1. Gas!
Very few debris disks have detected gas, and it is generally only found around the youngest systems.
So why should we consider gas here?
Desperation – some sort of additional physics is required to explain the near-IR observations.
Necessity – if dust is sublimating, then there is going to be gas; it’s just a question of how much.
Geoff, Karl, & Chas
representing
The DUNES Team

2. basic flow diagram GAS DUST sublimation P-R drag inner disk
outer reservoir

3. basic flow diagram GAS DUST MRI radiation pressure sublimation
P-R drag
inner disk
outer reservoir

4. basic flow diagram GAS DUST MRI radiation pressure blowout sublimation
deposition
DUST
P-R drag
blowout
inner disk
outer reservoir

5. basic flow diagram GAS DUST MRI radiation pressure blowout dust drag
sublimation
deposition
DUST
gas drag
P-R drag
blowout
inner disk
outer reservoir

6. 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*= has significant gas, it falls roughly in this phase space.
Potential to offset P-R drag and increase dust lifetime.
Geoff, Karl, & Chas
representing
The DUNES Team

7. literature examples Processes to consider:
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)
Radiative blowout of gas (Fernandez, Brandeker, & Wu 2006)
Gas creation via sublimation (Lebreton et al. 2012)
Geoff, Karl, & Chas
representing
The DUNES Team

8. Gas breaking
Lebreton et al considered gas breaking for dust blown out from Fomalhaut’s inner disk.
Geoff, Karl, & Chas
representing
The DUNES Team

9. 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 sublimation, collisions, and P-R drag.
Gas drag pushes particles outward, offsetting P-R drag.
Regularization of dust orbits lower their relative velocities, thereby reducing the effect of collisions.
Circularization of eccentric sublimating grains moves the grains outside of the sublimating region, helping to prolong their lifetime.
Lower inclination orbits can result in an optically thick disk, greatly reducing the radiative blowout.
Grains on blow out trajectories reach a lower terminal velocity (Lebreton et al. 2013).
Geoff, Karl, & Chas
representing
The DUNES Team

10. ______________________
Geoff, Karl, & Chas
representing
The DUNES Team

11. 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?
Spitzer/IRS from Su et al. 2006

12. IR Excess due to Gas (free-free) Emission
Reasons why we shouldn’t worry:
It’s only been seen around B stars. Later type stars may lack sufficiently strong winds.
Power-law SED: 1% near-IR excess should be stronger=detectable at longer wavelengths.
If it’s very close to the star (≤5 R*), it will not be resolved even with CHARA 34m baseline.
(see Roberge for discussion of gas & IR excess around A-type shell stars)
Spitzer/IRS from Su et al. 2006

13. ______________________
Geoff, Karl, & Chas
representing
The DUNES Team

14. 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.
Kennedy & Wyatt 2013
Representing

15. 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.
Kennedy & Wyatt 2013
Representing
Near-IR is different.
Excess measurements do not follow the trend/expectation, but rather show a pile up at flux ratios around 1%.

16. Why there so many near-IR-bright disks?
Why do near-IR observations show a pile-up at flux ratios of Fdisk/Fstar ~ 1% ?
Possibility #1: It’s all junk.
The disks are ~3-σ detections right at the edge of instrument capability.
Possibility #2: The flux distribution is highly bi-modal.
Disks either have a ton of dust or very little; it’s not clear 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.
Geoff, Karl, & Chas
representing
The DUNES Team

17. Optically thick?
To be optically thick (radially) the disk has to be very flat.
Ld/L* ~  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 ~ (AU Mic, Solar System)
Some dissipation of the velocity dispersion is likely required.
Effect on dust lifetime:
Low collisional speeds may reduce collisional destruction rate
Radiative blowout is greatly reduced
Geoff, Karl, & Chas
representing
The DUNES Team

18. ______________________
Geoff, Karl, & Chas
representing
The DUNES Team