12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 1 Ices in the Cosmos and the Laboratory Adwin Boogert NASA Herschel Science Center IPAC, Caltech Pasadena, CA, USA

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 2 Contents Major outstanding questions interstellar ices Observational results last ~10 years Role of laboratory simulations New laboratory experiments needed to analyze existing and future astronomical observations

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 3 Major Outstanding Questions What is the composition of interstellar ices? Identification many ice features still disputed! Any new ice feature detected has major impact on interstellar chemistry studies as it must represent abundant species. Example: 1% feature (~weakest detectable) represents column density 0.01*4 [cm -1 ]/1e-17 [cm/molecule]=4e15 cm -2...orders of magnitude higher than gas phase detections! CO, incl. 13 COfew-50% CO 2, incl. 13 CO % CH 4 2-4% CH 3 OH1-30% HCOOH1-5% NH % H 2 CO~6% HCOO-~0.3% OCS<0.05, 0.2% SO 2 <=3% NH % OCN - <0.2, 7% Ice abundances, relative to H 2 O ice. Uncertain identifications in red.

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 4 Major Outstanding Questions Thanks to combination of observations, laboratory work, and modeling, evidence is mounting for importance of grain surface chemistry presence of simple species thermal processing of ices Observational evidence for energetic processing of ices is meager at best. How can we observationally test and quantify energetic processing?

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 5 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

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 6 Ice Observations Last ~10 Years Sensitive wide field infrared imaging surveys (2MASS, Spitzer: c2d and GLIMPSE) drastically increased samples of embedded YSOs and (especially) background stars. Follow-up spectroscopy with Spitzer/IRS, AKARI, and large ground based telescopes over (almost) full 2-30 m m range significantly impacted ice studies (before, only handful of BG stars and limited YSO samples could be observed):  Inventory of ice features and carriers  Ice evolution before and during star formation process  Environment dependency: disk versus cloud, one cloud or core vs. another High spectral resolution observations for detailed line profile studies, in particular solid CO and CO 2 (Boogert et al. 2002, Pontoppidan et al. 2003, 2008) High spatial resolution observations using adaptive optics for studies of ices on small scales (Schegerer et al. 2010, Terada et al. 2007)

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 7 YSO and Background Star Selection Background stars selected using broad-band 2-25 μm colors from c2d catalogs. 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)

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 8 YSO and Background Star Selection YSO Background Star Isolated core L 1014; A V =2-35; Huard et al. 2006, ApJ 640, 391, 2006

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 9 Ice Features 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 Continuum determination critical step in study of ice bands. Not easy, time consuming. For background stars, continuum better constrained than YSOs (Boogert et al., ApJ 729, 92, 2011; Chiar et al., ApJ 731, 9, 2011) 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

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 10 Ice Inventory: Not Done Yet! Not all absorption 5-8 m m range caused by bending mode H 2 O ice. Known for massive YSOs since KAO and ISO (Schutte et al. 1996). Early in Spitzer mission became evident same issue for Low Mass YSOs (Boogert et al. 2004) and background stars (Knez et al. 2005) Laboratory work dominates progress in interpretation!

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 11 Ice Inventory: Diluted H 2 O Relatively simple laboratory experiment with high pay-off: dilution of H 2 O in, most relevantly, CO 2, results in destruction of bulk hydrogen bonding network and formation of monomer, dimer, and small multimer bonds: both band profile and integrated band strengths change. Absolute integrated strengths of stretch and bend modes decrease with CO 2 concentration, but stretch mode decreases faster: deeper 6.0 m m H 2 O bending mode, thus reducing 'excess' absorption. Oberg et al., 2007, A&A 462, 1187

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 12 Ice Inventory: CH 3 OH! 3.53 m m C-H stretch mode secure identifier of solid CH 3 OH, using laboratory spectrum. YSOs: CH 3 OH/H 2 O=1-25%. CH 3 OH contributes few to ~20% to 6.85 m m feature. Taurus background stars: CH 3 OH/H 2 O<1% (Chiar et al. 1996) Occasionally, CH 3 OH was suggested to be formed by heating or energetic processes related to YSO. Recent discovery of solid CH 3 OH in quiescent clouds shows CH 3 OH/H 2 O up to 12% (Boogert et al. 2011): forms away from heating or energetic radiation... grain surface chemistry? Why large abundance variation of 1- 12%? Background star Red: H 2 O:CH 3 OH:CO:NH 3 (Hudgins et al. 1993)

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 13 Ice Inventory: CH 3 OH! Abundance reproduced by Monte Carlo simulations of H atoms reacting with frozen CO, i.e. at T<16 K (Cuppen et al. A&A 508, 275, 2009) Taurus MC has CH 3 OH/H 2 O<1%. Some cores have higher atomic H/CO ratio or longer time of H accretion since CO froze out? Monte Carlo simulations depend critically on experimentally determined parameters: e.g., CO+H and H 2 CO+H reaction barriers and rates (Fuchs et al. 2009) as well as binding energies. H 2 CO/CH 3 OH 12 K 13.5 K 15 K 16 K Successful CH 3 OH story benefitted strongly from 'classic' laboratory transmission spectroscopy and 'modern' experimental determination reaction parameters used in models. Both top-down and bottom-up approaches (Linnartz et al. 2011) are needed!

