Building a sun-like star and a red giant. Image: ESO
Reading stars Stars with a planetary system are like a fine cappuccino: just as cocoa is emblazoned on the milk foam of the coffee drink, the starry atmospheres are sprinkled with iron. However, the iron dust pollutes the thin atmospheres only superficially, researchers have now found around Luca Pasquini of the European Southern Observatory in Garching. Since the discovery of the first exoplanets, astronomers have been puzzling over a curious phenomenon: Almost all stars that have a planetary system seem to be particularly rich in heavy elements like iron. Researchers wondered if iron abundance is the prerequisite for the formation of planets, or if the planets themselves cause pollution. The hen-egg problem could not be solved with the methods of astronomy without further ado, because you can not look under the surface with telescopes.

Pasquini and his colleagues circumvented this difficulty by examining stars at a later stage of development. Such so-called red giants arise when a star like the sun runs out of fuel. In the last phase of their lives, the aging light bulbs inflate into a huge ball of fire.

Strangely enough, red giants with planetary systems contain less iron and other heavy elements than their precursor stars. Pasquini and his colleagues explain it this way: In a normal star, only about two percent of the stellar mass in the outermost layer, the so-called convection zone, is mixed. Dust and debris from planetary formation remain trapped in this layer for billions of years. Only when a star turns into a red giant, the planetary debris is strongly diluted: in the convection zone, then about two-thirds of the star material is stirred.

"The simplest explanation for the data is that sun-like stars appear metal-rich because their atmospheres are contaminated, " explains co-author Artie Hatzes of the Thüringer Landessternwarte in Tautenburg. His colleague Luca Pasquini adds: "The anomaly simply disappears when the stars flare up to become red giants." Display

Luca Pasquini (European Southern Observatory, Garching) et al .: Astronomy & Astrophysics in press Ute Kehse

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