Object 9: HD 214714
Podcast release date: 02 December 2019
Right ascension: 22:39:34.3
Corresponding Earth location: About one-third of the distance from the Azores Islands to Portugal
HD 214714 is a peculiar star. That's one of its official classifications. Lying around 1000 light years or 330 pc away in the constellation Lacerta [1,2], the star doesn't look particularly peculiar to the eye, or even on photographs. At around 5500 K , it is close to temperature of the Sun, so shines like any other brilliant yellow star. It is about 300 times brighter than the Sun, but that isn't unusual; many other giant stars are brighter than this. But not many stars have this combination of temperature and luminosity. What makes it really peculiar though is its spectrum, which belies the composition of its atmosphere. It's a CH star, meaning it has an unusually high fraction of carbon . But, unlike conventional carbon stars, this carbon wasn't formed in the star. It comes from its dead twin. To find out why, we need to look at how binary stars interact, and what happens when a star dies.
Stars don't die quietly. It is only a star's radiation pressure that stops it collapsing under the force of gravity so, as stars burn their hydrogen fuel into denser ash, they must burn it faster and faster to sustain themselves. This makes them brighter and swollen, and they expand to become red giant stars. In massive stars, gravity wins. They simply run out of fuel and collapse, creating a supernova explosion. In less-massive stars, radiation pressure wins, and the atmosphere is blown off into space as stellar winds, forming a planetary nebula and exposing the star's dying core as a white dwarf.
In these less-massive stars, anything below about eight times the mass of the Sun, the star's thermonuclear engine doesn't cut out smoothly, but coughs and splutters as it is forced to burn its final, ash-laden fuel. Each cough is effectively a huge nuclear explosion, which can output in a second the energy as much energy as the Sun produces in a year. This explosion is mostly contained within the star, but it mixes the interior of the star together, bringing nuclear fallout from the core of the star to the surface. For stars below about five times the mass of the Sun, that nuclear debris is mostly carbon. After hydrogen, helium and oxygen, carbon is the most abundant element in the Universe. Like oxygen, it can bond to a wide variety of other elements, and if enough carbon is dredged up from the stellar core, it will turn the oxygen-based chemistry into a carbon-based chemistry, and a carbon star will be formed.
HD 214714 will undergo this process in the future, but it is not quite there yet. It is not yet bright enough to be undergoing this phase. It is also remarkably warm for a star of this brightness, so we know it is taking an unusual evolutionary path. The easiest way to explain this is if HD 214714 is not alone .
When a giant star expands, anything that is orbiting it is swallowed up. When the Sun becomes a red giant, it will swallow Mercury and Venus whole. We're still not sure whether the Earth will survive this process or not. However, if the star is being orbited by another star, it can't simply swallow it whole without getting some seriously stellar indigestion. Two stars can indeed merge through this process, but what normally happens is that the tides of the stars keep them apart, allowing them to keep orbiting each other, even if that means one star exists partly inside the other. This is known as a common envelope system, where two stars are physically touching each other and sharing their atmosphere, and it happens more commonly than you might think. So it could be that HD 214714 was once orbited by a more-massive star that shared its envelope with HD 214714 before dying and becoming a white dwarf companion to the star (that would now be invisible). But the carbon-laden winds of dying stars can also be funnelled onto orbiting companions through the companion's gravity, so it's possible that HD 214714 simply went around hoovering up the wind emitted by its twin as it died.
Whatever the mechanism by which its twin enriched HD 214714, it left an atmosphere with a peculiar composition. Like other CH stars, HD 214714 is rich in heavy elements too . These include many metals that are rare on Earth, including yttrium, praseodynium, neodynium and samarium. These are slow-neutron-capture-process elements, or s-process elements for short. They're formed when a light element, in this case carbon-13, gets bombarded with high-velocity neutrons. These get stuck in carbon nuclei in an unstable fashion, and decay into protons. As astrophysicists, we're taught that elements heavier than iron form in supernovae, but this only produces about half of elements heavier than iron - the so-called rapid- or r-process elements. The s-process elements, the other half, form during this thermonuclear spluttering during the death of intermediate-mass stars. The pollution that HD 214714 received from its dead twin preserves this chemical signature, allowing us to see it today in all its peculiar glory.
 Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1
 Gaia Collaboration et al., Gaia Data Release 2. Summary of the contents and survey properties, 2018, Astronomy & Astrophysics, 616, A1
 Karinkuzhi, Drisya and Goswami, Aruna, Chemical analysis of CH stars - I. Atmospheric parameters and elemental abundances, 2014, Monthly Notices of the Royal Astronomical Society, 440, 1095