arXiv subjects

Hans Baehr

Publications and source records attributed to Hans Baehr.

At least 19 recordsLinked to original sources

arXiv2026

From spirals to rings: dust dynamics in gravitoturbulent protoplanetary discs after late infall

Substructures appear to be a common trait of all extended protoplanetary discs. They are found notably in discs still surrounded by ambient, possibly infalling material. In this study, we revisit the observational signatures of gravitational instability (GI) in the dust and gas emission of protoplanetary discs. We focus on discs undergoing a short-lived episode of late infall that triggers spirals via GI, in order to investigate the long-term dynamics of millimetre-sized dust. We performed 2D hydrodynamical simulations modelling gas and millimetre-sized dust in a self-gravitating disc, with a simplified treatment of stellar irradiation and infall. Results were post-processed by 3D radiative transfer calculations. During infall, GI sets in and the disc develops a gravitoturbulent state characterised by the continuous formation of spirals. Shortly after infall ends, the millimetre-sized dust remains well coupled to the gas, and the dust emission exhibits large-scale spirals in both near-infrared scattered light and continuum emission. Even long after infall has ceased, two-armed spirals are persistently excited by GI in specific regions of the disc. Their dissipation entails the formation of multiple long-lasting pressure maxima, which can be viewed as vestiges of shocks induced by the spirals. They form persistent dust rings that are more or less axisymmetrical. Consequently, once late infall ends, a massive disc can gradually evolve from a disc with spirals in the continuum into one featuring multiple, potentially lopsided bright rings in the continuum. In near-infrared scattered light, the disc initially displays multiple spirals during infall or shortly after it ceases, before ultimately exhibiting multiple rings. The persistent excitation of spirals implies, however, that the residual line-of-sight velocity, derived from line emission, shows large-scale spirals across the disc.

astro-ph.EP↗

arXiv2026

The ALMA View of the Edge-on Gomez's Hamburger System: A Highly-Dynamic, Asymmetric Protoplanetary Disk Reveals the Earliest Phases of Giant Planet Formation

Chemical tracers provide some of the strongest observational signatures of ongoing planet formation and localized dynamical perturbations in protoplanetary disks. In particular, sulfur-bearing molecules are predicted to be enhanced in regions of shock heating, ice sublimation, and gravitational instability. Here, we present high-angular-resolution ($\approx$0.$^{\prime\prime}$2) Atacama Large Millimeter/submillimeter Array observations of $^{12}$CO J=3-2, $^{13}$CO J=3-2, CS J=7-6, and SO J$_{\rm N}$=8$_8$-7$_7$ toward the large, edge-on Gomez's Hamburger ('GoHam'; IRAS 18059-3211) disk. We detect a narrow, one-sided arc of SO emission that peaks near a previously-identified gas over-density, suggesting localized heating around an early-stage giant protoplanet or disk fragment. The edge-on geometry of GoHam enables us to place this chemical signature in the broader context of the disk gas and dust structure. To do so, we map the vertical distribution of molecular gas relative to millimeter- and (sub)-micron-sized dust, identify a pronounced north-south continuum asymmetry, and detect non-Keplerian $^{12}$CO and $^{13}$CO emission indicative of a disk wind. We also derive a dynamical stellar mass of 2.2 $\pm$ 0.5 M$_{\odot}$ and a revised dust-extinction-map-based distance of 139 $\pm$ 24 pc, which places GoHam in the outskirts of the Scorpius-Centaurus association. Together, these observations reveal a highly dynamic disk in which localized sulfur chemistry may trace one of the earliest observable stages of wide-separation giant planet formation.

