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Astronomers find massive supernova remnant closer than previously thought

An international team of astronomers have investigated a large Galactic supernova remnant designated G278.94+1.35. Results of the study, Dec. 30 on the pre-print server arXiv, shed more light on the properties of this remnant.

Supernova remnants (SNRs) are diffuse, expanding structures resulting from a supernova explosion. They contain ejected material expanding from the explosion and other interstellar material that has been swept up by the passage of the shockwave from the exploded star.

G278.94+1.35 is a supernova remnant in the Milky Way, discovered in 1988. It has an estimated linear diameter of about 320 light years and its age is assumed to be about 1 million years. The distance to G278.94+1.35 is estimated to be some 8,800 light years.

Recently, a group of astronomers led by Miroslav D. Filipović of the Western Sydney University in Australia, has conducted observations with the Australian Square Kilometer Array Pathfinder (ASKAP) as part of the ASKAP-Evolutionary Map of the Universe (EMU) project. During these observations they serendipitously found a large-scale object which was identified as G278.94+1.35.

According to the paper, ASKAP-EMU images show that G278.94+1.35 has an angular size of, at most, 200 by 194 arcminutes, which corresponds to a diameter of 512 by 495.5 light years. However, further inspection found that G278.94+1.35 is located at a much closer distance than previously thought—about 3,300 light years away. This suggests that its physical size is smaller, most likely 189 x 182 light years.

Filipović's team named G278.94+1.35 "Diprotodon," after an extinct genus of Australian marsupial from the Pleistocene period, resembling a giant wombat.

"We adopt this name for G278.94+1.35 to raise awareness of this extinct Australian animal and all of Australia's prehistorical megafauna, as well as to raise awareness of the current extinction rate of several other species in Australia. As G278.94+1.35 is potentially among the largest SNRs, it is fitting to liken it to this largest of wombats, Diprotodon," the astronomers explained.

Furthermore, the study found that Diprotodon is in the radiative evolutionary phase. Its almost circular shape observed across such a large field of view suggests that the remnant is still expanding.

Based on the collected data, the authors of the paper estimate that the progenitor mass of Diprotodon is about 15 solar masses and the SNR kinetic energy is at a level of 500 quindecillion erg. The spectral index of the remnant was measured to be approximately -0.55, which is representative and typical for the average shell-type SNRs observed in the Milky Way.