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Zhaohuan Zhu

Publications and source records attributed to Zhaohuan Zhu.

At least 73 records · Page 4Linked to original sources
arXiv2023

Growing Planet Envelopes in Spite of Recycling Flows

The hydrodynamic exchange of a protoplanet's envelope material with the background protoplanetary disk has been proposed as one mechanism to account for the diversity of observed planet envelopes which range in mass fractions of ~1% for super-Earths to ~90% for giants. Here we present and analyze 3D radiation-hydrodynamics models of protoplanet envelopes to understand how the exchange of mass and energy between the protoplanet and background disk influences the formation process. Our protoplanet envelope simulations show an exchange of material bringing the outer <0.4Rbondi envelope to steady state. This exchange provides a continuous source of energy, which acts to increase the observed luminosity beyond that inferred from the binding energy liberated from Kelvin-Helmholtz contraction alone -- a finding important for potential protoplanet observations. The inner <0.4Rbondi, on the other hand, appears insulated -- growing in accordance with 1D quasi-static theory. We incorporate these 3D hydrodynamic effects into an extensible 1D framework with a physically motivated three-layer recycling parameterization. Specializing to the case of Jupiter, recycling produces minimal changes to the growth rate with the planet still entering runaway accretion and becoming a gas giant in ~1 Myr. Even in the inner disk (0.1 AU), our 1D models suggest that recycling is not so robust and ubiquitous as to stop all cores from becoming giants. At the same time however, this recycling can delay a runaway phase by an order-of-magnitude depending on the inner disk conditions and core mass.

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arXiv2023

Changing-Look AGN Behaviour Induced by Disk-Captured Tidal Disruption Events

Recent observations of changing-look active galactic nuclei (AGN) hint at a frequency of accretion activity not fully explained by tidal disruption events (TDEs) stemming from relaxation processes in nucleus star clusters (NSCs), traditionally estimated to occur at rates of $10^{-4}$ to $10^{-5}$ yr$^{-1}$ per galaxy. In this letter, we propose an enhanced TDE rate through the AGN disk capture process, presenting a viable explanation for the frequent transitions observed in changing-look AGN. Specifically, we investigate the interaction between the accretion disk and retrograde stars within NSCs, resulting in the rapid occurrence of TDEs within a condensed time frame. Through detailed calculations, we derive the time-dependent TDE rates for both relaxation-induced TDE and disk-captured TDE. Our analysis reveals that TDEs triggered by the disk capture process can notably amplify the TDE rate by several orders of magnitude during the AGN phase. This mechanism offers a potential explanation for the enhanced high-energy variability characteristic of changing-look AGNs.

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arXiv2023

Stellar outbursts and chondrite composition

The temperatures of observed protoplanetary disks are not sufficiently high to produce the accretion rate needed to form stars, nor are they sufficient to explain the volatile depletion patterns in CM, CO, and CV chondrites and terrestrial planets. We revisit the role that stellar outbursts, caused by high accretion episodes, play in resolving these two issues. These outbursts provide the necessary mass to form the star during the disk lifetime, and provide enough heat to vaporize planet-forming materials. We show that these outbursts can reproduce the observed chondrite abundances at distances near one AU. These outbursts would also affect the growth of calcium-aluminum-rich inclusions (CAIs) and the isotopic compositions of carbonaceous and non-carbonaceous chondrites.

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arXiv2023

UV-Optical Emission of AB Aur b is Consistent with Scattered Stellar Light

The proposed protoplanet AB Aur b is a spatially concentrated emission source imaged in the mm-wavelength disk gap of the Herbig Ae/Be star AB Aur. Its near-infrared spectrum and absence of strong polarized light have been interpreted as evidence supporting the protoplanet interpretation. However, the complex scattered light structures in the AB Aur disk pose challenges in resolving the emission source and interpreting the true nature of AB Aur b. We present new images of the AB Aur system obtained using the Hubble Space Telescope Wide Field Camera 3 in the ultraviolet (UV) and optical bands. AB Aur b and the known disk spirals are recovered in the F336W, F410M, and F645N bands. The spectral energy distribution of AB Aur b shows absorption in the Balmer jump, mimicking those of early-type stars. By comparing the colors of AB Aur b to those of the host star, the disk spirals, and predictions from scattered light and self-luminous models, we find that the emission from AB Aur b is inconsistent with planetary photospheric or accretion shock models. Instead, it is consistent with those measured in the circumstellar disks that trace scattered light. We conclude that the UV and visible emission from AB Aur b does not necessitate the presence of a protoplanet. We synthesize observational constraints on AB Aur b and discuss inconsistent interpretations of AB Aur b among different datasets. Considering the significance of the AB Aur b discovery, we advocate for further observational evidence to verify its planetary nature.