astro-ph.EP↗

arXiv2026

Dust Morphology Under Changing Dust Mass Ratios in Protoplanetary Discs

Protoplanetary disc mass is one of the most fundamental properties of a planet-forming system, as it sets the total mass budget available for planet formation. However, obtaining disc mass measurements remain challenging, since it is not possible to directly detect H$_2$, and CO abundance ratios are poorly constrained. Dynamical measurements of the disc mass are now possible, but they are not suited to all discs since the measurements typically require well-behaved emission surfaces. A long-standing method is to obtain continuum flux measurements from the dust emission, and convert to a total disc mass by assumption of the dust-to-gas mass ratio, $\epsilon$. This quantity is poorly constrained in protoplanetary discs. % We investigate the impact of $\epsilon$ on the morphology of planet-containing hydrodynamical simulations of dusty protoplanetary accretion discs, and suggest that if a planet mass estimate can be obtained, then disc morphology could be used to constrain $\epsilon$ in observed systems relative to each other, improving the total disc mass estimates of protoplanetary discs.

astro-ph.EP↗

arXiv2025

On the Gravitational Collapse of Small Dust Grains in Self-gravitating Disk Structures

Planet formation may begin much earlier than previously expected, when the protoplanetary disk is still massive and gravitationally unstable. It has been proposed that solid grains can concentrate in the spiral arms of self-gravitating disks, leading to the formation of planetary embryos or cores that can greatly accelerate the process of planet formation. We perform hydrodynamic simulations of self-gravitating gas and even smaller dust grains than previously investigated in 3-dimensional shearing box simulations to explore the conditions necessary to form these planetary seeds. Focusing on small grains of dimensionless stopping time $\mathrm{St}=0.01$ and shorter, we find that disk metallicities $Z \gtrsim 0.02$ can overcome the disruptive effects of dust diffusion among these small dust grains. In the outer reaches of a gravitationally unstable disk, these models correspond to grains of approximately 1$\,mm$ and lead to planetary embryos between 0.1 and 1 Earth mass. The formation of these planetary embryos could therefore reduce the time needed for planet assembly, particularly in the outer regions of the disk where coagulation timescales are longer and solid growth is limited.

astro-ph.EP↗

arXiv2025

Entropy and diffusion characterize mutation accumulation and biological information loss

Aging is a universal consequence of life, yet researchers have identified no universal theme. This manuscript considers aging from the perspective of entropy, wherein things fall apart. We first examine biological information change as a mutational distance, analogous to physical distance. In this model, informational change over time is fitted to an advection-diffusion equation, a normal distribution with a time component. The solution of the advection-diffusion equation provides a means of measuring the entropy of diverse biological systems. The binomial distribution is also sufficient to demonstrate that entropy increases as mutations or epimutations accumulate. As modeled, entropy scales with lifespans across the tree of life. This perspective provides potential mechanistic insights and testable hypotheses as to how evolution has attained enhanced longevity: entropy management. We find entropy is an inclusive rather than exclusive aging theory.

q-bio.PE↗

arXiv2025

Dust density enhancements and the direct formation of planetary cores in gravitationally unstable discs

Planet formation via core accretion involves the growth of solids that can accumulate to form planetary cores. There are a number of barriers to the collisional growth of solids in protostellar discs, one of which is the drift, or metre, barrier. Solid particles experience a drag force that will tend to cause them to drift towards the central star in smooth, laminar discs, potentially removing particles before they grow large enough to decouple from the disc gas. Here we present 3-dimensional, shearing box simulations that explore the dynamical evolution of solids in a protostellar disc that is massive enough for the gravitational instability to manifest as spiral density waves. We expand on earlier work by considering a range of particle sizes and find that the spirals can still enhance the local solid density by more than an order of magnitude, potentially aiding grain growth. Furthermore, if solid particles have enough mass, and the particle size distribution extends to sufficiently large particle sizes, the solid component of the disc can undergo direct gravitational collapse to form bound clumps with masses typically between $1$ and $10$ M$_\oplus$. Thus, the concentration of dust in a self-gravitating disc could bypass the size barrier for collisional growth and directly form planetary cores early in the lifetime of the disc.

