X-ray And Extreme Ultraviolet Research Articles

Helium in Exoplanet Exospheres: Orbital and Stellar Influences

Searches for helium in the exospheres of exoplanets via the metastable near-infrared triplet have yielded 17 detections and 40 nondetections. We performed a comprehensive reanalysis of published studies to investigate the influence of stellar X-ray and extreme-ultraviolet (XUV) flux and orbital parameters on the detectability of helium in exoplanetary atmospheres. We identified a distinct “orbital sweet spot” for helium detection, 0.03 to 0.08 au from the host star, where the majority of detections occurred. This sweet spot is influenced by the stellar luminosity and planet size. Notably, a lower ratio of XUV flux to mid-UV flux is preferred for planets compared to nondetections. We also found that helium detections occur for planets around stars with effective temperatures of 4400–6500 K (i.e., spectral type K and G stars), with a sharp gap between 5400 and 6000 K, where no detections occur. We also report an upper-limit efficiency of 6% for energy-limited atmospheric escape from our analysis. Additionally, our analysis of the cumulative XUV flux versus escape velocity shows planets with helium detections above the “cosmic shoreline,” where atmospheres are not thought to be present, suggesting the shoreline needs revision. The unexpected trends revealed in our meta-analysis can contribute to a better understanding of star–planet interaction and exosphere evolution.

The elusive atmosphere of WASP–12 b

To date, the hot Jupiter WASP–12 b has been the only planet with confirmed orbital decay. The late F-type host star has been hypothesized to be surrounded by a large structure of circumstellar material evaporated from the planet. We obtained two high-resolution spectral transit time series with CARMENES and extensively searched for absorption signals by the atomic species Na, H, Ca, and He using transmission spectroscopy, thereby covering the He I λ10833 Å triplet with high resolution for the first time. We apply SYSREM for atomic line transmission spectroscopy, introduce the technique of signal protection to improve the results for individual absorption lines, and compare the outcomes to those of established methods. No transmission signals were detected and the most stringent upper limits as of yet were derived for the individual indicators. Nonetheless, we found variation in the stellar Hα and He I λ10833 Å lines, the origin of which remains uncertain but is unlikely to be activity. To constrain the enigmatic activity state of WASP–12, we analyzed XMM-Newton X-ray data and found the star to be moderately active at most. We deduced an upper limit for the X-ray luminosity and the irradiating X-ray and extreme ultraviolet (XUV) flux of WASP–12 b. Based on the XUV flux upper limit and the lack of the He I λ10833 Å signal, our hydrodynamic models slightly favor a moderately irradiated planet with a thermospheric temperature of ≲12 000 K, and a conservative upper limit of ≲4 × 1012 g s−1 on the mass-loss rate. Our study does not provide evidence for an extended planetary atmosphere or absorption by circumstellar material close to the planetary orbit.

Millimeter emission in photoevaporating disks is determined by early substructures

Context. Photoevaporation and dust-trapping are individually considered to be important mechanisms in the evolution and morphology of protoplanetary disks. However, it is not yet clear what kind of observational features are expected when both processes operate simultaneously. Aims. We studied how the presence (or absence) of early substructures, such as the gaps caused by planets, affects the evolution of the dust distribution and flux in the millimeter continuum of disks that are undergoing photoevaporative dispersal. We also tested if the predicted properties resemble those observed in the population of transition disks. Methods. We used the numerical code Dustpy to simulate disk evolution considering gas accretion, dust growth, dust-trapping at substructures, and mass loss due to X-ray and EUV (XEUV) photoevaporation and dust entrainment. Then, we compared how the dust mass and millimeter flux evolve for different disk models. Results. We find that, during photoevaporative dispersal, disks with primordial substructures retain more dust and are brighter in the millimeter continuum than disks without early substructures, regardless of the photoevaporative cavity size. Once the photoevaporative cavity opens, the estimated fluxes for the disk models that are initially structured are comparable to those found in the bright transition disk population (Fmm > 30 mJy), while the disk models that are initially smooth have fluxes comparable to the transition disks from the faint population (Fmm < 30 mJy), suggesting a link between each model and population. Conclusions. Our models indicate that the efficiency of the dust trapping determines the millimeter flux of the disk, while the gas loss due to photoevaporation controls the formation and expansion of a cavity, decoupling the mechanisms responsible for each feature. In consequence, even a planet with a mass comparable to Saturn could trap enough dust to reproduce the millimeter emission of a bright transition disk, while its cavity size is independently driven by photoevaporative dispersal.

