Supernova Remnants

NEW DISCOVERIES: SNR G321.3 -3.9/the Hidden SNR and PN Candidates StDrLu 1-7 & StDrLuLDû 1

I am very pleased to share the results of a project I have worked on over the past 18 months with Professor Robert Fesen from Dartmouth, Marcel Dreschler from Germany, and Xavier Strottner from France. During this time I took deep SHO images of the supernova remnant SNR G321.3 -3.9, which we refer to as the Hidden SNR. From this image we also made an incredible 8 new PN candidate discoveries: STROTTNER-DRECHSLER-LUDGATE (StDrLu) 1 through to 7 and STROTTNER-DRECHSLER-LUDGATE-LE Dû 1 (StDrLuLDû 1).

In June 2021, I was invited to be involved in a project on a potential new SNR located in Triangulum Australe. Marcel had detected a number of promising signs on SUMSS radio survey data and also on SHS H-alpha images suggesting a possible SNR. The next step was to obtain deep SHO imaging to both help confirm a SNR, and to identify suitable slit locations for future spectroscopic investigation with larger professional telescopes. Subsequently, I gathered over 100 hours of data during the end of 2021 and 2022 to capture this beautiful but extremely faint object.

Technical Details

Nikon 400mm f/2.8

ASI 6200mm

10Micron GM1000HPS

Chroma 5nm SHO + RGB



‍‍Dunedin, New Zealand

Closer crop of the Hidden SNR

SNR G321.3 -3.9

The Hidden SNR has been previously identified as a potential SNR candidate in galactic radio SNR survey literature, including the the Molonglo Survey of 2014 ( and the Parks 2.4 GHz radio survey ( However, it has never been confirmed as a SNR. This image is the first time the Hidden SNR has been detected in optical wavelengths, and provides strong supporting evidence that G321.3 is a true SNR.

Animation of the SUMSS radio shell and optical emission

The Hidden SNR has numerous OIII filaments, which form a well defined shell measuring 1.8 x 1.1 degrees. The OIII emission closely matches the radio morphology, which supports a SNR origin for the nebula. There are filaments to the North and West of the main SNR, which are located outside of the radio emission, and these likely represent large outbursts from the main SNR shell. In addition to the OIII emission, numerous Ha and SII filaments can be seen. These filaments have a high SII:Ha ratio demonstrating that this gas has been shock heated, further supporting a SNR origin.

Potential outbursts from the main SNR shell

Collecting further data on this SNR is challenging. The SNR filaments are very faint, and are quite diffuse. They are located in a dense star field and in a complex region with numerous molecular clouds and photo-ionised gas from the nearby Circinus OB association. As a result, larger scopes with a smaller FOV and much smaller pixel scales still require significant exposure times to detect the filaments. At the current time, high resolution spectroscopy with a 10 metre telescope using slit locations determined from this data is planned to confirm the SNR nature of the nebula.


In addition to the Hidden SNR, numerous other nebula were visible through the image field. Careful analysis of the image revealed a total of 8 new PN candidates, which have been successfully registered as STROTTNER-DRECHSLER-LUDGATE (StDrLu) 1-7 and STROTTNER-DRECHSLER-LUDGATE-LE Dû (StDrLuLDû) 1.

Annotated starless image showing Planetary Nebula candidates

Of special mention is the candidate StDrLu5. This fascinating candidate is located within the SNR shockwaves. It has characteristic inner OIII emission surrounded by some Ha emission. However there is also striking asymmetrical SII emission to the West of the candidate, which is the aspect is facing towards the expanding SNR shockwave. This raises the strong possibility that an interaction between the SNR shockwave and the PN candidate has occurred.

If the Hidden SNR is interacting with StDrLu5 then the SNR and PN must be located at the same distance. We identified the CSPN of StDrLu5 on DeCAM images, and this star has a g magnitude of 20.76 and a parallax of 0.712 milliarcseconds/year which equates a distance of 1.40 kpc. Given the Hidden SNR's angular size of 1.8 x 1.1 degrees, this translates into a physical size of ~ 44 x 27 pc, which is about the same size as the Cygnus Loop (Veil Nebula).



Many thanks to Prof Rob Fesen for his scientific support and for involving me in this project.A special thanks to Marcel Drechsler (Team StDr) for generously sharing his knowledge and processing expertise, and for his expert project organisation and assessment of potential candidates.

Thanks to Xavier Strottner (Team StDr) for his help in analysis and registering the new candidates.Thanks to Pascal Le Dû for his analysis and registration of the candidates.


The emission lines in this image are extremely faint, and have been processed using subtraction techniques with any small artefacts logically filled. For those interested, Vicent Peris has published an excellent overview of an implementation of the subtraction technique Noise reduction was performed using PS and Topaz DeNoise with careful attention to ensure no new detail was introduced by the NR process.

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