arXiv subjects

Ian Dobbs-Dixon

Publications and source records attributed to Ian Dobbs-Dixon.

At least 37 records · Page 2Linked to original sources

arXiv2018

Effects of non-Kozai mutual inclinations on two-planet system stability through all phases of stellar evolution

Previous full-lifetime simulations of single-star multi-planet systems across all phases of stellar evolution have predominately assumed coplanar or nearly-coplanar orbits. Here we assess the consequences of this assumption by removing it and exploring the effect of giant branch mass loss on the stability of two-planet systems with small to moderate non-Kozai (<40 degrees) relative inclinations. We run nearly 10^4 simulations over 14 Gyr for F-star, A-star and B-star planet hosts, incorporating main sequence stellar masses of 1.5, 2.0, 2.5, 3.0 and 5.0 solar masses, and initial planetary semimajor axis ratios that straddle their three-dimensional Hill stability limits. We find that the near-coplanar assumption can approximate well the stability frequencies and critical separations found for higher inclinations, except around strong mean-motion commensurabilities. Late instabilities -- after the star has become a white dwarf -- occur throughout the explored mutual inclination range. Consequently, non-Kozai mutual inclination should not be used as a predictive orbital proxy for determining which white dwarf multi-planet systems discovered by Gaia should represent high-priority follow-up targets for the detection of metal pollution and planetary debris discs.

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arXiv2018

Exocomets in the Proxima Centauri system and their importance for water transport

The scenario and efficiency of water transport by icy asteroids and comets are still amongst the most important unresolved questions of planetary systems. A better understanding of cometary dynamics in extrasolar systems shall provide information about cometary reservoirs and give an insight into water transport especially to planets in the habitable zone. The detection of Proxima Centauri-b (PCb), which moves in the habitable zone of this system, triggered a debate whether or not this planet can be habitable. In this work, we focus on the stability of an additional planet in the system and on water transport by minor bodies. We perform numerous N-body simulations with PCb and an outer Oort-cloud like reservoir of comets. We investigate close encounters and collisions with the planet, which are important for the transport of water. Observers found hints for a second planet with a period longer than 60 days. Our dynamical studies show that two planets in this system are stable even for a more massive second planet (~12 Earth masses). Furthermore, we perform simulations including exocomets, a second planet, and the influence of the binary Alpha Centauri. The studies on the dynamics of exocomets reveal that the outer limit for water transport is around 200 au. In addition we show that water transport would be possible from a close-in planetesimal cloud (1-4 au). From our simulations, based on typical M-star protoplanetary disks, we estimate the water mass delivered to the planets up to 51 Earth oceans.

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arXiv2018

Generation of a Circumstellar Gas Disk by Hot Jupiter WASP-12b

Observations of transiting extra-solar planets provide rich sources of data for probing the in-system environment. In the WASP-12 system, a broad depression in the usually-bright MgII h&k lines has been observed, in addition to atmospheric escape from the extremely hot Jupiter WASP-12b. It has been hypothesized that a translucent circumstellar cloud is formed by the outflow from the planet, causing the observed signatures. We perform 3D hydrodynamic simulations of the full system environment of WASP-12, injecting a planetary wind and stellar wind from their respective surfaces. We find that a torus of density high enough to account for the lack of MgII h&k line core emission in WASP-12 can be formed in approximately 13 years. We also perform synthetic observations of the Lyman-alpha spectrum at different points in the planet's orbit, which demonstrate that significant absorption occurs at all points in the orbit, not just during transits, as suggested by the observations.

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arXiv2018

Giant planets: good neighbors for habitable worlds?

