Understanding Of Planet Formation Research Articles

A giant planet shaping the disk around the very low-mass star CIDA 1

Context. Exoplanetary research has provided us with exciting discoveries of planets around very low-mass (VLM) stars (0.08 M⊙ ≲ M* ≲ 0.3 M⊙; e.g., TRAPPIST-1 and Proxima Centauri). However, current theoretical models still strive to explain planet formation in these conditions and do not predict the development of giant planets. Recent high-resolution observations from the Atacama Large Millimeter/submillimeter Array (ALMA) of the disk around CIDA 1, a VLM star in Taurus, show substructures that hint at the presence of a massive planet. Aims. We aim to reproduce the dust ring of CIDA 1, observed in the dust continuum emission in ALMA Band 7 (0.9 mm) and Band 4 (2.1 mm), along with its 12CO (J = 3−2) and 13CO (J = 3−2) channel maps, assuming the structures are shaped by the interaction of the disk with a massive planet. We seek to retrieve the mass and position of the putative planet, through a global simulation that assesses planet-disk interactions to quantitatively reproduce protoplanetary disk observations of both dust and gas emission in a self-consistent way. Methods. Using a set of hydrodynamical simulations, we model a protoplanetary disk that hosts an embedded planet with a starting mass of between 0.1 and 4.0 MJup and initially located at a distance of between 9 and 11 au from the central star. We compute the dust and gas emission using radiative transfer simulations, and, finally, we obtain the synthetic observations, treating the images as the actual ALMA observations. Results. Our models indicate that a planet with a minimum mass of ~1.4 MJup orbiting at a distance of ~9−10 au can explain the morphology and location of the observed dust ring in Band 7 and Band 4. We match the flux of the dust emission observation with a dust-to-gas mass ratio in the disk of ~10−2. We are able to reproduce the low spectral index (~2) observed where the dust ring is detected, with a ~40−50% fraction of optically thick emission. Assuming a 12CO abundance of 5 × 10−5 and a 13CO abundance 70 times lower, our synthetic images reproduce the morphology of the 12CO (J = 3−2) and 13CO (J = 3−2) observed channel maps where the cloud absorption allowed a detection. From our simulations, we estimate that a stellar mass M* = 0.2 M⊙ and a systemic velocity vsys = 6.25 km s−1 are needed to reproduce the gas rotation as retrieved from molecular line observations. Applying an empirical relation between planet mass and gap width in the dust, we predict a maximum planet mass of ~4−8 MJup. Conclusions. Our results suggest the presence of a massive planet orbiting CIDA 1, thus challenging our understanding of planet formation around VLM stars.

Discovery of Line Pressure Broadening and Direct Constraint on Gas Surface Density in a Protoplanetary Disk

The gas surface density profile of protoplanetary disks is one of the most fundamental physical properties to understanding planet formation. However, it is challenging to determine the surface density profile observationally, because the H2 emission cannot be observed in low-temperature regions. We analyzed the Atacama Large Millimeter/submillimeter Array (ALMA) archival data of the 12CO J = 3 − 2 line toward the protoplanetary disk around TW Hya and discovered extremely broad line wings due to the pressure broadening. In conjunction with a previously reported optically thin CO isotopologue line, the pressure broadened line wings enabled us to directly determine the midplane gas density for the first time. The gas surface density at ∼5 au from the central star reaches ∼103 g cm−2, which suggests that the inner region of the disk has enough mass to form a Jupiter-mass planet. Additionally, the gas surface density drops at the inner cavity by ∼2 orders of magnitude compared to outside the cavity. We also found a low CO abundance of ∼10−6 with respect to H2, even inside the CO snow line, which suggests conversion of CO to less volatile species. Combining our results with previous studies, the gas surface density jumps at r ∼ 20 au, suggesting that the inner region (3 < r < 20 au) might be the magnetorotational instability dead zone. This study sheds light on the direct gas surface density constraint without assuming the CO/H2 ratio using ALMA.

