05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 1 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) 2 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) 3 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) 4 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 More advanced: Chapter 10 in “The Physics and Chemistry of the Interstellar Medium”, A. G. G. M. Tielens, ISBN Cambridge, UK: Cambridge University Press, Current status of observational ice studies: Oberg et al. 2011, ApJ 740, 109

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 5 More realistic grain: Many molecules (H 2, H 2 O) 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. H 2 O, H 2 O mixtures, pure CO, but NOT H 2. Basics: Ice Mantle Formation

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 6 Basics: Ice Mantle Formation For gas at number density n, mean speed, mean particle mass, gas-to-grain sticking coefficient S, grain radius a, and grain density  : Grain mantle thickness: Mass growth rate: dm/dt=S*  *a 2 *n* * Radius growth rate: da/dt=(dm/dt)/(4*  *a 2 *  )  da/dt=S*n* * /(4*  ) 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 n(a)~a -3.5, a min =0.005 μm, a max =0.25 μm

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 7 Column Density CO H2OH2O Extinction (A V) Basics: Ice Mantle Formation Grain temperature and interstellar radiation field inhibit ice formation at low visual extinction (A V ): the ice formation threshold Taurus cloud: H 2 O ices absent below visual extinction A V ~3 and CO ices below A V ~7. Difference due to lower T sub of CO. Variation between clouds due to different conditions Extinction (A V)

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 8 Basics: Ice Column Densities and Abundances Ice column densities:  N=  peak *FWHM/A lab  A lab integrated band strength measured in laboratory  A[H 2 O 3  m]=2.0x cm/molecule Order of magnitude in quiescent dense clouds:  N(H 2 O-ice)=10 18 cm -2 along absorption 'pencil beam'.  This is ice layer of 0.3  m at 1 g/cm 3 in laboratory. Order of magnitude estimate of N H (for ice abundances):  A V =t 9.7 *18.5 mag (Roche & Aitken 1984)  N H =A V *2.0*10 21 cm -2 (Bohlin et al. 1977) Ice abundance:  X(H 2 O-ice)=N(H 2 O-ice)/N H ~10 -4  This is comparable to X(CO-gas)

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 9 Observing Ices Ices form anywhere T few magn. Visible against continuum YSO or background star. H2OH2O CO 2 silicates H2OH2O NH 4 + H2OH2O H2OH2O silicates CO 2 Star-forming dense core Foreground cloud(s) envelope outflow disk star Background star

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 10 Source Selection Taking advantage of large scale infrared imaging surveys, YSOs and background stars can be selected using broad-band 2-25 μ m colors. Extinction determined for many background stars, assuming average, intrinsic stellar colors (“NICE” method). L 1014; A V =2-35; 20” resol; Huard et al. (ApJ 640, 391, 2006)

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 11 Spectra of Spitzer-selected YSO and Background Star YSO Background Star Isolated core L 1014; A V =2-35; Huard et al. 2006, ApJ 640, 391, 2006

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 12 Continuum Determination Ice features are studies on optical depth scale 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 (H 2 O and silicates).

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 13 Continuum Determination Background Stars H2OH2O H2OH2O H2OH2O silicates CO 2 ? Red: M1 III model and featureless extinction curve at A K =1.5 magn Green: H 2 O ice and silicate model added c 2 minimization includes: Spectral type (CO and SiO bands) Stellar models (MARC; Decin et al.) Extinction laws Silicates model L-band spectra (H 2 O ice) H 2 O ice model 1-25 mm photometry

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 14 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) 15 Observing Solid State Molecules H 2 O 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 (O 2, N 2, H 2 ).

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

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 18 Ice Inventory [H 2 O and silicate subtracted!]

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 19 [H 2 O and silicate subtracted!] Ice Inventory

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 20 NH 3 /CH 3 OH=4 NH 3 /CH 3 OH<0.5 (SVS 4-5) Ice Inventory

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 21 Protostellar luminosity apparently not a dominant factor in ice formation and processing Low Mass vs High Mass Protostar Noriega-Crespo et al. ApJS 154, 352 (2004)

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 22 CO, incl 13COfew-50% CO 2, incl. 13CO % CH 4 2-4% CH 3 OH<8, 30% [HCOOH]3-8% NH 3 <10, 40% H 2 CO<2, 7% [HCOO-]0.3% OCS<0.05, 0.2% [SO 2 ]<=3% [NH 4 + ]3-12% [OCN - ]<0.2, 7% Factors of 2 abundance variations between sight- lines are common! Note uncertain NH 3 abundance. Will Spitzer spectra finally establish presence of NH 3 in interstellar ices? Ice Inventory ' Typical' abundances with respect to H 2 O ice. Species in brackets somewhat disputed.