The presence of giant planets influences potentially habitable worlds in numerous ways. Massive celestial neighbors can facilitate the formation of planetary cores and modify the influx of asteroids and comets towards Earth-analogs later on. Furthermore, giant planets can indirectly change the climate of terrestrial worlds by gravitationally altering their orbits. Investigating 147 well characterized exoplanetary systems known to date that host a main sequence star and a giant planet we show that the presence of 'giant neighbors' can reduce a terrestrial planet's chances to remain habitable, even if both planets have stable orbits. In a small fraction of systems, however, giant planets slightly increase the extent of habitable zones provided the terrestrial world has a high climate inertia. In providing constraints on where giant planets cease to affect the habitable zone size in a detrimental fashion, we identify prime targets in the search for habitable worlds.

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arXiv2018

The Implications of 3D Thermal Structure on 1D Atmospheric Retrieval

Using the atmospheric structure from a 3D global radiation-hydrodynamic simulation of HD 189733b and the open-source BART code, we investigate the difference between the secondary-eclipse temperature structure produced with a 3D simulation and the best-fit 1D retrieved model. Synthetic data are generated by integrating the 3D models over the Spitzer, HST, and JWST bandpasses, covering the wavelength range between 1 and 11 um. Using the data from different observing instruments, we present detailed comparisons between the temperature-pressure profiles recovered by BART and those from the 3D simulations. We calculate several averages of the 3D thermal structure and implement two temperature parameterizations to investigate different thermal profile shapes. To assess which part of the thermal structure is best constrained by the data, we generate contribution functions for both our theoretical model and each of our retrieved models. Our conclusions are strongly affected by the spectral resolution of the instruments included, their wavelength coverage, and the number of data points combined. We also see some limitations in each of the temperature parametrizations. The results show that our 1D retrieval is recovering a temperature and pressure profile that most closely matches the arithmetic average of the 3D thermal structure. When we use a higher resolution, more data points, and a parametrized temperature profile that allows more flexibility in the middle part of the atmosphere, we find a better match between the retrieved temperature and pressure profile and the arithmetic average.

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arXiv2018

Detection of a Westward Hotspot Offset in the Atmosphere of a Hot Gas Giant CoRoT-2b

Short-period planets exhibit day-night temperature contrasts of hundreds to thousands of degrees K. They also exhibit eastward hotspot offsets whereby the hottest region on the planet is east of the substellar point; this has been widely interpreted as advection of heat due to eastward winds. We present thermal phase observations of the hot Jupiter CoRoT-2b obtained with the IRAC instrument on the Spitzer Space Telescope. These measurements show the most robust detection to date of a westward hotspot offset of 23 $\pm$ 4 degrees, in contrast with the nine other planets with equivalent measurements. The peculiar infrared flux map of CoRoT-2b may result from westward winds due to non-synchronous rotation magnetic effects, or partial cloud coverage, that obscures the emergent flux from the planet's eastern hemisphere. Non-synchronous rotation and magnetic effects may also explain the planet's anomalously large radius. On the other hand, partial cloud coverage could explain the featureless dayside emission spectrum of the planet. If CoRoT-2b is not tidally locked, then it means that our understanding of star-planet tidal interaction is incomplete. If the westward offset is due to magnetic effects, our result represents an opportunity to study an exoplanet's magnetic field. If it has Eastern clouds, then it means that our understanding of large-scale circulation on tidally locked planets is incomplete.

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arXiv2017

Wavelength Does Not Equal Pressure: Vertical Contribution Functions and their Implications for Mapping Hot Jupiters

