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Register a new userEffect of dust rotational disruption by radiative torques on radiation pressure feedback from massive protostars
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Thiem Hoang
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Radiation pressure on dust is thought to play a crucial role in the formation process of massive stars by acting against gravitational collapse onto the central protostar. However, dust properties in dense regions irradiated by the intense radiation of massive protostars are poorly constrained. Previous studies usually assume the standard interstellar dust model to constrain the maximum mass of massive stars formed by accretion, which appears to contradict with dust evolution theory. In this paper, using the fact that stellar radiation exerts on dust simultaneous radiation pressure and radiative torques, we study the effects of grain rotational disruption by radiative torques (RATs) on radiation pressure and explore its implications for massive star formation. For this paper, we focus on the protostellar envelope and adopt a spherical geometry. We find that original large grains of micron-sizes presumably formed in very dense regions can be rapidly disrupted into small grains by RATs due to infrared radiation from the hot dust shell near the sublimation front induced by direct stellar radiation. Owing to the modification in the size distribution by rotational disruption, the radiation pressure opacity can be decreased by a factor of $sim 3$ from the value expected from the original dust model. However, to form massive stars via spherical accretion, the dust-to-gas mass ratio needs to be reduced by a factor of $sim 5$ as previously found.
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Dust clouds are ubiquitous in the atmospheres of hot Jupiters and affect their observable properties. The alignment of dust grains in the clouds and resulting dust polarization is a promising method to study magnetic fields of exoplanets. Moreover, the grain size distribution plays an important role in physical and chemical processes in the atmospheres, which is rather uncertain in atmospheres. In this paper, we first study grain alignment of dust grains in the atmospheres of hot Jupiters by RAdiative Torques (RATs). We find that silicate grains can be aligned by RATs with the magnetic fields (B-RAT) due to strong magnetic fields of hot Jupiters, but carbonaceous grains of diamagnetic material tend to be aligned with the radiation direction (k-RAT). At a low altitude of $r<2R_{rm p}$ with $R_{rm p}$ being the planet radius, only large grains can be aligned, but tiny grains of $asim 0.01mu$m can be aligned at a high altitude of $r>3R_{rm p}$. We then study rotational disruption of dust grains by the RAdiative Torque Disruption (RATD) mechanism. We find that large grains can be disrupted by RATD into smaller sizes. Grains of high tensile strength are disrupted at an altitude of $r>3R_{rm p}$, but weak grains can be disrupted at a lower altitude. We suggest that the disruption of large grains into smaller ones can facilitate dust clouds to escape to high altitudes due to lower gravity and may explain the presence of high-altitude clouds in hot Jupiter as well as super-puff atmospheres.
We reveal a deep connection between alignment of dust grains by RAdiative torques (RATs) and MEchanical Torques (METs) and rotational disruption of grains introduced by Hoang et al. (2019). The disruption of grains happens if they have attractor points corresponding to high angular momentum (high-J). We introduce {it fast disruption} for grains that are directly driven to the high-J attractor on a timescale of spin-up, and {it slow disruption} for grains that are first moved to the low-J attractor and gradually transported to the high-J attractor by gas collisions. The enhancement of grain magnetic susceptibility via iron inclusions expands the parameter space for high-J attractors and increases percentage of grains experiencing the disruption. The increase in the magnitude of RATs or METs can increase the efficiency of fast disruption, but counter-intuitively, decreases the effect of slow disruption by forcing grains towards low-J attractors, whereas the increase in gas density accelerates disruption by faster transporting grains to the high-J attractor. We also show that disruption induced by RATs and METs depends on the angle between the magnetic field and the anisotropic flow. We find that pinwheel torques can increase the efficiency of {it fast disruption} but may decrease the efficiency of {it slow disruption} by delaying the transport of grains from the low-J to high-J attractors via gas collisions. The selective nature of the rotational disruption opens a possibility of observational testing of grain composition as well as physical processes of grain alignment.
