The chemistry and physics of interstellar ices Klaus Pontoppidan Leiden Observatory Kees Dullemond (MPIA, Heidelberg) Helen Fraser (Leiden) Ewine van Dishoeck (Leiden) Neal Evans (Univ. of Texas) Geoff Blake (Caltech) The c2d team Cardiff, Jan ‘05

Abundance % of O% of C% of N H2H Oxygen- bearing ice 4x %20%- Carbon dust 3.3x %- Silicates 2.6x %-- Gas-phase CO 1x %15%- Nitrogen- bearing ice 3x10 -5 <1% 18% PAHs --10%- Other gas- phase molecules <1% Total ~60%~95%~18% Known molecular reservoirs in dense clouds (cores)

Grain mantles as chemical reservoirs Bare grain surface CO, O, N, H… H 2 O, CH 3 OH, CO 2, NH 3,… CH 3 OCH 3, CH 2 CH 3 CN,… Surface reactions Freeze-out Evaporation Gas-phase reactions Mostly hydrogenation Comets, planets Primitive cloud Circumstellar environment Mol. Cloud T=10-15 K n~10 5 cm -3

Main questions eFormation of interstellar ices. dWhat forms first? Water? CO 2 ? dWhat are the chemical pathways to form the most abundant ice species? dHow does the ice interact with the gas-phase? eEvolution of ices dWhich external processes are important - UV, heating, energetic particles? dWhat happens when prestellar ices are incorporated into a protostellar envelope and then a disk?

The big laboratory in the sky eMicroscopic properties dUnderstanding astronomical ice absorption spectra: Grain shape effects/distribution of ices within a grain mantle + inter- molecular interactions eMacroscopic properties dDistribution of ices in a cloud/envelope/disk. dDust temperatures, radiation fields, density and history of the above parameters.

Spectroscopy of ices VLT-ISAAC 3-5 micron mode H 2 O, CO, CH 3 OH, OCN-, (NH 3 ) --- ~50 lines of sight Spitzer-IRS 5-20 micron H 2 O, NH 4 +, CH 4, (NH 3 ), (CH 3 OH), --- ~100 lines of sight CO 2 ISOCAM-CVF 5-16 micron H 2 O, NH 4 +, CO 2

Single line of sight Traditional method of observing interstellar ices. Problem: almost impossible to couple the ice to the physical condition of the cloud

Multiple embedded lines of sight Good: Direct spatial information can be obtained. Sources are bright. Bad: Sources may interact With the ice on unresolved scales

Multiple background stars Good: Unbiased ice spectra. Bad: Stars are faint in the mid-IR

2MASS JHK SVS 4 - a cluster embedded in the outer envelope of a class 0 protostar. SVS 4SMM 4 Pontoppidan et al 2003, 2004 A&A

ISOCAM 6.7 micron SCUBA 850 micron (used to extract temperature+density profiles) Mapping of ice abundances SMM 4 Most of the stars in SVS4 have very little IR excess: Extinction estimates are accurate

H 2 O ice CH 3 OH ice Both H 2 O and CH 3 OH ices show a sudden jump in abundance At densities of 4x10 5 cm -3 and 1x10 5 cm -3, resp. -The formation of water seems to depend on density. -Methanol in high abundance is very localised.

CO ice seems to be divided into two (or three) basic components Pure CO CO+H 2 O Pontoppidan et al. 2003, A&A, 408, 981

Collings et al 2003 CO ice is mobile < 10 K K K Pontoppidan et al. 2003, A&A

Cold core Envelope? Large disk? 15.2 micron CO 2 bending mode with Spitzer

(+) indicates an observed line of sight. Ices in the Oph-F core CRBR Pontoppidan et al. 2005, in prep

NH 4 + Radial map of CO and CO 2 ices Density Spitzer-IRS VLT-ISAAC ISOCAM-CVF

The formation of ice mantles can be directly modeled. T0 x 3 T0 x 10 (equilibrium) T0 However, an accurate temperature-density model of The core is required for accurate age estimates. 50% 5%

Robert Hurt, SSC

CRBR model 2D Monte Carlo model to compute temperature + density structure of disk and envelope using JHK/(sub)mm imaging micron spectroscopy 90 AU flared disk (solar nebula style) + envelope/foreground material producing extinction to account for the near-infrared colours. Vary parameters by hand (a full grid would take years to compute).

Comparison between observed JHKs composite and model of CRBR ISAAC JHK s Model JHK s Pontoppidan et al. 2005, ApJ, in press

Model fit to the SED of CRBR ” 10”

Heated ice bands toward CRBR H 2 O+’6.85 micron’ bands Conclusion: Most of the ice, in particular the CO ice is Not located in the disk, in this case. However, the NH 4 + band Shows evidence for strong heating, requiring a significant part of this component to be located in the disk.

Summary Different methods of observing interstellar Ices: 1) Single line of sight toward embedded source. 2) multiple lines toward embedded and background stars. 3) disk ices coupled with a radiative transfer model. Examples given: 1)CRBR (disk) 2)SMM 4 (protostellar envelope) 3)Oph-F (dense core) 4)L723 (isolated dense core) Ices are important both for tracing the chemistry and physical conditions of dense clouds…