Towards the end of the four-week shutdown, we ventured up to the Compact Array to test the science-readiness of the system. For the first few days, we watched while the engineers worked tirelessly to complete the installation on schedule. When they finished late on the last night of the shutdown, and we were able to get our hands on the array, we found a beautifully working system with excellent characteristics! Figure 1 shows the receiver temperatures (upper panel) and the system temperatures (lower panel) as measured with a spectrum analyser for one polarisation (A) on one of the antennas (CA03). The characteristics are slightly different on each antenna and for each polarisaton. System temperatures of order 50 – 60 K are expected near the lower end of the band rising, due to the pressure-broadened atmospheric oxygen line at around 60 GHz, to around 100 K toward the upper edge of the 7-mm band. The sensitivity of the array for Fourier synthesis imaging may be estimated using the online sensitivity calculator ( www.atnf.csiro.au/observers/docs/at_sens/), which also accounts for atmospheric opacity.
Figure 1: 7-mm receiver and system temperatures (click on image for full-size display)
Figure 2: A 7-mm spectrum showing emission from silicon monoxide, methanol and ethylcyanide.
The resulting spectrum is shown in Figure 2. The spectrum is dominated by the strong SiO and methanol lines. Thermal transitions of ethylcyanide can also be reliably identified. However, we were unable to identify three spectral lines clearly present in the data (the rest frequencies are around 43016.7, 43447.7 and 44053.2 MHz). These transitions are not listed in the 2002 revision of the National Institute of Standards and Technology (NIST) recommended rest frequencies database (Lovas & Dragoset; physics.nist.gov/PhysRefData/Micro/Html/contents.html) and therefore their detection in star forming regions has probably never been reported before. It seems unlikely that these spectral features are high-velocity components of known strong transitions because velocities in excess of 100 km/s would be required. The 43016.7 and 44053.2 MHz spectral features could be caused by highly excited transitions of SO2. The third unidentified transition remains a mystery. New Compact Array observations are required to confirm the detection. In addition, imaging could help in associating this transition with one of the cores and, hence, in narrowing down the list of candidate species because the chemistry differs from core to core in OMC-1.