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 14 Ice Inventory: HCOOH? HCOOH identification controversial: Explains strengths 7.25 and 5.8 m m absorption bands OH+CO proceeds at 10 K on grains (Ioppolo et al. 2011, MNRAS 410, 1089). Energetic irradiation works too (Bennett et al. 2011, ApJ 727, 27).  Observed band narrower than laboratory simulations  Solid HCOOH abundance factor 10 4 higher than any gas phase measurement. If 7.25 and 5.8 m m not HCOOH, then what? Boogert et al., ApJ 678, 985 (2008) HCOOH identification disputed: Explains strengths 7.25 and 5.8 um absorption bands OH+CO proceeds at 10 K on grains (Ioppolo et al. 2011, MNRAS 410, 1089). Energetic irradiation works too (Bennett et al.  Observed band narrower than laboratory simulations  solid HCOOH abundance factor 10 4 higher than any gas phase measurement. see Bennett work. Why do we care about HCOOH? Somewhat controversial..looks good, but if not HCOOH THEN WHAT??

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 15 Ice Inventory: What's Left? Much of absorption explained by overlapping bands of simple species, but not all, especially for YSOs. If HCOOH identification is wrong, then the situation is much worse because of its strong mode at 5.8 m m.

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 16 Ice Inventory: What's Left? Known ices do not explain distinct features at 5.8, 6.2, 6.85 m m. Strength relative to H 2 O varies with factors of 2. Smooth absorption between m m (called component 5 in Boogert et al. 2008) varies strongly between different lines of sight and is absent toward background stars.

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 17 Ice Inventory: NH 4 + ? NH 4 + as carrier interstellar 6.85 μm band still debated: easily produced by 'warming' acid/base mixture NH 3 +HNCO, forming a salt (T~10 K; Raunier et al. A&A 416, 165, 2004) Subsequent heating shifts band to longer wavelengths Higher T sub of salt in agreement with increased t 6.85 /H 2 O ratio for warmer lines of sight.  However, band too broad and shallow in mixtures with H2O (Galvez et al., 2010, ApJ 724, 539), but not in isolated salt clusters.  Identification based on only one feature (6.85 m m). heated

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 18 Ice Inventory: More Salts? Is HNCO sufficiently available? Alternative formation route (Schutte & Khanna A&A 398, 1049, 2003): UV irradiation H 2 O:CO 2 :NH 3 :O 2 also produces NH 4 +, but many more ions, e.g., NO 2 - NO 3 - and HCO 3 -. This may explain not only 6.85 m m band, but also underlying broad absorption (“C5 component”). Salts may be important component of interstellar ices! UV-irradiated

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 19 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 after UV irradiation, some 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) Ice Inventory: Organic Residue?

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 20 Ice Inventory: Organic Residue? Organic residue fits diffuse medium 3.47 m m hydrocarbon feature. But: many diffuse-dense medium cycles would be needed. showstopper: interstellar feature is not polarized, while ices are (Adamson et al. 1999, ApJ 512, 224) Gibb and Whittet, 2002, ApJ 566, um On the other hand, organic residue shows feature similar to underlying broad absorption in 5-8 m m region (“C5 component”). Needed: quantitative understanding of UV or CR bombardment processes. Energy barriers, Monte Carlo simulations in the ice matrix.

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 21 Band Profile Analysis Sensitive, high spectral resolution (R=600-25,000) spectroscopy of CO and CO 2 allowed very detailed band profile studies. Laboratory transmission spectra and optical constants, combined with astrophysical models were pivotal in the analysis. Non-polar CO component is pure CO! We have not reached that level of detail yet for 5-8 m m region, but wait for SOFIA and JWST. First detection of solid 13 CO: Boogert, Blake & Tielens, ApJ 577, 271 (2002)

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 22 High S/N, high spectral resolution observations: 2/3 interstellar CO 2 in H 2 O environment 1/3 interstellar CO 2 in CO environment Double peak of pure CO 2 seen in 50% of sample: result of segregation As for CH 3 OH and OCN -, CO freeze out is critical moment in chemistry (Pontoppidan et al. 2008, ApJ 678, 1005; Oberg et al. 2011, ApJ 740, 109). Band Profile Analysis

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 23 Laboratory Experiments Ices Needed/Ongoing Modern wave of 'bottom-up' laboratory experiments (e.g., Linnartz et al., 2011, IAUS 280, 390: get quantitative information on energy barriers) yields robust results on processes of molecule formation and therewith helps with identification of interstellar features. 'Top-down' approach still necessary as identification cannot be correct if feature position and shape mismatch. Ongoing and new experiments:  reaction pathways, energy barriers  photo-desorption  formation of species in extreme environments, UV versus energetic particles  acid-base chemistry  simple absorption transmission spectra (e.g., PAHs in ices), optical constants  X-ray irradiation of ices Need agreement between experiments from different laboratories!

12 June 2012 AAS 220/Lab Astrophysics (Anchorage): Interstellar Ices (Boogert) 24 Conclusions New infrared imaging and spectroscopic surveys have drastically increased sample sizes of YSOs and background stars in different environments. Laboratory experiments pivotal in interpreting the observational results, both grain surface simulations and laboratory transmission spectroscopy. Complex mixture of simple species already formed in the quiescent cloud phase (e.g., CH 3 OH). Identification several species debated, e.g., HCOOH, NH m m region might show evidence of salts or organics formed by energetic processing Future sensitive, high spectral resolution observations need laboratory support!