9,813 research outputs found
Multiple Disk Gaps and Rings Generated by a Single Super-Earth: II. Spacings, Depths, and Number of Gaps, with Application to Real Systems
ALMA has found multiple dust gaps and rings in a number of protoplanetary
disks in continuum emission at millimeter wavelengths. The origin of such
structures is in debate. Recently, we documented how one super-Earth planet can
open multiple (up to five) dust gaps in a disk with low viscosity
(). In this paper, we examine how the positions, depths,
and total number of gaps opened by one planet depend on input parameters, and
apply our results to real systems. Gap locations (equivalently, spacings) are
the easiest metric to use when making comparisons between theory and
observations, as positions can be robustly measured. We fit the locations of
gaps empirically as functions of planet mass and disk aspect ratio. We find
that the locations of the double gaps in HL Tau and TW Hya, and of all three
gaps in HD 163296, are consistent with being opened by a sub-Saturn mass
planet. This scenario predicts the locations of other gaps in HL Tau and TW
Hya, some of which appear consistent with current observations. We also show
how the Rossby wave instability may develop at the edges of several gaps and
result in multiple dusty vortices, all caused by one planet. A planet as low in
mass as Mars may produce multiple dust gaps in the terrestrial planet forming
region.Comment: 16 pages; ApJ accepte
Multiple Disk Gaps and Rings Generated by a Single Super-Earth
We investigate the observational signatures of super-Earths (i.e.,
Earth-to-Neptune mass planets) in their natal disks of gas and dust. Combining
two-fluid global hydrodynamics simulations with a radiative transfer code, we
calculate the distributions of gas and of sub-mm-sized dust in a disk perturbed
by a super-Earth, synthesizing images in near-infrared scattered light and the
mm-wave thermal continuum for direct comparison with observations. In low
viscosity gas (), a super-Earth opens two annular gaps
to either side of its orbit by the action of Lindblad torques. This double gap
and its associated gas pressure gradients cause dust particles to be dragged by
gas into three rings: one ring sandwiched between the two gaps, and two rings
located at the gap edges farthest from the planet. Depending on system
parameters, additional rings may manifest for a single planet. A double gap
located at tens of AUs from a host star in Taurus can be detected in the dust
continuum by the Atacama Large Millimeter Array (ALMA) at an angular resolution
of ~0".03 after two hours of integration. Ring and gap features persist in a
variety of background disk profiles, last for thousands of orbits, and change
their relative positions and dimensions depending on the speed and direction of
planet migration. Candidate double gaps have been observed by ALMA in systems
like HL Tau (D5 and D6) and TW Hya (at 37 and 43 AU); we submit that each
double gap is carved by one super-Earth in nearly inviscid gas.Comment: 23 pages, 1 table, 14 figures, ApJ accepte
Standing sausage modes in coronal loops with plasma flow
Magnetohydrodynamic waves are important for diagnosing the physical
parameters of coronal plasmas. Field-aligned flows appear frequently in coronal
loops.We examine the effects of transverse density and plasma flow structuring
on standing sausage modes trapped in coronal loops, and examine their
observational implications. We model coronal loops as straight cold cylinders
with plasma flow embedded in a static corona. An eigen-value problem governing
propagating sausage waves is formulated, its solutions used to construct
standing modes. Two transverse profiles are distinguished, one being the
generalized Epstein distribution (profile E) and the other (N) proposed
recently in Nakariakov et al.(2012). A parameter study is performed on the
dependence of the maximum period and cutoff length-to-radius
ratio in the trapped regime on the density parameters
( and profile steepness ) and flow parameters (magnitude
and profile steepness ). For either profile, introducing a flow
reduces relative to the static case. depends
sensitively on for profile N but is insensitive to for profile E. By
far the most important effect a flow introduces is to reduce the capability for
loops to trap standing sausage modes: may be
substantially reduced in the case with flow relative to the static one. If the
density distribution can be described by profile N, then measuring the sausage
mode period can help deduce the density profile steepness. However, this
practice is not feasible if profile E better describes the density
distribution. Furthermore, even field-aligned flows with magnitudes
substantially smaller than the ambient Alfv\'en speed can make coronal loops
considerably less likely to support trapped standing sausage modes.Comment: 11 pages, 9 figures, to appear in Astronomy & Astrophysic
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