Woodward part of team that makes first-ever study of extragalactic recurrent nova

Unprecedented study yields some surprises and the hottest ever recorded temperature for a nova

Professor Charles, “Chick” Woodward of the School of Physics and Astronomy is a part of a group that published a paper that used near-infrared spectroscopy to make the first-ever study of a recurrent nova outside the Milky Way Galaxy. Most novae that have been recorded only ever erupt once, but a fraction have been observed to erupt more than once and are classed “recurrent” novae. Only a dozen of these have been observed within the Milky Way, with the majority of them being observed outside of our galaxy. Astronomers hope to gain a better understanding of the phenomenon of recurrent novae by studying these extragalactic events.

The group turned to the first recurrent extragalactic nova to be observed, LMC 1968-12a (LMC68), located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. This nova system consists of a white dwarf star and a companion red giant star (a star several times larger than the Sun). Eruptions of nova LMC68 are  fairly predictable with an event every four years, and astronomers captured its most recent eruption in August, 2024 at the Neil Gehrels Swift Observatory, which closely monitors this nova. Follow up observations were done by the Magellan Baade telescope and the Gemini South telescope at observatories in South America.

The team focused their observation on a study of the near region of infrared light, which has a wavelength longward of visible light, using spectroscopic techniques frequently used to study the chemical composition of different atmospheres. The team's goal was to record the dynamic changes of LMC68 getting a clearer picture of the nova’s ultra hot phase as the ejecta rapidly expanded from the white dwarf star surface where the thermonuclear explosion occurred. During the most extreme processes at play in the eruption, the light intensity emitted by all the silicon atoms in the hot ejecta (which were stripped of 9 out of their 14 total electrons, a process which requires an incredible amount of energy via radiation or violent collisions) alone was 95 times brighter that all the light emitted by the Sun across all of its wavelengths.

More surprising than the brightness was the fact that the silicon completely dominated the signal, and they did not find the expected signatures of highly energized sulfur, phosphorus, calcium and aluminum, which they would have expected even in a very violent nova eruption. One explanation for this surprising absence may be due to an unusually high gas temperature in the early-post-explosion phase of LMC68, which might be in excess of 5.4 million degrees Fahrenheit. The LMC has a lower metallicity than the Milky Way, meaning that the abundance elements heavier than hydrogen and helium are rarer there. In nova that happen in our galaxy (a higher metallicity system), it is typical for the heavy metals to trap heat on the surface of the white dwarf, causing faster eruptions as material builds on the surface of the white dwarf star. Without these metals, the more gas donated by the white dwarf's companion star builds up for a much longer time, before ignition of the explosion yields a more violent explosion.

The temperature recorded was 3 million degrees Celsius (5.4 million degrees Fahrenheit), the hottest ever recorded.