the chemistry and physics of interstellar ices klaus pontoppidan leiden observatory kees dullemond...

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…