We use the average radial velocity profile derived from simulated galaxies, ${\rm v_{rad}}(r)$, and the average velocity dispersion of galaxies at each redshift, ${\rm σ_v}(r)$, to explore cluster-centric dynamical radii that characterize the cluster infall region. We revisit the turnaround radius, the limiting outer radius of the infall region, and the radius where the infall velocity has a well-defined minimum. We also explore two new characteristic radii: (i) the point of inflection of ${\rm v_{rad}}(r)$ that lies within the velocity minimum, and (ii) the smallest radius where ${\rm σ_v}(r)$ = $|{\rm v_{rad}}(r)|$. These two, nearly coincident, radii mark the inner boundary of the infall region where radial infall ceases to dominate the cluster dynamics. Both of these galaxy velocity based radii lie within $1σ$ of the observable splashback radius. The minimum in the logarithmic slope of the galaxy number density is an observable proxy for the apocentric radius of the most recently accreted galaxies, the physical splashback radius. The two new dynamically derived radii relate the splashback radius to the inner boundary of the cluster infall region.

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arXiv2023

Galaxy cluster mass accretion rates from IllustrisTNG

We use simulated cluster member galaxies from Illustris TNG300-1 to develop a technique for measuring the galaxy cluster mass accretion rate (MAR) that can be applied directly to observations. We analyze 1318 IllustrisTNG clusters of galaxies with $M_{200c}>10^{14}$M$_\odot$ and $0.01\leq z \leq 1.04$. The MAR we derive is the ratio between the mass of a spherical shell located in the infall region and the time for the infalling shell to accrete onto the virialized region of the cluster. At fixed redshift, an $\sim 1$ order of magnitude increase in $M_{200c}$ results in a comparable increase in MAR. At fixed mass, the MAR increases by a factor of $\sim 5$ from $z=0.01$ to $z=1.04$. The MAR estimates derived from the caustic technique are unbiased and lie within 20% of the MARs based on the true mass profiles. This agreement is crucial for observational derivation of the MAR. The IllustrisTNG results are also consistent with (i) previous merger tree approaches based on N-body dark matter only simulations and with (ii) previously determined MARs of real clusters based on the caustic method. Future spectroscopic and photometric surveys will provide MARs of enormous cluster samples with mass profiles derived from both spectroscopy and weak lensing. Combined with future larger volume hydrodynamical simulations that extend to higher redshift, the MAR promises important insights into evolution of massive systems of galaxies.

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arXiv2023

Planetesimals drifting through dusty and gaseous white dwarf debris discs: Types I, II and III-like migration

The suite of over 60 known planetary debris discs which orbit white dwarfs, along with detections of multiple minor planets in these systems, motivate investigations about the migration properties of planetesimals embedded within the discs. Here, we determine whether any of the migration regimes which are common in (pre-)main-sequence protoplanetary discs, debris discs and ring systems could be active and important in white dwarf discs. We investigate both dust-dominated and gas-dominated regions, and quantitatively demonstrate that Type I and Type II migration, as well as their particulate disc analogues, are too slow to be relevant in white dwarf discs. However, we find that the analogue of Type III migration for particulate discs may be rapid in the dusty regions of asteroid- or moon-generated ($>10^{18}$ kg) white dwarf discs, where a planetesimal exterior to its Roche radius may migrate across the entire disc within its lifetime. This result holds over a wide range of disc boundaries, both within and exterior to $1R_{\odot}$, and such that the probability of migration occurring increases with higher disc masses.

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arXiv2023

SKYSURF-4: Panchromatic HST All-Sky Surface-Brightness Measurement Methods and Results

The diffuse, unresolved sky provides most of the photons that the Hubble Space Telescope (HST) receives, yet remains poorly understood. HST Archival Legacy program SKYSURF aims to measure the 0.2-1.6 $μ$m sky surface brightness (sky-SB) from over 140,000 HST images. We describe a sky-SB measurement algorithm designed for SKYSURF that is able to recover the input sky-SB from simulated images to within 1% uncertainty. We present our sky-SB measurements estimated using this algorithm on the entire SKYSURF database. Comparing our sky-SB spectral energy distribution (SED) to measurements from the literature shows general agreements. Our SKYSURF SED also reveals a possible dependence on Sun angle, indicating either non-isotropic scattering of solar photons off interplanetary dust or an additional component to Zodiacal Light. Finally, we update Diffuse Light limits in the near-IR based on the methods from Carleton et al. (2022), with values of 0.009 MJy sr$^{-1}$ (22 nW m$^{-2}$ sr$^{-1}$) at 1.25 $μ$m, 0.015 MJy sr$^{-1}$ (32 nW m$^{-2}$ sr$^{-1}$) at 1.4 $μ$m, and 0.013 MJy sr$^{-1}$ (25 nW m$^{-2}$ sr$^{-1}$) at 1.6 $μ$m. These estimates provide the most stringent all-sky constraints to date in this wavelength range. SKYSURF sky-SB measurements are made public on the official SKYSURF website and will be used to constrain Diffuse Light in future papers.

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