Stellar Radiation Research Articles

Examining astrophysical gas cloud collapse using an optical depth-scaled, x-ray-irradiated, carbon-foam sphere

When stellar radiation interacts with a molecular cloud, the cloud's fate depends on the strength of the incident radiation and the radiation's mean-free-path within the cloud [F. Bertoldi, Astrophys. J. 346, 735–755 (1989)]. Under the right conditions, the radiation compresses the cloud and a star formation may occur. Where and when the stellar formation occurs in the cloud's collapse are open questions. Direct observation of the complete star–cloud lifecycle is nearly impossible due to the immense timescales and distances over which the interaction occurs. Laboratory astrophysics offers a way to investigate such a system by scaling the important astrophysical parameters to the laboratory. This work describes laboratory experiments to study the radiation-driven implosion of clouds, using x rays from a laser-irradiated, thin, gold foil as a surrogate star and a carbon-foam sphere as a surrogate cloud. An optically thick system, theoretically corresponding to a star-forming regime, was selected by choice of the foam density. Gold foil and sphere motions were imaged by x-ray radiography. Radiographic images show the formation of an interface between rarefied gold and carbon plasmas, a shock moving into the sphere, and a blunting of the initial sphere's shape. Measurements show that the shock moved linearly around 64 μm/ns into the sphere, and the gold–carbon interface formed by 2 ns at the sphere edge remained stationary. The deformation of the sphere was driven by the incident radiation and not by mechanical pressures applied by gold plasma. The blunting of the sphere was likely due to the geometric reduction of flux near the sphere's poles. Higher x-ray flux near the sphere's equator caused high compression and a faster shock, which flattened the sphere. We will discuss the results and implications of our observations.

Numerical modeling of thermal dust polarization from aligned grains in the envelope of evolved stars with updated POLARIS

Abstract Magnetic fields are thought to influence the formation and evolution of circumstellar envelopes around evolved stars. Thermal dust polarization from aligned grains is a promising tool for probing magnetic fields and dust properties in these environments; however, a quantitative study on the dependence of thermal dust polarization on the physical properties of dust and magnetic fields for these circumstellar environments is still lacking. In this paper, we first perform the numerical modeling of thermal dust polarization in the IK Tau envelope using the magnetically enhanced radiative torque (MRAT) alignment mechanism implemented in our updated POLARIS code, accounting for the effect of grain drift relative to the gas. Despite experiencing grain drift and high gas density $n_{\rm gas} > 10^6\, \rm cm^{-3}$, the minimum grain size required for efficient MRAT alignment of silicate grains is $\sim 0.007 - 0.05\, \rm \mu m$ due to strong stellar radiation fields. Ordinary paramagnetic grains can achieve perfect alignment by MRAT in the inner envelope of $r < 500\, \rm au$ due to stronger magnetic fields of B ∼ 10 mG - 1G, producing the polarization degree of ∼10 %. The polarization degree can be enhanced to ∼20-40 % for superparamagnetic grains with embedded iron inclusions. The magnetic field geometry affects the resulting polarization degree due to the projection effect. We investigate the effect of rotational disruption by RATs (RAT-D) and find that the RAT-D effect decreases the dust polarization degree due to the decrease in the maximum grain size. Our modeling results motivate further observational studies at far-infrared/sub-millimeter to constrain the properties of magnetic fields and dust in evolved star’s envelopes.

