How can we probe gas in the planetforming region

1
How can we probe gas in the planet-forming region?
(pre-ALMA) The size scales are too small even for
the largest current near-term arrays.
Spectroscopy to the rescue?
Theory
Jupiter (5 AU) V_doppler 13 m/s V_orbit 13
km/s
H2 is difficult, what other gas tracers can be
studied?
Observation?
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High Resolution IR Spectroscopy Disks
Keck
R10,000-100,000 (30-3 km/s) echelles
(ISAAC,NIRSPEC, PHOENIX,TEXES) on 8-10 m
telescopes can now probe typical T Tauri/Herbig
Ae stars
CO M-band fundamental

NIRSPEC R25000
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As Ewine notes, orientation is pivotal to IR
spectra
Edge-on absorption.
L1489 Gas/Ice10/1, accretion. CRBR2422.8 Gas
/Ice1/1, velocity field? Elias
18 Gas/Icelt1/10 (Shuping et al.)
H2, H3 in absorption?
12Aug2004
4
In older systems, CO disk emission is common
Herbig Ae stars, from face-on (AB Aur) to
highly inclined (HD 163296). CO lines correlated
with inclination and much narrower than those of
H I Disk!
CO lines give distances slightly larger than
K-band interferometry, broad H I traces gas much
closer to star (see also Brittain Rettig 2002,
ApJ, 588, 535 Najita et al. 2003, ApJ, 589,
931). Can do 30-40 objects/night.
Pf b
5
How is the CO excited in these disks?
CO and 13CO rotation diagrams show curvature as a
result of tgt1. Still, small amounts of gas
since N(H2)5 x 1022 leads to dust opacities near
unity.
CO
13CO
Collisional excitation important, but cannot
explain line widths at low J values (too
broad). Resonant IR scattering
at larger radii! The vibrational
excitation is highly variable, likely due to
variations in the UV field. Disk shadowing?
12Aug2004
6
Where does the CO emission come from?
Flared disk models often possess 2-5 micron
deficiency in model SEDs, where a bump is
often observed for Herbig Ae stars.
Dullemond et al. 2002
Explanation Dust sublimation near the star
exposes the inner disk to direct stellar
radiation, heating the dust and puffing up
the disk.
12Aug2004
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How to model the CO emission?
Step 1 Take your favorite description of the
physical structure of the disk.
Step 2 From this description along with your
favorite grain opacity model and abundance for
CO, calculate the optical depth in the gas and in
the dust as you go into the disk. Step 3 For
now, we simply use thermal blackbody and v 0
LTE models to calculate the gas/dust emission and
resonant scattering.
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Systematic Line Width Trends

• Objects thought to be face on have the narrowest
  line widths, highly inclined systems the largest.
• As the excitation energy increases, so does the
  line width (small effect).
• Consistent with disk emission, radii range from
  0.5-5 AU at high J.
• Low J lines also resonantly scatter 5 mm photons
  to much larger distances.
• Asymmetries (VV Ser)?

Blake Boogert 2004, ApJL 606, L73.
12Aug2004
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SED Fits versus IR Interferometry
Fits to AB Aur SED yield an inner radius of 0.5
AU (and 0.06 AU for T Tau).
(Monnier Millan-Gabet 2002, ApJ)
Dullemond et al. 2002
This model can now be directly tested via YSO
size determinations with K-band
interferometry. Intense dust emission pumps
CO, rim shadowing can produce moderate Trot.
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CO Emission from Disks around T Tauri Stars
For dust sublimation alone, the lines from T
Tauri disks should be broader than those from
Herbig Ae starsdisks. Often observed, but
Calvet et al. 2002
The TW Hya lines are extremely narrow, even for a
disk with i7 degrees, imply Rgt1 AU. Gap tracer?
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Future work, Spitzer follow up

• For NIRSPEC at R25,000 limit is M8-9.
• Does CO follow dust? (Inner holes TW Hya)
• Can CO be seen toward wTTs? We have tried a few
  observations toward Ophiuchus, but K stars have
  very complex M-band spectra! Thus, we must be
  much more careful about standards.
• Are there other tracers we should be looking for?
• (esp. w/TEXES _at_ Gemini and VISIR _at_ VLT)