Debris Disks Research Articles

Using debris disk observations to infer substellar companions orbiting within or outside a parent planetesimal belt (Corrigendum)

Using debris disk observations to infer substellar companions orbiting within or outside a parent planetesimal belt (Corrigendum)

Correction to: Early stages of gap opening by planets in protoplanetary discs

Correction to: Early stages of gap opening by planets in protoplanetary discs

Deep Search for a Scattered Light Dust Halo Around Vega with the Hubble Space Telescope

We present a provisory scattered-light detection of the Vega debris disk using deep Hubble Space Telescope (HST) coronagraphy (PID 16666). At only 7.7 pc, Vega is immensely important in debris disk studies both for its prominence and also because it allows the highest physical resolution among all debris systems relative to temperature zones around the star. We employ the STIS coronagraph’s widest wedge position and classical reference differential imaging to achieve among the lowest surface-brightness sensitivities to date ( ∼4μJyarcsec−2 ) at wide separations using 32 orbits in Cycle 29. We detect a halo extending from the inner edge of our effective inner working angle at 10.″5 out to the photon noise floor at 30″ (80–230 au). The face-on orientation of the system and the lack of a perfectly color-matched point-spread function star have posed significant challenges to the reductions, particularly regarding artifacts from the imperfect color matching. However, we find that a halo of small dust grains provides the best explanation for the observed signal. Unlike Fomalhaut (a close twin to Vega in luminosity, distance, and age), there is no clear distinction in scattered light between the parent planetesimal belt observed with the Atacama Large Millimeter/submillimeter Array and the extended dust halo. These HST observations complement JWST GTO Cycle 1 observations of the system with NIRCam and MIRI.

Dust mass in protoplanetary disks with porous dust opacities

Atacama Large Millimeter/submillimeter Array surveys have suggested that protoplanetary disks are not massive enough to form the known exoplanet population, based on the assumption that the millimeter continuum emission is optically thin. In this work, we investigate how the mass determination is influenced when the porosity of dust grains is considered in radiative transfer models. The results show that disks with porous dust opacities yield similar dust temperatures, but systematically lower millimeter fluxes, as compared to disks that incorporate compact dust grains. Moreover, we have recalibrated the relation between dust temperature and stellar luminosity for a wide range of stellar parameters. We also calculated the dust masses of a large sample of disks using the traditionally analytic approach. The median dust mass from our calculation is about six times higher than the literature result, and this is mostly driven by the different opacities of porous and compact grains. A comparison of the cumulative distribution function between disk dust masses and exoplanet masses shows that the median exoplanet mass is about two times lower than the median dust mass when grains are assumed to be porous and there are no exoplanetary systems with masses higher than the most massive disks. Our analysis suggests that adopting porous dust opacities may alleviate the mass budget problem for planet formation. As an example illustrating the combined effects of optical depth and porous dust opacities on the mass estimation, we conducted new IRAM/NIKA-2 observations toward the IRAS\,04370+2559 disk and performed a detailed radiative transfer modeling of the spectral energy distribution (SED). The best-fit dust mass is roughly 100 times higher than the value given by a traditionally analytic calculation. Future spatially resolved observations at various wavelengths are required to better constrain the dust mass.

Mind the Kinematics Simulation of Planet–Disk Interactions: Time Evolution and Numerical Resolution

