It is known that the millimeter dust thermal emission of protoplanetary disks is affected by scattering, such that for optically thick disks, the emission decreases with respect to the pure ...absorption case and the spectral indices can reach values below 2. The latter can also be obtained with temperature gradients. Using simple analytical models of radiative transfer in thin slabs, we quantify the effect of scattering, vertical temperature gradients, and dust settling on the emission and spectral indices of geometrically thin face-on accretion disks around young stars. We find that, in vertically isothermal disks with large albedo ( ), the emergent intensity can increase at optical depths between 10−2 and 10−1. We show that dust settling has important effects on the spectral indices in the optically thick regime, since the disk emission mainly traces small dust grains in the upper layers of the disk. The λ = 870 m emission of these small grains can hide large grains at the disk midplane when the dust surface density is larger than ∼3.21 g cm−2. Finally, because of the change of the shape of the spectral energy distribution, optically thick disks at 1.3 mm and grains with sizes between mm have a 7 mm flux ∼60% higher than the extrapolation from higher millimeter frequencies, assumed when scattering is neglected. This effect could provide an explanation for the excess emission at reported in several disks.
We study the dust concentration and emission in protoplanetary disks vortices. We extend the Lyra-Lin solution for the dust concentration of a single grain size to a power-law distribution of grain ...sizes . Assuming dust conservation in the disk, we find an analytic dust surface density as a function of the grain radius. We calculate the increase of the dust-to-gas mass ratio ϵ and the slope p of the dust size distribution due to grain segregation within the vortex. We apply this model to a numerical simulation of a disk containing a persistent vortex. Due to the accumulation of large grains toward the vortex center, ϵ increases by a factor of 10 from the background disk value, and p decreases from 3.5 to 3.0. We find the disk emission at millimeter wavelengths corresponding to synthetic observations with ALMA and VLA. The simulated maps at 7 mm and 1 cm show a strong azimuthal asymmetry. This happens because, at these wavelengths, the disk becomes optically thin while the vortex remains optically thick. The large vortex opacity is mainly due to an increase in the dust-to-gas mass ratio. In addition, the change in the slope of the dust size distribution increases the opacity by a factor of two. We also show that the inclusion of the dust scattering opacity substantially changes the disks images.
A Circumplanetary Disk around PDS70c Benisty, Myriam; Bae, Jaehan; Facchini, Stefano ...
Astrophysical journal. Letters,
07/2021, Letnik:
916, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
PDS 70 is a unique system in which two protoplanets, PDS 70 b and c, have been discovered within the dust-depleted cavity of their disk, at ∼22 and 34 au, respectively, by direct imaging at ...infrared wavelengths. Subsequent detection of the planets in the H
α
line indicates that they are still accreting material through circumplanetary disks. In this Letter, we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of the dust continuum emission at 855
μ
m at high angular resolution (∼20 mas, 2.3 au) that aim to resolve the circumplanetary disks and constrain their dust masses. Our observations confirm the presence of a compact source of emission co-located with PDS 70 c, spatially separated from the circumstellar disk and less extended than ∼1.2 au in radius, a value close to the expected truncation radius of the circumplanetary disk at a third of the Hill radius. The emission around PDS 70 c has a peak intensity of ∼86 ± 16
μ
Jy beam
−1
, which corresponds to a dust mass of ∼0.031
M
⊕
or ∼0.007
M
⊕
, assuming that it is only constituted of 1
μ
m or 1 mm sized grains, respectively. We also detect extended, low surface brightness continuum emission within the cavity near PDS 70 b. We observe an optically thin inner disk within 18 au of the star with an emission that could result from small micron-sized grains transported from the outer disk through the orbits of b and c. In addition, we find that the outer disk resolves into a narrow and bright ring with a faint inner shoulder.
Understanding planet formation requires one to discern how dust grows in protoplanetary disks. An important parameter to measure in disks is the maximum dust grain size present. This is usually ...estimated through measurements of the dust opacity at different millimeter wavelengths assuming optically thin emission and dust opacity dominated by absorption. However, Atacama Large Millimeter/submillimeter Array (ALMA) observations have shown that these assumptions might not be correct in the case of protoplanetary disks, leading to overestimation of particle sizes and to underestimation of the disk's mass. Here, we present an analysis of high-quality ALMA and Very Large Array images of the HL Tau protoplanetary disk, covering a wide range of wavelengths, from 0.8 mm to 1 cm, and with a physical resolution of ∼7.35 au. We describe a procedure to analyze a set of millimeter images without any assumption about the optical depth of the emission, and including the effects of absorption and scattering in the dust opacity. This procedure allows us to obtain the dust temperature, the dust surface density, and the maximum particle size at each radius. In the HL Tau disk, we found that particles have already grown to a few millimeters in size. We detect differences in the dust properties between dark and bright rings, with dark rings containing low dust density and small dust particles. Different features in the HL Tau disk seem to have different origins. Planet-disk interactions can explain substructure in the external half of the disk, but the internal rings seem to be associated with the presence of snow lines of several molecules.
