arXiv subjects

Pin-Gao Gu

Publications and source records attributed to Pin-Gao Gu.

At least 19 recordsLinked to original sources

arXiv2026

Resonant Super-Earths Dancing With EKL Oscillations: TTV Phase Excitation and Resonance Disruption by EKL Interactions between a Cold Jupiter and Stellar Companion

Near-resonant Kepler planets are dynamically hot, as evidenced by nonzero transit timing variation (TTV) phases, indicating that free eccentricities are not damped. Recent observations suggest that circulating near-resonant planets tend to be dynamically unstable, and hence dynamically hot, likely representing an intermediate stage in the close-in super-Earth population at young ages. We investigate whether a cold Jupiter interacting with a stellar companion through the eccentric Kozai-Lidov mechanism (EKL) can excite TTV phases and increase the libration amplitude of resonant angles in close-in resonant pairs. We find that the EKL model that drives the observed eccentricity of cold Jupiters can also excite TTV phases, increase the libration amplitude of resonant angles away from ideal geometric alignment, and even disrupt them in a significant fraction of planetary systems in our simulated samples over 16 Myr. We also find that the TTV phases of the resonant pairs tend to be small (< 90 degrees), while the resonant angles are more easily elevated to become circulating during EKL excitation.

astro-ph.EP↗

arXiv2025

Peculiar Disk Substructures Associated with the Young Eruptive Star EX Lupi

Young eruptive stars such as EXors undergo dramatic accretion outbursts characterized by sudden optical brightenings, yet the underlying physical mechanism remains uncertain. We present high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 and 4 continuum observations of EX Lupi, the prototypical EXor-type variable, reconstructed using super-resolution imaging with sparse modeling. Our images reveal, for the first time, two distinct substructures: a compact, crescent-shaped inner arc within 10 au of the star, and a narrow outer ring at 30 au. The inner arc is strongly elongated and casts a shadow observed in the Very Large Telescope/SPHERE near-infrared scattered light. The outer ring exhibits a radial width comparable to the local pressure scale height, consistent with moderately efficient dust trapping. Geometric and thermal analysis of the disk surface, based on combined ALMA and SPHERE data, indicates that the disk is moderately flared with an average disk temperature consistent with that of classical T Tauri disks. The observed substructures suggest dynamical perturbations-plausibly induced by a massive companion companion-that may modulate accretion rates through gravitational interaction with the inner arc. These findings provide morphological evidence linking disk substructure to episodic accretion in the structurally mature disk.

astro-ph.EP↗

arXiv2025

Investigating the influence of the radiative torque disruption on the size evolution of dust in the heliosphere

In this paper, we conduct a detailed study on the effect of Radiative Torque Disruption (RATD) mechanism on the fragmentation of micrometer-sized dust grains into nanoparticles within the heliosphere. We start by estimating the disruption timescales for dust grains under various centrifugal stresses. Our numerical calculations demonstrate that RATD is a highly effective mechanism for breaking down micrometer-sized grains, producing nanoparticles more efficiently than other fragmentation processes. RATD also prevents micrometer-sized grains from being expelled by radiation pressure. Our findings indicate that the location of the present water snow line depends not only on temperature but also on the size of dust grains. For smaller grains, the snow line can shift outward beyond the position defined by thermal sublimation. Furthermore, we model the size distribution of dust grains modified by the RATD mechanism using a simplified model, showing that rotational disruption significantly decreases the number density of micrometer-sized grains while substantially increasing the number density of sub-micrometer-sized grains. However, the fraction of dust grains aligned at high-$J$ attractors by radiative torques less than 80\% can considerably weaken the effect of RATD on the grain size distribution. Finally, we suggest several experiments that could potentially test the RATD mechanism and discuss the uncertainties of our model in more realistic applications to heliospheric dust.

astro-ph.EP↗

arXiv2024

ALMA Observations of Proper Motions of the Dust Clumps in the Protoplanetary Disk MWC 758

