Imperial College London

DrJamesOwen

Faculty of Natural SciencesDepartment of Physics

Senior Lecturer in Exoplanet Physics
 
 
 
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Contact

 

+44 (0)20 7594 5785james.owen CV

 
 
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Location

 

Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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34 results found

Jankovic MR, Owen JE, Mohanty S, Tan JCet al., 2021, MRI-active inner regions of protoplanetary discs. I. A detailed model of disc structure, Monthly Notices of the Royal Astronomical Society, Vol: 504, Pages: 280-299, ISSN: 0035-8711

Short-period super-Earth-sized planets are common. Explaining how they form near their present orbits requires understanding the structure of the inner regions of protoplanetary discs. Previous studies have argued that the hot inner protoplanetary disc is unstable to the magnetorotational instability (MRI) due to thermal ionization of potassium, and that a local gas pressure maximum forms at the outer edge of this MRI-active zone. Here we present a steady-state model for inner discs accreting viscously, primarily due to the MRI. The structure and MRI-viscosity of the inner disc are fully coupled in our model; moreover, we account for many processes omitted in previous such models, including disc heating by both accretion and stellar irradiation, vertical energy transport, realistic dust opacities, dust effects on disc ionization, and non-thermal sources of ionization. For a disc around a solar-mass star with a standard gas accretion rate (⁠M˙∼10−8 M⊙ yr−1) and small dust grains, we find that the inner disc is optically thick, and the accretion heat is primarily released near the mid-plane. As a result, both the disc mid-plane temperature and the location of the pressure maximum are only marginally affected by stellar irradiation, and the inner disc is also convectively unstable. As previously suggested, the inner disc is primarily ionized through thermionic and potassium ion emission from dust grains, which, at high temperatures, counteract adsorption of free charges on to grains. Our results show that the location of the pressure maximum is determined by the threshold temperature above which thermionic and ion emission become efficient.

Journal article

Rogers JG, Owen JE, 2021, Unveiling the planet population at birth, Monthly Notices of the Royal Astronomical Society, Vol: 503, Pages: 1526-1542, ISSN: 0035-8711

The radius distribution of small, close-in exoplanets has recently been shown to be bimodal. The photoevaporation model predicted this bimodality. In the photoevaporation scenario, some planets are completely stripped of their primordial H/He atmospheres, whereas others retain them. Comparisons between the photoevaporation model and observed planetary populations have the power to unveil details of the planet population inaccessible by standard observations, such as the core mass distribution and core composition. In this work, we present a hierarchical inference analysis on the distribution of close-in exoplanets using forward models of photoevaporation evolution. We use this model to constrain the planetary distributions for core composition, core mass, and initial atmospheric mass fraction. We find that the core-mass distribution is peaked, with a peak-mass of ∼4M⊕. The bulk core-composition is consistent with a rock/iron mixture that is ice-poor and ‘Earth-like’; the spread in core-composition is found to be narrow (⁠≲16 per cent variation in iron-mass fraction at the 2σ level) and consistent with zero. This result favours core formation in a water/ice poor environment. We find the majority of planets accreted a H/He envelope with a typical mass fraction of ∼4 per cent⁠; only a small fraction did not accrete large amounts of H/He and were ‘born-rocky’. We find four times as many super-Earths were formed through photoevaporation, as formed without a large H/He atmosphere. Finally, we find core-accretion theory overpredicts the amount of H/He cores would have accreted by a factor of ∼5, pointing to additional mass-loss mechanisms (e.g. ‘boil-off’) or modifications to core-accretion theory.

Journal article

Robinson CE, Espaillat CC, Owen JE, 2021, Synthetic light curves of accretion variability in T Tauri stars, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 908, Pages: 1-15, ISSN: 0004-637X

Photometric observations of accreting, low-mass, pre-main-sequence stars (i.e., Classical T Tauri stars; CTTS) have revealed different categories of variability. Several of these classifications have been linked to changes in $\dot{M}$. To test how accretion variability conditions lead to different light-curve morphologies, we used 1D hydrodynamic simulations of accretion along a magnetic field line coupled with radiative transfer models and a simple treatment of rotation to generate synthetic light curves. We adopted previously developed metrics in order to classify observations to facilitate comparisons between observations and our models. We found that stellar mass, magnetic field geometry, corotation radius, inclination, and turbulence all play roles in producing the observed light curves and that no single parameter is entirely dominant in controlling the observed variability. While the periodic behavior of the light curve is most strongly affected by the inclination, it is also a function of the magnetic field geometry and inner disk turbulence. Objects with either pure dipole fields, strong aligned octupole components, or high turbulence in the inner disk all tend to display accretion bursts. Objects with anti-aligned octupole components or aligned, weaker octupole components tend to show light curves with slightly fewer bursts. We did not find clear monotonic trends between the stellar mass and empirical classification. This work establishes the groundwork for more detailed characterization of well-studied targets as more light curves of CTTS become available through missions such as the Transiting Exoplanet Survey Satellite (TESS).

