Publications
60 results found
Robinson A, Booth RA, Owen JE, 2024, Introducing CUDISC: a 2D code for protoplanetary disc structure and evolution calculations, Monthly Notices of the Royal Astronomical Society, Vol: 529, Pages: 1524-1541, ISSN: 0035-8711
We present a new two-dimensional (2D) axisymmetric code, CUDISC, for studying protoplanetary discs, focusing on the self-consistent calculation of dust dynamics, grain-size distribution and disc temperature. Self-consistently studying these physical processes is essential for many disc problems, such as structure formation and dust removal, given that the processes heavily depend on one another. To follow the evolution over substantial fractions of the disc lifetime, CUDISC uses the CUDA language and libraries to speed up the code through GPU acceleration. CUDISC employs a second-order finite-volume Godonuv solver for dust dynamics, solves the Smoluchowski equation for dust growth, and calculates radiative transfer using a multifrequency hybrid ray-tracing/flux-limited-diffusion method. We benchmark our code against current state-of-the-art codes. Through studying steady-state problems, we find that including 2D structure reveals that when collisions are important, the dust vertical structure appears to reach a diffusion-settling-coagulation equilibrium that can differ substantially from standard models that ignore coagulation. For low fragmentation velocities, we find an enhancement of intermediate-sized dust grains at heights of ∼1 gas scale height due to the variation in collision rates with height, and for large fragmentation velocities, we find an enhancement of small grains around the disc mid-plane due to collisional ‘sweeping’ of small grains by large grains. These results could be important for the analysis of disc spectral energy distributions or scattered light images, given these observables are sensitive to the vertical grain distribution.
Curry A, Booth R, Owen JE, et al., 2024, The evolution of catastrophically evaporating rocky planets, Monthly Notices of the Royal Astronomical Society, Vol: 528, Pages: 4314-4336, ISSN: 0035-8711
In this work, we develop a rocky planet interior model and use it to investigate the evolution of catastrophically evaporating rocky exoplanets. These planets, detected through the dust tails produced by evaporative outflows from their molten surfaces, can be entirely destroyed in a fraction of their host star’s lifetime. To allow for the major decrease in mass, our interior model can simultaneously calculate the evolution of the pressure and density structure of a planet alongside its thermal evolution, which includes the effects of conduction, convection and partial melting. We first use this model to show that the underlying planets are likely to be almost entirely solid. This means that the dusty tails are made up of material sampled only from a thin dayside lava pool. If one wishes to infer the bulk compositions of rocky exoplanets from their dust tails, it is important to take the localized origin of this material into account. Secondly, by considering how frequently one should be able to detect mass loss from these systems, we investigate the occurrence of sub-Earth mass exoplanets, which is difficult with conventional planet detection surveys. We predict that, depending on model assumptions, the number of progenitors of the catastrophically evaporating planets is either in line with, or higher than, the observed population of close-in (substellar temperatures around 2200 K) terrestrial exoplanets.
Householder A, Weiss LM, Owen JE, et al., 2024, Investigating the atmospheric mass loss of the Kepler-105 planets straddling the radius gap, The Astronomical Journal, Vol: 167, ISSN: 0004-6256
An intriguing pattern among exoplanets is the lack of detected planets between approximately 1.5 R⊕ and 2.0 R⊕. One proposed explanation for this "radius gap" is the photoevaporation of planetary atmospheres, a theory that can be tested by studying individual planetary systems. Kepler-105 is an ideal system for such testing due to the ordering and sizes of its planets. Kepler-105 is a Sun-like star that hosts two planets straddling the radius gap in a rare architecture with the larger planet closer to the host star (Rb = 2.53 ± 0.07 R⊕, Pb = 5.41 days, Rc = 1.44 ± 0.04 R⊕, Pc = 7.13 days). If photoevaporation sculpted the atmospheres of these planets, then Kepler-105b would need to be much more massive than Kepler-105c to retain its atmosphere, given its closer proximity to the host star. To test this hypothesis, we simultaneously analyzed radial velocities and transit-timing variations of the Kepler-105 system, measuring disparate masses of Mb = 10.8 ± 2.3 M⊕ (ρb = 3.68 ± 0.84 g cm−3) and Mc = 5.6 ± 1.2 M⊕ (ρc = 10.4 ± 2.39 g cm−3). Based on these masses, the difference in gas envelope content of the Kepler-105 planets could be entirely due to photoevaporation (in 76% of scenarios), although other mechanisms like core-powered mass loss could have played a role for some planet albedos.
