Workshop report:
ATCA millimetre science in 2002

Following the success of the 2001 Millimetre Science Workshop in Melbourne, we reconvened on 21 November 2002 at ATNF headquarters near Sydney to hear some results from the first full season of ATCA millimetre observations. Fifteen speakers presented talks on ATCA as well as Mopra observations (in this summary we focus on the ATCA results). As with the 2001 workshop, we have placed the talks from the meeting on the ATNF website; the address is www.atnf.csiro.au/whats_on/workshops/mm_science2002.

The workshop began with presentations of results from the 12-mm system. Perhaps the most ambitious project was a pilot survey of about 1200 square degrees of the radio sky at 16 - 20 GHz, making use of the single-baseline analogue wideband correlator (see article in the October 2002 issue of ATNF News). Such a blind search, aimed at characterizing the high-frequency spectra of known radio sources and possibly discovering new source populations, had never been attempted at so high a frequency. Of the 574 candidate sources identified, 226 distinct sources were confirmed (with fluxes > 60 mJy) in follow-up observations using the ATCA correlator and all three 12-mm baselines. Two source populations, one associated with the Galactic plane and LMC (presumably HII regions and planetary nebulae) and the other extragalactic, could be distinguished. All of the extragalactic sources could be identified in the 843-MHz SUMSS catalogue, with most also having bright optical counterparts (mainly QSOs). Thus, there is no evidence to date of a new, high-frequency source population.

Other 12-mm projects included a search for water masers in the Magellanic Clouds, which yielded nine new detections (seven associated with HII regions and two with oxygen-rich AGB stars), continued observations of the SNR 1987A, which suggest a possible increase in the 17 - 19 GHz flux over the past year, and an unsuccessful search for redshifted HCO+ absorption in z ~3 damped Lyman-alpha absorbers. The workshop antendees were encouraged by the current plan to equip by June all six ATCA antennas with 12-mm receivers, with an expanded tuning range of 16 - 26 GHz (including many of the ammonia transitions).

Some of the first images from the 3-mm system were also presented. A project to detect continuum emission from disks around the young stars TW Hya and HD 100546 was a success on both counts, with fluxes that were consistent with extrapolations from lower frequencies (see accompanying article). Moreover, HCO+ line emission was detected from TW Hya, with a very narrow velocity width (consistent with a face-on disk). A project to image the LMC molecular cloud near N113 in HCO+ was also successful, revealing a heavily resolved source (Figure 1) with FWHM of about 5arcseconds (1.5 pc). Even this only accounts for about 13% of the single-dish flux, suggesting that most of the HCO+ is in relatively diffuse gas associated with a photon-dominated region (PDR).





Figure 1: HCO+ intensity image (contours) for the dense molecular cloud associated with the N113 HII region in the LMC. The diffuse emission seen in the greyscale is H-alpha emission contributing to this R-band DSS image. The molecular cloud appears to be near the end of a dust lane obscuring the HII region. Dashed lines indicate the positions of the two pointing centres. The beam size is 4.7 x 2.8 arcsecond2.

In contrast, the HCN emission from the carbon star R Scl is very compact, with baselines of a few hundred meters required to significantly resolve it (Figure 2). No evidence is seen for the expanding spherical gas shell of radius 10arcseconds inferred from single-dish CO observations, perhaps because HCN is quickly dissociated to form CN.





Figure 2: Integrated HCN intensity image for R Scl. The contour levels are integer multiples of 2 Jy beam-1km s-1. The beam size is 2.7 x 1.8 arcsecond2.

Massive star-forming regions are natural targets for ATCA, as they tend to be found in the inner Galactic plane, which is best studied from the southern hemisphere. Widespread HCN and HCO+ emission was observed towards G291.3-0.7, although mapping was hindered somewhat by the complex source structure and limited coverage of the Fourier plane. Similarly, a plethora of molecular species (HCN, HCO+, HC3N, and CH3OH) was detected towards G318.9-0.17, revealing a dense core spatially coincident with the 6.7-GHz methanol maser. By comparing the observed molecular abundances with chemical models, it should be possible to determine the evolutionary state of the core.

One of the most exciting applications for the large collecting area afforded by ATCA is the search for biogenic molecules. Among the most interesting are glycine, one of the simplest amino acids, and propylene oxide, one of the simplest chiral molecules. Searches for these molecules conducted towards two molecule-rich sources in 2002 proved negative, although one of them, Sgr B2, shows a large number of unidentified lines. Thus, careful laboratory measurements to determine precise rest frequencies are essential to this effort, and have been undertaken by researchers at Monash University.

Unfortunately, completion of the 3-mm systems has been delayed until at least 2004, due to problems obtaining the necessary LO chain components. Nonetheless, the potential for useful observations has been demonstrated already, especially when data from several configurations can be combined. We look forward to being able to broaden the user community for both the 3-mm and 12-mm systems this year, in anticipation of an even more fruitful workshop in late 2003.

Tony Wong
(Tony.Wong@csiro.au)

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