Planet–disk interactions can produce kinematic signatures in protoplanetary disks. While recent observations have detected non-Keplerian gas motions in disks, their origins are still being debated. To explore this, we conduct 3D hydrodynamic simulations using the code FARGO3D to study nonaxisymmetric kinematic perturbations at two scale heights induced by Jovian planets in protoplanetary disks, followed by examinations of detectable signals in synthetic CO emission line observations at millimeter wavelengths. We advocate for using residual velocity or channel maps, generated by subtracting an azimuthally averaged background of the disk, to identify planet-induced kinematic perturbations. We investigate the effects of two basic simulation parameters, simulation duration and numerical resolution, on the simulation results. Our findings suggest that a short simulation (e.g., 100 orbits) is insufficient to establish a steady velocity pattern given our chosen viscosity (α = 10−3) and displays plenty of fluctuations on an orbital timescale. Such transient features could be detected in observations. By contrast, a long simulation (e.g., 1000 orbits) is required to reach steady state in kinematic structures. At 1000 orbits, the strongest detectable velocity structures are found in the spiral wakes close to the planet. Through numerical convergence tests, we find hydrodynamics results converge in spiral regions at a resolution of 14 cells per disk scale height or higher. Meanwhile, synthetic observations produced from hydrodynamic simulations at different resolutions are indistinguishable with 0.″1 angular resolution and 10 hr of integration time on Atacama Large Millimeter/submillimeter Array.

Effect of Time-varying X-Ray Emission from Stellar Flares on the Ionization of Protoplanetary Disks

X-rays have significant impacts on cold, weakly ionized protoplanetary disks by increasing the ionization rate and driving chemical reactions. Stellar flares are explosions that emit intense X-rays and are the unique source of hard X-rays with an energy of ≳10 keV in the protoplanetary disk systems. Hard X-rays should be carefully taken into account in models as they can reach the disk midplane as a result of scattering in the disk atmospheres. However, previous models are insufficient to predict the hard X-ray spectra because of simplifications in flare models. We develop a model of X-ray spectra of stellar flares based on observations and flare theories. The flare temperature and nonthermal electron emissions are modeled as functions of flare energy, which allows us to better predict the hard X-ray photon flux than before. Using our X-ray model, we conduct radiative transfer calculations to investigate the impact of flare hard X-rays on disk ionization, with a particular focus on the protoplanetary disk around a T Tauri star. We demonstrate that for a flare with an energy of 1035 erg, X-ray photons with ≳5 keV increase the ionization rates more than galactic cosmic rays down to z ≈ 0.1R. The contribution of flare X-rays to the ionization at the midplane depends on the disk parameters such as disk mass and dust settling. We also find that the 10 yr averaged X-rays from multiple flares could certainly contribute to the ionization. These results emphasize the importance of stellar flares on the disk evolution.

Dust Drift Timescales in Protoplanetary Disks at the Cusp of Gravitational Instability

Millimeter emitting dust grains have sizes that make them susceptible to drift in protoplanetary disks due to the difference between their orbital speed and that of the gas. The characteristic drift timescale depends on the surface density of the gas. By comparing disk radius measurements from Atacama Large Millimeter/submillimeter Array CO and continuum observations at millimeter wavelengths, the gas surface density profile and dust drift time can be self-consistently determined. We find that profiles which match the measured dust mass have very short drift timescales, an order of magnitude or more shorter than the stellar age, whereas profiles for disks that are on the cusp of gravitational instability, defined via the minimum value of the Toomre parameter, Qmin∼1−2 , have drift timescales comparable to the stellar lifetime. This holds for disks with masses of dust ≳5 M ⊕ across a range of absolute ages from less than 1 Myr to over 10 Myr. The inferred disk masses scale with stellar mass as Mdisk≈M*/5Qmin . This interpretation of the gas and dust disk sizes simultaneously solves two long standing issues regarding the dust lifetime and exoplanet mass budget, and suggests that we consider millimeter wavelength observations as a window into an underlying population of particles with a wide size distribution in secular evolution with a massive planetesimal disk.