Study of Atmospheric Ion Escape From Exoplanet TOI‐700 d: Venus Analogs

AbstractTOI‐700 d is the first Earth‐sized planet in the habitable zone (HZ) discovered by the Transiting Exoplanet Survey Satellite. Here, we assess whether a Venus‐like exoplanet at the TOI‐700 d location could retain an atmosphere for a time comparable to the age of the host star based on multispecies magnetohydrodynamics simulations. We investigate the effects of X‐ray and EUV (XUV) radiation from the host star, the interplanetary magnetic field (IMF) orientation, and the planetary intrinsic magnetic field. In unmagnetized cases, major ion loss is caused by O+ escape through a ring‐shaped region by the mass loading process after the ionization of the extended oxygen corona. As the IMF Parker spiral angle increases, the escape flux in the magnetotail shows stronger enhancement around the meridional current sheet, and the escape rate of molecular ions ( and ) increases by an order of magnitude due to acceleration in the ionosphere by magnetic tension forces. In magnetized cases, the intrinsic magnetic field suppresses ion pickup loss from the neutral oxygen corona by deflecting the stellar wind and preventing ion pickup while promoting cusp‐origin escape from the lower ionosphere. These results suggest that the unmagnetized exoplanet would have difficulty retaining its atmosphere over a few billion years under extreme conditions where XUV is 30 times stronger than at the current Earth. However, the dipole intrinsic magnetic field of 1,000 nT at the equatorial surface reduces the escape rate and would help the exoplanet to retain its atmosphere even under strong XUV conditions.

Planetary evolution with atmospheric photoevaporation

Context. Observations by the Kepler satellite have revealed a gap between larger sub-Neptunes and smaller super-Earths that atmospheric escape models had predicted as an evaporation valley prior to discovery. Aims. We seek to contrast results from a simple X-ray and extreme-ultraviolet (XUV)-driven energy-limited escape model against those from a direct hydrodynamic model. The latter calculates the thermospheric temperature structure self-consistently, including cooling effects such as thermal conduction. Besides XUV-driven escape, it also includes the boil-off escape regime where the escape is driven by the atmospheric thermal energy and low planetary gravity, catalysed by stellar continuum irradiation. We coupled these two escape models to an internal structure model and followed the planets’ temporal evolution. Methods. To examine the population-wide imprint of the two escape models and to compare it to observations, we first employed a rectangular grid, tracking the evolution of planets as a function of core mass and orbital period over gigayear timescales. We then studied the slope of the valley also for initial conditions derived from the observed Kepler planet population. Results. For the rectangular grid, we find that the power-law slope of the valley with respect to orbital period is −0.18 and −0.11 in the energy-limited and hydrodynamic model, respectively. For the initial conditions derived from the Kepler planets, the results are similar (−0.16 and −0.10). While the slope found with the energy-limited model is steeper than observed, the one of the hydrodynamic model is in excellent agreement with observations. The reason for the shallower slope is caused by the two regimes in which the energy-limited approximation fails. The first one are low-mass planets at low-to-intermediate stellar irradiation. For them, boil-off dominates mass loss. However, boil-off is absent in the energy-limited model, and thus it underestimates escape relative to the hydrodynamic model. The second one are massive compact planets at high XUV irradiation. For them, the energy-limited approximation overestimates escape relative to the hydrodynamic model because of cooling by thermal conduction, which is neglected in the energy-limited model. Conclusions. The two effects act together in concert to yield, in the hydrodynamic model, a shallower slope of the valley that agrees very well with observations. We conclude that a hydrodynamic escape model that includes boil-off and a more realistic treatment of cooling mechanisms can reproduce one of the most important constraints for escape models, the valley slope.

Proposal for Observing XUV-Induced Rabi Oscillation Using Superfluorescent Emission.

Intense x-ray and extreme ultraviolet (XUV) light sources have been available for decades, however, due to weak nonlinear interaction in the XUV photon energy range, observation of Rabi oscillation induced by XUV pulse remains a very challenging experimental task. Here we suggest a scheme where photoionization of a He medium by an intense XUV pump pulse is followed by a strong population inversion and Rabi oscillation at the He^{+}(1s-3p) transition and is accompanied by superfluorescence (SF) of the 7.56eV pulse at the He^{+}(3p-2s) transition. Our numerical simulations show that the Rabi oscillation at the He^{+}(1s-3p) transition induced by an XUV pulse with photon energy 48.36eV results in significant signatures in the SF spectra, allowing us to identify and characterize the XUV induced Rabi-oscillatory regime. The proposed scheme provides a sensitive tool to monitor and control ultrafast nonlinear dynamics in atoms and molecules triggered by intense XUV.

