84 research outputs found
An analytical model of radial dust trapping in protoplanetary disks
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
the 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 comparing it with a 3D magneto-hydrodynamical
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 , 1.3, 3.0 mm, we obtain a global dust-to-gas
mass ratio and a viscosity
coefficient . This model can be easily
implemented in numerical simulations of accretion disks
Understanding the Radio Emission from Eridani
Some solar-type stars are known to present faint, time-variable radio
continuum emission whose nature is not clearly established. We report on Jansky
Very Large Array observations of the nearby star Eridani at 10.0 and
33.0 GHz. We find that this star has flux density variations on scales down to
days, hours and minutes. On 2020 Apr 15 it exhibited a radio pulse at 10.0 GHz
with a total duration of about 20 minutes and a peak four times larger than the
plateau of 40 Jy present in that epoch. We were able to model the time
behavior of this radio pulse in terms of the radiation from shocks ramming into
the stellar wind. Such shocks can be produced by the wind interaction of
violently expanding gas heated suddenly by energetic electrons from a stellar
flare, similar to the observed solar flares. Because of the large temperature
needed in the working surface to produce the observed emission, this has to be
non thermal. It could be gyrosynchrotron or synchrotron emission.
Unfortunately, the spectral index or polarization measurements from the radio
pulse do not have enough signal-to-noise ratio to determine its nature.Comment: 7 pages, 4 figures. To appear in Astronomy & Astrophysic
Kinematics of the Outflow From The Young Star DG Tau B: Rotation in the vicinities of an optical jet
We present CO(2-1) line and 1300 m continuum observations made
with the Submillimeter Array (SMA) of the young star DG Tau B. We find, in the
continuum observations, emission arising from the circumstellar disk
surrounding DG Tau B. The CO(2-1) line observations, on the other hand,
revealed emission associated with the disk and the asymmetric outflow related
with this source. Velocity asymmetries about the flow axis are found over the
entire length of the flow. The amplitude of the velocity differences is of the
order of 1 -- 2 km s over distances of about 300 -- 400 AU. We interpret
them as a result of outflow rotation. The sense of the outflow and disk
rotation is the same. Infalling gas from a rotating molecular core cannot
explain the observed velocity gradient within the flow. Magneto-centrifugal
disk winds or photoevaporated disk winds can produce the observed rotational
speeds if they are ejected from a keplerian disk at radii of several tens of
AU. Nevertheless, these slow winds ejected from large radii are not very
massive, and cannot account for the observed linear momentum and angular
momentum rates of the molecular flow. Thus, the observed flow is probably
entrained material from the parent cloud. DG Tau B is a good laboratory to
model in detail the entrainment process and see if it can account for the
observed angular momentum.Comment: Accepted to Ap
Exploring the Formation of Resistive Pseudodisks with the GPU Code Astaroth
Pseudodisks are dense structures formed perpendicular to the direction of the
magnetic field during the gravitational collapse of a molecular cloud core.
Numerical simulations of the formation of pseudodisks are usually
computationally expensive with conventional CPU codes. To demonstrate the
proof-of-concept of a fast computing method for this numerically costly
problem, we explore the GPU-powered MHD code Astaroth, a 6th-order finite
difference method with low adjustable finite resistivity implemented with sink
particles. The formation of pseudodisks is physically and numerically robust
and can be achieved with a simple and clean setup for this newly adopted
numerical approach for science verification. The method's potential is
illustrated by evidencing the dependence on the initial magnetic field strength
of specific physical features accompanying the formation of pseudodisks, e.g.
the occurrence of infall shocks and the variable behavior of the mass and
magnetic flux accreted on the central object. As a performance test, we measure
both weak and strong scaling of our implementation to find most efficient way
to use the code on a multi-GPU system. Once suitable physics and
problem-specific implementations are realized, the GPU-accelerated code is an
efficient option for 3-D magnetized collapse problems.Comment: 29 pages, 1 table, 15 figures, Accepted for publication in the
Astrophysical Journa
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