Dust Segregation
Further evidence for planet-building activity
in the V4046 Sgr disk comes from a comparison of the GPI and SMA data (Figure 3). GPI
imaging traces light scattered off small dust
grains, which appear to prevail within 10-45
AU of the central stars. Data from the SMA
traces thermal emission from larger centimeter- to millimeter-sized dust grains, revealing
emission concentrated in a ring whose inner
edge lies at ~ 30 AU. Thus, the GPI data confirm the earlier Spitzer and Herschel spectroscopic observations, which show that the
gap seen at submillimeter wavelengths is indeed partially filled with small dust particles.
Modeling of planet formation in disks suggests that we should see this phenomenon
of grain size segregation. When a gas giant
planet forms in a disk, it creates local density waves that trap larger (mm- to cm-sized)
particles outside the planet-forming regions
of the disk. Smaller (micron-sized) grains
freely pass through these pressure traps, resulting in strong dust particle size gradients.
Our comparison of the GPI and submillimeter imaging of the V4046 Sgr disk provides
vivid evidence in support of these so-called
“dust filtration” models by describing the
structure of a circumbinary disk captured in
the process of actively forming planets.
In summary, our GPI images appear to provide powerful tests of two planet-forming
processes in the V4046 Sgr disk: (1) That
one or more young giant planets following
orbits similar to those of Saturn or Uranus
have simultaneously carved out a disk gap
and an inner disk hole; and (2) these gas-giant planets have generated large-scale density waves, resulting in dust particle filtration
and segregation by size. Together these results offer two possible ways giant planets
can form in debris disks around young, Sunlike stars.
January 2016
In addition, this evidence
for the presence of young
giant planets around
V4046 Sgr helps set essential constraints on simulations aimed at understanding the conditions in
which giant planets might
form around binary star
systems — a theoretical
question that is presently
of intense interest, given
the Kepler Mission’s detection of roughly 40 circumbinary planets to date.
Figure 3.
What’s Next?
The Gemini Planet Imager has provided us
with our first close look at some likely dynamic episodes in planet building activity
within a circumbinary disk orbiting a young
binary star system — especially in regions
where gas giant planets are known to form
around our Sun. Our team has subsequently obtained new time allocations with GPI
to obtain deeper imaging of the V4046 Sgr
disk. With these new observations, we hope
to image the massive planets we suspect are
forming, or have recently formed, in the disk.
Comparison of SMA
data (blue shading
and yellow contours)
and GPI J (green) and
K2 (red) data.
We also plan to image other nearby young
star-disk systems, to look for ring/gap features similar to those detected by GPI in the
V4046 Sgr disk. Any additional results will
further our understanding of the planet formation processes taking place in circumstellar disks orbiting young, Sun-like stars; they
might also teach us something new about
how gas giant planets formed in our own solar neighborhood.
Valerie Rapson recently completed her PhD at
the Rochester Institute of Technology. She can be
reached at: [email protected]
2015 Year in Review
GeminiFocus
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