astro-ph.EP↗

arXiv2025

Turbulence Inhibits Planetesimal Formation in Class 0/I Disks Subject to Infall

There is growing evidence that planet formation begins early, within the $\lesssim 1$Myr Class 0/I phase, when infall dominates disk dynamics. Our goal is to determine if Class 0/I disks reach the conditions needed to form planetesimals ($\sim 100$km planet building blocks) by the streaming instability (SI). We focus on a recent suggestion that early infall causes an ``inflationary'' phase in which dust grains are advected outward. We modified the \texttt{DustPy} code to build a 1D disk that includes dust evolution, infall, and heating and cooling sources. We ran six models and examined the implications for the SI, taking into account recent works on how the SI responds to external turbulence. In line with other works, we find that grains are advected outward, which leads to ``advection-condensation-drift'' loop that greatly enhances the dust density at the water snowline. However, we do not see this process at the silicate line. Instead, we find a new pile up at the edge of the expanding disk. However, despite these localized enhancements, even a modest amount of turbulence ($\alpha = 10^{-3}$) leaves planetesimal formation far out of reach. The midplane dust-to-gas ratio is at least an order of magnitude below the SI threshold, even taking into account recent results on how dust coagulation boosts the SI. For planetesimals to form in the Class 0/I phase may require a way to transport angular momentum without turbulence (e.g., disk winds) or a non-SI mechanism to form planetesimals.

astro-ph.EP↗

arXiv2023

Filling in the Gaps: Can Gravitationally Unstable Discs Form the Seeds of Gas Giant Planets?

Circumstellar discs likely have a short window when they are self-gravitating and prone to the effects of disc instability, but during this time the seeds of planet formation can be sown. It has long been argued that disc fragmentation can form large gas giant planets at wide orbital separations, but its place in the planet formation paradigm is hindered by a tendency to form especially large gas giants or brown dwarfs. We instead suggest that planet formation can occur early in massive discs, through the gravitational collapse of dust which can form the seeds of giant planets. This is different from the usual picture of self-gravitating discs, in which planet formation is considered through the gravitational collapse of the gas disc into a gas giant precursor. It is familiar in the sense that the core is formed first, and gas is accreted thereafter, as is the case in the core accretion scenario. However, by forming a $\sim 1 M_{\oplus}$ seed from the gravitational collapse of dust within a self-gravitating disc there exists the potential to overcome traditional growth barriers and form a planet within a few times $10^5$ years. The accretion of pebbles is most efficient with centimetre-sized dust, but the accretion of millimetre sizes can also result in formation within a Myr. Thus, if dust can grow to these sizes, planetary seeds formed within very young, massive discs could drastically reduce the timescale of planet formation and potentially explain the observed ring and gap structures in young discs.

astro-ph.EP↗

arXiv2023

Thermal instabilities in accretion disks II: Numerical Experiments for the Goldreich-Schubert-Fricke Instability and the Convective Overstability in disks around young stars

The linear stability analysis of a stratified rotating fluid (see paper I) showed that disks with a baroclinic stratification under the influence of thermal relaxation will become unstable to thermal instabilities. One instability is the Goldreich-Schubert-Fricke instability (GSF), which is the local version of the Vertical Shear Instability (VSI) and the other is a thermal overstability, the Convective Overstability (COS). In the present paper we reproduce the analytic predicted growth rates for both instabilities in numerical experiments of small axisymmetric sections of vertically isothermal disks with a radial temperature gradient, especially for cooling times longer than the critical cooling time for VSI. In this cooling time regime our simulations reveal the simultaneous and independent growth of both modes: COS and GSF. We consistently observe that GSF modes exhibit a faster growth rate compared to COS modes. Near the midplane, GSF modes eventually stop growing, while COS modes continue to grow and ultimately dominate the flow pattern. Away from the midplane, we find GSF modes to saturate, when bands of constant angular momentum have formed. In these bands we observe the formation and growth of eddies driven by the baroclinic term, further enhancing the velocity perturbations. In geophysics this effect is known as horizontal convection or sea-breeze instability. Three-dimensional simulations will have to show whether similar effects will occur when axisymmetry is not enforced. Our local simulations help to reveal the numerical resolution requirements to observe thermal instabilities in global simulations of disks around young stars.

