Vikram Dwarkadas , Lewis Ball , James Caswell , Anne Green , Simon Johnston , Brian Schmidt , Mark Wardle, PASA, 17 (1), 83.
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Masers associated with SNRs
An overview of the field was presented by Anne Green (USydney). The first observations revealing the likely association of 1720-MHz OH masers with SNRs were made 30 years ago, but the field then lay dormant for many years, since the detailed follow up observations of high spatial resolution and high sensitivity were beyond the reach of the available instruments. Interest in the field was revived 5 years ago (Frail, Goss & Slysh 1994), with a three pronged attack: first, high resolution observations of the stronger masers known from the pioneering work; second, a general single dish survey to see how widespread the phenomenon was in the light of the better catalogues of SNRs now available; third, a review of the theoretical explanation and implications. To date, about 75 per cent of the known SNRs have been searched (Frail et al. 1996; Green et al. 1997). Overall, a 10 per cent detection rate has been found, although the remnants containing masers are not uniformly distributed throughout the Galaxy; the detection rate is higher for SNRs located closer to the Galactic Centre, where there is a greatly increased density of molecular gas. Where high resolution observations have been made (with the VLA or with the ATCA), they reveal clusters of small diameter maser spots located predominantly at the periphery of the associated SNR. Zeeman splitting is often detectable, implying magnetic fields of typically 10-7 T or less. The masers tend to have only a small spread in radial velocity, irrespective of their location relative to the SNR boundary, and the inference has been drawn that they occur at points tangential to the shock front, and thus their velocity represents the systemic velocity of the remnant itself (but see later). If so, this provides a valuable distance indicator, and prompts the theoretical question of assessing the physical and chemical conditions needed, and whether the postulated shock provides them. These theoretical aspects were expanded on by Mark Wardle (SRCfTA). The basic pumping scheme was suggested 20 years ago and has required only minor refinements. It satisfactorily accounts for the fact that the 1720-MHz transition is seen without any accompanying masers at other OH transitions, occurring at a density too low for their excitation. More puzzling is how the required OH abundance, densities and temperatures arise. Remarkably good progress has been achieved with a strong consensus that non-dissociative (C-type) shock waves are a key factor. More contentious is the question of whether the soft X-ray emission from the SNR is also vital to the process. Although the framework of understanding is in place, it is largely based on the very small number of objects studied in detail. This is slowly being remedied with follow up observations of more remnants, and new results were presented by David Moffett (U Tasmania). He found it difficult in several cases to confirm at high resolution the preliminary detections made with single dishes. This may be because in these cases the emission is of a commonly found diffuse variety of very low gain maser, which lies in the direction of the SNR purely by chance. And even where the maser emission was confirmed, further puzzles arose. In the case of SNR 332.0+0.2, the velocity is large and if it represents the systemic velocity, then the implied distance is unexpectedly large. So perhaps the velocities can be significantly offset from the systemic velocity, a possibility that would reduce their value as distance indicators. Furthermore the location of the maser spots is slightly outside the shell as defined by the radio non-thermal emission; so how well does the radio shell delineate the outer shock front? These puzzles highlight the need to enlarge our sample of well studied objects, since generalisations drawn from only a few may be quite misleading.
Because the collisional excitation is believed to occur as a result of the SNR shock impinging on an adjacent or surrounding molecular cloud (Frail et al. 1996; Lockett et al. 1999), one expects to be able to explore this putative cloud by other means. For a few objects, this investigation has begun, and Jasmina Lazendic (SRCfTA) described her work to extend these investigations to more molecular species in a larger number of remnants, using mm radio observations. Studies of molecular hydrogen using IR observations can also be used in such studies, and work by Michael Burton (UNSW) with Jasmina Lazendic and others have revealed further unexpected phenomena. The Galactic object commonly known as the ``snake'' intersects a likely SNR shell almost at 90 degrees; at the point of intersection is a 1720-MHz maser, not in itself unexpected since the required shock conditions could well be fulfilled here. More surprising is the discovery of a molecular hydrogen outflow jet, apparently emanating from the intersection point. The difficulty of accounting for this perhaps indicates that this is a chance alignment without significance, and more study is clearly required.
Overall, the session was a lively reminder that this field is now making rapid and exciting progress after a 20 year dormant period while we waited for the appropriate investigative tools to be developed.
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