HIP 81208 is the first hierarchical quadruple system discovered using the SPHERE instrument

The HIP 81208 system seen by SPHERE
An article published in the journal “Astronomy & Astrophysics” reports the discovery of an object that could be a gas giant planet or a brown dwarf in the HIP 81208 system. A team of researchers analyzed data from the archive of the observations conducted with the SPHERE instrument mounted on the VLT in Chile discovering an object that was cataloged as HIP 81208 Cb orbiting the smaller of the two stars in this binary system. A brown dwarf was discovered around the more massive star together with the smaller and more distant star in an analysis also published some time ago in an article in “Astronomy & Astrophysics”. The new discovery makes HIP 81208 a so-called hierarchical quadruple system, the first of its kind discovered using a direct imaging system.

Since 2019, the B-star Exoplanet Abundance Study (BEAST) has been focusing on massive B-class stars in search of substellar companions to understand how common they are. The research concerns planets but also brown dwarfs, objects on the border between planet and star. This research takes advantage of the capabilities of the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument mounted on the Very Large Telescope (VLT) to photograph these objects and consequently offer the possibility of seeing them directly.

However, even for an instrument as extraordinary as SPHERE, it can be difficult to distinguish the light of a substellar object from that of its star. This problem arose in the study of the HIP 81208 system and required a reanalysis of the data obtained to find another object.

The original study showed that the HIP 81208 system is interesting with the more massive star, about 2.5 times the Sun, having two companions: another star with a mass that is only about 13% the Sun’s at a notable distance given that it’s about 230 times that of the Earth from the Sun and a brown dwarf at a distance that is about 50 times that of the Earth from the Sun. The analysis suggests that the three objects have orbits in a Kozai resonance, a type of configuration that makes it interesting for further studies.

This type of research is making progress with improvements in analysis techniques, and the authors of the study of HIP 81208 applied other processing algorithms to the already available images. The result was the discovery of another substellar object orbiting the smaller star, HIP 81208 C, which was consequently cataloged as HIP 81208 Cb.

The new analysis indicates that HIP 81208 Cb has a distance from its star that is about 20 times that of the Earth from the Sun. Its estimated mass is about 15 times Jupiter’s and this makes it interesting because it’s on the borderline between that of a gas giant planet and a brown dwarf.

The new discovery makes HIP 81208 a so-called hierarchical quadruple system given that the more massive star, HIP 81208 A, is at the center of a system with the relatively nearby brown dwarf HIP 81208 B and two companions that are more distant orbiting each other.

It’s the first time that such a system has been discovered using a direct imaging system, a great achievement for the BEAST project. Follow-up studies with different instruments can also be conducted thanks to the fact that the HIP 81208 system is relatively close, about 615 light-years from Earth. Those studies will ascertain the nature of HIP 81208 Cb and offer information on the possible origin and configuration of a system that proved to be very interesting for astronomers.

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