arXiv subjects

Jeffrey Fung

Publications and source records attributed to Jeffrey Fung.

At least 19 recordsLinked to original sources

arXiv2026

${}^{12}$CO Ro-vibrational Spectroscopy of AB Aurigae -- A Potential Point Source is Present

The Herbig Ae star AB Aurigae hosts a vast, low-inclination protoplanetary disk that exhibits a plethora of substructures, including the protoplanet candidate AB Aur b. We present M-band spectroscopic data taken with NASA IRTF from Feb 2024 covering multiple position angles that captured emission from an off-centered, low temperature, and compact source. Analysis of the ${}^{12}$CO $\nu=$1-0 low-J ro-vibrational emission line profiles and spectroastrometric signals localizes the source at around an orbital radius of 65 au and a position angle of 143$^\circ$. These coordinates are distinctly different from those of AB Aur b, which was not detected. Although there is no obvious explanation for the detected source, if we assume it was a circumplanetary disk, then its maximum temperature would be about 550 K and its maximum radius would be about 5 au. Our results alludes to a previously unknown companion that may be residing in the AB Aurigae system.

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arXiv2026

On Eccentric Protoplanetary Disks I -- How Eccentric are Planet-Perturbed Disks?

Protoplanetary disks can become eccentric when planets open deep gaps within, but how eccentric are they? We answer this question by analyzing two-dimensional hydrodynamical simulations of planet-disk interaction. The steady state eccentricity of the outer disk (outside of the planet's orbit) is described as a balance between eccentricity excitation by the 1:3 eccentric Lindblad resonance and eccentricity damping by gas pressure. This eccentricity scales with $q(\frac{h_p}{r_p})^{(-1)}(\frac{r_{gap}}{r_p})^{(a-\frac{b}{2}-2)}$, where $q$ is the planet-to-star mass ratio, $\frac{h_p}{r_p}$ is the disk aspect ratio, $\frac{r_{gap}}{r_p}$ is the radial position of the outer gap edge divided by the planet's position, and $a$ and $b$ are the negative exponents in the disk's surface density and temperature power law profiles, respectively. We derive a semi-analytic eccentricity profile that agrees with numerical simulations to within 30%. Our result is a first step to quantitatively interpret observations of eccentric protoplanetary disks, such as MWC 758, HD 142527, IRS 48, and CI Tau.

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arXiv2023

Spectroastrometric Survey of Protoplanetary Disks with Inner Dust Cavities

We present high-resolution spectra and spectroastrometric (SA) measurements of fundamental rovibrational CO emission from nine nearby ($\lesssim$300 pc) protoplanetary disks where large inner dust cavities have been observed. The emission line profiles and SA signals are fit with a slab disk model that allows the eccentricity of the disk and intensity of the emission to vary as power laws. Six of the sources are well fit with our model, and three of these sources show asymmetric line profiles that can be fit by adopting a non-zero eccentricity. The three other sources have components in either their line profile or SA signal that are not captured by our disk model. Two of these sources (V892 Tau, CQ Tau) have multi-epoch observations that reveal significant variability. CQ Tau and AB Aur have CO line profiles with centrally-peaked components that are similar to line profiles that have been interpreted as evidence of molecular gas arising from a wide-angle disk wind. Alternatively, emission from a circumplanetary disk (CPD) could also account for this component. The interpretations of these results can be clarified in the future with additional epochs that will test the variability timescale of these SA signals. We discuss the utility of using high-resolution spectroscopy for probing the dynamics of gas in the disk and the scenarios that can give rise to profiles that are not fit with a simple disk model.

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arXiv2023

A Systematic Study of Planetary Envelope Growth with 3D Radiation-Hydrodynamics Simulations