Multi-band phase variations in principle allow us to infer the longitudinal temperature distributions of planets as a function of height in their atmospheres. For example, 3.6 micron emission originates from deeper layers of the atmosphere than 4.5 micron due to greater water vapor absorption at the longer wavelength. Since heat transport efficiency increases with pressure, we expect thermal phase curves at 3.6 micron to exhibit smaller amplitudes and greater phase offsets than at 4.5 micron; this trend is not observed. Of the seven hot Jupiters with full-orbit phase curves at 3.6 and 4.5 micron, all have greater phase amplitude at 3.6 micron than at 4.5 micron, while four of seven exhibit a greater phase offset at 3.6 micron. We use a 3D radiative-hydrodynamic model to calculate theoretical phase curves of HD 189733b, assuming thermo-chemical equilibrium. The model exhibits temperature, pressure, and wavelength dependent opacity, primarily driven by carbon chemistry: CO is energetically favored on the dayside, while CH4 is favored on the cooler nightside. Infrared opacity therefore changes by orders of magnitude between day and night, producing dramatic vertical shifts in the wavelength-specific photospheres, which would complicate eclipse or phase mapping with spectral data. The model predicts greater relative phase amplitude and greater phase offset at 3.6 micron than at 4.5 micron, in agreement with the data. Our model qualitatively explains the observed phase curves, but is in tension with current thermo-chemical kinetics models that predict zonally uniform atmospheric composition due to transport of CO from the hot regions of the atmosphere.

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arXiv2017

Dynamic mineral clouds on HD 189733b II. Monte Carlo radiative transfer for 3D cloudy exoplanet atmospheres: combining scattering and emission spectra

As the 3D spatial properties of exoplanet atmospheres are being observed in increasing detail by current and new generations of telescopes, the modelling of the 3D scattering effects of cloud forming atmospheres with inhomogeneous opacity structures becomes increasingly important to interpret observational data. We model the scattering and emission properties of a simulated cloud forming, inhomogeneous opacity, hot Jupiter atmosphere of HD 189733b. We compare our results to available HST and Spitzer data and quantify the effects of 3D multiple-scattering on observable properties of the atmosphere. We discuss potential observational properties of HD 189733b for the upcoming TESS and CHEOPS missions. We develop a Monte Carlo radiative transfer code and apply it to post-process output of our 3D radiative-hydrodynamic, cloud formation simulation of HD 189733b. We employ three variance reduction techniques; next event estimation, survival biasing and composite emission biasing to improve signal-to-noise of the output.For cloud particle scattering events, a log-normal area distribution is constructed from the 3D cloud formation RHD results and stochastically sampled in order to model the Rayleigh and Mie scattering behaviour of a mixture of grain sizes. Stellar photon packets incident on the eastern dayside hemisphere show predominantly Rayleigh, single-scattering behaviour, while multiple-scattering occurs on the western hemisphere. Combined scattered and thermal emitted light predictions are consistent with published HST and Spitzer secondary transit observations. Our model predictions are also consistent with geometric albedo constraints from optical wavelength ground based polarimetry and HST B-band measurements. We predict an apparent geometric albedo for HD 189733b of 0.205 and 0.229, in the TESS and CHEOPS photometric bands respectively.

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arXiv2016

Binary star influence on post-main-sequence multi-planet stability

Nearly every star known to host planets will become a white dwarf, and nearly 100 planet-hosts are now known to be accompanied by binary stellar companions. Here, we determine how a binary companion triggers instability in otherwise unconditionally stable single-star two-planet systems during the giant branch and white dwarf phases of the planet host. We perform about 700 full-lifetime (14 Gyr) simulations with A0 and F0 primary stars and secondary K2 companions, and identify the critical binary distance within which instability is triggered at any point during stellar evolution. We estimate this distance to be about seven times the outer planet separation, for circular binaries. Our results help characterize the fates of planetary systems, and in particular which ones might yield architectures that are conducive to generating observable heavy metal pollution in white dwarf atmospheres.

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arXiv2016

Long term evolution of planetary systems with a terrestrial planet and a giant planet

We study the long term orbital evolution of a terrestrial planet under the gravitational perturbations of a giant planet. In particular, we are interested in situations where the two planets are in the same plane and are relatively close. We examine both possible configurations: the giant planet orbit being either outside or inside the orbit of the smaller planet. The perturbing potential is expanded to high orders and an analytical solution of the terrestrial planetary orbit is derived. The analytical estimates are then compared against results from the numerical integration of the full equations of motion and we find that the analytical solution works reasonably well. An interesting finding is that the new analytical estimates improve greatly the predictions for the timescales of the orbital evolution of the terrestrial planet compared to an octupole order expansion. Finally, we briefly discuss possible applications of the analytical estimates in astrophysical problems.

