arXiv subjects

James E. Owen

Publications and source records attributed to James E. Owen.

At least 109 records · Page 6Linked to original sources

arXiv2015

Atmospheres of low-mass planets: the "boil-off"

We show that, for a low-mass planet that orbits its host star within a few tenths of an AU (like the majority of the {\it Kepler} planets), the atmosphere it was able to accumulate while embedded in the proto-planetary disk may not survive unscathed after the disk disperses. This gas envelope, if more massive than a few percent of the core (with a mass below $10 M_\oplus$), has a cooling time that is much longer than the time-scale on which the planet exits the disk. As such, it could not have contracted significantly from its original size, of order the Bondi radius. So a newly exposed proto-planet would be losing mass via a Parker wind that is catalyzed by the stellar continuum radiation. This represents an intermediate stage of mass-loss, occurring soon after the disc has dispersed, but before the EUV/X-ray driven photoevaporation becomes relevant. The surface mass-loss induces a mass movement within the envelope that advects internal heat outward. As a result, the planet atmosphere rapidly cools down and contracts, until it has reached a radius of order $0.1$ Bondi radius, at which time the mass-loss effectively shuts down. Within a million years after the disk disperses, we find a planet that has only about ten percent of its original envelope, and a Kelvin-Helmholtz time that is much longer than its actual age. We suggest that this "boil-off" process may be partially responsible for the lack of planets above a radius of $2.5 R_\oplus$ in the {\it Kepler} data, provided planet formation results in initial envelope masses of tens of percent.

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arXiv2015

The origin and evolution of transition discs: successes, problems and open questions

Transition discs are protoplanetary discs that show evidence for large holes or wide gaps (with widths comparable to their radii) in their dust component. These discs could be giving us clues about the disc destruction mechanism or hints about the location and time-scales for the formation of planets. However, at the moment there remain key gaps in our theoretical understanding. The vast majority of transition discs are accreting onto their central stars, indicating that - at least close to the star - dust has been depleted from the gas by a very large amount. In this review, we discuss evidence for two distinct populations of transition discs: mm-faint - those with low mm-fluxes, small holes ($\lesssim$10AU) and low accretion rates (~1e-10-1e-9 Msun/yr) and mm-bright - discs with large mm-fluxes, large holes ($\gtrsim$20 AU) and high accretion rates ~1e-8 Msun/yr. MM-faint transition discs are consistent with what would naively be expected from a disc undergoing dispersal; however, mm-bright discs are not, and are likely to be rare and long-lived objects. We discuss the two commonly proposed mechanisms for creating transition discs: photoevaporation and planet-disc interactions, with a particular emphasis on how they would evolve in these models, comparing these predictions to the observed population. More theoretical work on explaining the lack of optically thick, non-accreting transition discs is required in both the photoevaporation and planetary hypothesis, before we can start to use transition discs to constrain models of planet formation. Finally, we suggest that the few discs with primordial looking SEDs, but serendipitously imaged, showing large cavities in the mm (e.g. MWC758 & WSB 60) may represent a hidden population of associated objects. Characterising and understanding how these objects fit into the overall paradigm may allow us to unravel the mystery of transition discs.

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arXiv2015

Hot Jupiter Breezes: Time-dependent Outflows from Extrasolar Planets

We explore the dynamics of magnetically controlled outflows from Hot Jupiters, where these flows are driven by UV heating from the central star. In these systems, some of the open field lines do not allow the flow to pass smoothly through the sonic point, so that steady-state solutions do not exist in general. This paper focuses on this type of magnetic field configuration, where the resulting flow becomes manifestly time-dependent. We consider the case of both steady heating and time-variable heating, and find the time scales for the corresponding time variations of the outflow. Because the flow cannot pass through the sonic transition, it remains subsonic and leads to so-called breeze solutions. One manifestation of the time variability is that the flow samples a collection of different breeze solutions over time, and the mass outflow rate varies in quasi-periodic fashion. Because the flow is subsonic, information can propagate inward from the outer boundary, which determines, in part, the time scale of the flow variability. This work finds the relationship between the outer boundary scale and the time scale of flow variations. In practice, the location of the outer boundary is set by the extent of the sphere of influence of the planet. The measured time variability can be used, in principle, to constrain the parameters of the system (e.g., the strengths of the surface magnetic fields).

