White dwarf has cool companion

Astronomers discover exotic double star pair

Size ratios of the newly discovered binary star system compared to the size of our sun. The arrows indicate the movement of the two stars around each other. © University of Tübingen
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An unusual binary star system has been proven by an international research team using the ultra-modern mirror telescopes of the Universities of Tübingen and Göttingen. It consists of a burnt white dwarf with a rare chemical composition and a star low in mass.

Together with colleagues from the University of Göttingen, the Tübingen astrophysicists observed the double star system with the name SDSSJ212531.92-010745.9 in the fall of last year and measured the fluctuation of its brightness.

Like the scientists around Dr. Thorsten Nagel from the Institute for Astronomy and Astrophysics of the University of Tübingen in the current issue of the journal "Astronomy & Astrophysics" reports that the burned-out white dwarf is a so-called PG1159 star. This hot star with surface temperatures between 75, 000 ° C and 200, 000 ° C is characterized by a very unusual chemical composition of its atmosphere. Unlike most stars, it is nearly free of hydrogen and is instead dominated by helium, carbon and oxygen.

The special thing about a white dwarf, according to the researchers, is that it ignites its nuclear fusion energy source one last time before it burns up to become a so-called red giant again and burn up as a white dwarf. A PG1159 star is in transition from the reborn Red Giant to the White Dwarf. At present, only 40 such stars are known.

Stars are close together

Since the spectrum of the investigated white dwarf not only shows the absorption lines of carbon typical for PG1159 stars, but also emission lines of hydrogen, the research team soon had the assumption that the light of the PG1159 star is superimposed by the light of another object. Therefore, the Tübingen and Göttingen astrophysicists conducted an observation campaign with their own telescopes and determined the brightness of the object. They found a regular brightness variation of about 25 percent with a period of just under seven hours. Further analysis showed that the observed period could only be the orbital period of a double star pair. display

The discovered double star system consists of a about 90, 000 ° C hot, about 0.6 solar mass heavy PG1159 star and a 3, 000 ° C cool, about 0.4 solar mass heavy companion. The surface of the companion star is heated to about 8, 200 ° C due to the intense irradiation by the PG1159 star, from its atmosphere comes the observed emission line spectrum of hydrogen.

The distance between the two stars is so small (about 1.4 million kilometers) that the entire system would almost have space in our sun. Due to this extremely short distance one speaks of a narrow double star system, and in the group of PG1159 stars it is the first ever found.

Theories about star evolution are being reviewed

The discovery of this binary star system gives researchers the opportunity to directly determine the masses of the two stars. This makes it possible to check existing theories about stellar evolution and to distinguish between different hypotheses.

For this purpose, the spectrum of the star must be observed over one orbital period. However, the low brightness of the object requires the sight of one of the largest 8m telescopes of the European Southern Observatory in Chile.

In a next step, astronomers want to find out how this binary system originated. Astronomically, not too long ago, a few hundred thousand years ago, the present PG1159 star was in its red-giant phase and had a diameter of a few hundred solar radii. At that time, its companion star was almost reversing within its atmosphere, slowing down sharply and approaching a distance of just under two solar radii, the present distance.

However, how this development progressed in detail, and how the companion star may have survived, is yet to be explored through further investigations of the object.

(idw - University of T bingen, 27.03.2006 - DLO)