Journal article

Bean JL, Raymond SN, Owen JE, 2021, The nature and origins of sub-Neptune size planets, Journal of Geophysical Research: Planets, Vol: 126, Pages: 1-20, ISSN: 2169-9097

Planets intermediate in size between the Earth and Neptune, and orbiting closer to their host stars than Mercury does the Sun, are the most common type of planet revealed by exoplanet surveys over the last quarter century. Results from NASA's Kepler mission have revealed a bimodality in the radius distribution of these objects, with a relative underabundance of planets between 1.5 and 2.0 urn:x-wiley:21699097:media:jgre21507:jgre21507-math-0001. This bimodality suggests that sub‐Neptunes are mostly rocky planets that were born with primary atmospheres a few percent by mass accreted from the protoplanetary nebula. Planets above the radius gap were able to retain their atmospheres (“gas‐rich super‐Earths”), while planets below the radius gap lost their atmospheres and are stripped cores (“true super‐Earths”). The mechanism that drives atmospheric loss for these planets remains an outstanding question, with photoevaporation and core‐powered mass loss being the prime candidates. As with the mass‐loss mechanism, there are two contenders for the origins of the solids in sub‐Neptune planets: the migration model involves the growth and migration of embryos from beyond the ice line, while the drift model involves inward‐drifting pebbles that coagulate to form planets close‐in. Atmospheric studies have the potential to break degeneracies in interior structure models and place additional constraints on the origins of these planets. However, most atmospheric characterization efforts have been confounded by aerosols. Observations with upcoming facilities are expected to finally reveal the atmospheric compositions of these worlds, which are arguably the first fundamentally new type of planetary object identified from the study of exoplanets.

Journal article

Owen JE, Shaikhislamov IF, Lammer H, Fossati L, Khodachenko MLet al., 2020, Hydrogen dominated atmospheres on terrestrial mass planets: evidence, origin and evolution, Space Science Reviews, Vol: 216, Pages: 1-24, ISSN: 0038-6308

The discovery of thousands of highly irradiated, low-mass, exoplanets has led to the idea that atmospheric escape is an important process that can drive their evolution. Of particular interest is the inference from recent exoplanet detections that there is a large population of low mass planets possessing significant, hydrogen dominated atmospheres, even at masses as low as ∼2 M⊕. The size of these hydrogen dominated atmospheres indicates the envelopes must have been accreted from the natal protoplanetary disc. This inference is in contradiction with the Solar System terrestrial planets, that did not reach their final masses before disc dispersal, and only accreted thin hydrogen dominated atmospheres. In this review, we discuss the evidence for hydrogen dominated atmospheres on terrestrial mass (≲2 M⊕) planets. We then discuss the possible origins and evolution of these atmospheres with a focus on the role played by hydrodynamic atmospheric escape driven by the stellar high-energy emission (X-ray and EUV; XUV).

Journal article

Owen JE, 2020, Constraining the entropy of formation from young transiting planet, Monthly Notices of the Royal Astronomical Society, Vol: 498, Pages: 5030-5040, ISSN: 0035-8711

Recently, K2 and TESS have discovered transiting planets with radii between ∼5 and 10 R⊕ around stars with ages <100 Myr. These young planets are likely to be the progenitors of the ubiquitous super-Earths/sub-Neptunes, which are well studied around stars with ages ≳1 Gyr. The formation and early evolution of super-Earths/sub-Neptunes are poorly understood. Various planetary origin scenarios predict a wide range of possible formation entropies. We show how the formation entropies of young (∼20–60 Myr), highly irradiated planets can be constrained if their mass, radius, and age are measured. This method works by determining how low-mass an H/He envelope a planet can retain against mass-loss, this lower bound on the H/He envelope mass can then be converted into an upper bound on the entropy. If planet mass measurements with errors ≲20 per cent can be achieved for the discovered young planets around DS Tuc A and V1298 Tau, then insights into their origins can be obtained. For these planets, higher measured planet masses would be consistent with the standard core-accretion theory. In contrast, lower planet masses (≲6–7 M⊕) would require a ‘boil-off’ phase during protoplanetary disc dispersal to explain.

Journal article

Turbet M, Bolmont E, Bourrier V, Demory B-O, Leconte J, Owen J, Wolf ETet al., 2020, A review of possible planetary atmospheres in the TRAPPIST-1 system, Space Science Reviews, Vol: 216, Pages: 1-48, ISSN: 0038-6308

TRAPPIST-1 is a fantastic nearby (∼39.14 light years) planetary system made of at least seven transiting terrestrial-size, terrestrial-mass planets all receiving a moderate amount of irradiation. To date, this is the most observationally favourable system of potentially habitable planets known to exist. Since the announcement of the discovery of the TRAPPIST-1 planetary system in 2016, a growing number of techniques and approaches have been used and proposed to characterize its true nature. Here we have compiled a state-of-the-art overview of all the observational and theoretical constraints that have been obtained so far using these techniques and approaches. The goal is to get a better understanding of whether or not TRAPPIST-1 planets can have atmospheres, and if so, what they are made of. For this, we surveyed the literature on TRAPPIST-1 about topics as broad as irradiation environment, planet formation and migration, orbital stability, effects of tides and Transit Timing Variations, transit observations, stellar contamination, density measurements, and numerical climate and escape models. Each of these topics adds a brick to our understanding of the likely—or on the contrary unlikely—atmospheres of the seven known planets of the system. We show that (i) Hubble Space Telescope transit observations, (ii) bulk density measurements comparison with H2-rich planets mass-radius relationships, (iii) atmospheric escape modelling, and (iv) gas accretion modelling altogether offer solid evidence against the presence of hydrogen-dominated—cloud-free and cloudy—atmospheres around TRAPPIST-1 planets. This means that the planets are likely to have either (i) a high molecular weight atmosphere or (ii) no atmosphere at all. There are several key challenges ahead to characterize the bulk composition(s) of the atmospheres (if present) of TRAPPIST-1 planets. The main one so far is characterizing and correcting for the effects of stellar contamination.