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arXiv2015

The long-term evolution of photoevaporating transition discs with giant planets

Photo-evaporation and planet formation have both been proposed as mechanisms responsible for the creation of a transition disc. We have studied their combined effect through a suite of 2d simulations of protoplanetary discs undergoing X-ray photoevaporation with an embedded giant planet. In a previous work we explored how the formation of a giant planet triggers the dispersal of the inner disc by photo-evaporation at earlier times than what would have happened otherwise. This is particularly relevant for the observed transition discs with large holes and high mass accretion rates that cannot be explained by photo-evaporation alone. In this work we significantly expand the parameter space investigated by previous simulations. In addition, the updated model includes thermal sweeping, needed for studying the complete dispersal of the disc. After the removal of the inner disc the disc is a non accreting transition disc, an object that is rarely seen in observations. We assess the relative length of this phase, to understand if it is long lived enough to be found observationally. Depending on the parameters, especially on the X-ray luminosity of the star, we find that the fraction of time spent as a non-accretor greatly varies. We build a population synthesis model to compare with observations and find that in general thermal sweeping is not effective enough to destroy the outer disc, leaving many transition discs in a relatively long lived phase with a gas free hole, at odds with observations. We discuss the implications for transition disc evolution. In particular, we highlight the current lack of explanation for the missing non-accreting transition discs with large holes, which is a serious issue in the planet hypothesis.

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arXiv2015

Torque on an exoplanet from an anisotropic evaporative wind

Winds from short-period Earth and Neptune mass exoplanets, driven by high energy radiation from a young star, may evaporate a significant fraction of a planet's mass. If the momentum flux from the evaporative wind is not aligned with the planet/star axis, then it can exert a torque on the planet's orbit. Using steady-state one-dimensional evaporative wind models we estimate this torque using a lag angle that depends on the product of the speed of the planet's upper atmosphere and a flow timescale for the wind to reach its sonic radius. We also estimate the momentum flux from time-dependent one-dimensional hydrodynamical simulations. We find that only in a very narrow regime in planet radius, mass and stellar radiation flux is a wind capable of exerting a significant torque on the planet's orbit. Similar to the Yarkovsky effect, the wind causes the planet to drift outward if atmospheric circulation is prograde (super-rotating) and in the opposite direction if the circulation is retrograde. A close-in super Earth mass planet that loses a large fraction of its mass in a wind could drift a few percent of its semi-major axis. While this change is small, it places constraints on the evolution of resonant pairs such as Kepler 36 b and c.

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arXiv2014

Astro & cosmo-chemical consequences of accretion bursts I: the D/H ratio of water

The D/H ratio of water in protostellar systems is a result of both inheritance from the parent molecular cloud and isotopic exchange in the disc. A possibly widespread feature of disc evolution, ignored in previous studies, is accretion bursts (or FU Orionis outbursts), which may thermally process a large fraction of the water. One proposed underlying mechanism for FU Orionis outbursts relies on the presence of a magnetically dead zone. Here we examine the evolution of (D/H)$_{\rm water}$ in 1D simulations of a disc's evolution that include dead zones and infall from an envelope with given D/H ratio in the infalling water ($\sim 10^{-3}$), and compare the results with similar calculations without dead zones. We find that the accretion bursts result in a significantly lower (D/H)$_{\rm water}$ ratio and a more extended region (radius up to $\sim 1-3$ AU) where water is equilibrated with hydrogen gas (D/H=$2\times 10^{-5}$), when compared to burst-free models. Solar system constraints suggest that our solar nebula either experienced no accretion bursts and had a Schmidt number $\lesssim 0.2$ or had a Schmidt number closer to "nominal" values ($\sim 1$) and experienced several accretion bursts. Finally, future observations of (D/H)$_{\rm water}$ in protoplanetary discs will allow inferences about angular momentum properties of the disc during disc building and the role of accretion bursts.