Modeling the Hα and He 10830 Transmission Spectrum of WASP-52b

Escaping atmosphere has been detected by the excess absorption of Lyα, Hα and He triplet (λ10830) lines. Simultaneously modeling the absorption of the Hα and He λ10830 lines can provide useful constraints about the exoplanetary atmosphere. In this paper, we use a hydrodynamic model combined with a non−local thermodynamic model and a new Monte Carlo simulation model to obtain the H(2) and He(23 S) populations. The Monte Carlo simulations of Lyα radiative transfer are performed with assumptions of a spherical stellar Lyα radiation and a spherical planetary atmosphere, for the first time, to calculate the Lyα mean intensity distribution inside the planetary atmosphere, necessary in estimating the H(2) population. We model the transmission spectra of the Hα and He λ10830 lines simultaneously in hot Jupiter WASP-52b. We find that models with many different H/He ratios can reproduce the Hα observations well if the host star has (1) a high X-ray and extreme-ultraviolet (XUV) flux (F XUV) and a relatively low X-ray fraction in XUV radiation (β m ) or (2) a low F XUV and a high β m . The simulations of the He λ10830 triplet suggest that a high H/He ratio (∼98/2) is required to fit the observation. The models that fit both lines well confine F XUV to be about 0.5 times the fiducial value and β m to have a value around 0.3. The models also suggest that hydrogen and helium originate from the escaping atmosphere, and the mass-loss rate is about 2.8 × 1011 g s−1.

Less Effective Hydrodynamic Escape of H2–H2O Atmospheres on Terrestrial Planets Orbiting Pre-main-sequence M Dwarfs

Terrestrial planets currently in the habitable zones around M dwarfs likely experienced a long-term runaway-greenhouse condition because of a slow decline in host-star luminosity in its pre-main-sequence phase. Accordingly, they might have lost significant portions of their atmospheres including water vapor at high concentration by hydrodynamic escape induced by the strong stellar X-ray and extreme ultraviolet (XUV) irradiation. However, the atmospheric escape rates remain highly uncertain due partly to a lack of understanding of the effect of radiative cooling in the escape outflows. Here we carry out 1D hydrodynamic escape simulations for an H2–H2O atmosphere on a planet with mass of 1M ⊕ considering radiative and chemical processes to estimate the atmospheric escape rate and follow the atmospheric evolution during the early runaway-greenhouse phase. We find that the atmospheric escape rate decreases with the basal H2O/H2 ratio due to the energy loss by the radiative cooling of H2O and chemical products such as OH and OH+: the escape rate of H2 becomes one order of magnitude smaller when the basal H2O/H2 = 0.1 than that of the pure hydrogen atmosphere. The timescale for H2 escape exceeds the duration of the early runaway-greenhouse phase, depending on the initial atmospheric amount and composition, indicating that H2 and H2O could be left behind after the end of the runaway-greenhouse phase. Our results suggest that temperate and reducing environments with oceans could be formed on some terrestrial planets around M dwarfs.

Ab Initio Prediction of Excited-State and Polaron Effects in Transient XUV Measurements of α-Fe2O3.

Transient X-ray and extreme ultraviolet (XUV) spectroscopies have become invaluable tools for studying photoexcited dynamics due to their sensitivity to carrier occupations and local chemical or structural changes. One of the most studied materials using transient XUV spectroscopy is α-Fe2O3 because of its rich photoexcited dynamics, including small polaron formation. The interpretation of carrier and polaron effects in α-Fe2O3 is currently carried out using a semi-empirical method that is not transferrable to most materials. Here, an ab initio, Bethe-Salpeter equation (BSE) approach is developed that can incorporate photoexcited-state effects into arbitrary material systems. The accuracy of this approach is proven by calculating the XUV absorption spectra for the ground, photoexcited, and polaron states of α-Fe2O3. Furthermore, the theoretical approach allows for the projection of the core-valence excitons and different components of the X-ray transition Hamiltonian onto the band structure, providing new insights into old measurements. From this information, a physical intuition about the origins and nature of the transient XUV spectra can be built. A route to extracting electron and hole energies is even shown possible for highly angular momentum split XUV peaks. This method is easily generalized to K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Irradiation-driven escape of primordial planetary atmospheres