astro-ph.EP↗

arXiv2022

Direct Formation of Planetary Embryos in Self-Gravitating Disks

Giant planets have been discovered at large separations from the central star. Moreover, a striking number of young circumstellar disks have gas and/or dust gaps at large orbital separations, potentially driven by embedded planetary objects. To form massive planets at large orbital separations through core accretion within disk lifetime, however, an early solid body to seed pebble and gas accretion is desirable. Young protoplanetary disks are likely self-gravitating, and these gravitoturbulent disks may efficiently concentrate solid material at the midplane driven by spiral waves. We run 3D local hydrodynamical simulations of gravitoturbulent disks with Lagrangian dust particles to determine whether particle and gas self-gravity can lead to the formation of dense solid bodies, seeding later planet formation. When self-gravity between dust particles is included, solids of size $\mathrm{St} = 0.1$ to $1$ concentrate within the gravitoturbulent spiral features and collapse under their own self-gravity into dense clumps up to several $M_{\oplus}$ in mass at wide orbits. Simulations with dust that drift most efficiently, $\mathrm{St}=1$, form the most massive clouds of particles, while simulations with smaller dust particles, $\mathrm{St}=0.1$, have clumps with masses an order of magnitude lower. When the effect of dust backreaction onto the gas is included, dust clumps become smaller by a factor of a few but more numerous. The existence of large solid bodies at an early stage of the disk can accelerate the planet formation process, particularly at wide orbital separations, and potentially explain planets distant from the central stars and young protoplanetary disks with substructures.

astro-ph.EP↗

arXiv2021

Global 3D Radiation Hydrodynamic Simulations of Proto-Jupiter's Convective Envelope

The core accretion model of giant planet formation has been challenged by the discovery of recycling flows between the planetary envelope and the disc that can slow or stall envelope accretion. We carry out 3D radiation hydrodynamic simulations with an updated opacity compilation to model the proto-Jupiter's envelope. To isolate the 3D effects of convection and recycling, we simulate both isolated spherical envelopes and envelopes embedded in discs. The envelopes are heated at given rates to achieve steady states, enabling comparisons with 1D models. We vary envelope properties to obtain both radiative and convective solutions. Using a passive scalar, we observe significant mass recycling on the orbital timescale. For a radiative envelope, recycling can only penetrate from the disc surface until $\sim$0.1-0.2 planetary Hill radii, while for a convective envelope, the convective motion can "dredge up" the deeper part of the envelope so that the entire convective envelope is recycled efficiently. This recycling, however, has only limited effects on the envelopes' thermal structure. The radiative envelope embedded in the disc has identical structure as the isolated envelope. The convective envelope has a slightly higher density when it is embedded in the disc. We introduce a modified 1D approach which can fully reproduce our 3D simulations. With our updated opacity and 1D model, we recompute Jupiter's envelope accretion with a 10 $M_{\oplus}$ core, and the timescale to runaway accretion is shorter than the disc lifetime as in prior studies. Finally, we discuss the implications of the efficient recycling on the observed chemical abundances of the planetary atmosphere (especially for super-Earths and mini-Neptunes).

astro-ph.EP↗

arXiv2021

Particle Dynamics in 3D Self-gravitating Disks II: Strong Gas Accretion and Thin Dust Disks

Observations suggest that protoplanetary disks have moderate accretion rates onto the central young star, especially at early stages (e.g. HL Tau), indicating moderate disk turbulence. However, recent ALMA observations suggest that dust is highly settled, implying weak turbulence. Motivated by such tension, we carry out 3D stratified local simulations of self-gravitating disks, focusing on settling of dust particles in actively accreting disks. We find that gravitationally unstable disks can have moderately high accretion rates while maintaining a relatively thin dust disk for two reasons. First, accretion stress from the self-gravitating spirals (self-gravity stress) can be stronger than the stress from turbulence (Reynolds stress) by a factor of 5-20. Second, the strong gravity from the gas to the dust decreases the dust scale height by another factor of $\sim 2$. Furthermore, the turbulence is slightly anisotropic, producing a larger Reynolds stress than the vertical dust diffusion coefficient. Thus, gravitoturbulent disks have unusually high vertical Schmidt numbers ($Sc_z$) if we scale the total accretion stress with the vertical diffusion coefficient (e.g. $Sc_z\sim$ 10-100). The reduction of the dust scale height by the gas gravity, should also operate in gravitationally stable disks ($Q>$1). Gravitational forces between particles become more relevant for the concentration of intermediate dust sizes, forming dense clouds of dust. After comparing with HL Tau observations, our results suggest that self-gravity and gravity among different disk components could be crucial for solving the conflict between the protoplanetary disk accretion and dust settling, at least at the early stages.

