arXiv subjects

Oded Aharonson

Publications and source records attributed to Oded Aharonson.

At least 19 recordsLinked to original sources

arXiv2025

The Influence of Stellar Chromospheres and Coronae on Exoplanet Transmission Spectroscopy

A main source of bias in transmission spectroscopy of exoplanet atmospheres is magnetic activity of the host star in the form of stellar spots, faculae or flares. However, the fact that main-sequence stars have a chromosphere and a corona, and that these optically thin layers are dominated by line emission may alter the global interpretation of the planetary spectrum, has largely been neglected. Using a JWST NIRISS/SOSS data set of hot Jupiter HAT-P-18 b, we show that even at near-IR and IR wavelengths, the presence of these layers leads to significant changes in the transmission spectrum of the planetary atmosphere. Accounting for these stellar outer layers thus improves the atmospheric fit of HAT-P-18 b, and increases its best-fit atmospheric temperature from 536 K to 736 K, a value much closer to the predicted equilibrium temperature of 852 K. Our analysis also decreases the best-fit abundance of CO2 by almost an order of magnitude. The approach provides a new window to the properties of chromospheres/corona in stars other than our Sun.

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arXiv2024

Planetary Mass Determinations from a Simplified Photodynamical Model -- Application To The Complete Kepler Dataset

We use PyDynamicaLC, a model using the least number of, and the least correlated, degrees of freedom needed to derive a photodynamical model, to describe some of the smallest -- and lowest TTV (transit timing variations) amplitude -- of the Kepler planets. We successfully analyze 64 systems containing 218 planets, for 88 of which we were able to determine significant masses (to better than $3\sigma$). We demonstrate consistency with literature results over two orders of magnitude in mass, and for the planets that already had literature mass estimations, we were able to reduce the relative mass error by $\sim22\%$ (median value). Of the planets with determined masses 23 are new mass determinations with no previous significant literature value, including a planet smaller and lighter than Earth (KOI-1977.02 / Kepler-345 b). These results demonstrate the power of photodynamical modeling with the appropriately chosen degrees of freedom. This will become increasingly more important as smaller planets are detected, especially as the TESS mission gathers ever longer-baseline light curves and for the analysis of the future PLATO mission data

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arXiv2023

Kepler Multi-Transiting Systems Physical Properties and Impact Parameter Variations

We fit a dynamical model to Kepler systems that contain four or more transiting planets using the analytic method AnalyticLC, and obtain physical and orbital parameters for 101 planets in 23 systems, of which 95 are of mass significance better than 3 sigma, and 46 are without previously reported mass constraints nor upper limits. In addition, we compile a list of 71 KOIs that display significant Impact Parameter Variations (TbVs), complementing our previously published work on two- and three-transiting planet systems. Together, these works include the detection of significant TbV signals of 130 planets, which is, to our knowledge, the largest catalog of this type to date. The results indicate that the typical detectable TbV rate in the Kepler population is of order 10^{-2} yr^{-1}, and that rapid TbV rates (>~0.05 yr^{-1}) are observed only in systems that contain a transiting planet of an orbital period less than ~20 days. The observed TbV rates are only weakly correlated with orbital period within Kepler's <~100 days-period planets. If this extends to longer periods, it implies a limit on the utility of the transit technique for long-period planets. The TbVs we find may not be detectable in direct impact parameter measurements but rather are inferred from the full dynamics of the system, encoded in all types of transit variations. Finally, we find evidence that the mutual inclinations distribution is qualitatively consistent with the previously suggested AMD (angular momentum deficit) model using an independent approach.