To study the dust dynamics in the dust trapping vortices in the protoplanetary disk around MWC~758, we analyzed the 1.3 mm continuum images of the MWC~758 disk obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) in 2017 and 2021. We detect proper motions of 22 mas and 24 mas in the two dust clumps at radii of 0\farcs32 and 0\farcs54 in the disk on the plane of the sky, respectively. On the assumption that the dust clumps are located in the disk midplane, the velocities of the observed proper motions along the azimuthal direction of the inner and outer dust clumps are sub- and super-Keplerian, respectively, and both have angular velocities corresponding to the Keplerian angular velocity at a radius of $0\farcs46\pm0\farcs04$. This deviation from the Keplerian motion is not expected in the conventional theory of vortices formed by the Rossby wave instability. The observed non-Keplerian proper motions of the dust clumps are unlikely due to the disk warp and eccentricity, nor be associated with any predicted planets. The two dust clumps are likely spatially coincident with the infrared spirals. In addition, we detect the changes in the intensity profiles of the dust clumps over the four-year span. Therefore, we suggest that the observed proper motions are possibly due to changes in the density distributions in the dust clumps caused by their interaction with the spirals in the disk.

astro-ph.EP↗

arXiv2023

Semi-analytic calculations for extended mid-Infrared emission associated with FU Ori-type objects

(Aims) Near-infrared imaging polarimetry at high-angular resolutions has revealed intriguing distribution of circumstellar dust towards FU Ori-type objects (FUors). These dust grains are probably associated with either an accretion disk or an infalling envelope. Follow-up observations in the mid-infrared would lead us to a better understanding of the hierarchy of the mass accretion processes onto FUors (i.e., envelope and disk accretion), which hold keys for understanding the mechanism of their accretion outbursts and the growth of low-mass young stellar objects in general. (Method) We have developed a semi-analytic method to estimate the mid-infrared intensity distributions using the observed polarized intensity (PI) distributions in H-band (lambda=1.65 micron). We have derived intensity distributions for two FUors, FU Ori and V1735 Cyg, at three wavelengths (lambda=3.5/4.8/12 micron) for various cases, i.e. with a star or a flat compact self-luminous disk as an illuminating source; an optically thick disk or an optically thin envelope for circumstellar dust grains; and three different dust models. (Results) We have been able to obtain self-consistent results for many cases and regions, in particular when the viewing angle of the disk/envelope is zero (face-on). Our calculations suggest that the mid-infrared extended emission at the above wavelengths is dominated by the single scattering process. The contribution of thermal emission is negligible unless we add an additional heating mechanism such as adiabatic heating in spiral structures and/or fragments. The uncertain nature of the central illuminating source, the distribution of circumstellar dust grains and the optical properties of dust grains yield uncertainties in the intensity levels on orders of magnitude, e.g., 20-800, for the disk/envelope aspect ratios of ~0.2 and lambda=3-13 micron.

astro-ph.SR↗

arXiv2023

Deuterium Escape on Photoevaporating Sub-Neptunes

We investigate the evolution of the deuterium-to-hydrogen (D/H) mass ratio driven by EUV photoevaporation of hydrogen-rich atmospheres of close-in sub-Neptunes around solar-type stars. For the first time, the diffusion-limited approach in conjunction with energy-limited photoevaporation is considered in evaluating deuterium escape from evolving exoplanet H/He envelopes. We find that the planets with smaller initial gas envelopes and thus smaller sizes can lead to weaker atmospheric escape, which facilitates hydrogen-deuterium fractionation. Specifically, in our grid of simulations with a low envelope mass fraction less than 0.005, a low-mass sub-Neptune (4-$5M_\oplus$) at about 0.25-0.4 au or a high-mass sub-Neptune (10-$15M_\oplus$) at about 0.1-0.25 au can increase the D/H values by greater than 20% over 7.5 Gyr. Akin to the helium-enhanced envelopes of sub-Neptunes due to photoevaporating escape, the planets along the upper boundary of the radius valley are the best targets to detect high D/H ratios. The ratio can rise by a factor of $\lesssim$ 1.65 within 7.5 Gyrs in our grid of evolutionary calculations. The D/H ratio is expected to be higher in thinner envelopes as long as the planets do not become bare rocky cores.

