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The Discovery of Interstellar Thioformaldehyde, Acetaldehyde, Methylamine, Deuterated water vapor and the Trans Ethyl Alcohol

Dr Nicholas Fourikis

Introduction

I was a member of a team that discovered interstellar thioformaldehyde [1], acetaldedyde [2] and methylamine [3] using the 64m Parkes Radiotelescope in the early seventies. The advantages we had over other teams searching for interstellar molecules were:
(i) The Parkes Radiotelescope has a respectable collecting area.
(ii) From Parkes, a small country town about 360 km West of Sydney, Australia, the Galactic Centre, a prolific source of interstellar molecules, is visible for more than six hours; and
(iii) My association with Japanese spectroscopists through Dr Masaki Morimoto of the Tokyo Astronomical Observatory. Masaki and I were members of a larger team that designed and realized the Culgoora Solar Radioheliograph in the late sixties [4].

Against these advantages, we had the following disadvantages:
(i) The microwave industry in Australia was at an embryonic state [5]; furthermore, the budgets we had for instrumentation at the Parkes Radiotelescope were proportional to our comparatively low population. Consequently, we could only explore narrow windows of the microwave spectrum.
(ii) The installation of receivers at the Parkes Radiotelescope, for each observing period was tedious and time consuming.

Our search strategy was based on two assumptions: (a) The rotational state populations for several molecules do not follow a thermal law; and (b) No one had a detailed knowledge of the excitation processes, related to undetected molecules.

Given our instrumentation constraints, we decided to search the K-doublet transitions of the molecules that were chemically related to the molecules already discovered, assuming cosmic abundance ratios for the elements; the transition frequencies of the K-doublets however had to be within the bandwidth of the receivers available at the Parkes Radiotelescope facility.

Morale among us was low before our successes but after our successes it was possible for me to get observing periods at the NRAO radiotelescopes at Green Bank and Kitt Peak and at the Mitaka, Japan.

The Discovery of Interstellar Thioformaldehyde.

The prospect of detecting the 2-2 transition of thioformaldehyde during our five day observing period was not good for the following reasons: (a) Two groups before us failed to detect the 1-1 transition of the molecule [6&7]; and (c) The phase-lock subsystem of the 9cm receiver we used was inoperative. Without phase-locking of the receiver’s local oscillator it was impossible to attain long integration times toward sources of interest.

As neither M. W. Sinclair who was the engineer in charge for the receiver, nor the local technical staff could restore the subsystem, I stepped in to investigate the problem. Tests I performed indicated that the fault was with the circuitry situated at the prime focus cabin of the 64m Parkes Radiotelescope. With the aid of a multimeter and a portable cathode ray oscilloscope I proceeded exploring the circuitry associated with the subsystem. Without any familiarity of the subsystem, it took me some time to locate the fault, a shorted resistor. After I replaced it, the subsystem worked and we were able to detect the 2-2 transition of thioformaldehyde during the remaining one and a half days of our allocated observing time.

After we returned to our Sydney base, we communicated the discovery to the participants of the NRAO Workshop on Interstellar Molecules in Charlottsville, Virginia.


The Detection of the 2-2 Transition of Interstellar Acetaldehyde.

We already knew of the discovery of interstellar acetaldehyde by its 1-1 transition [8] but decided to search for the 2-2 transition to increase our knowledge of the excitation process related to the molecule. Since our detection [2], several papers confirmed our hypothesis that the 2-2 transition required maser amplification.

The Discovery of Interstellar Methylamine.

Knowing that Japan had several world-class spectroscopists, I sent the frequency coverage of the Parkes Radiotelescope receivers to Dr Masaki Morimoto and asked him to consult with them. I was specifically interested in transitions of methylamine that fell within the bandwidth of our receivers. Methylamine is the terminal product of the hydrogenation series based on the cyanide radical, hydrogen cyanide and methanimine, interstellar species already discovered.

During a brief visit to Australia, Masaki and I decided to search for the 2-1 transition of methylamine after consultations he had with Dr Kojiro Takagi of Toyama University. In March 1974 I detected the transition we were interested in during an observing period I had at Parkes [9]. The detection was possible because the two states of the molecule, 202 and 110, are inverted.

