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

Star Formation Stages Image credit: Bill Saxton (NRAO)

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 ?

1. CO freeze-out/desorption probe ?

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

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

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

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]

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

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]

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

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

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

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

Slide credit: T. Birnstiel

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

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

2. The CO snow line location ?

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

CO (radial and) vertical structure HD 163296 CO multi-transition multi-isotope studies with SMA [Qi et al. 2011] , 12C17O

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

Locating CO snow line based on SMA 13CO 2-1 emission RCO = 155 AU HD 163296 [Qi et al. 2011]

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

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 163296 disk RCO=90 AU Have to consider the optical depth problem. Hard to distinguish from radial profile. [Qi et al. 2015]

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

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]

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]

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

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.

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

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.)

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