Variability of the inner dead zone edge in 2D radiation hydrodynamic simulations

Context. The inner regions of protoplanetary discs are prone to thermal instability (TI), which can significantly impact the thermal and dynamical evolution of planet-forming regions. Observable as episodic accretion outbursts, such periodic disturbances shape the disc’s vertical and radial structure. Aims. We have investigated the stability of the inner disc edge around a Class II T Tauri star and analysed the consequences of TI on the thermal and dynamic evolution in both the vertical and radial dimensions. A particular focus is laid on the emergence and destruction of solid-trapping pressure maxima. Methods. We conducted 2D axisymmetric radiation hydrodynamic simulations of the inner disc in a radial range of 0.05 AU to 10 AU. The models include a highly turbulent inner region, the transition to the dead zone, heating by both stellar irradiation and viscous dissipation, vertical and radial radiative transport, and tracking of the dust-to-gas mass ratio at every location. The simulated time frames include both the TI phase and the quiescent phase between TI cycles. We tracked the TI on S-curves of thermal stability. Results. Thermal instability can develop in discs with accretion rates of ≥3.6 ⋅ 10−9 M⊙ yr−1 and results from the activation of mag-netorotational instability (MRI) in the dead zone after the accumulation of material beyond the MRI transition. The TI creates an extensive MRI active region around the midplane and disrupts the stable pebble and migration trap at the inner edge of the dead zone. Our simulations consistently show the occurrence of TI reflares that, together with the initial TI, produce pressure maxima in the inner disc within 1 AU, possibly providing favourable conditions for streaming instability. During the TI phase, the dust content in the ignited regions adapts itself in order to create a new thermal equilibrium manifested in the upper branch of the S-curve. In these instances, we find a simple relation between the gas and dust-surface densities. Conclusions. On a timescale of a few thousand years, TI regularly disrupts the radial and vertical structure of the disc within 1 AU. While several pressure maxima are created, stable migration traps are destroyed and reinstated after the TI phase. Our models provide a foundation for more detailed investigations into phenomena such as the short-term variability of accretion rates.

Revisiting the conundrum of the sub-Jovian and Neptune desert

Context. The search for exoplanets has led to the identification of intriguing patterns in their distributions, one of which is the so-called sub-Jovian and Neptune desert. The occurrence rate of Neptunian exoplanets with an orbital period P ≲ 4 days sharply decreases in this region in period-radius and period-mass space. Aims. We present a novel approach to delineating the sub-Jovian and Neptune desert by considering the incident stellar flux F on the planetary surface as a key parameter instead of the traditional orbital period of the planets. Through this change of perspective, we demonstrate that the incident flux still exhibits a paucity of highly irradiated Neptunes, but also captures the proximity to the host star and the intensity of stellar radiation. Methods. Leveraging a dataset of confirmed exoplanets, we performed a systematic analysis to map the boundaries of the sub-Jovian and Neptune desert in the (F, Rp) and (F, Mp) diagrams, with Rp and Mp corresponding to the planetary radius and mass, respectively. By using statistical techniques and fitting procedures, we derived analytical expressions for these boundaries that offer valuable insights into the underlying physical mechanisms governing the dearth of Neptunian planets in close proximity to their host stars. Results. We find that the upper and lower bounds of the desert are well described by a power-law model in the (F, Rp) and (F, Mp) planes. We also obtain the planetary mass-radius relations for each boundary by combining the retrieved analytic expressions in the two planes. This work contributes to advancing our knowledge of exoplanet demographics and to refining theoretical models of planetary formation and evolution within the context of the sub-Jovian and Neptune desert.

Asymmetries in the Simulated Ozone Distribution on TRAPPIST-1e due to Orography

TRAPPIST-1e is a tidally locked rocky exoplanet orbiting the habitable zone of an M dwarf star. Upcoming observations are expected to reveal new rocky exoplanets and their atmospheres around M dwarf stars. To interpret these future observations we need to model the atmospheres of such exoplanets. We configured Community Earth System Model version 2–Whole Atmosphere Community Climate Model version 6, a chemistry climate model, for the orbit and stellar irradiance of TRAPPIST-1e assuming an initial Earth-like atmospheric composition. Our aim is to characterize the possible ozone (O3) distribution and explore how this is influenced by the atmospheric circulation shaped by orography, using the Helmholtz wind decomposition and meridional mass streamfunction. The model included Earth-like orography, and the substellar point was located over the Pacific Ocean. For such a scenario, our analysis reveals a north–south asymmetry in the simulated O3 distribution. The O3 concentration is highest at pressures >10 hPa (below ∼30 km) near the south pole. This asymmetry arises from the higher landmass fraction in the northern hemisphere, which causes drag in near-surface flows and leads to an asymmetric meridional overturning circulation. Catalytic species were roughly symmetrically distributed and were not found to be primary driver for the O3 asymmetry. The total O3 column density was higher for TRAPPIST-1e compared to Earth, with 8000 Dobson units (DUs) near the south pole and 2000 DU near the north pole. The results emphasize the sensitivity of O3 to model parameters, illustrating how incorporating Earth-like orography can affect atmospheric dynamics and O3 distribution. This link between surface features and atmospheric dynamics underlines the importance of how changing model parameters used to study exoplanet atmospheres can influence the interpretation of observations.

