Dying with style: merging white dwarfs can do it too

Author:

Miguel Santander-García holds a PhD in astrophysics (Universidad de La Laguna). After several years working as a support astronomer for the Isaac Newton Group of Telescopes in La Palma, he is currently a postdoctorate fellow at the Observatorio Astronómico Nacional in Madrid. His research focuses on the late stages of evolution of low- and intermediate-mass stars, namely post-Asymptotic Giant Branch stars and planetary nebulae.

Sad as it may be, every star must die. At some point during its life it stops burning nuclear elements to sustain itself via radiation pressure, and collapses. Its ultimate fate depends on its mass; most of them are not massive enough to produce a supernova by themselves, but they die in an equally spectacular and far less-known way: by ejecting and shaping their outer envelope into a beautiful cloud of gas known as a planetary nebula.

The inert core of a planetary nebula is known as a white dwarf. The surface of these stars gradually cools down from temperatures as high as 100,000 K, producing enough UV radiation to ionise their surrounding nebula, making it visible for around ten thousand years, until it is diluted into the interstellar medium.

No fusion reactions take place at the heart of a white dwarf. The only thing able to provide an outward force large enough to balance the star’s own huge weight star’s is the pressure of their electrons when trying to avoid, as the Pauli principle dictates, to occupy the same quantum state. In a white dwarf, this pressure is enough to prevent it from collapsing further into a singularity, although their density is enormous: when our Sun dies, the size of the resulting white dwarf will be that of Earth.

Now, some of these degenerate, white dwarf stars happen to have a close companion star. And some of these are so close that the tidal forces from the white dwarf distort the companion, making it fill its Roche lobe, a tear-shaped surface inside which matter is gravitationally bound to the star. Any material lingering around the Roche Lobe’s tip facing the white dwarf will spiral out and eventually fall onto it. The white dwarf is usually able to burn this material via thermonuclear burning, but a disaster might occur if the balance is broken and enough gas from the companion is accreted. For if the mass of the white dwarf surpasses the Chandrasekhar limit (1.4 times the mass of our Sun), the star quickly collapses and explodes as the brightest of supernovae, known as type Ia.

Supernova Ia are vital to our understanding of how the Universe works. Thanks to the regularity of its intrinsic brightness, they are perfect standard candles for reliably measuring distances at the extragalactic step of the cosmic ladder. In 1998, the accuracy attained by this method combined with systematic observations resulted in the discovery that the expansion of the Universe is actually accelerating due to some unknown physical mechanism which, in an appropriate reflection of our utter ignorance, we have called dark energy¹.

Despite their importance, we still don’t know much about supernovae Ia. In fact, there is an ongoing debate on the evolutionary paths stars can take in order to produce one. Apart from the scenario depicted above —known as single-degenerate scenario—, some theoreticians have proposed an alternate scenario in which not one but the two stars of a binary system would be degenerate cores, that is, white dwarfs. In this double-degenerate scenario, if the stars orbit each other close enough, they will spiral in towards their companion due to the emission of gravitational waves, as predicted by Einstein’s General Relativity. If the combined mass of the resulting merger is larger than the Chandrasekhar limit, a supernova Ia should happen.

There were some indications which suggested this might be the case for some systems like supernova remnant 0509-67.5, which can’t be explained by the single-degenerate scenario². However, no clear evidence existed beyond three systems in which the combined mass of the two white dwarfs might be, within uncertainties, slightly above the Chandrasekhar limit³. A work published today by Nature provides the first confirmation of such a system: the core of the planetary nebula Henize 2-428⁴.

Video: Artist’s impression of the likely evolution of the planetary nebula Henize 2-428. In 700 million years from now, the merging of its two white dwarfs will produce a type Ia supernova. | Credit: ESO/L. Calçada

The authors, an international group of researchers based in Spain, Chile and South Africa, were actually trying to answer a completely unrelated question: how stars, essentially round objects, manage themselves to eject a planetary nebula which is, most often, nowhere round but exhibit instead a clear axial symmetry. According to an hypothesis with growing support, angular momentum from an orbiting close companion star would help eject the nebula largely in a highly anisotropic way, shaping the planetary nebula as bipolar cloud of gas in expansion. Should that be the case, the equator of the nebula would be coincident with the orbital plane of the system.

Photometric monitoring of this system in 2009 revealed a periodic light variation compatible with a companion star orbiting the white dwarf every 4.2 hours. The shape of the light curve resulting from the periodic, orbital brightness variation showed the two stars were similarly distorted by their companion, suggesting they were both white dwarfs. More photometric and spectral data was gathered in the following years using different facilities which included world largest-class telescopes such as GTC (Gran Telescopio de Canarias) at La Palma, Spain, and VLT (Very Large Telescope) at Paranal, Chile.

The analysis of the periodic Doppler-shifts of characteristic Helium spectral lines from the atmosphere of the stars, together with the information from the light curve, allowed the determination of the orientation of the orbit. This resulted to be coincident, within a few degrees, with the equator of the planetary nebula, providing additional evidence to the binary hypothesis.

More strikingly, the two stars have almost the mass of our Sun, taking the total mass of the system to 1.8 solar masses, well beyond the Chandrasekhar limit even when considering measurement uncertainties. Also, the stars are close enough to each other so as to end up merging in some future time, and subsequently exploding as a deadly supernova Ia.

All in all, if a star can die with style, there is no doubt that Henize 2-428 is one of the most stylish ever: this system has already undergone two deaths as beautiful planetary nebulae —one of which unfortunately we missed, busy as we were inventing stone tools and striving to survive-. Yet it will die again some 700 million years from now, outshining the whole Galaxy when the two white dwarfs merge. This final death will be much harder to miss: the dying star will be visible with the naked eye and in broad daylight. So, in the improbable event that we are still around, be prepared for the show; we haven’t gotten many occasions to witness such a stylish death.