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 are not well understood. Molecular isomers, also prevalent in
astronomical environments, may play a key role in determining the formation routes of molecules in
space. Molecular isomers contain the same constituent atoms but 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 hypothesis using the isomers of C3H2O. C3H2O forms three structural
isomers; propadienione (CH2CCO), propynal (HCCCHO) and cyclopropenone (c-C3H2O), listed in
order of increasing bonding energy. We searched existing broadband spectral line surveys of SgrB2(N)
and found no evidence for propadienione and unpersuasive evidence for propynal, whereas
cyclopropenone is readily detected and confirmed. Although evidence is building that the lowest
energy C3H2O isomers are not present in the ISM, further observation is necessary for drawing solid
conclusions. An interstellar deficiency in the low energy isomers supports the postulate that formation
chemistry is more important than relative molecular stability for determining the abundance of
molecules in astronomical environments.
The astronomical molecular inventory currently stands at ~169 detected species and their
corresponding isotopologues. Given this vast molecular inventory however, the formation mechanisms
of even the simplest molecular species are not well understood. This is extremely problematic because
the models cannot predict the observed abundances of astronomical molecules and it questions the
ability of chemical formation models to guide the astronomical identification of new molecules.
Though there is progress in this area for cold sources (e.g. TMC-1), core chemistry remains a challenge
as abundances of several species are underpredicted in models by orders of magnitude (Quan & Herbst
2007, A&A, 474, 521). To overcome these challenges, new methods of predicting formation routes of
interstellar molecules must be developed.
Molecular isomers are also prevalent in astronomical environments and may play a role in determining
the formation routes 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 suggested that isomer abundance
ratios are directly proportional to their energy differences. Finally, given the number of interstellar
isomers detected in various 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 is
much more significant than the relative zero point bonding energies in establishing abundance
ratios. We set out to test the theory by Lattelais and colleagues using isomers of C3H2O. C3H2O forms
three structural isomers; propadienione (CH2CCO), propynal (HCCCHO) and cyclopropenone (cC3H2O), listed in order of increasing bonding energy. Figure 1 shows the structure of each molecule
and the bonding energy differences between the three isomers.
If the hypothesis of Lattelais and colleagues is correct, propadienione (CH2CCO) should be the most
abundant of the C3H2O isomers in astronomical environments. Yet there has never been an
astronomical detection of propadienione, whereas propynal (HCCCHO) and cyclopropenone (cC3H2O) have been reported as 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, cyclopropenone was first detected using the GBT towards SgrB2N and there have
been many claims of detected propynal transitions toward SgrB2N in various surveys.
Our objective was to first observe transitions of propynal (HCCCHO), a well-known and established
molecule since 1988, and then continue by searching for transitions of the other isomers. We searched
numerous broadband spectral line surveys to identify all detectable transitions of propynal (HCCCHO)
in order to compile a spectrum of this molecule that was as complete as possible. At the same time, this
would allow us to obtain a good measure of the physical conditions present in the molecule's
environment. We used spectral line data at 8-50 GHz from the PRIMOS survey on the Green Bank
telescope, 73-170 GHz data from the 12 m on Kitt Peak, and 218-260 GHz data from the Swedish ESO
Submillimeter Telescope (SEST). We summarize our results below.
The GBT data provided evidence of propynal towards Sgr B2N. Many transitions are detected
including b-type transitions. Figure 2 shows a strong absorption feature in the PRIMOS data which we
have identified as the 4(1,4) – 5(0,5) transition at 15146.06 MHz. Curiously however, no pure Ka=1
transitions were detected. This is surprising because the Ka=1 states are only ~4 K above the Ka=0
states, and thus should be as equally strong as the Ka=0 transitions. In addition, while many transitions
were detected, numerous other expected transitions were missing. The missing transitions prompted
us to explore the 1-3mm region as well, providing an added benefit of allowing us to better constrain
the physical environment.
Nearly all expected propynal lines in the 1-3mm region were contaminated by other strong interstellar
transitions (see, for example, sample spectra shown at the left of Figure 3). The only propynal
transition frequency that was not contaminated by strong lines is a missing transition at 166.647 GHz.
(Right Figure 3). This non-detection in the 2mm range leaves the interstellar detection of propynal
suspect. Therefore, we returned to the original detection of Irvine et al. 1988 toward TMC-1 to
determine if the initial detection was valid. The data from the original detection paper, as well as a
recent GBT observation taken from the archives are shown in Figure 4. (Fig 4: Mark where the two
lines should be. Cant read axes)
From analyzing the GBT, 12m, and SEST, we are left to conclude the following:
1. If propynal is present toward Sgr B2N, it is only detected at low frequencies. This implies that
identifications of ALL propynal transitions at millimeter wavelengths are erroneous.
2. The original detection from Irvine 1988 toward TMC-1 is quite suspect because subsequent
surveys with improved sensitivity and spectral resolution have not detected the Irvine 18GHz
line as well as other confirming transitions. Furthermore, the 18GHz line toward TMC-1
appears to have hyperfine structure which is inconsistent with propynal (see Figure 4).
3. The missing transitions in the GBT data, as well as in the mm region, suggest that the
interstellar detection of propynal is not supported by existing survey data.
Without a firm confirmation of propynal in astronomical environments, any determination of the
relative abundance ratios of C3H2O isomers to test the theory of Lattalais and colleagues would be
futile. The only way to accurately identify this molecule is through a dedicated search for a series of
transitions with a single instrument. In addition, given the line density and resulting line confusion in
the SgrB2N region, as well as discrepancies between survey data and existing detections of propynal,
further observations toward this source would likely provide no additional information. As such, we
propose a search for transitions of propynal toward the original source of the detection, TMC-1.
Using the GBT, we can easily observe all propynal transitions from 8-50 GHz. In addition, if the 18.65
GHz line 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, Kaifu
et al. (2004, PASJ, 56, 69) reported a line intensity of ~50mK which was assigned to the 4-3 transition
at 37.29 GHz. With several setups from X to Q band, we can cover several transitions of this molecule
and confirm (or dismiss) its existence in space. Finally, given the flexibility of the GBT spectrometer,
we will also conduct a dedicated search for the other C3H2O isomers towards our original goal of
testing the Lattalais and colleagues hypothesis that the lowest energy isomer, propadienione, should be
the most abundant. Irvine et al. 1988 also searched for this molecule but did not detect any transitions
from propadienione; deriving an upper limit on propadienione abundance that would be comparable to
the claimed estimated propynal abundance.
This is the first observational test of a predicted abundance ratio of isomers in astronomical
environments as well as a crucial confirmation or dismissal of an astronomical detection that has not
been followed up on in 22 years. As such, these observations will set a precedent for the continued
investigation of claimed astronomical detections of molecules as well as to test chemical formation
theory.
(Figures, make sure you can read the axes.)