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Earth and Planetary Astrophysics (astro-ph.EP)

Mon, 01 May 2023

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1.Small Planets Around Cool Dwarfs: Enhanced Formation Efficiency of Super-Earths around M dwarfs

Authors:Yayaati Chachan, Eve J. Lee

Abstract: Current measurements of planet population as a function of stellar mass show three seemingly contradictory signatures: close-in super-Earths are more prevalent around M dwarfs than FGK dwarfs; inner super-Earths are correlated with outer giants; and outer giants are less common around M dwarfs than FGK dwarfs. Here, we build a simple framework that combines the theory of pebble accretion with the measurements of dust masses in protoplanetary disks to reconcile all three observations. First, we show that cooler stars are more efficient at converting pebbles into planetary cores at short orbital periods. Second, when disks are massive enough to nucleate a heavy core at 5 AU, more than enough dust can drift in to assemble inner planets, establishing the correlation between inner planets and outer giants. Finally, while stars of varying masses are similarly capable of converting pebbles into cores at long orbital periods, hotter stars are much more likely to harbor more massive dust disks so that the giant planet occurrence rate rises around hotter stars. Our results are valid over a wide range of parameter space for a disk accretion rate that follows $\dot{M}_\star \sim 10^{-8}\,M_\odot\,{\rm yr}^{-1}(M_\star/M_\odot)^2$. We predict a decline in mini-Neptune population (but not necessarily terrestrial planets) around stars lighter than $\sim 0.3-0.5 \, M_\odot$. Cold giants ($\gtrsim$5 AU), if they exist, should remain correlated with inner planets even around lower mass stars.

2.Star-Planet Interaction at radio wavelengths in YZ Ceti: Inferring planetary magnetic field

Authors:Corrado Trigilio, Ayan Biswas, Paolo Leto, Grazia Umana, Innocenza Busa, Francesco Cavallaro, Barnali Das, Poonam Chandra, Miguel Perez-Torres, Gregg A. Wade, Cristobal Bordiu, Carla S. Buemi, Filomena Bufano, Adriano Ingallinera, Sara Loru, Simone Riggi

Abstract: In exoplanetary systems, the interaction between the central star and the planet can trigger Auroral Radio Emission (ARE), due to the Electron Cyclotron Maser mechanism. The high brightness temperature of this emission makes it visible at large distances, opening new opportunities to study exoplanets and to search for favourable conditions for the development of extra-terrestrial life, as magnetic fields act as a shield that protects life against external particles and influences the evolution of the planetary atmospheres. In the last few years, we started an observational campaign to observe a sample of nearby M-type stars known to host exoplanets with the aim to detect ARE. We observed YZ Ceti with the upgraded Giant Metrewave Radio Telescope (uGMRT) in band 4 (550-900 MHz) nine times over a period of five months. We detected radio emission four times, two of which with high degree of circular polarization. With statistical considerations we exclude the possibility of flares due to stellar magnetic activity. Instead, when folding the detections to the orbital phase of the closest planet YZ Cet b, they are at positions where we would expect ARE due to star-planet interaction (SPI) in sub-Alfvenic regime. With a degree of confidence higher than 4.37 sigma, YZ Cet is the first extrasolar systems with confirmed SPI at radio wavelengths. Modelling the ARE, we estimate a magnetic field for the star of about 2.4 kG and we find that the planet must have a magnetosphere. The lower limit for the polar magnetic field of the planet is 0.4 G.

3.DREAM II. The spin-orbit angle distribution of close-in exoplanets under the lens of tides

Authors:O. Attia, V. Bourrier, J. -B. Delisle, P. Eggenberger

Abstract: The spin-orbit angle, or obliquity, is a powerful observational marker that allows us to access the dynamical history of exoplanetary systems. Here, we have examined the distribution of spin-orbit angles for close-in exoplanets and put it in a statistical context of tidal interactions between planets and their stars. We confirm the observed trends between the obliquity and physical quantities directly connected to tides, namely the stellar effective temperature, the planet-to-star mass ratio, and the scaled orbital distance. We further devised a tidal efficiency factor combining critical parameters that control the strength of tidal effects and used it to corroborate the strong link between the spin-orbit angle distribution and tidal interactions. In particular, we developed a readily usable formula to estimate the probability that a system is misaligned, which will prove useful in global population studies. By building a robust statistical framework, we reconstructed the distribution of the three-dimensional spin-orbit angles, allowing for a sample of nearly 200 true obliquities to be analyzed for the first time. This realistic distribution maintains the sky-projected trends, and additionally hints toward a striking pileup of truly aligned systems. The comparison between the full population and a pristine subsample unaffected by tidal interactions suggests that perpendicular architectures are resilient toward tidal realignment, providing evidence that orbital misalignments are sculpted by disruptive dynamical processes that preferentially lead to polar orbits. On the other hand, star-planet interactions seem to efficiently realign or quench the formation of any tilted configuration other than for polar orbits, and in particular for antialigned orbits.

