On the cavity of a debris disc carved by a giant planet

On the cavity of a debris disc carved by a giant planet


Authors:

Regály et al

Abstract:
One possible explanation of the cavity in debris discs is the gravitational perturbation of an embedded giant planet. Planetesimals passing close to a massive body are dynamically stirred resulting in a cleared region known as the chaotic zone. Theory of overlapping mean-motion resonances predicts the width of this cavity. To test whether this cavity is identical to the chaotic zone, we investigate the formation of cavities by means of collisionless N-body simulations assuming a 1.25–10 Jupiter mass planet with eccentricities of 0–0.9. Synthetic images at millimetre wavelengths are calculated to determine the cavity properties by fitting an ellipse to 14 per cent contour level. Depending on the planetary eccentricity, epl, the elliptic cavity wall rotates as the planet orbits with the same (epl less than 0.2) or half (epl greater than 0.2) period that of the planet. The cavity centre is offset from the star along the semimajor axis of the planet with a distance of d=0.1q−0.17e0.5pl d=0.1q−0.17epl0.5 in units of cavity size towards the planet's orbital apocentre, where q is the planet-to-star mass ratio. Pericentre (apocentre) glow develops for epl less than 0.05 (epl greater than 0.1), while both are present for 0.05 ≤ epl ≤ 0.1. Empirical formulae are derived for the sizes of the cavities: δacav = 2.35q0.36 and
δacav=7.87q0.37e0.38pl δacav=7.87q0.37epl0.38 for epl ≤ 0.05 and epl greater 0.05, respectively. The cavity eccentricity, ecav, equals to that of the planet only for 0.3 ≤ epl ≤ 0.6. A new method based on Atacama Large Millimeter/submillimeter Array observations for estimating the orbital parameters and mass of the planet carving the cavity is also given.

A study of dust properties in the inner sub-au region of the Herbig Ae star HD 169142 with VLTI/PIONIER

A study of dust properties in the inner sub-au region of the Herbig Ae star HD 169142 with VLTI/PIONIER

Authors: 

Chen et al 

Abstract:

An essential step to understanding protoplanetary evolution is the study of disks that contain gaps or inner holes. The pretransitional disk around the Herbig star HD 169142 exhibits multi-gap disk structure, differentiated gas and dust distribution, planet candidates, and near-infrared fading in the past decades, which make it a valuable target for a case study of disk evolution. Using near-infrared interferometric observations with VLTI/PIONIER, we aim to study the dust properties in the inner sub-au region of the disk in the years 2011-2013, when the object is already in its near-infrared faint state. We first performed simple geometric modeling to characterize the size and shape of the NIR-emitting region. We then performed Monte-Carlo radiative transfer simulations on grids of models and compared the model predictions with the interferometric and photometric observations. We find that the observations are consistent with optically thin gray dust lying at Rin ~ 0.07 au, passively heated to T ~ 1500 K. Models with sub-micron optically thin dust are excluded because such dust will be heated to much higher temperatures at similar distance. The observations can also be reproduced with a model consisting of optically thick dust at Rin ~ 0.06 au, but this model is plausible only if refractory dust species enduring ~2400 K exist in the inner disk.

Gas dynamics in the inner few AU around the Herbig B[e] star MWC297: Indications of a disk wind from kinematic modeling and velocity-resolved interferometric imaging

Gas dynamics in the inner few AU around the Herbig B[e] star MWC297: Indications of a disk wind from kinematic modeling and velocity-resolved interferometric imaging 

Authors:

Hone et al 

Abstract:

