Stellar Accretion Rate Research Articles

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.

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.

A Far-ultraviolet-detected Accretion Shock at the Star–Disk Boundary of FU Ori

FU Ori objects are the most extreme eruptive young stars known. Their 4–5 mag photometric outbursts last for decades and are attributed to a factor of up to 10,000 increase in the stellar accretion rate. The nature of the accretion disk-to-star interface in FU Ori objects has remained a mystery for decades. To date, attempts to directly observe a shock or boundary layer have been thwarted by the apparent lack of emission in excess of the accretion disk photosphere down to λ = 2300 Å. We present a new near-ultraviolet and the first high-sensitivity far-ultraviolet (FUV) spectrum of FU Ori. The FUV continuum is detected for the first time and, at λ = 1400 Å, is more than 104 times brighter than predicted by a viscous accretion disk. We interpret the excess as arising from a shock at the boundary between the disk and the stellar surface. We model the shock emission as a blackbody and find that the temperature of the shocked material is T FUV ≈ 16,000 ± 2000 K. The shock temperature corresponds to an accretion flow along the surface of the disk that reaches a velocity of 40 km s−1 at the boundary, consistent with predictions from simulations.

Planet Formation Regulated by Galactic-scale Interstellar Turbulence

Planet formation occurs over a few Myr within protoplanetary disks of dust and gas, which are often assumed to evolve in isolation. However, extended gaseous structures have been uncovered around many protoplanetary disks, suggestive of late-stage infall from the interstellar medium (ISM). To quantify the prevalence of late-stage infall, we apply an excursion set formalism to track the local density and relative velocity of the ISM over the disk lifetime. We then combine the theoretical Bondi–Hoyle–Lyttleton (BHL) accretion rate with a simple disk evolution model, anchoring stellar accretion timescales to observational constraints. Disk lifetimes, masses, stellar accretion rates, and gaseous outer radii as a function of stellar mass and age are remarkably well reproduced by our simple model that includes only ISM accretion. We estimate that 20%−70% of disks may be mostly composed of material accreted in the most recent half of their lifetime, suggesting that disk properties are not a direct test of isolated evolution models. Our calculations indicate that BHL accretion can also supply sufficient energy to drive turbulence in the outer regions of protoplanetary disks with viscous α SS ∼ 10−5 to 10−1, although we emphasize that angular momentum transport and particularly accretion onto the star may still be driven by internal processes. Our simple approach can be easily applied to semianalytic models. Our results represent a compelling case for regulation of planet formation by large-scale turbulence, with broad consequences for planet formation theory. This possibility urgently motivates deep observational surveys to confirm or refute our findings.

Discovery of an Accretion Streamer and a Slow Wide-angle Outflow around FU Orionis

We present Atacama Large Millimeter/submillimeter Array 12-m, 7-m, and Total Power Array observations of the FU Orionis outbursting system, covering spatial scales ranging from 160 to 25,000 au. The high-resolution interferometric data reveal an elongated 12CO(2–1) feature previously observed at lower resolution in 12CO(3–2). Kinematic modeling indicates that this feature can be interpreted as an accretion streamer feeding the binary system. The mass infall rate provided by the streamer is significantly lower than the typical stellar accretion rates (even in quiescent states), suggesting that this streamer alone is not massive enough to sustain the enhanced accretion rates characteristic of the outbursting class prototype. The observed streamer may not be directly linked to the current outburst, but rather a remnant of a previous, more massive streamer that may have contributed enough to the disk mass to render it unstable and trigger the FU Orionis outburst. The new data detect, for the first time, a vast, slow-moving carbon monoxide molecular outflow emerging from this object. To accurately assess the outflow properties (mass, momentum, and kinetic energy), we employ 13CO(2–1) data to correct for optical depth effects. The analysis indicates that the outflow corresponds to swept-up material not associated with the current outburst, similar to the slow molecular outflows observed around other FUor and Class I protostellar objects.

