arXiv subjects

Ruth Murray-Clay

Publications and source records attributed to Ruth Murray-Clay.

At least 37 records · Page 2Linked to original sources

arXiv2024

Randomness and Retention: Using Weak Mean Motion Resonances to Constrain Neptune's Late-Stage Migration

Planet-planetesimal interactions cause a planet to migrate, manifesting as a random walk in semi-major axis. In models for Neptune's migration involving a gravitational upheaval, this planetesimal-driven migration is a side-effect of the dynamical friction required to damp Neptune's orbital eccentricitiy. This migration is noisy, potentially causing Trans Neptunian Objects (TNOs) in mean motion resonance to be lost. With Nbody simulations, we validate a previously-derived analytic model for resonance retention and determine unknown coefficients. We identify the impact of random-walk (noisy) migration on resonance retention for resonances up to fourth order lying between 39 au and 75 au. Using a population estimate for the weak 7:3 resonance from the well-characterized Outer Solar System Origins Survey (OSSOS), we rule out two cases: (1) a planetesimal disk distributed between 13.3 and 39.9 au with $\gtrsim$ 30 Earth masses in today's size distribution and $T_{\rm mig} \gtrsim$ 40Myr and (2) a top-heavy size distribution with $\gtrsim$ 2000 Pluto-sized TNOs and $T_{\rm mig} \gtrsim$ 10Myr, where $T_{\rm mig}$ is Neptune's migration timescale. We find that low-eccentricity TNOs in the heavily populated 5:2 resonance are easily lost due to noisy migration. Improved observations of the low-eccentricity region of the 5:2 resonance and of weak mean motion resonances with Rubin Observatory's Legacy Survey of Space and Time (LSST) will provide better population estimates, allowing for comparison with our model's retention fractions and providing strong evidence for or against Neptune's random interactions with planetesimals.

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arXiv2024

In Violation of the Prime Directive: Simulating detriments to Delta-Quadrant civilizations from the starship Voyager's impact on planetary rings

In the seven years that the starship Voyager spent in the Delta Quadrant, it used many questionable techniques to engage with alien civilizations and ultimately find its way home. From detailed studies of their logs and opening credits, we simulate Voyager's practice of orbiting a planet, to examine the effect on planetary rings. We outline a feasible planetary system and simulate the extent to which its rings would be disrupted. We find that Voyager's orbit could inflate the height of the rings in the vicinity of the spacecraft by a factor of 2, as well as increase the relative speeds of neighboring planetesimals within the rings. This increase in ring thickness has the potential to alter shadows on any moons of this planet, impacting ring-shadow based religions. Additionally, the acceleration of these planetesimals could rival their gravity, bucking any alien inhabitants and their tiny civilizations off of their planetesimal homeworlds. Finally, we posit that due to increased collisions amongst the planetesimals (which may harbor tiny intelligent life) the trajectory of these civilizations may be forever altered, violating the prime directive.

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arXiv2023

A Pathway for Collisional Planetesimal Growth in the Ice-Dominant Regions of Protoplanetary Disks

We present a semi-analytic model for the growth, drift, desorption, and fragmentation of millimeter- to meter-sized particles in protoplanetary disks. Fragmentation occurs where particle collision velocities exceed critical fragmentation velocities. Using this criterion, we produce fragmentation regions in disk orbital radius-particle size phase space for particles with a range of material properties, structures, and compositions (including SiO$_2$, Mg$_2$SiO$_4$, H$_2$O, CO$_2$, and CO). For reasonable disk conditions, compact aggregate H$_2$O, CO$_2$, and CO ice particles do not reach destructive relative velocities and are thus not likely to undergo collisional fragmentation. Uncoated silicate particles are more susceptible to collisional destruction and are expected to fragment in the inner disk, consistent with previous work. We then calculate the growth, drift, and sublimation of small particles, initially located in the outer disk. We find that ice-coated particles can avoid fragmentation as they grow and drift inward under a substantial range of disk conditions as long as the particles are aggregates composed of 0.1 $μ$m-sized monomers. Such particles may undergo runaway growth in disk regions abundant in H$_2$O or CO$_2$ ice depending on the assumed disk temperature structure. These results indicate that icy collisional growth to planetesimally-relevant sizes may happen efficiently throughout a disk's lifetime, and is particularly robust at early times when the disk's dust-to-gas ratio is comparable to that of the interstellar medium.

