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

1. Ice structures studied Acknowledgements Aims To 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. Pure Ices (Control Experiments) 1:1 and 19:1 mixtures Ice Mixtures: CO + X CO CO CO Layered Deposits: X X X = CH3OH, CO2, CH4, HCOOH, H2O All 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 The Physical Behaviour of CO On, In and Under Interstellar Ice AnaloguesW. Alsindi,1, 2 S. Bisschop1, 3 and H.J. Fraser11Raymond and Beverly Sackler Laboratory for Astrophysics, Sterrewacht Leiden,Universiteit Leiden, Niels Bohr Weg 2, 2300 RA Leiden, Netherlands.2School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K.3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A. Abstract The 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. Physical Behaviour Linear Growth and Decayof Spectral Components: observed in pure CO ice • Theory and Background • CO is the 2nd most abundant molecule in our galaxy (after H2). • 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 • 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? 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 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. • 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. Pure CO: Well-described by 2 Lorentzian components (2138 and 2140 cm-1): 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). 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. • 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) Deconvolution allows overall and individual component intensity to be tracked through thermal processing of ice mixtures. Trapping of CO in Hydrogen-Containing Ices: • 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. H2O dominated layer H2O + CO dominated layer CO dominated layer Thermal or Radiative Energy Freeze-out onto dust grains CO desorbing from CO CO desorbing from HCOOH surface • 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. CO desorbing during b to a phase change in HCOOH CO (s) HCOOH desorbs No CO No desorption ( )n CH3OH • Permanent dipole • H-bonding between OH groups and CO in amorphous ices assists trapping hn hn hn • No/negligible dipole • No H-bonding interactions and no trapping within matrix H2O CO HCOOH ? CO2 CH3OH ( )n This 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. CH4 CO2 CH3 CH4 • Therefore, CO can reside within grains above its sublimation temperature (~21 K), and is available for chemical reactions. HCOOH