Small Dust Grains Research Articles

Modeling Grain Rotational Disruption by Radiative Torques and Extinction of Active Galactic Nuclei

Abstract Extinction curves observed toward individual Active Galactic Nuclei (AGN) usually show a steep rise toward far-ultraviolet (FUV) wavelengths and can be described by the Small Magellanic Cloud (SMC)-like dust model. This feature suggests the dominance of small dust grains of size a ≤ 0.1 μm in the local environment of AGN, but the origin of such small grains is unclear. In this paper, we aim to explain this observed feature by applying the RAdiative Torque Disruption (RATD) to model the extinction of AGN radiation from FUV to mid-infrared (MIR) wavelengths. We find that in the intense radiation field of AGN, large composite grains of size a ≥ 0.1 μm are significantly disrupted to smaller sizes by RATD up to d RATD > 100 pc in the polar direction and d RATD ∼ 10 pc in the torus region. Consequently, optical–MIR extinction decreases, whereas FUV-near-ultraviolet extinction increases, producing a steep far-UV rise extinction curve. The resulting total-to-selective visual extinction ratio thus significantly drops to R V < 3.1 with decreasing distances to AGN center due to the enhancement of small grains. The dependence of R V with the efficiency of RATD will help us to study the dust properties in the AGN environment via photometric observations. In addition, we suggest that the combination of the strength between RATD and other dust destruction mechanisms that are responsible for destroying very small grains of a ≤ 0.05 μm is the key for explaining the dichotomy observed “SMC” and “gray” extinction curve toward many AGN.

Accretion of tidally disrupted asteroids on to white dwarfs: direct accretion versus disc processing

ABSTRACT Atmospheric heavy elements have been observed in more than a quarter of white dwarfs (WDs) at different cooling ages, indicating ongoing accretion of asteroidal material, whilst only a few per cent of the WDs possess a dust disc, and all these WDs are accreting metals. Here, assuming that a rubble-pile asteroid is scattered inside a WD’s Roche lobe by a planet, we study its tidal disruption and the long-term evolution of the resulting fragments. We find that after a few pericentric passages, the asteroid is shredded into its constituent particles, forming a flat, thin ring. On a time-scale of Myr, tens of per cent of the particles are scattered on to the WD, and are therefore directly accreted without first passing through a circularized close-in disc. Fragment mutual collisions are most effective for coplanar fragments, and are thus only important in 103−104 yr before the orbital coplanarity is broken by the planet. We show that for a rubble pile asteroid with a size frequency distribution of the component particles following that of the near earth objects, it has to be roughly at least 10 km in radius such that enough fragments are generated and $\ge 10{{\ \rm per\ cent}}$ of its mass is lost to mutual collisions. At relative velocities of tens of km s−1, such collisions grind down the tidal fragments into smaller and smaller dust grains. The WD radiation forces may shrink those grains’ orbits, forming a dust disc. Tidal disruption of a monolithic asteroid creates large km-size fragments, and only parent bodies ≥100 km are able to generate enough fragments for mutual collisions to be significant. Hence, those large asteroids experience a disc phase before being accreted.

Characterizing the morphology of the debris disk around the low-mass star GSC 07396-00759