astro-ph.EP↗

arXiv2022

Kinematical Constraint on Eccentricity in the Protoplanetary Disk MWC 758 with ALMA

We analyzed the archival data of the $^{13}\mathrm{CO}$ and $\mathrm{C}^{18}\mathrm{O}$ $J=3-2$ emission lines in the protoplanetary disk around MWC 758 obtained with the Atacama Large Millimeter/submillimeter Array to discuss possible planet-disk interaction and non-Keplerian motion in the disk. We performed fitting of a Keplerian disk model to the observational data and measured the velocity deviations from the Keplerian rotation. We found significant velocity deviations around the inner cavity in the MWC 758 disk. We examined several possibilities that may cause the velocity deviations, such as pressure gradient, height of the emitting layer, infall motion, inner warp, and eccentricity in the disk. We found that the combination of an eccentric orbital motion with eccentricity of $0.1\pm0.04$ at the radius of the inner cavity and an infalling flow best explains the observed velocity deviations. Our kinematically constrained eccentricity of the gas orbital motion close to the inner cavity is consistent with the eccentricity of the dust ring around the inner cavity measured in the submillimeter continuum emission. Our results hint at strong dust-gas coupling around the inner cavity and presence of a gas giant planet inside the inner cavity in the MWC 758 disk.

astro-ph.EP↗

arXiv2022

Revealing the drag instability in one-fluid nonideal MHD simulations of a 1D isothermal C-shock

C-type shocks are believed to be ubiquitous in turbulent molecular clouds thanks to ambipolar diffusion. We investigate whether the drag instability in 1D isothermal C-shocks, inferred from the local linear theory of Gu & Chen, can appear in non-ideal magnetohydrodynamic simulations. Two C-shock models (with narrow and broad steady-state shock widths) are considered to represent the typical environment of star-forming clouds. The ionization-recombination equilibrium is adopted for the one-fluid approach. In the 1D simulation, the inflow gas is continuously perturbed by a sinusoidal density fluctuation with a constant frequency. The perturbations clearly grow after entering the C-shock region until they start being damped at the transition to the postshock region. We show that the profiles of a predominant Fourier mode extracted locally from the simulated growing perturbation match those of the growing mode derived from the linear analysis. Moreover, the local growth rate and wave frequency derived from the predominant mode generally agree with those from the linear theory. Therefore, we confirm the presence of the drag instability in simulated 1D isothermal C-shocks. We also explore the nonlinear behavior of the instability by imposing larger-amplitude perturbations to the simulation. We find that the drag instability is subject to wave steepening, leading to saturated perturbation growth. Issues concerning local analysis, nonlinear effects, one-fluid approach, and astrophysical applications are discussed.

astro-ph.GA↗

arXiv2021

Size evolution of close-in super-Earths through giant impacts and photoevaporation

The Kepler transit survey with follow-up spectroscopic observations has discovered numerous super-Earth sized planets and revealed intriguing features of their sizes, orbital periods, and their relations between adjacent planets. For the first time, we investigate the size evolution of planets via both giant impacts and photoevaporation to compare with these observed features. We calculate the size of a protoplanet, which is the sum of its core and envelope sizes, by analytical models. $N$-body simulations are performed to evolve planet sizes during the giant impact phase with envelope stripping via impact shocks. We consider the initial radial profile of the core mass and the initial envelope mass fractions as parameters. Inner planets can lose their whole envelopes via giant impacts, while outer planets can keep their initial envelopes since they do not experience giant impacts. Photoevaporation is simulated to evolve planet sizes afterward. Our results suggest that the period-radius distribution of the observed planets would be reproduced if we perform simulations in which the initial radial profile of the core mass follows a wide range of power-law distributions and the initial envelope mass fractions are $\sim0.1$. Moreover, our model shows that the adjacent planetary pairs have similar sizes and regular spacings, with slight differences from detailed observational results such as the radius gap.