We present near-infrared AMBER (R = 12, 000) and CRIRES (R = 100, 000) observations of the Herbig B[e] star MWC297 in the hydrogen Br-gamma-line. Using the VLTI unit telescopes, we obtained a uv-coverage suitable for aperture synthesis imaging. We interpret our velocity-resolved images as well as the derived two-dimensional photocenter displacement vectors, and fit kinematic models to our visibility and phase data in order to constrain the gas velocity field on sub-AU scales. The measured continuum visibilities constrain the orientation of the near-infrared-emitting dust disk, where we determine that the disk major axis is oriented along a position angle of 99.6 +/- 4.8 degrees. The near-infrared continuum emission is 3.6 times more compact than the expected dust-sublimation radius, possibly indicating the presence of highly refractory dust grains or optically thick gas emission in the inner disk. Our velocity-resolved channel maps and moment maps reveal the motion of the Br-gamma-emitting gas in six velocity channels, marking the first time that kinematic effects in the sub-AU inner regions of a protoplanetary disk could be directly imaged. We find a rotation-dominated velocity field, where the blue- and red-shifted emissions are displaced along a position angle of 24 +/- 3 degrees and the approaching part of the disk is offset west of the star. The visibility drop in the line as well as the strong non-zero phase signals can be modeled reasonably well assuming a Keplerian velocity field, although this model is not able to explain the 3 sigma difference that we measure between the position angle of the line photocenters and the position angle of the dust disk. We find that the fit can be improved by adding an outflowing component to the velocity field, as inspired by a magneto-centrifugal disk-wind scenario.

Vortex stretching in self-gravitating protoplanetary discs

Vortex stretching in self-gravitating protoplanetary discs 

Authors: 

Regaly et al 

Abstract:
Horseshoe-shaped brightness asymmetries of several transitional discs are thought to be caused by large-scale vortices. Anticyclonic vortices are efficiently collect dust particles, therefore they can play a major role in planet formation. Former studies suggest that the disc self-gravity weakens vortices formed at the edge of the gap opened by a massive planet in discs whose masses are in the range of 0.01 less than M_disc/M_* less than 0.1. Here we present an investigation on the long-term evolution of the large-scale vortices formed at the viscosity transition of the discs' dead zone outer edge by means of two-dimensional hydrodynamic simulations taking disc self-gravity into account. We perform a numerical study of low mass, 0.001 less than M_disc/M_* less than 0.01, discs, for which cases disc self-gravity was previously neglected. The large-scale vortices are found to be stretched due to disc self-gravity even for low-mass discs with M_disc/M_*>=0.005 where initially the Toomre Q-parameter was less than or equal 50 at the vortex distance. As a result of stretching, the vortex aspect ratio increases and a weaker azimuthal density contrast develops. The strength of the vortex stretching is proportional to the disc mass. The vortex stretching can be explained by a combined action of a non-vanishing gravitational torque caused by the vortex, and the Keplerian shear of the disc. Self-gravitating vortices are subject to significantly faster decay than non-self-gravitating ones. We found that vortices developed at sharp viscosity transitions of self-gravitating discs can be described by a GNG model as long as the disc viscosity is low, i.e. alpha_dz less than or equal to 10^-5.

Unveiling the physical and chemical conditions in the young disk around L1527

Unveiling the physical and chemical conditions in the young disk around L1527

Authors:

van 't Hoff et al

Abstract:

Planets form in disks around young stars. The planet formation process may start when the protostar and disk are still deeply embedded within their infalling envelope. However, unlike more evolved protoplanetary disks, the physical and chemical structure of these young embedded disks are still poorly constrained. We have analyzed ALMA data for 13CO, C18O and N2D+ to constrain the temperature structure, one of the critical unknowns, in the disk around L1527. The spatial distribution of 13CO and C18O, together with the kinetic temperature derived from the optically thick 13CO emission and the non-detection of N2D+, suggest that this disk is warm enough (≳ 20 K) to prevent CO freeze-out.

Pebble Accretion in Turbulent Protoplanetary Disks

Pebble Accretion in Turbulent Protoplanetary Disks 

Authors: 

Xu et al 

Abstract: 

It has been realized in recent years that the accretion of pebble-sized dust particles onto planetary cores is an important mode of core growth, which enables the formation of giant planets at large distances and assists planet formation in general. The pebble accretion theory is built upon the orbit theory of dust particles in a laminar protoplanetary disk (PPD). For sufficiently large core mass (in the "Hill regime"), essentially all particles of appropriate sizes entering the Hill sphere can be captured. However, the outer regions of PPDs are expected to be weakly turbulent due to the magnetorotational instability (MRI), where turbulent stirring of particle orbits may affect the efficiency of pebble accretion. We conduct shearing-box simulations of pebble accretion with different levels of MRI turbulence (strongly turbulent assuming ideal magnetohydrodynamics, weakly turbulent in the presence of ambipolar diffusion, and laminar) and different core masses to test the efficiency of pebble accretion at a microphysical level. We find that accretion remains efficient for marginally coupled particles (dimensionless stopping time ${\tau }_{s}\sim 0.1\mbox{--}1$) even in the presence of strong MRI turbulence. Though more dust particles are brought toward the core by the turbulence, this effect is largely canceled by a reduction in accretion probability. As a result, the overall effect of turbulence on the accretion rate is mainly reflected in the changes in the thickness of the dust layer. On the other hand, we find that the efficiency of pebble accretion for strongly coupled particles (down to ${\tau }_{s}\sim 0.01$) can be modestly reduced by strong turbulence for low-mass cores.

