Water in protoplanetary disks: D/H ra4o

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1 Water in protoplanetary disks: D/H ra4o Kenji Furuya, Yuri Aikawa (Kobe Univ) Hideko Nomura (Kyoto Univ) Franck Hersant, Valen4ne Wakelam (Bordeaux Obs)

2 Water - Major Oxygen reservoire - Major ice (+ other volatiles as impurities) raw material for outer planets atmosphere ocean - 3 formation paths of water in ISM OH H 2 H 2 T ~ > 300K O H + 3 OH + H 2 H H 3 O + 2 O + H 2 e H 2 O low T Freeze on grain O ice H OH ice H H 2 Oice T<30K ~

3 Water in star-forming regions Evolu4onary stage Water is everywhere! Water vapor n(h 2 O)/n H Water ice column density [cm - 2 ] Starless core ~10-9 ~10 18 cm - 2 Protostellar core (Class 0- - I): 10% of elemental O Outer envelope (T < 100 K) ~10 18 cm - 2 Inner envelope (T > 100 K) Cavity wall by ouelow Circumstellar disk (Class II): Outer disk (>100 AU) <10-7 ~10 18 cm - 2 Inner disk (< 3 AU) 1.3 [rela4ve to CO] Ref. Whitet & Duley (1991); Terada et al. (2007); Najita & Carr (2008); Honda et al. (2009); Kristensen et al. (2010, 2012); Hogerheijde et al. (2011); Caselli et al. (2012); Coutens et al. (2012) Does water in disks & planetary systems originate in ISM?

4 HDO/H 2 O ra4o Probe of forma4on site of H 2 O forma4on at lower temperature à Higher HDO/H 2 O ra4o Exothermic Exchange Reac4ons - high H 2 D + /H H 2 D + + e à H 2 + D à high atomic D/H (e.g. Millar, Bennet & Herbst 1989) O OH H 2 H 2 H + 3 H OH + 2 H H 2 O + 2 H 3 O + e H 2 O Freeze on grain H H O ice OH ice H 2 Oice

5 HDO/H 2 O in comets and protostellar cores NGC1333 IRAS2A, IRAS4A (Taquet PhD thesis) NGC1333 IRAS2A, IRAS16293 (Liu et al. 2011; Coutens et al. 2012) IRAS16293 A (Persson et al. 2013) Varia4on? NGC1333-IRAS4B (Jorgensen & van Doshoeck 2010) HDO/H 2 O Protostellar envelope ~ Evolu4on? Comets ( )X10-4 (e.g. Mumma & Charnley 2011) Ocean@earth (SMOW, 1.56X10-4 ) 10-5 cosmic [D/H] 1.5X10-5 (Linsky 2003)

6 This Work Ø Can water be destroyed and re- formed in disks? à Does HDO/H 2 O ra4o change? Ø Effect of ver4cal mixing by turbulence Photo- dissocia4on (destruc4on) Re- forma4on (cf. Lyons & Young 2005)

7 Model Ø Disk model (Nomura et al. 2007) - Small dust (i.e. large surface area) well-mixed with gas - Stellar, interstellar, and induced UV - Stellar X-ray T(gas) T(dust) density UV flux

8 Model Ø Chemical model # Gas Grain network of Garrod & Herbst (2006) + mono- & multi-deuteration (Aikawa et al. 2012; Talukdar et al. 1996; Bergin et al. 1999; UMIST07) + high T reactions (Harada et al. 2010) # Photodesorption & photodissociation of ice (Oberg et al. 2007; Andersson & van Dishoeck 2008; Mason et al. 2006) - Desorption yield ~ Dissociation is allowed only in the surface 2 layers Ø Basic equa4on (e.g. Xie et al. 1995; Willacy et al. 2006) diffusion(ver4cal) : num density of species i Chemistry grain radius: 0.1μm : diff coef = 0, 10-3, 10-2

9 Photo-desorption Andersson & van Dishoeck (2008) Photo-dissociation of ice

10 Ø Ini4al Abundances - Collapsing star- forma4on core (Aikawa et al. 2012) - H 2 O is the major O carrier - HDO/H 2 O ~ 10-2 prestellar core (low-mass) protostellar 4 yr

11 Result: H 2 O abundances α=0: no diffusion time H 2 Ogas H 2 Oice 10 4 yr A V =1 A V =10 freeze-out T(dust) 10 5 yr 10 6 yr

12 Result: H 2 O abundances α=10-2 time 10 4 yr H 2 Ogas H 2 Oice A V =1 A V =10 T(dust) 10 5 yr - Water is gone at r < 40AU! Where is Oxygen?? CO 2 ice O 2 gas 10 6 yr

13 Result: Oxygen chemistry H 2 Oice Transported to disk surface à photodissocia4on O atom Transported to deeper T d <30K H 2 O ice is re- T d >30K H 2 O forma4on is inefficient à O is converted to O 2 & CO 2 ice H 2 Ogas H 2 Oice freeze-out CO 2 ice UV OHice O O 2, HCO 2 Oice 2 ice, H 2 Oice O 2 gas 10 6 yr T<30K 30K<T<40K 40K<T<100K

14 Result: Timescale of H 2 O destruc4on NH2Oice τ r H2O - Oatom boundary UV α=10-3 freeze-out O H 2 Oice α=10-2 alpha 10 6 yr A V =1 A V =10

15 Result: HDO/H 2 O ra4o H2 Oice - HDO/H 2 O ice ratio decreases ONLY when H 2 O ice is destroyed significantly Snow Line Snow Line H 2 Ogas - H 2 O gas is abundant at disk surface inside snow line - HDO HDO H2O < gas H2O ice ß Reformation in gas phase (see also Willacy & Woods 2009) log (HDO/H 2 O) log (H 2 Oice column) 10-3 α=10-2 column@ 1 Myr α=0 α= α=0 ini4al D/H α= HDO/H 2 O@ 1 Myr α=10-2 column@ 1 Myr HDO/H 2 O@ 1 Myr

16 Discussion: Dependence on E des (H) - H 2 O reformation is inefficient, because H atom is easily desorbed So far, we assumed E des (H)=450K H O H E des - E des (H) ~ 600 K on amorphous ice (Al-Halabi & van Dishoeck 2007; Watanabe et al 2010; Hama et al. 2012) à H 2 O can be reformed at higher temperature! à Abundant H 2 O ice with low D/H ratio ~10-4 H 2 O ice column E des (H)=600K E des (H)=450K H 2 O ice / HDOice E des (H)=450K E des (H)=600K

17 Summary on H 2 O & Implications H 2 O ice is photo-desorbed & dissociated at disk surface and reformed at T< 30K 40K if E HDO/H 2 O~ des (H)=600K converted to CO 2 at 30K < T <40K converted to O 2 at 40K < T < 100K à Sedimentation of large grains (while small grains are stirred up) will protect H 2 Oice. à But then (HDO/H 2 O) ice does not change (cf. Hogerheijde et al. 2011) HDO/H 2 O ~10-4 in comets could be set in turbulent disk (HDO/H 2 O) gas ~ HDO H2O gas HDO < H2O ice cf. (HDO/H 2 O) gas = IRAS4B (Jorgensen & van Dishoeck 2010)