Complex Organic Chemistry in Interstellar Ices

Susanna L. Widicus WeaverDepartment of Chemistry

Emory University

Detected Interstellar Molecules

2 atoms 3 atoms 4 atoms 5 atoms 6 atoms 7 atoms 8 atoms 9 atoms 10 atoms 11 atoms 12 atoms 13 atomsH2 C3 c-C3H C5 C5H C6H CH3C3N CH3C4H CH3C5N HC9N C6H6 HC11N

AlF C2H l-C3H C4H l-H2C4 CH2CHCN HC(O)OCH3 CH3CH2CN (CH3)2CO CH3C6H C2H5OCH3

AlCl C2O C3N C4Si C2H4 CH3C2H CH3COOH (CH3)2O (CH2OH)2

C2 C2S C3O l-C3H2 CH3CN HC5N C7H CH3CH2OH CH3CH2CHO

CH CH2 C3S c-C3H2 CH3NC CH3CHO H2C6 HC7N

CH+ HCN C2H2 H2CCN CH3OH CH3NH2 CH2OHCHO C8H

CN HCO NH3 CH4 CH3SH c-C2H4O l-HC6H CH3C(O)NH2

CO HCO+ HCCN HC3N HC3NH+ H2CCHOH CH2CHCHO C8H–

CO+ HCS+ HCNH+ HC2NC HC2CHO C6H– CH2CCHCN C3H6

CP HOC+ HNCO HCOOH NH2CHO

SiC H2O HNCS H2CNH C5N

HCl H2S HOCO+ H2C2O l-HC4H

KCl HNC H2CO H2NCN l-HC4N

NH HNO H2CN HNC3 c-H2C3O

NO MgCN H2CS SiH4 H2CCNH

NS MgNC H3O+ H2COH+

NaCl N2H+ c-SiC3 C4H–

OH N2O CH3

PN NaCN

SO OCS

SO+ SO2

SiN c-SiC2

SiO CO2

SiS NH2

CS H3+

HF H2D+, HD2+

SH SiCN

HD AlNC

FeO SiNC

O2 HCP

CF+

SiH

PO

Schematic of a Hot Core

UV

Hot Core

complex organics

T (gas) = 200 - 1000 K

~1016 cm

T (dust) ~90 K ~60 K ~45 K ~20 K

SiO

H2O, CH3OH, NH3

H2S

CH3CN

~5x1017 cm

H2O ice

CO2CON2O2ice

CO2ice

trappedCO

CH3OHice

CSO Orion Spectrum

5365 lines observed

79% of the lines are unassigned!

Blake et al. ApJ, 1986: RMS = 150 mK, integration time ~ 27 nightsOur survey: RMS = 20 mK, time ~ 4 nights

See poster by Radhuber et al. for more information!

THz Observational Astronomy

480 GHz -1.2 THz

1.4 – 1.9 THz

Herschel Observatory

Launched on May 14, 2009!

Stratospheric Observatory for Infrared AstronomyFlight tests began in 2007

Initial science observations begin in 2010500 GHz – 2.1 THz

Atacama Large Millimeter Array First antennas arrived in 2007

Scheduled for completion in 2012

80 GHz – 950 GHz64 antennas

The Murchison Meteorite

• amino acids

• sugars and polyols

• other organics

Organic Material in Meteorites

http://www.hermann-beer-ka.de/nucleosynthesis/abund/Murchison.jpg

Key Questions:

How far can chemistry go in the ISM??

Is a parent body required??

Possible Molecular Formation Schemes

vs.

Grain Surface Reactions

Charnley, S. (2001) Interstellar Organic Chemistry. In: The Proceedings of the Workshop The Bridge Between the Big Bang and Biology, (Consiglio Nazionale delle Ricerche, Italy).

Gas Phase Reactions

C

O

N

HH

H

HH

C

C

N

HH

HH

OO

H

C

O

N

HH

H

HH

H

aminomethanol glycine protonatedaminomethanol

HCOOH

or H3+

CH3OH2+ +

-H3O+

• Cannot form by ion-molecule reactions Horn et al. 2004, ApJ 611, 605

• Grain surface formation?Structural isomers would have similar abundances!

• Complex molecules observed in regions of grain mantle disruption:

Shocked regions in the GC (Martin-Pintado et al.)

Hot Corinos (Ceccarelli, Caselli, et al.)

