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Register a new userGrain alignment and rotational disruption by radiative torques in exoplanet atmospheres
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Thiem Hoang
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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.
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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.
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.
The mechanisms causing millimeter-wave polarization in protoplanetary disks are under debate. To disentangle the polarization mechanisms, we observe the protoplanetary disk around HL Tau at 3.1 mm with the Atacama Large Millimeter/submillimeter Array (ALMA), which had polarization detected with CARMA at 1.3 mm. We successfully detect the ring-like azimuthal polarized emission at 3.1 mm. This indicates that dust grains are aligned with the major axis being in the azimuthal direction, which is consistent with the theory of radiative alignment of elongated dust grains, where the major axis of dust grains is perpendicular to the radiation flux. Furthermore, the morphology of the polarization vectors at 3.1 mm is completely different from those at 1.3 mm. We interpret that the polarization at 3.1 mm to be dominated by the grain alignment with the radiative flux producing azimuthal polarization vectors, while the self-scattering dominates at 1.3 mm and produces the polarization vectors parallel to the minor axis of the disk. By modeling the total polarization fraction with a single grain population model, the maximum grain size is constrained to be $100{rm~mu m}$, which is smaller than the previous predictions based on the spectral index between ALMA at 3 mm and VLA at 7 mm.
Located in the Large Magellanic cloud and mostly irradiated by a massive-star cluster R$,$136, 30 Doradus is an ideal target to test the leading theory of the grain alignment and rotational disruption by RAdiative Torques (RATs). Here, we use publicly available polarized thermal dust emission observations of 30 Doradus at 89, 154, and 214$,mu$m using SOFIA/HAWC+. We analyse the variation of the dust polarization degree ($p$) with the total emission intensity ($I$), the dust temperature ($T_{rm d}$), and the gas column density ($N_{rm H}$) constructed from ${it Herschel}$ data. The 30 Doradus complex is divided into two main regions relative to R$,$136, namely North and South. In the North, we find that the polarization degree first decreases and then increases before decreasing again when the dust temperature increases toward the irradiating cluster R$,$136. The first depolarization likely arises from the decrease of grain alignment efficiency toward the dense medium due to the attenuation of the interstellar radiation field and the increase of the gas density. The second trend (the increase of $p$ with $T_{rm d}$) is consistent with the RAT alignment theory. The final trend (the decrease of $p$ with $T_{rm d}$) is consistent with the RAT alignment theory only when the grain rotational disruption by RATs is taken into account. In the South, we find that the polarization degree is nearly independent of the dust temperature, while the grain alignment efficiency is higher around the peak of the gas column density and decreases toward the radiation source. The latter feature is also consistent with the prediction of the rotational disruption by RATs.
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Christiane Helling Centre for Exoplanetn Science
, University of St Andrews & SRON Netherlands Institute for Spacen Research
2019
The study of the composition of brown dwarf atmospheres helped to understand their formation and evolution. Similarly, the study of exoplanet atmospheres is expected to constrain their formation and evolutionary states. We use results from 3D simulations, kinetic cloud formation and kinetic ion-neutral chemistry to investigate ionisation processes which will affect their atmosphere chemistry: The dayside of super-hot Jupiters is dominated by atomic hydrogen, and not H$_2$O. Such planetary atmospheres exhibit a substantial degree of thermal ionisation and clouds only form on the nightside where lightning leaves chemical tracers (e.g. HCN) for possibly long enough to be detectable. External radiation may cause exoplanets to be enshrouded in a shell of highly ionised, H$_3^+$-forming gas and a weather-driven aurora may emerge. Brown dwarfs enable us to study the role of electron beams for the emergence of an extrasolar, weather-system driven aurora-like chemistry, and the effect of strong magnetic fields on cold atmospheric gases. Electron beams trigger the formation of H$_3^+$ in the upper atmosphere of a brown dwarf (e.g. LSR-J1835) which may react with it to form hydronium, H$_3$O$^+$, as a longer lived chemical tracer. Brown dwarfs and super-hot gas giants may be excellent candidates to search for H$_3$O$^+$ as an H$_3^+$ product.
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