Solar Wind Power Likely Governs Uranus' Thermosphere Temperature

AbstractObservations of Uranus in the near‐infrared by ground‐based telescopes from 1992 to 2018 have shown that the planet's upper atmosphere (thermosphere) steadily cooled from ∼700 to ∼450 K. We explain this cooling as due to the concurrent decline in the power of the solar wind incident on Uranus' magnetic field, which has dropped by ∼50% over the same period due to solar activity trends longer than the 11‐year solar cycle. Uranus' thermosphere appears to be more strongly governed by the solar wind than any other planet where we have assessed this coupling so far. Uranus' total auroral power may also have declined, in contrast with the power of the radio aurora that we expect has been predominantly modulated by the solar cycle. In the absence of strong local driving, planets with sufficiently large magnetospheres may also have thermospheres predominantly governed by the stellar wind, rather than stellar radiation.

The origin and evolution of the CII deficit in HII regions and star-forming molecular clouds

We analyse synthetic emission maps of the CII 158 mu m line and far-infrared (FIR) continuum of simulated molecular clouds (MCs) within the SILCC-Zoom project to study the origin of the observed CII deficit, that is, the drop in the CII /FIR intensity ratio caused by stellar activity. All simulations include stellar radiative feedback and the on-the-fly chemical evolution of hydrogen species, CO, and C$^+$. We also account for further ionisation of C$^+$ into C$^ $ inside HII regions, which is crucial to obtain reliable results. Studying individual HII regions, we show that $I_ FIR $ is initially high in the vicinity of newly born stars, and then moderately decreases over time as the gas is compressed into dense and cool shells. In contrast, there is a large drop in $I_ CII $ over time, to which the second ionisation of C$^+$ into C$^ $ contributes significantly. This leads to a large drop in deficit inside HII regions, with deficit decreasing from 10$^ $ at scales above 10 pc to around 10$^ $ at scales below 2 pc. However, projection effects can significantly affect the radial profile of $I_ CII $, $I_ FIR $, and their ratio, and can create apparent HII regions without any stars. Considering the evolution on MC scales, we show that the luminosity ratio, deficitl , decreases from values of gtrsim 10$^ $ in MCs without star formation to values of around $ $ in MCs with star formation. We attribute this decrease and thus the origin of the CII deficit to two main contributors: (i) the saturation of the CII line and (ii) the conversion of C$^+$ into C$^ $ by stellar radiation. The drop in the deficitl ratio can be divided into two phases: (i) During the early evolution of HII regions, the saturation of CII and the further ionisation of C$^+$ limit the increase in $L_ CII $, while $L_ FIR $ increases rapidly, leading to the initial decline of deficitl . (ii) In more evolved HII regions, $L_ CII $ stagnates and even partially drops over time due to the aforementioned reasons. $L_ FIR $ also stagnates as the gas gets pushed into the cooler shells surrounding the HII region. In combination, this keeps the global deficitl ratio at low values of $

The Molecular Cloud Life Cycle. II. Formation and Destruction of Molecular Clouds Diagnosed via H2 Fluorescent Emission