Context. Debris disks have commonly been studied around intermediate-mass stars. Their intense radiation fields are believed to efficiently remove the small dust grains that are constantly replenished by collisions. For lower-mass central objects, in particular M stars, the dust removal mechanism needs to be further investigated given the much weaker radiation field produced by these objects. Aims. We present new observations of the nearly edge-on disk around the pre-main-sequence M-type star GSC 07396-00759, taken with VLT/SPHERE IRDIS in dual-beam polarimetric imaging mode, with the aim to better understand the morphology of the disk, its dust properties, and the star-disk interaction via the stellar mass-loss rate. Methods. We model the polarimetric observations to characterize the location and properties of the dust grains using the Henyey–Greenstein approximation of the polarized phase function. We use the estimated phase function to evaluate the strength of the stellar winds. Results. We find that the polarized light observations are best described by an extended and highly inclined disk (i ≈ 84.3 ° ± 0.3) with a dust distribution centered at a radius r0 ≈ 107 ± 2 au. Our modeling suggests an anisotropic scattering factor g ≈ 0.6 to best reproduce the polarized phase function S12. We also find that the phase function is reasonably well reproduced by small micron-sized dust grains with sizes s > 0.3μm. We discuss some of the caveats of the approach, mainly that our model probably does not fully recover the semimajor axis of the disk and that we cannot readily determine all dust properties due to a degeneracy between the grain size and the porosity. Conclusions. Even though the radius of the disk may be overestimated, our best-fit model not only reproduces the observations well but is also consistent with previous published data obtained in total intensity. Similarly to previous studies of debris disks, we suggest that using a given scattering theory might not be sufficient to fully explain key aspects, such as the shape of the phase function or the dust grain size. Taking into consideration the aforementioned caveats, we find that the average mass-loss rate of GSC 07396-00759 can be up to 500 times stronger than that of the Sun, supporting the idea that stellar winds from low-mass stars can evacuate small dust grains in an efficient way.

Gas-phase Formation of Cationic Fullerene/Amino Acid Clusters: Evidence for the “Magic Number” Chemical Reactivity of Fullerene Cations

Abstract An experimental and theoretical investigation on the chemical reactivity of fullerene cations (C n +, n = [36, 60]) with amino acid molecules (e.g., isoleucine, C6H13NO2) is performed. The results show that, in the gas phase, fullerene cations can react with amino acid molecules to form fullerene/amino acid cluster cations with high efficiency. The formation rate constants for the ion–molecule collision reactions between fullerene cations and isoleucine are estimated under the pseudo-first-order reaction condition. We find the formation rate constants increase gradually with a declining C-atom number of fullerene cations (C n +, n = [46, 60]), and depict a plateau in the reactivity for smaller fullerene cations (C n +, n = [36, 44]). More importantly, by comparing to its neighbor fullerenes, the magic number of C-atom counts (54, 58) that process with enhanced chemical reactivity are determined. We also obtained the molecular structures and binding energies for the fullerene-monoisoleucine adducts by quantum chemical calculations, which give a good explanation for the magic number chemical reactivity and the enhanced chemical reactivity of smaller fullerene cations. We infer that our results demonstrate the importance of ion–molecule reactions to the formation of large and complex fullerene-amino acid derivatives in the ISM. The high reactivity of fullerene species may indicate that amino acid molecules or other related prebiotic compounds can accrete on small interstellar carbon dust grains.

Investigating the Relative Gas and Small Dust Grain Surface Heights in Protoplanetary Disks

Abstract Dust evolution in protoplanetary disks from small dust grains to pebbles is key to the planet formation process. The gas in protoplanetary disks should influence the vertical distribution of small dust grains (∼1 μm) in the disk. Utilizing archival near-infrared polarized light and millimeter observations, we can measure the scale height and flare parameter β of the small dust grain scattering surface and 12CO gas emission surface for three protoplanetary disks: IM Lup, HD 163296, and HD 97048 (CU Cha). For two systems, IM Lup and HD 163296, the 12CO gas and small dust grains at small radii from the star have similar heights, but at larger radii (>100 au), the dust grain scattering surface height is lower than the 12CO gas emission surface height. In the case of HD 97048, the small dust grain scattering surface has similar heights to the 12CO gas emission surface at all radii. We ran a protoplanetary disk radiative transfer model of a generic protoplanetary disk with TORUS and showed that there is no difference between the observed scattering surface and 12CO emission surface. We also performed analytical modeling of the system and found that gas-to-dust ratios larger than 100 could explain the observed difference in IM Lup and HD 163296. This is the first direct comparison of observations of gas and small dust grain height distribution in protoplanetary disks. Future observations of gas emission and near-infrared scattered-light instruments are needed to look for similar trends in other protoplanetary disks.