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arXiv2016

Hot planetary winds near a star: dynamics, wind-wind interactions, and observational signatures

Signatures of "evaporative" winds from exo-planets on short (hot) orbits around their host star have been observed in a number of systems. In this paper we present global AMR simulations that track the launching of the winds, their expansion through the circumstellar environment, and their interaction with a stellar wind. We focus on purely hydrodynamic flows including the anisotropy of the wind launching and explore the orbital/fluid dynamics of the resulting flows in detail. In particular we find that a combination of the tidal and Coriolis forces strongly distorts the planetary "Parker" wind creating "up-orbit" and "down-orbit" streams. We characterize the flows in terms of their orbital elements which change depending on their launch position on the planet. We find that the anisotropy in the atmospheric temperature leads to significant backflow on to the planet. The planetary wind interacts strongly with the stellar wind creating instabilities that cause eventual deposition of planetary gas onto the star. We present synthetic observations of both transit and absorption line-structure for our simulations. For our initial conditions, we find that the orbiting wind material produces absorption signatures at significant distances from the planet and substantial orbit to orbit variability. Ly-α absorption shows red and blueshifted features out to 70 km/s. Finally, using semi-analytic models we constrain the effect of radiation pressure, given the approximation of uniform stellar absorption.

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arXiv2015

Spectral Eclipse Timing

We utilize multi-dimensional simulations of varying equatorial jet strength to predict wavelength dependent variations in the eclipse times of gas-giant planets. A displaced hot-spot introduces an asymmetry in the secondary eclipse light curve that manifests itself as a measured offset in the timing of the center of eclipse. A multi-wavelength observation of secondary eclipse, one probing the timing of barycentric eclipse at short wavelengths and another probing at longer wavelengths, will reveal the longitudinal displacement of the hot-spot and break the degeneracy between this effect and that associated with the asymmetry due to an eccentric orbit. The effect of time offsets was first explored in the IRAC wavebands by Williams et. al (2006). Here we improve upon their methodology, extend to a broad ranges of wavelengths, and demonstrate our technique on a series of multi-dimensional radiative-hydrodynamical simulations of HD 209458b with varying equatorial jet strength and hot-spot displacement. Simulations with the largest hot-spot displacement result in timing offsets of up to 100 seconds in the infrared. Though we utilize a particular radiative hydrodynamical model to demonstrate this effect, the technique is model independent. This technique should allow a much larger survey of hot-spot displacements with JWST then currently accessible with time-intensive phase curves, hopefully shedding light on the physical mechanisms associated with thermal energy advection in irradiated gas-giants.

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arXiv2014

3D Structures of equatorial waves and the resulting superrotation in the atmosphere of a tidally locked hot Jupiter

Three-dimensional equatorial trapped waves excited by stellar isolation and the resulting equatorial superrotating jet in a vertical stratified atmosphere of a tidally-locked hot Jupiter are investigated. Taking the hot Jupiter HD 189733b as a fiducial example, we analytically solve a set of linear equations subject to stationary stellar heating with a uniform zonal-mean flow included. We also extract wave information in the final equilibrium state of the atmosphere from the radiative hydrodynamical simulation for HD 189733b by Dobbs-Dixon & Agol (2013). We find that the analytic wave solutions are able to qualitatively explain the three-dimensional simulation results. Studying the vertical structure of waves allows us to explore new wave features such as the westward tilt of wavefronts related to the Rossby-wave resonance as well as double gyres of dispersive Rossby waves. We also make an attempt to apply our linear wave analysis to explain some numerical features associated with the equatorial jet development seen in the GCM by Showman & Polvani (2011). During the spin-up phase of the equatorial jet, the acceleration of the jet as a result of the divergence of the wave momentum flux can be in principle boosted by the Rossby-wave resonance as the superrotating jet speed is close to the phase speed of the Rossby waves. However, we also find that as the jet speed increases, the Rossby-wave structure shifts eastward, while the Kelvin-wave structure remains approximately stationary, leading to the decline of the acceleration rate. Our analytic model of jet evolution implies that there exists only one stable equilibrium state of the atmosphere, possibly implying that the final state of the atmosphere is independent of initial conditions in the linear regime. Limitations and future improvements are also discussed.