Linear Analysis of the Evolution of Nearly Polar Low Mass Circumbinary Discs

Linear Analysis of the Evolution of Nearly Polar Low Mass Circumbinary Discs

Authors:

Lubow et al

Abstract:

Martin & Lubow (2017) showed through simulations that an initially tilted disc around an eccentric binary can evolve to polar alignment in which the disc lies perpendicular to the binary orbital plane. We apply linear theory to show both analytically and numerically that a nearly polar aligned low mass circumbinary disc evolves to polar alignment and determine the alignment timescale. Significant disc evolution towards the polar state around moderately eccentric binaries can occur for typical protostellar disc parameters in less than a typical disc lifetime for binaries with orbital periods of order 100 years or less. Resonant torques are much less effective at truncating the inner parts of circumbinary polar discs than the inner parts of coplanar discs. For polar discs, they vanish for a binary eccentricity of unity. The results agree with the simulations in showing that discs can evolve to a polar state. Circumbinary planets may then form in such discs and reside on polar orbits.

High-cadence, High-resolution Spectroscopic Observations of Herbig Stars HD 98922 and V1295 Aquila

High-cadence, High-resolution Spectroscopic Observations of Herbig Stars HD 98922 and V1295 Aquila 

Authors:

Aarnio et al

Abstract:
Recent observational work has indicated that mechanisms for accretion and outflow in Herbig Ae/Be star–disk systems may differ from magnetospheric accretion (MA) as it is thought to occur in T Tauri star–disk systems. In this work, we assess the temporal evolution of spectral lines probing accretion and mass loss in Herbig Ae/Be systems and test for consistency with the MA paradigm. For two Herbig Ae/Be stars, HD 98922 (B9e) and V1295 Aql (A2e), we have gathered multi-epoch (~years) and high-cadence (~minutes) high-resolution optical spectra to probe a wide range of kinematic processes. Employing a line equivalent width evolution correlation metric introduced here, we identify species co-evolving (indicative of common line origin) via novel visualization. We interferometrically constrain often problematically degenerate parameters, inclination and inner-disk radius, allowing us to focus on the structure of the wind, magnetosphere, and inner gaseous disk in radiative transfer models. Over all timescales sampled, the strongest variability occurs within the blueshifted absorption components of the Balmer series lines; the strength of variability increases with the cadence of the observations. Finally, high-resolution spectra allow us to probe substructure within the Balmer series' blueshifted absorption components: we observe static, low-velocity features and time-evolving features at higher velocities. Overall, we find the observed line morphologies and variability are inconsistent with a scaled-up T Tauri MA scenario. We suggest that as magnetic field structure and strength change dramatically with increasing stellar mass from T Tauri to Herbig Ae/Be stars, so too may accretion and outflow processes.

The Architecture of the GW Ori Young Triple Star System and Its Disk: Dynamical Masses, Mutual Inclinations, and Recurrent Eclipses

The Architecture of the GW Ori Young Triple Star System and Its Disk: Dynamical Masses, Mutual Inclinations, and Recurrent Eclipses 

Authors: 

Czekala et al 

Abstract: 

