A Dedicated Effort to Investigate the Formation Chemistry of C3H2O Isomers in the ISM
Amy Robertson
University of Arizona and National Radio Astronomy Observatory
Anthony Remijan & Robin Pulliam
National Radio Astronomy Observatory
Joanna Corby
University of Virginia
The astronomical molecular inventory currently stands at ~169 detected species and their
corresponding isotopologues. Yet, given this vast molecular inventory, the formation
mechanisms of even the simplest molecular species detected in these varied environments
is still not well understood. Molecular isomers are also prevalent in astronomical
environments and may play a role in determining the formation route of molecules in
space. Molecular isomers contain the same constituent atoms yet have different
structures. Recently, Lattelais et al. 2009, suggested that the isomer with the lowest zero
point bonding energy (ie, the most stable isomer) should be the most abundant, and thus
most easily detected in astronomical environments. We explored this theory using the
C3H2O isomers. C3H2O forms three structural isomers; propadienione (CH2CCO),
propynal (HCCCHO) and cyclopropenone (c-C3H2O), each with increasing bonding
energy, respectively. We searched existing broadband spectral line surveys in the ISM
and all spectral explorations resulted in less then convincing proof of the presence of the
two lowest energy isomers in the ISM, yet cyclopropenone is readily detected and
confirmed. More observations are needed to make more solid conclusions yet the
evidence is building that the lowest energy C3H2O isomers are not present in the ISM
and supports the conclusion that the formation chemistry is more important than relative
molecular stability in determining the abundance of molecules in astronomical
environments.
The astronomical molecular inventory currently stands at ~169 detected species and their
corresponding isotopologues. Yet, given this vast molecular inventory, the formation
mechanisms of even the simplest molecular species detected in these varied environments
is still not well understood. This is extremely problematic given that chemical formation
models should provide predictions to guide the astronomical identification of new
molecules. Yet, there is progress in this area for cold sources (e.g. TMC-1); however,
core chemistry continues to remains a challenge as abundances of several species are
underestimated by orders of magnitude (Quan & Herbst 2007, A&A, 474, 521).
Therefore, new ways are needed to predict the formation route of potential astronomical
molecules.
Molecular isomers are also prevalent in astronomical environments and may play a role
in determining the formation route of molecules in space. Isomers contain the same
constituent atoms yet have different structures. Recently, Lattelais et al. (2009, ApJ, 696,
L133) investigated the relative abundances of molecular isomers in astronomical
environments and suggested that the isomer with the lowest zero point bonding energy
(ie, the most stable isomer) should be the most abundant, and thus most easily detected in
astronomical environments. In addition, they also suggested that the abundance ratio of
the most stable isomer to the others is directly related to their energy difference. Finally,
given the number of interstellar isomers detected in a variety of astronomical
environments, they suggested that this minimum energy principle can be verified with
existing observations.
It is our hypothesis that the chemical formation route to the production of astronomical
species in astronomical environments is much more significant than the relative zero
point bonding energies in establishing the relative abundance ratio. We set out to test the
theory by Lattelais and colleagues using the C3H2O isomers. C3H2O forms three
structural isomers; propadienione (CH2CCO), propynal (HCCCHO) and cyclopropenone
(c-C3H2O), each with increasing bonding energy, respectively. Figure 1 shows the
structure of each molecule and the relative bonding energy difference between the three
isomers.
Figure 1. Structures and Bonding Energy differences of the C3H2O isomers.
If the theory of Lattelais and colleagues is correct, the molecule of the C3H2O isomers
that should have the highest abundance in astronomical environments is propadienone
(CH2CCO). Yet, there has never been an astronomical detection of propadienone while
propynal (HCCCHO) and cyclopropenone (c-C3H2O) have well established detections
(See Irvine et al., 1988, ApJ 335, L89; Hollis et al., 2006, ApJ, 642, 933). Therefore, we
started a dedicated effort to search for all three C3H2O isomers toward the preeminent
source of molecules in the Galaxy, Sgr B2(N-LMH). Of all the molecules detected in
astronomical environments, more than half have been detected toward SgrB2N first. In
addition, the detection of cyclopropenone was first detected using the GBT towards
SgrB2N and there have been many claimed observations of transitions of propynal
toward SgrB2N in various surveys.