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arXiv2013

Infrared Transmission Spectroscopy of the Exoplanets HD209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope

Exoplanetary transmission spectroscopy in the near-infrared using Hubble/NICMOS is currently ambiguous because different observational groups claim different results from the same data, depending on their analysis methodologies. Spatial scanning with Hubble/WFC3 provides an opportunity to resolve this ambiguity. We here report WFC3 spectroscopy of the giant planets HD209458b and XO-1b in transit, using spatial scanning mode for maximum photon-collecting efficiency. We introduce an analysis technique that derives the exoplanetary transmission spectrum without the necessity of explicitly decorrelating instrumental effects, and achieves nearly photon-limited precision even at the high flux levels collected in spatial scan mode. Our errors are within 6-percent (XO-1) and 26-percent (HD209458b) of the photon-limit at a spectral resolving power of 70, and are better than 0.01-percent per spectral channel. Both planets exhibit water absorption of approximately 200 ppm at the water peak near 1.38 microns. Our result for XO-1b contradicts the much larger absorption derived from NICMOS spectroscopy. The weak water absorption we measure for HD209458b is reminiscent of the weakness of sodium absorption in the first transmission spectroscopy of an exoplanet atmosphere by Charbonneau et al. (2002). Model atmospheres having uniformly-distributed extra opacity of 0.012 cm^2 per gram account approximately for both our water measurement and the sodium absorption in this planet. Our results for HD209458b support the picture advocated by Pont et al. (2013) in which weak molecular absorptions are superposed on a transmission spectrum that is dominated by continuous opacity due to haze and/or dust. However, the extra opacity needed for HD209458b is grayer than for HD189733b, with a weaker Rayleigh component.

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arXiv2012

Stellar irradiated discs and implications on migration of embedded planets I: equilibrium discs

The strength and direction of migration of low mass planets depends on the disc's thermodynamics. In discs where the viscous heating is balanced by radiative transport, the migration can be directed outwards, a process which extends the lifetime of growing planetary embryos. We investigate the influence of opacity and stellar irradiation on the disc thermodynamics. Utilizing the resulting disc structure, we determine the regions of outward migration. We perform two-dimensional numerical simulations of equilibrium discs with viscous heating, radiative cooling and stellar irradiation. We use the hydrodynamical code NIRVANA that includes a full tensor viscosity and stellar irradiation, as well as a two temperature solver that includes radiation transport in the flux-limited diffusion approximation. The migration is studied by using torque formulae. In the constant opacity case, we reproduce the analytical results of a black-body disc: the stellar irradiation dominates in the outer regions -- leading to flaring -- while the viscous heating dominates close to the star. We find that the inner edge of the disc should not be significantly puffed-up by the stellar irradiation. If the opacity depends on the local density and temperature, the structure of the disc is different, and several bumps in the aspect ratio H/r appear, due to transitions between different opacity regimes. The bumps in the disc can shield the outer disc from stellar irradiation. Stellar irradiation is an important factor for determining the disc structure and has dramatic consequences for the migration of embedded planets. Compared to discs with only viscous heating, a stellar irradiated disc features a much smaller region of outward migration for a range of planetary masses. This suggests that the region where the formation of giant planet cores takes place is smaller, which in turn might lead to a shorter growth phase.