We present spatially and spectrally resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations of gas and dust orbiting the pre-main sequence hierarchical triple star system GW Ori. A forward-modeling of the 13CO and C18O J=2--1 transitions permits a measurement of the total stellar mass in this system, 5.29±0.09M⊙, and the circum-triple disk inclination, 137.6±2.0∘. Optical spectra spanning a 35 year period were used to derive new radial velocities and, coupled with a spectroscopic disentangling technique, revealed that the A and B components of GW Ori form a double-lined spectroscopic binary with a 241.50±0.05 day period; a tertiary companion orbits that inner pair with a 4218±50 day period. Combining the results from the ALMA data and the optical spectra with three epochs of astrometry in the literature, we constrain the individual stellar masses in the system (MA≈2.7M⊙, MB≈1.7M⊙, MC≈0.9M⊙) and find strong evidence that at least one (and likely both) stellar orbital planes are misaligned with the disk plane by as much as 45∘. A V-band light curve spanning 30 years reveals several new ∼30 day eclipse events 0.1-0.7 mag in depth and a 0.2 mag sinusoidal oscillation that is clearly phased with the AB-C orbital period. Taken together, these features suggest that the A-B pair may be partially obscured by material in the inner disk as tertiary approaches apoastron. Lastly, we conclude that stellar evolutionary models are consistent with our measurements of the masses and basic photospheric properties if the GW Ori system is ∼1 Myr old.

Three years of SPHERE: the latest view of the morphology and evolution of protoplanetary discs

Three years of SPHERE: the latest view of the morphology and evolution of protoplanetary discs 

Authors:

Garufi et al

Abstract:
Spatially resolving the immediate surroundings of young stars is a key challenge for the planet formation community. SPHERE on the VLT represents an important step forward by increasing the opportunities offered by optical or near-infrared imaging instruments to image protoplanetary discs. The Guaranteed Time Observation Disc team has concentrated much of its efforts on polarimetric differential imaging, a technique that enables the efficient removal of stellar light and thus facilitates the detection of light scattered by the disc within a few au from the central star. These images reveal intriguing complex disc structures and diverse morphological features that are possibly caused by ongoing planet formation in the disc. An overview of the recent advances enabled by SPHERE is presented.

Dynamical models to explain observations with SPHERE in planetary systems with double debris belts

Dynamical models to explain observations with SPHERE in planetary systems with double debris belts

Authors:
Lazzoni et al

Abstract:
A large number of systems harboring a debris disk show evidence for a double belt architecture. One hypothesis for explaining the gap between the belts is the presence of one or more planets dynamically carving it. This work aims to investigate this scenario in systems harboring two components debris disks. All the targets in the sample were observed with the SPHERE instrument which performs high-contrast direct imaging. Positions of the inner and outer belts were estimated by SED fitting of the infrared excesses or, when available, from resolved images of the disk. Very few planets have been observed so far in debris disks gaps and we intended to test if such non-detections depend on the observational limits of the present instruments. This aim is achieved by deriving theoretical predictions of masses, eccentricities and semi-major axes of planets able to open the observed gaps and comparing such parameters with detection limits obtained with SPHERE. The relation between the gap and the planet is due to the chaotic zone around the orbit of the planet. The radial extent of this zone depends on the mass ratio between the planet and the star, on the semi-major axis and on the eccentricity of the planet and it can be estimated analytically. We apply the formalism to the case of one planet on a circular or eccentric orbit. We then consider multi-planetary systems: 2 and 3 equal-mass planets on circular orbits and 2 equal-mass planets on eccentric orbits in a packed configuration. We then compare each couple of values (M,a), derived from the dynamical analysis of single and multiple planetary models, with the detection limits obtained with SPHERE. Our results show that the apparent lack of planets in gaps between double belts could be explained by the presence of a system of two or more planets possibly of low mass and on an eccentric orbits whose sizes are below the present detection limits.

Variable dynamics in the inner disk of HD 135344B revealed with multi-epoch scattered light imaging

Variable dynamics in the inner disk of HD 135344B revealed with multi-epoch scattered light imaging

 

Authors:

Stolker et al

Abstract:

We present multi-epoch VLT/SPHERE observations of the protoplanetary disk around HD 135344B (SAO 206462). The J-band scattered light imagery reveal, with high spatial resolution (∼41 mas, 6.4 au), the disk surface beyond ∼20 au. Temporal variations are identified in the azimuthal brightness distributions of all epochs, presumably related to the asymmetrically shading dust distribution in the inner disk. These shadows manifest themselves as narrow lanes, cast by localized density enhancements, and broader features which possibly trace the larger scale dynamics of the inner disk. We acquired visible and near-infrared photometry which shows variations up to 10% in the JHK bands, possibly correlated with the presence of the shadows. Analysis of archival VLTI/PIONIER H-band visibilities constrain the orientation of the inner disk to i=18.2deg+3.4−4.1 and PA=57.3deg±5.7deg, consistent with an alignment with the outer disk or a minor disk warp of several degrees. The latter scenario could explain the broad, quasi-stationary shadowing in N-NW direction in case the inclination of the outer disk is slightly larger. The correlation between the shadowing and the near-infrared excess is quantified with a grid of radiative transfer models. The variability of the scattered light contrast requires extended variations in the inner disk atmosphere (H/r≲0.2). Possible mechanisms that may cause asymmetric variations in the optical depth (Δτ≲1) through the atmosphere of the inner disk include turbulent fluctuations, planetesimal collisions, or a dusty disk wind, possibly enhanced by a minor disk warp. A fine temporal sampling is required to follow day-to-day changes of the shadow patterns which may be a face-on variant of the UX Orionis phenomenon.

Optically Thin Core Accretion: How Planets Get Their Gas in Nearly Gas-Free Disks

Optically Thin Core Accretion: How Planets Get Their Gas in Nearly Gas-Free Disks 

Authors:

Lee et al

Abstract:

Models of core accretion assume that in the radiative zones of accreting gas envelopes, radiation diffuses. But super-Earths/sub-Neptunes (1--4R⊕, 2--20M⊕) point to formation conditions that are optically thin: their modest gas masses are accreted from short-lived and gas-poor nebulae reminiscent of the transparent cavities of transitional disks. Planetary atmospheres born in such environments can be optically thin to both incident starlight and internally generated thermal radiation. We construct time-dependent models of such atmospheres, showing that super-Earths/sub-Neptunes can accrete their ∼1\%-by-mass gas envelopes, and super-puffs/sub-Saturns their ∼20\%-by-mass envelopes, over a wide range of nebular depletion histories requiring no fine tuning. Although nascent atmospheres can exhibit stratospheric temperature inversions effected by atomic Fe and various oxides that absorb strongly at visible wavelengths, the rate of gas accretion remains controlled by the radiative-convective boundary (rcb) at much greater pressures. For dusty envelopes, the temperature at the rcb Trcb≃2500 K is still set by H2 dissociation; for dust-depleted envelopes, Trcb tracks the temperature of the visible or thermal photosphere, whichever is deeper, out to at least ∼5 AU. The rate of envelope growth remains largely unchanged between the old radiative diffusion models and the new optically thin models, reinforcing how robustly super-Earths form as part of the endgame chapter in disk evolution.

Gas mass tracers in protoplanetary disks: CO is still the best

Gas mass tracers in protoplanetary disks: CO is still the best

Authors:

Molyarova et al

Abstract:
Protoplanetary disk mass is a key parameter controlling the process of planetary system formation. CO molecular emission is often used as a tracer of gas mass in the disk. In this study we consider the ability of CO to trace the gas mass over a wide range of disk structural parameters and search for chemical species that could possibly be used as alternative mass tracers to CO. Specifically, we apply detailed astrochemical modeling to a large set of models of protoplanetary disks around low-mass stars, to select molecules with abundances correlated with the disk mass and being relatively insensitive to other disk properties. We do not consider sophisticated dust evolution models, restricting ourselves with the standard astrochemical assumption of 0.1 μm dust. We find that CO is indeed the best molecular tracer for total gas mass, despite the fact that it is not the main carbon carrier, provided reasonable assumptions about CO abundance in the disk are used. Typically, chemical reprocessing lowers the abundance of CO by a factor of 3, compared to the case of photo-dissociation and freeze-out as the only ways of CO depletion. On average only 13% C-atoms reside in gas-phase CO, albeit with variations from 2 to 30%. CO2, H2O and H2CO can potentially serve as alternative mass tracers, the latter two being only applicable if disk structural parameters are known.