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arXiv2023

The maximum accretion rate of a protoplanet: how fast can runaway be?

The hunt is on for dozens of protoplanets hypothesised to reside in protoplanetary discs with imaged gaps. How bright these planets are, and what they will grow to become, depend on their accretion rates, which may be in the runaway regime. Using 3D global simulations we calculate maximum gas accretion rates for planet masses $M_{\rm p}$ from 1$\,M_{\oplus}$ to $10\,M_{\rm J}$. When the planet is small enough that its sphere of influence is fully embedded in the disc, with a Bondi radius $r_{\rm Bondi}$ smaller than the disc's scale height $H_{\rm p}$ -- such planets have thermal mass parameters $q_{\rm th} \equiv (M_{\rm p}/M_{\star}) / (H_{\rm p}/R_{\rm p})^3 \lesssim 0.3$, for host stellar mass $M_{\star}$ and orbital radius $R_{\rm p}$ -- the maximum accretion rate follows a Bondi scaling, with $\max \dot{M}_{\rm p} \propto ρ_{\rm g} M_{\rm p}^2 / (H_{\rm p}/R_{\rm p})^3$ for ambient disc density $ρ_{\rm g}$. For more massive planets with $0.3 \lesssim q_{\rm th} \lesssim 10$, the Hill sphere replaces the Bondi sphere as the gravitational sphere of influence, and $\max \dot{M}_{\rm p} \propto ρ_{\rm g} M_{\rm p}^1$, with no dependence on $H_{\rm p}/R_{\rm p}$. In the strongly superthermal limit when $q_{\rm th} \gtrsim 10$, the Hill sphere pops well out of the disc, and $\max \dot{M}_{\rm p} \propto ρ_{\rm g} M_{\rm p}^{2/3} (H_{\rm p}/R_{\rm p})^1$. Applied to the two confirmed protoplanets PDS 70b and c, our numerically calibrated maximum accretion rates imply their Jupiter-like masses may increase by up to a factor of $\sim$2 before their parent disc dissipates.

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arXiv2023

Resolving the Binary Components of the Outbursting Protostar HBC 494 with ALMA

Episodic accretion is a low-mass pre-main sequence phenomenon characterized by sudden outbursts of enhanced accretion. These objects are classified into two: protostars with elevated levels of accretion that lasts for decades or more, called FUors, and protostars with shorter and repetitive bursts, called EXors. HBC 494 is a FUor object embedded in the Orion Molecular Cloud. Earlier Atacama Large (sub-)Millimeter Array (ALMA) continuum observations showed an asymmetry in the disk at 0.''2 resolution. Here, we present follow-up observations at ~0.''03, resolving the system into two components: HBC 494 N (primary) and HBC 494 S (secondary). No circumbinary disk was detected. Both disks are resolved with a projected separation of ~0.''18 (75 au). Their projected dimensions are 84+/-1.8 x 66.9+/-1.5 mas for HBC 494 N and 64.6+/-2.5 x 46.0+/-1.9 mas for HBC 494 S. The disks are almost aligned and with similar inclinations. The observations show that the primary is ~5 times brighter/more massive and ~2 times bigger than the secondary. We notice that the northern component has a similar mass to the FUors, while the southern has to EXors. The HBC 494 disks show individual sizes that are smaller than single eruptive YSOs. In this work, we also report 12CO, 13CO, and C18O molecular line observations. At large scale, the 12CO emission shows bipolar outflows, while the 13CO and C18O maps show a rotating and infalling envelope around the system. At a smaller scale, the 12CO and 13CO moment zero maps show cavities within the continuum disks' area, which may indicate continuum over-subtraction or slow-moving jets and chemical destruction along the line-of-sight.

