109 research outputs found
Long-lived protoplanetary disks in multiple systems: the VLA view of HD 98800
The conditions and evolution of protoplanetary disks in multiple systems can
be considerably different from those around single stars, which may have
important consequences for planet formation. We present Very Large Array (VLA)
8.8 mm (34 GHz) and 5 cm (6 GHz) observations of the quadruple system HD 98800,
which consists of two spectroscopic binary systems (Aa-Ab, Ba-Bb). The Ba-Bb
pair is surrounded by a circumbinary disk, usually assumed to be a debris disk
given its 10 Myr age and lack of near infrared excess. The VLA 8.8 mm
observations resolve the disk size (5-5.5 au) and its inner cavity (3
au) for the first time, making it one of the smallest disks known. Its small
size, large fractional luminosity, and millimeter spectral index consistent
with blackbody emission support the idea that HD 98800 B is a massive,
optically thick ring which may still retain significant amounts of gas. The
disk detection at 5 cm is compatible with free-free emission from photoionized
material. The diskless HD 98800 A component is also detected, showing partial
polarization at 5 cm compatible with non-thermal chromospheric activity. We
propose that tidal torques from Ba-Bb and A-B have stopped the viscous
evolution of the inner and outer disk radii, and the disk is evolving via mass
loss through photoevaporative winds. This scenario can explain the properties
and longevity of HD 98800 B as well as the lack of a disk around HD 98800 A,
suggesting that planet formation could have more time to proceed in multiple
systems than around single stars in certain system configurations.Comment: 14 pages, 4 figures, 3 tables; Submitted to ApJ May 14 2018; Accepted
to ApJ August 3 2018. This version fixes a mistake in the reported position
angle. The order of the figures has been changed to match that of the
references in the tex
Dust Filtration by Planet-Induced Gap Edges: Implications for Transitional Disks
By carrying out two-dimensional two-fluid global simulations, we have studied
the response of dust to gap formation by a single planet in the gaseous
component of a protoplanetary disk - the so-called "dust filtration" mechanism.
We have found that a gap opened by a giant planet at 20 AU in a \alpha=0.01,
\dot{M}=10^{-8} Msun/yr disk can effectively stop dust particles larger than
0.1 mm drifting inwards, leaving a sub-millimeter dust cavity/hole. However,
smaller particles are difficult to filter by a planet-induced gap due to 1)
dust diffusion, and 2) a high gas accretion velocity at the gap edge. An
analytic model is also derived to understand what size particles can be
filtered by the gap edge. Finally, with our updated understanding of dust
filtration, we have computed Monte-Carlo radiative transfer models with
variable dust size distributions to generate the spectral energy distributions
(SEDs) of disks with gaps. By comparing with transitional disk observations
(e.g. GM Aur), we have found that dust filtration alone has difficulties to
deplete small particles sufficiently to explain the near-IR deficit of
transitional disks, except under some extreme circumstances. The scenario of
gap opening by multiple planets studied previously suffers the same difficulty.
One possible solution is by invoking both dust filtration and dust growth in
the inner disk. In this scenario, a planet induced gap filters large dust
particles in the disk, and the remaining small dust particles passing to the
inner disk can grow efficiently without replenishment from fragmentation of
large grains. Predictions for ALMA have also been made based on all these
scenarios. We conclude that dust filtration with planet(s) in the disk is a
promising mechanism to explain submm observations of transitional disks but it
may need to be combined with other processes (e.g. dust growth) to explain the
near-IR deficit.Comment: 23 Pages, 11 figures, Accepted by Ap
An Observational Perspective of Transitional Disks
Transitional disks are objects whose inner disk regions have undergone
substantial clearing. The Spitzer Space Telescope produced detailed spectral
energy distributions (SEDs) of transitional disks that allowed us to infer
their radial dust disk structure in some detail, revealing the diversity of
this class of disks. The growing sample of transitional disks also opened up
the possibility of demographic studies, which provided unique insights. There
now exist (sub)millimeter and infrared images that confirm the presence of
large clearings of dust in transitional disks. In addition, protoplanet
candidates have been detected within some of these clearings. Transitional
disks are thought to be a strong link to planet formation around young stars
and are a key area to study if further progress is to be made on understanding
the initial stages of planet formation. Here we provide a review and synthesis
of transitional disk observations to date with the aim of providing timely
direction to the field, which is about to undergo its next burst of growth as
ALMA reaches its full potential. We discuss what we have learned about
transitional disks from SEDs, color-color diagrams, and imaging in the (sub)mm
and infrared. We then distill the observations into constraints for the main
disk clearing mechanisms proposed to date (i.e., photoevaporation, grain
growth, and companions) and explore how the expected observational signatures
from these mechanisms, particularly planet-induced disk clearing, compare to
actual observations. Lastly, we discuss future avenues of inquiry to be pursued
with ALMA, JWST, and next generation of ground-based telescopes.Comment: 24 pages, 13 figures, Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. Klessen, C. Dullemond, Th. Hennin
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