1. Probing CO freeze-out and desorption in protoplanetary disks
Chunhua Qi
Harvard-Smithsonian CfA
[Qi et al. 2008]
[Qi et al. in prep]

2. Star Formation Stages
Image credit: Bill Saxton (NRAO)

3. Protoplanetary disk structure
[Henning & Semenov 2013]
[Öberg, Murray-Clay & Bergin 2011]
Sketch of the physical and chemical structure of a ∼1–5 Myr old protoplanetary disk around a Sun-like star.
CO freeze-out/desorption probe ?
CO snow line location ?

4. 1. CO freeze-out/desorption probe ?

5. CO disks are “huge” TW Hya HD 163296 Classical T Tauri star
8-12 Myr old
Inclination 7o
Herbig Ae star
3-5 Myr old
Inclination 44o

6. Disk temperature decreases radially away from the star
and vertically toward disk midplane
Temperature Contour
20 K

7. Optically thick CO lines on the surface hide the CO freezeout information at midplane

8. Chemical imaging of CO freeze-out: Ring structures
Chemical effect of the CO freeze-out to locate the co snow line, i.e. chemical imaging.
[Qi et al. 2013]

9. Chemical imaging of the CO snow line: N2H+ ring structure
[Qi et al. 2013]
TW Hya
Inner Edge
DCO+
N2H+ is destroyed by the gas CO and
enhanced by the freeze-out of gas CO
N2H+ + CO  HCO+ + N2

10. Chemical imaging – DCO+ ring structure
H2D+ + CO  DCO+ + H2
Outer Edge
DCO+
DCO+ abundance is balanced by CO freeze-out
and temperature-dependent D enhancement
[Mathews et al. 2013]

11. Probing CO photodesorption
IM Lup
[Öberg et al. 2015]

12. ALMA images of N2H+ and DCO+ toward TW Hya
[Qi et al. in prep]

13. ALMA images of N2H+ and DCO+ toward TW Hya
[Qi et al. in prep]

14. ALMA images of N2H+ and DCO+ toward TW Hya
[Qi et al. in prep]

15. Slide credit: T. Birnstiel

16. Impact of radial drift on the global dust temperature structure
[Cleeves 2016]
Drift of the mm grains allow the reprocessed radiation from the upper layer penetrating deeper.
The outer disk midplane directly heated by the upper layer

17. Imaging the CO desorption
N2H+ and DCO+
[Qi et al. in prep]
[Nomura et al. 2016]

18. 2. The CO snow line location ?

19. How to locate the CO snow line …
R [AU]
CO abundance
RCO
CO abundance drop
Chemical imaging

20. CO (radial and) vertical structure
HD CO multi-transition multi-isotope studies with SMA [Qi et al. 2011]
, 12C17O

21. Resolving protoplanetary disks spatially and spectrally
Figure+Movie credit:
Ian Czekala

22. Locating CO snow line based on SMA 13CO 2-1 emission
RCO = 155 AU HD
[Qi et al. 2011]

23. Locating CO snow line based on ALMA C18O 2-1 emission
RCO=155 AU [Qi11]
[Qi et al. submitted]

24. Locating CO snow line based on ALMA C18O 2-1 emission
RCO=155 AU [Qi+11]
CO snow line is at 90 AU in HD disk
RCO=90 AU
Have to consider the optical depth problem.
Hard to distinguish from radial profile.
[Qi et al. 2015]

25. The inner edge of N2H+ ring in HD 163296 disk
is around 90 AU, consistent with C18O analysis
[Qi et al. 2015] HD

26. However, the new 13C18O observation of TW Hya indicates the CO snow line around 21 AU, smaller than 30 AU found with N2H+ emission
[Qi et al. 2013]
TW Hya
[Zhang et al. 2017]

27. Model FD: considering only freeze-out and desorption
Model CH: considering simple chemical network for N2H+
Chemical model indicates the N2H+ emission can peak much further out beyond the CO snow line, and a rather smooth fall-off inward.
[Van ‘t Hoff et al. 2016]

28. However, the inner edge of N2H+ emission toward TW Hya found to be very sharp
[Qi in prep]

29. Summary
N2H+ is sensitive to the CO freeze-out but whether it can serve as a robust probe of the CO snow line is still under debate.
DCO+ can be used as a probe of the CO desorption, although more works are needed to disentangle the nature of desorption.
Optically thin CO isotopologue emission can be used locate the CO snow line directly but very tricky due to optical depth and sensitivity issue.

30. [Qi et al. 2013]
[Qi et al. 2008]
[Qi et al. in prep]

31. Collaborators: K. Öberg, D. Wilner, S. Andrews, L.I. Cleeves (CfA);
E. Bergin, N. Calvet (U. Michigan); A.M. Hughes (Wesleyan U.) ;
C. Espaillat (Boston U.); Michiel Hogerheijde (Leiden U.)

32. END

33. Impact of radial drift on the global dust temperature structure
[Andrews 2015]
[Cleeves 2016]