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arXiv2014

Magnetically Controlled Outflows from Planets

Hot Jupiters can experience mass loss driven by heating from UV radiation from their host stars, and this flow is often controlled by magnetic fields. More specifically, near the planetry surface, the magnetic pressure dominates the ram pressure of the outflow by several orders of magnitude. After leaving the vicinity of the planet, the flow must connect onto the background environment provided by the stellar wind and the stellar magnetic field. This contribution considers magnetically controlled planetary outflows and extends previous work by comparing two different geometries for the background magnetic field provided by the star. In the first case, stellar field is assumed to retain the form of a dipole, which is anti-aligned with the dipole field of the planet. In the second case, the stellar outflow opens up the stellar magnetic field structure so that the background field at the location of the planet is perpendicular to the planetary dipole. Using numerical simulations, we consider the launch of the planetary wind with these field configurations.

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arXiv2014

Importance of thermal diffusion in the gravo-magnetic limit cycle

We consider the role of thermal diffusion due to turbulence and radiation on accretion bursts that occur in protoplanetary discs which contain dead zones. Using 1D viscous disc models we show that diffusive radial transport of heat is important during the gravo-magnetic limit cycle, and can strongly modify the duration and frequency of accretion outbursts. When the Prandtl number is large - such that turbulent diffusion of heat is unimportant - radial radiative diffusion reduces the burst duration compared to models with no diffusive transport of heat. When the Prandtl number is small ($\lesssim 25$) we find that turbulent diffusion dominates the radial transport of heat, reducing the burst duration to $\lesssim 10^3$ years as well as increasing the burst frequency. Furthermore, inclusion of radial transport of heat extends the range of infall rates under which the disc undergoes accretion bursts from $10^{-8}$ to $10^{-6}$ M$_\odot$ yr$^{-1}$ with no diffusion, to $10^{-8}$ to $\gtrsim10^{-4}$ M$_\odot$ yr$^{-1}$ with radiative and strong turbulent diffusion. The relative roles of radiative and turbulent thermal diffusion are likely to vary during an accretion burst, but simple estimates suggest that the expected Prandtl numbers are of the order of 10 in protoplanetary discs, and hence that turbulent diffusion is likely to be an important process during accretion outbursts due to the gravo-magnetic limit cycle.

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arXiv2014

Magnetically controlled mass loss from extrasolar planets in close orbits

We consider the role magnetic fields play in guiding and controlling mass-loss via evaporative outflows from exoplanets that experience UV irradiation. First we present analytic results that account for planetary and stellar magnetic fields, along with mass-loss from both the star and planet. We then conduct series of numerical simulations for gas giant planets, and vary the planetary field strength, background stellar field strength, UV heating flux, and planet mass. These simulations show that the flow is magnetically controlled for moderate field strengths and even the highest UV fluxes, i.e., planetary surface fields $B_P\gtrsim 0.3$ gauss and fluxes $F_{UV}\sim10^{6}$ erg s$^{-1}$. We thus conclude that outflows from all hot Jupiters with moderate surface fields are magnetically controlled. The inclusion of magnetic fields highly suppresses outflow from the night-side of the planet. Only the magnetic field lines near the pole are open and allow outflow to occur. The fraction of open field lines depends sensitively on the strength (and geometry) of the background magnetic field from the star, along with the UV heating rate. The net effect of the magnetic field is to suppress the mass loss rate by (approximately) an order of magnitude. Finally, some open field lines do not allow the flow to pass smoothly through the sonic point; flow along these streamlines does not reach steady-state, resulting in time-variable mass-loss.