Our objective was to begin with propynal (HCCCHO), a well known and established
molecules since 1988, solidify the detection of that molecule and then move on to the
other isomers. We searched numerous broadband spectral line surveys to identify as
many transitions of propynal (HCCCHO) in order to compile a complete spectrum of this
molecule as possible in order to obtain a good measure of the physical conditions of the
environment around that molecule. We took spectral line data from the PRIMOS survey
using the Green Bank telescope, 8 – 50 GHz, the 12 m on Kitt Peak, 73-170 GHz, and
Swedish ESO Submillimeter Telescope (SEST), 218-260 GHz. The results of these
surveys are described below.
The data from the GBT give evidence of propynal present toward the Sgr B2N region.
Many transitions are detected including b-type transitions. However, no pure Ka=1
transitions were detected. This was suspicious because the Ka=1 states are only ~4 K
above the Ka=0 states. Figure 2 shows a strong absorption feature which we have
identified as the 4(1,4) – 5(0,5) transition at 15146.06 GHz detected with the GBT. The
lack of any Ka transitions in any of the data in the GBT, as well as some other
unexplained missing transitions, led us to explore the 1-3mm region as well.
Figure 2. Absorption feature assigned to the 4(1,4) – 5(0,5) transition at 15146.06 GHz of HCCCHO.
In addition, this search at higher frequencies would also allow us to get a better
understanding of the physical environment where this molecule is present.
Figure 3. Spectra of predicted transitions of propynal (red gaussians) overlayed on 12m observations (blue
trace). In this case, there is a strong transition absent at 166.5 GHz.
All the data in the 1-3mm region for propynal were either directly contaminated by other
strong interstellar transitions or were on the edge of a stronger transition from another
transition (Left Figure 3.) The only clear transition was the one that was missing (Right
Figure 3.) This leaves the interstellar detection of propynal very suspect given the
missing transition. Therefore, we went back to the original detection of Irvine et al. 1988
toward the TMC-1 to determine whether the initial detection was actually correct. The
data from the original detection paper, as well as a recent GBT observation taken from
the archives is shown in Figure 4.
Figure 4. Original detection by Irvine et al. 1988 (left) and recent GBT observation (right).Notice the
structure to the right and left of the supposed propynal peak in the GBT data that were disregarded as
frequency swiching artifacts from the original 140ft data.
Given these sets of data from the GBT, 12m and SEST, it is clear that:
1. if propynal is present toward Sgr B2N, it is only detected at low frequencies. This
implies any other identification of transition of propynal at mm wavelengths is
erroneous.
2. the original detection from Irvine 1988 toward TMC-1 is quite suspect because
subsequent surveys with other instruments have not detected the Irvine 18GHz
line with other confirming transitions. Furthermore, the 18GHz line toward
TMC-1 appears to have HFS that would be inconsistent with propynal. (see
Figure 4.)
3. Propynal is actually not present in the ISM. Confirmation of this molecule is
necessary given the existing data.
It is our supposition that propynal has yet to be confirmed in astronomical environments
given the datasets we have investigated. As such, any determination of the relative
abundance ratios of C3H2O isomers to test the theory of Lattalais and colleagues would
be futile if in fact the molecule is not present in space. The only way accurately identify
this molecule is with a dedicated search of a series of transitions with a single instrument.
In addition, given the line density of the SgrB2N region, and the discrepancies we find
with the existing detections (and non-detections) of propynal, further observations toward
this source would give no additional information. As such, we propose a dedicated
search of transitions of propynal toward the original source of the detection, TMC-1.
Using the GBT, we can easily observe all the transitions of propynal from 8-50 GHz. In
addition, if the 18.65 GHz lines shown in figure 4 is actually from propynal, the
measured line intensity is ~25 mK which is easily detected in 2 hours of observing time
toward TMC-1. Using the Nobeyma 45m telescope, they reported a line intensity of
~50mK which was assigned to the 4-3 transition at 37.29 GHz. Therefore, with several
setups form X to Q band, we can cover several transitions of this molecule and confirm
its existence in space. Finally, given the flexibility of the GBT spectrometer, we will also
conduct a dedicated search of the other C3H2O isomers to investigate our original goal of
testing the Lattalais and colleagues theory that the lowest energy isomer, propandienone,
should be the most abundant, and as such, easily detectable. Irvine et al. 1988 did not
detect any transitions from propandienone in their survey and from the upper limit they
derived on the abundance of propandienone, found a relative abundance ratio between the
2 species to be comparable.
This is the first test of a prediction of the relative abundance ratio of isomers in
astronomical environments as well as a confirmation of an astronomical detection that
has not been followed up in 22 years. As such, these observations will hopefully set a
precedent for the continued investigation of claimed astronomical detections of molecules
as well as tests of chemical formation theories.