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arXiv2023

The Advantages of Global Photometric Models in Fitting Transit Variations

Estimation of planetary orbital and physical parameters from light-curve data relies heavily on the accurate interpretation of Transit Timing Variations (TTV) measurements. In this letter, we review the process of TTV measurement and compare two fitting paradigms - one that relies on making transit-by-transit timing estimates and then fitting a TTV model to the observed timings and one that relies on fitting a global flux model to the entire light-curve data simultaneously. The latter method is achieved either by solving for the underlying planetary motion (often referred to as "photodynamics"), or by using an approximate or empirical shape of the TTV signal. We show that across a large range of the transit SNR regime, the probability distribution function (PDF) of the mid-transit time significantly deviates from a Gaussian, even if the flux errors do distribute normally. Treating the timing uncertainties as if they are distributed normally leads, in such a case, to a wrong interpretation of the TTV measurements. We illustrate these points using numerical experiments and conclude that a fitting process that relies on a global flux fitting rather than the derived TTVs, should be preferred.

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arXiv2021

An Accurate 3D Analytic Model for Exoplanetary Photometry, Radial Velocity, and Astrometry

We developed and provide AnalyticLC, a novel analytic method and code implementation for dynamical modeling of planetary systems, including non-coplanar interactions, based on a disturbing function expansion to fourth order in eccentricities and inclinations. AnalyticLC calculates the system dynamics in 3D and the resulting model light curve, radial velocity, and astrometry signatures, enabling simultaneous fitting of these data. We show that for a near-resonant chain of three planets, where the two super-periods are close to each other, the TTVs of the pair-wise interactions cannot be directly summed to give the full system TTVs because the super-periods themselves resonate. We derive the simultaneous three-planet correction and include it in AnalyticLC. We compare the model computed by AnalyticLC to synthetic data generated by an N-body integrator, and evaluate its accuracy. Depending on the maximal order of expansion terms kept, AnalyticLC computation time can be up to an order of magnitude faster than state-of-the-art published N-body integrator TTVFast, with smaller enhancement seen at higher order. The advantage increases for long-term observations as our approach's computation time does not depend on the time-span of the data. Depending on the system parameters, the photometric accuracy is typically a few ppm, significantly smaller than typical Kepler's and other observatories' data uncertainty. Our highly efficient and accurate implementation allows full inversion of a large number of observed systems for planetary physical and orbital parameters, presented in a companion paper.

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arXiv2021

Physical Properties and Impact Parameter Variations of Kepler Planets from Analytic Light Curve Modeling

We apply AnalyticLC, an analytic model described in an accompanying paper, to interpret Kepler data of systems that contain two or three transiting planets. We perform tests to verify that the obtained solutions agree with full N-body integrations, and that the number of model parameters is statistically justified. We probe non-coplanar interactions via impact parameter variations (TbVs), enabled by our analytic model. The subset of systems with a valid solution includes 54 systems composed of 140 planets, more than half of which without previously reported mass constraints. Overall we provide: (1) Estimates on physical and orbital properties for all systems analyzed. (2) 102 planets with mass detections significant to better than 3 standard deviations, 43 of which are lighter than five Earth masses. (3) 35 TbVs significant to better than 3 standard deviations. We focus on select systems showing strong TbVs, which can result from either interaction among the known transiting planets, or with a non-transiting object, and provide: (4) a method to constrain the parameters of such unseen companions. These results are enabled by an accurate, 3D, photodynamical model, of a kind expected to become increasingly important for modeling multi-decade photometric and composite (RV, astrometry) data sets.

