arXiv subjects

Ondrej Chrenko

Publications and source records attributed to Ondrej Chrenko.

6 recordsLinked to original sources

arXiv2026

Magnetically Driven Obliquity in Circumplanetary Disks and Twisted Bipolar-jet Formation

Circumplanetary disks (CPDs) regulate gas accretion onto forming giant planets and provide the environment in which their satellites may form. We use high-resolution, global three-dimensional simulations to investigate the early formation, orientation, and outflows of a CPD around a Jupiter-mass planet embedded in a turbulent magnetized protoplanetary disk. Within a locally isothermal, ideal-MHD framework, we evolve disks threaded by net vertical magnetic fields, corresponding to initial plasma parameters $875\leqβ\leq3500$, until magnetorotational-instability-driven turbulence is established before inserting the planet. We also perform a hydrodynamic control simulation. In the most strongly magnetized model, with $β=875$, the CPD forms already highly inclined and reaches a maximum tilt of approximately $87^\circ$. By contrast, the hydrodynamic CPD and the MHD models with $β\gtrsim1000$ remain nearly coplanar. A control simulation in which the planet is inserted before global MRI turbulence develops also remains coplanar, despite producing local turbulence and bipolar outflows. The large tilt is therefore associated with the pre-existing global turbulent state and its evolved velocity and toroidal magnetic-field structure, although our current diagnostics do not distinguish between a direct magnetic torque and the accretion of misaligned angular momentum. All MHD models launch bipolar outflows; in the highly tilted case, these develop a curved, helical morphology that persists until the end of our short-term simulations. These results identify pre-existing global magnetized turbulence as a viable route to generating strongly inclined CPDs and twisted planetary outflows.

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arXiv2026

Hourly radio variability of PDS70c from time-differential photometry

The radio emission mechanisms from accreting protoplanets, and their variability, link observations and physical properties. We revisit the variability of the ~343GHz (ALMA Band7) flux density from PDS70c (F_B7). The subtraction of the extended time-averaged signal may enable the measurement of the flux density from variable and embedded point sources. Visibility alignment and self-calibration yields close to thermal residuals in each execution block (EB) of ALMA observations, allowing the time-differential photometry of point-source in the visibility domain. The variability of PDS70c is checked against synthetic control point sources. In images of the 2017 ALMA dataset, with three ~1h EBs, PDS70c was detected only on 6 Dec. 2017, where F_B7 rose by 228%+-69% (3.3sigma). Time-differential photometry confirms a rise by 170%+-46% (3.7sigma). An application to ~2h EBs from the 2023 dataset resulted in constant flux densities, within a scatter of ~15%. However, F_B7(t) shows some scatter when splitting the deep 2023 EBs in 20min intervals, with a chi2 test significant at 2.6sigma, and an intrinsic dispersion of 49%21%. The radio variability of PDS70c, observed over hours but averaged out on longer timescales, is indeed expected if the signal is due to HI free-free from an accretion shock on a circum-planetary disk surface. A planet-to-environment mass ratio <1E-4 is required to avoid smoothing by radiative diffusion if the signal is due to thermal emission from the environment.

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arXiv2025

Multi-frequency observations of PDS 70c: Radio emission mechanisms in the circum-planetary environment

PDS 70c is a source of Ha emission and variable sub-mm signal. Understanding its emission mechanisms may enable observations of accretion rates and physical conditions in the circum-planetary environment. We report ALMA observations of PDS 70 at 145 GHz (Band 4), 343.5 GHz (Band 7) and 671 GHz (Band 9) and compare with data at 97.5 GHz (Band 3), taken within two months. The radio spectrum (SED) is analyzed with an analytical circumplanetary disk (CPD) model. In a novel approach including the free-free continuum from H I, metals (e.g. K I) and H-. New detections in Bands 3 (tentative at 2.6sigma), 4 (5sigma), and 7 (re-detected at 9sigma) are consistent with optically thick thermal emission from PDS 70c (spectral index 2+-0.2). However, a Band 9 non-detection lies 2.6sigma below an optically thick extrapolation. A viscous dusty disk is inconsistent with the data, even with the inclusion of ionised jets. Interestingly, the central temperatures in such CPD models are high enough to ionise H I, with huge emission measures and an optically thick spectrum that marginally accounts for the SED (within 3sigma of Band 9). By contrast, uniform-slab models suggest much lower emission measures to account for the Band 9 drop, with ionisation fractions ~1e-7, and an outer radius ~0.1 au. Such conditions are recovered if the CPD interacts with a planetary magnetic field, leading to a radially variable viscosity alpha(R)<~1 and midplane temperatures ~1e3 K that regulate metal ionisation. However, the H- opacity still results in an optically thick SED, overshooting Band 9. We find that the optically thin turnover at ~600 GHz is only recovered if a thin shocked layer is present at the CPD surface, as suggested by simulations. A photospheric shock or accretion funnels are ruled out as radio emission sources because their small solid angles would require T~1e6 K, which is unrealistic for planetary accretion.

