An international team of astrophysicists, led by researchers from the Université de Montréal, has identified a star on which a sodium-rich asteroid has crashed, drastically transforming its apparent color. These results raise many questions about the origin of this strange object. The research that led to this discovery was directed by Simon Blouin and Patrick Dufour of the Center for Research in Astrophysics of Quebec (CRAQ) and appears in the latest issue of The Astrophysical Journal.
The star that received this massive amount of sodium is a white dwarf star known as WD J2356-209. A white dwarf consists of the bare core of a star that has exhausted all its nuclear fuel. Since there are no more nuclear reactions in its core, this type of star is doomed to cool forever. Therefore, as WD J2356-209 is a very cold white dwarf, it is also very old.
White dwarfs are very compact objects – typically half the mass of the Sun compressed into a volume comparable to that of the Earth. As a result, the gravitational field at their surface is 100,000 times stronger than on Earth. Under these conditions, the lightest chemicals, such as hydrogen and helium, float to the surface while the heavier elements sink towards the center. Most white dwarfs are thus surrounded by a thin layer exclusively composed of hydrogen or helium. However, there are exceptions. For example, if some of them accrete rocky material – following the impact of comets, asteroids, or dwarf planets – heavy elements such as calcium, iron, magnesium, or sodium can be briefly detected at their surface. The presence of these additional elements affects the spectrum of the white dwarf, thus making it possible to accurately measure the chemical composition of the rocky body that crashed on its surface.
Almost 20 years ago, WD J2356-209 was observed using the 10-meter telescope at the W. M. Keck observatory in Hawaii. At that time, it was already obvious that there was something off with this star: its apparent color was abnormal. Normally, a white dwarf as cool as WD J2356-209 (about 4,000 K) should appear orange. However, it seems rather bluish, which is usually the signature of a very hot star. For example, the star Betelgeuse, in the Orion constellation, appears red-orange with a surface temperature of about 3500 K, while the surface of the star Rigel, in the same constellation, is warmer, at about 12 000 K, and appears bluish-white.
Several years of work were needed to develop appropriate theoretical models, simulating the physical conditions on the surface of cold white dwarfs, before solving this puzzle. It is within the framework of his doctoral thesis that Simon Blouin developed these models, finally allowing researchers to reveal the secrets of WD J2356-209.
The strange color of WD J2356-209 comes from the presence of a never seen amount of sodium on its surface. Under the conditions found at the surface of this old and cold white dwarf, sodium absorbs the orange light so efficiently that WD J2356-209 seems bluish. By analogy, we can observe the opposite phenomenon every night in our streets, where sodium streetlights produce an orange light. If WD J2356-029 had been younger and thus hotter, the presence of sodium would not have caused the surface to turn blue.
This is the first time such a chemical composition has been observed among the hundreds of white dwarfs that are known to have accreted rocky material. The total amount of sodium scattered on the surface of WD J2356-209 would be enough to fill all the Great Lakes to the brim! Note however that this is sodium and not table salt (i.e. sodium chloride – NaCl) since no trace of chlorine was detected.
The team of astrophysicists has determined that the disintegration of an object with a mass of at least 1018kg (corresponding to an asteroid of about 100 km in diameter), containing a large amount of minerals rich in sodium, is needed to account for the observations. To date, no asteroid of this kind has ever been spotted in our Solar System.
Finally, the observations of WD J2356-209 show that it is one of the oldest white dwarfs having accreted matter; the object that crashed onto it must have been in orbit for at least 9 billion years. By setting a lower limit on the duration during which a planetary body can remain in orbit around its star, objects such as WD J2356-209 give us a better understanding of the long-term evolution of planetary systems like ours.
A copy of the paper, entitled “A New Generation of Cool White Dwarf Atmosphere Models. III. WD J2356−209: Accretion of a Planetesimal with an Unusual Composition”, is available here: http://arxiv.org/abs/1902.03219
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