Origins Astrobiology Interstellar medium planets life http www
Origins & Astrobiology: Interstellar medium --> planets --> life http: //www. spaceflight. esa. int/users/images/commonpic/ISM. jpe
Interstellar dust (carbonaceous, silicate, etc. ) Infrared emission from diffuse ISM Composition • Submicron dust (in thermal equilibrium) (BGs; ~100 nm) • Polycyclic Aromatic Hydrocarbons (PAHs; ~10Å) • Very Small Grains (graphites, small silicate ) (VSGs; 1~10 nm, stochastic heating) Carbonaceous Dust PAH molecule H H H C C C C C C PAH platelet (≡) PAH cluster (≡) H H Amorphous carbon
Unidentified Infrared Bands Ubiquitous presence of UIR emission • HII region, Reflection Nebulae, Planetary Nebulae, etc. (Tokunaga et al. 1997) • Diffuse ISM (Onaka et al. 1996; Mattila et al. 1996) • External Galaxies (e. g. Helou et al. 2000) even at z~2 (Yan et al. 2004) CH out-of-plane UV photon CC stretching bending duo trio CH in-plane quartet C C H H bending C C H H C C C C H CH stretching C C C H C C H solo energy IR photons UV photon
Polycyclic Aromatic Hydrocarbons (PAHs) Strongly bound pi-bonded cyclic hydrocarbons (‘aromatic’) Prominent nonthermal emission features Form in carbon stars Reaction mechanism is very well studied experimentally Extremely stable: • oxidizing/reducing conditions • high temperatures • UV radiation In ISM: ~10% of C is in PAHs
PAHs in astrophysical environments Ames Astrochemistry Lab
“protoplanetary disks”
Geers et al, A&A 2008 Observations of disks around young stars: PAHs are modified in disk environments PAHs are at lower abundance in disks than in diffuse ISM
Carbonaceous chondrites contain abundant aromatic carbon (G. Cody, Carnegie) http: //www. gl. ciw. edu/~cody/meteorite_files/IMAGE 006. JPG
SED characteristics of dust emission in the CO Molecular Clouds in the LMC CO Molecular Clouds; NANTEN 12 CO(J=1 ->0) LMC survey data at l=2. 7 mm (Fukui 2002; Yamaguchi et al. 2001; Mizuno et al. 2001) HPBW 2. 6’ at 115 GHz Infrared SED; IRAS Sky Survey Atlas 12 mm, 25 mm, 60 mm, 100 mm beam size ; 4’ x 4’ -> only the data more than 16’ apart from the young (<30 Myr) star clusters in the LMC are used large scatter in 12/25 color for I 12 CO < 5 K km s-1
SED characteristics of dust emission in the CO Molecular Clouds in the LMC ISRF distribution; f(U)∝U-b for Umin < Umax Umin ; CO 分子雲に特徴的な輻射場環境 Umin~0. 1; TBGs~13 K (Miville-Deschenes et al. 2002) Umax ; CO 分子雲の外部の輻射場環境によって決まる IRAS 12 mm / 25 mm vs. b for various (Umin , Umax ) b 大; CO分子雲領域が 領域内で支配的 ( I 12 CO 大に相当) b 小; CO分子雲領域 外側の輻射場 環境が支配的 ( I 12 CO 小に相当) (Umin , Umax ) = (0. 1, 10) ; Arc I, Molecular Ridge South etc. (Umin , Umax ) = (0. 1, 1000) ; Arc III, Molecular Ridge North etc. (Sakon et al. 2006)
Focal plane of AKARI/IRC channels (Onaka et al. 2007)
System spectral response curves of AKARI/IRC imaging filters
System spectral response curves of AKARI/IRC spectroscopic filters (Ohyama et al. 2007) NP/NG: UIR 3. 3 mm SG 1 : UIR 6. 2 mm, 7. 7 mm SG 2 : UIR 8. 6 mm, 11. 2 mm