Large Fluctuations within 1 au in Protoplanetary Disks

Protoplanetary disks are often assumed to change slowly and smoothly during planet formation. Here, we investigate the time evolution of isolated disks subject to viscosity and a disk wind. The viscosity is assumed to increase rapidly at around 900 K due to thermal ionization of alkali metals, or thermionic and ion emission from dust, and the onset of magnetorotational instability (MRI). The disks generally undergo large, rapid fluctuations for a wide range of time-averaged mass accretion rates. Fluctuations involve coupled waves in temperature and surface density that move radially in either direction through the inner 1.5 au of the disk. Two types of waves are seen with radial speeds of roughly 50 and 1000 cm s−1, respectively. The pattern of waves repeats with a period of roughly 10,000 yr that depends weakly on the average mass accretion rate. Viscous transport due to MRI is confined to the inner disk. This region is resupplied by mass flux from the outer disk driven by the disk wind. Interior to 1 au, the temperature and surface density can vary by a factor of 2–10 on timescales of years to kiloyears. The stellar mass accretion rate varies by 3 orders of magnitude on a similar timescale. This behavior lasts for at least 1 Myr for initial disks comparable to the minimum-mass solar nebula.

Determining stellar accretion rates from Paα and Brβ emission lines with JWST NIRSpec

In this Letter, we present the first systematic spectroscopic measurements of the near-infrared (NIR) hydrogen recombination lines Paschen alpha (Paαλ = 1.875 μm) and Brackett beta (Brβλ = 2.626 μm), produced by pre-main-sequence (PMS) stars. Such stars include T Tauri and Herbig AeBe stars, located in the massive Galactic star-forming region NGC 3603. We used measurements obtained from JWST NIRSpec, using multi-object spectroscopy (MOS) mode. Based on the existing empirical relations between Lacc and LBrγ from the literature, we used our new measurements to formulate, for the first time, an empirical relationship between the accretion luminosity, Lacc, of the stars and the line luminosities, Lline, of both Paα and Brβ. These relationships are: log10(Lacc/L⊙) = 1.42(±0.18) × log10(LPaα/L⊙ + 3.33(±0.42) and log10(Lacc/L⊙) = 1.47(±0.18) × log10(LBrβ/L⊙) + 4.60(±0.57). These new relationships are key to establishing rough estimates of the accretion rates for large samples of PMS stars with JWST.

A 4,565-My-old record of the solar nebula field

Magnetic fields in protoplanetary disks are thought to play a prominent role in the formation of planetary bodies. Acting upon turbulence and angular momentum transport, they may influence the motion of solids and accretion onto the central star. By searching for the record of the solar nebula field preserved in meteorites, we aim to characterize the strength of a disk field with a spatial and temporal resolution far superior to observations of extrasolar disks. Here, we present a rock magnetic and paleomagnetic study of the andesite meteorite Erg Chech 002 (EC002). This meteorite contains submicron iron grains, expected to be very reliable magnetic recorders, and carries a stable, high-coercivity magnetization. After ruling out potential sources of magnetic contamination, we show that EC002 most likely carries an ancient thermoremanent magnetization acquired upon cooling on its parent body. Using the U-corrected Pb-Pb age of the meteorite's pyroxene as a proxy for the timing of magnetization acquisition, we estimate that EC002 recorded a field of 60 ± 18 µT at a distance of ~2 to 3 astronomical units, 2.0 ± 0.3 My after the formation of calcium-aluminum-rich inclusions. This record can only be explained if EC002 was magnetized by the field prevalent in the solar nebula. This makes EC002's record, particularly well resolved in time and space, one of the two earliest records of the solar nebula field. Such a field intensity is consistent with stellar accretion rates observed in extrasolar protoplanetary disks.

Testing Magnetospheric Accretion as an Hα Emission Mechanism of Embedded Giant Planets: The Case Study for the Disk Exhibiting Meridional Flow Around HD 163296

Recent high-sensitivity observations reveal that accreting giant planets embedded in their parental circumstellar disks can emit Hα at their final formation stages. While the origin of this emission is not yet determined, magnetospheric accretion is currently the most plausible hypothesis. In order to test this hypothesis further, we develop a simplified but physics-based model and apply it to our observations taken toward HD 163296 with Subaru/SCExAO+VAMPIRES. We specify under which conditions embedded giant planets can undergo magnetospheric accretion and emit hydrogen lines. We find that when the stellar accretion rates are high, magnetospheric accretion becomes energetic enough to self-regulate the resulting emission. On the other hand, when massive planets are embedded in disks with low accretion rates, earlier formation histories determine whether magnetospheric accretion occurs. We explore two different origins for the hydrogen emission lines (magnetospheric accretion flow heated by accretion-related processes versus planetary surfaces via accretion shock). The corresponding relationships between the accretion and line luminosities dictate that the emission from accretion flow achieves higher line flux than that from accretion shock, and the flux decreases with increasing wavelengths (i.e., from Hα to Paβ and up to Brγ). Our observations do not detect any point-like source emitting Hα, and they are used to derive the 5σ detection limit. The observations are therefore not sensitive enough, and a reliable examination of our model becomes possible when the observational sensitivity is improved by a factor of 10 or more. Multi-band observations increase the possibility of efficiently detecting embedded giant planets and carefully determining the origin of the hydrogen emission lines.

