How can we probe gas in the planet-forming 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?

2. AB Aur HD 163296 High Resolution IR Spectroscopy & Disks R=10,000-100,000 (30-3 km/s) echelles (ISAAC,NIRSPEC, PHOENIX,TEXES) on 8-10 m telescopes can now probe “typical” T Tauri/Herbig Ae stars: Keck CO M-band fundamental NIRSPEC R=25000

3. As Ewine notes, orientation is pivotal to IR spectra: Edge-on absorption. L1489: Gas/Ice~10/1, accretion. CRBR2422.8: Gas/Ice~1/1, velocity field? Elias 18 Gas/Ice<1/10 (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 t>1. 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

7. 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.

8. 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

9. 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.

10. 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 stars+disks. Often observed, but… Calvet et al. 2002 The TW Hya lines are extremely narrow, even for a disk with i~7 degrees, imply R>1 AU. Gap tracer?

11. Future work, Spitzer follow up: • For NIRSPEC at R~25,000; limit is M~8-9. • 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 @ Gemini and VISIR @ VLT)