Transcript Trapping mechanisms: the effect of gas

Gas!
Very few debris disks have detected gas, and
generally only found around the youngest systems.
So why should we consider gas here?
it is
1. Desperation – some sort of additional physics is required
to explain the near-IR observations.
Geoff, Karl, & Chas
1. Necessity – if dust is sublimating, then thererepresenting
is going to
be gas; it’s just a question of how much. The DUNES Team
Pasadena – May 2015
basic flow diagram
GAS
sublimation
DUST
P-R drag
inner disk
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outer
reservoir
basic flow diagram
MRI
radiation
pressure
GAS
sublimation
DUST
P-R drag
inner disk
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outer
reservoir
basic flow diagram
MRI
radiation
pressure
GAS
blowout
sublimation
deposition
DUST
P-R drag
inner disk
blowout
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outer
reservoir
basic flow diagram
MRI
GAS
radiation
pressure
blowout
dust drag
sublimation
DUST
deposition
gas drag
P-R drag
inner disk
blowout
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outer
reservoir
Outward gas drag
In between gas-free debris disks
and gas-rich protostellar disks, there
is a little explored range of gas
densities where
gas drag
pushes outward.
If hot dust with Ld/L*=10-3 has
significant gas, it falls roughly in this
phase space.
Potential to offset P-R drag
increase dust lifetime.
and
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Geoff, Karl, & Chas
representing
The DUNES Team
literature examples
Processes to consider:
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Radial diffusion of gas via MRI (Turner et al. 2007)
Gas drag (Klahr & Lin 2001, Lyra et al. 2012)
Gas-dust feedback (Johansen & Youdin 2007)
Reformation of dust beyond the “ice line” (Kretke & Lin 2007)
Karl, & Chas
Radiative blowout of gas (Fernandez, Brandeker, & Geoff,
Wu 2006)
representing
Gas creation via sublimation (Lebreton et al. 2012)
The DUNES Team
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Gas breaking
Lebreton et al. 2012 considered gas breaking for dust blown out
from Fomalhaut’s inner disk.
Geoff, Karl, & Chas
representing
The DUNES Team
Pasadena – May 2015
Net effect of Gas
How might gas help explain hot dust with high Ld/L*?
The gas can reduce the rate of dust removal via
collisions, and P-R drag.
sublimation,
1. Gas drag pushes particles outward, offsetting P-R drag.
2. Regularization of dust orbits lower their relative velocities, thereby
Geoff, Karl, & Chas
reducing the effect of collisions.
representing
3. Circularization of eccentric sublimating grains moves the grains
The DUNES Team
outside of the sublimating region, helping to prolong their lifetime.
4. Lower inclination orbits can result in an optically thick disk, greatly
reducing the radiative blowout.
5. Grains on blow out trajectories reach a lower terminal velocity
(Lebreton et al. 2013).
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Geoff, Karl, & Chas
representing
The DUNES Team
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IR Excess due to
Gas (free-free) Emission
Be stars are identified based on their optical
emission lines (‘e’=emission).
A strong stellar wind supplies ionized
hydrogen-rich gas around these fast rotating
stars.
Spitzer debris disk surveys (Su+ 2006)
determined that some mid-IR excesses are
due to gas, not dust.
The SED signature is distinctive:
- power-law emission, not blackbody
- IR emission lines, e.g. 7.5um HI
 Should we worry about this?
Is
it possible that near-IR excesses
are due
to free-free emission from a gaseous stellar
wind?
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Spitzer/IRS from Su et al. 2006
IR Excess due to
Gas (free-free) Emission
Reasons why we shouldn’t worry:
1. It’s only been seen around B stars. Later Spitzer/IRS from Su et al. 2006
type stars may lack sufficiently strong
winds.
2. Power-law SED: 1% near-IR excess
should be stronger=detectable at longer
wavelengths.
3. If it’s very close to the star (≤5 R*), it will
not be resolved even with CHARA 34m
baseline.
(see Roberge+ 2008 for discussion of gas & IR
excess around A-type shell stars)
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Geoff, Karl, & Chas
representing
The DUNES Team
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Flux-ratio Distributions
At most wavelengths, the flux ratios are
consistent with a broad, log-normal
distribution where the solar system is
roughly close to the median.
Similarly, a random collisional model
predicts a smaller number of bright disks
decaying toward more frequent faint disks.
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Kennedy & Wyatt 2013
Representing
Flux-ratio Distributions
At most wavelengths, the flux ratios are
consistent with a broad, log-normal
distribution where the solar system is
roughly close to the median.
Similarly, a random collisional model
predicts a smaller number of bright disks
decaying toward more frequent faint disks.
 Near-IR is different.
Excess measurements do not
follow the trend/expectation, but
rather show a pile up at flux
ratios around 1%.
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Kennedy & Wyatt 2013
Representing
Why there so many
near-IR-bright disks?
Why do near-IR observations show a pile-up at
ratios of Fdisk/Fstar ~ 1% ?
flux
Possibility #1: It’s all junk.
The disks are ~3-σ detections right at the edge of
instrument capability.
Geoff, Karl, & Chas
Possibility #2: The flux distribution is highly bi-modal.
representing
Disks either have a ton of dust or very little;The
it’s DUNES
not clear
Team
how they cycle between the high and low states.
Possibility #3: The disks are optically thick.
A wide range of disk masses would give the same
fractional luminosity.
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Optically thick?
To be optically thick (radially) the disk has to be very flat.
Ld/L* ~ 10-3  H/R ~ 10-3
Is this reasonable?
Flared protostellar: H/R ~ 0.02 at 0.1 AU (solar-type star)
Saturn’s rings:
H/R < 10-5
Debris disks:
H/R ~ 0.05-0.15 (AU Mic, Solar System)
Geoff, Karl, & Chas
Some dissipation of the velocity dispersion is likely required.
representing
The DUNES Team
Effect on dust lifetime:
- Low collisional speeds may reduce collisional destruction rate
- Radiative blowout is greatly reduced
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______________________
Geoff, Karl, & Chas
representing
The DUNES Team
Pasadena – May 2015