The chemistry and physics of interstellar ices
Klaus PontoppidanLeiden 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 NH2 1 - - -Oxygen-bearing ice
4x10-4 29% 20% -
Carbon dust 3.3x10-4 - 50% -Silicates 2.6x10-4 23% - -Gas-phase CO
1x10-4 9% 15% -
Nitrogen-bearing ice
3x10-5 <1% <1% 18%
PAHs - - 10% -Other gas-phase molecules
10-7-10-6 <1% <1% <1%
Total ~60% ~95% ~18%
Known molecular reservoirs in dense clouds (cores)
Grain mantles as chemical reservoirs
Bare grain surface
CO, O, N, H…
H2O, CH3OH, CO2, NH3,…
CH3OCH3, CH2CH3CN,…
Surface reactions
Freeze-out Evaporation
Gas-phase reactions
Mostly hydrogenation
Comets, planets
Primitive cloud Circumstellar environment
Mol. CloudT=10-15 Kn~105 cm-3
Main questionsFormation 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 icesSpectroscopy of ices
VLT-ISAAC 3-5 micron modeH2O, CO, CH3OH, OCN-, (NH3) --- ~50 lines of sight
Spitzer-IRS 5-20 micronH2O, NH4
+, CH4, (NH3), (CH3OH), --- ~100 lines of sightCO2
ISOCAM-CVF 5-16 micronH2O, NH4
+, CO2
Single line of sight
Traditional methodof observing interstellarices. Problem: almostimpossible to couple theice to the physicalcondition of the cloud
Multiple embedded lines of sight
Good: Direct spatialinformation 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 envelopeof a class 0 protostar.
SVS 4
SMM 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
H2O ice
CH3OH ice
Both H2O and CH3OHices show a suddenjump in abundanceAt densities of4x105 cm-3 and 1x105 cm-3, resp.
-The formation of water seems to dependon density.-Methanol in high abundance is verylocalised.
CO ice seems to be dividedinto two (or three) basic components
Pure CO
CO+H2O
Pontoppidan et al. 2003, A&A, 408, 981
Collings et al 2003
CO ice is mobile
< 10 K
10-20 K
30-70 K
Pontoppidan et al. 2003, A&A
Cold core
Envelope?
Large disk?
15.2 micron CO2 bending mode with Spitzer
(+) indicates an observed line of sight.
Ices in the Oph-F core
CRBR 2422.8-3423
Pontoppidan et al. 2005, in prep
NH4+
Radial map of CO and CO2 icesDensity
Spitzer-IRSVLT-ISAACISOCAM-CVF
The formation of ice mantles can be directly modeled.
T0 x 3
T0 x 10 (equilibrium)
T0
However, an accurate temperature-density model ofThe core is required for accurate age estimates.
50%
5%
Robert Hurt, SSC
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 wouldtake years to compute).
Comparison between observed JHKs composite and modelof CRBR 2422.8-3423
ISAAC JHKs Model JHKs
Pontoppidan et al. 2005, ApJ, in press
Model fit to the SED of CRBR 2422.8-3423
30”
10”
Heated ice bands toward CRBR 2422.8-3423
H2O+’6.85 micron’ bands
Conclusion:
Most of the ice, in particular the CO ice isNot located in the disk, in this case. However, the NH4
+ bandShows evidence for strong heating, requiring a significant part of this component to be located in the disk.
Summary
• Different methods of observing interstellarIces: 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) CRBR2422.8-3423 (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…