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arXiv2023

High Angular Resolution Imaging of the V892 Tau Binary System: A New Circumprimary Disk Detection and Updated Orbital Constraints

We present a direct imaging study of V892 Tau, a young Herbig Ae/Be star with a close-in stellar companion and circumbinary disk. Our observations consist of images acquired via Keck 2/NIRC2 with non-redundant masking and the pyramid wavefront sensor at K$^\prime$ band (2.12$μ$m) and L$^\prime$ band (3.78$μ$m). Sensitivity to low-mass accreting companions and cool disk material is high at L$^\prime$ band, while complimentary observations at K$^\prime$ band probe hotter material with higher angular resolution. These multi-wavelength, multi-epoch data allow us to differentiate the secondary stellar emission from disk emission and deeply probe the structure of the circumbinary disk at small angular separations. We constrain architectural properties of the system by fitting geometric disk and companion models to the K$^\prime$ and L$^\prime$ band data. From these models, we constrain the astrometric and photometric properties of the stellar binary and update the orbit, placing the tightest estimates to date on the V892 Tau orbital parameters. We also constrain the geometric structure of the circumbinary disk, and resolve a circumprimary disk for the first time.

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arXiv2023

Systematic Multi-Epoch Monitoring of LkCa 15: Dynamic Dust Structures on Solar-System Scales

We present the highest angular resolution infrared monitoring of LkCa 15, a young solar analog hosting a transition disk. This system has been the subject of a number of direct imaging studies from the millimeter through the optical, which have revealed multiple protoplanetary disk rings as well as three orbiting protoplanet candidates detected in infrared continuum (one of which was simultaneously seen at H$α$). We use high-angular-resolution infrared imaging from 2014-2020 to systematically monitor these infrared signals and determine their physical origin. We find that three self-luminous protoplanets cannot explain the positional evolution of the infrared sources, since the longer time baseline images lack the coherent orbital motion that would be expected for companions. However, the data still strongly prefer a time-variable morphology that cannot be reproduced by static scattered-light disk models. The multi-epoch observations suggest the presence of complex and dynamic substructures moving through the forward-scattering side of the disk at $\sim20$ AU, or quickly-varying shadowing by closer-in material. We explore whether the previous H$α$ detection of one candidate would be inconsistent with this scenario, and in the process develop an analytical signal-to-noise penalty for H$α$ excesses detected near forward-scattered light. Under these new noise considerations, the H$α$ detection is not strongly inconsistent with forward scattering, making the dynamic LkCa 15 disk a natural explanation for both the infrared and H$α$ data.

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arXiv2023

Can the orbital distribution of Neptune's 3:2 mean motion resonance result from stability sculpting?

We explore a simplified model of the outcome of an early outer Solar System gravitational upheaval during which objects were captured into Neptune's 3:2 mean motion resonance via scattering rather than smooth planetary migration. We use N-body simulations containing the Sun, the four giant planets, and test particles in the 3:2 resonance to determine whether long-term stability sculpting over 4.5 Gyr can reproduce the observed 3:2 resonant population from an initially randomly scattered 3:2 population. After passing our simulated 3:2 resonant objects through a survey simulator, we find that the semimajor axis (a) and eccentricity (e) distributions are consistent with the observational data (assuming an absolute magnitude distribution constrained by prior studies), suggesting that these could be a result of stability sculpting. However, the inclination (i) distribution cannot be produced be stability sculpting and thus must result from a distinct process that excited the inclinations. Our simulations modestly under-predict the number of objects with high libration amplitudes (Aϕ), possibly because we do not model transient sticking. Finally, our model under-populates the Kozai subresonance compared to both observations and to smooth migration models. Future work is needed to determine whether smooth migration occurring as Neptune's eccentricity damped to its current value can resolve this discrepancy.