When I communicated our discovery to Masaki, he informed me that members of the Tokyo Astronomical Observatory detected two mm-wave transitions of methylamine using the Mitaka 6m radiotelescope and the 11m Kitt Peak radiotelescope. After many considerations, we decided that they publish their detections in one paper [10] and we publish our detection in another [9]. Naturally I insisted that K Takagi who was at the time at Rice University, Houston Texas be my co-author together with M Morimoto and the two papers appeared back to back in the Astrophysical Journal Letters. In our paper [9] we sited paper [10] but the authors of the latter paper did not cite our paper although M Morimoto and K Takagi were co-authors of both papers.

The Detection of Interstellar Deuterated Water Vapor and Trans ethyl alcohol

Soon after the detection of interstellar methylamine, the Japan Society for the Promotion of Science awarded me a Fellowship and an attachment to the Tokyo Astronomical Observatory for a year - 1975-6. I decided to take up the fellowship after a detour to the US, where I had an observing period with the 11m Kitt Peak mm-wave radiotelescope to search for interstellar HNO. To maximize the chances of detecting new interstellar molecules during my session, I decided to join forces with Ben Zuckerman and Barry Turner.

That observing session offered us a chance to explore the mm-wave spectrum using a new super sensitive receiver engineered by A Kerr, a fellow Australian working at NRAO.

During that observing session we did not detect interstellar HNO but detected interstellar deteurated water vapor [11] and a number of unknown emission lines. Other observing teams after us established that one of the emission lines we detected was due to the trans ethyl alcohol species and they detected two more lines of the molecule [12]. While I was in the US, I contributed to the writing of the two papers, before I headed to Tokyo to take up my fellowship.

Soon after I arrived in Japan, I took the train to Toyama, where Dr Kojiro Takagi met me at the station. From there he took me to the Physics Department of Toyama University and showed me the spectrum of the methylamine transition, I detected from Parkes displayed on a cathode ray oscilloscope. His students worked all morning to re-measure the spectrum of methylamine for my benefit. Seeing the spectrum of the transition of methylamine with its hyperfine components, I shook hands with Kojiro and the members of his team.

References
[1] Sinclair MW, Fourikis N, Ribes JC, Robinson BJ, Brown RD and Godfrey PD. Detection of interstellar thioformaldehyde. Aust J Phys , Vol 26, 1973.
[2] Fourikis N, Sinclair MW, Robinson BJ, Godfrey PD and Brown RD. Microwave emission of the 211-212 rotational transition in interstellar acetaldehyde. Aust J Phys , Vol 27. No 3, 1974.
[3] Fourikis N, Takagi K and Morimoto M. detection of interstellar methylamine by its 202-110 AA- state transition. Astrophys Lett 191. No3 Part 2, 1974.
[4] Special Edition on the Culgoora Radioheliograph, Proc IREE (Aust), Vol28. No9, Sep 1967.
[5] Fourikis N. Microwave engineering education in Australia. Invited paper IEEE Trans Micr Theory & Tech, Vol 41, No6, June 1993.
[6] Davies RD, Booth RS and Pedlar A. Mon Nt R Astr Soc 152, 7P, 1971.
[7] Evans NJ, Townes CH, Weaver HF and Williams DR. Science NY 169, 680, 1970.
[8] Gottlieb CA. In Molecules in the Galactic Environment. Edit by MA Gordon and LE Snyder. P182, Wiley Interscience, New York, 1973.
[9] Fourikis N, Takagi K and Morimoto M. Detection of interstellar methylamine by its 202-110 AA-state transition. Astrophys J Lett, L139-141, 191, 1974.
[10] Kaifu N, Morimoto M, Nagane K, Akabane K, Iguchi T, Takagi K. Detection of interstellar methylamine. Astrophys J Lett, L135-137, 191, 1974.
[11] Turner BE, Zuckerman B, Fourikis N, Morris M and Palmer P. Microwave detection of interstellar HDO. Astrophys J Lett, Vol198. No 3, 15 June 1975.
[12] Zuckerman B, Turner BE, Johnson DR, Clark FO, Lovas FJ, Fourikis N, Palmer P, Lilley AE, Ball JA, Gottlieb CA. Litvak MM and Penfield. Detection of interstellar trans ethyl alcohol. Astrophys J Lett, Vol196, No 3, March 15 1975.