Molecular hydrogen (H2) formation and dissociation are key processes that drive the gas life cycle in galaxies. Using the SImulating the LifeCycle of Molecular Clouds zoom-in simulation suite, we explore the utility of future observations of H2 dissociation and formation for tracking the life cycle of molecular clouds. The simulations used in this work include nonequilibrium H2 formation, stellar radiation, sink particles, and turbulence. We find that at early times in the cloud evolution H2 formation rapidly outpaces dissociation and molecular clouds build their mass from the atomic reservoir in their environment. Rapid H2 formation is also associated with a higher early star formation rate. For the clouds studied here, H2 is strongly out of chemical equilibrium during the early stages of cloud formation but settles into a bursty chemical steady state about 2 Myr after the first stars form. At the latest stage of cloud evolution, dissociation outweighs formation and the clouds enter a dispersal phase. We discuss how theories of the molecular cloud life cycle and star formation efficiency may be distinguished with observational measurements of H2 fluorescence with a space-based high-resolution far-UV spectrometer, such as the proposed Hyperion and Eos NASA Explorer missions. Such missions would enable measurements of the H2 dissociation and formation rates, which we demonstrate can be connected to different phases in a molecular cloud’s star-forming life, including cloud building, rapidly star forming, H2 chemical equilibrium, and cloud destruction.

Spectral reconstruction for radiation hydrodynamic simulations of galaxy evolution

Radiation from stars and active galactic nuclei (AGN) plays an important role in galaxy formation and evolution, and profoundly transforms the intergalactic, circumgalactic, and interstellar medium (IGM, CGM, and ISM). On-the-fly radiative transfer (RT) has started being incorporated in cosmological simulations, but the complex evolving radiation spectra are often crudely approximated with a small number of broad bands with piece-wise constant intensity and a fixed photo-ionisation cross-section. Such a treatment is unable to capture the changes to the spectrum as light is absorbed while it propagates through a medium with non-zero opacity. This can lead to large errors in photo-ionisation and heating rates. In this work we present a novel approach of discretising the radiation field at discrete photon energies, at the edges of the typically used photo-ionising bands, in order to capture the power-law slope of the radiation field. In combination with power-law approximations for the photo-ionisation cross-sections, this model allows us to self-consistently combine radiation from sources with different spectra and accurately follow the ionisation states of primordial and metal species through time. The method is implemented in GASOLINE2 in connection with TREVR2, a fast reverse ray tracing algorithm with 𝒪(Nactive log2 N) scaling. We compare our new piece-wise power-law reconstruction to the piece-wise constant method in calculating the primordial chemistry photo-ionisation and heating rates under an evolving UV background (UVB) and stellar spectrum, and find that our method reduces errors significantly, by up to two orders of magnitude in the case of HeII ionisation. We apply our new spectral reconstruction method in RT post-processing of a cosmological zoom-in simulation, MUGS2 g1536, including radiation from stars and a live UVB, and find a significant increase in total neutral hydrogen (HI) mass in the ISM and the CGM due to shielding of the UVB and a low escape fraction of the stellar radiation. This demonstrates the importance of RT and an accurate spectral approximation in simulating the CGM-galaxy ecosystem.

Out of the Darkness: High-resolution Detection of CO Absorption on the Nightside of WASP-33b

We observed the ultrahot Jupiter WASP-33b with the SpectroPolarimètre InfraRouge on the Canada–France–Hawaii Telescope. Previous observations of the dayside of WASP-33b show evidence of CO and Fe emission indicative of a thermal inversion. We observed its nightside over five Earth nights to search for spectral signatures of CO in the planet’s thermal emission. Our three pretransit observations and two posttransit observations are sensitive to regions near the morning or evening terminators, respectively. From spectral retrievals, we detect CO molecular absorption in the planet’s emission spectrum after transit at ∼6.6σ. This is the strongest ground-based detection of nightside thermal emission from an exoplanet and only the third ever. CO appearing in absorption suggests that the nightside near the evening terminator does not have a temperature inversion; this makes sense if the dayside inversion is driven by absorption of stellar radiation. On the contrary, we do not detect CO from the morning terminator. This may be consistent with heat advection by an eastward jet. Phase-resolved high-resolution spectroscopy offers an economical alternative to space-based full-orbit spectroscopic phase curves for studying the vertical and horizontal atmospheric temperature profiles of short-period exoplanets.