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arXiv2014

Snow-lines as probes of turbulent diffusion in protoplanetary discs

Sharp chemical discontinuities can occur in protoplanetary discs, particularly at `snow-lines' where a gas-phase species freezes out to form ice grains. Such sharp discontinuities will diffuse out due to the turbulence suspected to drive angular momentum transport in accretion discs. We demonstrate that the concentration gradient - in the vicinity of the snow-line - of a species present outside a snow-line but destroyed inside is strongly sensitive to the level of turbulent diffusion (provided the chemical and transport time-scales are decoupled) and provides a direct measurement of the radial `Schmidt number' (the ratio of the angular momentum transport to radial turbulent diffusion). Taking as an example the tracer species N$_2$H$^+$, which is expected to be destroyed inside the CO snow-line (as recently observed in TW Hya) we show that ALMA observations possess significant angular resolution to constrain the Schmidt number. Since different turbulent driving mechanisms predict different Schmidt numbers, a direct measurement of the Schmidt number in accretion discs would allow inferences about the nature of the turbulence to be made.

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arXiv2014

Accreting planets as dust dams in `transition' discs

We investigate under what circumstances an embedded planet in a protoplanetary disc may sculpt the dust distribution such that it observationally presents as a `transition' disc. We concern ourselves with `transition' discs that have large holes ($\gtrsim 10$ AU) and high accretion rates ($\sim 10^{-9}-10^{-8}$ M$_\odot$ yr$^{-1}$). Particularly, those discs which photoevaporative models struggle to explain. Assuming the standard picture for how massive planets sculpt their parent discs, along with the observed accretion rates in `transition' discs, we find that the accretion luminosity from the forming planet is significant, and can dominate over the stellar luminosity at the gap edge. This planetary accretion luminosity can apply a significant radiation pressure to small ($s\lesssim 1μ$m) dust particles provided they are suitably decoupled from the gas. Secular evolution calculations that account for the evolution of the gas and dust components in a disc with an embedded, accreting planet, show that only with the addition of the radiation pressure can we explain the full observed characteristics of a `transition' disc (NIR dip in the SED, mm cavity and high accretion rate). At suitably high planet masses ($\gtrsim 3-4$ M$_J$), radiation pressure from the accreting planet is able to hold back the small dust particles, producing a heavily dust-depleted inner disc that is optically thin (vertically and radially) to Infra-Red radiation. We use our models to calculate synthetic observations and present a observational evolutionary scenario for a forming planet, sculpting its parent disc. The planet-disc system will present as a `transition' disc with a dip in the SED, only when the planet mass and planetary accretion rate is high enough. At other times it will present as a disc with a primordial SED, but with a cavity in the mm, as observed in a handful of protoplanetary discs.

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arXiv2013

Characterising thermal sweeping: a rapid disc dispersal mechanism

(Abridged) We consider the properties of protoplanetary discs that are undergoing inside-out clearing by photoevaporation. In particular, we aim to characterise the conditions under which a protoplanetary disc may undergo `thermal sweeping', a rapid (< 1e4 years) disc destruction mechanism proposed to occur when a clearing disc reaches sufficiently low surface density at its inner edge and where the disc is unstable to runaway penetration by the X-rays. We use a large suite of 1D radiation-hydrodynamic simulations to probe the observable parameter space, which is unfeasible in higher dimensions. These models allow us to determine the surface density at which thermal sweeping will take over the disc's evolution and to evaluate this critical surface density as a function of X-ray luminosity, stellar mass and inner hole radius. We find that this critical surface density scales linearly with X-ray luminosity, increases with inner hole radius and decreases with stellar mass and we develop an analytic model that reproduces these results. This surface density criterion is then used to determine the evolutionary state of protoplanetary discs at the point that they become unstable to destruction by thermal sweeping. We find that transition discs created by photoevaporation will undergo thermal sweeping when their inner holes reach 20-40 AU, implying that transition discs with large holes and no accretion (which were previously a predicted outcome of the later stages of all flavours of photoevaporation model) will not form. We emphasise that the surface density criteria that we have developed apply to all situations where the disc develops an inner hole that is optically thin to X-rays. It thus applies not only to the case of holes originally created by photoevaporation but also to holes formed, for example, by the tidal influence of planets.