In the core accretion model of planet formation, envelope cooling regulates the accretion of material and ultimately sets the timescale to form a giant planet. Given the diversity of planet-forming environments, opacity uncertainties, and the advective transport of energy by 3-dimensional recycling flows, it is unclear whether 1D models can adequately describe envelope structure and accretion in all regimes. Even in 3D models, it is unclear whether approximate radiative transfer methods sufficiently model envelope cooling particularly at the planetary photosphere. To address these uncertainties, we present a suite of 3D radiation hydrodynamics simulations employing methods that directly solve the transfer equation. We perform a parameter space study, formulated in terms of dimensionless parameters, for a variety of envelope optical depths and cooling times. We find that the thermodynamic structure of the envelope ranges from adiabatic to isothermal based on the cooling time and by extension, the background disk temperature and density. Our models show general agreement with 1D static calculations, suggesting a limited role of recycling flows in determining envelope structure. By adopting a dimensionless framework, these models can be applied to a wide range of formation conditions and assumed opacities. In particular, we dimensionalize them to the case of a super-Earth and proto-Jupiter and place upper limits on the 3D mass accretion rates prior to runaway growth. Finally, we evaluate the fidelity of approximate radiative transfer methods and find that even in the most challenging cases, more approximate methods are sufficiently accurate and worth their savings in computational cost.

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arXiv2023

Ro-vibrational Spectroscopy of CI Tau -- Evidence of a Multi-Component Eccentric Disk Induced by a Planet

CI Tau is currently the only T Tauri star with an inner protoplanetary disk that hosts a planet, CI Tau b, that has been detected by a radial velocity survey. This provides the unique opportunity to study disk features that were imprinted by that planet. We present multi-epoch spectroscopic data, taken with NASA IRTF in 2022, of the ${}^{12}$CO and hydrogen Pf$β$ line emissions spanning 9 consecutive nights, which is the proposed orbital period of CI Tau b. We find that the star's accretion rate varied according to that 9~d period, indicative of companion driven accretion. Analysis of the ${}^{12}$CO emission lines reveals that the disk can be described with an inner and outer component spanning orbital radii 0.05-0.13~au and 0.15-1.5~au, respectively. Both components have eccentricities of about 0.05 and arguments of periapses that are oppositely aligned. We present a proof-of-concept hydrodynamic simulation that shows a massive companion on a similarly eccentric orbit can recreate a similar disk structure. Our results allude to such a companion being located around an orbital distance of 0.14~au. However, this planet's orbital parameters may be inconsistent with those of CI Tau b whose high eccentricity is likely not compatible with the low disk eccentricities inferred by our model.

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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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arXiv2022

Planet-Disk Interactions

Planet-disk interactions, where an embedded massive body interacts gravitationally with the protoplanetary disk it was formed in, can play an important role in reshaping both the disk and the orbit of the planet. Spiral density waves are launched into the disk by the planet, which, if they are strong enough, can lead to the formation of a gap. Both effects are observable with current instruments. The back-reaction of perturbations induced in the disk, both wave-like and non-wavelike, is a change in orbital elements of the planet. The efficiency of orbital migration is a long-standing problem in planet formation theory. We discuss recent progress in planet-disk interactions for different planet masses and disk parameters, in particular the level of turbulence, and progress in modeling observational signatures of embedded planets.

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arXiv2022

Dust Dynamics in Transitional Disks: Clumping and Disk Recession

The role of radiation pressure in dust migration and the opening of inner cavities in transitional disks is revisited in this paper. Dust dynamics including radiation pressure is often studied in axisymmetric models, but in this work, we show that highly non-axisymmetric features can arise from an instability at the inner disk edge. Dust grains clump into high density features there, allowing radiation to leak around them and penetrate deeper into the disk, changing the course of dust migration. Our proof-of-concept, two-dimensional, vertically-averaged simulations show that the combination of radiation pressure, shadowing, and gas drag can produce a net outward migration, or recession, of the dust component of the disk. The recession speed of the inner disk edge is on the order of $10^{-5}$ times Keplerian speed in our parameter space, which is faster than the background viscous flow, assuming a Shakura & Sunyaev viscosity alpha $\lesssim 10^{-3}$. This speed, if sustained over the lifetime of the disk, can result in a dust cavity as large as tens of au.