Does Processing or Formation of Water Ice Mantles Affect the Capacity of Nanosilicates to Be the Source of Anomalous Microwave Emission?

Anomalous microwave emission (AME) is detected in many astrophysical environments as a foreground feature typically peaking between 20–30 GHz and extending over a 10–60 GHz range. One of the leading candidates for the source of AME is small spinning dust grains. Such grains should be very small (approx. ≤1 nm diameter) in order for the rotational emission to fall within the observed frequency range. In addition, these nanosized grains should possess a significant dipole moment to account for the observed emissivities. These constraints have been shown to be compatible with spinning bare nanosilicate clusters, assuming that ∼1% of the total Si mass budget is held in these ultrasmall grains. Silicate dust can be hydroxylated by processing in the interstellar medium and is generally known to provide seeds for molecular water ice nucleation in denser regions. Herein, we use quantum chemical calculations to investigate how the dipole moment of Mg-rich pyroxenic (MgSiO3) nanoclusters is affected by both accretion of molecular water and dissociative hydration. Our work thus provides an indication of how the formation of water ice mantles is likely to affect the capacity of nanosilicates to generate AME.

The Sweeping-out of Dust by Radiation Pressure of Stars and Chemical Composition Peculiarities of Disc Galaxies

—We consider the drift of dust grains of various sizes and chemical compositions caused by the stellar radiation pressure in the vicinity of the Milky Way. When integrating the equations of motion, in addition to the radiation pressure, we consider the gravitational attraction from various components of the Galaxy and the gas drag. It has been shown that carbonaceous grains of medium sizes (~0.01 μm) are swept out of the galactic disc most effectively. Smaller dust grains are swept out to a substantially lesser extent, or they are not swept out at all. We also consider the motion of silicate dust grains, including those with porous structure. It has been shown that silicate grains experience a considerably weaker impact of the radiation pressure. The simulation result of their motion does not essentially depend on whether their porosity is accounted for or ignored. The total rate of the Galaxy’s dust loss has turned out to be high—approximately 0.03 M⊙ per year, which is comparable to the effect produced by the other mechanisms ejecting heavy elements to the circumgalactic space. We discuss the potential of the sweeping of dust out of the Galaxy in formation of the radial metallicity gradient, as well as the prospects of detecting extensive dust structures in elliptical galaxies.

The Anatomy of an Unusual Edge-on Protoplanetary Disk. I. Dust Settling in a Cold Disk

Abstract As the earliest stage of planet formation, massive, optically thick, and gas-rich protoplanetary disks provide key insights into the physics of star and planet formation. When viewed edge-on, high-resolution images offer a unique opportunity to study both the radial and vertical structures of these disks and relate this to vertical settling, radial drift, grain growth, and changes in the midplane temperatures. In this work, we present multi-epoch Hubble Space Telescope and Keck scattered light images, and an Atacama Large Millimeter/submillimeter Array 1.3 mm continuum map for the remarkably flat edge-on protoplanetary disk SSTC2DJ163131.2–242627, a young solar-type star in ρ Ophiuchus. We model the 0.8 μm and 1.3 mm images in separate Markov Chain Monte Carlo (MCMC) runs to investigate the geometry and dust properties of the disk using the MCFOST radiative transfer code. In scattered light, we are sensitive to the smaller dust grains in the surface layers of the disk, while the submillimeter dust continuum observations probe larger grains closer to the disk midplane. An MCMC run combining both data sets using a covariance-based log-likelihood estimation was marginally successful, implying insufficient complexity in our disk model. The disk is well characterized by a flared disk model with an exponentially tapered outer edge viewed nearly edge-on, though some degree of dust settling is required to reproduce the vertically thin profile and lack of apparent flaring. A colder than expected disk midplane, evidence for dust settling, and residual radial substructures all point to a more complex radial density profile to be probed with future, higher-resolution observations.

