grains, which coagulate over time, grow in
size, and settle towards the disk midplane.
Present theories hold that giant planet formation in disks likely occurs via one of two
mechanisms: core accretion or gravitational
instability. In the core accretion scenario,
ice-covered dust particles collide and stick
together, growing into ever-larger rocky
bodies; planetesimals form from this buildup
of rocky material, and their collisions eventually build super-Earth-mass planetary “cores.”
Such massive cores can then rapidly collect
gas from the disk to form giant planets.
In contrast, the gravitational instability theory holds that planets form when a perturbation in the disk causes a large amount of disk
material to collapse and form a planet essentially all at once. Hence, this rapid process is
similar to that by which a young central star
forms from its birth cloud.
Under either scenario, once a massive planet
forms, co-orbiting material either accretes
onto the planet or is accelerated radially
in the disk via spiral density waves, which
cause material approaching them to speed
up — until they reach the perturbed regions,
where they slow down and linger. These
mechanisms result in ring-like or spiral structures in the disk characterized by sharp radial gradients in both surface density and
particle size.
The predicted spiral and ring structures have
indeed been observed in disks around young
stars in nearby star-forming clouds with
telescopes such as the Sub-Millimeter Array (SMA) and Atacama Large Millimeter Array (ALMA), and with near-infrared cameras
on 8-meter-class telescopes, such as Subaru
and Gemini. However, these observations are
typically probing planet formation around
young stars at orbital separations many times
that of the gas giants in our Solar System.
formation around young nearby stars within
~ 30 AU, where we can search for evidence
for gas giant planet formation on scales
similar to that of our Solar System. We do so
by focusing on a handful of solar mass stars
within ~ 300 light years (ly) of Earth that are
surrounded by, and actively accreting material from, gas-rich circumstellar disks.
Target: V4046 Sagittarii
Our team has been closely scrutinizing one
such star-disk system: V4046 Sagittarii (Sgr).
Lying at a distance of just ~ 240 ly, V4046 Sgr
is an extraordinary binary star system surrounded by a massive disk of gas and dust
roughly 0.1 solar mass. With an age of only
~ 20 million years, this system provides us
with an excellent opportunity to search for
evidence of recent or ongoing planet formation. The V4046 Sgr system also offers
an intriguing twist: Any planets spawned in
its disk would have to orbit twin stars (both
only slightly less massive than our Sun) separated by just ~ 9 solar radii.
Interestingly, the V4046 Sgr disk exhibits a
central clearing out to ~ 30 AU (i.e., ~ 1/2 arcsecond) at submillimeter wavelengths. The
presence of this inner “hole” suggests that
one or more planets may be forming close
to the central stars, carving out an opening
within the disk (Figure 1 shows an artist’s
rendering of the V4046 Sgr disk based on
Figure 1.
Artist’s impression of
the the binary stardisk system V4046 Sgr.
Image credit: NASA/
JPL-Caltech/T. Pyle
(SSC).
Our team is interested in studying planet
January 2016
2015 Year in Review
GeminiFocus
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