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arXiv2021

The influence of Equation of State on impact dynamics between Pluto-like bodies

Impacts between planetary-sized bodies can explain the origin of satellites orbiting large ($R>500$~km) trans-Neptunian objects. Their water rich composition, along with the complex phase diagram of water, make it important to accurately model the wide range of thermodynamic conditions material experiences during an impact event and in the debris disk. Since differences in the thermodynamics may influence the system dynamics, we seek to evaluate how the choice of an equation of state (EOS) alters the system's evolution. Specifically, we compare two EOSs that are constructed by different approaches: either by a simplified analytic description (Tillotson), or by interpolation of tabulated data (Sesame). Approximately $50$ pairs of Smoothed Particle Hydrodynamics impact simulations were performed, with similar initial conditions but different EOSs, in the parameter space in which the Pluto-Charon binary is thought to form (slow impacts between Pluto-size, water rich bodies). Generally, we show that impact outcomes (e.g., circumplanetary debris disk) are consistent between EOSs. Some differences arise, importantly in the production of satellitesimals (large intact clumps) that form in the post-impact debris disk. When utilizing an analytic EOS, the emergence of satellitesimals is highly certain, while when using the tabulated EOS it is less common. This is because for the typical densities and energies experienced in these impacts, the analytic EOS predicts very low pressure values, leading to particles artificially aggregating by a tensile instability.

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arXiv2020

A Simplified Photodynamical Model for Planetary Mass Determination in Low-Eccentricity Multi-Transiting Systems

Inferring planetary parameters from transit timing variations is challenging for small exoplanets because their transits may be so weak that determination of individual transit timing is difficult or impossible. We implement a useful combination of tools which together provide a numerically fast global photodynamical model. This is used to fit the TTV-bearing light-curve, in order to constrain the masses of transiting exoplanets in low eccentricity, multi-planet systems - and small planets in particular. We present inferred dynamical masses and orbital eccentricities in four multi-planet systems from Kepler's complete long-cadence data set. We test our model against Kepler-36 / KOI-277, a system with some of the most precisely determined planetary masses through TTV inversion methods, and find masses of 5.56 +0.41 -0.45 and 9.76 +0.79 -0.89 m_earth for Kepler-36 b and c, respectively - consistent with literature in both value and error. We then improve the mass determination of the four planets in Kepler-79 / KOI-152, where literature values were physically problematic to 12.5 +4.5 -3.6, 9.5 +2.3 -2:1, 11.3 +2.2 -2.2 and 6.3 +1.0 -1.0 m_earth for Kepler-79 b, c, d and e, respectively. We provide new mass constraints where none existed before for two systems. These are 12.5 +3.2 -2.6 m_earth for Kepler-450 c, and 3.3 +1.7 -1.0 and 17.4 +7.1 -3.8 m_earth for Kepler-595 c (previously KOI-547.03) and b, respectively. The photodynamical code used here, called PyDynamicaLC, is made publicly available.

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arXiv2020

Light-Curve Evolution due to Secular Dynamics and the Vanishing Transits of KOI 120.01

Non-Keplerian dynamics of planetary orbits manifest in the transit light-curve as variations of different types. In addition to Transit Timing Variations (TTV's), the shape of the transits contains additional information on variations in the geometry of the orbit. This study presents an analytic approach to light-curve fitting: dynamical variations in the orbital elements are transformed to a light-curve using an analytic function with a restricted set of fitting parameters. Our method requires no N-body integration, resulting in a smaller number of degrees of freedom and a faster calculation. The approach described here is for the case of secular perturbations. By assuming that the orbital motion is dominated by nodal and apsidal precessions, analytic expressions for the light-curve transit parameters are derived as a function of the orbital variations. Detecting and characterizing such dynamical scenarios provides information regarding the possible existence of non-transiting companions, or the non-spherical mass distribution of the host star. The variations may imply forces out of the orbital plane, and thus probe mutual inclinations among components of the system. The derived models successfully reproduce the vanishing transit signals of KOI 120.01, and suggest a possible interesting scenario of a planet orbiting one member of a close-in binary system undergoing unusually rapid nodal regression. The model parameters are degenerate, so we provide relevant information for followup observations, which are suggested in order to place further constraints on this unique Kepler object.

