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Register a new userPhysical processes in protoplanetary disks
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Philip J. Armitage
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This review introduces physical processes in protoplanetary disks relevant to accretion and the initial stages of planet formation. After a brief overview of the observational context, I introduce the elementary theory of disk structure and evolution, review the gas-phase physics of angular momentum transport through turbulence and disk winds, and discuss possible origins for the episodic accretion observed in Young Stellar Objects. Turning to solids, I review the evolution of single particles under aerodynamic forces, and describe the conditions necessary for the development of collective gas-particle instabilities. Observations show that disks can exhibit pronounced large-scale structure, and I discuss the types of structures that may form from gas and particle interactions at ice lines, vortices and zonal flows, prior to the formation of large planetary bodies. I conclude with disk dispersal.
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Magnetic fields are fundamental to the accretion dynamics of protoplanetary disks and they likely affect planet formation. Typical methods to study the magnetic field morphology observe the polarization of dust or spectral lines. However, it has recently become clear that dust-polarization in ALMAs spectral regime not always faithfully traces the magnetic field structure of protoplanetary disks, which leaves spectral line polarization as a promising method of mapping the magnetic field morphologies of such sources. We aim to model the emergent polarization of different molecular lines in the ALMA wavelength regime that are excited in protoplanetary disks. We explore a variety of disk models and molecules to identify those properties that are conducive to the emergence of polarization in spectral lines and may therefore be viably used for magnetic field measurements in protoplanetary disks. Methods. We use PORTAL (POlarized Radiative Transfer Adapted to Lines) in conjunction with LIME (Line Emission Modeling Engine). Together, they allow us to treat the polarized line radiative transfer of complex three-dimensional physical and magnetic field structures. We present simulations of the emergence of spectral line polarization of different molecules and molecular transitions in the ALMA wavelength regime and we comment on the observational feasibility of ALMA linear polarization observations of protoplanetary disks. We find that molecules that thermalize at high densities, such as HCN, are also most susceptible to polarization. We find that such molecules are expected to be significantly polarized in protoplanetary disks, while molecules that thermalize at low densities, such as CO, are only significantly polarized in the outer disk regions.
We present arcsecond-scale Submillimeter Array observations of the CO(3-2) line emission from the disks around the young stars HD 163296 and TW Hya at a spectral resolution of 44 m/s. These observations probe below the ~100 m/s turbulent linewidth inferred from lower-resolution observations, and allow us to place constraints on the turbulent linewidth in the disk atmospheres. We reproduce the observed CO(3-2) emission using two physical models of disk structure: (1) a power-law temperature distribution with a tapered density distribution following a simple functional form for an evolving accretion disk, and (2) the radiative transfer models developed by DAlessio et al. that can reproduce the dust emission probed by the spectral energy distribution. Both types of models yield a low upper limit on the turbulent linewidth (Doppler b-parameter) in the TW Hya system (<40 m/s), and a tentative (3-sigma) detection of a ~300 m/s turbulent linewidth in the upper layers of the HD 163296 disk. These correspond to roughly <10% and 40% of the sound speed at size scales commensurate with the resolution of the data. The derived linewidths imply a turbulent viscosity coefficient, alpha, of order 0.01 and provide observational support for theoretical predictions of subsonic turbulence in protoplanetary accretion disks.
Aims and Methods. Accretion bursts triggered by the magnetorotational instability (MRI) in the innermost disk regions were studied for protoplanetary gas-dust disks formed from prestellar cores of various mass $M_{rm core}$ and mass-to-magnetic flux ratio $lambda$. Numerical magnetohydrodynamics simulations in the thin-disk limit were employed to study the long-term ($sim 1.0$~Myr) evolution of protoplanetary disks with an adaptive turbulent $alpha$-parameter, which depends explicitly on the strength of the magnetic field and ionization fraction in the disk. The numerical models also feature the co-evolution of gas and dust, including the back-reaction of dust on gas and dust growth. Results. Dead zone with a low ionization fraction $x <= 10^{-13}$ and temperature on the order of several hundred Kelvin forms in the inner disk soon after its formation, extending from several to several tens of astronomical units depending on the model. The dead zone features pronounced dust rings that are formed due to the concentration of grown dust particles in the local pressure maxima. Thermal ionization of alkaline metals in the dead zone trigger the MRI and associated accretion burst, which is characterized by a sharp rise, small-scale variability in the active phase, and fast decline once the inner MRI-active region is depleted of matter. The burst occurrence frequency is highest in the initial stages of disk formation, and is driven by gravitational instability (GI), but declines with diminishing disk mass-loading from the infalling envelope. There is a causal link between the initial burst activity and the strength of GI in the disk fueled by mass infall from the envelope. Abridged.
The volatile composition of a planet is determined by the inventory of gas and ice in the parent disk. The volatile chemistry in the disk is expected to evolve over time, though this evolution is poorly constrained observationally. We present ALMA observations of C18O, C2H, and the isotopologues H13CN, HC15N, and DCN towards five Class 0/I disk candidates. Combined with a sample of fourteen Class II disks presented in Bergner et al. (2019b), this data set offers a view of volatile chemical evolution over the disk lifetime. Our estimates of C18O abundances are consistent with a rapid depletion of CO in the first ~0.5-1 Myr of the disk lifetime. We do not see evidence that C2H and HCN formation are enhanced by CO depletion, possibly because the gas is already quite under-abundant in CO. Further CO depletion may actually hinder their production by limiting the gas-phase carbon supply. The embedded sources show several chemical differences compared to the Class II stage, which seem to arise from shielding of radiation by the envelope (impacting C2H formation and HC15N fractionation) and sublimation of ices from infalling material (impacting HCN and C18O abundances). Such chemical differences between Class 0/I and Class II sources may affect the volatile composition of planet-forming material at different stages in the disk lifetime.
We present ALMA observations of 101 protoplanetary disks within the star-forming region Lynds 1641 in the Orion Molecular Cloud A. Our observations include 1.33 mm continuum emission and spectral windows covering the J=2-1 transition of $^{12}$CO, $^{13}$CO, and C$^{18}$O. We detect 89 protoplanetary disks in the dust continuum at the 4$sigma$ level ($sim$88% detection rate) and 31 in $^{12}$CO, 13 in $^{13}$CO, and 4 in C$^{18}$O. Our sample contains 23 transitional disks, 20 of which are detected in the continuum. We target infrared-bright Class II objects, which biases our sample towards massive disks. We determine dust masses or upper limits for all sources in our sample and compare our sample to protostars in this region. We find a decrease in dust mass with evolutionary state. We also compare this sample to other regions surveyed in the (sub-)millimeter and find that Lynds 1641 has a relatively massive dust disk population compared to regions of similar and older ages, with a median dust mass of 11.1$^{+32.9}_{-4.6}$ $M_oplus$ and 27% with dust masses equal to or greater than the minimum solar nebula dust mass value of $sim$30 $M_oplus$. We analyze the disk mass-accretion rate relationship in this sample and find that the viscous disk lifetimes are similar to the age of the region, however with a large spread. One object, [MGM2012] 512, shows large-scale ($>$5000 AU) structure in both the dust continuum and the three gas lines. We discuss potential origins for this emission, including an accretion streamer with large dust grains.
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