Owen JE, Schlichting HE, 2024, Mapping out the parameter space for photoevaporation and core-powered mass-loss, Monthly Notices of the Royal Astronomical Society, Vol: 528, Pages: 1615-1629, ISSN: 0035-8711
Understanding atmospheric escape in close-in exoplanets is critical to interpreting their evolution. We map out the parameter space over which photoevaporation and core-powered mass-loss dominate atmospheric escape. Generally, the transition between the two regimes is determined by the location of the Bondi radius (i.e. the sonic point of core-powered outflow) relative to the penetration depth of extreme ultra-violet (XUV) photons. Photoevaporation dominates the loss when the XUV penetration depth lies inside the Bondi radius (RXUV < RB) and core-powered mass-loss when XUV radiation is absorbed higher up in the flow (RB < RXUV). The transition between the two regimes occurs at a roughly constant ratio of the planet’s radius to its Bondi radius, with the exact value depending logarithmically on planetary and stellar properties. In general, core-powered mass-loss dominates for lower gravity planets with higher equilibrium temperatures, and photoevaporation dominates for higher gravity planets with lower equilibrium temperatures. However, planets can transition between these two mass-loss regimes during their evolution, and core-powered mass-loss can ‘enhance’ photoevaporation over a significant region of parameter space. Interestingly, a planet that is ultimately stripped by core-powered mass-loss has likely only ever experienced core-powered mass-loss. In contrast, a planet that is ultimately stripped by photoevaporation could have experienced an early phase of core-powered mass-loss. Applying our results to the observed super-Earth population suggests that it contains significant fractions of planets where each mechanism controlled the final removal of the H/He envelope, although photoevaporation appears to be responsible for the final carving of the exoplanet radius valley.
CamposEstrada B, Owen JE, Jankovic MR, et al., 2024, On the likely magnesium–iron silicate dusty tails of catastrophically evaporating rocky planets, Monthly Notices of the Royal Astronomical Society, Vol: 528, Pages: 1249-1263, ISSN: 0035-8711
Catastrophically evaporating rocky planets provide a unique opportunity to study the composition of small planets. The surfacecomposition of these planets can be constrained via modelling their comet-like tails of dust. In this work, we present a newself-consistent model of the dusty tails: we physically model the trajectory of the dust grains after they have left the gaseousoutflow, including an on-the-fly calculation of the dust cloud’s optical depth. We model two catastrophically evaporating planets:KIC 1255 b and K2-22 b. For both planets, we find the dust islikely composed of magnesium–iron silicates(olivine and pyroxene),consistent with an Earth-like composition. We constrain the initial dust grain sizes to be ∼ 1.25–1.75 μm and the average (dusty)planetary mass-loss rate to be ∼ 3 M⊕Gyr−1. Our model shows that the origin of the leading tail of dust of K2-22 b is likely acombination of the geometry of the outflow and a low radiation pressure force to stellar gravitational force ratio. We find theoptical depth of the dust cloud to be a factor of a few in the vicinity of the planet. Our composition constraint supports therecently suggested idea that the dusty outflows of these planets go through a greenhouse effect–nuclear winter cycle, which givesorigin to the observed transit depth time variability. Magnesium–iron silicates have the necessary visible-to-infrared opacityratio to give origin to this cycle in the high mass-loss state.
Schreyer E, Owen JE, Spake JJ, et al., 2024, Using helium 10 830 Å transits to constrain planetary magnetic fields, Monthly Notices of the Royal Astronomical Society, Vol: 527, Pages: 5117-5130, ISSN: 0035-8711
Planetary magnetic fields can affect the predicted mass-loss rate for close-in planets that experience large amounts of ultraviolet irradiation. In this work, we present a method to detect the magnetic fields of close-in exoplanets undergoing atmospheric escape using transit spectroscopy at the 10 830 Å line of helium. Motivated by previous work on hydrodynamic and magnetohydrodynamic photoevaporation, we suggest that planets with magnetic fields that are too weak to control the outflow’s topology lead to blueshifted transits due to dayside-to-nightside flows. In contrast, strong magnetic fields prevent this day-to-night flow, as the gas is forced to follow the magnetic field’s roughly dipolar topology. We post-process existing 2D photoevaporation simulations, computing synthetic transit profiles in helium to test this concept. As expected, we find that hydrodynamically dominated outflows lead to blueshifted transits of the order of the sound speed of the gas. Strong surface magnetic fields lead to unshifted or slightly redshifted transit profiles. High-resolution observations can distinguish between these profiles; however, eccentricity uncertainties generally mean that we cannot conclusively say that velocity shifts are due to the outflow for individual planets. The majority of helium observations are blueshifted, which could be a tentative indication that close-in planets generally have surface dipole magnetic field strengths