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arXiv2023

Gap Opening in Protoplanetary Disks: Gas Dynamics from Global Non-ideal MHD Simulations with Consistent Thermochemistry

Recent high angular resolution ALMA observations have revealed numerous gaps in protoplanetary disks. A popular interpretation has been that planets open them. Most previous investigations of planet gap-opening have concentrated on viscous disks. Here, we carry out 2D (axisymmetric) global simulations of gap opening by a planet in a wind-launching non-ideal MHD disk with consistent thermochemistry. We find a strong concentration of poloidal magnetic flux in the planet-opened gap, where the gas dynamics are magnetically dominated. The magnetic field also drives a fast (nearly sonic) meridional gas circulation in the denser disk regions near the inner and outer edges of the gap, which may be observable through high-resolution molecular line observations. The gap is more ionized than its denser surrounding regions, with a better magnetic field-matter coupling. In particular, it has a much higher abundance of molecular ion HCO$^+$, consistent with ALMA observations of the well-studied AS 209 protoplanetary disk that has prominent gaps and fast meridional motions reaching the local sound speed. Finally, we provide fitting formulae for the ambipolar and Ohmic diffusivities as a function of the disk local density, which can be used for future 3D simulations of planet gap-opening in non-ideal MHD disks where thermochemistry is too computationally expensive to evolve self-consistently with the magneto-hydrodynamics.

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arXiv2023

Porous Dust Particles in Protoplanetary Disks: Application to the HL Tau Disk

Dust particle sizes constrained from dust continuum and polarization observations by radio interferometry are inconsistent by at least an order of magnitude. Motivated by porous dust observed in small Solar System bodies (e.g., from the Rosetta mission), we explore how the dust particle's porosity affects the estimated particle sizes from these two methods. Porous particles have lower refractive indices, which affect both opacity and polarization fraction. With weaker Mie interference patterns, the porous particles have lower opacity at mm wavelengths than the compact particles if the particle size exceeds several hundred microns. Consequently, the inferred dust mass using porous particles can be up to a factor of six higher. The most significant difference between compact and porous particles is their scattering properties. The porous particles have a wider range of particle sizes with high linear polarization from dust self-scattering, allowing mm-cm-sized particles to explain polarization observations. With a Bayesian approach, we use porous particles to fit HL Tau disk's multi-wavelength continuum and mm-polarization observations from ALMA and VLA. The moderately porous particles with sizes from 1 mm-1 m can explain both continuum and polarization observations, especially in the region between 20-60 au. If the particles in HL Tau are porous, the porosity should be from 70% to 97% from current polarization observations. We also predict that future observations of the self-scattering linear polarization at longer wavelengths (e.g., ALMA B1 and ngVLA) have the potential to further constrain the particle's porosity and size.

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arXiv2023

Substructures in Compact Disks of the Taurus Star-forming Region

Observations of substructure in protoplanetary disks have largely been limited to the brightest and largest disks, excluding the abundant population of compact disks which are likely sites of planet formation. Here, we reanalyze ~0.1'', 1.33 mm ALMA continuum observations of 12 compact protoplanetary disks in the Taurus star-forming region. By fitting visibilities directly, we identify substructures in 6 of the 12 compact disks. We then compare the substructures identified in the full Taurus sample of 24 disks in single star systems and the ALMA DSHARP survey, differentiating between compact (R_eff,90% < 50 au) and extended (R_eff,90% > 50 au) disk sources. We find that substructures are detected at nearly all radii in both small and large disks. Tentatively, we find fewer wide gaps in intermediate-sized disks with R_eff,90% between 30 and 90 au. We perform a series of planet-disk interaction simulations to constrain the sensitivity of our visibility-fitting approach. Under an assumption of planet-disk interaction, we use the gap widths and common disk parameters to calculate potential planet masses within the Taurus sample. We find that the young planet occurrence rate peaks near Neptune masses, similar to the DSHARP sample. For 0.01 $M_J/M_\odot$ $\lesssim$ $M_p/M_*$ $\lesssim$ 0.1 $M_J/M_\odot$, the rate is 17.4$\pm$8.3%; for 0.1 $M_J/M_\odot$ $\lesssim$ $M_p/M_*$ $\lesssim$ 1 $M_J/M_\odot$, it is 27.8$\pm$8.3%. Both of them are consistent with microlensing surveys. For gas giants more massive than 5 $M_J$, the occurrence rate is 4.2$\pm$4.2%, consistent with direct imaging surveys.