Depletion of gaseous CO in protoplanetary disks by surface-energy-regulated ice formation

Empirical constraints of fundamental properties of protoplanetary disks are essential for understanding planet formation and planetary properties1,2. Carbon monoxide (CO) gas is often used to constrain disk properties3. However, estimates show that the CO gas abundance in disks is depleted relative to expected values4,5,6,7, and models of various disk processes impacting the CO abundance could not explain this depletion on observed ~1 Myr timescales8,9,10,11,12,13,14. Here we demonstrate that surface energy effects on particles in disks, such as the Kelvin effect, that arise when ice heterogeneously nucleates onto an existing particle can efficiently trap CO in its ice phase. In previous ice formation models, CO gas was released when small ice-coated particles were lofted to warmed disk layers. Our model can reproduce the observed abundance, distribution and time evolution of gaseous CO in the four most studied protoplanetary disks7. We constrain the solid and gaseous CO inventory at the midplane and disk diffusivities and resolve inconsistencies in estimates of the disk mass—three crucial parameters that control planetary formation.

The Ariel Target List: The Impact of TESS and the Potential for Characterizing Multiple Planets within a System

The ESA Ariel mission has been adopted for launch in 2029 and will conduct a survey of around 1000 exoplanetary atmospheres during its primary mission life. By providing homogeneous data sets with a high signal-to-noise ratio and wide wavelength coverage, Ariel will unveil the atmospheric demographics of these faraway worlds, helping to constrain planet formation and evolution processes on a galactic scale. Ariel seeks to undertake a statistical survey of a diverse population of planets; therefore, the sample of planets from which this selection can be made is of the utmost importance. While many suitable targets have already been found, hundreds more will be discovered before the mission is operational. Previous studies have used predictions of exoplanet detections to forecast the available planet population by the launch date of Ariel, with the most recent noting that the Transiting Exoplanet Survey Satellite (TESS) alone should provide over 1000 potential targets. In this work, we consider the planet candidates found to date by TESS to show that, with the addition of already confirmed planets, Ariel will already have a more than sufficient sample to choose its target list from once these candidates are validated. We showcase the breadth of this population, as well as exploring, for the first time, the ability of Ariel to characterize multiple planets within a single system. Comparative planetology of worlds orbiting the same star, as well as across the wider population, will undoubtedly revolutionize our understanding of planet formation and evolution.

Asteroids accretion, differentiation, and break-up in the Vesta source region: Evidence from cosmochemistry of mesosiderites

The cosmochemistry of meteorites provides unique clues on asteroids accretion, differentiation, collisional break-up and reassembly - processes of critical importance for understanding planet formation in the early solar system. Mesosiderites are a complex group of achondrites whose nearly 50:50 metal-silicate composition is interpreted in the literature as resulting from the mixing of core and crustal materials derived from differentiated asteroids. Because of their complex nature and contrasting geochemical, isotopic, and spectroscopic data, the formation mechanism of mesosiderites is still poorly understood and is open to a large variety of planetary differentiation scenarios and collisional histories. In this study, based on new petrographic and geochemical data of 16 mesosiderites, we investigate in detail the proposal that mesosiderites are related to the howardite-eucrite-diogenite (HED) meteorite group, whose parent body is widely considered to be asteroid 4 Vesta (∼500 km diameter), the target of the recent NASA’s Dawn mission. We present the first high precision oxygen isotope analyses on the matrix of a set of mesosiderite samples, coupled with new chemical and petrographic analyses of mesosiderites Um Hadid, Estherville, and Mount Padbury. Concordant Δ17O values between mesosiderites (–0.241 ± 0.015 (2σ)) and howardite-eucrite-diogenites (−0.241 ± 0.017‰ (2σ)) indicate that they derived from the same oxygen isotope reservoir, but petrological evidence, in particular the distinctly lower Fe/Mn ratios and the larger lithological diversity in mesosiderites, indicates that they formed within different parent bodies. This suggests that mesosiderites and howardite-eucrite-diogenites originated in distinct parent bodies that accreted in the 4 Vesta source region but experienced different geologic evolution in terms of crustal differentiation and impact history, which were more complex and catastrophic in the mesosiderite parent body.