Shadows Wreak Havoc in Transition Disks

We demonstrate that shadows cast on a protoplanetary disk can drive it eccentric. Stellar irradiation dominates heating across much of these disks, so an uneven illumination can have interesting dynamical effects. Here, we focus on transition disks. We carry out 3D Athena++ simulations, using a constant thermal relaxation time to describe the disk’s response to changing stellar illumination. We find that an asymmetric shadow, a feature commonly observed in real disks, perturbs the radial pressure gradient and distorts the fluid streamlines into a set of twisted ellipses. Interactions between these streamlines have a range of consequences. For a narrow ring, an asymmetric shadow can sharply truncate its inner edge, possibly explaining the steep density drop-offs observed in some disks and obviating the need for massive perturbers. For a wide ring, such a shadow can dismantle it into two (or possibly more) eccentric rings. These rings continuously exert torque on each other and drive gas accretion at a healthy rate, even in the absence of disk viscosity. Signatures of such twisted eccentric rings may have already been observed as, e.g., twisted velocity maps inside gas cavities. We advocate for more targeted observations and for a better understanding on the origin of such shadows.

The missing rings around Solar System moons

Context. Rings are complex structures that surround various bodies within the Solar System, such as giant planets and certain minor bodies. While some formation mechanisms could also potentially promote their existence around (regular or irregular) satellites, none of these bodies currently bear these structures. Aims. We aim to understand the underlying mechanisms that govern the potential formation, stability, and/or decay of hypothetical circumsatellital rings (CSRs) orbiting the largest moons in the Solar System. This extends to the exploration of short-term morphological features within these rings, providing insights into the ring survival timescales and the interactions that drive their evolution. Methods. To conduct this study, we used numerical N-body simulations under the perturbing influence of the host planet and other moon companions. Results. We found that, as suspected, moons with a lower Roche-to-Hill radius can preserve their rings over extended periods. Moreover, the gravitational environment in which these rings are immersed influences the morphological evolution of the system (e.g. ring size), inducing gaps through the excitation of eccentricity and inclination of constituent particles. Specifically, our results show that the rings of Iapetus and Rhea experience minimal variations in their orbital parameters, enhancing their long-term stability. This agrees with the hypothesis that some of the features of Iapetus and Rhea were produced by ancient ring systems, for example, the huge ridge in the Iapetus equator as a result of a decaying ring. Conclusions. From a dynamical perspective, we found that there are no mechanisms that preclude the existence of CSRs, and we attribute their current absence to non-gravitational phenomena. Effects such as stellar radiation, magnetic fields, and the influence of magnetospheric plasma can significantly impact the dynamics of constituent particles and trigger their decay. This highlights the importance of future studies of these effects.

Dynamical Consequence of Shadows Cast to the Outer Protoplanetary Disks. I. Two-dimensional Simulations

There has been increasing evidence of shadows from scattered light observations of outer protoplanetary disks (PPDs) cast from the (unresolved) disk inner region, while in the meantime these disks present substructures of various kinds in the submillimeter. As stellar irradiation is the primary heating source for the outer PPDs, the presence of such shadows thus suggests inhomogeneous heating of the outer disk in azimuth, leading to a “thermal forcing” with dynamical consequences. We conduct a suite of idealized two-dimensional disk simulations of the outer disk with azimuthally varying cooling prescription to mimic the effect of shadows, generally assuming the shadow is static or slowly rotating. The linear response to such shadows is two-armed spirals with the same pattern speed as the shadow. Toward the nonlinear regime, we find that shadows can potentially lead to the formation of a variety of types of substructures including rings, spirals, and crescents, depending on viscosity, cooling time, etc. We have conducted systematic and statistical characterization of the simulation suite, and as thermal forcing from the shadow strengthens, the dominant form of shadow-induced disk substructures change from spirals to rings, and eventually to crescents/vortices. Our results highlight the importance of properly modeling the dynamical impact of inhomogeneous stellar irradiation, while calling for more detailed modeling incorporating more realistic disk physics.