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arXiv2022

Depletion of gaseous CO in protoplanetary disks by surface-energy-regulated ice formation

Empirical constraints of fundamental properties of protoplanetary disks are essential for understanding planet formation and planetary properties (1,2). Carbon monoxide (CO) gas is often used to constrain disk properties (3). However, estimates show that the CO gas abundance in disks is depleted relative to expected values (4,5,6,7), and models of various disk processes impacting the CO abundance could not explain this depletion on observed 1Myr timescales (8,9,10,11,12,13,14). Here we demonstrate that surface energy effects on particles in disks, such as the Kelvin effect, that arise when ice heterogeneously nucleates onto an existing particle can efficiently trap CO in its ice phase. In previous ice formation models, CO gas was released when small ice-coated particles were lofted to warmed disk layers. Our model can reproduce the observed abundance, distribution and time evolution of gaseous CO in the four most studied protoplanetary disks (7). We constrain the solid and gaseous CO inventory at the midplane and disk diffusivities and resolve inconsistencies in estimates of the disk mass -- three crucial parameters that control planetary formation.

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arXiv2022

The UV-SCOPE Mission: Ultraviolet Spectroscopic Characterization Of Planets and their Environments

UV-SCOPE is a mission concept to determine the causes of atmospheric mass loss in exoplanets, investigate the mechanisms driving aerosol formation in hot Jupiters, and study the influence of the stellar environment on atmospheric evolution and habitability. As part of these investigations, the mission will generate a broad-purpose legacy database of time-domain ultraviolet (UV) spectra for nearly 200 stars and planets. The observatory consists of a 60 cm, f/10 telescope paired to a long-slit spectrograph, yielding simultaneous, almost continuous coverage between 1203 Å and 4000 Å, with resolutions ranging from 6000 to 240. The efficient instrument provides throughputs > 4% (far-UV; FUV) and > 15% (near-UV; NUV), comparable to HST/COS and much better than HST/STIS, over the same spectral range. A key design feature is the LiF prism, which serves as a dispersive element and provides high throughput even after accounting for radiation degradation. The use of two delta-doped Electron-Multiplying CCD detectors with UV-optimized, single-layer anti-reflection coatings provides high quantum efficiency and low detector noise. From the Earth-Sun second Lagrangian point, UV-SCOPE will continuously observe planetary transits and stellar variability in the full FUV-to-NUV range, with negligible astrophysical background. All these features make UV-SCOPE the ideal instrument to study exoplanetary atmospheres and the impact of host stars on their planets. UV-SCOPE was proposed to NASA as a Medium Explorer (MidEx) mission for the 2021 Announcement of Opportunity. If approved, the observatory will be developed over a 5-year period. Its primary science mission takes 34 months to complete. The spacecraft carries enough fuel for 6 years of operations.

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arXiv2022

Effects of Charge Exchange on the Evaporative Wind of HD 209458b

The role of charge exchange in shaping exoplanet photoevaporation remains a topic of contention. Exchange of electrons between stellar wind protons from the exoplanet's host star and neutral hydrogen from the planet's wind has been proposed as a mechanism to create "energetic neutral atoms" (ENAs), which could explain the high absorption line velocities observed in systems where mass loss is occurring. In this paper we present results from 3D hydrodynamic simulations of the mass loss of a planet similar to HD 209458b. We self-consistently launch a planetary wind by calculating the ionization and heating resulting from incident high-energy radiation, inject a stellar wind into the simulation, and allow electron exchange between the stellar and planetary winds. We predict the potential production of ENAs by the wind-wind interaction analytically, then present the results of our simulations, which confirm the analytic limits. Within the limits of our hydrodynamic simulation, we find that charge exchange with the stellar wind properties examined here is unable to explain the absorption observed at high Doppler velocities.

