1. Photodesorption of Ices Karin Öberg, Ruud Visser Ewine van Dishoeck, Harold Linnartz

2. Cold Gas in Disks and Cloud Cores Cold CO gas detected in several disks - too much for turbulence Cold HCO 2 + detected towards envelope of L1527 IRS Cold H 2 O observations will be possible with Herschel MWC 480 1-0 13 CO L1527 IRS Pietu et al. 2006, Sakai et al. 2008, ESA- AEOS Medialab

3. Photodesorpti on Non-thermal desorption yields needed CO and H 2 O photodesorpti on efficient under some conditions i.e. >10 -3 molecules photon -1 Westley et al. 1995, Öberg et al. 2007

4. CRYOP AD H 2 UV lamp QMS RAIRS Ultra high vacuum (~10 7 H 2 cm -3 ) 15 - 100 K 2 - 20 ice monolayers 7 - 10.5 eV photons (~10 13 photons s -1 = 100 yrs inside cloud)

5. CRYOP AD Total photodesorption rate from RAIRS Relative photodesorption rates from QMS CO-from-CO 2 D 2 -from-D 2 O H 2 O-from-H 2 O

6. Surface desorption of CO - Öberg et al. 2007: CO photo- desorption yield is ice thickness and flux independent at ~3x10 -3 photon -1 - Öberg et al. 2008: Inversely dependent on annealing temperature and slow N 2 co- desorption Temperatu re

7. CO and N 2 Mechanisms CO photodesorbs through internal conversion of CO following excitation (van Hemert & Takahashi in prep.). The mechanism of the co-desorption of N 2 remains to be investigated CO * * * * ?

8. Photo-dissociation and - desorption of CO 2 4.6 ML 2.9 ML Total yield CO-from- CO 2

9. 9 4.5 ML 2.6 ML Thickness dependence due to diffusion of CO and CO 2 Temperature step-function (due to mobility of CO) Photo-dissociation and - desorption of CO 2

10. H 2 O dependencies Total photodesorption rate at 100 K: ~4 x 10 -3 photon -1 –8 x 10 -3 photon -1 in Westley et al. No dependence on flux, time or photon dose Thickness and temperature dependence of total yield and OH to H 2 O ratio

11. Astrophysical Implications I: keeping ~1% in the gas phase Disk midplane Cloud core

12. Astrophysical Implications II: The impact on chemistry in disks Model of H 2 O gas to gas+ice ratio in pre-main sequence star and disk without and with photo- desorption turned on. Photodesorption has a large impact on the gas chemistry, which should be easily detectable with Herschel

13. Conclusio ns Different photodesorption mechanisms, different dependencies on ice thickness and temperature and yet… … similar photodesorption yields for CO, CO 2 and H 2 O ices ~10 -3 molecules photon -1 Yields of 10 -3 molecules photon -1 are large enough to impact gas phase chemistry Similar yields suggests a gas phase signature of ice composition

14. Surface desorption of CO - From Öberg et al. 2007: CO photo-desorption yield is ice thickness and flux independent at ~3x10 -3 - Öberg et al. 2007: Inversely dependent on annealing temperature Temperatu re Compositi on