Damasso M, Rodrigues J, Castro-González A, et al., 2023, A compact multi-planet system transiting HIP 29442 (TOI-469) discovered by TESS and ESPRESSO. Radial velocities lead to the detection of transits with low signal-to-noise ratio, Astronomy and Astrophysics: a European journal, Vol: 679, ISSN: 0004-6361
Context. One of the goals of the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) Guaranteed Time Observations (GTO) consortium is the precise characterisation of a selected sample of planetary systems discovered by TESS. One such target is the K0V star HIP 29442 (TOI-469), already known to host a validated sub-Neptune companion TOI-469.01, which we followed-up with ESPRESSO.Aims. We aim to verify the planetary nature of TOI-469.01 by obtaining precise mass, radius, and ephemeris, and constraining its bulk physical structure and composition.Methods. Following a Bayesian approach, we modelled radial velocity and photometric time series to measure the dynamical mass, radius, and ephemeris, and to characterise the internal structure and composition of TOI-469.01.Results. We confirmed the planetary nature of TOI-469.01 (now renamed HIP 29442 b), and thanks to the ESPRESSO radial velocities we discovered two additional close-in companions. Through an in-depth analysis of the TESS light curve, we could also detect their low signal-to-noise transit signals. We characterised the additional companions, and conclude that HIP 29442 is a compact multi-planet system. The three planets have orbital periods Porb,b = 13.63083 ± 0.00003, Porb,c = 3.53796 ± 0.00003, and Porb,d = 6.42975−0.00010+0.00009 days, and we measured their masses with high precision: mp,b = 9.6 ± 0.8 M⊕, mp,c = 4.5 ± 0.3 M⊕, and mp,d = 5.1 ± 0.4 M⊕. We measured radii and bulk densities of all the planets (the 3σ confidence intervals are shown in parentheses): Rp,b = 3.48−0.08(−0.28)+0.07(+0.19) R⊕ and ρp,b = 1.3 ± 0.2(0.3)g cm−3; Rp,c = 1.58−0.11(−0.34)+0.10(+0.30) R⊕ and ρp,c = 6.3−1.3(−2.7)+1.7(+6.0)g cm−3; Rp,d = 1.37 ± 0.11(−0.43)(+0.32) R⊕ and ρp,d = 11.0−2.4(−6.3)+3.4(+21.0)g cm−3. Due
Rab C, Weber ML, Picogna G, et al., 2023, High-resolution [O i] line spectral mapping of TW Hya consistent with X-ray-driven photoevaporation, Letters of the Astrophysical Journal, Vol: 955, ISSN: 2041-8205
Theoretical models indicate that photoevaporative and magnetothermal winds play a crucial role in the evolution and dispersal of protoplanetary disks and affect the formation of planetary systems. However, it is still unclear what wind-driving mechanism is dominant or if both are at work, perhaps at different stages of disk evolution. Recent spatially resolved observations by Fang et al. of the [O i] 6300 Å spectral line, a common disk wind tracer in TW Hya, revealed that about 80% of the emission is confined to the inner few astronomical units of the disk. In this work, we show that state-of-the-art X-ray-driven photoevaporation models can reproduce the compact emission and the line profile of the [O i] 6300 Å line. Furthermore, we show that the models also simultaneously reproduce the observed line luminosities and detailed spectral profiles of both the [O i] 6300 Å and the [Ne ii] 12.8 μm lines. While MHD wind models can also reproduce the compact radial emission of the [O i] 6300 Å line, they fail to match the observed spectral profile of the [O i] 6300 Å line and underestimate the luminosity of the [Ne ii] 12.8 μm line by a factor of 3. We conclude that, while we cannot exclude the presence of an MHD wind component, the bulk of the wind structure of TW Hya is predominantly shaped by a photoevaporative flow.
Nayakshin S, Owen JE, Elbakyan V, 2023, Extreme evaporation of planets in hot thermally unstable protoplanetary discs: the case of FU Ori, Monthly Notices of the Royal Astronomical Society, Vol: 523, Pages: 385-403, ISSN: 0035-8711
Disc accretion rate onto low mass protostar FU Ori suddenly increased hundreds of times 85 yr ago and remains elevated to this day. We show that the sum of historic and recent observations challenges existing FU Ori models. We build a theory of a new process, Extreme Evaporation (EE) of young gas giant planets in discs with midplane temperatures of ≳ 30 000 K. Such temperatures are reached in the inner 0.1 AU during thermal instability bursts. In our 1D time-dependent code the disc and an embedded planet interact through gravity, heat, and mass exchange. We use disc viscosity constrained by simulations and observations of dwarf novae instabilities, and we constrain planet properties with a stellar evolution code. We show that dusty gas giants born in the outer self-gravitating disc reach the innermost disc in a ∼O(104) yr with radius of ∼10RJ. We show that their EE rates are ≳10−5M⊙ yr−1; if this exceeds the background disc accretion activity then the system enters a planet-sourced mode. Like a stellar secondary in mass-transferring binaries, the planet becomes the dominant source of matter for the star, albeit for ∼O(100) yr. We find that a ∼6 Jupiter mass planet evaporating in a disc fed at a time-averaged rate of ∼10−6M⊙ yr−1 appears to explain all that we currently know about FU Ori accretion outburst. More massive planets and/or planets in older less massive discs do not experience EE process. Future FUOR modelling may constrain planet internal structure and evolution of the earliest discs.