Origin of Ca II emission around polluted white dwarfs

Dozens of white dwarfs with anomalous metal polluted atmospheres are currently known to host dust and gas discs. The line profiles of the Ca II triplet emitted by the gas discs show a significant asymmetry. In recent decades, researchers have also discovered several minor planets orbiting such white dwarf stars. The most challenging burden of modelling gas discs around metal polluted white dwarfs is to simultaneously explain the asymmetry and metal pollution of the star's atmosphere over a certain period of time. Furthermore, models should also be consistent with other aspects of the observations, such as the morphology of the emission lines. This paper aims to construct a self-consistent model to explain the simultaneous white dwarf pollution and Ca II line asymmetry over at least three years. In our model, an asteroid disintegrates in an eccentric orbit, periodically entering below the star's Roche limit. The debris resulting from the disintegration sublimates at a temperature of 1500 K, producing gas that viscously spreads to form a disc. The evolution of the disc is studied over a period of 1.2 years (over 21000 orbits) using two-dimensional hydrodynamic simulations. Synthetic Ca II line profiles are calculated using the surface mass density and velocity distributions provided by the simulations, taking into account for the first time the asymmetric velocity distribution in the disc. An asteroid disintegrating on an eccentric orbit gives rise to the formation of an asymmetric disc and asymmetric Ca II triplet emission. Our model can explain the periodic reversal of the redshifted and blueshifted peak of the Ca II lines caused by the precession of the disc on timescales of 10.6 to 177.4 days. Our work suggests that the persistence of Ca II asymmetry over decades and its periodic change in the peaks can be explained by asteroids on eccentric orbits in two scenarios. In the first case, the asteroid disrupts on a short timescale (a couple of orbits), and the gas has a low viscosity range ($0.001< to maintain the Ca II signal for decades. In the other scenario, the asteroid disrupts on a timescale of a year, and the viscosity of the gas is required to be high, $

The Search for Disk Perturbing Planets Around the Asymmetrical Debris Disk System HD 111520 Using REBOUND

Debris disks, which are optically thin, dusty disks around main-sequence stars, are often found to have structures and/or asymmetries associated with planet–disk interactions. Debris disk morphologies can hence be used as probes for planets in these systems, which are unlikely to be detected with other current exoplanet detection methods. In this study we take a look at the very asymmetrical debris disk around HD 111520, which harbours several signs of perturbation such as a “fork”-like structure in the NW, as well as a 4° warp from the midplane on either side of the disk. We simulate the complicated disk morphology using the code REBOUND, with the goal of constraining the possible mass and orbit of the planet responsible for the observed structures. We find that an ∼1 M jup, eccentric planet that is inclined relative to the disk and is orbiting outside the warp location is able to reproduce the majority of disk features including the warp, fork, and radial extent asymmetry. To create the surface brightness asymmetry, a second eccentric planet is required inside the disk inner edge (50 au), although we are unable to produce the 2:1 brightness asymmetry observed, suggesting that a second mechanism may be required. Our work demonstrates how debris disk morphologies alone can be used to learn more about the architecture and evolution of a system as a whole, and can provide planet constraints to determine potential targets for current/future instruments such as JWST/NIRCam and the Gemini Planet Imager 2.0.

A Search for Collisions and Planet–Disk Interactions in the Beta Pictoris Disk with 26 Years of High-precision HST/STIS Imaging

β Pictoris's well-studied debris disk and two known giant planets, in combination with the stability of the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (HST/STIS) (and now also JWST), offers a unique opportunity to test planet–disk interaction models and to observe recent planetesimal collisions. We present HST/STIS coronagraphic imaging from two new epochs of data taken between 2021 and 2023, complementing earlier data taken in 1997 and 2012. This data set enables a temporal comparison with the longest baseline and highest precision of any debris disk to date, with sensitivity to variations in temporal surface brightness of sub-percent levels in the midplane of the disk. While no localized changes in surface brightness are detected, which would be indicative of a recent planetesimal collision, there is a tentative brightening of the southeast side of the disk over the past decade. We link the constraints on surface brightness variations to dynamical models of the planetary system’s evolution and to the collisional history of planetesimals. Using a coupled collisional model and injection/recovery framework, we estimate sensitivity to expanding collisional debris down to a Ceres mass per progenitor in the most sensitive regions of the disk midplane. These results demonstrate the capabilities of long-baseline, temporal studies with HST (and also soon with JWST) for constraining the physical processes occurring within debris disks.

