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Infrared Observations of Interstellar Ices

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Infrared Observations of Interstellar Ices Infrared Observations of Interstellar Ices Adwin Boogert NASA Herschel Science Center IPAC, Caltech Pasadena, CA, USA 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 1 Scope ●Lecture 1 (Monday): What you need to know when planning, reducing, or analyzing infrared spectroscopic observations of dust and ices. ●Lecture 2 (Tuesday): Basic physical and chemical information derived from interstellar ice observations. Not discussed: laboratory techniques (see Palumbo lectures) and surface chemistry (see Cuppen lectures). ●Lecture 3 (Tuesday): Infrared spectroscopic databases. What's in them and how (not) to use them. ●Drylabs (Tuesday): Using databases of interstellar infrared spectra and of laboratory ices. Deriving ice abundances and analyzing ice band profiles. NOTE: Please download all presentations and drylab tar file: spider.ipac.caltech.edu/~aboogert/Cuijk/ 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 2 Topics ●Basics ● Ice mantle formation ● Deriving ice column densities and abundances ●YSO and background source selection ●Continuum determination ●Vibrational modes ●The interstellar ice inventory ●Ice band profile analysis: ● Polar versus apolar ices ● Amorphous versus crystalline ices ● Segregation in the ices ● Grain size and shape effects ●Location of ices ●Processing of ices by YSOs ●Complex molecules in ices? 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 3 Background Reading ● For the basics: Dust in the galactic environment, 2nd ed. by D.C.B. Whittet. Bristol: Institute of Physics (IOP) Publishing, 2003 Series in Astronomy and Astrophysics, ISBN 0750306246. ● More advanced: Chapter 10 in “The Physics and Chemistry of the Interstellar Medium”, A. G. G. M. Tielens, ISBN 0521826349. Cambridge, UK: Cambridge University Press, 2005. ● Current status of observational ice studies: Oberg et al. 2011, ApJ 740, 109 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 4 Basics: Ice Mantle Formation Many molecules (H2, H2O) much more easily formed on grain surfaces. Freeze out <100 K. Keywords: Physisorption, chemisorption, tunneling (see lectures by Cuppen) Interstellar ‘ice’ or ‘dirty ice’: any frozen volatile, e.g. H2O, H2O mixtures, pure CO, but NOT H2. More realistic grain: 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 5 Basics: Ice Mantle Formation For gas at number density n, mean speed <v>, mean particle mass <m>, gas-to-grain sticking coefficient S, grain radius a, and grain density r: ●Grain mantle thickness: Mass growth rate: dm/dt=S*p*a2*n*<v>*<m> Radius growth rate: da/dt=(dm/dt)/(4*p*a2*r) da/dt=S*n*<v>*<m>/(4*r) Mantle thickness independent of grain radius ● Dense clouds can have mantles as thick as 0.1 um, and in deeply embedded protostars even more. ● Mantle thicker than most grain cores according to MRN grain size distribution -3.5 n(a)~a , amin=0.005 μm, amax=0.25 μm 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 6 Basics: Ice Mantle Formation ●Grain temperature and interstellar radiation field inhibit ice formation at low visual extinction (AV): the ice formation threshold ●Taurus cloud: H2O ices absent below visual extinction AV~3 and CO ices below AV~7. ●Difference due to lower Tsub of CO. ●Variation between clouds due to different conditions H2O CO y t i s n e D n m u l o C Extinction (A Extinction (AV) V) 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 7 Basics: Ice Column Densities and Abundances Ice column densities: – N=tpeak*FWHM/Alab – Alab integrated band strength measured in laboratory – -16 A[H2O 3 mm]=2.0x10 cm/molecule Order of magnitude in quiescent dense clouds: 18 -2 – N(H2O-ice)=10 cm along absorption 'pencil beam'. – This is ice layer of 0.3 mm at 1 g/cm3 in laboratory. Order of magnitude estimate of NH (for ice abundances): – AV=t9.7*18.5 mag (Roche & Aitken 1984) – NH=AV*2.0*1021 cm-2 (Bohlin et al. 1977) Ice abundance: -4 – X(H2O-ice)=N(H2O-ice)/NH~10 – This is comparable to X(CO-gas) 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 8 Observing Ices Ices form anywhere T<90 K and Av>few magn. Visible against H O 2 Foreground continuum YSO or background star. + NH4 cloud(s) H2O Star-forming dense core silicates CO2 Background star + H2O NH4 H O 2 envelope silicates CO2 star disk outflow 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 9 Source Selection Taking advantage of large scale infrared imaging surveys, YSOs and background stars can be selected using broad-band 2-25 μm colors. Extinction determined for many background stars, assuming average, intrinsic stellar colors (“NICE” method). L 1014; AV=2-35; 20” resol; Huard et al. (ApJ 640, 391, 2006) 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 10 Spectra of Spitzer-selected YSO and Background Star Isolated core L 1014; AV=2-35; Huard et al. 2006, ApJ 640, 391, 2006 YSO Background Star 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 11 Continuum Determination Ice features are studies on optical depth scale flux =−ln continuum Critical step in the analysis of ice bands is continuum determination. This can be done: ●Locally, on a limited wavelength range (for single or few features), usually done with polynomial fit ●Globally: ● Physical model of source (can be done for background stars, rarely feasible for YSOs) ● Polynomial or spline. Relatively subjective. Fits can be guided by taking into account models and laboratory spectra of dominant absorbers (H2O and silicates). 