Five Key Exoplanet Questions Answered via the Analysis of 25 Hot-Jupiter Atmospheres in Eclipse

Population studies of exoplanets are key to unlocking their statistical properties. So far, the inferred properties have been mostly limited to planetary, orbital, and stellar parameters extracted from, e.g., Kepler, radial velocity, and Gaia data. More recently an increasing number of exoplanet atmospheres have been observed in detail from space and the ground. Generally, however, these atmospheric studies have focused on individual planets, with the exception of a couple of works that have detected the presence of water vapor and clouds in populations of gaseous planets via transmission spectroscopy. Here, using a suite of retrieval tools, we analyze spectroscopic and photometric data of 25 hot Jupiters, obtained with the Hubble and Spitzer Space Telescopes via the eclipse technique. By applying the tools uniformly across the entire set of 25 planets, we extract robust trends in the thermal structure and chemical properties of hot Jupiters not obtained in past studies. With the recent launch of the James Webb Space Telescope and the upcoming missions Twinkle and Ariel, population-based studies of exoplanet atmospheres, such as the one presented here, will be a key approach to understanding planet characteristics, formation, and evolution in our galaxy.

The mass and size of Herbig disks as seen by ALMA

Context. Many population studies have been performed over the past decade with the Atacama Large millimeter/submillimeter Array (ALMA) to understand the bulk properties of protoplanetary disks around young stars. The studied populations have mostly consisted of late spectral type (i.e., G, K &amp; M) stars, with relatively few more massive Herbig stars (spectral types B, A &amp; F). With Gaia-updated distances, now is a good time to use ALMA archival data for a Herbig disk population study and take an important step forward in our understanding of planet formation. Aims. The aim of this work is to determine the masses and sizes of all Herbig dust disks observed with ALMA to date in a volume-limited sample out to 450 pc. These masses and sizes are put in the context of the Lupus and Upper Sco T Tauri disk populations. Methods. ALMA Band 6 and Band 7 archival data of 36 Herbig stars are used, making this work 64% complete out to 225 pc, and 38% complete out to 450 pc also including Orion. Using stellar parameters and distances, the dust masses and sizes of the disks are determined via a curve-of-growth method. Survival analysis is used to obtain cumulative distributions of the dust masses and radii. Results. Herbig disks have a higher dust mass than the T Tauri disk populations of Lupus and Upper Sco by factors of ~3 and ~7 respectively. In addition, Herbig disks are often larger than the typical T Tauri disk. Although the masses and sizes of Herbig disks extend over a similar range to those of T Tauri disks, the distributions of masses and sizes of Herbig disks are significantly skewed toward higher values. Lastly, group I disks are more massive than group II disks. An insufficient number of group II disks are observed at sufficient angular resolution to determine whether or not they are also small in size compared to group I disks. Conclusions. Herbig disks are skewed towards more massive and larger dust disks compared to T Tauri disks. Based on this we speculate that these differences find their origin in an initial disk mass that scales with the stellar mass, and that subsequent disk evolution enlarges the observable differences, especially if (sub)millimeter continuum optical depth plays a role. Moreover, the larger disk masses and sizes of Herbig stars could be linked to the increasing prevalence of giant planets with host star mass.

Understanding the interior structure of gaseous giant exoplanets with machine learning techniques

Context. Characterizing the interiors of gaseous giant exoplanets is currently one of the main objectives in exoplanetary sciences. In particular, the planetary heavy-element mass provides a critical constraint on planet formation from exoplanetary systems. However, gas giant exoplanets show large diversities in thermal states and their interior properties vary across a wide magnitude range. Forward modeling of their interiors exhibits a larger degeneracy with respect to rocky exoplanets. Aims. We applied machine learning techniques based on mixture density networks (MDNs) to investigate the interiors of gaseous giant exoplanets. We aim to provide a well-trained MDN for quick and efficient predictions. Methods. Based on our current knowledge of gas giants in the Solar System, we discussed an effect of model uncertainties on planetary interiors and presented a data set for gas giants with masses between 0.1 and 10 Jupiter masses using two-layer interior models. Then, MDNs were constructed to train the generated data set and their performance was evaluated in order to achieve a well-trained one. Results. The MDN using planetary mass and radius as inputs exhibits the well-known degeneracy of interior models. The surface temperature of a planet bears constraints on the thermal state of planetary interiors, and adding it as additional input considerably breaks the degeneracy of possible interior structures. The MDN with inputs of mass, radius, and surface temperature is found to show excellent performance in predicting the interior properties of gaseous giant exoplanets, although these interior properties span over a very wide range. We also applied the well-trained MDN to four gas giants in the Solar System and beyond. The MDN predictions are in good agreement with the interior model solutions within the observational and systematic uncertainties. Conclusions. We offer a convenient and powerful tool available online providing knowledge of the interiors of gaseous giant exoplanets in addition to rocky exoplanets, which could be helpful for our understanding of planet formation in diverse protoplanetary environments.

