Spectacular explosions keep occurring in the binary star system named RS Ophiuchi. Every 20 years or … [+]
More than a year after its latest outburst, teams of global astronomers have completed the most extensive mapping sequence of an active stellar nova ever achieved. Actually, a binary star system, the ongoing aftermath of RS Ophiuchi’s (RS Oph) August 2021 thermonuclear outburst has been exhaustively observed from the radio spectrum all the way up to very high energy gamma rays.
Usually, hundreds of times too faint to see with the naked eye, RS Oph is a rare example of a recurrent nova, only a handful of which are known in the Milky Way, says the American Astronomical Society (AAS).
Not to be confused with more destructive supernovae, novae outbursts occur when a white dwarf stellar remnant in a binary system steals enough material from the atmosphere of its puffed-up red giant companion to ignite a brief flash of nuclear fusion on its surface, the AAS notes. This, in turn, releases a thousand times the Sun’s yearly energy output in just a few days.
In the case of RS Oph, its binary components —- the stellar remnant white dwarf core and the red giant —- are only separated by 1.48 astronomical units (or roughly one and a half times the distance from the Earth to our Sun). That’s sufficiently close for the white dwarf to continually accrete material from the surface of its expanding red giant companion.
RS Oph is known as a recurrent nova because it has continually been observed to flare up to a thousand times brighter than normal on timescales of roughly a decade or two. Located some 5000 light years away in the constellation of Ophiuchus, its first recorded explosion was in 1898. The two most recent outbursts took place in 2006 and 2021.
“Over a year after the 2021 explosion, we’re still following the expansion of material that was blasted out of this star,” Tim O’Brien, associate director of the U.K.’s Jodrell Bank Observatory, recently told me at the observatory’s historic Lovell Telescope.
O’Brien and colleagues are still using the 250-foot Lovell radio telescope to observe radio emissions coming from the aftermath of RS Oph’s August 2021 thermonuclear outburst.
We’ve been using our telescope here, and a network of other telescopes across the U.K. and Europe, to make radio images of material from the outburst expanding outwards, says O’Brien, an astrophysicist at the nearby University of Manchester. We’re able to see this stuff plowing through the wind of the red giant, accelerating particles to nearly lightspeed, he says. This produces copious amounts of radio emission that appear to be shooting out in opposite directions above and below the plane of this binary star system, says O’Brien.
How does this continual buildup of gas on the surface of the white dwarf create nuclear fusion?
Material from the red giant falls onto this earth-sized white dwarf, says O’Brien. And about every 20 years or so, that material gets thick enough, heavy enough, dense enough, and hot enough to start nuclear reactions, he says. O’Brien likens it to covering Earth’s entire surface in hydrogen bombs and setting them all off at once.
Our own star is an almost perfect balance of gravity and thermonuclear fusion. But imagine living next to one of these binary cataclysmic variables. Although most recurrent novae erupt on timeframes of 10 to 100 years, their white dwarf components are so hyperdense, that even these massive thermonuclear surface outbursts can’t destroy them.
But eventually, these odd binary combinations of dying red giants and stellar remnant white dwarfs will end their lives as Type 1a supernovae.
The Lovell Radio Telescope at Jodrell Bank, Macclesfield, England. (Photo by: Loop Images/Universal … [+]
There’s a very interesting recurrent nova over in the Andromeda Galaxy that explodes every Autumn, plus or minus a month or two, says O’Brien. Because it explodes every year, it’s probably close to the Chandrasekhar limit, he says. That is, the maximum mass of a stable white dwarf. Above this 1.4 solar mass limit, carbon fusion is triggered within the white dwarf’s core creating a rare Type 1a supernova.
O’Brien is interested in these recurrent novae in part because he’s intrigued by the idea that a thousand years ago, early sky watchers were historically “observing the same systems.” But today, with modern technology, we are learning about what these objects actually are, he says.
O’Brien notes that by looking at the way their light changes on their runup to an explosion, we’re possibly getting closer to even predicting when one of these novae will explode.