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 12 Continuum Determination Background Stars Red: M1 III model and featureless extinction curve at AK=1.5 magn Green: H2O ice and silicate model added H2O H2O ? c2 minimization includes: H O ● Spectral type (CO and SiO bands) 2 ● Stellar models (MARC; Decin et al.) ● Extinction laws ● Silicates model silicates ● L-band spectra (H2O ice) CO ● H2O ice model 2 ● 1-25 mm photometry 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 13 Continuum Determination YSOs Continuum determination YSOs much harder because models have many poorly constrained degrees of freedom. 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 14 Observing Solid State Molecules ●H2O ice has many broad absorption bands: ● Symmetric stretch ● Asymmetric stretch ● Bending mode ● Libration mode ● Combination modes ● Lattice mode (can be in emission) ● etc... ●CO:one vibrational mode ●No features for species without permanent dipole moment (O2, N2, H2). 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 15 Ice Inventory 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 16 Ice Inventory 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 17 Ice Inventory [H2O and silicate subtracted!] 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 18 Ice Inventory [H2O and silicate subtracted!] 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 19 Ice Inventory NH3/CH3OH=4 (SVS 4-5) NH3/CH3OH<0.5 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 20 Low Mass vs High Mass Protostar Protostellar luminosity apparently not a dominant factor in ice formation and processing Noriega-Crespo et al. ApJS 154, 352 (2004) 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 21 Ice Inventory CO, incl 13CO few-50% 15-35% CO2, incl. 13CO2 'Typical' abundances 2-4% CH4 with respect to H2O CH OH <8, 30% ice. Species in 3 brackets somewhat [HCOOH] 3-8% disputed. <10, 40% NH3 <2, 7% H2CO [HCOO-] 0.3% OCS <0.05, 0.2% <=3% [SO2] + 3-12% [NH4 ] [OCN-] <0.2, 7% 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 22 Factors of 2 abundance variations between sight- lines are common! Note uncertain NH3 abundance. Will Spitzer spectra finally establish presence of NH3 in interstellar ices? Ice Versus Gas Phase Inventory Gas phase molecules detected in space (123 listed here). Currently up to ~150? Far less ices than gas phase species detected because ices can only be detected by absorption spectroscopy: weakest features (1%) represent column density 0.01*4 [cm-1]/1e-17 [cm/molecule]=4e15 cm-2, orders of magnitude higher www.cv.nrao.edu/~awootten/allmols.html than gas phase detections! 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 23 Ice Band Profiles Analysis Ice band profiles contain wealth of information because they depend on dipole interactions (bond lengths are modified by attractive or repulsive electric forces). Strong effects caused by: ●Ice composition, pure ices versus mixtures ●Matrix structure: amorphous versus crystalline. Temperature. ●Grain size and shape: surface charge induced by external light (polarizability) Comparison with laboratory analogs powerful tool, but at same time fitting is subtle and solutions not unique. Instead of fitting 1000 lab spectra to the interstellar spectra, best to use trends in peak position versus width to draw general conclusions. 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 24 Ice Band Profiles Polar vs Apolar Ices Peak position and width of CO ice band depends strongly on what other molecules are present in the ice. (also note strong dependence on thermal history and grain shape) 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 25 Ice Band Profiles Polar vs Apolar Ices · Interstellar CO ice band consists of 3 components, explained by laboratory simulations as originating from CO in 3 distinct mixtures: ± 'polar' H2O:CO ± 'apolar' CO2:CO ± 'apolar' pure CO (Boogert, Hogerheijde & Blake, ApJ 568,761, 2002) 05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 26 Ice Band Profiles Polar vs Apolar Ices ●CO ice profiles vary in different sight-lines, as a result of different sublimation temperatures: ~90 K for H2O-rich and ~18 K for CO- rich.
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