The HD 137496 system: A dense, hot super-Mercury and a cold Jupiter

Context. Most of the currently known planets are small worlds with radii between that of the Earth and that of Neptune. The characterization of planets in this regime shows a large diversity in compositions and system architectures, with distributions hinting at a multitude of formation and evolution scenarios. However, many planetary populations, such as high-density planets, are significantly under-sampled, limiting our understanding of planet formation and evolution. Aims. NCORES is a large observing program conducted on the HARPS high-resolution spectrograph that aims to confirm the planetary status and to measure the masses of small transiting planetary candidates detected by transit photometry surveys in order to constrain their internal composition. Methods. Using photometry from the K2 satellite and radial velocities measured with the HARPS and CORALIE spectrographs, we searched for planets around the bright (Vmag = 10) and slightly evolved Sun-like star HD 137496. Results. We precisely estimated the stellar parameters, M* = 1.035 ± 0.022 M⊙, R* = 1.587 ± 0.028 R⊙, Teff = 5799 ± 61 K, together with the chemical composition (e.g. [Fe/H] = −0.027 ± 0.040 dex) of the slightly evolved star. We detect two planets orbiting HD 137496. The inner planet, HD 137496 b, is a super-Mercury (an Earth-sized planet with the density of Mercury) with a mass of Mb = 4.04 ± 0.55 M⊕, a radius of Rb = 1.31−0.05+0.06 R⊕, and a density of ρb = 10.49−1.82+2.08 g cm-3. With an interior modeling analysis, we find that the planet is composed mainly of iron, with the core representing over 70% of the planet’s mass (Mcore / Mtotal = 0.73−0.12+0.11). The outer planet, HD 137496 c, is an eccentric (e = 0.477 ± 0.004), long period (P = 479.9−1.1+1.0 days) giant planet (Mc sinic = 7.66 ± 0.11 MJup) for which we do not detect a transit. Conclusions. HD 137496 b is one of the few super-Mercuries detected to date. The accurate characterization reported here enhances its role as a key target to better understand the formation and evolution of planetary systems. The detection of an eccentric long period giant companion also reinforces the link between the presence of small transiting inner planets and long period gas giants.

Exoplanets in the Galactic context: planet occurrence rates in the thin disc, thick disc, and stellar halo of Kepler stars

ABSTRACT In order to gain a better understanding of planet formation and evolution, it is important to examine the statistics of exoplanets in the Galactic context. By combining information on stellar elemental abundances and kinematics, we constructed separate samples of Kepler stars according to their affiliation to the Galactic components of thin disc, thick disc, and stellar halo. Using a Bayesian analysis with conjugate priors, we then investigated how planet occurrence rates differ in different regions of planet properties. We find that young, slow, and metal-rich stars, associated mainly with the thin disc, host on average more planets (especially close-in super Earths) compared to the old, fast, and metal-poor thick disc stars. We further assess the dependence between stellar properties such as spectral type and metallicity, and planet occurrence rates. The trends we find agree with those found by other authors as well. We argue that in the Galactic context, these are probably not the main properties that affect planet occurrence rates, but rather the dynamical history of stars, and especially stellar age and kinematics, impact the current distribution of planets in the Galaxy.

Exploring the link between star and planet formation with Ariel

The goal of the Ariel space mission is to observe a large and diversified population of transiting planets around a range of host star types to collect information on their atmospheric composition. The planetary bulk and atmospheric compositions bear the marks of the way the planets formed: Ariel’s observations will therefore provide an unprecedented wealth of data to advance our understanding of planet formation in our Galaxy. A number of environmental and evolutionary factors, however, can affect the final atmospheric composition. Here we provide a concise overview of which factors and effects of the star and planet formation processes can shape the atmospheric compositions that will be observed by Ariel, and highlight how Ariel’s characteristics make this mission optimally suited to address this very complex problem.