Kotorashvili K, Blackman EG, Owen JE, 2023, Why the observed spin evolution of older-than-solar-like stars might not require a dynamo mode change, Monthly Notices of the Royal Astronomical Society, Vol: 522, Pages: 1583-1590, ISSN: 0035-8711
The spin evolution of main-sequence stars has long been of interest for basic stellar evolution, stellar ageing, stellar activity, and consequent influence on companion planets. Observations of older-than-solar late-type main-sequence stars have been interpreted to imply that a change from a dipole-dominated magnetic field to one with more prominent higher multipoles might be necessary to account for the data. The spin-down models that lead to this inference are essentially tuned to the Sun. Here, we take a different approach that considers individual stars as fixed points rather than just the Sun. We use a time-dependent theoretical model to solve for the spin evolution of low-mass main-sequence stars that includes a Parker-type wind and a time-evolving magnetic field coupled to the spin. Because the wind is exponentially sensitive to the stellar mass over radius and the coronal base temperature, the use of each observed star as a separate fixed point is more appropriate and, in turn, produces a set of solution curves that produces a solution envelope rather than a simple line. This envelope of solution curves, unlike a single line fit, is consistent with the data and does not unambiguously require a modal transition in the magnetic field to explain it.
Rogers JGG, Schlichting HEE, Owen JEE, 2023, Conclusive evidence for a population of water worlds around M dwarfs remains elusive, Letters of the Astrophysical Journal, Vol: 947, ISSN: 2041-8205
The population of small, close-in exoplanets is bifurcated into super-Earths and sub-Neptunes. We calculate physically motivated mass–radius relations for sub-Neptunes, with rocky cores and H/He-dominated atmospheres, accounting for their thermal evolution, irradiation, and mass loss. For planets ≲10 M⊕, we find that sub-Neptunes retain atmospheric mass fractions that scale with planet mass and show that the resulting mass–radius relations are degenerate with results for "water worlds" consisting of a 1:1 silicate-to-ice composition ratio. We further demonstrate that our derived mass–radius relation is in excellent agreement with the observed exoplanet population orbiting M dwarfs and that planet mass and radii alone are insufficient to determine the composition of some sub-Neptunes. Finally, we highlight that current exoplanet demographics show an increase in the ratio of super-Earths to sub-Neptunes with both stellar mass (and therefore luminosity) and age, which are both indicative of thermally driven atmospheric escape processes. Therefore, such processes should not be ignored when making compositional inferences in the mass–radius diagram.
Rogers JG, Munoz CJ, Owen JE, et al., 2023, Exoplanet atmosphere evolution: emulation with neural networks, Monthly Notices of the Royal Astronomical Society, Vol: 519, Pages: 6028-6043, ISSN: 0035-8711
Atmospheric mass-loss is known to play a leading role in sculpting the demographics of small, close-in exoplanets. Knowledge of how such planets evolve allows one to ‘rewind the clock’ to infer the conditions in which they formed. Here, we explore the relationship between a planet’s core mass and its atmospheric mass after protoplanetary disc dispersal by exploiting XUV photoevaporation as an evolutionary process. Historically, this inference problem would be computationally infeasible due to the large number of planet models required; however, we use a novel atmospheric evolution emulator which utilizes neural networks to provide three orders of magnitude in speedup. First, we provide a proof of concept for this emulator on a real problem by inferring the initial atmospheric conditions of the TOI-270 multi-planet system. Using the emulator, we find near-indistinguishable results when compared to the original model. We then apply the emulator to the more complex inference problem, which aims to find the initial conditions for a sample of Kepler, K2, and TESS planets with well-constrained masses and radii. We demonstrate that there is a relationship between core masses and the atmospheric mass they retain after disc dispersal. This trend is consistent with the ‘boil-off’ scenario, in which close-in planets undergo dramatic atmospheric escape during disc dispersal. Thus, it appears that the exoplanet population is consistent with the idea that close-in exoplanets initially acquired large massive atmospheres, the majority of which is lost during disc dispersal, before the final population is sculpted by atmospheric loss over 100 Myr to Gyr time-scales.
Owen JE, Lin DNC, 2023, The evolution of circumstellar discs in the galactic centre: an application to the G-clouds, Monthly Notices of the Royal Astronomical Society, Vol: 519, Pages: 397-417, ISSN: 0035-8711
The Galactic Centre is known to have undergone a recent star formation episode a few Myr ago, which likely produced many T Tauri stars hosting circumstellar discs. It has been suggested that these discs may be the compact and dusty ionized sources identified as ‘G-clouds’. Given the Galactic Centre’s hostile environment, we study the possible evolutionary pathways these discs experience. We compute new external photoevaporation models applicable to discs in the Galactic Centre that account for the subsonic launching of the wind and absorption of UV photons by dust. Using evolutionary disc calculations, we find that photoevaporation’s rapid truncation of the disc causes them to accrete onto the central star rapidly. Ultimately, an accreting circumstellar disc has a lifetime ≲ 1 Myr, which would fail to live long enough to explain the G-clouds. However, we identify a new evolutionary pathway for circumstellar discs in the Galactic Centre. Removal of disc material by photoevaporation prevents the young star from spinning down due to magnetic braking, ultimately causing the rapidly spinning young star to torque the disc into a ‘decretion disc’ state which prevents accretion. At the same time, any planetary companion in the disc will trap dust outside its orbit, shutting down photoevaporation. The disc can survive for up to ∼10 Myr in this state. Encounters with other stars are likely to remove the planet on Myr time-scales, causing photoevaporation to restart, giving rise to a G-cloud signature. A giant planet fraction of ∼10 per cent can explain the number of observed G-clouds.