Making use of the publicly available 1D photoionization hydrodynamics code ATES we set out to investigate the combined effects of specific planetary gravitational potential energy (ϕp ≡ GMp/Rp) and stellar X-ray and extreme ultraviolet (XUV) irradiation (FXUV) on the evaporation efficiency (η) of moderately-to-highly irradiated gaseous planets, from sub-Neptunes through hot Jupiters. We show that the (known) existence of a threshold potential above which energy-limited thermal escape (i.e., η ≃ 1) is unattainable can be inferred analytically, by means of a balance between the ion binding energy and the volume-averaged mean excess energy. For log ϕp ≳ log ϕpthr ≈ [12.9 − 13.2] (in cgs units), most of the energy absorption occurs within a region where the average kinetic energy acquired by the ions through photo-electron collisions is insufficient for escape. This causes the evaporation efficiency to plummet with increasing ϕp, by up to 4 orders of magnitude below the energy-limited value. Whether or not planets with ϕp ≲ ϕpthr exhibit energy-limited outflows is primarily regulated by the stellar irradiation level. Specifically, for low-gravity planets, above FXUVthr ≃ 104–5 erg cm−2 s−1, Lyα losses overtake adiabatic and advective cooling and the evaporation efficiency of low-gravity planets drops below the energy-limited approximation, albeit remaining largely independent of ϕp. Further, we show that whereas η increases as FXUV increases for planets above ϕpthr, the opposite is true for low-gravity planets (i.e., for sub-Neptunes). This behavior can be understood by examining the relative fractional contributions of advective and radiative losses as a function of atmospheric temperature. This novel framework enables a reliable, physically motivated prediction of the expected evaporation efficiency for a given planetary system; an analytical approximation of the best-fitting η is given in the appendix.

Multispecies MHD Study of Ion Escape at Ancient Mars: Effects of an Intrinsic Magnetic Field and Solar XUV Radiation

AbstractIon escape is one of the key processes responsible for drastic climate change on ancient Mars. Ion escape is affected by the solar X‐ray and EUV (X‐ray and extreme ultraviolet (XUV)) flux, the solar wind, and the presence of a planetary intrinsic magnetic field, all of which was much different at ancient Mars. We investigated how the presence and strength of a dipole field affects the ion escape under 50 or 10 times higher solar XUV flux and strong solar wind with multispecies magnetohydrodynamics model. The results showed two opposite effects on the escape rates, which is associated with the pressure ratio of the dipolar magnetic pressure at the equatorial surface to the solar wind dynamic pressure. The escape rates increase by up to a factor of 6 for O2+ and CO2+ but change little for O+ if the pressure ratio is below 0.1. On the other hand, the escape rates decrease by more than one order of magnitude for the three ions if the pressure ratio is above 0.1. The threshold can be described by the pressure balance between the solar wind flow and the dipole field at high latitudes, where the ionospheric outflow emerges in the unmagnetized cases. The effects on the escape rates are stronger under lower solar XUV cases. The total escape rate reaches 1027 s−1 in the unmagnetized case, which may lead to a large contribution to atmospheric loss at ancient Mars, but it can be reduced by an order of magnitude in the presence of a dipole field.

The harsh environment where exoplanets live

AbstractThe X‐ray and Extreme Ultraviolet (XUV) actvity of the Sun has determined important effects on the planets around it. In particular, high doses of X‐rays, flares, and coronal mass ejections (CMEs) combined with masses and sizes of the planets and the shielding action of the planetary magnetic fields have determined the diverse habitability of rocky planets like Venus, Earth, and Mars. In exoplanetary systems, the same could apply, especially in those stars with close‐in planets like hot Jupiter's or Earth analogs around M dwarfs. These planets live in the harsh environment of the hot coronal fringes. This has a consequence on the evaporation and the photochemistry of the upper atmospheres of such planets. Furthermore, phenomena related to gravitational and magnetic interaction between stars and close‐in planets could be at work. Effects due to star–planet interaction could be the transfer of rotational momentum between planet and star, magnetic interaction between the fields of planet and star that could potentially lead to flares in the outer corona, evaporation with the formation of a cloud of planetary gas around the star, and accretion onto the star itself. In this review, I will report on the search for such phenomena at XUV bands with X‐ray observatories like XMM‐Newton.