The –M disk Relationship for Herbig Ae/Be Stars: A Lifetime Problem for Disks with Low Masses?

The accretion of material from protoplanetary disks onto their central stars is a fundamental process in the evolution of these systems and a key diagnostic in constraining the disk lifetime. We analyze the relationship between the stellar accretion rate and the disk mass in 32 intermediate-mass Herbig Ae/Be systems and compare them to their lower-mass counterparts, T Tauri stars. We find that the –M disk relationship for Herbig Ae/Be stars is largely flat at ∼10−7 M ☉ yr−1 over 3 orders of magnitude in dust mass. While most of the sample follows the T Tauri trend, a subset of objects with high accretion rates and low dust masses are identified. These outliers (12 out of 32 sources) have an inferred disk lifetime of less than 0.01 Myr and are dominated by objects with low infrared excess. This outlier sample is likely identified in part by the bias in classifying Herbig Ae/Be stars, which requires evidence of accretion that can only be reliably measured above a rate of ∼10−9 M ☉ yr−1 for these spectral types. If the disk masses are not underestimated and the accretion rates are not overestimated, this implies that these disks may be on the verge of dispersal, which may be due to efficient radial drift of material or outer disk depletion by photoevaporation and/or truncation by companions. This outlier sample likely represents a small subset of the larger young, intermediate-mass stellar population, the majority of which would have already stopped accreting and cleared their disks.

Co-evolution of dust grains and protoplanetary disks

Abstract We propose a new evolutionary process for protoplanetary disks, the co-evolution of dust grains and protoplanetary disks, revealed by dust–gas two-fluid non-ideal magnetohydrodynamics simulations considering the growth of dust grains and associated changes in magnetic resistivity. We found that the dust growth significantly affects disk evolution by changing the coupling between the gas and the magnetic field. Moreover, once the dust grains grow sufficiently large and the adsorption of charged particles on to them becomes negligible, the physical quantities (e.g., density and magnetic field) of the disk are well described by characteristic power laws. In this disk structure, the radial profile of density is steeper and the disk mass is smaller than those of the model ignoring dust growth. We analytically derive these power laws from the basic equations of non-ideal magnetohydrodynamics. The analytical power laws are determined only by observable physical quantities, e.g., central stellar mass and mass accretion rate, and do not include difficult-to-determine parameters, e.g., the viscous parameter α. Therefore, our model is observationally testable and this disk structure is expected to provide a new perspective for future studies on protostar and disk evolution.

The distribution of accretion rates as a diagnostic of protoplanetary disc evolution

ABSTRACT We show that the distribution of observed accretion rates is a powerful diagnostic of protoplanetary disc physics. Accretion due to turbulent (‘viscous’) transport of angular momentum results in a fundamentally different distribution of accretion rates than accretion driven by magnetized disc winds. We find that a homogeneous sample of ≳300 observed accretion rates would be sufficient to distinguish between these two mechanisms of disc accretion at high confidence, even for pessimistic assumptions. Current samples of T Tauri star accretion rates are not this large, and also suffer from significant inhomogeneity, so both viscous and wind-driven models are broadly consistent with the existing observations. If accretion is viscous, the observed accretion rates require low rates of disc photoevaporation (≲10−9 M⊙ yr−1). Uniform, homogeneous surveys of stellar accretion rates can therefore provide a clear answer to the long-standing question of how protoplanetary discs accrete.