Booth RA, Owen JE, Schulik M, 2023, Dust formation in the outflows of catastrophically evaporating planets, Monthly Notices of the Royal Astronomical Society, Vol: 518, Pages: 1761-1775, ISSN: 0035-8711
Ultra-short period planets offer a window into the poorly understood interior composition of exoplanets through material evaporated from their rocky interiors. Among these objects are a class of disintegrating planets, observed when their dusty tails transit in front of their host stars. These dusty tails are thought to originate from dust condensation in thermally-driven winds emanating from the sublimating surfaces of these planets. Existing models of these winds have been unable to explain their highly variable nature and have not explicitly modelled how dust forms in the wind. Here we present new radiation-hydrodynamic simulations of the winds from these planets, including a minimal model for the formation and destruction of dust, assuming that nucleation can readily take place. We find that dust forms readily in the winds, a consequence of large dust grains obtaining lower temperatures than the planet’s surface. As hypothesised previously, we find that the coupling of the planet’s surface temperature to the outflow properties via the dust’s opacity can drive time-variable flows when dust condensation is sufficiently fast. In agreement with previous work, our models suggest that these dusty tails are a signature of catastrophically evaporating planets that are close to the end of their lives. Finally, we discuss the implications of our results for the dust’s composition. More detailed hydrodynamic models that self-consistently compute the nucleation and composition of the dust and gas are warranted in order to use these models to study the planet’s interior composition.
Owen JE, Murray-Clay RA, Schreyer E, et al., 2023, The fundamentals of Lyman alpha exoplanet transits, Monthly Notices of the Royal Astronomical Society, Vol: 518, Pages: 4357-4371, ISSN: 0035-8711
Lyman α transits have been detected from several nearby exoplanets and are one of our best insights into the atmospheric escape process. However, due to ISM absorption, we typically only observe the transit signature in the blue-wing, making them challenging to interpret. This challenge has been recently highlighted by non-detections from planets thought to be undergoing vigorous escape. Pioneering 3D simulations have shown that escaping hydrogen is shaped into a cometary tail receding from the planet. Motivated by this work, we develop a simple model to interpret Lyman α transits. Using this framework, we show that the Lyman α transit depth is primarily controlled by the properties of the stellar tidal field rather than details of the escape process. Instead, the transit duration provides a direct measurement of the velocity of the planetary outflow. This result arises because the underlying physics is the distance a neutral hydrogen atom can travel before it is photoionized in the outflow. Thus, higher irradiation levels, expected to drive more powerful outflows, produce weaker, shorter Lyman α transits because the outflowing gas is ionized more quickly. Our framework suggests that the generation of energetic neutral atoms may dominate the transit signature early, but the acceleration of planetary material produces long tails. Thus, Lyman α transits do not primarily probe the mass-loss rates. Instead, they inform us about the velocity at which the escape mechanism is ejecting material from the planet, providing a clean test of predictions from atmospheric escape models.
Ardila DR, Shkolnik E, Ziemer J, et al., 2022, The UV-SCOPE mission: ultraviolet spectroscopic characterization of planets and their environments, Conference on Space Telescopes and Instrumentation - Ultraviolet to Gamma Ray Part of SPIE Astronomical Telescopes and Instrumentation Conference, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, Pages: 1-12, ISSN: 0277-786X
UV-SCOPE is a mission concept to determine the causes of atmospheric mass loss in exoplanets, investigate the mechanisms driving aerosol formation in hot Jupiters, and study the influence of the stellar environment on atmospheric evolution and habitability. As part of these investigations, the mission will generate a broad-purpose legacy database of time-domain ultraviolet (UV) spectra for nearly 200 stars and planets. The observatory consists of a 60 cm, f/10 telescope paired to a long-slit spectrograph, yielding simultaneous, almost continuous coverage between 1203 Å and 4000 Å, with resolutions ranging from 6000 to 240. The efficient instrument provides throughputs < 4% (far-UV; FUV) and < 15% (near-UV; NUV), comparable to HST/COS and much better than HST/STIS, over the same spectral range. A key design feature is the LiF prism, which serves as a dispersive element and provides high throughput even after accounting for radiation degradation. The use of two delta-doped Electron-Multiplying CCD detectors with UV-optimized, single-layer anti-reflection coatings provides high quantum efficiency and low detector noise. From the Earth-Sun second Lagrangian point, UV-SCOPE will continuously observe planetary transits and stellar variability in the full FUV-to-NUV range, with negligible astrophysical background. All these features make UV-SCOPE the ideal instrument to study exoplanetary atmospheres and the impact of host stars on their planets. UV-SCOPE was proposed to NASA as a Medium Explorer (MidEx) mission for the 2021 Announcement of Opportunity. If approved, the observatory will be developed over a 5-year period. Its primary science mission takes 34 months to complete. The spacecraft carries enough fuel for 6 years of operations.
