Thirteen years of aeolian dust dynamics in a desert region (Negev desert, Israel): analysis of horizontal and vertical dust flux, vertical dust distribution and dust grain size

ABSTRACT When imaged at high resolution, many protoplanetary discs show gaps and rings in their dust sub-mm continuum emission profile. These structures are widely considered to originate from local maxima in the gas pressure profile. The properties of the underlying gas structures are however unknown. In this paper, we present a method to measure the dust–gas coupling α/St and the width of the gas pressure bumps affecting the dust distribution, applying high-precision techniques to extract the gas rotation curve from emission line data cubes. As a proof of concept, we then apply the method to two discs with prominent substructure, HD 163296 and AS 209. We find that in all cases the gas structures are larger than in the dust, confirming that the rings are pressure traps. Although the grains are sufficiently decoupled from the gas to be radially concentrated, we find that the degree of coupling of the dust is relatively good (α/St ∼ 0.1). We can therefore reject scenarios in which the disc turbulence is very low and the dust has grown significantly. If we further assume that the dust grain sizes are set by turbulent fragmentation, we find high values of the α turbulent parameter (α ∼ 10−2). Alternatively, solutions with smaller turbulence are still compatible with our analysis if another process is limiting grain growth. For HD 163296, recent measurements of the disc mass suggest that this is the case if the grain size is 1 mm. Future constraints on the dust spectral indices will help to discriminate between the two alternatives.

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Low dust emissivities and radial variations in the envelopes of Class 0 protostars: possible signature of early grain growth

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936342 ◽

2019 ◽

Vol 632 ◽

pp. A5 ◽

Cited By ~ 5

Author(s):

M. Galametz ◽

A. J. Maury ◽

V. Valdivia ◽

L. Testi ◽

A. Belloche ◽

...

Keyword(s):

Grain Size ◽

Star Formation ◽

Grain Growth ◽

Central Region ◽

Radial Gradient ◽

Central Regions ◽

Dust Composition ◽

Dust Grain ◽

Early Phases ◽

Inner Envelope

Context. Analyzing the properties of dust and its evolution in the early phases of star formation is crucial to put constraints on the collapse and accretion processes as well as on the pristine properties of planet-forming seeds. Aims. In this paper, we aim to investigate the variations of the dust grain size in the envelopes of the youngest protostars. Methods. We analyzed Plateau de Bure interferometric observations at 1.3 and 3.2 mm for 12 Class 0 protostars obtained as part of the CALYPSO survey. We performed our analysis in the visibility domain and derived dust emissivity index (β1−3mm) profiles as a function of the envelope radius at 200–2000 au scales. Results. Most of the protostellar envelopes show low dust emissivity indices decreasing toward the central regions. The decreasing trend remains after correction of the (potentially optically thick) central region emission, with surprisingly low β1−3mm < 1 values across most of the envelope radii of NGC 1333-IRAS 4A, NGC 1333-IRAS 4B, SVS13B, and Serpens-SMM4. Conclusions. We discuss the various processes that could explain such low and varying dust emissivity indices at envelope radii 200–2000 au. Our observations of extremely low dust emissivity indices could trace the presence of large (millimeter-size) grains in Class 0 envelopes, in which case our results would point to a radial increase of the dust grain size toward the inner envelope regions. While it is expected that large grains in young protostellar envelopes could be built via grain growth and coagulation, we stress that the typical timescales required to build millimeter grains in current coagulation models are at odds with the youth of our Class 0 protostars. Additional variations in the dust composition could also partly contribute to the low β1−3mm we observe. We find that the steepness of the β1−3mm radial gradient depends strongly on the envelope mass, which might favor a scenario in which large grains are built in high-density protostellar disks and transported to the intermediate envelope radii, for example with the help of outflows and winds.

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The time evolution of dusty protoplanetary disc radii: observed and physical radii differ

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1190 ◽

2019 ◽

Vol 486 (4) ◽

pp. 4829-4844 ◽

Cited By ~ 20

Author(s):

Giovanni P Rosotti ◽

Marco Tazzari ◽

Richard A Booth ◽

Leonardo Testi ◽

Giuseppe Lodato ◽

...

Keyword(s):

Outer Part ◽

Theoretical Models ◽

Outer Edge ◽

Continuum Emission ◽

Multiple Star ◽

Star Forming ◽

Star Forming Regions ◽

Dust Distribution ◽

Dust Grain ◽

Dust Grain Growth

ABSTRACT Protoplanetary disc surveys conducted with Atacama Large Millimetre Array (ALMA) are measuring disc radii in multiple star-forming regions. The disc radius is a fundamental quantity to diagnose whether discs undergo viscous spreading, discriminating between viscosity or angular momentum removal by winds as drivers of disc evolution. Observationally, however, the sub-mm continuum emission is dominated by the dust, which also drifts inwards, complicating the picture. In this paper we investigate, using theoretical models of dust grain growth and radial drift, how the radii of dusty viscous protoplanetary discs evolve with time. Despite the existence of a sharp outer edge in the dust distribution, we find that the radius enclosing most of the dust mass increases with time, closely following the evolution of the gas radius. This behaviour arises because, although dust initially grows and drifts rapidly on to the star, the residual dust retained on Myr time-scales is relatively well coupled to the gas. Observing the expansion of the dust disc requires using definitions based on high fractions of the disc flux (e.g. 95 per cent) and very long integrations with ALMA, because the dust grains in the outer part of the disc are small and have a low sub-mm opacity. We show that existing surveys lack the sensitivity to detect viscous spreading. The disc radii they measure do not trace the mass radius or the sharp outer edge in the dust distribution, but the outer limit of where the grains have significant sub-mm opacity. We predict that these observed radii should shrink with time.

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Small dust grain dynamics on adaptive mesh refinement grids

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834147 ◽

2019 ◽

Vol 626 ◽

pp. A96 ◽

Cited By ~ 10

Author(s):

U. Lebreuilly ◽

B. Commerçon ◽

G. Laibe

Keyword(s):

Finite Volume ◽

Adaptive Mesh Refinement ◽

Mesh Refinement ◽

Adaptive Mesh ◽

Uniform Grids ◽

Dust Dynamics ◽

Protostellar Collapse ◽

Dust Grain ◽

Grain Dynamics ◽

Small Dust

Context. Small dust grains are essential ingredients of star, disk and planet formation. Aims. We present an Eulerian numerical approach to study small dust grain dynamics in the context of star and protoplanetary disk formation. It is designed for finite volume codes. We use it to investigate dust dynamics during the protostellar collapse. Methods. We present a method to solve the monofluid equations of gas and dust mixtures with several dust species in the diffusion approximation implemented in the adaptive-mesh-refinement code RAMSES. It uses a finite volume second-order Godunov method with a predictor-corrector MUSCL scheme to estimate the fluxes between the grid cells. Results. We benchmark our method against six distinct tests, DUSTYADVECT, DUSTYDIFFUSE, DUSTYSHOCK, DUSTYWAVE, SETTLING, and DUSTYCOLLAPSE. We show that the scheme is second-order accurate in space on uniform grids and intermediate between second- and first-order on non-uniform grids. We apply our method on various DUSTYCOLLAPSE simulations of 1 M⊙ cores composed of gas and dust. Conclusions. We developed an efficient approach to treat gas and dust dynamics in the diffusion regime on grid-based codes. The canonical tests were successfully passed. In the context of protostellar collapse, we show that dust is less coupled to the gas in the outer regions of the collapse where grains larger than ≃100 μm fall significantly faster than the gas.

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