A possible mechanism for overcoming the electrostatic barrier against dust growth in protoplanetary disks

Ivlev

Caselli

2020

ApJ

Self Cite

The collisional evolution of solid material in protoplanetary disks is a crucial step in the formation of planetesimals, comets, and planets. Although dense protoplanetary environments favor fast dust coagulation, there are several factors that limit the straightforward pathway from interstellar micron-size grains to pebble-size aggregates. Apart from the grain bouncing, fragmentation, and fast drift to the central star, a notable limiting factor is the electrostatic repulsion of like-charged grains. In this study we aim at theoretical modeling of the dust coagulation coupled with the dust charging and disk ionization calculations. We show that the electrostatic barrier is a strong restraining factor to the coagulation of micrometer-size dust in dead zones of the disk (where the turbulence is suppressed). While the sustained turbulence helps to overcome the electrostatic barrier, low fractal dimensions of dust aggregates can potentially block their further coagulation even in this case. Coulomb repulsion may keep a significant fraction of small dust in the disk atmosphere and outer regions.

Section: Model Descriptionmentioning

confidence: 99%

Section: Model Descriptionmentioning

confidence: 99%

Inhibited Coagulation of Micron-size Dust Due to the Electrostatic Barrier

Ivlev

Caselli

2020

ApJ

Self Cite

“…Also grown dust makes disk more transparent to stellar radiation. However, the theoretical description of dust evolution is a complicated task, especially taking into account such poorly studied effects as grain charging (Okuzumi 2009;Akimkin 2015;Ivlev et al 2016). Therefore we neglect dust evolution effects and focus on varying disk macrophysical parameters.…”

Section: Chemical Modelmentioning

confidence: 99%

Gas Mass Tracers in Protoplanetary Disks: CO is Still the Best

Molyarova

Semenov

et al. 2017

ApJ

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%. CO 2 , H 2 O and H 2 CO can potentially serve as alternative mass tracers, the latter two being only applicable if disk structural parameters are known.

“…An accurate calculation of ionization-recombination balance in dense protoplanetary conditions is essential for understanding various fundamental problems, such as coupling of the gas with magnetic field (Li et al 2014), accretion processes (Turner et al 2014), chemistry (Semenov et al 2004;Larsson et al 2012) and dust evolution (Okuzumi et al 2011b;Akimkin 2015). Both the ionization and recombination processes can arise from several sources.…”

Section: Introductionmentioning

confidence: 99%

“…Along with the plasma charging, other charging mechanisms can operate in protoplanetary disks. In Akimkin (2015) the photoelectric emission from grains, induced by stellar radiation and leading to their positive charging, was considered as a mechanism to overcome the electrostatic barrier in upper disk regions. A similar mechanism -photoelectric charging due to H 2 fluorescence induced by cosmic rays (CRs) -operates in much deeper regions at the disk periphery (Ivlev et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Ionization and Dust Charging in Protoplanetary Disks

Ivlev

Caselli

2016

ApJ

Self Cite

Ionization-recombination balance in dense interstellar and circumstellar environments is a key factor for a variety of important physical processes, such as chemical reactions, dust charging and coagulation, coupling of the gas with magnetic field and development of instabilities in protoplanetary disks. We determine a critical gas density above which the recombination of electrons and ions on the grain surface dominates over the gas-phase recombination. For this regime, we present a self-consistent analytical model which allows us to exactly calculate abundances of charged species in dusty gas, without making assumptions on the grain charge distribution. To demonstrate the importance of the proposed approach, we check whether the conventional approximation of low grain charges is valid for typical protoplanetary disks, and discuss the implications for dust coagulation and development of the "dead zone" in the disk. The presented model is applicable for arbitrary grain-size distributions and, for given dust properties and conditions of the disk, has only one free parameter -the effective mass of the ions, shown to have a low effect on results. The model can be easily included in numerical simulations following the dust evolution in dense molecular clouds and protoplanetary disks.