Density of Uranus moons: Evidence for ice/rock fractionation during planetary accretion

Current models suggest the five regular moons of Uranus formed in a single stage from a primary planetary disk or a secondary impact disk. Using latest estimates of moon masses (Jacobson, 2014), we find a power-law relationship between size and density of the moons due to varying rock/ice ratios caused by fractionation processes. This relationship is better explained by mild enrichment of rock with respect to ice in the solids that aggregate to form the moons following Rayleigh law for distillation (Rayleigh, 1896) than by differential diffusion in the disk, although the two mechanisms are not exclusive. Rayleigh fractionation requires that moon composition and density reflect their order of formation in a closed-system circumplanetary disk. For Uranus, the largest and densest moons Titania and Oberon (R ∼ 788 and 761 km, respectively) first formed, then the mid-sized Umbriel and Ariel (585 and 579 km), satellites in each pair forming simultaneously with similar composition, and finally the small rock-depleted Miranda (236 km). Fractionation likely occurred through impact vaporization during planetesimal accretion. This mechanism would add to those affecting the composition of accreting planets and moons in disks such as temporal/spatial variation of disk composition due to temperature gradients, advection, and large impacts. In the outer solar nebula, Rayleigh fractionation may account for the separation of a rock-dominated reservoir, and an ice-dominated reservoir, currently represented by CI carbonaceous chondrite/type-C asteroids and comets, respectively. Potential consequences for Uranus moons' composition are discussed.

A Close Asteroid Encounter May Have Once Given Earth a Ring

An unusual concentration of impact craters suggests that they may have been caused by the breakup of an asteroid that created a temporary debris ring around Earth.

Origin of transition disk cavities: Pebble-accreting protoplanets vs super-Jupiters

Protoplanetary disks surrounding young stars are the birth places of planets. Among them, transition disks with inner dust cavities of tens of au are sometimes suggested to host massive companions. Yet, such companions are often not detected. Some transition disks exhibit a large amount of gas inside the dust cavity and relatively high stellar accretion rates, which contradicts typical models of gas-giant-hosting systems. Therefore, we investigate whether a sequence of low-mass planets can create the appearance of cavities in the dust disk. We evolve the disks with low-mass growing embryos in combination with 1D dust transport and 3D pebble accretion, to investigate the reduction of the pebble flux at the embryos' orbits. We vary the planet and disk properties to understand the resulting dust profile. We find that multiple pebble-accreting planets can efficiently decrease the dust surface density, resulting in dust cavities consistent with transition disks. The number of low-mass planets necessary to sweep up all pebbles decreases with decreasing turbulent strength and is preferred when the dust Stokes number is $10^ $. Compared to dust rings caused by pressure bumps, those by efficient pebble accretion exhibit more extended outer edges. We also highlight the observational reflections: the transition disks with rings featuring extended outer edges tend to have a large gas content in the dust cavities and rather high stellar accretion rates. We propose that planet-hosting transition disks consist of two groups. In Group A disks, planets have evolved into gas giants, opening deep gaps in the gas disk. Pebbles concentrate in pressure maxima, forming dust rings. In Group B, multiple Neptunes (unable to open deep gas gaps) accrete incoming pebbles, causing the appearance of inner dust cavities and distinct ring-like structures near planet orbits. The morphological discrepancy of these rings may aid in distinguishing between the two groups using high-resolution ALMA observations.

Using meteorite magnetism to elucidate early solar system history – MMESSH

Using meteorite magnetism to elucidate early solar system history – MMESSH By utilising several recent breakthroughs in extraterrestrial magnetism, MMESSH aims to unlock pivotal insights into the protoplanetary disk of dust and gas that surrounded the young Sun. Because the behaviour of this disk underpinned the process of planet building, this project has the potential to revolutionise our understanding of the formation and habitability of the Earth.

