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Investigating Prebiotic Organic Chemistry using
Broadband Centimeter Spectral Line Surveys
Anthony J. Remijan
Associate Scientist – NRAO
Division Head – NRAO Scientific User Support Services
[email protected]
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A Hodgepodge of Surveys (just Orion!):
• 34-50, 83.5-84.5, 86-91 GHz Ohishi et at. 1986
• 47, 87 GHz Madden et al. 1989
• 70-115 GHz Turner 1991
• 86 GHz Plambeck et al. 1982
• 87-108 GHz Friedel et al. 2003
• 98 GHz Murata et al 1992
• 138-151 GHz Lee et al. 2001
• 150-160 GHz Ziurys and McGonagle 1993
• 160-165 GHz Lee et al. 2002
• 172-256 GHz Dickens et al. 1997
• 190-900GHz (with gaps) Serabyn et al. 1995
• 225-262 GHz Liu, S.-Y. et al. 2002
• 215-263 GHz Sutton et al. 1985; Blake et al.
1987
• 325-360GHz Schilke et al. 1997
• 334-343 GHz Sutton et al. 1995
• 455-507 GHz White et al. 2003
• 486-492 and 541-577 GHz Persson et al. 2007
• 607-725GHz Schilke et al. 2001
• 795-903GHz Comito et al. 2005
• 795-903 GHz Comito et al. 2005
• Etc…
• Etc…
A complete survey with HIFI/PACS
Investigate chemical and physical
conditions toward Orion
ALMA Orion Band 6 Survey
(214-246GHz)
NOTE: Not many at cm wavelengths!
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A Spectral-Line Survey
Observation of IRC +10216
between 28 and 50 GHz
(Kawaguchi et al. 1995)
Full spectral scan
188 features detected
150 features identified to 22
molecular species.
38 unidentified features
including the “B1377”
Laboratory and Astronomical
Identification of the Negative
Molecular Ion C6H(McCarthy et al. 2006)
Confirmed the molecule both in
IRC+10216 but also in TMC-1
Went on to detect several other
molecular ions including C8H-,
C4H-, CN- and C3N-
Original Survey data from Kawaguchi et al.
1995)
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Why waste our time in the cm?
1)  building up the chemical inventory of astronomical molecules is important to
understand how organic matter is produced in interstellar space
2)  thorough, sensitive survey will allow scientists to utilize these databases to make firm,
multitransition identifications of species
3)  surveys are not only to discover new interstellar species but also provide probes of
physical, kinematic and chemical conditions in astronomical environments
Most of the “most favorable”
transitions should be in mm
Certainly many more transitions
To select from…
BUT
Major line confusion problem and
What if the transitions are not in
LTE?
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The Green Bank Telescope
l 
θB = 740”/ν [GHz]
l 
7854 m2 - ~2 ACRES!
l 
l 
l 
l 
l 
l 
At 17 million pounds, it is one
of the largest moving
structures on land!
OFF(2 min)-ON(2 min)‫‏‬
Toward Sgr, use 4x200 MHz
windows for a spectral
resolution of ~24.4 kHz
These limitations should
disappear with the new
spectrometer.
The GBT has been
instrumental in searching for
large, organic and prebiotic
species.
11 17! new species have
been detected since 2004
PRebiotic Interstellar MOlecular Survey
• Target: Sgr B2(N-LMH)
• Coverage: 40.4 GHz of Bandwidth from 300 MHz - 50 GHz
• Noise level of ~2 mK
• Publicly available with no proprietary period
http://www.cv.nrao.edu/~aremijan/SLiSE/
Jan M. Hollis, Anthony J. Remijan, Philip R. Jewell, Frank J. Lovas, Joanna F. Corby
www.cv.nrao.edu/~aremijan/PRIMOS/
Ballpark number of available lines for
SKA mid up to 13500 MHz
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(Some) New Molecule Detections
propanal
propenal
cyanoformaldehyde
ketenimine
ethanimine
carbodiimide
acetamide
methylisocyanate
methyltriacetylene
cyclopropenone
Ethanimine
§  CH3CHNH has two stereoisomers, E (μa=0.834, μb=1.882) and Z
(μa=2.446, μb=0)
§  Microwave spectra first taken by Frank Lovas in 1980, then RD Brown in
1981
§  -CH3 rotamer yields A-E splitting, assisting in interstellar detection
Lovas, F. J. (1980). J. Chem. Phys., 72(9), 4964-4972
Interstellar Detection and Analysis
• 
The detection of
ethanimine is significant
because of its possible
role in the formation of
alanine .
• 
In this mechanism for
amino acid formation, the
key step is the formation
of a primary aldimine that
can be co-deposited in the
interstellar ice with HCN
to permit subsequent
formation of the
aminonitrile precursor.