The Methyl Formate Problem

Acetic AcidMethyl Formate Glycolaldehyde

52 2 1

C

O

OCH3

H

C

O

OHCH3C

OCOH

H

H H

Bottinelli et al. ApJL 617, 2004

Grain Surface Formation

hν

H2O, CO, CH3OH, NH3 , H2COIce mantle

H2O + hν OH + HH2 + O

CH3OH + hν CH3 + OHCH3O + HCH2OH + H

NH3 + hν NH2 + H

H2CO + hν HCO + H

HCO + CH3O CH3OCHO (methyl formate)

HCO + CH2OH HOCH2CHO (glycolaldehyde)

HCO

CH3O

HCOOCH3

Garrod, Widicus Weaver, & Herbst, ApJ 682, 2008

Two-Stage Hot Core Model

1. Cloud Collapse (isothermal free-fall)

2. Warm-up (second-order power law)

nH = 3x103 cm-3 nH = 1x107 cm-3

106 years

5x104 years

(high mass)

10 K 200 K

time

New Model

Previous Models

Tem

pera

ture

Garrod, Widicus Weaver, & Herbst, ApJ 682, 2008

-12-11-10

-9-8-7-6-5-4-3

glyco

lalde

hyde

aceti

c acid

formic

acid

formald

ehyd

e

ethyle

ne gl

ycol

ethan

ol

methyl

formate

dimeth

yl eth

er

formam

ide

aceta

ldehy

de

methan

ol

methyla

mine

log(

n/n H

)

Sgr B2(N-LMH) (Observed)

Model

Initial Results: Ice Composition

Garrod, Widicus Weaver, & Herbst, ApJ 682, 2008

Lingering Questions

1. Methanol photolysis branching ratios?Quantitative lab measurements.

2. ID of key intermediates to trace chemistry?Laboratory studies to support observational search.

3. Varying physical and chemical parameters for interstellar clouds?Spectral line surveys of many sources.

4. Spectral interference from “interstellar weeds?”Complete laboratory spectral cataloging.

Motivation: Understanding COMs in the ISMGrain surface formation

• Simple molecules form in ice via single-atom addition reactions

• Organic radicals form in ice via photolysis of simple molecules

• Radicals react during warm-up to form larger organics

Gas phase formation• Molecules are released from ices

• Gas-phase molecules are ionized

• Ion-molecule reactions drivegas-phase organic chemistry

hν

H2O, CO, CH3OH, NH3 , H2COIce mantle

C

O

N

HH

H

HH

C

C

N

HH

HH

OO

H

C

O

N

HH

H

HH

H

aminomethanol

glycine

protonatedaminomethanol

HCOOH

or H3+

CH3OH2+ +

-H2O

Transient molecules are the driving forces for both grain-surface and gas-phase chemistry.

Detected Interstellar Molecules

2 atoms 3 atoms 4 atoms 5 atoms 6 atoms 7 atoms 8 atoms 9 atoms 10 atoms 11 atoms 12 atoms 13 atomsH2 C3 c-C3H C5 C5H C6H CH3C3N CH3C4H CH3C5N HC9N C6H6 HC11N

AlF C2H l-C3H C4H l-H2C4 CH2CHCN HC(O)OCH3 CH3CH2CN (CH3)2CO CH3C6H C2H5OCH3

AlCl C2O C3N C4Si C2H4 CH3C2H CH3COOH (CH3)2O (CH2OH)2

C2 C2S C3O l-C3H2 CH3CN HC5N C7H CH3CH2OH CH3CH2CHO

CH CH2 C3S c-C3H2 CH3NC CH3CHO H2C6 HC7N

CH+ HCN C2H2 H2CCN CH3OH CH3NH2 CH2OHCHO C8H

CN HCO NH3 CH4 CH3SH c-C2H4O l-HC6H CH3C(O)NH2

CO HCO+ HCCN HC3N HC3NH+ H2CCHOH CH2CHCHO C8H–

CO+ HCS+ HCNH+ HC2NC HC2CHO C6H– CH2CCHCN C3H6

CP HOC+ HNCO HCOOH NH2CHO

SiC H2O HNCS H2CNH C5N

HCl H2S HOCO+ H2C2O l-HC4H

KCl HNC H2CO H2NCN l-HC4N

NH HNO H2CN HNC3 c-H2C3O

NO MgCN H2CS SiH4 H2CCNH

NS MgNC H3O+ H2COH+

NaCl N2H+ c-SiC3 C4H–

OH N2O CH3

PN NaCN

SO OCS

SO+ SO2

SiN c-SiC2

SiO CO2

SiS NH2

CS H3+

HF H2D+, HD2+

SH SiCN

HD AlNC

FeO SiNC

O2 HCP

CF+

SiH

PO

Radicals and Ions

Production Methods

Discharges

Photolysis

• Small quantities (low efficiency)

• High temperatures = weak signals

• Interference from stable molecules

• Reactivity/instability of products

Matrix Isolation

hν

Supersonic Expansions

High-Sensitivity Cavity-Enhanced Spectroscopy

> 1000 cm-1

~2 – 50 GHz

THzFTMW CRDS

1 cm 10 µm 1 µm 100 nm1 mm 100 µm

30 GHz1 cm-1

30 THz1000 cm-1

300 THz10,000 cm-1

3000 THz100,000 cm-1

300 GHz10 cm-1

3 THz100 cm-1

10 cm

3 GHz0.1 cm-1

The ‘THz Gap’

1 – 50 GHzFrequencySynthesizer

VDI Multiplier chain50 GHz – 1.2 THz Detector

Gas Flow Cell

To Computer

Sample Input To Vacuum Pump

Laboratory Spectral Cataloging

Methanol

First light April 1, 2009!