Spatially correlated stellar accretion in the Lupus star-forming region. Evidence for ongoing infall from the interstellar medium

Growing evidence suggests that protoplanetary discs may be influenced by late stage infall from the interstellar medium (ISM). It remains unclear the degree to which infall shapes disc populations at ages $ 1$ Myr. We explored possible spatial correlations between stellar accretion rates in the Lupus star-forming region, which would support the hypothesis that infall can regulate stellar accretion. We considered both the `clustered' stars towards the centre of Lupus 3, and the `distributed' stars that are more sparsely distributed across the Lupus complex. We took the observed accretion rates in the literature and explore spatial correlations. In particular, we tested whether the clustered stars exhibit a radial gradient in normalised accretion rates, and whether the distributed stars have spatially correlated accretion rates. We found statistically significant correlations for both the clustered and distributed samples. The clustered sample exhibits higher accretion rates in the central region, consistent with the expected Bondi-Hoyle-Lyttleton accretion rate. Stars that are spatially closer among the distributed population also exhibit more similar accretion rates. These results cannot be explained by the stellar mass distribution for either sample. Age gradients are disfavoured, though not discounted because normalised disc dust masses are not spatially correlated across the region. Spatially correlated stellar accretion rates within the Lupus star-forming region argue in favour of an environmental influence on stellar accretion, possibly combined with internal processes in the inner disc. Refined age measurements and searches for evidence of infalling material are potential ways to further test this finding.

An Analytic Characterization of the Limb Asymmetry—Transit Time Degeneracy

Atmospheres are not spatially homogeneous. This is particularly true for hot, tidally locked exoplanets with large day-to-night temperature variations, which can yield significant differences between the morning and evening terminators—known as limb asymmetry. Current transit observations with the James Webb Space Telescope (JWST) are precise enough to disentangle the separate contributions of these morning and evening limbs to the overall transmission spectrum in certain circumstances. However, the signature of limb asymmetry in a transit light curve is highly degenerate with uncertainty in the planet’s time of conjunction. This raises the question of how precisely transit times must be measured to enable accurate studies of limb asymmetry, in particular with JWST. Although this degeneracy has been discussed in the literature, a general description of it has not been presented. In this work, we show how this degeneracy results from apparent changes in the transit contact times when the planetary disk has asymmetric limb sizes. We derive a general formula relating the magnitude of limb asymmetry to the amount by which it would cause the apparent time of conjunction to vary, which can reach tens of seconds. Comparing our formula to simulated observations, we find that numerical fitting techniques add additional bias to the measured time, of generally less than a second, resulting from the occultation geometry. We also derive an analytical formula for this extra numerical bias. These formulae can be applied to planning new observations or interpreting literature measurements, and we show examples for commonly studied exoplanets.

TEMPORARY REMOVAL: Origin of Mars's moons by disruptive partial capture of an asteroid

The origin of Mars’s small moons, Phobos and Deimos, remains unknown. They are typically thought either to be captured asteroids or to have accreted from a debris disk produced by a giant impact. Here, we present an alternative scenario wherein fragments of a tidally disrupted asteroid are captured and evolve into a collisional proto-satellite disk. We simulate the initial disruption and the fragments’ subsequent orbital evolution. We find that tens of percent of an unbound asteroid’s mass can be captured and survive beyond collisional timescales, across a broad range of periapsis distances, speeds, masses, spins, and orientations in the Sun–Mars frame. Furthermore, more than one percent of the asteroid’s mass could evolve to circularise in the moons’ accretion region. This implies a lower mass requirement for the parent body than that for a giant impact, which could increase the likelihood of this route to forming a proto-satellite disk that, unlike direct capture, could also naturally explain the moons’ orbits. These three formation scenarios each imply different properties of Mars’s moons to be tested by upcoming spacecraft missions.