• 
As such, aminonitriles
may serve as a reservoir
species. For the formation
of the simplest amino
acid, glycine, interstellar
production of the two key
species (methanimine and
aminoacetonitrile) has
been confirmed (Godfrey
1973, Belloche et al. 2008)
Interstellar spectra from GBT PRIMOS survey; A species hyperfine
fiducials shown in blue, E species hyperfine fiducials shown in
red
Loomis, et al. 2013, ApJL, 765, L10
E-Cyanomethanimine
Zaleski et al. 2013 ApJ, 765, L9
• 
Precursor to prebiotic
oligomers of HCN
• 
Formation of adenine
using only addition of
HCN
•  Adenine is a
nucleobase with a
variety of roles in
organic chemistry
including cellular
respiration.
•  energy-rich
adenosine
triphosphate (ATP)
•  Protein synthesis, as
a chemical
component of DNA
and RNA
Carbodiimide (HNCNH)
~2 x 1014 cm-2
(Turner et al. 1975)
~2 x 1013 cm-2
In water ice
~4% @ 80 K
~13% @140 K
+4 kcal mol-1
~2000 K
~1% @ 300 K
McGuire et al. 2012, ApJ, 758, L33
Duvernay et al., 2004 J. Am. Chem. Soc., 126, 7772.
Duvernay et al., 2005, J. Phys. Chem. A., 109, 603.
Carbodiimide - observations
Lines at 4 GHz
+64 and +82 km s-1
+64 and +82 km s-1
CH3OH Line
H Recomb Line
CH3OH Line
No lines!
CH3OH Line
+64 km s-1
Without any other observations, there is a 0.002% chance we are wrong
with just the 4GHz transitions.
Carbodiimide - energy level structure
4.8 GHz Maser Line
-log(Aij) ~ 2.5
-log(Aij) ~ 2.5
-log(Aij) ~ 9.0
J = 18
J = 17
J = 19
J = 16
J = 18
~ 0.75 1.5 THz
𝚫J = 0, ±1
𝚫K
Ka = ±1
J = 18
J = 17
J = 16
Ka = 0
Ka = 1
Ka = 2
Carbodiimide - energy level structure
J = 16
36.6 GHz Line
-log(Aij) ~ 2.5
-log(Aij) ~ 2.5
-log(Aij) ~ 6.3
J = 15
J = 14
J = 17
J = 16
J = 15
𝚫J = 0, ±1
𝚫K
Ka = ±1
J = 16
J = 14
Ka = 0
J = 17
Ka = 1
Ka = 2
Carbodiimide - analysis
Transition
Population
Inversion?
TA* (mK)
Observed
TA* (mK)
(2 x 1013 cm-2)
TA* (mK)
(5 x 1014 cm-2)
4 GHz
Yes
85
0.1
3.8
15 GHz
No
-
-
-
25 GHz
Yes
27
0.8
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36 GHz
No
< 11
1.4
39
45 GHz
Yes
25
0.9
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Emission arising from a thermal population of HNCNH would
be undetectable in (any?) survey Methyl Formate Maser Emission
These results are in sharp contrast with
interferometric observations showing that
HCOOCH3 resides predominantly in the
compact (6.5") LMH hot core near Sgr B2(N)
(Miao et al. 1995).
This source is indeed characterized by very high
densities (> 107 cm-3) and gas temperatures
(>100 K). Assuming a source size of 4“ and a
rotational temperature of 80 K, a column density
of 4.5x1017 cm-2 (Belloche et al. 2009, 2013)
Methyl Formate Maser Emission
The conclusion is that all
detected methyl formate
lines below 30 GHz are
masers!
Q. What mechanisms are
pumping these new masers?
A. Can mapping the
distribution give insight to
excitation and possible
formation?
Where do the cm emission/absorption
features come from?
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Where do the cm emission/absorption
features come from?
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Conclusions
Centimeter-wave observations are a powerful tool for the
identification of new molecules
At least 1-2 new
molecule detections
a year come from
PRIMOS for the last
7 years
Low line density
makes definitive
detections possible
with fewer lines
Non-LTE emission
can enable detection
of otherwise
undetectable species
So what do we want:
•  Large spectral bandwidth with possible microJansky spectral line sensitivity…
hit mJy RMS with the GBT for some methyl formate transitions but still
could not be detected. Helps to greatly constrain physical conditions!
•  Discovery space will be vast for complex organic molecules (COMs) because
there will be maser type transitions for a myriad of prebiotic molecules. We
just need to fully characterize the energy level diagrams for these species of
interest…may also be able to start detecting COMs in disks.
•  High spatial resolution observations to locate and characterize the
distribution of these molecular species in order to again, characterize the
physical environment and to possible ascertain chemical formation pathways.
•  Investigate these molecules in a myriad of sources. No need to continue to
look at the “best and the brightest” sources in the sky. This will help us to
better understand the chemistry from the ISM to planetary systems.
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