Cummins DP, Owen JE, Booth RA, 2022, Extreme pebble accretion in ringed protoplanetary discs, Monthly Notices of the Royal Astronomical Society, Vol: 515, Pages: 1276-1295, ISSN: 0035-8711
Axisymmetric dust rings containing tens to hundreds of Earth masses of solids have been observed in protoplanetary discs with (sub-)millimetre imaging. Here, we investigate the growth of a planetary embryo in a massive (150M⊕) axisymmetric dust trap through dust and gas hydrodynamics simulations. When accounting for the accretion luminosity of the planetary embryo from pebble accretion, the thermal feedback on the surrounding gas leads to the formation of an anticyclonic vortex. Since the vortex forms at the location of the planet, this has significant consequences for the planet’s growth: as dust drifts towards the pressure maximum at the centre of the vortex, which is initially co-located with the planet, a rapid accretion rate is achieved, in a distinct phase of “vortex-assisted” pebble accretion. Once the vortex separates from the planet due to interactions with the disc, it accumulates dust, shutting off accretion onto the planet. We find that this rapid accretion, mediated by the vortex, results in a planet containing ≈100M⊕ of solids. We follow the evolution of the vortex, as well as the efficiency with which dust grains accumulate at its pressure maximum as a function of their size, and investigate the consequences this has for the growth of the planet as well as the morphology of the protoplanetary disc. We speculate that this extreme formation scenario may be the origin of giant planets which are identified to be significantly enhanced in heavy elements.
Zicher N, Barragán O, Klein B, et al., 2022, One year of AU Mic with HARPS: I - measuring the masses of the two transiting planets, Monthly Notices of the Royal Astronomical Society, Vol: 512, Pages: 3060-3078, ISSN: 0035-8711
The system of two transiting Neptune-sized planets around the bright, young M-dwarf AU Mic provides a unique opportunity to test models of planet formation, early evolution, and star-planet interaction. However, the intense magnetic activity of the host star makes measuring the masses of the planets via the radial velocity (RV) method very challenging. We report on a 1-year, intensive monitoring campaign of the system using 91 observations with the HARPS spectrograph, allowing for detailed modelling of the ∼600 m s−1 peak-to-peak activity-induced RV variations. We used a multidimensional Gaussian Process framework to model these and the planetary signals simultaneously. We detect the latter with semi-amplitudes of Kb = 5.8 ± 2.5 m s−1 and Kc = 8.5 ± 2.5 m s−1, respectively. The resulting mass estimates, Mb = 11.7 ± 5.0 M⊕ and Mc = 22.2 ± 6.7 M⊕, suggest that planet b might be less dense, and planet c considerably denser than previously thought. These results are in tension with the current standard models of core-accretion. They suggest that both planets accreted a H/He envelope that is smaller than expected, and the trend between the two planets’ envelope fractions is the opposite of what is predicted by theory.
Haworth TJ, Kim JS, Qiao L, et al., 2022, An APEX search for carbon emission from NGC 1977 proplyds, Monthly Notices of the Royal Astronomical Society, Vol: 512, Pages: 2594-2603, ISSN: 0035-8711
We used the Atacama Pathfinder Experiment (APEX) telescope to search for C I 1-0 (492.16 GHz) emission towards 8 proplyds in NGC 1977, which is an FUV radiation environment two orders of magnitude weaker than that irradiating the Orion Nebular Cluster (ONC) proplyds. C I is expected to enable us to probe the wind launching region of externally photoevaporating discs. Of the 8 targets observed, no 3σ detections of the C I line were made despite reaching sensitivities deeper than the anticipated requirement for detection from prior APEX CI observations of nearby discs and models of external photoevaporation of quite massive discs. By comparing both the proplyd mass loss rates and C I flux constraints with a large grid of external photoevaporation simulations, we determine that the non-detections are in fact fully consistent with the models if the proplyd discs are very low mass. Deeper observations in C I and probes of the disc mass with other tracers (e.g. in the continuum and CO) can test this. If such a test finds higher masses, this would imply carbon depletion in the outer disc, as has been proposed for other discs with surprisingly low C I fluxes, though more massive discs would also be incompatible with models that can explain the observed mass loss rates and C I non-detections. The expected remaining lifetimes of the proplyds are estimated to be similar to those of proplyds in the ONC at 0.1 Myr. Rapid destruction of discs is therefore also a feature of common, intermediate UV environments.
