Heavy-metal subdwarf

An international team of astronomers led by Professor Simon Jeffery at the Armagh Observatory in Northern Ireland has discovered a small, very blue helium-rich, and hot star called UVO 0825+15, which has a surface extremely rich in lead and other heavy metals and varies in brightness by up to 1% every eleven hours. Only the fourth “heavy-metal subdwarf” discovered, and the second to be variable, the new star raises major questions about how these stars form and work.

The overwhelming majority of stars, like the Sun, consist mostly of hydrogen, from the surface through to the core, where nuclear reactions convert hydrogen into helium and keep the star shining for billions of years. When hydrogen in the core is used up, a star starts to burn helium. However, its surface remains almost unchanged, even in rare cases when the surface hydrogen has been stripped away to leave less than 1% of the original star. These stripped helium-burning stars are small, being about half the mass and one tenth the size of the Sun — but having surfaces five times hotter and 25 times brighter than the Sun, are known as “hot subdwarfs” (stars like the Sun are known as dwarfs, to distinguish them from the much larger giants).

The new discovery was made during a search for pulsating hot subdwarfs using NASA’s planet-hunting Kepler spacecraft. Serendipitously, the same star was also observed in a search for chemically peculiar stars using Japan’s 8-m Subaru telescope on Hawaii.

Apart from hydrogen, star surfaces usually consist of about 28% helium and up to 2% other elements (by mass), being mostly oxygen, nitrogen, carbon, and iron. The abundances of exotic heavy elements such as zirconium, strontium, or lead are so small as to be measured in parts per billion. UVO 0825+15 had already been identified as a helium-rich hot subdwarf, having two or three times more surface helium than the Sun, and tens of times more than normal hot subdwarfs.

The new Subaru observations of UVO 0825+15 revealed the signature of triply-ionized lead, implying that the star’s surface contains 5,000 times more lead than the Sun. Yttrium and germanium are also enriched by factors of several thousand. The chemistry is thought to be due to intense ultraviolet light, produced by the high surface temperature and brightness.

The surfaces of hot subdwarfs are quite unlike that of the Sun. In the latter, heat is transported upwards by convection and the surface layers are a bubbling cauldron of hot plasma rising, spreading, cooling, and sinking. These motions give rise to sunspots and flares which occur on timescales of months and days. In contrast, hot subdwarfs have no convection; plasma moves very, very slowly … being heavier than hydrogen, helium can sink out of sight on timescales of 100,000 years.

The ultraviolet light at the surface of UVO 0825+15 reacts with ionised atoms in the star’s atmosphere, producing a very small upward force that changes with temperature and from element to element. The result is a multi-layered atmosphere, with exotic ions concentrated into strata at precise temperatures by light pushing upwards. If an atom sinks beneath its equilibrium position, light pushes it back up; if it floats too high, light pressure weakens and it sinks back down. Thus we find a star where a thin layer of lead may be floating on light.

Observations of UVO 0825+15 made over 75 days with the Kepler spacecraft revealed unusual light variations. Most stars seem not to vary at all, while stars similar to the Sun vary almost imperceptibly by a few parts per million over a timescale of minutes. Stars which vary by a few percent or more are said to be pulsating. The period (or periods) of their pulsations can reveal much about their internal density and structure, through the techniques of asteroseismology. The periods in UVO 0825+15 are between 10 and 14 hours and are much longer than is believed possible for a star of this size. Other explanations, such as surface spots, or reflection from a companion body, cannot explain the complicated light curve. A solution is still wanting.

The atmospheres of most hot subdwarfs are extremely hydrogen-rich, although their interiors from just below the surface down are mostly helium, which is converted to carbon and oxygen during a lifetime of 100 million years or less. Less than 10% of hot subdwarfs have helium-rich surfaces. With UVO 0825+15, four now show extreme overabundance of heavy elements, including zirconium, strontium, germanium, yttrium, and lead up to 10,000 times more abundant than in the Sun. We call these the “heavy-metal” subdwarfs. These four stars present yet another puzzle. The Sun is a well-behaved star which travels in a nearly circular orbit around the Galactic center, along with most other stars in the Galactic disk. In contrast, the heavy-metal subdwarfs have highly elliptical, high-velocity orbits. This implies that they are either very old, or have been given a kick. This might be a clue as to how, where, and when they were made, but we are still a long way from solving the puzzle.

The discovery of the fourth heavy-metal subdwarf is very exciting, especially because it is variable. It will help explore an important stage in the life-cycle of these very rare stars. But UVO 0825+15 has left astronomers scratching their heads. The surface chemistry – cloud layers of lead and yttrium – is amazing. But there is no clue to what causes the light variations, or why they seem to be so old.

Featured image credit: The author’s impression of the surface of UVO 0825+15, showing a very hot star covered by thin clouds of exotic elements, including lead. Image by Simon Jeffery. Used with permission.