Probing protoplanetary disk evolution in the Chamaeleon II region

Context. Characterizing the evolution of protoplanetary disks is necessary to improve our understanding of planet formation. Constraints on both dust and gas are needed to determine the dominant disk dissipation mechanisms. Aims. We aim to compare the disk dust masses in the Chamaeleon II (Cha II) star-forming region with other regions with ages between 1 and 10 Myr. Methods. We use ALMA band 6 observations (1.3 mm) to survey 29 protoplanetary disks in Cha II. Dust mass estimates are derived from the continuum data. Results. Out of our initial sample of 29 disks, we detect 22 sources in the continuum, 10 in 12CO, 3 in 13CO, and none in C18O (J = 2−1). Additionally, we detect two companion candidates in the continuum and 12CO emission. Most disk dust masses are lower than 10 M⊕, assuming thermal emission from optically thin dust. Including non-detections, we derive a median dust mass of 4.5 ± 1.5 M⊕ from survival analysis. We compare consistent estimations of the distributions of the disk dust mass and the disk-to-stellar mass ratios in Cha II with six other low mass and isolated star-forming regions in the age range of 1–10 Myr: Upper Sco, CrA, IC 348, Cha I, Lupus, and Taurus. When comparing the dust-to-stellar mass ratio, we find that the masses of disks in Cha II are statistically different from those in Upper Sco and Taurus, and we confirm that disks in Upper Sco, the oldest region of the sample, are statistically less massive than in all other regions. Performing a second statistical test of the dust mass distributions from similar mass bins, we find no statistical differences between these regions and Cha II. Conclusions. We interpret these trends, most simply, as a sign of decline in the disk dust masses with time or dust evolution. Different global initial conditions in star-forming regions may also play a role, but their impact on the properties of a disk population is difficult to isolate in star-forming regions lacking nearby massive stars.

Six transiting planets and a chain of Laplace resonances in TOI-178

Determining the architecture of multi-planetary systems is one of the cornerstones of understanding planet formation and evolution. Resonant systems are especially important as the fragility of their orbital configuration ensures that no significant scattering or collisional event has taken place since the earliest formation phase when the parent protoplanetary disc was still present. In this context, TOI-178 has been the subject of particular attention since the first TESS observations hinted at the possible presence of a near 2:3:3 resonant chain. Here we report the results of observations from CHEOPS, ESPRESSO, NGTS, and SPECULOOS with the aim of deciphering the peculiar orbital architecture of the system. We show that TOI-178 harbours at least six planets in the super-Earth to mini-Neptune regimes, with radii ranging from 1.152−0.070+0.073 to 2.87−0.13+0.14 Earth radii and periods of 1.91, 3.24, 6.56, 9.96, 15.23, and 20.71 days. All planets but the innermost one form a 2:4:6:9:12 chain of Laplace resonances, and the planetary densities show important variations from planet to planet, jumping from 1.02−0.23+0.28 to 0.177−0.061+0.055 times the Earth’s density between planets c and d. Using Bayesian interior structure retrieval models, we show that the amount of gas in the planets does not vary in a monotonous way, contrary to what one would expect from simple formation and evolution models and unlike other known systems in a chain of Laplace resonances. The brightness of TOI-178 (H = 8.76 mag, J = 9.37 mag, V = 11.95 mag) allows for a precise characterisation of its orbital architecture as well as of the physical nature of the six presently known transiting planets it harbours. The peculiar orbital configuration and the diversity in average density among the planets in the system will enable the study of interior planetary structures and atmospheric evolution, providing important clues on the formation of super-Earths and mini-Neptunes.

