Exploring Molecular Complexity with ALMA (EMoCA): High-Angular-Resolution Observations

of Sagittarius B2(N) at 3 mm

Holger S. P. MüllerA. Belloche (PI), K. M. Menten; MPIfR

R. T. Garrod; UVA

70th ISMS, Urbana-Champaign, IL, 22 – 26 June 2015, RI05

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 (NH2CH2COOH) in samples from comet 81P/Wild2

Newly detected molecules may evolve to tracers of specific chemical or physical conditions in space

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

The Star-forming Region Sagittarius B2

most massive star-forming region in our Galaxy (~ 107 M)

~100 pc from Galactic Center

very high column densities (> 1025 cm–2) key for detection of COMs

2 massive clumps, (M) and (N), hosting clusters of UC HII regions

Sgr B2(N)

many COMs first detected there

2 hot cores: N1 (or LMH) & N2 different vlsr (10 km/s); 5" apart (0.2 pc)

Sgr B2(N) at 850 µm (SMA; Qin et al., 2011)

Central Molecular Zone at 870 µmATLASGAL/LABOCA & Planck; © MPIfR/A. Weiß

Our previous IRAM 30 m survey of Sgr B2(N)

80 90 100 110Frequency / GHz

3700 lines > 4 (100 lines/GHz)70 % identified56 molecules66 minor isotopologs59 vibrationally excited states

Belloche et al., A&A 559 (2013) A47

Highlights: 3 molecules newly detected

aminoacetonitrile (NH2CH2CN)

ethyl formate (C2H5OCHO)

n-propyl cyanide (n-C3H7CN)

Spectrum close to confusion limit higher angular resolution needed

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

Sensitivity and Resolution with ALMA

N2 (secondary hot core): narrower line width confusion limit lowered

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 > 108 cm–3 LTE is a very good approximation

initial focus on N2 (secondary) hot core

On propyl cyanides

gauche lower than anti by 0.48 ± 0.04 kJ/mol (PCCP 3 (2001) 766)

anti 2 conformers gauchen-C3H7CN

2 isomers: normal (n) and iso (i)

i-C3H7CN

rot. spectrum, HFS, dipole: H. S. P. Müller, A. Coutens, A. Walters, J.-U.Grabow, S. Schlemmer, J. Mol. Spectrosc. 267 (2011) 100

ALMA detection of iso-propyl cyanide

~50 lines of i-C3H7CN detected and ~120 of n-C3H7CN

rotation temperature ~150 K, emission size ~1"

A. Belloche, R. T. Garrod, H. S. P. Müller, K. M. Menten, Science 345 (2014) 1584

Abundance of iso-propyl cyanide

i-C3H7CN : n-C3H7CN ~ 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-C3H7CN: CH3CH2 + CH2CN (no equivalent reaction to produce i-C3H7CN)

• dominant route to i-C3H7CN: CH3CHCH3 + CN (CH3CH=CH2 + H strongly favors CH3CHCH3 over CH3CH2CH2)

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-C3H7CN bodes well for the presence of more complex molecules in the ISM, such as amino acids

"Laboratory Spectroscopy" with ALMA:Ethanol Intensities

Vibration-Rotation-Interaction and Signs of Dipole Components

(relative) signs may matter and may be determined by

– Stark effect measurement, e.g. NH2D, Cohen & Pickett, JMSp 93 (1982) 83; HOONO2 & CH2FOH, Suenram et al., JMSp 116/119 (1986) 406/446– intensity measurements, e.g. (CH2OH)2, Christen & Müller, PCCP 5 (2003) 3600; H2DO+, Müller et al., PCCP 12 (2010) 8362

Component Value Sign

Initial Present µa(a a) 0. 046 (14) + (+) µb(a a) 1. 438 (7) + + µc(a a) 0.

µa(g± g±) 1. 246 (10) − + µb(g± g±) 0. 104 (8) + (+) µc(g± g–±) 1. 101 (16) − −

anti gauche

Initial signs: Pearson et al., JMSp 251 (2008) 394

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 (C4H9CN) ? n-BuCN, t-BuCN studied; two more on the agenda in Köln. what about C3H7OH ?