Journal article

Owen JE, 2020, Snow lines can be thermally unstable, Monthly Notices of the Royal Astronomical Society, Vol: 495, Pages: 3160-3174, ISSN: 0035-8711

Volatile species in protoplanetary discs can undergo a phase change from vapour to solid. These ‘snow lines’ can play vital roles in planet formation at all scales, from dust coagulation to planetary migration. In the outer regions of protoplanetary discs, the temperature profile is set by the absorption of reprocessed stellar light by the solids. Further, the temperature profile sets the distribution of solids through sublimation and condensation at various snow lines. Hence, the snow line position depends on the temperature profile and vice versa. We show that this coupling can be thermally unstable, such that a patch of the disc at a snow line will produce either runaway sublimation or condensation. This thermal instability arises at moderate optical depths, where heating by absorption of reprocessed stellar light from the disc’s atmosphere is optically thick, yet cooling is optically thin. Since volatiles in the solid phase drift much faster than volatiles in the vapour phase, this thermal instability results in a limit cycle. The snow line progressively moves in, condensing volatiles, before receding, as the volatiles sublimate. Using numerical simulations, we study the evolution of the carbon monoxide (CO) snow line. We find the CO snow line is thermally unstable under typical disc conditions and evolves inwards from ∼50 to ∼30 au on time-scales from 1000 to 10 000 yr. The CO snow line spends between ∼10 per centand50 per cent of its time at smaller separations, where the exact value is sensitive to the total opacity and turbulent viscosity. The evolving snow line also creates ring-like structures in the solid distribution interior to the snow line. Multiple ring-like structures created by moving snow lines could potentially explain the substructures seen in many ALMA images.

Journal article

Dempsey R, Zakamska NL, Owen JE, 2020, Formation of Orion fingers, Monthly Notices of the Royal Astronomical Society, Vol: 495, Pages: 1172-1187, ISSN: 0035-8711

‘Orion fingers’ are a system of dozens of bow shocks, with the wings of shocks pointing to a common system of origin, which is centred on a dynamically disintegrating system of several massive stars. The shock heads propagate with velocities of up to 300–400 km s−1, but the formation and physical properties of the ‘bullets’ leading the shocks are not known. Here, we summarize two possible scenarios for the formation of the ‘bullets’ and the resulting bow shocks (‘fingers’). In the first scenario, bullets are self-gravitating, Jupiter-mass objects that were formed rapidly and then ejected during the strong dynamical interactions of massive stars and their discs. This scenario naturally explains the similar time-scales for the outflow of bullets and for the dynamical interaction of the massive stars, but has some difficulty explaining the observed high velocities of the bullets. In the second scenario, bullets are formed via hydrodynamic instabilities in a massive, infrared-driven wind, naturally explaining the high velocities and the morphology of outflow, but the bullets are not required to be self-gravitating. The processes that created the Orion fingers are likely not unique to this particular star-forming region and may result in free-floating, high-velocity, core-less planets.

Journal article

Haworth TJ, Cadman J, Meru F, Hall C, Albertini E, Forgan D, Rice K, Owen JEet al., 2020, Massive discs around low-mass stars, Monthly Notices of the Royal Astronomical Society, Vol: 494, Pages: 4130-4148, ISSN: 0035-8711

We use a suite of smoothed particle hydrodynamic simulations to investigate the susceptibility of protoplanetary discs to the effects of self-gravity as a function of star–disc properties. We also include passive irradiation from the host star using different models for the stellar luminosities. The critical disc-to-star mass ratio for axisymmetry (for which we produce criteria) increases significantly for low-mass stars. This could have important consequences for increasing the potential mass reservoir in a proto Trappist-1 system, since even the efficient Ormel et al. formation model will be influenced by processes like external photoevaporation, which can rapidly and dramatically deplete the dust reservoir. The aforementioned scaling of the critical Md/M* for axisymmetry occurs in part because the Toomre Q parameter has a linear dependence on surface density (which promotes instability) and only an M1/2∗ dependence on shear (which reduces instability), but also occurs because, for a given Md/M*, the thermal evolution depends on the host star mass. The early phase stellar irradiation of the disc (for which the luminosity is much higher than at the zero age main sequence, particularly at low stellar masses) can also play a key role in significantly reducing the role of self-gravity, meaning that even solar mass stars could support axisymmetric discs a factor two higher in mass than usually considered possible. We apply our criteria to the DSHARP discs with spirals, finding that self-gravity can explain the observed spirals so long as the discs are optically thick to the host star irradiation.

Journal article

Booth RA, Owen JE, 2020, Fingerprints of giant planets in the composition of solar twins, Monthly Notices of the Royal Astronomical Society, Vol: 493, Pages: 5079-5088, ISSN: 0035-8711

The Sun shows a ∼10 per cent depletion in refractory elements relative to nearby solar twins. It has been suggested that this depletion is a signpost of planet formation. The exoplanet statistics are now good enough to show that the origin of this depletion does not arise from the sequestration of refractory material inside the planets themselves. This conclusion arises because most sun-like stars host close-in planetary systems that are on average more massive than the Sun’s. Using evolutionary models for the protoplanetary discs that surrounded the young Sun and solar twins, we demonstrate that the origin of the depletion likely arises due to the trapping of dust exterior to the orbit of a forming giant planet. In this scenario, a forming giant planet opens a gap in the gas disc, creating a pressure trap. If the planet forms early enough, while the disc is still massive, the planet can trap ≳100 M⊕ of dust exterior to its orbit, preventing the dust from accreting on to the star in contrast to the gas. Forming giant planets can create refractory depletions of ∼5−15 per cent⁠, with the larger values occurring for initial conditions that favour giant planet formation (e.g. more massive discs that live longer). The incidence of solar twins that show refractory depletion matches both the occurrence of giant planets discovered in exoplanet surveys and ‘transition’ discs that show similar depletion patterns in the material that is accreting on to the star.