Population study on MHD wind-driven disc evolution

Context. Current research has established magnetised disc winds as a promising way of driving accretion in protoplanetary discs. Aims. We investigate the evolution of large protoplanetary disc populations under the influence of magnetically driven disc winds as well as internal and external photoevaporation. We aim to constrain magnetic disc wind models through comparisons with observations. Methods. We ran 1D vertically integrated evolutionary simulations for low-viscosity discs, including magnetic braking and various outflows. The initial conditions were varied and chosen to produce populations that are representative of actual disc populations inferred from observations. We then compared the observables from the simulations (e.g. stellar accretion rate, disc mass evolution, disc lifetime, etc.) with observational data. Results. Our simulations show that to reach stellar accretion rates comparable to those found by observations (~10−8 M⊙ yr−1), it is necessary to have access not only to strong magnetic torques, but weak magnetic winds as well. The presence of a strong magnetic disc wind, in combination with internal photoevaporation, leads to the rapid opening of an inner cavity early on, allowing the stellar accretion rate to drop while the disc is still massive. Furthermore, our model supports the notion that external photoevaporation via the ambient far-ultraviolet radiation of surrounding stars is a driving force in disc evolution and could potentially exert a strong influence on planetary formation. Conclusions. Our disc population syntheses show that for a subset of magnetohydrodynamic wind models (weak disc wind, strong torque), it is possible to reproduce important statistical observational constraints. The magnetic disc wind paradigm thus represents a novel and appealing alternative to the classical α-viscosity scenario.

Disk fragmentation around a massive protostar: Comparison of two 3D codes

Context. Most massive stars are located in multiple stellar systems. The modeling of disk fragmentation, a mechanism that may plausibly lead to stellar multiplicity, relies on parallel 3D simulation codes whose agreement remains to be evaluated. Aims. Cartesian adaptive-mesh refinement (AMR) and spherical codes have frequently been used in the past decade to study massive star formation. We aim to study how the details of collapse and disk fragmentation depend on these codes. Methods. Using the Cartesian AMR code RAMSES within its self-gravity radiation-hydrodynamical framework, we compared disk fragmentation in a centrally condensed protostellar system to the findings of earlier studies performed on a grid in spherical coordinates using PLUTO. Results. To perform the code comparison, two RAMSES runs were considered, effectively giving qualitatively distinct pictures. On the one hand, when allowing for unlimited sink particle creation with no initial sink, Toomre instability and subsequent gas fragmentation leads to a multiple stellar system whose multiplicity is affected by the grid when triggering fragmentation and via numerically assisted mergers. On the other hand, using a unique, central, fixed-sink particle, a centrally-condensed system forms that is similar to that reported by PLUTO. Hence, the RAMSES-PLUTO comparison was performed with the latter and an agreement between the two codes is found as to the first rotationally supported disk formation, the presence of an accretion shock onto it, and the first fragmentation phase. Gaseous fragments form. The properties of the fragments (i.e., number, mass, and temperature) are dictated by local thermodynamics and are in agreement between the two codes given that the system has entered a highly nonlinear phase. Over the simulations, the stellar accretion rate is made of accretion bursts and continuous accretion on the same order of magnitude. As a minor difference between both codes, the dynamics of the fragments causes the disk structure to be sub-Keplerian in RAMSES, whereas it is found to be Keplerian, thus reaching quiescence, in PLUTO. We attribute this discrepancy to the central star being twice less massive in RAMSES because of the different stellar accretion subgrid models in use - rather than the potential grid effects. Conclusions. In a centrally condensed system, the agreement between RAMSES and PLUTO regarding many of the collapse properties and fragmentation process is good. In contrast, fragmentation occurring in the innermost region and given specific numerical choices (use of sink particles, grid, etc.) have a crucial impact when similar but smooth initial conditions are employed. These aspects prove more crucial than the choice of code, with regard to the system being multiple or centrally condensed.

Migration of pairs of giant planets in low-viscosity discs

Context. When considering the migration of Jupiter and Saturn, a classical result is to find the planets migrating outwards and locked in the 3:2 mean motion resonance (MMR). These results were obtained in the framework of viscously accreting discs, in which the observed stellar accretion rates constrained the viscosity values. However, it has recently been shown observationally and theoretically that discs are probably less viscous than previously thought. Aims. Therefore, in this paper, we explore the dynamics of pairs of giant planets in low-viscosity discs. Methods. We performed two-dimensional hydrodynamical simulations using the grid-based code FARGOCA. Results. In contrast to classical viscous discs, we find that the outer planet never crosses the 2:1 resonance and the pair does not migrate outwards. After a wide parameter exploration, including the mass of the outer planet, we find that the planets are primarily locked in the 2:1 MMR and in some cases in the 5:2 MMR. We explain semi-analytically why it is not possible for the outer planet to cross the 2:1 MMR in a low-viscosity disc. Conclusions. We find that pairs of giant planets migrate inwards in low-viscosity discs. Although, in some cases, having a pair of giant planets can slow down the migration speed with respect to a single planet. Such pairs of slowly migrating planets may be located, at the end of the disc phase, in the population of exoplanets of ’warm Jupiters’. However, the planets never migrate outwards. These results could have strong implications on the Solar System’s formation scenarios if the Sun’s protoplanetary disc had a low viscosity.

