3C Sugars in Interstellar Hot Cores? The Laboratory Rotational Spectroscopy of

and Observational Search for Dihydroxyacetone

Susanna L. Widicus

August 22, 2003

What is Dihydroxyacetone?

• It is the simplest 3C sugar.

• It is a white crystalline powder in dimer form at room temperature.

• Its major use is as the active ingredient in sunless tanning products.

What do we know spectroscopically?

• Ab initio calculations predict:

doubly H-bonded conformer = ground stateb = 1.8 Dsingly H-bonded conformer ~ 750 cm-1

lowest torsional modes ~ 190 cm-1 , 280 cm-1 , 285 cm-1

• Previously unpublished microwave work now in press:

Lovas, Suenram, Plusquellic, and Møllendal (J. Mol. Spec. 2003)

ground state assignments from 10 - 20 GHzb = 1.767 D

• No vibrational work has been done.

Why Dihydroxyacetone? • Glycolaldehyde detected in Sgr B2(N-LMH)

Hollis, Lovas, and Jewell (ApJ 540, 2000)

• Acetone detection confirmed in Sgr B2(N-LMH) Snyder et al. (ApJ 578, 2002)

• Sugars (DHA) detected in Murchison meteoriteCooper et al. (Nature 414, 2001)

UV

Hot Core

complex organics

T (gas) = 200 - 1000 K

~1016 cm

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

SiO

H2O, CH3OH, NH3

H2S

CH3CN

~5x1017 cm

H2O ice

CO2

CON2

O2

iceCO2

icetrappedCO

CH3OHice

Schematic of a Hot Core

• Prebiotic materials form in hot cores and are assimilated into meteorites and comets.

• Meteorite or comet parent body forms from cloud and prebiotic materials form in situ.

Key Questions:

How far can prebiotic chemistry go in the ISM??

Is a parent body required for prebiotic chemistry to occur??

Possible Prebiotic Species Formation Schemes

Grain Surface ReactionsCharnley, S. (1999) Interstellar Organic Chemistry. In: The Proceedings of the Workshop The Bridge Between the Big Bang and Biology, (Consiglio Nazionale delle Ricerche, Italy).

No sugars!

Again, no sugars!

Gas Phase Reactions

Alanine

Laboratory Work: 1. Original Balle-Flygare FTMW Spectrometer

Valve Driver

Local Oscillator

Timing Control

Freq.Stabilizer

Freq. Standard

Master Oscillator

Amp

Mixer

Isolator

Mixer

Mixer

Mixer Amp

Switch

PINDiode

PINDiode

Freq.Stabilizer

+ 30 MHz

m

30 MHz

Pump

Molecular Nozzle

30 MHz

To Computer

The Heated Nozzle

heater

Sample Holder

Top View

Cross-Sectional View

Ar + DHA

Ar

wire mesh

DHA

DHA

DHA 2 1 2 1 0 1

15006.7695 MHz

Flygare Spectra of DHA Transition Frequency (MHz)

1 1 1 0 0 0 11536.4474

2 1 2 1 0 1 15006.7695

5 0 5 4 1 4 12302.5023

5 1 4 5 0 5 10540.6400

6 0 6 5 1 5 16596.6445

6 1 5 6 0 6 11731.7461

DHA 1 1 1 0 0 0

11536.4474 MHz

Laboratory Work: 2. Caltech and JPL Millimeter and Submillimeter

Flow Cell Spectrometers

Frequency Synthesizer

Lock In Amp.

SourceFlow Cell

Polarizer

Detector

To Computer

Multiplier

Rooftop Reflector

• Heating required for mm scans (~ 50 °C).

• Cell contamination a problem due to relatively weak DHA linestrengths.

• Harmonic contamination for submm scans.

3 mm Flow Cell Spectrum of DHA

10185

7916

5626

3376

1107

-1163

-3433

-5702

-7972

112000 112800 113600 114400 115200 116000 116800 117600 118400 119300 120000

Frequency (MHz)

Transition Frequency (MHz)

31 2 30 30 1 29 112558.8289

15 4 11 14 3 12 112580.2057

41 5 36 40 6 35 112590.8853

54 7 48 54 6 49 112600.2144

32 0 32 31 1 31 112612.5095

32 1 32 31 0 31 112636.6087

Parameter 0

1

2

3

Lines Assigned 1256 457 292 239

Energy (cm-1) 0 93 147 150

J max 104 86 70 72

Ka max 23 13 10 13

A 9801.29720( 37) 9764.47769(145) 9701.6815( 44) 9662.11405(274)