The Effects of Protostellar Disk Turbulence on CO Emission Lines: A Comparison Study of Disks with Constant CO Abundance vs. Chemically Evolving Disks

The Effects of Protostellar Disk Turbulence on CO Emission Lines: A Comparison Study of Disks with Constant CO Abundance vs. Chemically Evolving Disks 

Authors:

Yu et al

Abstract:
Turbulence is the leading candidate for angular momentum transport in protoplanetary disks and therefore influences disk lifetimes and planet formation timescales. However, the turbulent properties of protoplanetary disks are poorly constrained observationally. Simon et al. (2015) suggested that the ratio of the peak line flux to the flux at line center of the CO J=3-2 transition is a reasonable diagnostic of turbulence, while Flaherty et al. (2015) and Flaherty et al. (2017) found turbulent speeds in HD 163296 smaller than what fully-developed MRI would produce based on the Simon et al. (2015) simulation results. Yet Simon et al. (2015) and Flaherty et al. (2015) assumed a constant CO/H2 ratio of 0.0001 in locations where CO is not frozen-out or photodissociated. Yu et al. (2016) found that the CO abundance varies both with distance from the star and as a function of time because CO molecules are gradually dissociated, with the liberated carbon forming complex organic molecules that freeze out on grain surfaces. We simulate the emission lines of CO based on chemical evolution models presented in Yu et al. (2016), and find that the peak-to-trough ratio changes as a function of time as CO is destroyed. Specifically, a CO-depleted disk with high turbulent velocity mimics the peak-to-trough ratios of a non-CO-depleted disk with lower turbulent velocity. We suggest that disk observers and modelers take into account the possibility of CO depletion when using line peak-to-trough ratios to constrain the degree of turbulence in disks. Assuming that CO/H2 = 0.0001 at all disk radii can lead to underestimates of turbulent speeds in the disk by at least 0.2 km/s.

X-ray photoevaporation’s limited success in the formation of planetesimals by the streaming instability

X-ray photoevaporation’s limited success in the formation of planetesimals by the streaming instability

Authors:

Ercolano et al

Abstract:

The streaming instability is often invoked as solution to the fragmentation and drift barriers in planetesimal formation, catalysing the aggregation of dust on kyr time-scales to grow km-sized cores. However, there remains a lack of consensus on the physical mechanism(s) responsible for initiating it. One potential avenue is disc photoevaporation, wherein the preferential removal of relatively dust-free gas increases the disc metallicity. Late in the disc lifetime, photoevaporation dominates viscous accretion, creating a gradient in the depleted gas surface density near the location of the gap. This induces a local pressure maximum that collects drifting dust particles, which may then become susceptible to the streaming instability. Using a one-dimensional viscous evolution model of a disc subject to internal X-ray photoevaporation, we explore the efficacy of this process to build planetesimals. Over a range of parameters, we find that the amount of dust mass converted into planetesimals is often <1 a="" across="" an="" and="" are="" at="" au.="" be="" best="" common="" conclude="" considered="" contrast="" cores.="" debris="" discs="" driven="" far-ultra-violet="" few="" for="" formation="" in="" investigation="" large="" m="" may="" mechanism="" model="" most="" of="" our="" photoevaporation="" planetary="" rather="" recent="" relevant="" reported="" results="" similar="" spread="" tens="" than="" that="" the="" to="" we="">100 au) disc radii. The discrepancies are primarily a consequence of the different photoevaporation profiles assumed. Until observations more tightly constrain photoevaporation models, the relevance of this process to the formation of planets remains uncertain.

The structure of young embedded protostellar discs

The structure of young embedded protostellar discs

Authors:

MacFarlane et al

Abstract:
Young protostellar discs provide the initial conditions for planet formation. The properties of these discs may be different from those of late-phase (T Tauri) discs due to continuing infall from the envelope and protostellar variability resulting from irregular gas accretion. We use a set of hydrodynamic simulations to determine the structure of discs forming in collapsing molecular clouds. We examine how radiative feedback from the host protostar affects the disc properties by examining three regimes: without radiative feedback, with continuous radiative feedback and with episodic feedback, similar to FU Ori-type outbursts. We find that the radial surface density and temperature profiles vary significantly as the disc accretes gas from the infalling envelope. These profiles are sensitive to the presence of spiral structure, induced by gravitational instabilities, and the radiative feedback provided by the protostar, especially in the case when the feedback is episodic. We also investigate whether mass estimates from position–velocity (PV) diagrams are accurate for early-phase discs. We find that the protostellar system mass (i.e. the mass of the protostar and its disc) is underestimated by up to 20 per cent, due to the impact of an enhanced radial pressure gradient on the gas. The mass of early-phase discs is a significant fraction of the mass of the protostar, so PV diagrams cannot accurately provide the mass of the protostar alone. The enhanced radial pressure gradient expected in young discs may lead to an increased rate of dust depletion due to gas drag, and therefore to a reduced dust-to-gas ratio.