Mann AW, Wood ML, Schmidt SP, et al., 2022, TESS hunt for young and maturing exoplanets (THYME). VI. an 11 Myr giant planet transiting a very-low-mass star in lower centaurus crux, The Astronomical Journal, Vol: 163, Pages: 156-156, ISSN: 0004-6256
Mature super-Earths and sub-Neptunes are predicted to be ≃ Jovian radius when younger than 10 Myr. Thus, we expect to find 5–15 R⊕ planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227b, a 0.85 ± 0.05 RJ (9.5 R⊕) planet transiting a very-low-mass star (0.170 ± 0.015 M⊙) every 27.4 days. TOI 1227's kinematics and strong lithium absorption confirm that it is a member of a previously discovered subgroup in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11 ± 2 Myr for Musca, based on lithium, rotation, and the color–magnitude diagram of Musca members. The TESS data and ground-based follow-up show a deep (2.5%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of ≃0.5 MJ. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227b is still contracting and will eventually turn into one of the more common <5 R⊕ planets.
Petigura EA, Rogers JG, Isaacson H, et al., 2022, The California-Kepler survey. X. The radius gap as a function of stellar mass, metallicity, and age, The Astronomical Journal, Vol: 163, Pages: 179-179, ISSN: 0004-6256
In 2017, the California-Kepler Survey (CKS) published its first data release (DR1) of high-resolution optical spectra of 1305 planet hosts. Refined CKS planet radii revealed that small planets are bifurcated into two distinct populations, super-Earths (smaller than 1.5 R⊕) and sub-Neptunes (between 2.0 and 4.0 R⊕), with few planets in between (the "radius gap"). Several theoretical models of the radius gap predict variation with stellar mass, but testing these predictions is challenging with CKS DR1 due to its limited M⋆ range of 0.8–1.4 M⊙. Here we present CKS DR2 with 411 additional spectra and derived properties focusing on stars of 0.5–0.8 M⊙. We found that the radius gap follows Rp ∝ Pm with m = −0.10 ± 0.03, consistent with predictions of X-ray and ultraviolet- and core-powered mass-loss mechanisms. We found no evidence that m varies with M⋆. We observed a correlation between the average sub-Neptune size and M⋆. Over 0.5–1.4 M⊙, the average sub-Neptune grows from 2.1 to 2.6 R⊕, following ${R}_{p}\propto {M}_{\star }^{\alpha }$ with α = 0.25 ± 0.03. In contrast, there is no detectable change for super-Earths. These M⋆–Rp trends suggest that protoplanetary disks can efficiently produce cores up to a threshold mass of Mc, which grows linearly with stellar mass according to Mc ≈ 10 M⊕(M⋆/M⊙). There is no significant correlation between sub-Neptune size and stellar metallicity (over −0.5 to +0.5 dex), suggesting a weak relationship between planet envelope opacity and stellar metallicity. Finally, there is no significant variation in sub-Neptune size with stellar age (over 1–10 Gyr), which suggests that the majority of envelope contraction concludes after ∼1 Gyr.
Jankovic MR, Mohanty S, Owen JE, et al., 2021, MRI-active inner regions of protoplanetary discs – II. Dependence on dust, disc, and stellar parameters, Monthly Notices of the Royal Astronomical Society, Vol: 509, Pages: 5974-5991, ISSN: 0035-8711
Close-in super-Earths are the most abundant exoplanets known. It has been hypothesized that they form in the inner regions of protoplanetary discs, out of the dust that may accumulate at the boundary between the inner region susceptible to the magneto-rotational instability (MRI) and an MRI-dead zone further out. In Paper I, we presented a model for the viscous inner disc which includes heating due to both irradiation and MRI-driven accretion; thermal and non-thermal ionization; dust opacities; and dust effects on ionization. Here, we examine how the inner disc structure varies with stellar, disc, and dust parameters. For high accretion rates and small dust grains, we find that: (1) the main sources of ionization are thermal ionization and thermionic and ion emission; (2) the disc features a hot, high-viscosity inner region, and a local gas pressure maximum at the outer edge of this region (in line with previous studies); and (3) an increase in the dust-to-gas ratio pushes the pressure maximum outwards. Consequently, dust can accumulate in such inner discs without suppressing the MRI, with the amount of accumulation depending on the viscosity in the MRI-dead regions. Conversely, for low accretion rates and large dust grains, there appears to be an additional steady-state solution in which: (1) stellar X-rays become the main source of ionization; (2) MRI-viscosity is high throughout the disc; and (3) the pressure maximum ceases to exist. Hence, if planets form in the inner disc, larger accretion rates (and thus younger discs) are favoured.
