Astronomers observe ultra-hot nova with unexpected chemistry
Using the Gemini South telescope, one half of the International Gemini Observatory, astronomers have for the first time observed a recurring nova outside of the Milky Way in near-infrared light. The data revealed highly unusual chemical emissions as well as one of the hottest temperatures ever reported for a nova, both indicative of an extremely violent eruption. Image courtesy of the NSF NOIRLab
A team of astronomers, including Arizona State University Regents Professor Sumner Starrfield, has uncovered an exceptionally hot and violent eruption through the first-ever near-infrared analysis of an extragalactic recurrent nova.
The results of their study have recently been published in the Monthly Notices of the Royal Astronomical Society.
Nova explosions occur in binary star systems where a white dwarf — a dense remnant of a deceased star — continuously accretes stellar material from a nearby companion star. As the companion's outer atmosphere accumulates on the white dwarf's surface, temperatures rise to a critical threshold, triggering an explosive eruption.
Most observed novae erupt only once, but a subset, classified as recurrent novae, undergo multiple eruptions over varying timescales, ranging from one year to several decades.
Fewer than a dozen recurrent novae have been identified within the Milky Way, whereas significantly more exist in extragalactic environments. Studying these extragalactic novae provides astronomers with critical insights into how different astrophysical conditions influence nova eruptions.
The first extragalactic recurrent nova, LMC 1968-12a (LMC68), was identified in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. With a recurrence interval of approximately four years — the third-shortest recorded — LMC68 consists of a white dwarf paired with a red subgiant companion star. Since its discovery in 1968, astronomers have observed its eruptions with increasing regularity, particularly since 1990.
The most recent eruption of LMC68 occurred in August 2024, initially detected by the Neil Gehrels Swift Observatory, which has conducted monthly monitoring since the nova's 2020 eruption. Anticipating this event based on LMC68's established recurrence pattern, astronomers captured timely observations.
Nine days after the initial outburst, follow-up studies were conducted using the Carnegie Institution's Magellan Baade Telescope, with additional observations performed 22 days later using Gemini South telescope — part of the International Gemini Observatory, funded in part by the U.S. National Science Foundation and operated by NSF NOIRLab.
Employing spectroscopic techniques, researchers analyzed LMC68's near-infrared light to study the nova's ultra-hot phase, characterized by the extreme excitation of various elements. This investigation represents the first-ever near-infrared spectroscopic study of an extragalactic recurrent nova.
Following its eruption, LMC68's luminosity declined rapidly; however, the FLAMINGOS-2 instrument on Gemini South detected a strong emission signal from ionized silicon, specifically from atoms that had lost nine of their 14 electrons — a process requiring substantial energy input via radiation or high-velocity collisions.
Earlier spectroscopic data from Magellan revealed that ionized silicon's near-infrared emission was 95 times more intense than the sun's total energy output across all wavelengths (X-ray, ultraviolet, visible, infrared and radio). Although this emission diminished in subsequent Gemini observations, silicon remained the dominant spectral feature.
"This surprising absence, combined with the presence and great strength of the silicon signature, implied an unusually high gas temperature, which our modeling confirmed," said Starrfield, a professor at ASU’s School of Earth and Space Exploration.
“The ionized silicon shining at almost 100 times brighter than the sun is unprecedented,” said Tom Geballe, NOIRLab emeritus astronomer. “And while this signal is shocking, it’s also shocking what’s not there.”
Unlike Milky Way novae, which typically exhibit multiple near-infrared signatures from highly excited elements, LMC68's spectrum displayed only ionized silicon.
The research team estimates that in the early post-explosion phase, LMC68's expelled gas reached temperatures of approximately 3 million degrees Celsius (5.4 million degrees Fahrenheit), making it one of the hottest novae on record. This extreme temperature suggests an exceptionally violent eruption, likely influenced by the nova's astrophysical environment.
The Large Magellanic Cloud exhibits lower metallicity than the Milky Way, meaning it contains fewer elements heavier than hydrogen and helium. In high-metallicity environments, heavy elements trap heat on the white dwarf's surface, triggering eruptions earlier in the accretion process. However, more significant amounts of material accumulate before ignition in low-metallicity conditions, resulting in more explosive outbursts. Additionally, interactions between the expelled gas and the companion red subgiant's atmosphere likely generated powerful shocks, further elevating temperatures.
Before gathering observational data, Starrfield hypothesized that low-metallicity accretion onto a white dwarf would produce more violent nova explosions. This study's findings align closely with that prediction.
“With only a small number of recurrent novae detected within our own galaxy, understanding of these objects has progressed episodically,” said Martin Still, NSF program director for the International Gemini Observatory. “By broadening our range to other galaxies using the largest astronomical telescopes available, like Gemini South, astronomers will increase the rate of progress and critically measure the behavior of these objects in different chemical environments.”
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