An HST/STIS view of protoplanetary discs in Upper Scorpius: observations of three young M stars

ABSTRACT We present observations of three protoplanetary discs in visible scattered light around M-type stars in the Upper Scorpius OB association using the Space Telescope Imaging Spectrograph (STIS) instrument on the Hubble Space Telescope (HST). The discs around stars 2MASS J16090075–1908526, 2MASS J16142029–1906481, and 2MASS J16123916–1859284 have all been previously detected with the Atacama Large Millimeter/submillimeter Array (ALMA), and 2MASS J16123916–1859284 has never previously been imaged at scattered light wavelengths. We process our images using reference star differential imaging, comparing and contrasting three reduction techniques – classical subtraction, Karhunen–Loève Image Projection, and non-negative matrix factorization, selecting the classical method as the most reliable of the three for our observations. Of the three discs, two are tentatively detected (2MASS J16142029–1906481 and 2MASS J16123916–1859284), with the third going undetected. Our two detections are shown to be consistent when varying the reference star or reduction method used, and both detections exhibit structure out to projected distances of ≳200 au. Structures at these distances from the host star have never been previously detected at any wavelength for either disc, illustrating the utility of visible-wavelength observations in probing the distribution of small dust grains at large angular separations.

MRI-active inner regions of protoplanetary discs. I. A detailed model of disc structure

ABSTRACT Short-period super-Earth-sized planets are common. Explaining how they form near their present orbits requires understanding the structure of the inner regions of protoplanetary discs. Previous studies have argued that the hot inner protoplanetary disc is unstable to the magnetorotational instability (MRI) due to thermal ionization of potassium, and that a local gas pressure maximum forms at the outer edge of this MRI-active zone. Here we present a steady-state model for inner discs accreting viscously, primarily due to the MRI. The structure and MRI-viscosity of the inner disc are fully coupled in our model; moreover, we account for many processes omitted in previous such models, including disc heating by both accretion and stellar irradiation, vertical energy transport, realistic dust opacities, dust effects on disc ionization, and non-thermal sources of ionization. For a disc around a solar-mass star with a standard gas accretion rate ($\dot{M}\, \sim \, 10^{-8}$ M⊙ yr−1) and small dust grains, we find that the inner disc is optically thick, and the accretion heat is primarily released near the mid-plane. As a result, both the disc mid-plane temperature and the location of the pressure maximum are only marginally affected by stellar irradiation, and the inner disc is also convectively unstable. As previously suggested, the inner disc is primarily ionized through thermionic and potassium ion emission from dust grains, which, at high temperatures, counteract adsorption of free charges on to grains. Our results show that the location of the pressure maximum is determined by the threshold temperature above which thermionic and ion emission become efficient.

How Does Temperature Affect the Infrared Vibrational Spectra of Nanosized Silicate Dust?

Magnesium-rich silicates are pervasive throughout the universe and are found in many astronomical environments as small dust grains. Most of the information we have about such silicate dust comes from infrared (IR) observations, whereby the prominent ∼9.7 μm Si-O stretching and 18–20 μm O–Si–O bending vibrational bands are used as identifying signatures. During the initial stages of silicate dust nucleation around aging stars, the nascent grains are nanosized and heated to 800–1000 K. Later, the larger fully formed grains are ejected into the cold interstellar medium and are subsequently heavily processed (e.g., by shattering and sputtering), likely leading to an abundant population of nanosilicate grains. Such ultrasmall grains can eventually form a dust component of protoplanetary disks, where they can be again heated to a few hundred degrees. Although nanosilicate grains are astrochemically important and are likely carriers of the ubiquitous anomalous microwave emission, they have yet to be unambiguously identified through observation. The advent of the James Webb Space Telescope, with IR instruments of unrivalled sensitivity covering the 5-28 μm wavelength range, promises to open a window on these elusive, but presumably abundant, species. Here, we examine Mg-rich ultrasmall silicate clusters with both olivinic (Mg2SiO4) and pyroxenic (MgSiO3) stoichiometries and accurately calculate their IR spectra at temperatures between 0 and 800 K. Although this temperature range is below that required to anneal bulk magnesium silicates (∼1000 K), we show that nanosized grains can exhibit significant anharmonic motions and premelted structural conformational fluxionality even at 400 K. As a result, we demonstrate that the IR spectra of nanosilicates are highly sensitive to temperature, which should thus be considered when attempting to identify these species.