astro-ph.EP↗

arXiv2021

The Drag Instability in a 2D Isothermal C-shock

We extend the linear analysis of the drag instability in a 1D perpendicular isothermal C-shock by Gu & Chen to 2D perpendicular and oblique C-shocks in the typical environment of star-forming clouds. Simplified dispersion relations are derived for the unstable modes. We find that the mode property of the drag instability generally depends on the ratio of the transverse (normal to the shock flow) to longitudinal (along the shock flow) wavenumber. For the transversely large-scale mode, the growth rate and wave frequency of the drag instability in a 2D shock resemble those in a 1D shock. For the transversely small-scale mode, the drag instability is characterized by an unstable mode coupled with an acoustic mode primarily along the transverse direction. When the shock is perpendicular or less oblique, there exists a slowly propagating mode, which can potentially grow into a nonlinear regime and contribute to the maximum growth of the instability. In contrast, when the shock is more oblique, this slowly propagating unstable mode disappears, and the maximum growth of the drag instability is likely contributed from the transversely large-scale mode (i.e., almost 1D mode). In all cases that we consider, the magnitude of the density perturbations is significantly larger than that of the velocity and magnetic field perturbations, implying that the density enhancement governs the dynamics in the linear regime of the instability. A few issues in the linear analysis, as well as the possible astrophysical implications, are also briefly discussed.

astro-ph.SR↗

arXiv2020

Formation of multiple-planet systems in resonant chains around M dwarfs

Recent observations have revealed the existence of multiple-planet systems composed of Earth-mass planets around late M dwarfs. Most of their orbits are close to commensurabilities, which suggests that planets were commonly trapped in resonant chains in their formation around low-mass stars. We investigate the formation of multiple-planet systems in resonant chains around low-mass stars. A time-evolution model of the multiple-planet formation via pebble accretion in the early phase of the disk evolution is constructed based on the formation model for the TRAPPIST-1 system by Ormel et al. (2017). Our simulations show that knowing the protoplanet appearance timescale is important for determining the number of planets and their trapped resonances: as the protoplanet appearance timescale increases, fewer planets are formed, which are trapped in more widely separated resonances. We find that there is a range of the protoplanet appearance timescale for forming the stable multiple-planet systems in resonant chains. This range depends on the stellar mass and disk size. We suggest that the protoplanet appearance timescale is a key parameter for studying the formation of multiple-planet systems with planets in resonant chains around low-mass stars. The composition of the planets in our model is also discussed.

astro-ph.EP↗

arXiv2020

Kinematical signs of dust trapping and feedback in a local pressure bump in the protoplanetary disk around HD 142527 revealed with ALMA

We analyzed the archival data of the continuum emission at six wavelengths from 3 to 0.4 mm and 13CO and C18O (1-0, 2-1, and 3-2) lines in the protoplanetary disk around HD 142527 obtained with the Atacama Large Millimeter/submillimeter Array. We performed fitting to the spectral energy distributions obtained at the six wavelengths with the gray-body slab models to estimate the distributions of the dust surface density and spectral index of dust absorption coefficient beta. We also estimated the distribution of the gas column density by fitting the C18O spectra and measured the disk rotation by fitting the Keplerian disk models to the C18O data. We found super- and sub-Keplerian rotation inside and outside the dust ring in the northwest in the HD 142527 disk, suggestive of the presence of a local pressure bump. In comparison with our estimated dust and gas distributions, the location of the pressure bump is coincident with the region showing a three times higher dust density and a three times lower gas-to-dust mass ratio than the mean values in the disk, suggesting dust trapping in the pressure bump. Nevertheless, there is no correlation between our derived beta distribution and the location of the pressure bump. In addition, we found that the width of the dust ring is comparable or larger than the width of the pressure bump, which could suggest that dust feedback is significant in the pressure bump.

astro-ph.EP↗

arXiv2020

Ejection of close-in super-Earths around low-mass stars in the giant impact stage