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arXiv2021

OSSOS: The Eccentricity and Inclination Distributions of the Stable Neptunian Trojans

The minor planets on orbits that are dynamically stable in Neptune's 1:1 resonance on Gyr timescales were likely emplaced by Neptune's outward migration. We explore the intrinsic libration amplitude, eccentricity, and inclination distribution of Neptune's stable Trojans, using the detections and survey efficiency of the Outer Solar System Origins Survey (OSSOS) and Pan-STARRS1. We find that the libration amplitude of the stable Neptunian Trojan population can be well modeled as a Rayleigh distribution with a libration amplitude width $σ_{A_phi}$ of 15$^\circ$. When taken as a whole, the Neptune Trojan population can be acceptably modeled with a Rayleigh eccentricity distribution of width $σ_e$ of 0.045 and a typical sin(i) x Gaussian inclination distribution with a width $σ_i$ of 14 +/- 2 degrees. However, these distributions are only marginally acceptable. This is likely because, even after accounting for survey detection biases, the known large Hr < 8 and small Hr >= 8 Neptune Trojans appear to have markedly different eccentricities and inclinations. We propose that like the classical Kuiper belt, the stable intrinsic Neptunian Trojan population have dynamically `hot' and dynamically `cold' components to its eccentricity/inclination distribution, with $σ_{e-cold}$ ~ 0.02 / $σ_{i-cold}$ ~ 6$^\circ$ and $σ_{e-hot}$~ 0.05 / $σ_{i-hot}$ ~ 18$^\circ$. In this scenario, the `cold' L4 Neptunian Trojan population lacks the Hr >= 8 members and has 13 +11/-6 `cold' Trojans with Hr < 8. On the other hand, the `hot' L4 Neptunian Trojan population has 136 +57/-48 Trojans with Hr < 10 -- a population 2.4 times greater than that of the L4 Jovian Trojans in the same luminosity range.

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arXiv2020

How Consumption and Repulsion Set Planetary Gap Depths and the Final Masses of Gas Giants

Planets open gaps in discs. Gap opening is typically modeled by considering the planetary Lindblad torque which repels disc gas away from the planet's orbit. But gaps also clear because the planet consumes local material. We present a simple, easy-to-use, analytic framework for calculating how gaps deplete and how the disc's structure as a whole changes by the combined action of Lindblad repulsion and planetary consumption. The final mass to which a gap-embedded gas giant grows is derived in tandem. The analytics are tested against 1D numerical experiments and calibrated using published multi-dimensional simulations. In viscous alpha discs, the planet, while clearing a gap, initially accretes practically all of the gas that tries to diffuse past, rapidly achieving super-Jupiter if not brown dwarf status. By contrast, in inviscid discs---that may still accrete onto their central stars by, say, magnetized winds---planets open deep, repulsion-dominated gaps. Then only a small fraction of the disc accretion flow is diverted onto the planet, which grows to a fraction of a Jupiter mass. Transitional disc cavities might be cleared by families of such low-mass objects opening inviscid, repulsion-dominated, overlapping gaps which allow most of the outer disc gas to flow unimpeded onto host stars.

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arXiv2020

Effects of Radiation Pressure on the Evaporative Wind of HD 209458b

The role of radiation pressure in shaping exoplanet photoevaporation remains a topic of contention. Radiation pressure from the exoplanet's host star has been proposed as a mechanism to drive the escaping atmosphere into a "cometary" tail and explain the high velocities observed in systems where mass loss is occurring. In this paper we present results from high-resolution 3-D hydrodynamic simulations of a planet similar to HD 209458b. We self-consistently launch a wind flowing outward from the planet by calculating the ionization and heating resulting from incident high-energy radiation, and account for radiation pressure. We first present a simplified calculation, setting a limit on the Lyman-$α$ flux required to drive the photo-evaporated planetary material to larger radii and line-of-sight velocities. We then present the results of our simulations, which confirm the limits determined by our analytic calculation. We thus demonstrate that, within the limits of our hydrodynamic simulation and for the Lyman-$α$ fluxes expected for HD 209458, radiation pressure is unlikely to significantly affect photoevaporative winds or to explain the high velocities at which wind material is observed, though further possibilities remain to be investigated.