astro-ph.EP↗

arXiv2021

Particle Dynamics in 3D Self-gravitating Disks I: Spirals

Spiral arms are distinctive features of many circumstellar disks, observed in scattered light, which traces the disk surface, millimeter dust emission, which probes the disk midplane, as well as molecular emission. The two leading explanations for spirals are wakes generated by a massive planet and the density waves excited by disk self-gravity. We use stratified 3D hydrodynamic shearing-box simulations including dust particles and disk self-gravity to investigate how gas and dust spirals in a self-gravitating disk depend on the simulation size, the cooling efficiency, and the aerodynamics properties of particles. We find that opening angles of spirals are universal ($\sim10^o$), and not significantly affected by the size of the computational domain, the cooling time, or the particle size. In simulations with the biggest domain, the spirals in the gaseous disk become slightly more open with a higher cooling efficiency. Small dust follows the gaseous spirals very well, while intermediate-sized dust with dimensionless stopping time $(\mathrm{St})$ close to 1 concentrates to the spirals more and shows stronger spirals. However, large dust with $\mathrm{St} > 1$ also shows spirals, which is different from some previous simulations. We identify that this is due to the gravity from the gas to the dust component. We show that when $\mathrm{St} \gtrsim Q$, the gravitational force from the gaseous spirals to the dust particles becomes stronger than the particles' aerodynamic drag force, so that the gas significantly affects these large particles through gravitational interaction. This has important implications for both spiral observations and planetesimal formation/dynamics.

astro-ph.EP↗

arXiv2021

Kozai-Lidov oscillations triggered by a tilt instability of detached circumplanetary discs

Circumplanetary discs can be linearly unstable to the growth of disc tilt in the tidal potential of the star-planet system. We use three-dimensional hydrodynamical simulations to characterize the disc conditions needed for instability, together with its long term evolution. Tilt growth occurs for disc aspect ratios, evaluated near the disc outer edge, of $H/r\gtrsim 0.05$, with a weak dependence on viscosity in the wave-like regime of warp propagation. Lower mass giant planets are more likely to have circumplanetary discs that satisfy the conditions for instability. We show that the tilt instability can excite the inclination to above the threshold where the circumplanetary disc becomes unstable to Kozai--Lidov (KL) oscillations. Dissipation in the Kozai--Lidov unstable regime caps further tilt growth, but the disc experiences large oscillations in both inclination and eccentricity. Planetary accretion occurs in episodic accretion events. We discuss implications of the joint tilt--KL instability for the detectability of circumplanetary discs, for the obliquity evolution of forming giant planets, and for the formation of satellite systems.

astro-ph.EP↗

arXiv2020

EDEN: Sensitivity Analysis and Transiting Planet Detection Limits for Nearby Late Red Dwarfs

Small planets are common around late-M dwarfs and can be detected through highly precise photometry by the transit method. Planets orbiting nearby stars are particularly important as they are often the best-suited for future follow-up studies. We present observations of three nearby M-dwarfs referred to as EIC-1, EIC-2, and EIC-3, and use them to search for transits and set limits on the presence of planets. On most nights our observations are sensitive to Earth-sized transiting planets, and photometric precision is similar to or better than TESS for faint late-M dwarfs of the same magnitude (I=15 mag). We present our photometry and transit search pipeline, which utilizes simple median detrending in combination with transit least squares based transit detection (Hippke & Heller 2019).For these targets, and transiting planets between one and two Earth radii, we achieve an average transit detection probability of 60% between periods of 0.5 and 2 days, 30% between 2 and 5 days,and 10% between 5 and 10 days. These sensitivities are conservative compared to visual searches.

astro-ph.EP↗

arXiv2020

Requirements for gravitational collapse in planetesimal formation --- the impact of scales set by Kelvin-Helmholtz and nonlinear streaming instability