Collision rates of planetesimals near mean-motion resonances

ABSTRACT In circumstellar discs, collisional grinding of planetesimals produces second-generation dust. While it remains unclear whether this ever becomes a major component of the total dust content, the presence of such dust, and potentially the substructure within it, can be used to explore a disc’s physical conditions. A perturbing planet produces non-axisymmetric structures and gaps in the dust, regardless of its origin. The dynamics of planetesimals, however, will be very different than that of small dust grains due to weaker gas interactions. Therefore, planetesimal collisions could create dusty disc structures that would not exist otherwise. In this work, we use N-body simulations to investigate the collision rate profile of planetesimals near mean-motion resonances. We find that a distinct bump or dip feature is produced in the collision profile, the presence of which depends on the libration width of the resonance and the separation between the peri- and apocentre distances of the edges of the resonance. The presence of one of these two features depends on the mass and eccentricity of the planet. Assuming that the radial dust emission traces the planetesimal collision profile, the presence of a bump or dip feature in the dust emission at the 2:1 mean-motion resonance can constrain the orbital properties of the perturbing planet. This assumption is valid, so long as radial drift does not play a significant role during the collisional cascade process. Under this assumption, these features in the dust emission should be marginally observable in nearby protoplanetary discs with ALMA.

Dust Growth in Molecular Cloud Envelopes: A Numerical Approach

Abstract Variations in the grain size distribution are to be expected in the interstellar medium (ISM) owing to grain growth and destruction. In this work, we present a dust collision model to be implemented inside a magnetohydrodynamic (MHD) code that takes into account grain growth and shattering of charged dust grains of a given composition (silicate or graphite). We integrate this model in the MHD code Athena and build on a previous implementation of the dynamics of charged dust grains in the same code. To demonstrate the performance of this coagulation model, we study the variations in the grain size distribution of a single-sized population of dust with radius 0.05 μm inside several dust filaments formed during a 2D MHD simulation. We also consider a realistic dust distribution with sizes ranging from 50 Å to 0.25 μm and analyze the variations in both the size distribution for graphite and silicates and the far-ultraviolet extinction curve. From the obtained results, we conclude that the methodology here presented, based on the MHD evolution of the equation of motion for a charged particle, is optimal for studying the coagulation of charged dust grains in a diffuse regime such as a molecular cloud envelope. Observationally, these variations in the dust size distribution are translated into variations in the far-ultraviolet extinction curve, and they are mainly caused by small graphite dust grains.

ALMA Observations of the Inner Cavity in the Protoplanetary Disk around Sz 84

Abstract We present Atacama Large Millimeter/submillimeter Array observations of a protoplanetary disk around the T Tauri star Sz 84 and analyses of the structures of the inner cavity in the central region of the dust disk. Sz 84's spectral energy distribution (SED) has been known to exhibit negligible infrared excess at λ ≲ 10 μm due to the disk’s cavity structure. Analyses of the observed visibilities of dust continuum at 1.3 mm and the SED indicate that the size of the cavity in the disk of large (millimeter size) dust grains is 8 au in radius and that in the disk of small (submicron size) dust grains is 60 au in radius. Furthermore, from the SED analyses, we estimate that the upper limit mass of small dust grains at r < 60 au is less than ∼10−3 M ⊕, which is ≲0.01% of the total (small + large) dust mass at r < 60 au. These results suggest that large dust grains are dominant at r < 60 au, implying that dust grains efficiently grow with less efficient fragmentation in this region, potentially due to weak turbulence and/or stickier dust grains. The balance of grain growth and dust fragmentation is an important factor for determining the size of large dust grains in protoplanetary disks, and thus Sz 84 could serve as a good testbed for investigations of grain growth in such disks.