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 23 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 than gas phase detections!

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 24 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) 25 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) 26 Interstellar CO ice band consists of 3 components, explained by laboratory simulations as originating from CO in 3 distinct mixtures: – 'polar' H 2 O:CO – 'apolar' CO 2 :CO – 'apolar' pure CO (Boogert, Hogerheijde & Blake, ApJ 568,761, 2002) Ice Band Profiles Polar vs Apolar Ices

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 27 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 H 2 O-rich and ~18 K for CO-rich. Profile good indicator of thermal processing. Polar/non-polar distinction relevant for outgassing behavior of comets. 'Pockets' of apolar CO may result in sudden sublimation.

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 28 Ice Band Profiles: Amorphous vs. Crystalline Interstellar H 2 O ices formed in amorphous phase, as evidenced by prominent 'blue' wing. Crystallization by protostellar heat. [long wavelength wing originates from scattering on large grains and NH 3 :H 2 O complexes] Crystallization temperature ~120 K in laboratory, but ~70 K in space due to longer time scales. [Time scale ~exp(-T/E barrier ) (~1 hour in lab, 10 5 yr in space). For same reason sublimation temperature in lab (~180 K) higher than in space (~90 K)]. Interstellar H 2 O ices formed in amorphous phase, as evidenced by great width. Crystallization by YSO heat. Crystallization temperature ~120 K in laboratory, but ~70 K in space due to longer time scales: ~1 hour in lab and yr in space: Time scale ~exp(E barrier /T) [For same reason sublimation temperature in lab (~180 K) higher than in space (~90 K)]

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 29 Ice Band Profiles Segregation Interstellar H 2 O ices formed in amorphous phase, as evidenced by prominent 'blue' wing. Crystallization by protostellar heat. [long wavelength wing originates from scattering on large grains and NH 3 :H 2 O complexes] Crystallization temperature ~120 K in laboratory, but ~70 K in space due to longer time scales. [Time scale ~exp(-T/E barrier ) (~1 hour in lab, 10 5 yr in space). For same reason sublimation temperature in lab (~180 K) higher than in space (~90 K)]. If average bond strength in pure ice is stronger than in a mixture, mixture will segregate into clusters of pure ice as the ice is heated. This is how 'pure CO 2 ' ice is formed in space: CO 2, H 2 O, and CH 3 OH formation are linked through OH. The characteristic double peak of the 15 mm CO 2 bending mode appears when the mixture of CO 2 with H 2 O and/or CH 3 OH is heated. Oberg et al., 2009, A&A 505, 183

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 30 Ice Band Profiles Grain Shape and Size Effects Laboratory and interstellar absorption spectra cannot always be compared directly: Scattering on large (micron sized) grains leads to 3 μm red wing (often observed) Surface modes in small grains may lead to large absorption profile variations For ice refractive index m=n+ik, absorption cross section ellipsoidal grain proportional to (Mie theory) (2nk/L 2 )/[(1/L-1+n 2 -k 2 ) 2 +(2nk) 2 ] Resonance for sphere (L=1/3) occurs at k 2 -n 2 =2, so at large k (=strong transitions) Important for pure CO, but not for CO diluted in H 2 O and also not for 13 CO. Laboratory and interstellar absorption spectra cannot always be directly compared: – Scattering on large (micron sized) grains leads to 3 um red wing (often observed) – Surface modes in small grains may lead to large absorption profile variations: Ice refractive index m=n+ik Absorption cross section ellipsoidal grain proportional to (Mie theory): (2nk/L 2 )/[(1/L-1+n 2 -k 2 ) 2 +(2nk) 2 ] Resonance for sphere (L=1/3) occurs at k 2 -n 2 =2, so at large k (=strong transitions): important for pure CO, but not for CO diluted in H 2 O and also not for 13 CO.