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arXiv2021

Cooling-Induced Vortex Decay in Keplerian Disks

Vortices are readily produced by hydrodynamical instabilities, such as the Rossby wave instability, in protoplanetary disks. However, large-scale asymmetries indicative of dust-trapping vortices are uncommon in sub-millimeter continuum observations. One possible explanation is that vortices have short lifetimes. In this paper, we explore how radiative cooling can lead to vortex decay. Elliptical vortices in Keplerian disks go through adiabatic heating and cooling cycles. Radiative cooling modifies these cycles and generates baroclinicity that changes the potential vorticity of the vortex. We show that the net effect is typically a spin down, or decay, of the vortex for a sub-adiabatic radial stratification. We perform a series of two-dimensional shearing box simulations, varying the gas cooling (or relaxation) time, $t_{\rm cool}$, and initial vortex strength. We measure the vortex decay half-life, $t_{\rm half}$, and find that it can be roughly predicted by the timescale ratio $t_{\rm cool}/t_{\rm turn}$, where $t_{\rm turn}$ is the vortex turnaround time. Decay is slow in both the isothermal ($t_{\rm cool}\ll t_{\rm turn}$) and adiabatic ($t_{\rm cool}\gg t_{\rm turn}$) limits; it is fastest when $t_{\rm cool}\sim0.1\,t_{\rm turn}$, where $t_{\rm half}$ is as short as $\sim300$ orbits. At tens of au where disk rings are typically found, $t_{\rm turn}$ is likely much longer than $t_{\rm cool}$, potentially placing vortices in the fast decay regime.

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arXiv2021

Observational Signatures of Planets in Protoplanetary Disks: Temperature structures in spiral arms

High-resolution imaging of protoplanetary disks has unveiled a rich diversity of spiral structure, some of which may arise from disk-planet interaction. Using 3D hydrodynamics with $β$-cooling to a vertically-stratified background, as well as radiative-transfer modeling, we investigate the temperature rise in planet-driven spirals. In rapidly cooling disks, the temperature rise is dominated by a contribution from stellar irradiation, 0.3-3% inside the planet radius but always <0.5% outside. When cooling time equals or exceeds dynamical time, however, this is overwhelmed by hydrodynamic PdV work, which introduces a 10-20% perturbation within a factor of 2 from the planet's orbital radius. We devise an empirical fit of the spiral amplitude $Δ(T)$ to take into account both effects. Where cooling is slow, we find also that temperature perturbations from buoyancy spirals -- a strictly 3D, non-isothermal phenomenon -- become nearly as strong as those from Lindblad spirals, which are amenable to 2D and isothermal studies. Our findings may help explain observed thermal features in disks like TW Hydrae and CQ Tauri, and underscore that 3D effects have a qualitatively important effect on disk structure.

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arXiv2020

The Three Dimensional Flow Field Around Planets on Eccentric Orbits

We investigate the properties of the hydrodynamic flow around eccentric protoplanets and compare them with the often assumed case of a circular orbit. To this end, we perform a set of 3D hydrodynamic simulations of protoplanets with small eccentricities ($e\leq 0.1$). We adopt an isothermal equation of state and concentrate resolution on the protoplanet to investigate flows down to the scale of the protoplanet's circumplanetary disk (CPD). We find enhanced prograde rotation exterior to the CPD for low planet masses undergoing subsonic eccentric motion. If the eccentricity is made large enough to develop a bow shock, this trend reverses and rotation becomes increasingly retrograde. The instantaneous eccentric flow field is dramatically altered compared to circular orbits. Whereas the latter exhibit a generic pattern of polar inflow and midplane outflow, the flow geometry depends on orbital phase in the eccentric case. For even the modest eccentricities tested here, the dominant source of inflow can come from the midplane instead of the poles. We find that the amount of inflow and outflow increases for higher $e$ and lower protoplanet masses, thereby recycling more gas through the planet's Bondi radius. These increased fluxes may increase the pebble accretion rate for eccentric planets up to several times that of the circular orbit rate. In response to eccentric motion, the structure and rotation of the planet's bound CPD remains unchanged. Because the CPD regulates the eventual accretion of gas onto the planet, we predict little change to the gas accretion rates between eccentric and circular planets.