The CHEOPS view of the climate of WASP-3 b

Hot Jupiters are giant planets subject to intense stellar radiation. The physical and chemical properties of their atmosphere make them the most amenable targets for atmospheric characterization. In this paper we analyze the photometry collected during the secondary eclipses of the hot Jupiter by and Our aim is to characterize the atmosphere of the planet by measuring the secondary eclipse depth in several passbands and constrain the planetary dayside spectrum. We updated the radius and the ephemeris of by analyzing the transit photometry collected by and We also analyzed the CHEOPS, TESS, and photometry of the occultations of the planet, measuring the eclipse depth at different wavelengths. Our update of the stellar and planetary properties is consistent with previous works. The analysis of the occultations returns an eclipse depth of 92pm 21 ppm in the CHEOPS passband, 83pm 27 ppm for TESS, and $>$2000 ppm in the IRAC 1-2-4 passbands. Using the eclipse depths in the bands, we propose a set of likely emission spectra that constrain the emission contribution in the CHEOPS and TESS passbands to approximately a few dozen parts per million. This allowed us to measure a geometric albedo of 0.21pm 0.07 in the CHEOPS passband, while the TESS data lead to a 95<!PCT!> upper limit of sim 0.2. belongs to the group of ultra-hot Jupiters that are characterized by a low Bond albedo (<0.3pm 0.1), as predicted by different atmospheric models. On the other hand, it seems to efficiently recirculate the absorbed stellar energy, which is not typical for similar, highly irradiated planets. To explain this inconsistency, we propose that other energy recirculation mechanisms are at play besides advection (for example, the dissociation and recombination of H$_2$). Another possibility is that the observations in different bandpasses probe different atmospheric layers; this would make the atmospheric analysis difficult without an appropriate modeling of the thermal emission spectrum of which is not feasible with the limited spectroscopic data available to date.

Stellar Wind Contribution to the Origin of Water on the Surface of Oxygen-containing Minerals

The origin of water and volatile compounds on planets including Earth is a hotly debated topic in planetary science. For example, many dynamic models suggest that the majority of Earth’s water and volatile elements were added from an external source. The stellar wind irradiation of rocky oxygen-containing minerals results in a reaction between H+ ions and silicate minerals to produce water and OH, which could explain the presence of water in the regoliths of airless worlds such as the Moon, as well as the water abundances in asteroids. Here, we used the method of high-resolution infrared spectrometry and temperature-programmed desorption (TPD) with mass detection to observe and for the first time quantify water formation on the surfaces of oxygen-bearing minerals. We tested 14 different mineral and natural samples and observed the formation of water on their surfaces upon exposure to H+ or D+ irradiation. The samples, including two meteorite samples (RAS 445 and SAU 567), were shown to have a water adsorption capacity between 0.09 and 0.7 wt%. The adsorbed water (likely dissociatively adsorbed) remains on the surface at pressures as low as 10−9 mbar (in the TPD experiment) and temperatures as high as 600 K, which suggests a possible transfer over long distances and timescales. Our article has a general character and demonstrates that any interaction of oxygen-containing minerals with stellar radiation (H+ ions) leads to the generation of water adsorbed on the surface of the minerals. The case of the origin of water on Earth is taken as a prime example.