Modeling Nitrogen Fractionation in the Protoplanetary Disk around TW Hya: Model Constraints on Grain Population and Carbon-to-oxygen Elemental Abundance Ratio

Abstract Observations conducted using the Atacama Large Millimeter/submillimeter Array on the protoplanetary disk around TW Hya show the nitrogen fractionation of HCN molecules in HC14N/HC15N ∼ 120 at a radius of ∼20 au. In this study, we investigate the physical and chemical conditions that control this nitrogen fractionation process. To this end, a new disk model was developed, in which the isotope-selective photodissociation of N2 and isotope-exchange chemical reactions have been incorporated. Our model can successfully reproduce the observed HCN column density when the elemental abundances of gas-phase carbon and oxygen are depleted by two orders of magnitude relative to those in the interstellar medium and carbon is more abundant than oxygen ([C/O]elem > 1). The isotope-selective photodissociation of N2 is the dominant nitrogen fractionation process in our models. The observed HC14N/HC15N ratio, which increases outwards, can also be reproduced by the model by assuming that the small dust grains in the atmosphere of the outer disk are depleted more than those in the inner disk. This is consistent with grain evolution models, according to which small dust grains are continuously replenished in the inner disk due to fragmentation of the large dust grains that radially drift from the outer disk.

Formation of Dust Filaments in the Diffuse Envelopes of Molecular Clouds

Abstract The path to understanding star formation processes begins with the study of the formation of molecular clouds. The outskirts of these clouds are characterized by low column densities that allow the penetration of ultraviolet radiation, resulting in a nonnegligible ionization fraction and the charging of the small dust grains that are mixed with the gas; this diffuse phase is then coupled to the ambient magnetic field. Despite the general assumption that dust and gas are tightly correlated, several observational and theoretical studies have reported variations in the dust-to-gas ratio toward diffuse and cold clouds. In this work, we present the implementation of a new charged particles module for analyzing the dust dynamics in molecular cloud envelopes. We study the evolution of a single population of small charged grains (0.05 μm) in the turbulent, magnetized molecular cloud envelope using this module. We show that variations in the dust-to-gas ratio arise due to the coupling of the grains with the magnetic field, forming elongated dust structures decoupled from the gas. This emphasizes the importance of considering the dynamics of charged dust when simulating the different phases of the interstellar medium, especially for star formation studies.

Evidence for localized onset of episodic mass loss in Mira

Context. Mass loss from long-period variable stars (LPV) is an important contributor to the evolution of galactic abundances. Dust formation is understood to play an essential role in mass loss. It has, however, proven difficult to develop measurements that strongly constrain the location and timing of dust nucleation and acceleration. Aims. Interferometric imaging has the potential to constrain the geometry and dynamics of mass loss. High angular resolution studies of various types have shown that LPVs have a distinct core-halo structure. These have also shown that LPV images commonly exhibit a non-circular shape. The nature of this shape and its implications are yet to be understood. Methods. Multi-telescope interferometric measurements taken with the Interferometric Optical Telescope Array (IOTA) provide imagery of the LPV Mira in the H-band. This wavelength region is well suited to studying mass loss given the low continuum opacity, which allows for emission to be observed over a very long path in the stellar atmosphere and envelope. Results. The observed visibilities are consistent with a simple core-halo model to represent the central object and the extended molecular layers but, in addition, they demonstrate a substantial asymmetry. An analysis with image reconstruction software shows that the asymmetry is consistent with a localized absorbing patch. The observed opacity is tentatively associated with small dust grains, which will grow substantially during a multi-year ejection process. Spatial information along with a deduced dust content of the cloud, known mass loss rates, and ejection velocities provide evidence for the pulsational pumping of the extended molecular layers. The cloud may be understood as a spatially local zone of enhanced dust formation, very near to the pulsating halo. The observed mass loss could be provided by several such active regions around the star. Conclusions. This result provides an additional clue for better understanding the clumpiness of dust production in the atmosphere of AGB stars. It is compatible with scenarios where the combination of pulsation and convection play a key role in the process of mass loss.