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arXiv2024

Low-mass planets falling into gaps with cyclonic vortices

We investigate the planetary migration of low-mass planets ($M_p\in[1,15]M_\oplus$, here $M_\oplus$ is the Earth mass) in a gaseous disc containing a previously formed gap. We perform high-resolution 3D simulations with the FARGO3D code. To create the gap in the surface density of the disc, we use a radial viscosity profile with a bump, which is maintained during the entire simulation time. We find that when the gap is sufficiently deep, the spiral waves excited by the planet trigger the Rossby wave instability, forming cyclonic (underdense) vortices at the edges of the gap. When the planet approaches the gap, it interacts with the vortices, which produce a complex flow structure around the planet. Remarkably, we find a widening of the horseshoe region of the planet produced by the vortex at the outer edge of the gap, which depending on the mass of the planet differs by at least a factor of two with respect to the standard horseshoe width. This inevitably leads to an increase in the corotation torque on the planet and produces an efficient trap to halt its inward migration. In some cases, the planet becomes locked in corotation with the outer vortex. Under this scenario, our results could explain why low-mass planets do not fall towards the central star within the lifetime of the protoplanetary disc. Lastly, the development of these vortices produces an asymmetric temporal evolution of the gap, which could explain the structures observed in some protoplanetary discs.

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arXiv2023

Turbulent stress within dead zones and magnetic field dragging induced by Rossby vortices

By means of three dimensional resistive-magnetohydrodynamical models, we study the evolution of the so-called dead zones focused on the magnitude of the Reynolds and Maxwell stresses. We consider two different types of static resistivity radial profiles which give rise to an intermediate dead zone or an intermediate active zone. As we are interested in analyzing the strength of angular momentum transport in these intermediate regions of the disc, we use as free parameters the radial extent of the intermediate dead ($Δr_\mathrm{idz}$) or active ($Δr_\mathrm{iact}$) zones, and the widths of the inner ($H_{b_1}$) and outer ($H_{b_2}$) transitions. We find that regardless of the width or radial extent of the intermediate zones, Rossby wave instability (RWI) develops at these transition boundaries, leading to the emergence of vortices and spiral waves. In the case of an intermediate dead zone, when $H_{b_1}\,,H_{b_2}\leq0.8$, the vortices are almost completely confined to the dead zone. Remarkably, we find that the formation of vortices at the inner transition can drag magnetic field lines into the dead zone stirring up the region that the vortex covers (reaching an $α\approx10^{-2}$ value similar to that of an active zone). Vortices formed in the outer transition only modify the Reynolds stress tensor. Our results can be important to understanding angular momentum transport in poorly ionized regions within the disc due to magnetized vortices within dead zones.

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arXiv2021

TOI-216: Resonant Constraints on Planet Migration

TOI-216 is a pair of close-in planets with orbits deep in the 2:1 mean motion resonance. The inner, Neptune-class planet (TOI-216b) is near 0.12 au (orbital period $P_{\rm b} \simeq 17$ d) and has a substantial orbital eccentricity ($e_{\rm b} \simeq 0.16$), and large libration amplitude ($A_ψ\simeq 60^\circ$) in the resonance. The outer planet (TOI-216c) is a gas giant on a nearly circular orbit. We carry out $N$-body simulations of planet migration in a protoplanetary gas disk to explain the orbital configuration of TOI-216 planets. We find that TOI-216b's migration must have been halted near its current orbital radius to allow for a convergent migration of the two planets into the resonance. For the inferred damping-to-migration timescale ratio $τ_e/τ_a \simeq 0.02$, overstable librations in the resonance lead to a limit cycle with $A_ψ\simeq 80^\circ$ and $e_{\rm b}<0.1$. The system could have remained in this configuration for the greater part of the protoplanetary disk lifetime. If the gas disk was removed from inside out, this would have reduced the libration amplitude to $A_ψ\simeq 60^\circ$ and boosted $e_{\rm b}$ via the resonant interaction with TOI-216c. Our results suggest a relatively fast inner disk removal ($\sim 10^5$ yr). Another means of explaining the large libration amplitude is stochastic stirring from a (turbulent) gas disk. For that to work, overstable librations would need to be suppressed, $τ_e/τ_a \simeq 0.05$, and very strong turbulent stirring (or some other source of large stochastic forcing) would need to overcome the damping effects of gas. Hydrodynamical simulations can be performed to test these models.

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