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arXiv2019

OSSOS XVIII: Constraining migration models with the 2:1 resonance using the Outer Solar System Origins Survey

Resonant dynamics plays a significant role in the past evolution and current state of our outer Solar System. The population ratios and spatial distribution of Neptune's resonant populations are direct clues to understanding the history of our planetary system. The orbital structure of the objects in Neptune's 2:1 mean-motion resonance (\emph{twotinos}) has the potential to be a tracer of planetary migration processes. Different migration processes produce distinct architectures, recognizable by well-characterized surveys. However, previous characterized surveys only discovered a few twotinos, making it impossible to model the intrinsic twotino population. With a well-designed cadence and nearly 100\% tracking success, the Outer Solar System Origins Survey (OSSOS) discovered 838 trans-Neptunian objects, of which 34 are securely twotinos with well-constrained libration angles and amplitudes. We use the OSSOS twotinos and the survey characterization parameters via the OSSOS Survey Simulator to inspect the intrinsic population and orbital distributions of twotino. The estimated twotino population, 4400$^{+1500}_{-1100}$ with $H_r<8.66$ (diameter$\sim$100km) at 95\% confidence, is consistent with the previous low-precision estimate. We also constrain the width of the inclination distribution to a relatively narrow value of $σ_i$=6$^\circ$$^{+1}_{-1}$, and find the eccentricity distribution is consistent with a Gaussian centered on $e_\mathrm{c}=0.275$ with a width $e_\mathrm{w}=0.06$. We find a single-slope exponential luminosity function with $α=0.6$ for the twotinos. Finally, we for the first time meaningfully constrain the fraction of symmetric twotinos, and the ratio of the leading asymmetric islands; both fractions are in a range of 0.2--0.6. These measurements rule out certain theoretical models of Neptune's migration history.

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arXiv2019

Using Ice and Dust Lines to Constrain the Surface Densities of Protoplanetary Disks

We present a novel method for determining the surface density of protoplanetary disks through consideration of disk 'dust lines' which indicate the observed disk radial scale at different observational wavelengths. This method relies on the assumption that the processes of particle growth and drift control the radial scale of the disk at late stages of disk evolution such that the lifetime of the disk is equal to both the drift timescale and growth timescale of the maximum particle size at a given dust line. We provide an initial proof of concept of our model through an application to the disk TW Hya and are able to estimate the disk dust-to-gas ratio, CO abundance, and accretion rate in addition to the total disk surface density. We find that our derived surface density profile and dust-to-gas ratio are consistent with the lower limits found through measurements of HD gas. The CO ice line also depends on surface density through grain adsorption rates and drift and we find that our theoretical CO ice line estimates have clear observational analogues. We further apply our model to a large parameter space of theoretical disks and find three observational diagnostics that may be used to test its validity. First we predict that the dust lines of disks other than TW Hya will be consistent with the normalized CO surface density profile shape for those disks. Second, surface density profiles that we derive from disk ice lines should match those derived from disk dust lines. Finally, we predict that disk dust and ice lines will scale oppositely, as a function of surface density, across a large sample of disks.

