Astronomers explained almost a century-old mystery of the origin of the stellar red giants

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Astronomers from the Astronomical Observatory of the University of Warsaw have explained almost a century-old mystery of the origin of the so-called long secondary period in stellar red giants. An article on this topic appeared in The Astrophysical Journal Letters.

A group of astronomers from the Astronomical Observatory of the University of Warsaw under the supervision of prof. Igor Soszyński has been studying the phenomenon of the so-called Long Secondary Period (LSP), using the world's largest photometric database created for almost 30 years by the Sky Survey Optical Gravitational Lensing Experiment (OGLE). Chaired by prof. Andrzej Udalski, the OGLE project conducts regular observations of about 2 billion stars belonging to our Galaxy and the nearby Magellanic Clouds. The huge OGLE database was used to select an unprecedentedly large sample of around 16,000 LSP stars and study their properties.

An article explaining the mystery of the origin of LSP in the red giants was published in The Astrophysical Journal Letters.

The mystery of the red giants

When solar-type stars run out of nuclear fuel, they turn into red giants. This means that they increase their radius hundreds of times, thereby consuming any nearby planets orbiting them. At the same time, these stars begin to lose mass as a result of the intense stellar wind. Moreover, in the last stages of their lives, red giants begin to pulsate as evidenced by the regular changes in the brightness of these stars.

About 30% of bright red giants show additional variability with periods ranging from several months to several years - typically ten times longer than the pulsation period of these stars. The origin of the so-called The Long Secondary Period (LSP) has remained a mystery for almost a century since the phenomenon was first observed. Astronomers have tried to explain the variability of LSPs in red giants by the induction of non-radial stellar oscillations, the formation of giant convection cells, episodic dust outbursts from the star's surface, or the presence of dark spots in its photosphere. However, none of the proposed models could explain all the observed properties of LSP stars.

Research by astronomers from the University of Warsaw

For the first time, scientists from the Astronomical Observatory of the University of Warsaw analyzed changes in the brightness of LSP stars in the infrared range. For this purpose, the researchers used observations conducted for several years by the American space telescope WISE (Wide-field Infrared Survey Explorer). A comparison of the OGLE optical glow curves with the infrared WISE photometric data showed one major difference: the additional brightness minimum visible only in the infrared range. This behavior can be explained by the presence of an additional object orbiting just above the giant's surface and surrounded by a vast dust cloud obscuring the star once per orbital period. The matter forming the orbiting cloud heats up to temperatures of about 1000-1500 K, which causes the emission of infrared radiation, so when the cloud is covered by a red giant.

Measurements of changes in the radial velocity of LSP stars indicate that the companions of red giants immersed in a dust cloud are most often bodies with substellar masses, the so-called brown dwarfs. Objects of this type probably arose as a result of the pulling of matter by planets that initially orbit their stars in distant orbits. This explanation opens up new perspectives for studying the spatial arrangements of planetary systems in our and other galaxies.