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arXiv2013

Kepler planets: a tale of evaporation

(Abridged) Inspired by the Kepler planet discoveries, we consider the thermal contraction of planets close to their parent star, under the influence of evaporation. The mass-loss rates are based on hydrodynamic models of evaporation that include both X-ray and EUV irradiation. We find that only low-mass planets with hydrogen envelopes are significantly affected by evaporation, with evaporation being able to remove massive hydrogen envelopes inward of 0.1 AU for Neptune-mass objects. We construct a theoretical population of planets with varying core masses, envelope masses, orbital separations, and stellar spectral types, and compare these against the sizes and densities measured for low-mass planets, both in the Kepler mission and from radial velocity surveys. This exercise leads us to conclude that evaporation is the driving force of evolution for close-in Kepler planets. In fact, some 50% of the Kepler planet candidates may have been significantly eroded. Evaporation explains two striking correlations observed in these objects: a lack of large radius/low density planets close to the stars, and a bimodal distribution in planet sizes with a deficit of planets around 2R_E. Planets that have experienced high X-ray exposures are generally smaller than this size, and those with lower X-ray exposures are typically larger. A bimodal distribution is naturally explained by the evaporation model, where, depending on their X-ray exposure, close-in planets can either hold on to hydrogen envelopes 1% in mass, or be stripped entirely. To quantitatively reproduce the observed features, we argue that not only do low-mass Kepler planets need to be made of rocky cores overlaid with hydrogen envelopes, but few of them should have initial masses above 20 M_E, and the majority of them should have core masses of a few Earth masses.

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arXiv2013

Testing protoplanetary disc dispersal with radio emission

We consider continuum free-free radio emission from the upper atmosphere of protoplanetary discs as a probe of the ionized luminosity impinging upon the disc. Making use of previously computed hydrodynamic models of disc photoevaporation within the framework of EUV and X-ray irradiation, we use radiative transfer post-processing techniques to predict the expected free-free emission from protoplanetary discs. In general, the free-free luminosity scales roughly linearly with ionizing luminosity in both EUV and X-ray driven scenarios, where the emission dominates over the dust tail of the disc and is partial optically thin at cm wavelengths. We perform a test observation of GM Aur at 14-18 Ghz and detect an excess of radio emission above the dust tail to a very high level of confidence. The observed flux density and spectral index are consistent with free-free emission from the ionized disc in either the EUV or X-ray driven scenario. Finally, we suggest a possible route to testing the EUV and X-ray driven dispersal model of protoplanetary discs, by combining observed free-free flux densities with measurements of mass-accretion rates. On the point of disc dispersal one would expect to find a M_dot^2 scaling with free-free flux in the case of EUV driven disc dispersal or a M_dot scaling in the case of X-ray driven disc dispersal.

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arXiv2013

The interplay between X-ray photoevaporation and planet formation

We assess the potential of planet formation instigating the early formation of a photoevaporation driven gap, up to radii larger than typical for photoevaporation alone. For our investigation we make use of hydrodynamics models of photoevaporating discs with a giant planet embedded. We find that, by reducing the mass accretion flow onto the star, discs that form giant planets will be dispersed at earlier times than discs without planets by X-ray photoevaporation. By clearing the portion of the disc inner of the planet orbital radius, planet formation induced photoevaporation (PIPE) is able to produce transition disc that for a given mass accretion rate have larger holes when compared to standard X-ray photoevaporation. This constitutes a possible route for the formation of the observed class of accreting transition discs with large holes, which are otherwise difficult to explain by planet formation or photoevaporation alone. Moreover, assuming that a planet is able to filter dust completely, PIPE produces a transition disc with a large hole and may provide a mechanism to quickly shut down accretion. This process appears to be too slow however to explain the observed desert in the population of transition disc with large holes and low mass accretion rates.