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arXiv2023

Exciting spiral arms in protoplanetary discs from flybys

Spiral arms are observed in numerous protoplanetary discs. These spiral arms can be excited by companions, either on bound or unbound orbits. We simulate a scenario where an unbound perturber, i.e. a flyby, excites spiral arms during a periastron passage. We run three-dimensional hydrodynamical simulations of a parabolic flyby encountering a gaseous protoplanetary disc. The perturber mass ranges from $10\, \rm M_J$ to $1\, \rm M_{\odot}$. The perturber excites a two-armed spiral structure, with a more prominent spiral feature for higher mass perturbers. The two arms evolve over time, eventually winding up, consistent with previous works. We focus on analysing the pattern speed and pitch angle of these spirals during the whole process. The initial pattern speed of the two arms are close to the angular velocity of the perturber at periastron, and then it decreases over time. The pitch angle also decreases over time as the spiral winds up. The spirals disappear after several local orbital times. An inclined prograde orbit flyby induces similar disc substructures as a coplanar flyby. A solar-mass flyby event causes increased eccentricity growth in the protoplanetary disc, leading to an eccentric disc structure which dampens over time. The spirals' morphology and the disc eccentricity can be used to search for potential unbound stars or planets around discs where a flyby is suspected. Future disc observations at high resolution and dedicated surveys will help to constrain the frequency of such stellar encounters in nearby star-forming regions.

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arXiv2023

Effective two-body scatterings around a massive object

Two-body scatterings under the potential of a massive object are very common in astrophysics. If the massive body is far enough away that the two small bodies are in their own gravitational sphere of influence, the gravity of the massive body can be temporarily ignored. However, this requires the scattering process to be fast enough that the small objects do not spend too much time at distances near the surface of the sphere of influence. In this paper, we derive the validation criteria for effective two-body scattering and establish a simple analytical solution for this process, which we verify through numerical scattering experiments. We use this solution to study star-black hole scatterings in the disks of Active Galactic Nuclei and planet-planet scatterings in planetary systems, and calculate their one-dimensional cross-section analytically. Our solution will be valuable in reducing computational time when treating two-body scatterings under the potential of a much more massive third body, provided that the problem settings are in the valid parameter space region identified by our study.

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arXiv2023

Grid-Based Simulations of Polar Circumbinary Disks: Polar Alignment and Vortex Formation

We describe the first grid-based simulations of the polar alignment of a circumbinary disk. We simulate the evolution of an inclined disk around an eccentric binary using the grid-based code ATHENA++. The use of a grid-based numerical code allows us to explore lower disk viscosities than have been examined in previous studies. We find that the disk aligns to a polar orientation when the $α$ viscosity is high, while disks with lower viscosity nodally precess with little alignment over 1000 binary orbital periods. The timescales for polar alignment and disk precession are compared as a function of disk viscosity, and are found to be in agreement with previous studies. At very low disk viscosities (e.g. $α= 10^{-5}$), anticyclonic vortices are observed along the inner edge of the disk. These vortices can persist for thousands of binary orbits, creating azimuthally localized overdensities as well as multiple pairs of spiral arms. The vortex is formed at $\sim 3-4$ times the binary semi-major axis, close to the inner edge of the disk, and orbits at roughly the local Keplerian speed. The presence of a vortex in the disk may play an important role in the evolution of circumbinary systems, such as driving episodic accretion and accelerating the formation of polar circumbinary planets.

