Coagulation Instability in Protoplanetary Disks: A Novel Mechanism Connecting Collisional Growth and Hydrodynamical Clumping of Dust Particles

Abstract

We present a new instability driven by a combination of coagulation and radial drift of dust particles. We refer to this instability as “coagulation instability” and regard it as a promising mechanism to concentrate dust particles and assist planetesimal formation in the very early stages of disk evolution. Because of dust-density dependence of collisional coagulation efficiency, dust particles efficiently (inefficiently) grow in a region of positive (negative) dust density perturbations, leading to a small radial variation of dust sizes and as a result radial velocity perturbations. The resultant velocity perturbations lead to dust concentration and amplify dust density perturbations. This positive feedback makes a disk unstable. The growth timescale of coagulation instability is a few tens of orbital periods even when dust-to-gas mass ratio is on the order of 10−3. In a protoplanetary disk, radial drift and coagulation of dust particles tend to result in dust depletion. The present instability locally concentrates dust particles even in such a dust-depleted region. The resulting concentration provides preferable sites for dust–gas instabilities to develop, which leads to further concentration. Dust diffusion and aerodynamical feedback tend to stabilize short-wavelength modes, but do not completely suppress the growth of coagulation instability. Therefore, coagulation instability is expected to play an important role in setting up the next stage for other instabilities, such as streaming instability or secular gravitational instability, to further develop toward planetesimal formation.

No Evidence of the Significant Grain Growth but Tentative Discovery of Disk Substructure in a Disk around the Class I Protostar L1489 IRS

“…If the substructure is real, one possibility of the substructure is a ring because several protostellar disks with the ring-like substructures have been identified (e.g., Alves et al 2020;Segura-Cox et al 2020;Sheehan et al 2020). Various mechanisms for the ring formation have been studied such as a growth front (Ohashi et al 2021), snowlines of volatile gas species freezing out onto dust grains (Zhang et al 2015;Okuzumi et al 2016), magnetorotational instabilities (Flock et al 2015), disk winds in unevenly ionized disks (Takahashi & Muto 2018), unseen planets (Goldreich & Tremaine 1980;Nelson et al 2000;Zhu et al 2012), secular gravitational instability (Takahashi & Inutsuka 2014), and coagulation instability (Tominaga et al 2021). Our results show that grain growth has not occurred yet outside of the ring-like position.…”

Formation of Dust Clumps with Sub-Jupiter Mass and Cold Shadowed Region in Gravitationally Unstable Disk around Class 0/I Protostar in L1527 IRS

“…It is thus important to clarify the formation mechanism. Various mechanisms for ring formation have been studied, including those involving unseen planets (Goldreich & Tremaine 1980;Nelson et al 2000;Zhu et al 2012), snowlines of volatile gas species freezing out onto dust grains (Zhang et al 2015;Okuzumi et al 2016), magnetorotational instability (Flock et al 2015), disk winds in unevenly ionized disks (Takahashi & Muto 2018), a growth front (Ohashi et al 2021), secular gravitational instability (Takahashi & Inutsuka 2014), and coagulation instability (Tominaga & Inutsuka 2021). In addition to the ring formation mechanisms described above, gravitational instability has been theoretically suggested to be important in Class 0/I protostellar disks (e.g., Nakamoto & Nakagawa 1994;Vorobyov & Basu 2006Machida et al 2011;Tsukamoto et al 2017), and it is a possible mechanism for gas giant planet formation (e.g., Gammie 2001;Kratter & Lodato 2016;Vigan et al 2017).…”

No evidence of the significant grain growth but tentative discovery of disk substructure in a disk around the Class I Protostar L1489 IRS

“…If the substructure is real, one possibility of the substructure is a ring because the several protostellar disks with the ringlike substructures have been identified (e.g., Segura-Cox et al 2020;Alves et al 2020). Various mechanisms for the ring formation have been studied such as a growth front (Ohashi et al 2021), snowlines of volatile gas species freezing-out onto dust grains (Zhang et al 2015;Okuzumi et al 2016), magneto-rotational instabilities (Flock et al 2015), disk winds in unevenly ionized disks (Takahashi & Muto 2018), unseen planets (Goldreich & Tremaine 1980;Nelson et al 2000;Zhu et al 2012), secular gravitational instability (Takahashi & Inutsuka 2014), and coagulation instability (Tominaga et al 2021).…”