Journal article

Haworth TJ, Owen JE, 2020, The observational anatomy of externally photoevaporating planet-forming discs - I. Atomic carbon, Monthly Notices of the Royal Astronomical Society, Vol: 492, Pages: 5030-5040, ISSN: 0035-8711

We demonstrate the utility of C I as a tracer of photoevaporative winds that are being driven from discs by their ambient UV environment. Commonly observed CO lines only trace these winds in relatively weak UV environments and are otherwise dissociated in the wind at the intermediate to high UV fields that most young stars experience. However, C I traces unsubtle kinematic signatures of a wind in intermediate UV environments (∼1000 G0) and can be used to place constraints on the kinematics and temperature of the wind. In C I position–velocity (PV) diagrams external photoevaporation results in velocities that are faster than those from Keplerian rotation alone, as well as emission from quadrants of PV space in which there would be no Keplerian emission. This is independent of viewing angle because the wind has components that are perpendicular to the azimuthal rotation of the disc. At intermediate viewing angles (∼30–60°) moment 1 maps also exhibit a twisted morphology over large scales (unlike other processes that result in twists, which are typically towards the inner disc). C I is readily observable with ALMA, which means that it is now possible to identify and characterize the effect of external photoevaporation on planet-forming discs in intermediate UV environments.

Journal article

Owen JE, Estrada BC, 2020, Testing exoplanet evaporation with multitransiting systems, Monthly Notices of the Royal Astronomical Society, Vol: 491, Pages: 5287-5297, ISSN: 0035-8711

The photoevaporation model is one of the leading explanations for the evolution of small, close-in planets and the origin of the radius-valley. However, without planet mass measurements, it is challenging to test the photoevaporation scenario. Even if masses are available for individual planets, the host star’s unknown EUV/X-ray history makes it difficult to assess the role of photoevaporation. We show that systems with multiple transiting planets are the best in which to rigorously test the photoevaporation model. By scaling one planet to another in a multitransiting system, the host star’s uncertain EUV/X-ray history can be negated. By focusing on systems that contain planets that straddle the radius-valley, one can estimate the minimum masses of planets above the radius-valley (and thus are assumed to have retained a voluminous hydrogen/helium envelope). This minimum mass is estimated by assuming that the planet below the radius-valley entirely lost its initial hydrogen/helium envelope, then calculating how massive any planet above the valley needs to be to retain its envelope. We apply this method to 104 planets above the radius gap in 73 systems for which precise enough radii measurements are available. We find excellent agreement with the photoevaporation model. Only two planets (Kepler-100c and 142c) appear to be inconsistent, suggesting they had a different formation history or followed a different evolutionary pathway to the bulk of the population. Our method can be used to identify TESS systems that warrant radial-velocity follow-up to further test the photoevaporation model.

Journal article

Owen JE, Adams FC, 2019, Effects of magnetic fields on the location of the evaporation valley for low-mass exoplanets, Monthly Notices of the Royal Astronomical Society, Vol: 490, Pages: 15-20, ISSN: 0035-8711

The observed distribution of radii for exoplanets shows a bimodal form that can be explained by mass-loss from planetary atmospheres due to high-energy radiation emitted by their host stars. The location of the minimum of this radius distribution depends on the mass–radius relation, which in turn depends on the composition of planetary cores. Current studies suggest that super-Earth and mini-Neptune planets have iron-rich and hence largely Earth-like composition cores. This paper explores how non-zero planetary magnetic fields can decrease the expected mass-loss rates from these planets. These lower mass-loss rates, in turn, affect the location of the minimum of the radius distribution and the inferred chemical composition of the planetary cores.

Journal article

Owen JE, Kollmeier JA, 2019, Radiation pressure clear-out of dusty photoevaporating discs, Monthly Notices of the Royal Astronomical Society, Vol: 487, Pages: 3702-3714, ISSN: 0035-8711

Theoretical models of protoplanetary disc dispersal predict a phase where photoevaporation has truncated the disc at several au, creating a pressure trap which is dust-rich. Previous models predicted this phase could be long-lived (∼Myr), contrary to the observational constraints. We show that dust in the pressure trap can be removed from the disc by radiation pressure exerting a significant acceleration, and hence radial velocity, on small dust particles that reside in the surface layers of the disc. The dust in the pressure trap is not subject to radial drift so it can grow to reach sizes large enough to fragment. Hence small particles removed from the surface layers are replaced by the fragments of larger particles. This link means radiation pressure can deplete the dust at all particle sizes. Through a combination of 1D and 2D models, along with secular models that follow the disc’s long-term evolution, we show that radiation pressure can deplete dust from pressure traps created by photoevaporation in ∼105 yr, while the photoevaporation created cavity still resides at 10 s of au. After this phase of radiation pressure removal of dust, the disc is gas-rich and dust depleted and radially optically thin to stellar light, having observational signatures similar to a gas-rich, young debris disc. Indeed many of the young stars (≲10 Myr old) classified as hosting a debris disc may rather be discs that have undergone this process.

