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Register a new userTwo timescale dispersal of magnetized protoplanetary disks
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Protoplanetary disks are likely to be threaded by a weak net flux of vertical magnetic field that is a remnant of the much larger fluxes present in molecular cloud cores. If this flux is approximately conserved its dynamical importance will increase as mass is accreted, initially by stimulating magnetorotational disk turbulence and subsequently by enabling wind angular momentum loss. We use fits to numerical simulations of ambipolar dominated disk turbulence to construct simplified one dimensional evolution models for weakly magnetized protoplanetary disks. We show that the late onset of significant angular momentum loss in a wind can give rise to two timescale disk evolution in which a long phase of viscous evolution precedes rapid dispersal as the wind becomes dominant. The wide dispersion in disk lifetimes could therefore be due to varying initial levels of net flux. Magnetohydrodynamic (MHD) wind triggered dispersal differs from photoevaporative dispersal in predicting mass loss from small (less that 1 AU) scales, where thermal winds are suppressed. Our specific models are based on a limited set of simulations that remain uncertain, but qualitatively similar evolution appears likely if mass is lost from disks more quickly than flux, and if MHD winds become important as the plasma beta decreases.
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We present a model for the dispersal of protoplanetary disks by winds from either the central star or the inner disk. These winds obliquely strike the flaring disk surface and strip away disk material by entraining it in an outward radial-moving flow at the wind-disk interface which lies several disk scale heights above the mid-plane. The disk dispersal time depends on the entrainment velocity at which disk material flows into this turbulent shear layer interface. If the entrainment efficiency is ~10% of the local sound speed, a likely upper limit, the dispersal time at 1 AU is ~6 Myr for a disk with a surface density of 10^3 g cm^{-2}, a solar mass central star, and a wind with an outflow rate 10^{-8} Msun/yr and terminal velocity 200 km/s. When compared to photoevaporation and viscous evolution, wind stripping can be a dominant mechanism only for the combination of low accretion rates (< 10^{-8} Msun/yr) and wind outflow rates approaching these accretion rates. This case is unusual since generally outflow rates are < 0.1 of of accretion rates.
We aim to understand the effect of stellar evolution on the evolution of protoplanetary disks. We focus in particular on the disk evolution around intermediate-mass (IM) stars, which evolve more rapidly than low-mass ones. We numerically solve the long-term evolution of disks around 0.5-5 solar-mass stars considering viscous accretion and photoevaporation (PE) driven by stellar far-ultraviolet (FUV), extreme-ultraviolet (EUV), and X-ray emission. We also take stellar evolution into account and consider the time evolution of the PE rate. We find that the FUV, EUV, and X-ray luminosities of IM stars evolve by orders of magnitude within a few Myr along with the time evolution of stellar structure, stellar effective temperature, or accretion rate. Therefore, the PE rate also evolves with time by orders of magnitude, and we conclude that stellar evolution is crucial for the disk evolution around IM stars.
The dispersal of protoplanetary disks sets the timescale available for planets to assemble, and thus it is one of the fundamental parameters in theories of planetary formation. Disk dispersal is determined by several properties of the central star, the disk itself, and the surrounding environment. In particular, the metallicity of disks may impact their evolution, even if to date controversial results exist: in low-metallicity clusters disks seem to rapidly disperse, while in the Magellanic Clouds some evidence supports the existence of accreting disks few tens of Myrs old. In this paper we study the dispersal timescale of disks in Dolidze~25, the young cluster in proximity of the Sun with lowest metallicity, with the aim of understanding whether disk evolution is impacted by the low-metallicity of the cluster. We have analyzed Chandra/ACIS-I observations of the cluster and combined the resulting source catalog with existing optical and infrared catalogs of the region. We selected the disk-bearing population and the disk-less population of Dolidze 25. We have derived stellar parameters from isochrones fitted to color-magnitude diagrams. We derived a disk fraction of about 34% and a median age of 1.2 Myrs. By comparing this estimate with existing estimates of the disk fraction of clusters younger than 10 Myrs, our study suggests that the disk fraction of Dolidze 25 is lower than what is expected from its age alone. Even if our results are not conclusive given the intrinsic uncertainty on stellar ages estimated from isochrones fitting to color-magnitude diagrams, we suggest that disk evolution in Dolidze 25 may be impacted by the environment. Given the poor O star population and low stellar density of the cluster, it is more likely that disks dispersal timescale is dictated more by the low metallicity of the cluster rather than external photoevaporation or dynamical encounters.
We investigate the roles of magnetically driven disk wind (MDW) and thermally driven photoevaporative wind (PEW) in the long-time evolution of protoplanetary disks. We start simulations from the early phase in which the disk mass is $0.118,{mathrm{M}_{odot}}$ around a $1,{mathrm{M}_{odot}}$ star and track the evolution until the disk is completely dispersed. We incorporate the mass loss by PEW and the mass loss and magnetic braking (wind torque) by MDW, in addition to the viscous accretion, viscous heating, and stellar irradiation. We find that MDW and PEW respectively have different roles: magnetically driven wind ejects materials from an inner disk in the early phase, whereas photoevaporation has a dominant role in the late phase in the outer ($gtrsim1,$au) disk. The disk lifetime, which depends on the combination of MDW, PEW, and viscous accretion, shows a large variation of $sim1$-$20,$Myr; the gas is dispersed mainly by the MDW and the PEW in the cases with a low viscosity and the lifetime is sensitive to the mass-loss rate and torque of the MDW, whereas the lifetime is insensitive to these parameters when the viscosity is high. Even in disks with very weak turbulence, the cooperation of MDW and PEW enables the disk dispersal within a few Myr.
The vertical shear instability (VSI) is a robust phenomenon in irradiated protoplanetary disks (PPDs). While there is extensive literature on the VSI in the hydrodynamic limit, PPDs are expected to be magnetized and their extremely low ionization fractions imply that non-ideal magneto-hydrodynamic (MHD) effects should be properly considered. To this end, we present linear analyses of the VSI in magnetized disks with Ohmic resistivity. We primarily consider toroidal magnetic fields, which are likely to dominate the field geometry in PPDs. We perform vertically global and radially local analyses to capture characteristic VSI modes with extended vertical structures. To focus on the effect of magnetism, we use a locally isothermal equation of state. We find that magnetism provides a stabilizing effect to dampen the VSI, with surface modes, rather than body modes, being the first to vanish with increasing magnetization. Subdued VSI modes can be revived by Ohmic resistivity, where sufficient magnetic diffusion overcome magnetic stabilization, and hydrodynamic results are recovered. We also briefly consider poloidal fields to account for the magnetorotational instability (MRI), which may develop towards surface layers in the outer parts of PPDs. The MRI grows efficiently at small radial wavenumbers, in contrast to the VSI. When resistivity is considered, we find the VSI dominates over the MRI for Ohmic Els{a}sser numbers $lesssim 0.09$ at plasma beta parameter $beta_Z sim 10^4$.
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