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arXiv2023

Structured Distributions of Gas and Solids in Protoplanetary Disks

Recent spatially-resolved observations of protoplanetary disks revealed a plethora of substructures, including concentric rings and gaps, inner cavities, misalignments, spiral arms, and azimuthal asymmetries. This is the major breakthrough in studies of protoplanetary disks since Protostars and Planets VI and is reshaping the field of planet formation. However, while the capability of imaging substructures in protoplanetary disks has been steadily improving, the origin of many substructures are still largely debated. The structured distributions of gas and solids in protoplanetary disks likely reflect the outcome of physical processes at work, including the formation of planets. Yet, the diverse properties among the observed protoplanetary disk population, for example, the number and radial location of rings and gaps in the dust distribution, suggest that the controlling process may differ between disks and/or the outcome may be sensitive to stellar or disk properties. In this review, we (1) summarize the existing observations of protoplanetary disk substructures collected from the literature; (2) provide a comprehensive theoretical review of various processes proposed to explain observed protoplanetary disk substructures; (3) compare current theoretical predictions with existing observations and highlight future research directions to distinguish between different origins; and (4) discuss implications of state-of-the-art protoplanetary disk observations to protoplanetary disk and planet formation theory.

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arXiv2022

Prograde and Retrograde Gas Flow around Disk-embedded Companions: Dependence on Eccentricity, Mass and Disk Properties

We apply 3D hydrodynamical simulations to study the rotational aspect of gas flow patterns around eccentric companions embedded in an accretion disk around its primary host. We sample a wide range of companion mass ratio q and disk aspect ratio h, and confirm a generic transition from prograde (steady tidal interaction dominated) to retrograde (background Keplerian shear dominated) circum-companion flow when orbital eccentricity exceeds a critical value et. We find et \sim h for sub-thermal companions while et \sim (q/h)^1/3 for super-thermal companions, and propose an empirical formula to unify the two scenarios. Our results also suggest that et is insensitive to modest levels of turbulence, modeled in the form of a kinematic viscosity term. In the context of stellar-mass Black Holes (sBHs) embedded in AGN accretion disks, the bifurcation of their circum-stellar disk (CSD) rotation suggest the formation of a population of nearly anti-aligned sBHs, whose relevance to low spin gravitational waves (GW) events can be probed in more details with future population models of sBH evolution in AGN disks, making use of our quantitative scaling for et; In the context of circum-planetary disks (CPDs), our results suggest the possibility of forming retrograde satellites in-situ in retrograde CPDs around eccentric planets.

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arXiv2022

Directly Detecting the Envelopes of Low-mass Planets Embedded In Protoplanetary Discs and The Case For TW Hydrae

Despite many methods developed to find young massive planets in protoplanetary discs, it is challenging to directly detect low-mass planets that are embedded in discs. On the other hand, the core-accretion theory suggests that there could be a large population of embedded low-mass young planets at the Kelvin-Helmholtz (KH) contraction phase. We adopt both 1-D models and 3-D simulations to calculate the envelopes around low-mass cores (several to tens of $M_{\oplus}$) with different luminosities, and derive their thermal fluxes at radio wavelengths. We find that, when the background disc is optically thin at radio wavelengths, radio observations can see through the disc and probe the denser envelope within the planet's Hill sphere. When the optically thin disc is observed with the resolution reaching one disc scale height, the radio thermal flux from the planetary envelope around a 10 M$_{\oplus}$ core is more than 10 % higher than the flux from the background disc. The emitting region can be extended and elongated. Finally, our model suggests that the au-scale clump at 52 au in the TW Hydrae disc revealed by ALMA is consistent with the envelope of an embedded 10-20 $M_{\oplus}$ planet, which can explain the detected flux, the spectral index dip, and the tentative spirals. The observation is also consistent with the planet undergoing pebble accretion. Future ALMA and ngVLA observations may directly reveal more such low-mass planets, enabling us to study core growth and even reconstruct the planet formation history using the embedded "protoplanet" population.

