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Exploring Molecular Complexity with ALMA (EMoCA): High-Angular-Resolution Observations of Sagittarius B2(N) at 3 mm Holger S. P. Müller A. Belloche (PI), K. M. Menten; MPIfR R. T. Garrod; UVA 70th ISMS, Urbana-Champaign, IL, 22 – 26 June 2015, RI05
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Motivation Which level of molecular complexity in space can be detected by radio astronomical means ? Did interstellar chemistry contribute to the formation of life on Earth or other planets ? – e.g. > 80 amino acids found in meteorites on Earth; isotopic and racemic composition suggests extraterrestrial origin – Glycine (NH 2 CH 2 COOH) in samples from comet 81P/Wild2 Newly detected molecules may evolve to tracers of specific chemical or physical conditions in space
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How do complex organic molecules (COMs) form ? Gas phase ion molecule reactions ineffective Grain surface chemistry at low T not sufficient (hydrogenation of atoms and small molecules) Radical radical reactions upon warm-up required Observational results needed to test model predictions
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The Star-forming Region Sagittarius B2 most massive star-forming region in our Galaxy (~ 10 7 M ) ~100 pc from Galactic Center very high column densities (> 10 25 cm –2 ) key for detection of COMs 2 massive clumps, (M) and (N), hosting clusters of UC H II regions Sgr B2(N) many COMs first detected there 2 hot cores: N1 (or LMH) & N2 different v lsr (10 km/s); 5" apart (0.2 pc) Sgr B2(N) at 850 µm (SMA; Qin et al., 2011) Central Molecular Zone at 870 µm ATLASGAL/LABOCA & Planck; © MPIfR/A. Weiß
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Our previous IRAM 30 m survey of Sgr B2(N) 3700 lines > 4 (100 lines/GHz) 70 % identified 56 molecules 66 minor isotopologs 59 vibrationally excited states Belloche et al., A&A 559 (2013) A47 Highlights: 3 molecules newly detected aminoacetonitrile (NH 2 CH 2 CN) ethyl formate (C 2 H 5 OCHO) n-propyl cyanide (n-C 3 H 7 CN) Spectrum close to confusion limit higher angular resolution needed
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Exploring Molecular Complexity with ALMA 3 mm spectral line survey of Sgr B2(N) in Cycles 0 and 1 (84.0 – 114.4 GHz) angular resolution: 1.8" and 1.4" sensitivity to compact emission: factor ~20 compared to our IRAM 30 m survey status: observations completed, data reduced
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Sensitivity and Resolution with ALMA N2 (secondary hot core): narrower line width confusion limit lowered
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Analysis of EMoCA survey modeling with CLASS extension WEEDS (Maret et al., 2011) catalogs: CDMS, JPL, + private LTE model for each identified molecule plus isotopologs and vibrational states emission of COMs is compact (~1", 0.04 pc, 8300 AU); densities > 10 8 cm –3 LTE is a very good approximation initial focus on N2 (secondary) hot core
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On propyl cyanides gauche lower than anti by 0.48 ± 0.04 kJ/mol (PCCP 3 (2001) 766) anti 2 conformers gauche n-C 3 H 7 CN 2 isomers: normal (n) and iso (i) i-C 3 H 7 CN rot. spectrum, HFS, dipole: H. S. P. Müller, A. Coutens, A. Walters, J.-U.Grabow, S. Schlemmer, J. Mol. Spectrosc. 267 (2011) 100
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ALMA detection of iso-propyl cyanide ~50 lines of i-C 3 H 7 CN detected and ~120 of n-C 3 H 7 CN rotation temperature ~150 K, emission size ~1" A. Belloche, R. T. Garrod, H. S. P. Müller, K. M. Menten, Science 345 (2014) 1584
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Abundance of iso-propyl cyanide i-C 3 H 7 CN : n-C 3 H 7 CN ~ 0.40 ± 0.06 : 1 ratio reproduced by Garrod's hot-core chemical model; formation on grain surfaces 0.375 : 1 with new model dominant route to n-C 3 H 7 CN: CH 3 CH 2 + CH 2 CN (no equivalent reaction to produce i-C 3 H 7 CN) dominant route to i-C 3 H 7 CN: CH 3 CHCH 3 + CN (CH 3 CH=CH 2 + H strongly favors CH 3 CHCH 3 over CH 3 CH 2 CH 2 )
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Branched molecules in the ISM are now known to exist in the ISM ! were proposed to exist there since the 1980s stability of tertiary radical sites over secondary ones over primary ones may favor branched molecules in even larger cases amino acids in meteorites: branched ones dominate our detection of i-C 3 H 7 CN bodes well for the presence of more complex molecules in the ISM, such as amino acids
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"Laboratory Spectroscopy" with ALMA: Ethanol Intensities
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Vibration-Rotation-Interaction and Signs of Dipole Components (relative) signs may matter and may be determined by – Stark effect measurement, e.g. NH 2 D, Cohen & Pickett, JMSp 93 (1982) 83; HOONO 2 & CH 2 FOH, Suenram et al., JMSp 116/119 (1986) 406/446 – intensity measurements, e.g. (CH 2 OH) 2, Christen & Müller, PCCP 5 (2003) 3600; H 2 DO +, Müller et al., PCCP 12 (2010) 8362 anti gauche Initial signs: Pearson et al., JMSp 251 (2008) 394
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Conclusion and Outlook ALMA: high sensitivity, broad bandwidth, high spatial resolution prime instrument to investigate molecular complexity can even contribute to laboratory spectroscopy EMoCA: will provide data to test and calibrate chemical models and the means to investigate the evolutionary states of the two hot cores branched molecules: what is the distribution of the BuCN isomers (C 4 H 9 CN) ? n-BuCN, t-BuCN studied; two more on the agenda in Köln. what about C 3 H 7 OH ?
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