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Register a new userConstraining disk evolution prescriptions of planet population synthesis models with observed disk masses and accretion rates
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Carlo Felice Manara
Publication date
2019
fields
Physics
and research's language is
English
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While planets are commonly discovered around main-sequence stars, the processes leading to their formation are still far from being understood. Current planet population synthesis models, which aim to describe the planet formation process from the protoplanetary disk phase to the time exoplanets are observed, rely on prescriptions for the underlying properties of protoplanetary disks where planets form and evolve. The recent development in measuring disk masses and disk-star interaction properties, i.e., mass accretion rates, in large samples of young stellar objects demand a more careful comparison between the models and the data. We performed an initial critical assessment of the assumptions made by planet synthesis population models by looking at the relation between mass accretion rates and disk masses in the models and in the currently available data. We find that the currently used disk models predict mass accretion rate in line with what is measured, but with a much lower spread of values than observed. This difference is mainly because the models have a smaller spread of viscous timescales than what is needed to reproduce the observations. We also find an overabundance of weakly accreting disks in the models where giant planets have formed with respect to observations of typical disks. We suggest that either fewer giant planets have formed in reality or that the prescription for planet accretion predicts accretion on the planets that is too high. Finally, the comparison of the properties of transition disks with large cavities confirms that in many of these objects the observed accretion rates are higher than those predicted by the models. On the other hand, PDS70, a transition disk with two detected giant planets in the cavity, shows mass accretion rates well in line with model predictions.
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We report on the impact of a probabilistic prescription for compact remnant masses and kicks on massive binary population synthesis. We find that this prescription populates the putative mass gap between neutron stars and black holes with low-mass black holes. However, evolutionary effects reduce the number of X-ray binary candidates with low-mass black holes, consistent with the dearth of such systems in the observed sample. We further find that this prescription is consistent with the formation of heavier binary neutron stars such as GW190425, but over-predicts the masses of Galactic double neutron stars. The revised natal kicks, particularly increased ultra-stripped supernova kicks, do not directly explain the observed Galactic double neutron star orbital period--eccentricity distribution. Finally, this prescription allows for the formation of systems similar to the recently discovered extreme mass ratio binary GW190814, but only if we allow for the survival of binaries in which the common envelope is initiated by a donor crossing the Hertzsprung gap, contrary to our standard model.
We report new global ideal MHD simulations for thin accretion disks (with thermal scale height H/R=0.1 and 0.05) threaded by net vertical magnetic fields. Our computations span three orders of magnitude in radius, extend all the way to the pole, and are evolved for more than one viscous time over the inner decade in radius. Static mesh refinement is used to properly resolve MRI. We find that:(1) inward accretion occurs mostly in the upper magnetically dominated regions of the disk, similar to the predictions from some previous analytical work and the coronal accretion in previous GRMHD simulations. Rapid inflow in the upper layers combined with slow outflow at the midplane creates strong $Rphi$ and $zphi$ stresses in the mean field; the vertically integrated $alphasim 0.5-1$ when the initial field has $beta_{0}=10^3$ at the midplane. (2) A quasi-static global field geometry is established in which flux transport by inflows at the surface is balanced by turbulent diffusion. The field is strongly pinched inwards at the surface. A steady-state advection-diffusion model, with turbulent magnetic Prandtl number of order unity, reproduces this geometry well. (3) Weak unsteady disk winds are launched at $z/Rsim1$ with the Alfven radius $R_{A}/R_{0}sim3$. Although the wind is episodic, the time averaged properties are well described by steady wind theory. Wind is not efficient at transporting angular momentum. Even with $beta_{0}=10^3$, only 5% of the angular momentum transport is driven by torque from the wind, and the wind mass flux from the inner decade of radius is only $sim$ 0.4% of the mass accretion rate. With weaker fields or thinner disks, the wind contributes even less. (4) Most of the disk accretion is driven by the $Rphi$ stress from the MRI and global magnetic fields. Our simulations have many applications to astrophysical accretion disk systems.
With the increasing number of exoplanets discovered, statistical properties of the population as a whole become unique constraints on planet formation models provided a link between the description of the detailed processes playing a role in this formation and the observed population can be established. Planet population synthesis provides such a link. The approach allows to study how different physical models of individual processes (e.g., proto-planetary disc structure and evolution, planetesimal formation, gas accretion, migration, etc.) affect the overall properties of the population of emerging planets. By necessity, planet population synthesis relies on simplified descriptions of complex processes. These descriptions can be obtained from more detailed specialised simulations of these processes. The objective of this chapter is twofold: 1) provide an overview of the physics entering in the two main approaches to planet population synthesis and 2) present some of the results achieved as well as illustrate how it can be used to extract constraints on the models and to help interpret observations.
We present a near-infrared direct imaging search for accretion signatures of possible protoplanets around the young stellar object (YSO) TW Hya, a multi-ring disk exhibiting evidence of planet formation. The Pa$beta$ line (1.282 $mu$m) is an indication of accretion onto a protoplanet, and its intensity is much higher than that of blackbody radiation from the protoplanet. We focused on the Pa$beta$ line and performed Keck/OSIRIS spectroscopic observations. Although spectral differential imaging (SDI) reduction detected no accretion signatures, the results of the present study allowed us to set 5$sigma$ detection limits for Pa$beta$ emission of $5.8times10^{-18}$ and $1.5times10^{-18}$ erg/s/cm$^2$ at 0farcs4 and 1farcs6, respectively. We considered the mass of potential planets using theoretical simulations of circumplanetary disks and hydrogen emission. The resulting masses were $1.45pm 0.04$ M$_{rm J}$ and $2.29 ^{+0.03}_{-0.04}$ M$_{rm J}$ at 25 and 95 AU, respectively, which agree with the detection limits obtained from previous broadband imaging. The detection limits should allow the identification of protoplanets as small as $sim$1 M$_{rm J}$, which may assist in direct imaging searches around faint YSOs for which extreme adaptive optics instruments are unavailable.
Atmospheric heavy elements have been observed in more than a quarter of white dwarfs (WDs) at different cooling ages, indicating ongoing accretion of asteroidal material, whilst only a few per cent of the WDs possess a dust disk, and all these WDs are accreting metals. Here, assuming that a rubble-pile asteroid is scattered inside a WDs Roche lobe by a planet, we study its tidal disruption and the long-term evolution of the resulting fragments. We find that after a few pericentric passages, the asteroid is shredded into its constituent particles, forming a flat, thin ring. On a timescale of Myr, tens of per cent of the particles are scattered onto the WD. Fragment mutual collisions are most effective for coplanar fragments, and are thus only important in $10^3-10^4$ yr before the orbital coplanarity is broken by the planet. We show that for a rubble pile asteroid with a size frequency distribution of the component particles following that of the near earth objects, it has to be roughly at least 10 km in radius such that enough fragments are generated and $ge10%$ of its mass is lost to mutual collisions. At relative velocities of tens of km/s, such collisions grind down the tidal fragments into smaller and smaller dust grains. The WD radiation forces may shrink those grains orbits, forming a dust disk. Tidal disruption of a monolithic asteroid creates large km-size fragments, and only parent bodies $ge100$ km are able to generate enough fragments for mutual collisions to be significant.
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