Volatile Sample Return in the Solar System

We advocate for the realization of volatile sample return from various destinations including: small bodies, the Moon, Mars, ocean worlds/satellites, and plumes. As part of recent mission studies (e.g., Comet Astrobiology Exploration SAmple Return (CAESAR) and Mars Sample Return), new concepts, technologies, and protocols have been considered for specific environments and cost. Here we provide a plan for volatile sample collection and identify the associated challenges with the environment, transit/storage, Earth re-entry, and curation. Laboratory and theoretical simulations are proposed to verify sample integrity during each mission phase. Sample collection mechanisms are evaluated for a given environment with consideration for alteration. Transport and curation are essential for sample return to maximize the science investment and ensure pristine samples for analysis upon return and after years of preservation. All aspects of a volatile sample return mission are driven by the science motivation: isotope fractionation, noble gases, organics and prebiotic species; plus planetary protection considerations for collection and for the sample. The science value of sample return missions has been clearly demonstrated by previous sample return programs and missions. Sample return of volatile material is key to understanding (exo)planet formation, evolution, and habitability. Returning planetary volatiles poses unique and potentially severe technical challenges. These include preventing changes to samples between (and including) collection and analyses, and meeting planetary protection requirements.

Modeling the nonaxisymmetric structure in the HD 163296 disk with planet-disk interaction

Context. High-resolution ALMA observations such as the DSHARP campaign have revealed a variety of rich substructures in numerous protoplanetary disks. These structures consist of rings, gaps, and asymmetric features. It has been debated whether planets can be accounted for among these substructures in the dust continuum. Characterizing the origin of asymmetries, as seen in HD 163296, might lead to a better understanding of planet formation and the underlying physical parameters of the system. Aims. We test the possibility that the formation of the crescent-shaped asymmetry in the HD 163296 disk can be attributed to planet-disk interaction. The goal is to obtain constraints on planet masses, eccentricities, and disk viscosities. Furthermore, we test the reproducibility of the two prominent rings in the HD 163296 disk at 67 and 100 au. Methods. We performed two-dimensional, multi-fluid, hydrodynamical simulations with the FARGO3D code, including three embedded planets in the setup. Dust is described via the pressureless fluid approach and distributed over eight size bins. The resulting grids were post-processed with the radiative transfer code RADMC-3D and CASA software to model the synthetic observations. Results. We find that the crescent-shaped asymmetry can be qualitatively modeled with a Jupiter mass planet at a radial distance of 48 au. Dust is trapped in the trailing Lagrange point L5, preferably, with a mass of between 10 and 15 earth masses. The observation of such a feature constrains the level of viscosity and planetary mass. Increased values of eccentricity of the innermost Jupiter mass planet negatively impacts the stability of the crescent-shaped feature and does not reproduce the observed radial proximity to the first prominent ring in the system. Generally, a low level of viscosity (α ≤ 2 × 10−3) is necessary to allow for the existence of such a feature. Including dust feedback in the leading point, L4, can dominantly capture dust for dust grains with an initial Stokes number ≤ 3.6 × 10−2. In the synthetic ALMA observation of the model with dust feedback, two crescent-shaped features are visible. The observational results suggest a negligible effect on the part of dust feedback since only one such feature has been detected so far. The dust-to-gas ratio may thus be overestimated in the models. Additionally, the planet mass growth time scale does not strongly affect the formation of such asymmetries in the co-orbital region.

A photospheric and chromospheric activity analysis of the quiescent retrograde-planet host ν Octantis A

ABSTRACT The single-lined spectroscopic binary ν Octantis provided evidence of the first conjectured circumstellar planet demanding an orbit retrograde to the stellar orbits. The planet-like behaviour is now based on 1437 radial velocities (RVs) acquired from 2001 to 2013. ν Oct’s semimajor axis is only 2.6 au with the candidate planet orbiting $\nu ~{\rm Oct\, A}$ about mid-way between. These details seriously challenge our understanding of planet formation and our decisive modelling of orbit reconfiguration and stability scenarios. However, all non-planetary explanations are also inconsistent with numerous qualitative and quantitative tests including previous spectroscopic studies of bisectors and line-depth ratios, photometry from Hipparcos and the more recent space missions TESS and Gaia (whose increased parallax classifies $\nu ~{\rm Oct\, A}$ closer still to a subgiant, ∼K1 IV). We conducted the first large survey of $\nu ~{\rm Oct\, A}$’s chromosphere: 198 $\rm Ca\,{\small II}$ H-line and 1160 $\rm {H}\, \alpha$ indices using spectra from a previous RV campaign (2009–2013). We also acquired 135 spectra (2018–2020) primarily used for additional line-depth ratios, which are extremely sensitive to the photosphere’s temperature. We found no significant RV-correlated variability. Our line-depth ratios indicate temperature variations of only ±4 K, as achieved previously. Our atypical $\rm Ca\,{\small II}$ analysis models the indices in terms of S/N and includes covariance significantly in their errors. The $\rm {H}\, \alpha$ indices have a quasi-periodic variability that we demonstrate is due to telluric lines. Our new evidence provides further multiple arguments realistically only in favour of the planet.