Journal article

Picogna G, Ercolano B, Owen JE, Weber MLet al., 2019, The dispersal of protoplanetary discs - I. A new generation of X-ray photoevaporation models, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 487, Pages: 691-701, ISSN: 0035-8711

Photoevaporation of planet-forming discs by high-energy radiation from the central star is potentially a crucial mechanism for disc evolution and it may play an important role in the formation and evolution of planetary systems. We present here a new generation of X-ray photoevaporation models for solar-type stars, based on hydrodynamical simulations, which account for stellar irradiation via a significantly improved parametrization of gas temperatures, based on detailed photoionization and radiation transfer calculations. This is the first of a series of papers aiming at providing a library of models which cover the observed parameter space in stellar and disc mass, metallicity, and stellar X-ray properties. We focus here on solar-type stars (0.7 M⊙) with relatively low-mass discs (1 per cent of the stellar mass) and explore the dependence of the wind mass-loss rates on stellar X-ray luminosity. We model primordial discs and transition discs at various stages of evolution. Our two-dimensional hydrodynamical models are then used to derive simple recipes for the mass-loss rates that are suitable for one-dimensional disc evolution and/or planet formation models typically employed for population synthesis studies. Line profiles from typical wind diagnostics ([O I]6300 Å and [Ne II]12.8 μm) are also calculated for our models and found to be roughly in agreement with previous studies. Finally, we perform a population study of transition discs by means of one-dimensional viscous evolution models including our new photoevaporation prescription and find that roughly a half of observed transition discs cavities and accretion rates could be reproduced by our models.

Journal article

Owen JE, 2019, Atmospheric escape and the evolution of close-in exoplanets, Annual Review of Earth and Planetary Sciences, Vol: 47, Pages: 67-90, ISSN: 0084-6597

Exoplanets with substantial hydrogen/helium atmospheres have been discovered in abundance, many residing extremely close to their parent stars.The extreme irradiation levels that these atmospheres experience cause themto undergo hydrodynamic atmospheric escape. Ongoing atmospheric escape has been observed to be occurring in a few nearby exoplanet systems through transit spectroscopy both for hot Jupiters and for lower-masssuper-Earths and mini-Neptunes. Detailed hydrodynamic calculations thatincorporate radiative transfer and ionization chemistry are now common inone-dimensional models, and multidimensional calculations that incorporate magnetic fields and interactions with the interstellar environment arecutting edge. However, comparison between simulations and observationsremains very limited.While hot Jupiters experience atmospheric escape, themass-loss rates are not high enough to affect their evolution. However, forlower-mass planets, atmospheric escape drives and controls their evolution,sculpting the exoplanet population that we observe today.

Journal article

Jankovic MR, Owen JE, Mohanty S, 2019, Close-in super-Earths: The first and the last stages of planet formation in an MRI-accreting disc, Monthly Notices of the Royal Astronomical Society, Vol: 484, Pages: 2296-2308, ISSN: 0035-8711

We explore in situ formation and subsequent evolution of close-in super-Earths and mini-Neptunes. We adopt a steady-state inner protoplanetary gas disc structure that arises from viscous accretion due to the magneto-rotational instability (MRI). We consider the evolution of dust in the inner disc, including growth, radial drift, and fragmentation, and find that dust particles that radially drift into the inner disc fragment severely due to the MRI-induced turbulence. This result has two consequences: (1) radial drift of grains within the inner disc is quenched, leading to an enhancement of dust in the inner regions that scales as dust-to-gas-mass-flux-ratio at ∼1 au; (2) however, despite this enhancement, planetesimal formation is impeded by the small grain size. Nevertheless, assuming that planetary cores are present in the inner disc, we then investigate the accretion of atmospheres on to cores and their subsequent photoevaporation. We then compare our results to the observed exoplanet mass–radius relationship. We find that (1) the low gas surface densities and high temperatures in the inner disc reduce gas accretion on to cores compared to the minimum mass solar nebula, preventing the cores from growing into hot Jupiters, in agreement with the data; (2) however, our predicted envelope masses are still typically larger than observed ones. Finally, we sketch a qualitative picture of how grains may grow and planetesimals form in the inner disc if grain effects on the ionization levels and the MRI and the back reaction of the dust on the gas (both neglected in our calculations) are accounted for.

Journal article

Owen JE, Lai D, 2018, Photoevaporation and high-eccentricity migration created the sub-Jovian desert, Monthly Notices of the Royal Astronomical Society, Vol: 479, Pages: 5012-5021, ISSN: 0035-8711

The mass–period or radius–period distribution of close-in exoplanets shows a paucity of intermediate mass/size (sub-Jovian) planets with periods ≲3 d. We show that this sub-Jovian desert can be explained by the photoevaporation of highly irradiated sub-Neptunes and the tidal disruption barrier for gas giants undergoing high-eccentricity migration. The distinctive triangular shape of the sub-Jovain desert results from the fact that photoevaporation is more effective closer to the host star, and that in order for a gas giant to tidally circularize closer to the star without tidal disruption it needs to be more massive. Our work indicates that super-Earths/mini-Neptunes and hot-Jupiters had distinctly separate formation channels and arrived at their present locations at different times.