Is There a Temperature Limit in Planet Formation at 1000 K?

Is There a Temperature Limit in Planet Formation at 1000 K?

Authors:

Demirci et al

Abstract:
Dust drifting inward in protoplanetary disks is subject to increasing temperatures. In laboratory experiments, we tempered basaltic dust between 873 K and 1273 K and find that the dust grains change in size and composition. These modifications influence the outcome of self-consistent low speed aggregation experiments showing a transition temperature of 1000\,K. Dust tempered at lower temperatures grows to a maximum aggregate size of 2.02±0.06 mm, which is 1.49±0.08 times the value for dust tempered at higher temperatures. A similar size ratio of 1.75±0.16 results for a different set of collision velocities. This transition temperature is in agreement with orbit temperatures deduced for observed extrasolar planets. Most terrestrial planets are observed at positions equivalent to less than 1000 K. Dust aggregation on the millimeter-scale at elevated temperatures might therefore be a key factor for terrestrial planet formation.

Episodic accretion: the interplay of infall and disc instabilities

Episodic accretion: the interplay of infall and disc instabilities

Authors:

Kuffmeier et al

Abstract:
Using zoom-simulations carried out with the adaptive mesh-refinement code RAMSES with a dynamic range of up to 227≈1.34×108 we investigate the accretion profiles around six stars embedded in different environments inside a (40 pc)3 giant molecular cloud, the role of mass infall and disc instabilities on the accretion profile, and thus on the luminosity of the forming protostar. Our results show that the environment in which the protostar is embedded determines the overall accretion profile of the protostar. Infall onto the circumstellar disc may trigger gravitational disc instabilities in the disc at distances of around ~10 to ~50 AU leading to rapid transport of angular momentum and strong accretion bursts. These bursts typically last for about ~10 to a ~100 years, consistent with typical orbital times at the location of the instability, and enhance the luminosity of the protostar. Calculations with the stellar evolution code \mesa\ show that the accretion bursts induce significant changes in the protostellar properties, such as the stellar temperature and radius. We apply the obtained protostellar properties to produce synthetic observables with RadMC and predict that accretion bursts lead to observable enhancements around 20 to 200 μm in the spectral energy distribution of Class 0 type young stellar objects.

Mottled protoplanetary disk ionization by magnetically-channeled T Tauri star energetic particles

Mottled protoplanetary disk ionization by magnetically-channeled T Tauri star energetic particles 

Authors: 

Fraschetti et al 

Abstract:
The evolution of protoplanetary disks is believed to be driven largely by angular momentum transport resulting from magnetized disk winds and turbulent viscosity. The ionization of the disk that is essential for these processes has been thought due to host star coronal X-rays but could also arise from energetic particles produced by coronal flares or by travelling shock waves and advected by the stellar wind. We have performed test-particle numerical simulations of energetic protons propagating into a realistic T~Tauri stellar wind, including a superposed small-scale magnetostatic turbulence. The isotropic (Kolmogorov power spectrum) turbulent component is synthesised along the individual particle trajectories. We have investigated the particle energy range, [0.1−10] GeV, consistent with expectations from {\it Chandra} X-ray observations of large flares on T~Tauri stars and with recent indications by the {\it Herschel} Space Observatory of a significant contribution of energetic particles to the disk ionization of young stars. In contrast with a previous theoretical study finding dominance of energetic particle ionization over X-ray ionization throughout the disk, we find that the disk ionization is likely dominated by X-rays over much of its area except within narrow regions where the energetic particles are channeled onto the disk by the strongly-tangled and turbulent magnetic field lines. The radial thickness of such regions is ∼5 stellar radii close to the star and broadens with increasing radial distance. In these regions, the disk ionization due to energetic particles can locally dominate the stellar X-ray contribution. This likely continues out to large distances from the star (10 AU or greater) where particles can be copiously advected and diffused by the turbulent wind.