The formation of planetesimals is an unsolved problem in planet formation theory. A prominent scenario for overcoming dust growth barriers in dead zones is the gravitational collapse of locally over-dense regions, shown to robustly produce $\sim$100 km sized objects. Still, the conditions under which planetesimal formation occurs remain unclear. For collapse to proceed, the self-gravity of an overdensity must overcome stellar tidal disruption on large scales and turbulent diffusion on small scales. Here, we relate the scales of streaming and Kelvin-Helmholtz instability, which both regulate particle densities on the scales of gravitational collapse, directly to planetesimal formation. We support our analytic findings by performing 3D hydrodynamical simulations of streaming and Kelvin-Helmholtz instability and planetesimal formation. We find that the vertical extent of the particle mid-plane layer and the radial width of streaming instability filaments are set by the same characteristic length scale, thus governing the strength of turbulent diffusion on the scales of planetesimal formation. We present and successfully test a collapse criterion: $0.1 Q βε^{-1}Z^{-1} \lesssim 1$ and show that even for Solar metallicities, planetesimals can form in dead zones of sufficiently massive disks. For a given gas Toomre-parameter $Q$, pressure gradient $β$, metallicity $Z$ and local particle enhancement $ε$, the collapse criterion also provides a range of unstable scales, instituting a promising path for studying initial planetesimal mass distributions. Streaming instability is not required for planetesimal collapse, but by increasing $ε$, can evolve a system to instability.

astro-ph.EP↗

arXiv2019

The Concentration and Growth of Solids in Fragmenting Circumstellar Disks

Due to the gas rich environments of early circumstellar disks, the gravitational collapse of cool, dense regions of the disk form fragments largely composed of gas. During formation, disk fragments may attain increased metallicities as they interact with the surrounding disk material, whether through particle migration to pressure maxima or through mutual gravitational interaction. In this paper, we investigate the ability of fragments to collect and retain a significant solid component through gas-particle interactions in high-resolution 3D self-gravitating shearing box simulations. The formation of axissymmetric perturbations associated with gravitational instabilities allows particles of intermediate sizes to concentrate through aerodynamic drag forces. By the onset of fragmentation, the mass of local particle concentrations within the fragment are comparable to that of the gas component and the sebsequent gravitational collapse results in the formation of a solid core. We find that these cores can be up to several tens of Earth masses, depending on grain size, before the fragment center reaches temperatures which would sublimate solids. The solid fraction and total mass of the fragment also depend on the metallicity of the young parent protoplanetary disk, with higher initial metallicities resulting in larger fragments and larger solid cores. Additionally, the extended atmospheres of these soon-to-be gas giants or brown dwarfs are occasionally enriched above the initial metallicity, provided no solid core forms in the center and are otherwise lacking in heavier elements when a core does form.

astro-ph.EP↗

arXiv2017

The fragmentation criteria in local vertically stratified self-gravitating disk simulations

Massive circumstellar disks are prone to gravitational instabilities, which trigger the formation of spiral arms that can fragment into bound clumps under the right conditions. Two dimensional simulations of self-gravitating disks are useful starting points for studying fragmentation, allowing for high-resolution simulations of thin disks. However, convergence issues can arise in 2D from various sources. One of these sources is the 2D approximation of self-gravity, which exaggerates the effect of self-gravity on small scales when the potential is not smoothed to account for the assumed vertical extent of the disk. This effect is enhanced by increased resolution, resulting in fragmentation at longer cooling timescales $β$. If true, it suggests that the 3D simulations of disk fragmentation may not have the same convergence problem and could be used to examine the nature of fragmentation without smoothing self-gravity on scales similar to the disk scale height. To that end, we have carried out local 3D self-gravitating disk simulations with simple $β$ cooling with fixed background irradiation to determine if 3D is necessary to properly describe disk fragmentation. Above a resolution of $\sim 40$ grid cells per scale height, we find that our simulations converge with respect to the cooling timescale. This result converges in agreement with analytic expectations which place a fragmentation boundary at $β_\mathrm{crit} = 3$.

astro-ph.EP↗