Journal article

Van Eylen V, Agentoft C, Lundkvist M, Kjeldsen H, Owen JE, Fulton B, Petigura E, Snellen Iet al., 2018, An asteroseismic view of the radius valley: stripped cores, not born rocky, Monthly Notices of the Royal Astronomical Society, Vol: 479, Pages: 4786-4795, ISSN: 0035-8711

Various theoretical models treating the effect of stellar irradiation on planetary envelopes predict the presence of a radius valley, i.e. a bimodal distribution of planet radii, with super-Earths and sub-Neptune planets separated by a valley at around≈2R⊕. Such a valley has been observed recently, owing to an improvement in the precision of stellar and therefore planetary radii. Here, we investigate the presence, location, and shape of such a valley using a small sample with highly accurate stellar parameters determined from asteroseismology, which includes 117 planets with a median uncertainty on the radius of 3.3 per cent. We detect a clear bimodal distribution, with super-Earths (≈1.5R⊕) and sub-Neptunes (≈2.5 R⊕) separated by a deficiency around2R⊕. We furthermore characterize the slope of the valley as a power law R∝Pγ withγ=−0.09+0.02−0.04. A negative slope is consistent with models of photoevaporation, but not with the late formation of rocky planets in a gas-poor environment, which would lead to a slope of opposite sign. The exact location of the gap further points to planet cores consisting of a significant fraction of rocky material.

Journal article

Owen JE, Murray-Clay R, 2018, Metallicity-dependent signatures in the Kepler planets, Monthly Notices of the Royal Astronomical Society, Vol: 480, Pages: 2206-2216, ISSN: 0035-8711

Using data from the California-Kepler Survey (CKS), we study trends in planetary properties with host star metallicity for close-in planets. By incorporating knowledge of the properties of the planetary radius gap identified by the CKS survey, we are able to investigate the properties of planetary cores and their gaseous envelopes separately. Our primary findings are that the solid core masses of planets are higher around higher metallicity stars and that these more massive cores were able to accrete larger gas envelopes. Furthermore, investigating the recently reported result that planets with radii in the range ( 2–6 R⊕) are more common at short periods around higher metallicity stars in detail, we find that the average host star metallicity of H/He atmosphere-hosting planets increases smoothly inside an orbital period of ∼20 d. We interpret the location of the metallicity increase within the context of atmospheric photoevaporation: higher metallicity stars are likely to host planets with higher atmospheric metallicity, which increases the cooling in the photoevaporative outflow, lowering the mass-loss rates. Therefore, planets with higher metallicity atmospheres are able to resist photoevaporation at shorter orbital periods. Finally, we find evidence at 2.8 σ that planets that do not host H/He atmospheres at long periods are more commonly found around lower metallicity stars. Such planets are difficult to explain by photoevaporative stripping of planets which originally accreted H/He atmospheres. Alternatively, this population of planets could be representative of planets that formed in a terrestrial-like fashion, after the gas disc dispersed.

Journal article

Mohanty S, Jankovic MR, Tan JC, Owen JEet al., 2018, Inside-out planet formation. V. structure of the inner disk as implied by the MRI, Astrophysical Journal, Vol: 861, Pages: 1-27, ISSN: 0004-637X

The large population of Earth to super-Earth sized planets found very closeto their host stars has motivated consideration of $in$ $situ$ formationmodels. In particular, Inside-Out Planet Formation is a scenario in whichplanets coalesce sequentially in the disk, at the local gas pressure maximumnear the inner boundary of the dead zone. The pressure maximum arises from adecline in viscosity, going from the active innermost disk (where thermalionization of alkalis yields high viscosities via the magneto-rotationalinstability (MRI)) to the adjacent dead zone (where the MRI is quenched).Previous studies of the pressure maximum, based on $\alpha$-disk models, haveassumed ad hoc values for the viscosity parameter $\alpha$ in the active zone,ignoring the detailed physics of the MRI. Here we explicitly couple the MRIcriteria to the $\alpha$-disk equations, to find steady-state (constantaccretion rate) solutions for the disk structure. We consider the effects ofboth Ohmic and ambipolar resistivities, and find solutions for a range of diskaccretion rates ($\dot{M}$ = $10^{-10}$ - $10^{-8}$ ${\rm M}_{\odot}$/yr),stellar masses ($M_{\ast}$ = 0.1 - 1 ${\rm M}_{\odot}$), and fiducial values ofthe $non$-MRI $\alpha$-viscosity in the dead zone ($\alpha_{\rm {DZ}} =10^{-5}$ - $10^{-3}$). We find that: (1) A midplane pressure maximum formsradially $outside$ the inner boundary of the dead zone; (2) Hall resistivitydominates near the midplane in the inner disk, which may explain why close-inplanets do $not$ form in $\sim$50% of systems; (3) X-ray ionization can becompetitive with thermal ionization in the inner disk, because of the lowsurface density there in steady-state; and (4) our inner disk solutions areviscously unstable to surface density perturbations.