Ca II H&amp;K stellar activity parameter: a proxy for extreme ultraviolet stellar fluxes

Atmospheric escape is an important factor shaping the exoplanet population and hence drives our understanding of planet formation. Atmospheric escape from giant planets is driven primarily by the stellar X-ray and extreme ultraviolet (EUV) radiation. Furthermore, EUV and longer wavelength UV radiation power disequilibrium chemistry in the middle and upper atmospheres. Our understanding of atmospheric escape and chemistry, therefore, depends on our knowledge of the stellar UV fluxes. While the far-ultraviolet (FUV) fluxes can be observed for some stars, most of the EUV range is unobservable due to the lack of a space telescope with EUV capabilities and, for the more distant stars, due to interstellar medium absorption. Therefore, it becomes essential to have an indirect means for inferring EUV fluxes from features observable at other wavelengths. We present here analytic functions for predicting the EUV emission of F-, G-, K-, and M-type stars from the log R′HK activity parameter that is commonly obtained from ground-based optical observations of the Ca II H&amp;K lines. The scaling relations are based on a collection of about 100 nearby stars with published log R′HK and EUV flux values, the latter of which are either direct measurements or inferences from high-quality FUV spectra. The scaling relations presented here return EUV flux values with an accuracy of about a factor of three, which is slightly lower than that of other similar methods based on FUV or X-ray measurements.

The Separation and Hα Contrasts of Massive Accreting Planets in the Gaps of Transitional Disks: Predicted Hα Protoplanet Yields for Adaptive Optics Surveys

Abstract We present a massive accreting gap planet model that ensures large gaps in transitional disks are kept dust free by the scattering action of three coplanar quasi-circular planets in a 1:2:4 mean motion resonance (MMR). This model uses the constraint of the observed gap size, and the dust-free nature of the gap, to determine within ∼10% the possible orbits for three massive planets in an MMR. Calculated orbits are consistent with the observed orbits and Hα emission (the brightest line to observe these planets) for LkCa 15 b, PDS 70 b, and PDS 70 c within observational errors. Moreover, the model suggests that the scarcity of detected Hα planets is likely a selection effect of the current limitations of non-coronagraphic, low (&lt;10%) Strehl, Hα imaging with adaptive optics (AO) systems used in past Hα surveys. We predict that as higher Strehl AO systems (with high-performance custom coronagraphs; like the 6.5 m Magellan Telescope MagAO-X system) are utilized at Hα, the number of detected gap planets will substantially increase by more than tenfold. For example, we show that &gt;25 ± 5 new Hα “gap planets” are potentially discoverable by a survey of the best 19 transitional disks with MagAO-X. Detections of these accreting protoplanets will significantly improve our understanding of planet formation, planet growth and accretion, solar system architectures, and planet–disk interactions.