Rapid elimination of small dust grains in molecular clouds

We argue that impact velocities between dust grains with sizes of less than ∼0.1 μm in molecular cloud cores are dominated by drift arising from ambipolar diffusion. This effect is due to the size dependence of the dust coupling to the magnetic field and the neutral gas. Assuming perfect sticking in collisions up to ≈50 m s−1, we show that this effect causes rapid depletion of small grains, consistent with starlight extinction and IR and microwave emission measurements, both in the core center (n ∼ 106 cm−3) and envelope (n ∼ 104 cm−3). The upper end of the size distribution does not change significantly if only velocities arising from this effect are considered. We consider the impact of an evolved grain-size distribution on the gas temperature, and argue that if the depletion of small dust grains occurs as expected from our model, then the cosmic ray ionization rate must be well below 10−16 s−1 at a number density of 105 cm−3.

Dynamics of small grains in transitional discs

ABSTRACT Transitional discs have central regions characterized by significant depletion of both dust and gas compared to younger, optically thick discs. However, gas and dust are not depleted by equal amounts: gas surface densities are typically reduced by factors of ∼100, but small dust grains are sometimes depleted by far larger factors, to the point of being undetectable. While this extreme dust depletion is often attributed to planet formation, in this paper we show that another physical mechanism is possible: expulsion of grains from the disc by radiation pressure. We explore this mechanism using 2D simulations of dust dynamics, simultaneously solving the equation of radiative transfer with the evolution equations for dust diffusion and advection under the combined effects of stellar radiation and hydrodynamic interaction with a turbulent, accreting background gas disc. We show that, in transition discs that are depleted in both gas and dust fraction by factors of ∼100–1000 compared to minimum mass Solar nebular values, and where the ratio of accretion rate to stellar luminosity is low ($\dot{M}/L \lesssim 10^{-10}\, \mathrm{ M}_\odot$ yr$^{-1}\, \mathrm{ L}_\odot ^{-1}$), radiative clearing of any remaining ${\sim}0.5\, \mu\mathrm{ m}$ and larger grains is both rapid and inevitable. The process is size-dependent, with smaller grains removed fastest and larger ones persisting for longer times. Our proposed mechanism thus naturally explains the extreme depletion of small grains commonly found in transition discs. We further suggest that the dependence of this mechanism on grain size and optical properties may explain some of the unusual grain properties recently discovered in a number of transition discs. The simulation code we develop is freely available.

The MOSDEF Survey: The Variation of the Dust Attenuation Curve with Metallicity

Abstract We derive a UV-optical stellar dust attenuation curve of galaxies at z = 1.4–2.6 as a function of gas-phase metallicity. We use a sample of 218 star-forming galaxies, excluding those with very young or heavily obscured star formation, from the MOSFIRE Deep Evolution Field survey with Hα, Hβ, and [N ii]λ 6585 spectroscopic measurements. We constrain the shape of the attenuation curve by comparing the average flux densities of galaxies sorted into bins of dust obscuration using Balmer decrements, i.e., Hα-to-Hβ luminosities. The average attenuation curve for the high-metallicity sample ( , corresponding to ) has a shallow slope, identical to that of the Calzetti local starburst curve, and a significant UV 2175 Å extinction bump that is ∼0.5× the strength of the Milky Way bump. On the other hand, the average attenuation curve of the low-metallicity sample ( ) has a steeper slope similar to that of the SMC curve, only consistent with the Calzetti slope at the 3σ level. The UV bump is not detected in the low-metallicity curve, indicating the relative lack of the small dust grains causing the bump at low metallicities. Furthermore, we find that on average the nebular reddening (E(B − V)) is a factor of 2 times larger than that of the stellar continuum for galaxies with low metallicities, while the nebular and stellar reddening are similar for galaxies with higher metallicities. The latter is likely due to a high surface density of dusty clouds embedding the star-forming regions but also reddening the continuum in the high-metallicity galaxies.