Journal article

Hendler NP, Pinilla P, Pascucci I, Pohl A, Mulders G, Henning T, Dong R, Clarke C, Owen J, Hollenbach Det al., 2018, A likely planet-induced gap in the disc around T Cha, Monthly Notices of the Royal Astronomical Society: Letters, Vol: 475, Pages: L62-L66, ISSN: 1745-3925

We present high-resolution (0.11 × 0.06 arcsec2) 3 mm ALMA observations of the highly inclined transition disc around the star T Cha. Our continuum image reveals multiple dust structures: an inner disc, a spatially resolved dust gap, and an outer ring. When fitting sky-brightness models to the real component of the 3 mm visibilities, we infer that the inner emission is compact (≤1 au in radius), the gap width is between 18 and 28 au, and the emission from the outer ring peaks at ∼36 au. We compare our ALMA image with previously published 1.6 μm VLT/SPHERE imagery. This comparison reveals that the location of the outer ring is wavelength dependent. More specifically, the peak emission of the 3 mm ring is at a larger radial distance than that of the 1.6 μm ring, suggesting that millimeter-sized grains in the outer disc are located farther away from the central star than micron-sized grains. We discuss different scenarios to explain our findings, including dead zones, star-driven photoevaporation, and planet-disc interactions. We find that the most likely origin of the dust gap is from an embedded planet, and estimate – for a single planet scenario – that T Cha's gap is carved by a 1.2MJup planet.

Journal article

de Wit J, Wakeford HR, Lewis NK, Delrez L, Gillon M, Selsis F, Leconte J, Demory B-O, Bolmont E, Bourrier V, Burgasser AJ, Grimm S, Jehin E, Lederer SM, Owen JE, Stamenkovic V, Triaud AHMJet al., 2018, Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1, NATURE ASTRONOMY, Vol: 2, Pages: 214-219, ISSN: 2397-3366

Seven temperate Earth-sized exoplanets readily amenable for atmospheric studies transit the nearby ultracool dwarf star TRAPPIST-1 (refs 1,2). Their atmospheric regime is unknown and could range from extended primordial hydrogen-dominated to depleted atmospheres3,4,5,6. Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones6,7,8. An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in the near-infrared detectable during transits. Observations of the innermost planets have ruled out such signatures9. However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere10, 11. Here, we report observations for the four planets within or near the system’s habitable zone, the circumstellar region where liquid water could exist on a planetary surface12,13,14. These planets do not exhibit prominent spectroscopic signatures at near-infrared wavelengths either, which rules out cloud-free hydrogen-dominated atmospheres for TRAPPIST-1 d, e and f, with significance of 8σ, 6σ and 4σ, respectively. Such an atmosphere is instead not excluded for planet g. As high-altitude clouds and hazes are not expected in hydrogen-dominated atmospheres around planets with such insolation15, 16, these observations further support their terrestrial and potentially habitable nature.

Journal article

Ercolano B, Weber ML, Owen JE, 2017, Accreting Transition Discs with large cavities created by X-ray photoevaporation in C and O depleted discs, Monthly Notices of the Royal Astronomical Society: Letters, Vol: 473, Pages: L64-L68, ISSN: 1745-3933

Circumstellar discs with large dust depleted cavities and vigorous accretiononto the central star are often considered signposts for (multiple) giantplanet formation. In this letter we show that X-ray photoevaporation operatingin discs with modest (factors 3-10) gas-phase depletion of Carbon and Oxygen atlarge radii (> 15 AU) yield the inner radius and accretion rates for most ofthe observed discs, without the need to invoke giant planet formation. Wepresent one-dimensional viscous evolution models of discs affected by X-rayphotoevaporation assuming moderate gas-phase depletion of Carbon and Oxygen,well within the range reported by recent observations. Our models use asimplified prescription for scaling the X-ray photoevaporation rates andprofiles at different metallicity, and our quantitative result depends on thisscaling. While more rigorous hydrodynamical modelling of mass loss profiles atlow metallicities is required to constrain the observational parameter spacethat can be explained by our models, the general conclusion that metalsequestering at large radii may be responsible for the observed diversity oftransition discs is shown to be robust. Gap opening by giant planet formationmay still be responsible for a number of observed transition discs with largecavities and very high accretion rate.

Journal article

Owen JE, Wu Y, 2017, The evaporation valley in the Kepler planets, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 847, ISSN: 0004-637X

A new piece of evidence supporting the photoevaporation-driven evolution model for low-mass, close-in exoplanets was recently presented by the California–Kepler Survey. The radius distribution of the Kepler planets is shown to be bimodal, with a "valley" separating two peaks at 1.3 and 2.6 R ⊕. Such an "evaporation valley" had been predicted by numerical models previously. Here, we develop a minimal model to demonstrate that this valley results from the following fact: the timescale for envelope erosion is the longest for those planets with hydrogen/helium-rich envelopes that, while only a few percent in weight, double its radius. The timescale falls for envelopes lighter than this because the planet's radius remains largely constant for tenuous envelopes. The timescale also drops for heavier envelopes because the planet swells up faster than the addition of envelope mass. Photoevaporation therefore herds planets into either bare cores (~1.3 R ⊕), or those with double the core's radius (~2.6 R ⊕). This process mostly occurs during the first 100 Myr when the stars' high-energy fluxes are high and nearly constant. The observed radius distribution further requires the Kepler planets to be clustered around 3 M ⊕ in mass, born with H/He envelopes more than a few percent in mass, and that their cores are similar to the Earth in composition. Such envelopes must have been accreted before the dispersal of the gas disks, while the core composition indicates formation inside the ice line. Lastly, the photoevaporation model fails to account for bare planets beyond ~30–60 days; if these planets are abundant, they may point to a significant second channel for planet formation, resembling the solar system terrestrial planets.