X-Ray Emission of Nearby Low-mass and Sunlike Stars with Directly Imageable Habitable Zones

Stellar X-ray and UV radiation can significantly affect the survival, composition, and long-term evolution of the atmospheres of planets in or near their host star’s habitable zone (HZ). Especially interesting are planetary systems in the solar neighborhood that may host temperate and potentially habitable surface conditions, which may be analyzed by future ground- and space-based direct-imaging surveys for signatures of habitability and life. To advance our understanding of the radiation environment in these systems, we leverage ∼3 Ms of XMM-Newton and Chandra observations in order to measure three fundamental stellar properties at X-ray energies for 57 nearby FGKM stellar systems: the shape of the stellar X-ray spectrum, the luminosity, and the timescales over which the stars vary (e.g., due to flares). These systems possess HZs that will be directly imageable to next-generation telescopes such as the Habitable Worlds Observatory and ground-based Extremely Large Telescopes. We identify 29 stellar systems with L X/L bol ratios similar to (or less than) that of the Sun; any potential planets in the HZs of these stars therefore reside in present-day X-ray radiation environments similar to (or less hostile than) modern Earth, though a broader set of these targets could host habitable planets. An additional 19 stellar systems have been observed with the Swift X-ray Telescope; in total, only ∼30% of potential direct imaging target stars has been observed with XMM-Newton, Chandra, or Swift. The data products from this work (X-ray light curves and spectra) are available via a public Zenodo repository (doi:10.5281/zenodo.11490574).

The Feasibility of Asynchronous Rotation via Thermal Tides for Diverse Atmospheric Compositions

The equilibrium rotation rate of a planet is determined by the sum of torques acting on its solid body. For planets with atmospheres, the dominant torques are usually the gravitational tide, which acts to slow the planet’s rotation rate, and the atmospheric thermal tide, which acts to spin up the planet. Previous work demonstrated that rocky planets with thick atmospheres may produce strong enough thermal tides to avoid tidal locking, but a study of how the strength of the thermal tide depends on atmospheric properties has not been done. In this work, we use a combination of simulations from a global climate model and analytic theory to explore how the thermal tide depends on the shortwave and longwave optical depth of the atmosphere, the surface pressure, and the absorbed stellar radiation. We find that for planets in the habitable zones of M stars only high-pressure but low-opacity atmospheres permit asynchronous rotation owing to the weakening of the thermal tide at high longwave and shortwave optical depths. We conclude that asynchronous rotation may be very unlikely around low-mass stars, which may limit the potential habitability of planets around M stars.

Spiral excitation in protoplanetary disks through gap-edge illumination

The advent of high-resolution, near-infrared (NIR) instruments such as VLT/SPHERE and Gemini/GPI has helped uncover a wealth of substructure in planet-forming disks, including large, prominent spiral arms in MWC 75 8, SAO 206462, and V1247 Ori. In the classical theory of disk-planet interaction, these arms are consistent with Lindblad-resonance driving by companions of multiple Jupiter masses. Despite improved detection limits, evidence for massive bodies like this in connection with spiral substructure has been inconclusive. In search of an alternative explanation, we used the PLUTO code to run 3D hydrodynamical simulations with two comparatively low planet masses (Saturn mass and Jupiter mass) and two thermodynamic prescriptions (three-temperature radiation hydrodynamics, and the more traditional β-cooling) in a low-mass disk. In the radiative cases, an m = 2 mode, potentially attributable to the interaction of stellar radiation with gap-edge asymmetries, creates an azimuthal pressure gradient, which in turn gives rise to prominent spiral arms in the upper layers of the disk. Monte Carlo radiative transfer post-processing with RADMC3D revealed that in NIR scattered light, these gap-edge spirals are significantly more prominent than the traditional Lindblad spirals for planets in the mass range we tested. Our results demonstrate that even intermediate-mass protoplanets, which are less detectable, but more ubiquitous than super-Jupiters, are capable of indirectly inducing large-scale spiral disk features, and underscore the importance of including radiation physics in any efforts to reproduce observations.