Earth-sized planets were observed in close-in orbits around M dwarfs. While more and more planets are expected to be uncovered around M dwarfs, theories of their formation and dynamical evolution are still in their infancy. We investigate the giant impact growth of protoplanets, which includes strong scattering around low-mass stars. The aim is to clarify whether strong scattering around low-mass stars affects the orbital and mass distributions of the planets. We perform $N$-body simulation of protoplanets by systematically surveying the parameter space of the stellar mass and surface density of protoplanets. We find that protoplanets are often ejected after twice or three times close-scattering around late M dwarfs. The ejection sets the upper limit of the largest planet mass. Adopting the surface density scaling linearly with the stellar mass, we find that as the stellar mass decreases less massive planets are formed in orbits with higher eccentricities and inclinations. Under this scaling, we also find that a few close-in protoplanets are generally ejected. The ejection of protoplanets plays an important role in the mass distribution of super-Earths around late M dwarfs. The mass relation of observed close-in super-Earths and their central star mass is well reproduced by ejection.

astro-ph.EP↗

arXiv2020

The Drag Instability in a 1D Isothermal C-Shock

We investigate whether the drag instability, proposed by Gu et al., occurs in a one-dimensional (1D) C-shock. The 1D background model proposed by Chen and Ostriker for a steady isothermal C-shock is adopted, and a 1D isothermal linear analysis is performed. We confirm the postulation of Gu et al. that the drift velocity between the ions and the neutrals is sufficiently high within a C-shock to allow for the drag instability. We also study the underlying physics of the decaying modes in the shock and post-shock regions. The drag instability is an overstability phenomenon associated with an exponentially growing mode of a propagating wave. We find that the growing wave mode can only propagate downstream within the shock and subsequently decay in the post-shock region. The maximum total growth (MTG) for such an unstable wave before it is damped is estimated in typical environments of star-forming clouds, which is approximately 10-30 times larger than the initial perturbation at the modest shock velocities and can be significantly enhanced several hundred times for a stronger C-shock with a larger width.

astro-ph.SR↗

arXiv2020

Effect of Dust Rotational Disruption by Radiative Torques and Implications for F-corona decrease revealed by the Parker Solar Probe

The first-year results from the Parker Solar Probe (PSP) reveal a gradual decrease of F-coronal dust from distances of $D=0.166-0.336$ AU (or the inner elongations of $\sim 9.22- 18.69~R_{\odot}$) to the Sun (Howard et al. 2019). Such a F-corona decrease cannot be explained by the dust sublimation scenario of the popular silicate composition that implies a dust-free-zone of boundary at heliocentric radius $R\lesssim 4-5R_{\odot}$, but may be explained by appealing to various dust compositions with different sublimation fronts. In this paper, we present an additional explanation for the F-corona decrease using our newly introduced mechanism of dust destruction so-called Radiative Torque Disruption (RATD) mechanism. We demonstrate that RATD rapidly breaks large grains into nanoparticles so that they can be efficiently destroyed by nonthermal sputtering induced by bombardment of energetic protons from slow solar winds, which extends the dust-free-zone established by thermal sublimation to $R_{dfz}\sim 8R_{\odot}$. Beyond this extended dust-free-zone, we find that the dust mass decreases gradually from $R\sim 42R_{\odot}$ toward the Sun due to partial removal of nanodust by nonthermal sputtering. The joint effect of RATD and nonthermal sputtering can successfully reproduce the gradual decrease of the F-corona between $19-9R_{\odot}$ observed by the PSP. Finally, the RATD mechanism can efficiently produce nanoparticles usually observed in the inner solar system.