Rogers JG, Gupta A, Owen JE, et al., 2021, Photoevaporation versus core-powered mass-loss: model comparison with the 3D radius gap, Monthly Notices of the Royal Astronomical Society, Vol: 508, Pages: 5886-5902, ISSN: 0035-8711
The extreme ultraviolet (EUV)/X-ray photoevaporation and core-powered mass-loss models are both capable of reproducing the bimodality in the sizes of small, close-in exoplanets observed by the Kepler space mission, often referred to as the ‘radius gap’. However, it is unclear which of these two mechanisms dominates the atmospheric mass-loss that is likely sculpting the radius gap. In this work, we propose a new method of differentiating between the two models, which relies on analysing the radius gap in 3D parameter space. Using models for both mechanisms, and by performing synthetic transit surveys we predict the size and characteristics of a survey capable of discriminating between the two models. We find that a survey of ≳5000 planets, with a wide range in stellar mass and measurement uncertainties at a ≲5 per cent level is sufficient. Our methodology is robust against moderate false positive contamination of ≲10 per cent. We perform our analysis on two surveys (which do not satisfy our requirements): the California-KeplerSurvey and the Gaia–KeplerSurvey and find, unsurprisingly, that both data sets are consistent with either model. We propose a hypothesis test to be performed on future surveys that can robustly ascertain which of the two mechanisms formed the radius gap, provided one dominates over the other.
Van Eylen V, Astudillo-Defru N, Bonfils X, et al., 2021, Masses and compositions of three small planets orbiting the nearby M dwarf L231-32 (TOI-270) and the M dwarf radius valley, Monthly Notices of the Royal Astronomical Society, Vol: 507, Pages: 2154-2173, ISSN: 0035-8711
We report on precise Doppler measurements of L231-32 (TOI-270), a nearby M dwarf (d = 22 pc, M⋆ = 0.39 M⊙, R⋆ = 0.38 R⊙), which hosts three transiting planets that were recently discovered using data from the Transiting Exoplanet Survey Satellite (TESS). The three planets are 1.2, 2.4, and 2.1 times the size of Earth and have orbital periods of 3.4, 5.7, and 11.4 d. We obtained 29 high-resolution optical spectra with the newly commissioned Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) and 58 spectra using the High Accuracy Radial velocity Planet Searcher (HARPS). From these observations, we find the masses of the planets to be 1.58 ± 0.26, 6.15 ± 0.37, and 4.78 ± 0.43 M⊕, respectively. The combination of radius and mass measurements suggests that the innermost planet has a rocky composition similar to that of Earth, while the outer two planets have lower densities. Thus, the inner planet and the outer planets are on opposite sides of the ‘radius valley’ – a region in the radius-period diagram with relatively few members – which has been interpreted as a consequence of atmospheric photoevaporation. We place these findings into the context of other small close-in planets orbiting M dwarf stars, and use support vector machines to determine the location and slope of the M dwarf (Teff < 4000 K) radius valley as a function of orbital period. We compare the location of the M dwarf radius valley to the radius valley observed for FGK stars, and find that its location is a good match to photoevaporation and core-powered mass-loss models. Finally, we show that planets below the M dwarf radius valley have compositions consistent with stripped rocky cores, whereas most planets above have a lower density consistent with the presence of a H-He atmosphere.
Owen JE, Altaf N, 2021, A slim disc approach to external photoevaporation of discs, Monthly Notices of the Royal Astronomical Society, Vol: 508, Pages: 2493-2504, ISSN: 0035-8711
The photoevaporation of protoplanetary discs by nearby massive stars present in their birth cluster plays a vital role in their evolution. Previous modelling assumes that the disc behaves like a classical Keplerian accretion disc out to a radius where the photoevaporative outflow is launched. There is then an abrupt change in the angular velocity profile, and the outflow is modelled by forcing the fluid parcels to conserve their specific angular momenta. Instead, we model externally photoevaporating discs using the slim disc formalism. The slim disc approach self-consistently includes the advection of radial and angular momentum as well as angular momentum redistribution by internal viscous torques. Our resulting models produce a smooth transition from a rotationally supported Keplerian disc to a photoevaporative driven outflow, where this transition typically occurs over ∼4–5 scale heights. The penetration of ultraviolet photons predominately sets the radius of the transition and the viscosity’s strength plays a minor role. By studying the entrainment of dust particles in the outflow, we find a rapid change in the dust size and surface density distribution in the transition region due to the steep gas density gradients present. This rapid change in the dust properties leaves a potentially observable signature in the continuum spectral index of the disc at mm wavelengths. Using the slim disc formalism in future evolutionary calculations will reveal how both the gas and dust evolve in their outer regions and the observable imprints of the external photoevaporation process.
Jankovic MR, Owen JE, Mohanty S, et 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.
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.
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).
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.
Owen JE, Shaikhislamov IF, Lammer H, et 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).
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