CARMENES detection of the Ca II infrared triplet and possible evidence of He I in the atmosphere of WASP-76b

Ultra-hot Jupiters are highly irradiated gas giants with equilibrium temperatures typically higher than 2000 K. Atmospheric studies of these planets have shown that their transmission spectra are rich in metal lines, with some of these metals being ionised due to the extreme temperatures. Here, we use two transit observations of WASP-76b obtained with the CARMENES spectrograph to study the atmosphere of this planet using high-resolution transmission spectroscopy. Taking advantage of the two channels and the coverage of the red and near-infrared wavelength ranges by CARMENES, we focus our analysis on the study of the Ca II infrared triplet (IRT) at 8500 Å and the He I triplet at 10 830 Å. We present the discovery of the Ca II IRT at 7σ in the atmosphere of WASP-76b using the cross-correlation technique, which is consistent with previous detections of the Ca II H&K lines in the same planet, and with the atmospheric studies of other ultra-hot Jupiters reported to date. The low mass density of the planet, and our calculations of the XUV (X-ray and EUV) irradiation received by the exoplanet, show that this planet is a potential candidate to have a He I evaporating envelope and, therefore, we performed further investigations focussed on this aspect. The transmission spectrum around the He I triplet shows a broad and red-shifted absorption signal in both transit observations. However, due to the strong telluric contamination around the He I lines and the relatively low signal-to-noise ratio of the observations, we are not able to unambiguously conclude if the absorption is due to the presence of helium in the atmosphere of WASP-76b, and we consider the result to be only an upper limit. Finally, we revisit the transmission spectrum around other lines such as Na I, Li I, Hα, and K I. The upper limits reported here for these lines are consistent with previous studies.

The key impact of the host star’s rotational history on the evolution of TOI-849b

Context. TOI-849b is one of the few planets populating the hot-Neptune desert and it is the densest Neptune-sized one discovered so far. Its extraordinary proximity to the host star, together with the absence of a massive H/He envelope on top of the 40.8 M⊕ rocky core, calls into question the role played by the host star in the evolution of the system. Aims. We aim to study the impact of the host star’s rotational history on the evolution of TOI-849b, particularly focussing on the planetary migration due to dynamical tides dissipated in the stellar convective envelope, and on the high-energy stellar emission. Methods. Rotating stellar models of TOI-849 are coupled to our orbital evolution code to study the evolution of the planetary orbit. The evolution of the planetary atmosphere is studied by means of the JADE code, which uses realistic X-ray and extreme-ultraviolet (XUV) fluxes provided by our rotating stellar models. Results. Assuming that the planet was at its present-day position (ain = 0.01598 AU) at the protoplanetary disc dispersal, with mass 40.8 M⊕, and considering a broad range of host star initial surface rotation rates (Ωin ∈ [3.2, 18] Ω⊙), we find that only for Ωin ≤ 5 Ω⊙ do we reproduce the current position of the planet, given that for Ωin > 5 Ω⊙ its orbit is efficiently deflected by dynamical tides within the first ∼40 Myr of evolution. We also simulated the evolution of the orbit for values of ain ≠ 0.01598 AU for each of the considered rotational histories, confirming that the only combination suited to reproduce the current position of the planet is given by ain = 0.01598 AU and Ωin ≤ 5 Ω⊙. We tested the impact of increasing the initial mass of the planet on the efficiency of tides, finding that a higher initial mass (Min = 1 MJup) does not change the results reported above. Based on these results we computed the evolution of the planetary atmospheres with the JADE code for a large range of initial masses above a core mass of 40.8 M⊕, finding that the strong XUV-flux received by the planet is able to remove the entirety of the envelope within the first 50 Myr, even if it formed as a Jupiter-mass planet.