astro-ph.SR↗

arXiv2019

Modeling the Thermal Bulge of A Hot Jupiter with the Two-Stream Approximation

We revisit the problem of thermal bulge of asynchronous hot Jupiters, using HD 209458 b as a fiducial study. We improve upon previous works by using a double-gray atmosphere model and interior structure from MESA as the background state, and then solve for the thermal bulge in response to the semidiurnal component of stellar insolation. The atmosphere model is based on the radiative transfer with Eddington's two-stream approximation. Two opacity cases are considered: the first introduces a greenhouse effect and the second exhibits a strong temperature inversion. We find that for the predominant thermal bulges excited by g-modes of lower orders, our results are qualitatively similar to the adiabatic results from Arras and Socrates (2010). It arises because the perturbed heating due to self-absorption of thermal emissions can be significant (i.e., greenhouse effect) against Newtonian damping, thereby leading to almost undamped thermal bulges. We also find that the contribution to the thermal bulge from the evanescent waves in the convective zone is not negligible, implying that the thermal bulge is not merely confined in the atmosphere and radiative envelope. Assuming the torque balance between the thermal and gravitational bulges, we estimate the tidal quality factor of the planet for gravitational tides to match the observed radius. The limitations of our model are also briefly discussed.

astro-ph.EP↗

arXiv2019

HL Tau disk in HCO+ (3-2) and (1-0) with ALMA: gas density, temperature, gap, and one-arm spiral

We present our observational results of the 1.1 mm continuum and the HCO+ (3-2) line in HL Tau at angular resolutions of 0.1" obtained with ALMA and our data analysis of the 2.9 mm and 1.1 mm continuum and the HCO+ (3-2) and (1-0) lines of the HL Tau disk. The Keplerian rotation of the HL Tau disk is well resolved in the HCO+ (3-2) emission, and the stellar mass is estimated to be 2.1+/-0.2 Msun with a disk inclination angle of 47 deg. The radial profiles of the HCO+ column density and excitation temperature are measured with the LTE analysis of the two transitions of the HCO+ emission. An HCO+ gas gap at a radius of 30 au, where the column density drops by a factor of 4-8, is found in the HCO+ column density profile, coincident with the dust gap traced by the continuum emission. No other clear HCO+ gas gap is seen. This HCO+ gas gap can be opened by a planet with mass of 0.5-0.8 Mj, which is comparable to the planet mass adopted in numerical simulations to form the dust gap at the same radius in the HL Tau disk. In addition to the disk component, a one-arm spiral with a length of ~3" (520 au) stretching out from the inner disk is observed in the HCO+ (3-2) emission. The observed velocity structures along the spiral suggest an infalling and rotating gas stream toward the inner disk.

astro-ph.SR↗

arXiv2019

Signs of outflow feedback from a nearby young stellar object on the protostellar envelope around HL Tau

HL Tau is a Class I-II protostar embedded in an infalling and rotating envelope and possibly associated with a planet forming disk, and it is co-located in a 0.1 pc molecular cloud with two nearby young stellar objects. Our ALMA observations revealed two arc-like structures on a 1000 au scale connected to the disk, and their kinematics could not be explained with any conventional model of infalling and rotational motions. In this work, we investigate the nature of these arc-like structures connected to the HL Tau disk. We conducted new observations in the 13CO and C18O (3-2; 2-1) lines with JCMT and IRAM 30m, and obtained the ACA data with the 7-m array. With the single-dish, ACA, and ALMA data, we analyzed the gas motions on both 0.1 pc and 1000 au scales in the HL Tau region. We constructed new kinematical models of an infalling and rotating envelope with the consideration of relative motion between HL Tau and the envelope. By including the relative motion between HL Tau and its protostellar envelope, our kinematical model can explain the observed velocity features in the arc-like structures. The morphologies of the arc-like structures can also be explained with an asymmetric initial density distribution in our model envelope. In addition, our single-dish results support that HL Tau is located at the edge of a large-scale (0.1 pc) expanding shell driven by the wind or outflow from XZ Tau, as suggested in the literature. The estimated expanding velocity of the shell is comparable to the relative velocity between HL Tau and its envelope in our kinematical model. These results hints that the large-scale expanding motion likely impacts the protostellar envelope around HL Tau and affects its gas kinematics. We found that the mass infalling rate from the envelope onto the HL Tau disk can be decreased by a factor of two due to this impact by the large-scale expanding shell.

astro-ph.SR↗