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
Grain mantles as chemical reservoirs Comets, planets CH3OCH3, CH2CH3CN,… Gas-phase reactions Circumstellar environment Primitive cloud Mol. Cloud T=10-15 K n~105 cm-3 CO, O, N, H… Freeze-out Evaporation Surface reactions Bare grain surface H2O, CH3OH, CO2, NH3,… Mostly hydrogenation
Main questions • Formation of interstellar ices. • What forms first? Water? CO2? • What are the chemical pathways to form the most abundant ice species? • How does the ice interact with the gas-phase? • Evolution of ices • Which external processes are important - UV, heating, energetic particles? • What happens when prestellar ices are incorporated into a protostellar envelope and then a disk?
The big laboratory in the sky • Microscopic properties • Understanding astronomical ice absorption spectra: Grain shape effects/distribution of ices within a grain mantle + inter-molecular interactions • Macroscopic properties • Distribution of ices in a cloud/envelope/disk. • Dust temperatures, radiation fields, density and history of the above parameters.
Spectroscopy of ices VLT-ISAAC 3-5 micron mode H2O, CO, CH3OH, OCN-, (NH3) --- ~50 lines of sight Spitzer-IRS 5-20 micron H2O, NH4+, CH4, (NH3), (CH3OH), --- ~100 lines of sight CO2 ISOCAM-CVF 5-16 micron H2O, NH4+, CO2
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
SVS 4 SMM 4 SVS 4 - a cluster embedded in the outer envelope of a class 0 protostar. 2MASS JHK Pontoppidan et al 2003, 2004 A&A
Mapping of ice abundances SMM 4 SCUBA 850 micron (used to extract temperature+density profiles) ISOCAM 6.7 micron Most of the stars in SVS4 have very little IR excess: Extinction estimates are accurate
Both H2O and CH3OH ices show a sudden jump in abundance At densities of 4x105 cm-3 and 1x105 cm-3, resp. -The formation of water seems to depend on density. -Methanol in high abundance is very localised. H2O ice CH3OH ice
Pure CO CO ice seems to be dividedinto two (or three) basic components CO+H2O Pontoppidan et al. 2003, A&A, 408, 981
CO ice is mobile < 10 K 10-20 K 30-70 K Collingsetal2003 Pontoppidan et al. 2003, A&A
15.2 micron CO2 bending mode with Spitzer Cold core Envelope? Large disk?
Ices in the Oph-F core CRBR 2422.8-3423 (+) indicates an observed line of sight. Pontoppidan et al. 2005, in prep
Radial map of CO and CO2 ices Density Spitzer-IRS VLT-ISAAC ISOCAM-CVF NH4+
The formation of ice mantles can be directly modeled. However, an accurate temperature-density model of The core is required for accurate age estimates. 50% T0 x 10 (equilibrium) CO depletion T0 x 3 T0 5%
CRBR 2422.8-3423 model 2D Monte Carlo model to compute temperature + density structure of disk and envelope using JHK/(sub)mm imaging + 2-40 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 2422.8-3423 ISAAC JHKs Model JHKs Pontoppidan et al. 2005, ApJ, in press
Heated ice bands toward CRBR 2422.8-3423 H2O+’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 NH4+ 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: • CRBR2422.8-3423 (disk) • SMM 4 (protostellar envelope) • Oph-F (dense core) • L723 (isolated dense core) • Ices are important both for tracing the chemistry and physical conditions of dense clouds…