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arXiv2022

ALMA Detection of Dust Trapping around Lagrangian Points in the LkCa 15 Disk

We present deep high-resolution ($\sim$50 mas, 8 au) ALMA 0.88 and 1.3 mm continuum observations of the LkCa 15 disk. The emission morphology shows an inner cavity and three dust rings at both wavelengths, but with slightly narrower rings at the longer wavelength. Along a faint ring at 42 au, we identify two excess emission features at $\sim$10$σ$ significance at both wavelengths: one as an unresolved clump and the other as an extended arc, separated by roughly 120 degrees in azimuth. The clump is unlikely to be a circumplanetary disk (CPD) as the emission peak shifts between the two wavelengths even after accounting for orbital motion. Instead, the morphology of the 42 au ring strongly resembles the characteristic horseshoe orbit produced in planet--disk interaction models, where the clump and the arc trace dust accumulation around Lagrangian points $L_{4}$ and $L_{5}$, respectively. The shape of the 42 au ring, dust trapping in the outer adjacent ring, and the coincidence of the horseshoe ring location with a gap in near-IR scattered light, are all consistent with the scenario of planet sculpting, with the planet likely having a mass between those of Neptune and Saturn. We do not detect point-like emission associated with a CPD around the putative planet location ($0.''27$ in projected separation from the central star at a position angle of $\sim$60\degr), with upper limits of 70 and 33 $μ$Jy at 0.88 and 1.3 mm, respectively, corresponding to dust mass upper limits of 0.02--0.03 $M_{\oplus}$.

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arXiv2022

Engine-fed Kilonovae (Mergernovae) -- I. Dynamical Evolution and Energy Injection / Heating Efficiencies

A binary neutron star merger is expected to be associated by a kilonova, transient optical emission powered by radioactive decay of the neutron-rich ejecta. If the post-merger remnant is a long-lived neutron star, additional energy injection to the ejecta is possible. In this first paper of a series, we study the dynamical evolution of the engine-fed kilonova (mergernova) ejecta in detail. We perform a semi-analytical study of the problem by adopting a modified mechanical blastwave model that invokes interaction between a Poynting-flux-dominated flow and a non-magnetized massive ejecta. Shortly after the engine is turned on, a pair of shocks would be excited. The reverse shock quickly reaches the wind-acceleration region and disappears (in a few seconds), whereas the forward shock soon breaks out from the ejecta (in $10^2$ - $10^3$ seconds) and continues to propagate in the surrounding interstellar medium. Most of the energy injected into the blastwave from the engine is stored as magnetic energy and kinetic energy. The internal energy fraction is $f_{\rm int} < 0.3$ for an ejecta mass equal to $10^{-3}M_{\odot}$. Overall, the energy injecting efficiency $ξ$ is at most $\sim 0.6$ and can be as small as $\sim 0.04$ at later times. Contrary to the previous assumption, efficient heating only happens before the forward shock breaks out of the ejecta with a heating efficiency $ξ_t \sim (0.006 - 0.3)$, which rapidly drops to $\sim 0$ afterwards. The engine-fed kilonova lightcurves will be carefully studied in Paper II.

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arXiv2022

Thermal instability in radiation hydrodynamics: instability mechanisms, position-dependent S-curves, and attenuation curves

Local thermal instability can plausibly explain the formation of multiphase gas in many different astrophysical environments, but the theory is only well understood in the optically thin limit of the equations of radiation hydrodynamics (RHD). Here we lay groundwork for transitioning from this limit to a full RHD treatment assuming a gray opacity formalism. We consider a situation where the gas becomes thermally unstable due to the hardening of the radiation field when the main radiative processes are free-free cooling and Compton heating. We identify two ways in which this can happen: (i) when the Compton temperature increases with time, through a rise in either the intensity or energy of a hard X-ray component; and (ii) when attenuation reduces the flux of the thermal component so that the Compton temperature increases with depth through the slab. Both ways likely occur in the broad line region of active galactic nuclei where columns of gas can be ionization bounded. In such instances where attenuation is significant, thermal equilibrium solution curves become position-dependent and it no longer suffices to assess the stability of an irradiated column of gas at all depths using a single equilibrium curve. We demonstrate how to analyze a new equilibrium curve -- the attenuation curve -- for this purpose, and we show that by Field's instability criterion, a negative slope along this curve indicates that constant density slabs are thermally unstable whenever the gas temperature increases with depth.

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