Unique achondritic impact debris in the CH3 chondrite Acfer 182

The metal-rich CH carbonaceous chondrites contain abundant xenolithic clasts originating from different regions of the Solar System. In the CH3 chondrite Acfer 182, we identified two phosphide spherules (one 95-μm in diameter and the other 50μm×60μm) of schreibersite ((Fe,Ni)3P) and barringerite ((Fe,Ni)2P) with kamacite eutectic structures. These objects are likely to have formed during an impact between planetesimals during the debris-disk phase of the protoplanetary disk before being incorporated into the CH chondrite parent body. In the same sample we identified a 130μm×60μm heideite grain (iron‑titanium sulfide: (Fe,Cr)1.15(Ti,Fe)2S4) with exsolution lamellae of calcium-rich titanium oxide. Thin veins of shock-induced kamacite cross-cut the oxide lamellae, suggesting that it was ejected into the protoplanetary debris disk during an impact event before eventually being accreted by the CH chondrite parent body. This assemblage is distinct from heideite grains found in enstatite chondrites, aubrites, and the Kaidun meteorite. We propose that this object originated from a highly-reduced planetesimal in the inner Solar System that may have been similar to proto-Mercury.

The ALMA Legacy Survey of Class 0/I Disks in Corona australis, Aquila, chaMaeleon, oPhiuchus north, Ophiuchus, Serpens (CAMPOS). I. Evolution of Protostellar Disk Radii

We surveyed nearly all the embedded protostars in seven nearby clouds (Corona Australis, Aquila, Chamaeleon I and II, Ophiuchus North, Ophiuchus, Serpens) with the Atacama Large Millimeter/submillimeter Array at 1.3 mm observations with a resolution of 0.″1. This survey detected 184 protostellar disks, 90 of which were observed at a resolution of 14–18 au, making it one of the most comprehensive high-resolution disk samples across various protostellar evolutionary stages to date. Our key findings include the detection of new annular substructures in two Class I and two flat-spectrum sources, while 21 embedded protostars exhibit distinct asymmetries or substructures in their disks. We find that protostellar disks have a substantially large variability in their radii across all evolutionary classes. In particular, the fraction of large disks with sizes above 60 au decreases as the protostar evolves from Class 0 to Class I. Compiling the literature data, we discovered an increasing trend of the gas disk radii to dust disk radii ratio (R gas,Kep/R mm) with increasing bolometric temperature (T bol). Our results indicate that the dust and gas disk radii decouple during the early Class I stage. However, in the Class 0 stage, the dust and gas disk sizes are similar, which allows for a direct comparison between models and observational data at the earliest stages of protostellar evolution. We show that the distribution of radii in the 52 Class 0 disks in our sample is in high tension with various disk formation models, indicating that protostellar disk formation remains an unsolved question.

MIRI MRS Observations of Beta Pictoris. II. The Spectroscopic Case for a Recent Giant Collision

Modeling observations of the archetypal debris disk around β Pic, obtained in 2023 January with the MIRI Medium Resolution Spectrograph on board JWST, reveals significant differences compared with that obtained with the Infrared Spectrograph on board Spitzer. The bright 5–15 μm continuum excess modeled using a ∼600 K black body has disappeared. The previously prominent 18 and 23 μm crystalline forsterite emission features, arising from cold dust (∼100 K) in the Rayleigh limit, have disappeared and been replaced by very weak features arising from the hotter 500 K dust population. Finally, the shape of the 10 μm silicate feature has changed, consistent with a shift in the temperature of the warm dust population from ∼300 to ∼500 K and an increase in the crystalline fraction of the warm, silicate dust. Stellar radiation pressure may have blown both the hot and the cold crystalline dust particles observed in the Spitzer spectra out of the planetary system during the intervening 20 yr between the Spitzer and JWST observations. These results indicate that the β Pic system has a dynamic circumstellar environment, and that periods of enhanced collisions can create large clouds of dust that sweep through the planetary system.