AKARI/IRC spectroscopy of Diffuse Molecular Cloud with Young ISM in the LMC star cluster near N 159 Molecular Cloud without young star clusters IRAS 12 mm/25 mm
AKARI/IRC spectroscopy of Diffuse ① ISM in the LMC [Ne. II] ② ① Molecular Cloud with Young star cluster near N 159 [Ar. III] UIR 6. 2 UIR 7. 7 UIR 8. 6 -69 [SIV] UIR 11. 2 UIR 12. 7 [Ne. II] DEC (B 1950) (deg) ② [Ar. III] -70 ③ [SIV] Molecular Cloud without young star clusters ③ -71 -72 90 89 88 87 R. A. (B 1950) (deg) 86 Molecular Cloud with Young star cluster (①, ②) : small 12 mm/25 mm, large UIR 7. 7/11. 2 Molecular Cloud without Young star cluster (③) : large 12 mm/25 mm, small UIR 7. 7/11. 2
AKARI/IRC Slit-spectroscopy of diffuse ISM in NGC 6946
UIR bands in NGC 6946 IRC MIR-S/SG 1 & SG 2 spectra: Arm region, Inter-arm region ※ Zodiacal emission is not subtracted as for (a) and (b) [Ar. III] [SIV] Band strengths of UIR 6. 2 mm and 7. 7 mm features relative to that of UIR 11. 2 mm feature become stronger in the arm region than in the inter-arm region : UIR 6. 2, 7. 7 mm UIR 11. 2 mm inter-arm < UIR 6. 2, 7. 7 mm UIR 11. 2 mm star forming region
Properties of PAHs in NGC 6946 UIR band ratios of 6. 2 mm/11. 2 mm, 7. 7 mm/11. 2 mm, 8. 6 mm/11. 2 mm UIR band ratio Inter-Arm*1 Arm*2 S. F. Region*2 [6. 2 / 11. 2] ext. corr. 0. 84(± 0. 14) 1. 33(± 0. 13) 2. 36(± 0. 64) [7. 7 / 11. 2] ext. corr. 2. 27(± 0. 32) 3. 32(± 0. 35) 5. 51(± 1. 44) [8. 6 / 11. 2] ext. corr. 0. 50(± 0. 14) 0. 56(± 0. 09) 0. 66(± 0. 28) *1 no extinction correction for the extreme case *2 Taking account of the extinction of Av=3 mag (Ferguson, Gallagher & Wyse 1998). - As a valid measure of the on-going star formation UIR band / Plateau Emission in 7 -9 mm cf. second plateau component in Peeters et al. 2002 ⇒ gas-phase PAHs (20 - 100 NC) / PAH clusters (~500 NC) Feature UIR band / plateau emission in 7 -9 mm Inter-Arm S. F. Region UIR* / plateau 0. 60(± 0. 018) 0. 65(± 0. 009) 0. 70(± 0. 016) * Total intensity of UIR 6. 2 mm + 7. 7 mm + 8. 6 mm features More abundant small sized (free-flying) PAHs in Star forming Region possibly as a result of photo-evaporation of PAH clusters into free-flying PAHs
PAHs and PAH clusters (Rapacioli, Joblin and Boissel 2005, A&A, 429, 193) A B C UIR 7. 7/11. 2 A : ionized PAHs large B : neutral PAHs small C : PAH clusters -- Peak position of “ 7. 7 mm” complex 7. 57 mm 7. 63 mm 7. 85 mm -> 7. 6 mm / 7. 8 mm ratios: PAHs/PAH clusters
PAHs and PAH clusters Results of the fit Arm region Inter-arm region 7. 6 mm component lcenter(fixed) h 7. 6(free) D 7. 6(free) 7. 8 mm component lcenter(fixed) h 7. 8(free) D 7. 8(free) (mm) (W/m 2/um/pixel) ratio I(7. 6) / I(7. 8) (mm) (W/m 2/um/pixel) (mm) 7. 60 1. 34 E-16 0. 16 7. 83 1. 40 E-16 0. 29 0. 53 7. 60 5. 84 E-17 0. 16 7. 83 6. 95 E-17 0. 36 0. 37 Larger ratio of I(7. 6)/I(7. 8) in star forming region -> More abundant small sized (free-flying) PAHs in Star forming Region as a result of photo-evaporation of PAH clusters into free-flying PAHs
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