BOW TIE IN THE SKY: The rapidly expanding shell of V445 Puppis marks a violent explosion that likely arose from a white dwarf accreting material from a nearby helium star. The image above shows the ejected shell from the nova as it appeared in 2005. Image: ESO/P.A. Woudt
A stellar explosion known as a nova that was detected in 2000 formed a two-lobed shell of material ejected from the star. Shaped like a bow tie, it continues to swell at great velocity. But, curiously, the coat of ejecta flowing outward from the star lacks hydrogen, the most common gas in the universe.
Such a nova had never been observed before, says Danny Steeghs, an astrophysicist at the University of Warwick in England. The object, known as V445 Puppis, is known as a helium nova after the gas that dominates its makeup in the absence of hydrogen.
Helium novae are thought to arise from a binary star system in which one member is a hydrogen-depleted star, which relies mostly on helium to generate light, and one is a white dwarf, an ultradense remnant of an exhausted star. The white dwarf steadily accretes helium from its neighbor until it reaches a density and temperature sufficient to trigger a nuclear explosion. If the white dwarf grows large enough to exceed the critical so-called Chandrasekhar limit—roughly 1.4 times the mass of the sun—it will ignite in a catastrophic nuclear blast known as a type Ia supernova. But if the explosion is localized to a compressed shell of accreted helium on a somewhat smaller white dwarf, a helium nova is the result, and the dwarf survives. Such novae had been predicted to occur but had never been seen before the 2000 V445 Puppis detonation.
In the November 20 issue of the Astrophysical Journal, Steeghs and his colleagues, led by astronomer Patrick Woudt of the University of Cape Town in South Africa, describe the rapidly expanding shell of the helium nova V445 Puppis and what it indicates about the progenitor system. Drawing on years of ground-based observations, Steeghs and his co-authors estimate that V445 Puppis is some 27,000 light-years distant. The lobes of the shell, the authors conclude from their campaign, are moving at more than 6,000 kilometers per second, with knots at the end of each lobe zooming outward even faster.
One consideration making helium novae more than just an astronomical curiosity is the suggestion that they might serve to explain anomalously young binary star systems that yield type Ia supernovae. That class of explosions is hotly studied, because as so-called standard candles, type Ia supernovae form the basis of cosmological distance measurements, although their underlying mechanisms are not well understood.
The white dwarf in V445 Puppis has been estimated to be close to the 1.4–solar mass limit, the point at which a white dwarf is thought to explode in a type Ia supernova. So assuming the white dwarf continues to feed on its helium-rich neighbor, and that it did not eject too much mass in the nova outburst, it might one day become a type Ia. "While we can't guarantee it, all the ingredients are there in the V445 Puppis system," Steeghs says. "You are basically loading up a white dwarf in this binary system, and it's receiving material at a rather large rate from this helium star."
University of Oklahoma astrophysicist David Branch calls the observations of the nova "exquisite" but stops short of calling the binary a compelling candidate for a type Ia precursor. The rate of mass transfer in the system is unclear, so it is not known how the binary will evolve. But the new study provides "significant observational support" for the model in which a helium star, bound in an orbital pair with a white dwarf, could indeed lend the compact white dwarf enough matter to initiate an explosion.
If V445 Puppis is on the path to supernova, just how long that path will be is an open question, Steeghs acknowledges. The problem in solidly pinning down the system's future is that astronomers have yet to take a clear look into the heart of the nova, where its stellar progenitors reside. The stars remain clouded by a haze of obscuring dust. "We need to wait for that to become visible when the shell thins," Steeghs says. "If we knew the mass of the white dwarf and the mass of the star next to it, and its orbital period, then we could sort of forward calculate how long it would take in that configuration to explode. But at this point we don't know that number."
It may be a few years yet before the binary system in V445 Puppis becomes visible. Ordinary novae containing hydrogen usually open up to view in a year or two, Steeghs notes. "It's been nine years, and we still can't see the binary whatsoever," he says. "Because it's the first time we've seen a helium nova, there is no real benchmark. I guess we should maybe not be surprised that we don't quite know how long it will take and what it's doing."