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arXiv2020

Micro Cold Traps on the Moon

Water ice is thought to be trapped in large permanently shadowed regions (PSRs) in the Moon's polar regions, due to their extremely low temperatures. Here, we show that many unmapped cold traps exist on small spatial scales, substantially augmenting the areas where ice may accumulate. Using theoretical models and data from the Lunar Reconnaissance Orbiter, we estimate the contribution of shadows on scales from 1 km down to 1 cm, the smallest distance over which we find cold-trapping to be effective for water ice. Approximately 10-20\% of the permanent cold trap area for water is found to be contained in these "micro cold traps," which are the most numerous cold traps on the Moon. Consideration of all spatial scales therefore substantially increases the number of cold traps over previous estimates, for a total area of ~40,000 km^2. A majority of cold traps for water ice is found at latitudes >80° because permanent shadows equatorward of 80° are typically too warm to support ice accumulation. Our results show that water trapped at the lunar poles may be more accessible as a resource for future missions than previously thought.

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arXiv2019

Surface slopes of asteroid pairs as indicators of mechanical properties and cohesion

Asteroid pairs had a single progenitor that split due to rotational-fission of a weak, rubble-pile structured body. By constructing shape models of asteroid pairs from multiple-apparition observations and using a lightcurve inversion technique, we mapped the gravitational and rotational accelerations on the surfaces of these asteroids. This allows us to construct a map of local slopes on the asteroids' surfaces. In order to test for frictional failure, we determine the maximum rotation rate at which an area larger than half the surface area of the secondary member (assumed to be the ejected component) has a slope value greater than 40 degrees, the angle of friction of lunar regolith, where loose material will begin sliding. We use this criterion to constrain the failure stress operating on the body, just before disruption at the commonly observed spin barrier of 2.2 h. Our current sample includes shape models of eleven primary members of asteroid pairs, observed from the Wise Observatory in the last decade. In the studied parameter space we find that the shape models only reach the spin barrier when their bulk density is larger than the ~2 gr cm-3 measured for the rubble pile structured 25143 Itokawa, suggesting that km-sized asteroid pairs are dense compared to sub-km bodies. Assuming ejection of secondary components that are larger than those observed (up to the maximal size allowing separation), can also increase the spin barrier of the asteroids, thus supporting the previously suggested scenario of continuous disruption of the secondary. In addition, cohesion levels of hundreds of Pascals are also required to prevent these shape models from disrupting at spin rates slower than the usual spin barrier.

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arXiv2019

Impact Dynamics of Moons Within a Planetary Potential

Current lunar origin scenarios suggest that Earth's Moon may have resulted from the merger of two (or more) smaller moonlets. Dynamical studies of multiple moons find that these satellite systems are not stable, resulting in moonlet collision or loss of one or more of the moonlets. We perform Smoothed Particle Hydrodynamic (SPH) impact simulations of two orbiting moonlets inside the planetary gravitational potential and find that the classical outcome of two bodies impacting in free space is altered as erosive mass loss is more significant with decreasing distance to the planet. Depending on the conditions of accretion, each moonlet could have a distinct isotopic signature, therefore, we assess the initial mixing during their merger, in order to estimate whether future measurements of surface variations could distinguish between lunar origin scenarios (single vs. multiple moonlets). We find that for comparable-size impacting bodies in the accretionary regime, surface mixing is efficient, but in the hit-and-run regime, only a small amount of material is transferred between the bodies. However, sequences of hit-and-run impacts are expected, which will enhance the surface mixing. Overall, our results show that large scale heterogeneities can arise only from the merger of drastically different component masses. Surfaces of moons resulting from the merger of comparable-sized components have little material heterogeneities, and such impacts are preferred, as the relatively massive impactor generates more melt, extending the lunar magma ocean phase.