A 4000 Mʘ supermassive star as a possible source for the W1 kilomaser

AbstractSupermassive stars have been proposed as the solution to a number of longstanding problems in globular cluster formation. The hypothetical stars have been suggested as potential polluters responsible for the observed chemical peculiarities within those clusters. In recent hydrodynamic simulations, we have demonstrated that accretion discs around such stars are stable even with large stellar accretion and flyby rates and produce H2O kilomasers. We propose that the W1 kilomaser, associated with a super star cluster in the starburst galaxy NGC 253, may arise in an accretion disc around a supermassive star with a mass of around 4000 Mʘ.

A semi-analytical model for the temporal evolution of the episodic disc-to-star accretion rate during star formation

ABSTRACT We develop a semi-analytical formalism for the determination of the evolution of the stellar mass accretion rate for specified density and velocity profiles that emerge from the runaway collapse of a prestellar cloud core. In the early phase, when the infall of matter from the surrounding envelope is substantial, the star accumulates mass primarily because of envelope-induced gravitational instability in a protostellar disc. In this phase, we model the envelope mass accretion rate from the isothermal free-fall collapse of a molecular cloud core. The disc gains mass from the envelope, and transports matter to the star via a disc accretion mechanism that includes episodic gravitational instability and mass accretion bursts according to the Toomre Q-criterion. In a later phase, mass is accreted on to the star due to gravitational torques within the spiral structures in the disc, in a manner that analytical theory suggests has a mass accretion rate ∝t−6/5. Our model provides a self-consistent evolution of the mass accretion rate by joining the spherical envelope accretion (dominant at the earlier stage) with the disc accretion (important at the later stage), and accounts for the presence of episodic accretion bursts at appropriate times. We show using a simple example that the burst mode can provide a good match to the observed distribution of bolometric luminosities. Our framework reproduces key elements of detailed numerical simulations of disc accretion and can aid in developing intuition about the basic physics as well as to compare theory with observations.

CO isotopolog line fluxes of viscously evolving disks

Context. Protoplanetary disks are thought to evolve viscously, where the disk mass – the reservoir available for planet formation – decreases over time as material is accreted onto the central star over a viscous timescale. Observations have shown a correlation between disk mass and the stellar mass accretion rate, as expected from viscous theory. However, this happens only when using the dust mass as a proxy of the disk mass; the gas mass inferred from CO isotopolog line fluxes, which should be a more direct measurement, shows no correlation with the stellar mass accretion rate. Aims. We investigate how 13CO and C18O J = 3−2 line fluxes, commonly used as gas mass tracers, change over time in a viscously evolving disk and use them together with gas disk sizes to provide diagnostics of viscous evolution. In addition, we aim to determine if the chemical conversion of CO through grain-surface chemistry combined with viscous evolution can explain the CO isotopolog observations of disks in Lupus. Methods. We ran a series of thermochemical DALI models of viscously evolving disks, where the initial disk mass is derived from observed stellar mass accretion rates. Results. While the disk mass, Mdisk, decreases over time, the 13CO and C18O J = 3−2 line fluxes instead increase over time due to their optically thick emitting regions growing in size as the disk expands viscously. The C18O 3–2 emission is optically thin throughout the disk for only for a subset of our models (M*≤ 0.2 M⊙ and αvisc ≥ 10−3, corresponding to Mdisk(t = 1 Myr) ≤ 10−3 M⊙). For these disks the integrated C18O flux decreases with time, similar to the disk mass. Observed 13CO and C18O 3–2 fluxes of the most massive disks (Mdisk ≳ 5 × 10−3 M⊙) in Lupus can be reproduced to within a factor of ~2 with viscously evolving disks in which CO is converted into other species through grain-surface chemistry with a moderate cosmic-ray ionization rate of ζcr ~ 10−17 s−1. The C18O 3–2 fluxes for the bulk of the disks in Lupus (with Mdisk ≲ 5 × 10−3 M⊙) can be reproduced to within a factor of ~2 by increasing ζcr to ~ 5 × 10−17−10−16 s−1, although explaining the stacked upper limits requires a lower average abundance than our models can produce. In addition, increasing ζcr cannot explain the observed 13CO fluxes for lower mass disks, which are more than an order of magnitude fainter than what is predicted. In our models the optically thick 13CO emission originates from a layer higher up in the disk (z∕r ~ 0.25−0.4) where photodissociation stops the conversion of CO into other species. Reconciling the 13CO fluxes of viscously evolving disks with the observations requires either efficient vertical mixing or low mass disks (Mdust ≲ 3 × 10−5 M⊙) being much thinner and/or smaller than their more massive counterparts. Conclusions. The 13CO model flux predominantly traces the disk size, but the C18O model flux traces the disk mass of our viscously evolving disk models if chemical conversion of CO is included. The discrepancy between the CO isotopolog line fluxes of viscously evolving disk models and the observations suggests that CO is efficiently vertically mixed or that low mass disks are smaller and/or colder than previously assumed.