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arXiv2012

Three Dimensional Radiative Hydrodynamical Simulations of the Highly Irradiated Short Period Exoplanet HD189733b

We present a detailed three-dimensional radiative-hydrodynamical simulation of the well known irradiated exoplanet HD189733b. Our model solves the fully compressible Navier-Stokes equations coupled to wavelength-dependent radiative transfer throughout the entire planetary envelope. We provide detailed comparisons between the extensive observations of this system and predictions calculated directly from the numerical models. The atmospheric dynamics is characterized by supersonic winds that fairly efficiently advect energy from the dayside to the nightside. Super-rotating equatorial jets form for a wide range of pressures from 10^-5 to 10 bars while counter rotating jets form at higher latitudes. Calculated transit spectrum agree well with the data from the infrared to the UV including the strong Rayleigh scattering seen at short wavelength, though we slightly under-predict the observations at wavelengths shorter then ~0.6 microns. Our predicted emission spectrum agrees remarkably well at 5.8 and 8 microns, but slightly over-predicts the emission at 3.6 and 4.5 microns when compared to the latest analysis by Knutson et. al (2012). Our simulated IRAC phasecurves agree fairly well with the amplitudes of variations, shape, and phases of minimum and maximum flux. However, we over-predict the peak amplitude at 3.6 and 4.5 microns, and slightly under-predict the location of the phasecurve maximum and minimum. These simulations include, for the first time in a multi-dimensional simulation, a strong Rayleigh scattering component to the absorption opacity, necessary to explain observations in the optical and UV. The agreement between our models and observations suggest that including the effects of condensates in simulations as the dominant form of opacity will be very important in future models.

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arXiv2011

The Impact of Circumplantary Jets on Transit Spectra and Timing Offsets for Hot-Jupiters

We present theoretical wavelength-dependent transit light curves for the giant planet HD209458b based on a number of state of the art 3D radiative hydrodynamical models. By varying the kinematic viscosity in the model we calculate observable signatures associated with the emergence of a super-rotating circumplanetary jet that strengthens with decreased viscosity. We obtain excellent agreement between our mid-transit transit spectra and existing data from Hubble and Spitzer, finding the best fit for intermediate values of viscosity. We further exploit dynamically driven differences between eastern and western hemispheres to extract the spectral signal imparted by a circumplanetary jet. We predict that: (i) the transit depth should decrease as the jet becomes stronger; (ii) the measured transit times should show timing offsets of up to 6 seconds at wavelengths with higher opacity, which increases with jet strength; (iii) wavelength-dependent differences between ingress and egress spectra increase with jet strength; (iv) the color-dependent transit shape should vary more strongly with wavelength for stronger jets. These techniques and trends should be valid for other hot Jupiters as well. Observations of transit timing offsets may be accessible with current instrumentation, though the other predictions may require the capabilities JWST and other future missions. Hydrodynamical models utilized solve the 3D Navier-Stokes equations together with decoupled thermal and radiative energy equations and wavelength dependent stellar heating.

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arXiv2010

Radiative Hydrodynamic Simulations of HD209458b: Temporal Variability

We present a new approach for simulating the atmospheric dynamics of the close-in giant planet HD209458b that allows for the decoupling of radiative and thermal energies, direct stellar heating of the interior, and the solution of the full 3D Navier Stokes equations. Simulations reveal two distinct temperature inversions (increasing temperature with decreasing pressure) at the sub-stellar point due to the combined effects of opacity and dynamical flow structure and exhibit instabilities leading to changing velocities and temperatures on the nightside for a range of viscosities. Imposed on the quasi-static background, temperature variations of up to 15% are seen near the terminators and the location of the coldest spot is seen to vary by more than 20 degrees, occasionally appearing west of the anti-solar point. Our new approach introduces four major improvements to our previous methods including simultaneously solving both the thermal energy and radiative equations in both the optical and infrared, incorporating updated opacities, including a more accurate treatment of stellar energy deposition that incorporates the opacity relevant for higher energy stellar photons, and the addition of explicit turbulent viscosity.

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