TOI-3568 b: A super-Neptune in the sub-Jovian desert

The sub-Jovian desert is a region in the mass-period and radius-period parameter space that typically encompasses short-period ranges between super-Earths and hot Jupiters, and exhibits an intrinsic dearth of planets. This scarcity is likely shaped by photoevaporation caused by the stellar irradiation received by giant planets that have migrated inward. We report the detection and characterization of TOI-3568 b, a transiting super-Neptune with a mass of 26.4 ± 1.0 M⊕, a radius of 5.30 ± 0.27 R⊕, a bulk density of 0.98 ± 0.15 g cm−3, and an orbital period of 4.417965 (5) d situated in the vicinity of the sub-Jovian desert. This planet orbiting a K dwarf star with solar metallicity was identified photometrically by the Transiting Exoplanet Survey Satellite (TESS). It was characterized as a planet by our high-precision radial-velocity (RV) monitoring program using MAROON-X at Gemini North, supplemented with additional observations from the SPICE large program with SPIRou at CFHT. We performed a Bayesian MCMC joint analysis of the TESS and ground-based photometry, and MAROON-X and SPIRou RVs, to measure the orbit, radius, and mass of the planet, as well as a detailed analysis of the high-resolution flux and polarimetric spectra to determine the physical parameters and elemental abundances of the host star. Our results reveal TOI-3568 b to be a hot super-Neptune rich in hydrogen and helium, with a core of heavier elements of between 10 and 25 M⊕ in mass. We analyzed the photoevaporation status of TOI-3568 b and find that it experiences one of the highest extreme-ultraviolet (EUV) luminosities among planets with a mass of Mp < 2 MNep, yet it has an evaporation lifetime exceeding 5 Gyr. Positioned in the transition between two significant populations of exoplanets on the mass-period and energy diagrams, this planet presents an opportunity to test theories concerning the origin of the sub-Jovian desert.

Gas dynamics around a Jupiter-mass planet

Context. Giant planets grow and acquire their gas envelope during the disk phase. At the time of the discovery of giant planets in their host disk, it is important to understand the interplay between the host disk and the envelope and circum-planetary disk properties of the planet. Aims. Our aim is to investigate the dynamical and physical structure of the gas in the vicinity of a Jupiter-mass planet and study how protoplanetary disk properties, such as disk mass and viscosity, determine the planetary system as well as the accretion rate inside the planet’s Hill sphere. Methods. We ran global 3D simulations with the grid-based code fargOCA, using a fully radiative equation of state and a dust-to-gas ratio of 0.01. We built a consistent disk structure starting from vertical thermal equilibrium obtained by including stellar irradiation. We then let a gap open with a sequence of phases, whereby we deepened the potential and increased the resolution in the planet’s neighbourhood. We explored three models. The nominal one features a disk with surface density, ∑, corresponding to the minimum mass solar nebula at the planet’s location (5.2 au), characterised by an α viscosity value of 4 10−3 at the planet’s location. The second model has a surface density that is ten times smaller than the nominal one and the same viscosity. In the third model, we also reduced the viscosity value by a factor of 10. Results. During gap formation, giant planets accrete gas inside the Hill sphere from the local reservoir. Gas is heated by compression and cools according to opacity, density, and temperature values. This process determine the thermal energy budget inside the Hill sphere. In the analysis of our disks, we find that the gas flowing into the Hill sphere is approximately scaled as the product ∑ν, as expected from viscous transport. The accretion rate of the planetary system (envelope plus circum-planetary disk) is instead scaled as √Σv, with its efficiency depending on the thermal energy budget inside the Hill sphere. Conclusions. Previous studies have shown that pressure-supported or rotationally supported structures are formed around giant planets, depending on the equation of state (EoS) or on the opacity; namely, on the dust content within the Hill sphere. In the case of a fully radiative EoS and a constant dust to gas ratio of 0.01, we find that low-mass and low-viscosity circum-stellar disks favour the formation of a rotationally supported circum-planetary disk. Gas accretion leading to the doubling time of the planetary system of > 105 years has only been found in the case of a low-viscosity disk.