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arXiv2019

Multiple Impact Origin for the Moon

The hypothesis of lunar origin by a single giant impact can explain some aspects of the Earth-Moon system. However, it is difficult to reconcile giant impact models with the compositional similarity of the Earth and Moon without violating angular momentum constraints. Furthermore, successful giant impact scenarios require very specific conditions such that they have a low probability of occurring. Here we present numerical simulations suggesting that the Moon could instead be the product of a succession of a variety of smaller collisions. In this scenario, each collision forms a debris disk around the proto-Earth that then accretes to form a moonlet. The moonlets tidally advance outward, and may coalesce to form the Moon. We find that sub-lunar moonlets are a common result of impacts expected onto the proto-Earth in the early solar system and find that the planetary rotation is limited by impact angular momentum drain. We conclude that, assuming efficient merger of moonlets, a multiple impact scenario can account for the formation of the Earth-Moon system with its present properties.

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arXiv2019

Dynamic and Isotopic Evolution of Ice Reservoirs on Mars

The layered polar caps of Mars have long been thought to be related to variations in orbit and axial tilt. We dynamically link Mars's past climate variations with the stratigraphy and isotopic composition of its ice by modeling the exchange of H2O and HDO among three reservoirs. The model shows that the interplay among equatorial, mid-latitude, and north-polar layered deposits (NPLD) induces significant isotopic changes in the cap. The diffusive properties of the sublimation lags and dust content in our model result in a cap size consistent with current Mars. The layer thicknesses are mostly controlled by obliquity variations, but the precession period of 50 kyr dominates the variations in the isotopic composition during epochs of relatively low and nearly constant obliquity such as at present. Isotopic sampling of the top 100 meters may reveal climate oscillations unseen in the layer thicknesses and would thus probe recent precession-driven climate cycles.

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arXiv2018

The role of multiple giant impacts in the formation of the Earth-Moon system

The Earth-Moon system is suggested to have formed through a single giant collision, in which the Moon accreted from the impact-generated debris disk. However, such giant impacts are rare, and during its evolution the Earth experienced many more smaller impacts, producing smaller satellites that potentially coevolved. In the multiple-impact hypothesis of lunar formation, the current Moon was produced from the mergers of several smaller satellites (moonlets), each formed from debris disks produced by successive large impacts. In the Myrs between impacts, a pre-existing moonlet tidally evolves outward until a subsequent impact forms a new moonlet, at which point both moonlets will tidally evolve until a merger or system disruption. In this work, we examine the likelihood that pre-existing moonlets survive subsequent impact events, and explore the dynamics of Earth-moonlet systems that contain two moonlets generated Myrs apart. We demonstrate that pre-existing moonlets can tidally migrate outward, remain stable during subsequent impacts, and later merge with newly created moonlets (or re-collide with the Earth). Formation of the Moon from the mergers of several moonlets could therefore be a natural byproduct of the Earth's growth through multiple impacts. More generally, we examine the likelihood and consequences of Earth having prior moons, and find that the stability of moonlets against disruption by subsequent impacts implies that several large impacts could post-date Moon formation.

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arXiv2017

Stratigraphy of Aeolis Dorsa, Mars: stratigraphic context of the great river deposits

Unraveling the stratigraphic record is the key to understanding ancient climate and past climate changes on Mars. River deposits when placed in stratigraphic order could constrain the number, magnitudes, and durations of the wettest climates in Mars history. We establish the stratigraphic context of river deposits in Aeolis Dorsa sedimentary basin, 10E of Gale crater. Here, wind has exhumed a stratigraphic section of >=4 unconformity-bounded sedimentary rock packages, recording >=3 distinct episodes of surface runoff. Early deposits (>700m thick) are embayed by river deposits (>400m), which are in turn unconformably draped by fan-shaped deposits (<100m) which we interpret as alluvial fans. Yardang-forming deposits (>900 m) unconformably drape all previous deposits. River deposits embay a dissected sedimentary-rock landscape, and comprise >=2 distinguishable units. The total interval spanned by river deposits is >(1x10^6-2x10^7) yr; more if we include alluvial-fan deposits. Alluvial-fan deposits unconformably postdate thrust faults which crosscut river deposits. We infer a relatively dry interval of >4x10^7 yr after river deposits formed and before fan-shaped deposits formed. The time gap between the end of river deposition and the onset of yardang-forming deposits is constrained to >10^8 yr by the density of impact craters embedded at the unconformity. We correlate yardang-forming deposits to the upper layers of Gale crater's mound (Mt. Sharp/Aeolis Mons), and fan-shaped deposits to Peace Vallis fan. Alternations between periods of low vs. high mean obliquity may have modulated erosion-deposition cycling in Aeolis. This is consistent with results from an ensemble of simulations of Solar System orbital evolution and the resulting history of Mars obliquity. Almost all simulations yield intervals of continuously low mean Mars obliquity that are long enough to match our unconformity data.