B 2051.525463( 84) 2049.846447(286) 2051.54944( 42) 2050.02125( 44)

C 1735.164761( 87) 1736.322042(255) 1737.92890( 35) 1739.41896( 36)

J 0.1823549(102)E-03 0.183262( 35)E-03 0.184902( 59)E-03 0.187034( 61)E-03

JK 0.657431( 99)E-03 0.84808( 44)E-03 0.50435( 98)E-03 0.60889( 81)E-03

K 5.36997( 58)E-03 5.4587( 82)E-03 3.507( 39)E-03 7.1326(190)E-03

J 0.02767141(203)E-03 0.0274030(148)E-03 0.0274835(281)E-03 0.0265713(293)E-03

K 0.569369(157)E-03 0.64404(107)E-03 0.35858(199)E-03 0.31770(214)E-03

Rotational and Centrifugal Distortion Constants for Dihydroxyacetone

Energies determined by relative line strengths.

Global fit wave RMS = 135 kHz. ~ 85 % of strong lines (> 2) assigned.Additional 4 assignments underway.

Proposed Observational Searches• Sagittarius B2(N-LMH)

• T ~ 200 K Note: Boltzmann peak for DHA ~ 250 GHz at this T.

• Glycolaldehyde, acetone detected at column densities of ~1015 cm-2

• Orion Hot Core, Compact Ridge• T ~ 150 K • High abundance of many complex molecules.

• W51 e2• T ~ 120 K • Similar abundances of complex molecules to Sgr and Orion.

• IRAS 16293 - 2422• T ~ 90 K

Note: Low T reduces partition function considerably, lowers expected detection limits.

• Similar abundances of complex molecules to Sgr and Orion.

Initial Observational Searches with the Caltech Submillimeter Observatory

• 10.4 meter dish• 230 GHz receiver (strong DHA lines)• Predicted detection limits for DHA ~ 1012 cm-2

• Double sideband system

The Susannas at the CSO!

The Difficulty with Double Sideband Observing of Sagittarius B2(N-LMH)

+=

Image sideband

Frequency sideband

Observed Double Sideband Spectrum

desired line position

desired line position

Spectra from Nummelin et al. (ApJ Supp. Series 117, 1998)

Detection of DHA in Sgr B2(N-LMH)?!

CH3CHO (LSB)

No frequency offset, 50 MHz AOS

61 4 58 60 3 57

Frequency offset, 500 MHz AOS

61 4 58 60 3 5715 11 5 14 10 4

60 5 56 59 4 55

67 3 64 66 4 63

Determination of Trot and Column Density (N)via a Rotation Diagram

The integrated intensity of a transition u l is:The integrated intensity of a transition u l is:

Therefore:Therefore:

So a plot of So a plot of versus E versus Euu yields a line yields a line

withwith slope = -1/Tslope = -1/Trotrot and y-intercept = and y-intercept =

19

20

21

22

23

24

25

0 100 200 300 400 500 600

Eu (K)

ln [

8pk

2 IT

mbd

v/h

c3 Ag

]

Trot = 182 K

N = 2.45E15 cm -2

Rotation Diagram for DHA

Other Observational ToolsThe Owen’s Valley Radio Observatory Millimeter Array• 6 10 meter dishes• 3 mm receiver: strong lines at

112 GHz with expected S/N ~ 6• Predicted detection limits for DHA < 1013 cm-2

The Green Bank Telescope• 110 meter dish• K and Q band (microwave) receivers online in fall 2003 (lower line confusion limit)• Predicted detection limits for DHA < 1013 cm-2

Future Work

1. Additional observational work to confirm detection:

• 3 mm line searches, mapping at OVRO.• Microwave line searches at GBT.

2. Structure Determination:

Isotopic substitution of the hydroxyl protons and 13C isotopomers in natural abundance.

2. Assignment of Higher Vibrational States.

4. Torsional Mode Spectroscopic Measurement:

Tunable Far-IR experiments.

Acknowledgements• The Blake Group -- especially Geoff!

– Rogier Braakman– Kathryn Dyl– Maryam Ali– Suzanne Bisschop

• The JPL Millimeter and Submillimeter Spectroscopy Group– Brian Drouin

• Tryggvi Emilsson

• The CSO, GBT, and OVRO

• The Goddard Group (Ab Initio Calculations)– Chip Kent

• The NASA Exobiology program, grant number NAG5-8822

• The NASA SARA program, grant number NAG5-11423