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arXiv2020

Visualizing the Kinematics of Planet Formation

A stunning range of substructures in the dust of protoplanetary disks is routinely observed across a range of wavelengths. These gaps, rings and spirals are highly indicative of a population of unseen planets, hinting at the possibility of current observational facilities being able to capture planet-formation in action. Over the last decade, our understanding of the influence of a young planet on the dynamical structure of its parental disk has progressed significantly, revealing a host of potentially observable features which would betray the presence of a deeply embedded planet. In concert, recent observations have shown that subtle perturbations in the kinematic structure of protoplanetary disks are found in multiple sources, potentially the characteristic disturbances associated with embedded planets. In this work, we review the theoretical background of planet-disk interactions, focusing on the kinematical features, and the current methodologies used to observe these interactions in spatially and spectrally resolved observations. We discuss the potential pit falls of such kinematical detections of planets, providing best-practices for imaging and analysing interferometric data, along with a set of criteria to use as a benchmark for any claimed detection of embedded planets. We finish with a discussion on the current state of simulations in regard to planet-disk interactions, highlighting areas of particular interest and future directions which will provide the most significant impact in our search for embedded planets. This work is the culmination of the 'Visualizing the Kinematics of Planet Formation' workshop, held in October 2019 at the Center for Computational Astrophysics at the Flatiron Institute in New York City.

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arXiv2019

Circumplanetary Disk Dynamics in the Isothermal and Adiabatic Limits

Circumplanetary disks (CPDs) may be essential to the formation of planets, regulating their spin and accretion evolution. We perform a series of 3D hydrodynamics simulations in both the isothermal and adiabatic limits to systematically measure the rotation rates, sizes, and masses of CPDs as functions of $q_{\rm thermal}$, the ratio of the planet mass to the disk thermal mass. Our $q_{\rm thermal}$ ranges from 0.1 to 4; for our various disk temperatures, this corresponds to planet masses between 1 Earth mass and 4 Jupiter masses. Within this parameter space, we find that isothermal CPDs are disky and bound within $\sim$10\% of the planet's Bondi radius $r_{\rm B}$, with the innermost $\sim0.05\,r_{\rm B}$ in full rotational support. Adiabatic CPDs are spherical (therefore not actually "disks"), bound within $\sim0.2\,r_{\rm B}$, and mainly pressure-supported with rotation rates scaling linearly with $q_{\rm thermal}$; extrapolation suggests full rotational support of adiabatic envelopes at $\sim10\,q_{\rm thermal}$. Fast rotation and 3D supersonic flow render isothermal CPDs significantly different in structure from --- and orders of magnitude less massive than --- their 1D isothermal hydrostatic counterparts. Inside a minimum-mass solar nebula, even a maximally cooled, isothermal CPD around a 10 Earth-mass core may have less than 1 Earth mass, suggesting that gas giant formation may hinge on angular momentum transport processes in CPDs. Our CPD sizes and masses appear consistent with the regular satellites orbiting solar system giants.

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arXiv2019

A Staggered Semi-Analytic Method for Simulating Dust Grains Subject to Gas Drag

Numerical simulations of dust-gas dynamics are one of the fundamental tools in astrophysical research, such as the study of star and planet formation. It is common to find tightly coupled dust and gas in astrophysical systems, which demands that any practical integration method be able to take time steps $Δt$ much longer than the stopping time $t_{\rm s}$ due to drag. A number of methods have been developed to ensure stability in this stiff ($Δt\gg t_{\rm s}$) regime, but there remains large room for improvement in terms of accuracy. In this paper, we describe an easy-to-implement method, the "staggered semi-analytic method" (SSA), and conduct numerical tests to compare it to other implicit and semi-analytic methods, including the $2^{\rm nd}$ order implicit method and the Verlet method. SSA makes use of a staggered step to better approximate the terminal velocity in the stiff regime. In applications to protoplanetary disks, this not only leads to orders-of-magnitude higher accuracy than the other methods, but also provides greater stability, making it possible to take time steps 100 times larger in some situations. SSA is also $2^{\rm nd}$ order accurate and symplectic when $Δt \ll t_{\rm s}$. More generally, the robustness of SSA makes it applicable to linear dust-gas drag in virtually any context.

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arXiv2019

PDS 70: A transition disk sculpted by a single planet

The wide, deep cavities of transition disks are often believed to have been hollowed out by nascent planetary systems. PDS 70, a ${\sim}5$ Myr old transition disk system in which a multi-Jupiter-mass planet candidate at 22 au coexists with a ${\sim}30$ au gas and ${\sim}60$ au dust-continuum gap, provides a valuable case study for this hypothesis. Using the PEnGUIn hydrodynamics code, we simulate the orbital evolution and accretion of PDS 70b in its natal disk over the lifetime of the system. When the accreting planet reaches about 2.5 Jupiter masses, it spontaneously grows in eccentricity and consumes material from a wide swathe of the PDS 70 disk; radiative transfer post-processing with DALI shows that this accurately reproduces the observed gap profile. Our results demonstrate that super-Jupiter planets can single-handedly carve out transition disk cavities, and indicate that the high eccentricities measured for such giants may be a natural consequence of disk-planet interaction.