4.SHAMPOO: A stochastic model for tracking dust particles under the influence of non-local disk processes

Authors:M. Oosterloo, I. Kamp, W. van Westrenen, C. Dominik

Abstract: The abundances of CHNOS are crucial for the composition of planets. At the onset of planet formation, large amounts of these elements are stored in ices on dust grains in planet-forming disks. The evolution of this ice is affected by dynamical transport, collisional processes, and the formation and sublimation of ice. We aim to constrain the disk regions where these processes are fully coupled, and develop a flexible modelling approach that is able to predict the effects of these processes acting simultaneously on the CHNOS budgets of the dust in these regions. We compared timescales associated with these disk processes to constrain the disk regions where this approach is necessary, and developed the SHAMPOO code, which tracks the CHNOS abundances in the ice mantle of a single monomer dust particle, embedded in a larger aggregate and undergoing these processes simultaneously. The adsorption and photodesorption of monomer ices depend on the depth of the monomer in the aggregate. We investigated the effect of fragmentation velocity and aggregate filling factor on the amount of ice on monomers residing at r = 10 AU. The locations where disk processes are fully coupled depend on both grain size and ice species. Monomers embedded in aggregates with fragmentation velocities of 1 m/s are able to undergo adsorption and photodesorption more often compared to a fragmentation velocity of 5 m/s or 10 m/s. Aggregates with a filling factor of $10^{-3}$ are able to accumulate ice 22 times faster on average than aggregates with a filling factor of 1. As different grain sizes are coupled through collisions and the grain ice consists of multiple ice species, it is difficult to isolate the locations where disk processes are fully coupled, necessitating the development of the SHAMPOO code. The processing of ice may not be spatially limited to dust aggregate surfaces for either fragile or porous aggregates.

5.High Tide or Riptide on the Cosmic Shoreline? A Water-Rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ~486b from JWST Observations

Authors:Sarah E. Moran, Kevin B. Stevenson, David K. Sing, Ryan J. MacDonald, James Kirk, Jacob Lustig-Yaeger, Sarah Peacock, L. C. Mayorga, Katherine A. Bennett, Mercedes López-Morales, E. M. May, Zafar Rustamkulov, Jeff A. Valenti, Jéa I. Adams Redai, Munazza K. Alam, Natasha E. Batalha, Guangwei Fu, Junellie Gonzalez-Quiles, Alicia N. Highland, Ethan Kruse, Joshua D. Lothringer, Kevin N. Ortiz Ceballos, Kristin S. Sotzen, Hannah R. Wakeford

Abstract: Planets orbiting M-dwarf stars are prime targets in the search for rocky exoplanet atmospheres. The small size of M dwarfs renders their planets exceptional targets for transmission spectroscopy, facilitating atmospheric characterization. However, it remains unknown whether their host stars' highly variable extreme-UV radiation environments allow atmospheres to persist. With JWST, we have begun to determine whether or not the most favorable rocky worlds orbiting M dwarfs have detectable atmospheres. Here, we present a 2.8-5.2 micron JWST NIRSpec/G395H transmission spectrum of the warm (700 K, 40.3x Earth's insolation) super-Earth GJ 486b (1.3 R$_{\oplus}$ and 3.0 M$_{\oplus}$). The measured spectrum from our two transits of GJ 486b deviates from a flat line at 2.2 - 3.3 $\sigma$, based on three independent reductions. Through a combination of forward and retrieval models, we determine that GJ 486b either has a water-rich atmosphere (with the most stringent constraint on the retrieved water abundance of H2O > 10% to 2$\sigma$) or the transmission spectrum is contaminated by water present in cool unocculted starspots. We also find that the measured stellar spectrum is best fit by a stellar model with cool starspots and hot faculae. While both retrieval scenarios provide equal quality fits ($\chi^2_\nu$ = 1.0) to our NIRSpec/G395H observations, shorter wavelength observations can break this degeneracy and reveal if GJ 486b sustains a water-rich atmosphere.

6.Formation of Gaps in Self-gravitating Debris Disks by Secular Resonance in a Single-planet System. II. Towards a Self-consistent Model

Authors:Antranik A. Sefilian, Roman R. Rafikov, Mark C. Wyatt

Abstract: High-resolution observations of several debris disks reveal structures such as gaps and spirals, suggestive of gravitational perturbations induced by underlying planets. Most existing studies of planet--debris disk interactions ignore the gravity of the disk, treating it as a reservoir of massless planetesimals. In this paper, we continue our investigation into the long-term interaction between a single eccentric planet and an external, massive debris disk. Building upon our previous work, here we consider not only the axisymmetric component of the disk's gravitational potential, but also the non-axisymmetric torque that the disk exerts on the planet (ignoring for now only the non-axisymmetric component of the disk \textit{self}-gravity). To this goal, we develop and test a semi-analytic `$N$-ring' framework that is based on a generalized (softened) version of the classical Laplace--Lagrange secular theory. Using this tool, we demonstrate that even when the disk is less massive than the planet, not only can a secular resonance be established within the disk that leads to the formation of a wide non-axisymmetric gap (akin to those observed in HD 107146, HD 92945, and HD 206893), but that the very same resonance also damps the planetary eccentricity via a process known as resonant friction. We also develop analytic understanding of these findings, finding good quantitative agreement with the outcomes of the $N$-ring calculations. Our results may be used to infer both the dynamical masses of gapped debris disks and the dynamical history of the planets interior to them, as we exemplify for HD 206893.