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Photodesorption of Ices
Karin Öberg, Ruud VisserEwine van Dishoeck, Harold
Linnartz
Cold Gas in Disks and Cloud Cores
• Cold CO gas detected in several disks - too much for turbulence
• Cold HCO2+
detected towards envelope of L1527 IRS
• Cold H2O observations will be possible with Herschel
MWC 480
1-0 13CO
L1527 IRS
Pietu et al. 2006, Sakai et al. 2008, ESA-AEOS Medialab
Photodesorption
• Non-thermal desorption yields needed
• CO and H2O photodesorption efficient under some conditions i.e. >10-3 molecules photon-1
Westley et al. 1995, Öberg et al. 2007
CRYOPAD
H2 UV lamp
QMS
RAIRS
Ultra high vacuum (~107 H2 cm-3)15 - 100 K2 - 20 ice monolayers7 - 10.5 eV photons (~1013 photons s-1
= 100 yrs inside cloud)
CRYOPAD
Total photodesorptio
n rate from RAIRS Relative
photodesorption rates from QMS
CO-from-CO2
D2-from-D2O
H2O-from-H2O
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 N2 co-desorption
Temperature
CO and N2 Mechanisms
CO photodesorbs through internal conversion of CO following excitation (van Hemert & Takahashi in prep.).
The mechanism of the co-desorption of N2 remains to be investigated
CO
* *
CO
* *
?
Photo-dissociation and -desorption of CO2
€
c × 1− e−x
l ⎛
⎝ ⎜
⎞
⎠ ⎟
4.6 ML
2.9 ML
Total yield
CO-from-CO2
9
€
c × 1− e−x
l ⎛
⎝ ⎜
⎞
⎠ ⎟
4.5 ML
2.6 ML
Thickness dependence due to diffusion of CO and CO2
Temperature step-function (due to mobility of CO)
Photo-dissociation and -desorption of CO2
H2O dependenci
es• 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 H2O ratio
Astrophysical Implications I:keeping ~1% in the gas phase
Disk midplane
Cloud core
€
RPD = IISRFe−γAV + ICR( )YPDσ gr
Astrophysical Implications II:The impact on chemistry in disks
Model of H2O 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
Conclusions
• Different photodesorption mechanisms, different dependencies on ice thickness and temperature and yet…
• … similar photodesorption yields for CO, CO2 and H2O 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