The fate of planetesimals formed at planetary gap edges

The presence of rings and gaps in protoplanetary disks are often ascribed to planet–disk interactions, where dust and pebbles are trapped at the edges of planetary-induced gas gaps. Recent works have shown that these are likely sites for planetesimal formation via the streaming instability. Given the large amount of planetesimals that potentially form at gap edges, we address the question of their fate and their ability to radially transport solids in protoplanetary disks. We performed a series ofN-body simulations of planetesimal orbits, taking into account the effect of gas drag and mass loss via ablation. We considered two planetary systems: one that is akin to the young Solar System and another inspired by the structures observed in the protoplanetary disk around HL Tau. In both systems, the proximity to the gap-opening planets results in large orbital excitations, causing the planetesimals to leave their birth locations and spread out across the disk soon after formation. We find that collisions between pairs of planetesimals are rare and should not affect the outcome of our simulations. Collisions with planets occur for ~1% of the planetesimals in the Solar System and for ~20% of the planetesimals in the HL Tau system. Planetesimals that end up on eccentric orbits interior of ~10 au experience efficient ablation and lose all mass before they reach the innermost disk region. In our nominal Solar System simulation, with a stellar gas accretion rate ofṀ0= 10−7M⊙yr−1andα= 10−2, we find that 70% of the initial planetesimal mass has been ablated after 500 kyr. Since the protoplanets are located further away from the star in the HL Tau system, the ablation rate is lower and only 11% of the initial planetesimal mass has been ablated after 1 Myr using the same disk parameters. The ablated material consist of a mixture of solid grains and vaporized ices, where a large fraction of the vaporized ices re-condense to form solid ice. Assuming that the solid grains and ices grow to pebbles in the disk midplane, this results in a pebble flux of ~10−100M⊕Myr−1through the inner disk. This occurred in the Solar System at a time so early in its evolution that there is not likely to be any record of it. Our results demonstrate that scattered planetesimals can carry a significant flux of solids past planetary-induced gaps in young and massive protoplanetary disks.

Testing the models of X-ray driven photoevaporation with accreting stars in the Orion Nebula Cluster

Context. Recent works highlight the importance of stellar X-rays on the evolution of the circumstellar disks of young stellar objects, especially for disk photoevaporation. Aims. A signature of this process may be seen in the so far tentatively observed dependence of stellar accretion rates on X-ray luminosities. According to models of X-ray driven photoevaporation, stars with higher X-ray luminosities should show lower accretion rates, on average, in a sample with similar masses and ages. Methods. To this aim, we have analyzed X-ray properties of young stars in the Orion Nebula Cluster determined with Chandra during the COUP observation as well as accretion data obtained from the photometric catalog of the HST Treasury Program. With these data, we have performed a statistical analysis of the relation between X-ray activity and accretion rates using partial linear regression analysis. Results. The initial anticorrelation found with a sample of 332 young stars is considerably weaker compared to previous studies. However, excluding flaring activity or limiting the X-ray luminosity to the soft band (0.5−2.0 keV) leads to a stronger anticorrelation, which is statistically more significant. Furthermore, we have found a weak positive correlation between the higher component of the plasma temperature gained in the X-ray spectral fitting and the accretion rates, indicating that the hardness of the X-ray spectra may influence the accretion process. Conclusions. There is evidence for a weak anticorrelation, as predicted by theoretical models, suggesting that X-ray photoevaporation modulates the accretion rate through the inner disk at late stages of disk evolution, leading to a phase of photoevaporation-starved accretion.