Revisiting Kepler-444

Context.Kepler-444 is one of the oldest planetary systems known thus far. Its peculiar configuration consisting of five sub-Earth-sized planets orbiting the companion to a binary stellar system makes its early history puzzling. Moreover, observations of HI-Lyαvariations raise many questions about the potential presence of escaping atmospheres today.Aims.We aim to study the orbital evolution of Kepler-444-d and Kepler-444-e and the impact of atmospheric evaporation on Kepler-444-e.Methods.Rotating stellar models of Kepler-444-A were computed with the Geneva stellar evolution code and coupled to an orbital evolution code, accounting for the effects of dynamical, equilibrium tides and atmospheric evaporation. The impacts of multiple stellar rotational histories and X-ray and extreme ultraviolet (XUV) luminosity evolutionary tracks are explored.Results.Using detailed rotating stellar models able to reproduce the rotation rate of Kepler-444-A, we find that its observed rotation rate is perfectly in line with what is expected for this old K0-type star, indicating that there is no reason for it to be exceptionally active as would be required to explain the observed HI-Lyαvariations from a stellar origin. We show that given the low planetary mass (~0.03 M⊕) and relatively large orbital distance (~0.06 AU) of Kepler-444-d and e, dynamical tides negligibly affect their orbits, regardless of the stellar rotational history considered. We point out instead how remarkable the impact is of the stellar rotational history on the estimation of the lifetime mass loss for Kepler-444-e. We show that, even in the case of an extremely slow rotating star, it seems unlikely that such a planet could retain a fraction of the initial water-ice content if we assume that it formed with a Ganymede-like composition.

Atmosphere Escape Inferred from Modeling the Hα Transmission Spectrum of WASP-121b

Abstract The escaping atmospheres of hydrogen driven by stellar X-ray and extreme ultraviolet (XUV) have been detected around some exoplanets by the excess absorption of Lyα in the far-ultraviolet band. In the optical band the excess absorption of Hα is also found by ground-based instruments. However, it is not certain if the escape of the atmosphere driven by XUV can result in such absorption. Here we present the XUV-driven hydrodynamic simulation coupled with the calculation of detailed level population and the process of radiative transfer for WASP-121b. Our fiducial model predicts a mass-loss rate of ∼1.28 × 1012 g s−1 for WASP-121b. Due to the high temperature and Lyα intensity predicted by the fiducial model, many hydrogen atoms are populated into the first excited state. As a consequence, the transmission spectrum of Hα simulated by our model is broadly consistent with the observation. Compared with the absorption of Hα at different observation times, the stellar XUV emission varies in the range of 0.5–1.5 times fiducial value, which may reflect the variation of the stellar activity. Finally, we find that the supersonic regions of the planetary wind contribute a prominent portion to the absorption of Hα by comparing the equivalent width of Hα, which hints that a transonic outflow of the upper atmosphere driven by XUV irradiation of the host star can be detected by a ground-based telescope and that Hα can be a good indicator of escaping atmosphere.

Keeping M-Earths habitable in the face of atmospheric loss by sequestering water in the mantle

ABSTRACT Water cycling between Earth’s mantle and surface has previously been modelled and extrapolated to rocky exoplanets, but these studies neglected the host star. M-dwarf stars are more common than Sun-like stars and at least as likely to host temperate rocky planets (M-Earths). However, M dwarfs are active throughout their lifetimes; specifically, X-ray and extreme ultraviolet (XUV) radiation during their early evolution can cause rapid atmospheric loss on orbiting planets. The increased bolometric flux reaching M-Earths leads to warmer, moister upper atmospheres, while XUV radiation can photodissociate water molecules and drive hydrogen and oxygen escape to space. Here, we present a coupled model of deep-water cycling and water loss to space on M-Earths to explore whether these planets can remain habitable despite their volatile evolution. We use a cycling parametrization accounting for the dependence of mantle degassing on seafloor pressure, the dependence of regassing on mantle temperature, and the effect of water on mantle viscosity and thermal evolution. We assume the M dwarf’s XUV radiation decreases exponentially with time, and energy-limited water loss with 30 per cent efficiency. We explore the effects of cycling and loss to space on planetary water inventories and water partitioning. Planet surfaces desiccated by loss can be rehydrated, provided there is sufficient water sequestered in the mantle to degas once loss rates diminish at later times. For a given water loss rate, the key parameter is the mantle overturn time-scale at early times: if the mantle overturn time-scale is longer than the loss time-scale, then the planet is likely to keep some of its water.

