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