A Dedicated Effort to Investigate the Formation Chemistry of C3H2O Isomers in the ISM

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

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