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arXiv2017

Low paleopressure of the Martian atmosphere estimated from the size distribution of ancient craters

The decay of the martian atmosphere - which is dominated by carbon dioxide - is a component of the long-term environmental change on Mars from a climate that once allowed rivers to flow to the cold and dry conditions of today. The minimum size of craters serves as a proxy for palaeopressure of planetary atmospheres, because thinner atmospheres permit smaller objects to reach the surface at high velocities and form craters. The Aeolis Dorsa region near Gale crater on Mars contains a high density of preserved ancient craters interbedded with river deposits and thus can provide constraints on atmospheric density around the time of fluvial activity. Here we use high-resolution orthophotos and digital terrain models from the Mars Reconnaissance Orbiter to identify ancient craters in Aeolis Dorsa that date to about 3.6 Gyr ago and compare their size distribution with models of atmospheric filtering of impactors. We obtain an upper limit of 0.9$\pm$0.1 bar, rising to 1.9$\pm$0.2 bar if rimmed circular mesas - interpreted to be erosionally-resistant fills of floors of impact craters - are excluded. We assume target properties appropriate for desert alluvium: if sediment had rock-like rock-mass strength similar to bedrock at the time of impact, the upper limit increases by a factor of up to two. If Mars did not have a stable multibar atmosphere at the time that the rivers were flowing - as suggested by our results - then the warm and wet CO2/H2O greenhouse is ruled out, and long-term average temperatures were most likely below freezing.

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arXiv2017

Global regolith thermophysical properties of the Moon from the Diviner Lunar Radiometer Experiment

We used infrared data from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment to globally map thermophysical properties of the Moon's regolith fines layer. Thermal conductivity varies from 7.4$\times$10$^{-4}$ W m$^{-1}$ K$^{-1}$ at the surface, to 3.4$\times$10$^{-3}$ W m$^{-1}$ K$^{-1}$ at depths of $\sim$1 m, given density values of 1100 kg m$^{-3}$ at the surface, to 1800 kg m$^{-3}$ at 1-m depth. On average, the scale height of these profiles is $\sim$7 cm, corresponding to a thermal inertia of 55 $\pm$2 J m$^{-2}$ K$^{-1}$ s$^{-1/2}$ at 273 K, relevant to the diurnally active near-surface layer, $\sim$4-7 cm. The temperature-dependence of thermal conductivity and heat capacity leads to a $\sim$2$\times$ diurnal variation in thermal inertia at the equator. On global scales, the regolith fines are remarkably uniform, implying rapid homogenization by impact gardening of this layer on timescales $<$ 1 Gyr. Regional and local scale variations show prominent impact features $<$ 1 Gyr old, including higher thermal inertia ($>$ 100 J m$^{-2}$ K$^{-1}$ s$^{-1/2}$) in the interiors and ejecta of Copernican-aged impact craters, and lower thermal inertia ($<$ 50 J m$^{-2}$ K$^{-1}$ s$^{-1/2}$) within the lunar cold spots identified by Bandfield et al. (2014). Observed trends in ejecta thermal inertia provide a potential tool for age-dating craters of previously unknown age, complementary to the approach suggested by Ghent et al. (2014). Several anomalous regions are identified in the global 128 pixels-per-degree maps presented here, including a high-thermal inertia deposit near the antipode of Tycho crater.

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