3C Sugars in Interstellar Hot Cores? e Laboratory Rotational Spectroscopy o Observational Search for Dihydroxyacet Susanna L. Widicus August 22, 2003
Dec 20, 2015
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• Previously unpublished microwave work now in press: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