The GRAVITY young stellar object survey

Context. Studies of the dust distribution, composition, and evolution of protoplanetary disks provide clues for understanding planet formation. However, little is known about the innermost regions of disks where telluric planets are expected to form. Aims. We aim constrain the geometry of the inner disk of the T Tauri star RY Lup by combining spectro-photometric data and interferometric observations in the near-infrared (NIR) collected at the Very Large Telescope Interferometer. We use PIONIER data from the ESO archive and GRAVITY data that were obtained in June 2017 with the four 8m telescopes. Methods. We use a parametric disk model and the 3D radiative transfer code MCFOST to reproduce the spectral energy distribution (SED) and match the interferometric observations. MCFOST produces synthetic SEDs and intensity maps at different wavelengths from which we compute the modeled interferometric visibilities and closure phases through Fourier transform. Results. To match the SED from the blue to the millimetric range, our model requires a stellar luminosity of 2.5 L⊙, higher than any previously determined values. Such a high value is needed to accommodate the circumstellar extinction caused by the highly inclined disk, which has been neglected in previous studies. While using an effective temperature of 4800 K determined through high-resolution spectroscopy, we derive a stellar radius of 2.29 R⊙. These revised fundamental parameters, when combined with the mass estimates available (in the range 1.3–1.5 M⊙), lead to an age of 0.5–2.0 Ma for RY Lup, in better agreement with the age of the Lupus association than previous determinations. Our disk model (that has a transition disk geometry) nicely reproduces the interferometric GRAVITY data and is in good agreement with the PIONIER ones. We derive an inner rim location at 0.12 au from the central star. This model corresponds to an inclination of the inner disk of 50°, which is in mild tension with previous determinations of a more inclined outer disk from SPHERE (70° in NIR) and ALMA (67 ± 5°) images, but consistent with the inclination determination from the ALMA CO spectra (55 ± 5°). Increasing the inclination of the inner disk to 70° leads to a higher line-of-sight extinction and therefore requires a higher stellar luminosity of 4.65 L⊙ to match the observed flux levels. This luminosity would translate to a stellar radius of 3.13 R⊙, leading to an age of 2–3 Ma, and a stellarmass of about 2 M⊙, in disagreement with the observed dynamical mass estimate of 1.3–1.5 M⊙. Critically, this high-inclination inner disk model also fails to reproduce the visibilities observed with GRAVITY. Conclusions. The inner dust disk, as traced by the GRAVITY data, is located at a radius in agreement with the dust sublimation radius. An ambiguity remains regarding the respective orientations of the inner and outer disk, coplanar and mildly misaligned, respectively.As our datasets are not contemporary and the star is strongly variable, a deeper investigation will require a dedicated multi-technique observing campaign.

Observations of edge-on protoplanetary disks with ALMA

Aims. We aim to study vertical settling and radial drift of dust in protoplanetary disks from a different perspective: an edge-on view. An estimation of the amplitude of settling and drift is highly relevant to understanding planet formation. Methods. We analyze a sample of 12 HST-selected edge-on protoplanetary disks (i.e., seen with high inclinations) for which the vertical extent of the emission layers can be constrained directly. We present ALMA high angular resolution continuum images (~0.1′′) of these disks at two wavelengths, 0.89 and 2.06 mm (respectively ALMA bands 7 and 4), supplemented with archival band 6 data (1.33 mm) where available. Results. Several sources show constant brightness profiles along their major axis with a steep drop at the outer edges. Two disks have central holes with additional compact continuum emission at the location of the central star. For most sources, the millimeter continuum emission is more compact than the scattered light, both in the vertical and radial directions. Six sources are resolved along their minor axis in at least one millimetric band, providing direct information on the vertical distribution of the millimeter grains. For the second largest disk of the sample, Tau 042021, the significant difference in vertical extent between band 7 and band 4 suggests efficient size-selective vertical settling of large grains. Furthermore, the only Class I object in our sample shows evidence of flaring in the millimeter. Along the major axis, all disks are well resolved. Four of them are larger in band 7 than in band 4 in the radial direction, and three have a similar radial extent in all bands. These three disks are also the ones with the sharpest apparent edges (between 80% and 20% of the peak flux, Δr∕r ~ 0.3), and two of them are binaries. For all disks, we also derive the millimeter brightness temperature and spectral index maps. We find that all edge-on disks in our sample are likely optically thick and that the dust emission reveals low brightness temperatures in most cases (brightness temperatures ≤10 K). The integrated spectral indices are similar to those of disks at lower inclination. Conclusions. The comparison of a generic radiative transfer disk model with our data shows that at least three disks are consistent with a small millimeter dust scale height, of a few au (measured at r = 100 au). This is in contrast with the more classical value of hg ~ 10 au derived from scattered light images and from gas line measurements. These results confirm, by direct observations, that large (millimeter) grains are subject to significant vertical settling in protoplanetary disks.