Ethyl Cyanide

CRDS High Finesse Cavity

Mode MatchingOptics

SupersonicSource

DetectorRadiationSource

IR mirrors → dielectric coatedLosses due to transmission

THz mirrors → metal coatedLosses due to skin depth

R ~99.99%

cavity ringdownrecorded

FTMW High Finesse Cavity

Detector

Switch

SupersonicSource

RadiationSource

Aperture: r << λ

X

microwave mirrors → apertureLarge λ, small losses

THz mirrors → ?Small λ, large losses with any aperture!

R ≈ 98%

free-induction decay recorded

Proposed THz-CRDS Spectrometer

Wire GridPolarizers

1 – 50 GHzFrequencySynthesizer

VDI multiplier chain50 GHz – 1.2 THz

Mode MatchingOptics

Off-Axis Parabolic Mirror

HEB Detector

To Computer

SupersonicSource

Transmission = 10-4

≤700 GHzR = 99.99%

Progress Toward THz-CRDSBeam profiling completed, mode-matching optimized

Polarizer reflectivity tested up to 300 GHz

R = 99.9 – 99.99%

Benchtop THz-CRDS Setup

Cavity Modes at 180 GHz!

Cavity length = 33 cmMode FWHM = 1.7 MHzPolarizer R =0.988Cavity FSR = 445.5 MHz

Need longer cavity, higher R for CRDS!

Next Steps in THz-CRDS Development

• Narrow cavity modes, increase pathlength

• Incorporate translation stage; trigger ringdown events

• Extend system to higher frequencies

• Incorporate cavity into vacuum chamber

• Test fully-integrated system on known molecules

• Begin molecular spectroscopy on ions, reactive organic intermediates

• Extend to broadband spectral acquisition

Using THz Spectroscopy to Trace Prebiotic Chemistry in Space

Grain Surface Formation

hν

H2O, CO, CH3OH, NH3 , H2COIce mantle

H2O + hν OH + HH2 + O

CH3OH + hν CH3 + OHCH3O + HCH2OH + H

NH3 + hν NH2 + H

H2CO + hν HCO + H

HCO + CH3O CH3OCHO (methyl formate)

HCO + CH2OH HOCH2CHO (glycolaldehyde)

Garrod, Widicus Weaver, & Herbst, ApJ 682, 2008

••

••

•• •

Ener

gy (k

cal/m

ol) •

Methanol Dissociation

Chang and Lin 2004, Chem. Phys. Lett., 384, 229

THz Spectroscopy as a Probe

Mass Spec alone cannot distinguish products

THz spectroscopy can!

See poster by Laas et al. for more information!

Astrochemical Modeling• A series of methanol photolysis branching ratios were used, and peak

abundances were compared to Sgr B2(N) abundances

• Ice photolysis branching ratios of CH3:CH2OH:CH3O = 18:1:1 give the best match to Sgr observations

• Longer warm-up timescales give better agreement with observed abundances of more complex organic species.

CO

O

H H

HH

O C

HH

HCH H

O

H

C

O

N

HH

H

HH

OC

HH

H

OH

NHH

CH H

OH

CH H

OH

CH H

OH

.

methoxymethanol

methanediol

aminomethanol

.

+

+

+.

.

.

.

Prebiotic

Molecule Formation

O(1D) Insertion

Reactions

C

H

N

HH

HH

CO

O

H H

HH

O C

HH

HCH H

O

H

COH

HH

H

O C

HH

HCH H

H

C

O

N

HH

H

HH

O(1D)

O(1D)

O(1D)

dimethyl ether methoxymethanol

methanol methanediol

methylamine aminomethanol

hν

Photolysis Fast-Mixing Nozzle

quartz capillary

CH3OH

N2O + Ar

O(1D)

O(1D) + CH3OH → HOCH2OH

HOCH2OH

+ Ar

Interaction region

Kinetics StudiesSee poster by Anderson et al. for more information!

Initial Photlysis Results: N2O

N2O photolysis monitored by rotational line signal depletion

Photolysis Fast-Mixing

Sourcehν

mm/submm

12% reduction in N2O signal observed

Using THz Spectroscopy to Trace Prebiotic Chemistry in Space

What do we plan to measure?• Photolysis branching ratios for complex organics• Spectra of small, reactive organics produced via O(1D) insertion• Spectra of molecular ions with complex internal motion• THz spectral catalogs of “interstellar weeds”

AcknowledgementsThe Widicus Weaver Group:Mary Radhuber, Jake Laas, Brandon Carroll, Brett McGuire, Thomas AndersonJay Kroll, Patrick Lanter

Eric Herbst, OSURobin Garrod, CornellGeoffrey Blake, CaltechThom Orlando, GA TechCSO/Caltech: Matthew Sumner, Frank Rice, Jonas Zmuidzinas, & Tom Phillips

Virginia Diodes, Inc. QMC Instruments, Ltd.