Astronomers have a new explanation for the strange behavior of the double star system V Sagittae, which has puzzled them for more than a century. The discovery suggests that Earthlings could soon be treated to two separate shows, once when the system’s white dwarf undergoes a nova explosion, and then again when it becomes a supernova. Regrettably, however, “soon” in this case is measured on astronomical scales, so we could be talking decades or even centuries, not months.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.
At first sight, V Sagittae looks like a classic example of the sort of system that gives us novae. A white dwarf and a main-sequence star are locked in a tight orbit. Material is being pulled off the main sequence star by the white dwarf’s intense gravity to form an accretion disk, which gradually spirals down to the dwarf.
In a standard nova, when material reaches the heat and pressure of the white dwarf’s surface, fusion is ignited, causing the white dwarf to suddenly become hundreds or thousands of times brighter. Some novae do this once and have not been seen again on a timescale of centuries. Others appear more regularly, although, as the case of T Coronae Borealis has shown, it’s best not to place too much reliance on their reappearance on schedule.
However, V Sagittae is an anomaly: it has sustained an unusual brightness rather than a brief explosion, but Dr Pasi Hakala of the University of Turku led a team that thinks they have it figured out.
“V Sagittae is no ordinary star system – it’s the brightest of its kind and has baffled experts since it was first discovered in 1902,” Dr Phil Charles of the University of Southampton said in a statement. Although intensely luminous, at 10,000 light-years away, V Sagittae is too faint to be seen with binoculars, let alone the naked eye, varying between 10th and 13th magnitude.
We have 123 years of observations of V Sagittae, and others have noted that the rate at which material is being transferred appears to be increasing exponentially. Based on this, one team concluded the two would merge late this century, becoming brighter than Sirius, despite being more than a thousand times further away. Although the brightness would more closely resemble a supernova than an ordinary nova, that study concluded there would be no explosion; instead, a new star would form composed of a white dwarf core surrounded by fusing hydrogen.
However, Charles and co-authors have reached a different conclusion. They observed the system with the Very Large Telescope in Chile and detected a ring of gas surrounding the pair of stars. They think the system is among the brightest sources in the galaxy of “superpsoft X-rays”, those closest to the ultraviolet part of the spectrum. “The white dwarf cannot consume all the mass being transferred from its hot star twin, so it creates this bright cosmic ring,” Hakala said.
The authors think this ring is generating extreme winds that explain some of the features astronomers have previously struggled to explain. They also argue the ring’s brightness has led others to overestimate the mass of the system, with the two stars having combined masses of no more than 2.1 times the Sun, less than half some previous estimates.
Some time in the next few years, they think, enough material will accumulate on the white dwarf that its already unusual brightness will increase much further, making the system visible to the naked eye. Such events excite both professional and amateur astronomers – thus the numerous articles pinning hopes on T Coronae Borealis – but they’re not super rare. Just this year, two were briefly visible at once, albeit to those with excellent eyesight away from light pollution.
That, however, is just a foretaste of what the team thinks is the system’s ultimate fate. “When the two stars finally smash into each other and explode, this would be a supernova explosion so bright it’ll be visible from Earth even in the daytime,” said Dr Rodríguez-Gil of Spain’s Instituto de Astrofisica de Canarias. Although at this distance such an event will not be as impressive as Betelgeuse going supernova, it would at least end the four-century-long wait for a supernova in our own galaxy.
The study is open access in the Monthly Notices of the Royal Astronomical Society.