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 31 First detection of solid 13 CO High spectral resolution required! New insights into nature apolar ices: – 13 CO well fitted with pure CO, but 12 CO requires ellipsoidal grains – Most CO not mixed with CO 2 (Boogert, Blake & Tielens, ApJ 577, 271 (2002)) Ice Band Profiles Grain Shape Effects First detection of solid 13 CO High spectral resolution required! New insights into nature apolar ices: – 13 CO well fitted with pure CO, but 12 CO..... –...requires ellipsoidal grains – CO and CO 2 not mixed Finally: isotope ratios

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 32 Solid 13 CO 2 : Processing of YSO Ices Bill Saxton (NRAO/AUI/NSF) Ice processing 'hot topic' in astrobiology. Which processing mechanism (heating, UV light, cosmic rays) is relevant where? Do processed ices survive and how?

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 33 Ices formed anywhere T<90 K and extinction large enough. Proper interpretation of ice features, such as the causes of processing history, requires knowledge of location of ices along the line of sight. This can be achieved by complementary gas phase observations, by high spatial resolution spectroscopy, and/or physical models. Where ARE the Ices?

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 34 Spatial distribution CO 2 in circumstellar envelope. (Pontoppidan et al. 2008)  (R)~R -1.5 Where ARE the Ices? Physical conditions vary with radius from the star and vertically in disk. Understanding ice processing of proto-planetary environment requires disk modelling.

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 35 – Protostellar disks provide crucial link between evolution of ices from molecular clouds to planetary systems (comets). – Major difficulty: does line of sight pass through disk and which part of disk? Disk needs to be edge-on. Ices on small scales can nowadays be studied using infrared spectrometers behind adaptive optics systems on large (>=8 m) telescopes, approaching diffraction limit (<0.1''). E.g., IRCS at Subaru and CONICA at VLT. Where ARE the Ices? Crystalline H 2 O ice from upper disk layers YLW16A detected by Schegerer et al. (2010) 1X1'' FOV Edge on disk HV Tau C can be separated by binary component and slit can be oriented at desired angle w.r.t. disk. H 2 O ice band depth varies on few year time scale (Terada et al. 2007). slit

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 36 Solid 13 CO 2 : Processing Ices YSO Envelopes Observational evidence for thermal processing of ices near YSOs: Solid 13 CO 2 band profile varies toward different protostars…

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 37 Solid 13 CO 2 : Processing Ices YSO Envelopes Observational evidence for thermal processing of ices near YSOs: Solid 13 CO 2 band profile varies toward different protostars… …and laboratory simulated spectra show this is due to CO 2 :H 2 O mixture progressively heated by young star (Boogert et al. 2000; Gerakines et al. 1999)

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 38 Processing Ices YSO Envelopes Observational evidence for thermal processing of ices near YSOs: Solid CO 2 band profile varies toward different protostars… …and laboratory simulated spectra show this is due to CO 2 :H 2 O mixture progressively heated by young star (Boogert et al. 2000; Gerakines et al. 1999) H 2 O crystallization (Smith et al. 1989) gas/solid ratio increases (van Dishoeck et al. 1997) Detailed modelling gas phase mm- wave observations (van der Tak et al. 2000) Little evidence for energetic processing of ices, however......

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 39 Complex Molecules? Greenberg et al. ApJ 455, L177 (1995): launched processed ice sample in earth orbit exposing directly to solar radiation (EURECA experiment). Yellow stuff turned brown: highly carbonaceous residue, also including PAH. Complex species formed, some are of biological interest: POM (polyoxymethylene, -(CH 2 -O)n- HMT (hexamethylenetetramine, C 6 H 12 N 4 ) Amino acids (glycine) Urea (H 2 NCONH 2 ) PAHs (polycyclic aromatic hydrocarbons)

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 40 Greenberg et al. ApJ 455, L177 (1995): launched processed ice sample in earth orbit exposing directly to solar radiation (EUREKA experiment). Yellow stuff turned brown: highly carbonaceous residue, also including PAH. Complex Molecules? Observational evidence for complex products of UV/CR bombardment of simple ices is weak at best: Triple peak 3.4 mm band seen only in diffuse medium and it is not polarized as opposed to silicates/ices: not in processed mantle but separate grains Perhaps organic residue in 5-8 mm region?

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 41 Molecular Evolution: Hot Cores Formic acid Methyl formate Formic acid Dimethyl ether Ceccarelli et al. A&A 521, L22 (2010) High spectral and spatial resolution observations of Hot cores. Trace the ices indirectly, after sublimation.

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 42 Future of Astrochemistry is Bright.... Thirty Meter Telescope Atacama Large MM Array James Webb Space Telescope ….plus a lot more……