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arXiv2012

Radiative Transfer in Star Formation: Testing FLD and Hybrid Methods

We perform a comparison between two radiative transfer algorithms commonly employed in hydrodynamical calculations of star formation: grey flux limited diffusion and the hybrid scheme, in addition we compare these algorithms to results from the Monte-Carlo radiative transfer code MOCASSIN. In disc like density structures the hybrid scheme performs significantly better than the FLD method in the optically thin regions, with comparable results in optically thick regions. In the case of a forming high mass star we find the FLD method significantly underestimates the radiation pressure by a factor of ~100.

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arXiv2012

Planetary evaporation by UV & X-ray radiation: basic hydrodynamics

We consider the evaporation of close in planets by the star's intrinsic EUV and X-ray radiation. We calculate evaporation rates by solving the hydrodynamical problem for planetary evaporation including heating from both X-ray and EUV radiation. We show that most close-in planets (a<0.1 AU) are evaporating hydrodynamically, with the evaporation occurring in two distinct regimes: X-ray driven, in which the X-ray heated flow contains a sonic point, and EUV driven, in which the X-ray region is entirely sub-sonic. The mass-loss rates scale as L_X/a^2 for X-ray driven evaporation, and as Phi_*^{1/2}/a for EUV driven evaporation at early times, with mass-loss rates of order 10e10-10e14 g/s. No exact scaling exists for the mass-loss rate with planet mass and planet radius, however, in general evaporation proceeds more rapidly for planets with lower densities and higher masses. Furthermore, we find that in general the transition from X-ray driven to EUV driven evaporation occurs at lower X-ray luminosities for planets closer to their parent stars and for planets with lower densities. Coupling our evaporation models to the evolution of the high energy radiation - which falls with time - we are able to follow the evolution of evaporating planets. We find that most planets start off evaporating in the X-ray driven regime, but switch to EUV driven once the X-ray luminosity falls below a critical value. The evolution models suggest that while `hot Jupiters' are evaporating, they are not evaporating at a rate sufficient to remove the entire gaseous envelope on Gyr time-scales. However, we do find that close in Neptune mass planets are more susceptible to complete evaporation of their envelopes. Thus we conclude that planetary evaporation is more important for lower mass planets, particularly those in the `hot Neptune'/`super Earth' regime.

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arXiv2011

On the theory of disc photoevaporation

We discuss a hydrodynamical model for the dispersal of protoplanetary discs around young, low mass (<1.5 M_sun) stars by photoevaporation from the central object's energetic radiation, which considers the far-ultraviolet as well as the X-ray component of the radiation field. We present analytical scaling relations and derive estimates for the total mass-loss rates, as well as discussing the existence of similarity solutions for flows from primordial discs and discs with inner holes. Furthermore, we perform numerical calculations, which span a wide range of parameter space and allow us to provide accurate scalings of the mass-loss rates with the physical parameters of the systems (X-ray and FUV luminosity, stellar mass, disc mass, disc temperature and inner hole radius). The model suggest that the X-ray component dominates the photoevaporative mass-loss rates from the inner disc. The mass-loss rates have values in the range from 10e-11 to 10e-7 M_sun/yr and scale linearly with X-ray luminosity, with only a weak dependence on the other parameters explored. However, in the case of high FUV to X-ray (L_FUV/L_X>100) luminosity ratios, the FUV constricts the X-ray flow and may dominate the mass-loss. Simulations of low mass discs with inner holes demonstrate a further stage of disc clearing, which we call `thermal sweeping'. This process occurs when the mid-plane pressure drops to sufficiently low values. At this stage a bound, warm, X-ray heated region becomes sufficiently large and unstable, such that the remaining disc material is cleared on approximately dynamical time-scales. This process significantly reduces the time taken to clear the outer regions of the disc, resulting in an expected transition disc population that will be dominated by accreting objects, as indicated by recent observations.

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