Journal article

Owen JE, Lai D, 2017, Generating large misalignments in gapped and binary discs, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: 2834-2844, ISSN: 0035-8711

Many protostellar gapped and binary discs show misalignments between their inner and outer discs; in some cases, ∼70° misalignments have been observed. Here, we show that these misalignments can be generated through a secular resonance between the nodal precession of the inner disc and the precession of the gap-opening (stellar or massive planetary) companion. An evolving protostellar system may naturally cross this resonance during its lifetime due to disc dissipation and/or companion migration. If resonance crossing occurs on the right time-scale, of the order of a few million years, characteristic for young protostellar systems, the inner and outer discs can become highly misaligned, with misalignments ≳ 60° typical. When the primary star has a mass of order a solar mass, generating a significant misalignment typically requires the companion to have a mass of ∼0.01–0.1 M⊙ and an orbital separation of tens of astronomical units. The recently observed companion in the cavity of the gapped, highly misaligned system HD 142527 satisfies these requirements, indicating that a previous resonance crossing event misaligned the inner and outer discs. Our scenario for HD 142527's misaligned discs predicts that the companion's orbital plane is aligned with the outer disc's; this prediction should be testable with future observations as the companion's orbit is mapped out. Misalignments observed in several other gapped disc systems could be generated by the same secular resonance mechanism.

Journal article

Bolmont E, Selsis F, Owen JE, Ribas I, Raymond SN, Leconte J, Gillon Met al., 2017, Water loss from terrestrial planets orbiting ultracool dwarfs: implications for the planets of TRAPPIST-1, Monthly Notices of the Royal Astronomical Society, Vol: 464, Pages: 3728-3741, ISSN: 0035-8711

Ultracool dwarfs (UCD; Teff < ∼3000 K) cool to settle on the main sequence after ∼1 Gyr. For brown dwarfs, this cooling never stops. Their habitable zones (HZ) thus sweeps inward at least during the first Gyr of their lives. Assuming they possess water, planets found in the HZ of UCDs have experienced a runaway greenhouse phase too hot for liquid water prior to enter the HZ. It has been proposed that such planets are desiccated by this hot early phase and enter the HZ as dry worlds. Here, we model the water loss during this pre-HZ hot phase taking into account recent upper limits on the XUV emission of UCDs and using 1D radiation-hydrodynamic simulations. We address the whole range of UCDs but also focus on the planets recently found around the 0.08 M⊙ dwarf TRAPPIST-1. Despite assumptions maximizing the FUV photolysis of water and the XUV-driven escape of hydrogen, we find that planets can retain significant amount of water in the HZ of UCDs, with a sweet spot in the 0.04–0.06 M⊙ range. We also studied the TRAPPIST-1 system using observed constraints on the XUV flux. We find that TRAPPIST-1b and c may have lost as much as 15 Earth oceans and planet d – which might be inside the HZ – may have lost less than 1 Earth ocean. Depending on their initial water contents, they could have enough water to remain habitable. TRAPPIST-1 planets are key targets for atmospheric characterization and could provide strong constraints on the water erosion around UCDs.

Journal article

Owen JE, Mohanty S, 2016, Habitability of terrestrial-mass planets in the HZ of M Dwarfs - I. H/He-dominated atmospheres, Monthly Notices of the Royal Astronomical Society, Vol: 459, Pages: 4088-4108, ISSN: 1365-2966

The ubiquity of M dwarfs, combined with the relative ease of detecting terrestrial-mass planets around them, has made them prime targets for finding and characterizing planets in the ‘habitable zone’ (HZ). However, Kepler finds that terrestrial-mass exoplanets are often born with voluminous H/He envelopes, comprising mass-fractions (Menv/Mcore) ≳1 per cent. If these planets retain such envelopes over Gyr time-scales, they will not be ‘habitable’ even within the HZ. Given the strong X-ray/UV fluxes of M dwarfs, we study whether sufficient envelope mass can be photoevaporated away for these planets to become habitable. We improve upon previous work by using hydrodynamic models that account for radiative cooling as well as the transition from hydrodynamic to ballistic escape. Adopting a template active M dwarf XUV spectrum, including stellar evolution, and considering both evaporation and thermal evolution, we show that: (1) the mass-loss is (considerably) lower than previous estimates that use an ‘energy-limited’ formalism and ignore the transition to Jeans escape; (2) at the inner edge of the HZ, planets with core mass ≲ 0.9 M⊕ can lose enough H/He to become habitable if their initial envelope mass-fraction is ∼1 per cent; (3) at the outer edge of the HZ, evaporation cannot remove a ∼1 per cent H/He envelope even from cores down to 0.8 M⊕. Thus, if planets form with bulky H/He envelopes, only those with low-mass cores may eventually be habitable. Cores ≳1 M⊕, with ≳1 per cent natal H/He envelopes, will not be habitable in the HZ of M dwarfs.

Journal article

Owen JE, Wu Y, 2016, Atmospheres of low-mass planets: the "boil-off", Astrophysical Journal, Vol: 817, ISSN: 0004-637X

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 Kepler planets), the atmosphere it was able to accumulate while embedded in the protoplanetary 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 timescale on which the planet exits the disk. As such, it could not have contracted significantly from its original size, of the order of the Bondi radius. So a newly exposed protoplanet 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 disk 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 the order of 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 10% 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 Kepler data, provided planet formation results in initial envelope masses of tens of percent.

Journal article

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