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TESS finds a surviving star-expanding planet

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Astronomers have analyzed the TESS space telescope observations of stars HD 212771 and HD 203949 – the first luminaries with already discovered exoplanets, in which the apparatus recorded noticeable fluctuations. It turned out that in one of them the planet is so close to the star that it should have been absorbed at the stage of expansion of the sun. Most likely, earlier the planet was farther and migrated to the centre relatively recently.

The TESS telescope is designed to study stars in a sunny neighbourhood. First of all, it is aimed at the search for exoplanets by the transit method, that is, based on changes in the brightness of the bodies when objects pass through their disk. However, extremely accurate measurements of radiation fluxes by TESS instruments allow many other studies, including in the field of astroseismology, that is, the science of star oscillations. This is also facilitated by the observational program of the telescope, in which it will cover almost the entire sky and collect information about a large number of objects.

Astroseismeismology is particularly productive in the case of sun-like stars or red giants, since the outer shells of such luminaries are in convective motion, due to which large-scale oscillations with a noticeable amplitude can occur. In the context of the study of exoplanets, astroseismology may indicate both the mere presence of such objects in orbit around the star, as well as on detailed properties such as the reciprocal orientation of angular and orbital moments, as well as the eccentricity of the planets.

The work of the international team of astronomers with the participation of Tiago Campante from the University of Porto in Portugal is dedicated to the study of stars HD 212771 and HD 203949 using the TESS telescope. These stars had previously been discovered on one exoplanet using the radial velocity method, and now scientists have also measured their fluctuations.

The analysis of the oscillations allowed to make new estimates of masses, ages, surface temperatures, radii, densities and gravity for both luminaries. It turned out that the mass of HD 203949 was significantly overestimated in the initial analysis a few years ago: instead of 2.1 mass of the Sun in the new work turned out to be 1.0-1.2. This change required a recalculation of the parameters of the exoplanet, as the method of radial velocities, which it was originally discovered, strongly depends on the mass of the central object.

By calculating the planet’s influence on the light of a smaller mass, it turned out that to explain the observational data it was necessary to be at a distance of about 0.65 astronomical units from the star, and its mass exceeded Jupiter’s five times or a little more.

Since the parent star HD 203949 can refer to red condensation, that is, a group of red giants already surviving an expansion and helium flash, the planet is in such close orbit should have been destroyed. The numerical simulations carried out by the researchers indicate that the planet could initially be much further away, at a distance of about 3.1-3.5 astronomical units, and then during the active loss of mass by the star when passing the top of the branches of red giants due to tidal interactions the planet could migrate closer to the star.

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