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arXiv2019

New Constraints From Dust Lines On The Surface Densities Of Protoplanetary Disks

We present new determinations of disk surface density, independent of an assumed dust opacity, for a sample of 7 bright, diverse protoplanetary disks using measurements of disk dust lines. We develop a robust method for determining the location of dust lines by modeling disk interferometric visibilities at multiple wavelengths. The disks in our sample have newly derived masses that are 9-27% of their host stellar mass, substantially larger than the minimum mass solar nebula. All are stable to gravitational collapse except for one which approaches the limit of Toomre-Q stability. Our mass estimates are 2-15 times larger than estimates from integrated optically thin dust emission. We derive depleted dust-to-gas ratios with typical values of ~$10^{-3}$ in the outer disk. Using coagulation models we derive dust surface density profiles that are consistent with millimeter dust observations. In these models, the disks formed with an initial dust mass that is a factor of ~10 greater than is presently observed. Of the three disks in our sample with resolved CO line emission, the masses of HD 163296, AS 209, and TW Hya are roughly 3, 115, and 40 times more massive than estimates from CO respectively. This range indicates that CO depletion is not uniform across different disks and that dust is a more robust tracer of total disk mass. Our method of determining surface density using dust lines is robust even if particles form as aggregates and is useful even in the presence of dust substructure caused by pressure traps. The low Toomre-Q values observed in this sample indicate that at least some disks do not accrete efficiently.

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arXiv2019

Wide-Orbit Exoplanet Demographics

The Kepler, K2 and TESS transit surveys are revolutionizing our understanding of planets orbiting close to their host stars and our understanding of exoplanet systems in general, but there remains a gap in our understanding of wide-orbit planets. This gap in our understanding must be filled if we are to understand planet formation and how it affects exoplanet habitability. We summarize current and planned exoplanet detection programs using a variety of methods: microlensing (including WFIRST), radial velocities, Gaia astrometry, and direct imaging. Finally, we discuss the prospects for joint analyses using results from multiple methods and obstacles that could hinder such analyses. We endorse the findings and recommendations published in the 2018 National Academy report on Exoplanet Science Strategy. This white paper extends and complements the material presented therein.

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arXiv2019

Imaging Giant Protoplanets with the ELTs

We have now accumulated a wealth of observations of the planet-formation environment and of mature planetary systems. These data allow us to test and refine theories of gas-giant planet formation by placing constraints on the conditions and timescale of this process. Yet a number of fundamental questions remain unanswered about how protoplanets accumulate material, their photospheric properties and compositions, and how they interact with protoplanetary disks. While we have begun to detect protoplanet candidates during the last several years, we are presently only sensitive to the widest separation, highest mass / accretion rate cases. Current observing facilities lack the angular resolution and inner working angle to probe the few-AU orbital separations where giant planet formation is thought to be most efficient. They also lack the contrast to detect accretion rates that would form lower mass gas giants and ice giants. Instruments and telescopes coming online over the next decade will provide high contrast in the inner giant-planet-forming regions around young stars, allowing us to build a protoplanet census and to characterize planet formation in detail for the first time.

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arXiv2018

Photoevaporative Flows From Exoplanet Atmospheres: A 3-D Radiative Hydrodynamic Parameter Study

The photoionization-driven evaporation of planetary atmospheres has emerged as a potentially fundamental process for planets on short period orbits. While 1-D studies have proven the effectiveness of stellar fluxes at altering the atmospheric mass and composition for sub-Jupiter mass planets, there remains much that is uncertain with regard to the larger-scale, multidimensional nature of such "planetary wind" flows. In this paper we use a new radiation-hydrodynamic platform to simulate atmospheric evaporative flows. Using the AstroBEAR AMR multiphysics code in a co-rotating frame centered on the planet, we model the transfer of ionizing photons into the atmosphere, the subsequent launch of the wind and the wind's large scale evolution subject to tidal and non-inertial forces. We run simulations for planets of 0.263 and 0.07 Jupiter masses and stellar fluxes of $2 \times 10^{13}$ and $2 \times 10^{14}$ photons/cm^2/s. Our results reveal new, potentially observable planetary wind flow patterns, including the development, in some cases, of an extended neutral tail lagging behind the planet in its orbit.

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