Further work

Some of the most interesting discoveries made will be from work further to and in parallel with the H survey. In relation to supernova remnants this will involve spectroscopy, radio observations and optical imaging.

One goal of this project is to discover new supernova remnants, and so we need methods to confirm objects belong to this class. The spectra of supernova remnants are dominated by emission lines. Hydrogen lines are nearly always seen, along with forbidden lines produced by a wide variety of other elements (Fesen & Hurford 1996). One such pair often observed are the 6717Å, 6731Å [SII] lines. If these are observed to be strong in comparison with the H line, the object is likely to be a supernova remnant. This method works best with evolved SNR, which are probably the most common. Lower values of the H/[SII] ratio can be produced by SNR, HII regions and planetary nebula; with Balmer-dominated and oxygen-rich SNR the [SII] lines are very weak or absent (Fesen et al. 1985, Weiler & Sramek 1988). Therefore spectral data will often need to be used with other information to establish an object's identity.

For example, by comparing radio maps with IRAS Sky Survey Atlas images (Beichman et al. 1985), or using multi-frequency radio observations, it is possible to determine the thermal or non-thermal nature of objects. The Molongolo Observatory Synthesis Telescope (MOST) has been very successful using IRAS data to identify supernova remnants. Its new wide-field survey runs over the same timespan as the H survey, allowing discoveries made in one to be followed up using the other.

In the near future we will be concentrating on completing a comprehensive examination of H films produced by the survey. This will be in conjunction with spectroscopy and radio observations to confirm the identification of new supernova remnants. Eventually we will be in a position to produce a catalogue of Galactic optical supernova remnants and candidate objects, bringing together information which is already available from a wide range of sources. This will help in deciding possible directions for future research.

Optical identifications of supernova remnants allow us to study their properties in much greater detail than otherwise possible. Here we list some of the main areas in which the efforts of researchers may be directed.

Optical emission lines have been used in a variety of ways to study the physical and chemical properties of supernova remnants. Simple line-ratio diagnostics can be used to estimate properties such as temperature, pressure, electron density and shock velocity. Velocity measurements may also allow the determination of age and distance. Computer modelling has also been used to study the chemical makeup of observed regions.

There is also a lot of future work in investigating the morphology of new supernova remnants, which have been observed to come in a wide variety of types. Higher resolution work will allow the association between optical and radio features to be studied, as well as possible new pulsar/remnant associations to be discovered. CO and other molecular lines may also be used to find new cases of supernova remnant triggered star formation.

CCD imaging will have less detail but will detect features fainter than visible in the H survey. Imaging in other lines such as [SII] and various oxygen lines, will show the different conditions present across a remnant, and the differences in structure between remnants.

© Copyright Astronomical Society of Australia 1997