A
bstract
We investigate symmetry breaking in two-dimensional field theories which have a holographic gravity dual. Being at large
N
, the Coleman theorem does not hold and Goldstone bosons are ...expected. We consider the minimal setup to describe a conserved current and a charged operator, and we perform holographic renormalization in order to find the correct Ward identities describing symmetry breaking. This involves some subtleties related to the different boundary conditions that a vector can have in the three-dimensional bulk. We establish which is the correct prescription that yields, after renormalization, the same Ward identities as in higher dimensions.
T-duality equivalences beyond string theory Edelstein, José D.; Sfetsos, Konstantinos; Sierra-Garcia, J. Anibal ...
The journal of high energy physics,
05/2019, Letnik:
2019, Številka:
5
Journal Article
Recenzirano
Odprti dostop
A
bstract
We examine a two parameter family of gravitational actions which contains higher-derivative terms. These are such that the entire action is invariant under corrected T-duality rules, which ...we derive explicitly. Generically this action does not describe low energy string backgrounds except for isolated choices for the parameters. Nevertheless, we demonstrate that in this theory the entropy and the temperature of generic non-extremal black hole solutions are T-duality invariant. This further supports the idea put forward in our previous work that T-duality might provide physical equivalences beyond the realm of string theory.
Abstract
The polarimetric observations of the protoplanetary disk around HL Tau have shown the scattering-induced polarization at ALMA Band 7, which indicates that the maximum dust size is ∼100
μ
m, ...while the spectral energy distribution (SED) has suggested that the maximum dust size is approximately a millimeter. To solve the contradiction, we investigate the impact of differential settling of dust grains on the SED and polarization. If the disk is optically thick, a longer observing wavelength traces more interior layers, which would be dominated by larger grains. We find that the SED of the center part of the HL Tau disk can be explained with millimeter-sized grains for a broad range of turbulence strength, while 160
μ
m–sized grains cannot be explained unless the turbulence strength parameter
α
t
is lower than 10
−5
. We also find that the observed polarization fraction can be potentially explained with a maximum dust size of 1 mm if
α
t
≲ 10
−5
, although models with 160
μ
m–sized grains are also acceptable. However, if the maximum dust size is ∼3 mm, the simulated polarization fraction is too low to explain the observations even if the turbulence strength is extremely small, indicating a maximum dust size of ≲1 mm. The degeneracy between 100
μ
m– and millimeter-sized grains can be solved by improving the ALMA calibration accuracy or polarimetric observations at (sub)centimeter wavelengths.
We study dust concentration in axisymmetric gas rings in protoplanetary disks. Given the gas surface density, we derived an analytical total dust surface density by taking into account the ...differential concentration of all grain sizes. This model allows us to predict the local dust-to-gas mass ratio and the slope of the particle size distribution, as a function of radius. We test this analytical model by comparing it with a 3D magnetohydrodynamical simulation of dust evolution in an accretion disk. The model is also applied to the disk around HD 169142. By fitting the disk continuum observations simultaneously at λ = 0.87, 1.3, and 3.0 mm, we obtain a global dust-to-gas mass ratio and a viscosity coefficient = 1.35 × 10−2. This model can be easily implemented in numerical simulations of accretion disks.
A
bstract
We show that the temperature and entropy of a BTZ black hole are invariant under T-duality to next to leading order in
M
⋆
− 2
,
M
⋆
being the scale suppressing higher-curvature/derivative ...terms in the Lagrangian. We work in the framework of a twoparameter family of theories exhibiting T-duality, which includes (but goes beyond) String Theory. Interestingly enough, the AdS/CFT correspondence enforces quantization conditions on these parameters. In the particular case of bosonic/heterotic string theory, our results extend those of a classical paper by Horowitz and Welch. For generic (albeit quantized) values of the parameters, it suggests that T-duality might be an interesting tool to constrain consistent low-energy effective actions while entailing physical equivalences outside String Theory. Moreover, it generates a new family of regular asymptotically flat black string solutions in three-dimensions.
Abstract
Dust particle sizes constrained from dust continuum and polarization observations by radio interferometry are inconsistent by at least an order of magnitude. Motivated by porous dust ...observed in small solar system bodies (e.g., from the Rosetta mission), we explore how the dust particle’s porosity affects the estimated particle sizes from these two methods. Porous particles have lower refractive indices, which affect both opacity and polarization fraction. With weaker Mie interference patterns, the porous particles have lower opacity at millimeter wavelengths than the compact particles if the particle size exceeds several hundred microns. Consequently, the inferred dust mass using porous particles can be up to a factor of six higher. The most significant difference between compact and porous particles is their scattering properties. The porous particles have a wider range of particle sizes with high linear polarization from dust self-scattering, allowing millimeter- to centimeter-sized particles to explain polarization observations. With a Bayesian approach, we use porous particles to fit HL Tau disk’s multiwavelength continuum and millimeter-polarization observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Array (VLA). The moderately porous particles with sizes from 1 mm–1 m can explain both continuum and polarization observations, especially in the region between 20 and 60 au. If the particles in HL Tau are porous, the porosity should be from 70%–97% from current polarization observations. We also predict that future observations of the self-scattering linear polarization at longer wavelengths (e.g., ALMA B1 and ngVLA) have the potential to further constrain the particle’s porosity and size.
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