Dust properties within a galaxy are known to change from the diffuse medium to dense clouds due to increased local gas density. However, the question of whether dust properties change with redshift remains elusive. In this paper, using the fact that the mean radiation intensity of the interstellar medium (ISM) of star-forming galaxies increases with redshift, we show that dust properties should change due to increasing efficiency of rotational disruption by radiative torques, an effect named RAdiative Torque Disruption (RATD). We first show that, due to RATD, the size distribution of interstellar dust varies with redshift, such as dust grains become smaller at higher $z$. We model the extinction curves and find that the curve becomes steeper with increasing redshift. The ratio of total-to-selective extinction, $R_{V}$, decreases with redshift and achieves low values of $R_{V}sim 1.5-2.5$ for grains having a composite structure. We also find that dust properties change with the local gas density due to RATD, but the change is dominated by the radiation field for the diffuse ISM. The low values of $R_{V}$ implied by RATD of interstellar dust could reproduce anomalous dust extinction observed toward type Ia supernovae (SNe Ia) and Small Magellanic Cloud (SMC)-like extinction curves with a steep far-UV rise toward high-z galaxies. Fluctuations in $R_{V}$ due to interstellar turbulence and varying radiation intensity may resolve the tension in measurements of the Hubble constant using SNe Ia. We finally discuss the implications of evolving dust properties for high-z astrophysics.
Interstellar dust is an essential component of the interstellar medium (ISM) and plays critical roles in astrophysics. Achieving an accurate model of interstellar dust is therefore of great importance. Interstellar dust models are usually built based on observational constraints such as starlight extinction and polarization, but dynamical constraints such as grain rotation are not considered. In this paper, we show that a newly discovered effect by Hoang et al., so-called RAdiative Torque Disruption (RATD), can act as an important dynamical constraint for dust models. Using this dynamical constraint, we derive the maximum size of grains that survive in the ISM for different dust models, including contact binary, composite, silicate-core, and amorphous carbon mantle, and compact grain model for the different radiation fields. We find that the different dust models have different maximum size due to their different tensile strengths, and the largest maximum size corresponds to compact grains with the highest tensile strength. We show that the composite grain model cannot be ruled out if constituent particles are very small with radius $a_{p}le$ 25 nm, but large composite grains would be destroyed if the particles are large with $a_{p}ge 50$ nm. We suggest that grain internal structures can be constrained with observations using the dynamical RATD constraint for strong radiation fields such as supernova, nova, or star-forming regions. Finally, our obtained results suggest that micron-sized grains perhaps have compact/core-mantle structures or have composite structures but located in regions with slightly higher gas density and weaker radiation intensity than the average ISM.
Complex Organic Molecules (COMs) are believed to form in the ice mantle of dust grains and are released to the gas by thermal sublimation when grain mantles are heated to temperatures of $T_{rm d}gtrsim 100,rm K$. However, some COMs are detected in regions with temperatures below 100 K. Recently, a new mechanism of rotational desorption due to centrifugal stress induced by radiative torques (RATs) is proposed by Hoang & Tram 2020 that can desorb COMs at low temperatures. In this paper, we report observational evidence for rotational desorption of COMs toward the nearest massive star-forming region Orion BN/KL. We compare the abundance of three representative COMs which have very high binding energy computed by the rotational desorption mechanism with observations by ALMA, and demonstrate that the rotational desorption mechanism can explain the existence of such COMs. We also analyze the polarization data from SOFIA/HAWC+ and JCMT/SCUBA-2 and find that the polarization degree at far-infrared/submm decreases with increasing the grain temperature for $T_{rm d}gtrsim 71,rm K$. This is consistent with the theoretical prediction using the Radiative Torque (RAT) alignment theory and Radiative Torque Disruption (RATD) mechanism. Such an anti-correlation between dust polarization and dust temperature supports the rotational disruption as well as rotational desorption mechanism of COMs induced by RATs.
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