Modeling the Lyα transit absorption of the hot Jupiter HD 189733b

Context. Hydrogen-dominated atmospheres of hot exoplanets expand and escape hydrodynamically due to the intense heating by the X-ray and extreme ultraviolet (XUV) irradiation of their host stars. Excess absorption of neutral hydrogen has been observed in the Lyα line during transits of several close-in gaseous exoplanets, indicating such extended atmospheres. Aims. For the hot Jupiter HD 189733b, this absorption shows temporal variability. We aim to study if variations in stellar XUV emission and/or variable stellar wind conditions may explain this effect. Methods. We applied a 1D hydrodynamic planetary upper atmosphere model and a 3D magnetohydrodynamic stellar wind flow model to study the effect of variations of the stellar XUV irradiation and wind conditions at the planet’s orbit on the neutral hydrogen distribution. This includes the production of energetic neutral atoms (ENAs) and the related Lyα transit signature. Results. We obtain comparable, albeit slightly higher Lyα absorption than that observed in 2011 with a stellar XUV flux of 1.8 × 104 erg cm−2 s−1, rather typical activity conditions for this star. Flares with parameters similar to that observed eight hours before the transit are unlikely to have caused a significant modulation of the transit signature. We find that the resulting Lyα absorption is dominated by atmospheric broadening, whereas the contribution of ENAs is negligible, as they are formed inside the bow shock from decelerated wind ions that are heated to high temperatures. Thus, within our modeling framework and assumptions, we find an insignificant dependence of the absorption on the stellar wind parameters. Conclusions. Since the transit absorption can be modeled with typical stellar XUV and wind conditions, it is possible that the nondetection of the absorption in 2010 was affected by less typical stellar activity conditions, such as a very different magnitude and/or shape of the star’s spectral XUV emission, or temporal and/or spatial variations in Lyα affecting the determination of the transit absorption.

Dust entrainment in photoevaporative winds: The impact of X-rays

Context. X-ray- and extreme ultraviolet (XEUV) driven photoevaporative winds acting on protoplanetary disks around young T Tauri stars may crucially impact disk evolution, affecting both gas and dust distributions. Aims. We investigate the dust entrainment in XEUV-driven photoevaporative winds and compare our results to existing magnetohydrodynamic and EUV-only models. Methods. We used a 2D hydrodynamical gas model of a protoplanetary disk irradiated by both X-ray and EUV spectra from a central T Tauri star to trace the motion of passive Lagrangian dust grains of various sizes. The trajectories were modelled starting at the disk surface in order to investigate dust entrainment in the wind. Results. For an X-ray luminosity of LX = 2 × 1030 erg s−1 emitted by a M* = 0.7 M⊙ star, corresponding to a wind mass-loss rate of Ṁw ≃ 2.6 × 10−8 M⊙ yr−1, we find dust entrainment for sizes a0 ≲ 11 μm (9 μm) from the inner 25 AU (120 AU). This is an enhancement over dust entrainment in less vigorous EUV-driven winds with Ṁw ≃ 10−10 M⊙ yr−1. Our numerical model also shows deviations of dust grain trajectories from the gas streamlines even for μm-sized particles. In addition, we find a correlation between the size of the entrained grains and the maximum height they reach in the outflow. Conclusions. X-ray-driven photoevaporative winds are expected to be dust-rich if small grains are present in the disk atmosphere.

A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system

Measures of exoplanet bulk densities indicate that small exoplanets with radius less than 3 Earth radii ($R_\oplus$) range from low-density sub-Neptunes containing volatile elements to higher density rocky planets with Earth-like or iron-rich (Mercury-like) compositions. Such astonishing diversity in observed small exoplanet compositions may be the product of different initial conditions of the planet-formation process and/or different evolutionary paths that altered the planetary properties after formation. Planet evolution may be especially affected by either photoevaporative mass loss induced by high stellar X-ray and extreme ultraviolet (XUV) flux or giant impacts. Although there is some evidence for the former, there are no unambiguous findings so far about the occurrence of giant impacts in an exoplanet system. Here, we characterize the two innermost planets of the compact and near-resonant system Kepler-107. We show that they have nearly identical radii (about $1.5-1.6~R_\oplus$), but the outer planet Kepler-107c is more than twice as dense (about $12.6~\rm g\,cm^{-3}$) as the innermost Kepler-107b (about $5.3~\rm g\,cm^{-3}$). In consequence, Kepler-107c must have a larger iron core fraction than Kepler-107b. This imbalance cannot be explained by the stellar XUV irradiation, which would conversely make the more-irradiated and less-massive planet Kepler-107b denser than Kepler-107c. Instead, the dissimilar densities are consistent with a giant impact event on Kepler-107c that would have stripped off part of its silicate mantle. This hypothesis is supported by theoretical predictions from collisional mantle stripping, which match the mass and radius of Kepler-107c.