Physical Behaviour

Linear Growth and Decay of Spectral Components: observed in pure CO ice

CO desorbing from CO

Nodesorption

CO desorbing from HCOOH surface CO desorbing

during to phase change

in HCOOH

No CO

HCOOHdesorbs

Theory and Background•CO is the 2nd most abundant molecule in our galaxy (after H 2).

•It is observed in the gas phase and solid state towards high, medium and low mass SFR’s.

•CO vibrational band (at around 2139 cm -1) is a good candidate for use as a solid state probe or tracer molecule.

•Spectral profile is thought to reflect prevailing physical and chemical conditions during CO condensation, and subsequent processing.

Observations of Solid CO

CO spectra towards low mass YSOs TPSC 78, IRS 43 and Reipurth 50, obtained at the VLT by Pontoppidan et al. (A&A 2003, submitted). Fits

are phenomenological, at 3 fixed wavelengths: 2143.7, 2139.9 and 2136.5 cm-1, and fixed linewidths.

• Recent observations using high-sensitivity, high-resolution, 8 m-class telescopes reveal clear sub-structure in the solid CO band, previously unresolved.

• Traditionally band is deconvolved into two components, that may be spatially separated on the grains:

CO in ice matrix dominated by Van der Waals bonding (2139 cm-1); CO in ice matrix dominated by hydrogen bonding (2136 cm-1)

• Boogert et al. and Pontoppidan et al. deconvolve further substructure, at around 2143 cm-1 using different analysis techniques. The identity of the carrier has been suggested as CO2-rich ice or LO-TO splitting in crystalline CO respectively.

CO and Solid-State Synthesis

Solid-State Chemistry (Postulated)

CO Spectrum of L1489 IRS; fit is a composite of 3 laboratory ice mixtures –

H2O:CO (4:1) at 50 K (dotted), N2:O2:CO2:CO

(1:5:0.5:1) at 10 K (dashed) and CDE-

corrected pure CO at 10 K (grey). Obtained at the Keck Observatory by

Boogert et al. (ApJ, 2002, 568, 761).

 

 

 

Astronomical Implications•All our “medium-resolution” spectroscopy shows that CO spectra can be deconvolved into discrete components that do not vary significantly from matrix to matrix, whether dominated by Van der Waals’ or hydrogen-bonds.

•Certain crystalline phases do not trap CO; all low-temperature hydrogen-bonded amorphous phases studied can.

The Physical Behaviour of CO The Physical Behaviour of CO OnOn, , InIn and and UnderUnder Interstellar Ice Analogues Interstellar Ice Analogues

W. Alsindi,W. Alsindi,1, 21, 2 S. Bisschop S. Bisschop1, 31, 3 and H.J. Fraser and H.J. Fraser11 11Raymond and Beverly Sackler Laboratory for Astrophysics, Sterrewacht Leiden,Raymond and Beverly Sackler Laboratory for Astrophysics, Sterrewacht Leiden,

Universiteit Leiden, Niels Bohr Weg 2, 2300 RA Leiden, Netherlands.Universiteit Leiden, Niels Bohr Weg 2, 2300 RA Leiden, Netherlands.22School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K.School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K.

33Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.

AbstractThe spectral behaviour of solid- state carbon monoxide has been studied both in ‘solid mixtures’ and ‘layered deposits’ of interstellar ice components typical of those found in star-forming regions.

0.5 cm-1 resolution Fourier Transform Infrared (FTIR) Spectroscopy was used, with a view to understanding the physical significance of the components which constitute astronomical solid-state CO bands.Through deconvolution of the spectra obtained during deposition and thermal processing, we have monitored the growth and decay of individual components within the CO vibrational transition. Trapping,

translational diffusion, and phase transitions of solid CO have all been observed. The astronomical implications of this physical behaviour are discussed.

Translational Diffusion: CH4-CO system

•Displays translational diffusion at low temperatures;

•Change in CO environment evinced by change in profile – differential growth and decay of components (without desorption of CO). Formation of different CO environments with thermal processing; domains of pure, crystalline CO formed?

Phase Transformations: CH3OH-CO system

•Phase transformations can allow trapped volatile species to escape and enter the gas phase. Here, the amorphous-crystalline phase change leads to the loss of remaining CO trapped within the structure well above its pure desorption temperature.

•Prior to this, slow and gradual loss of CO intensity indicates translational mobility of CO within the solid, diffusing through pores and subliming.

Effect of Ice Structure on CO Trapping: HCOOH-CO•The extent of CO trapping within hydrogen-bonded ices depends on deposition conditions and behaviour of surrounding ice matrix.

•By tracking summed component intensity against temperature for each ice type, these relationships can be correlated to changes in ice structures through thermal processing.

Spectral Analysis: Deconvolution

Spectra were deconvoluted with graphical analysis software utilising Levenberg-Marquardt non-linear least squares regression fitting. Two dominant line profiles observed:

Lorentzian: Gaussian:

where ‘c’ is the peak centre and ‘σ’ is the FWHM

Pure CO: Well-described by 2 Lorentzian components (2138 and 2140 cm -1):

CH3OH-CO: H-bonding leads to a broader, Gaussian distribution (centred at 2136 cm-1 and FWHM ~9 cm-1

CH4-CO: Complex multicomponent spectrum which is significantly temperature-dependent. We can clearly see growth and decay of three components (1 Gaussian, 2 Lorentzian) upon thermal processing.

Deconvolution allows overall and individual component intensity to be tracked through thermal processing of ice mixtures.

Trapping of CO in Hydrogen-Containing Ices:

22/2)(

2

1)(

cx

exf

22)(

1)(

cxxf

Ice structures studied

X = CH3OH, CO2, CH4, HCOOH, H2OAll ices formed as amorphous networks and growth of pure CO layers was

carried out differentially. Thermal processing monitored from 14 K to sublimation in 3-5 K increments.

X

CO

CO X

CO + X

1:1 and 19:1

mixtures

Pure Ices (Control Experiments)

Ice Mixtures:

Layered Deposits: CO

X

CH4

CO dominated layer

H2O dominated layer

Thermal or Radiative Energy

H2O + CO dominated

layer Freeze-out onto dust

grains

AimsTo use ‘medium’-resolution laboratory spectroscopy and

thermal processing of pure CO and CO-containing ices, to understand underlying physical influences on the spectra

observed towards YSO’s.

HCOOH

CO

CH4

CO2CH3OH

H2O

?

•No/negligible dipole

•No H-bonding interactions and no trapping within matrix

• Permanent dipole

•H-bonding between OH groups and CO in amorphous ices assists trapping

CO (s)

CH3OH

CO2

HCOOH

CH3

( )n

( )n

h h

•Therefore, CO can reside within grains above its sublimation temperature (~21 K), and is available for chemical reactions.

AcknowledgementsThis work was supported by NOVA, the Dutch Research School

for Astronomy, and the Spinoza Fund. We thank E. F. van Dishoeck for useful discussions and K. Pontoppidan for sharing

results with us prior to publication.

0 20 40 60 80 100 120 140 160 180 200

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lis

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sit

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of

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str

etc

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Temperature / K

CO alone CO above HCOOH CO mixed with HCOOH CO below HCOOH

h