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arXiv2019

Observational Signatures of Planets in Protoplanetary Disks: Planet-Induced Line Broadening in Gaps

Protoplanets can produce structures in protoplanetary disks via gravitational disk-planet interactions. Once detected, such structures serve as signposts of planet formation. Here we investigate the kinematic signatures in disks produced by multi-Jupiter mass ($M_{\rm J}$) planets using 3D hydrodynamics and radiative transfer simulations. Such a planet opens a deep gap, and drives transonic vertical motions inside. Such motions include both a bulk motion of the entire half-disk column, and turbulence on scales comparable to and smaller than the scale height. They significantly broaden molecular lines from the gap, producing double-peaked line profiles at certain locations, and a kinematic velocity dispersion comparable to thermal after azimuthal averaging. The same planet does not drive fast vertical motions outside the gap, except at the inner spiral arms and the disk surface. Searching for line broadening induced by multi-$M_{\rm J}$ planets inside gaps requires an angular resolution comparable to the gap width, an assessment of the gap gas temperature to within a factor of 2, and a high sensitivity needed to detect line emission from the gap.

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arXiv2018

Subaru/HiCIAO $HK_{\rm s}$ imaging of LkH$α$ 330 - multi-band detection of the gap and spiral-like structures

We present $H$- and $K_{\rm s}$-bands observations of the LkH$α$ 330 disk with a multi-band detection of the large gap and spiral-like structures. The morphology of the outer disk ($r\sim$$0\farcs3$) at PA=0--45$^\circ$ and PA=180--290$^\circ$ are likely density wave-induced spirals and comparison between our observational results and simulations suggests a planet formation. We have also investigated the azimuthal profiles at the ring and the outer-disk regions as well as radial profiles in the directions of the spiral-like structures and semi-major axis. Azimuthal analysis shows a large variety in wavelength and implies that the disk has non-axisymmetric dust distributions. The radial profiles in the major-axis direction (PA=$271^\circ$) suggest that the outer region ($r\geq0\farcs25$) may be influenced by shadows of the inner region of the disk. The spiral-like directions (PA=10$^\circ$ and 230$^\circ$) show different radial profiles, which suggests that the surfaces of the spiral-like structures are highly flared and/or have different dust properties. Finally, a color-map of the disk shows a lack of an outer eastern region in the $H$-band disk, which may hint the presence of an inner object that casts a directional shadow onto the disk.

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arXiv2018

The Eccentric Cavity, Triple Rings, Two-Armed Spirals, and Double Clumps of the MWC 758 Disk

Spatially resolved structures in protoplanetary disks hint at unseen planets. Previous imaging observations of the transitional disk around MWC 758 revealed an inner cavity, a ring-like outer disk, emission clumps, and spiral arms, all possibly generated by companions. We present ALMA dust continuum observations of MWC 758 at 0.87 millimeter (mm) wavelength with 43$\times$39 mas angular resolution (6.9$\times$6.2 AU) and 20 $μ$Jy beam$^{-1}$ rms. The central sub-mm emission cavity is revealed to be eccentric; once deprojected, its outer edge can be well-fitted by an ellipse with an eccentricity of 0.1 and one focus on the star. The broad ring-like outer disk is resolved into three narrow rings with two gaps in between. The outer two rings tentatively show the same eccentricity and orientation as the innermost ring bounding the inner cavity. The two previously known dust emission clumps are resolved in both the radial and azimuthal directions, with radial widths equal to $\sim$4$\times$ the local scale height. Only one of the two spiral arms previously imaged in near-infrared (NIR) scattered light is revealed in ALMA dust emission, at a slightly larger stellocentric distance owing to projection effects. We also submit evidence of disk truncation at $\sim$100 AU based on comparing NIR imaging observations with models. The spirals, the north clump, and the truncated disk edge are all broadly consistent with the presence of one companion exterior to the spirals at roughly 100 AU.

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