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…