Old stars in young body
An international study with several Italian researchers discovered red giant stars that are older if we look at their chemical composition, but considered young by the internal structure. Comment by Cristina Chiappini (AIP) and Josefina Montalban (INAF).
Red giant stars with the chemical clock malfunctioning. And this was discovered by an international team of astrophysicists, led by Cristina Chiappini from the Leibniz Institute for Astrophysics in Potsdam, Germany. The chemical characteristics are peculiar of older stars, but if their age is inferred through asteroseismology – an approach that allows us to reconstruct the internal structure of stars by observing and interpreting the pulse – then alleged ancient prove surprisingly younger.
Credit: AIP/ J. Fohlmeister
The existence of this type of stars, little known so far, cannot be explained by standard models of the chemical evolution of the Milky Way, so scientists assume that the chemical enrichment history of the galactic disk is to be more complex than expected.
In this context, the term galactic archeology is used by researchers to refer to the fact that the history of the Milky Way can be gauged not only by the relative abundances of the different chemical elements observable in stellar atmospheres, but also by movements that shake the stars themselves.
One of the pillars of archeology Galactic is the use of chemical abundance ratios as indirect indicators of age. This is possible thanks to the fact that the supernova explosions enriched the interstellar medium of the chemical elements heavier than helium, produced at different speeds in the various reactions of nucleosynthesis. In particular, the relative abundance of alpha elements (such as carbon, nitrogen, oxygen and others) and iron (slower to occur) can be used as a chemical clock. A clock that has proven quite regular for many stars observed.
However, the authors of the new study – published today in Astronomy & Astrophysics – have shown that the so-called enrichment alpha / iron, or hand position in the watch chemical above, is not a guarantee that the star has actually an age as advanced as it appears. By measuring the “pulse” of a group of red giant stars with asteroseismology, researchers have found that many young people appeared, despite being the richest in alpha elements relative to the Sun, and therefore theoretically older.
“At the moment there are several hypotheses to explain the origin of these objects,” told Cristina Chiappini. “The interesting point is that these stars are for the most part to the inner regions of the galactic disk, where the interaction between the center bar and the spiral arms may have given rise to a scenario of more complex chemical enrichment. The possibility exists that in this region the gas can remain virtually inert for a long time: then we would find an old gas – with alpha / high iron – but that began to form stars only recently. In order to confirm this hypothesis will be necessary to study the numerous samples and, above all, in other directions. ”
The sample of stars used to the new study is the result of collaboration between APOGEE, a sampling of the sky in infrared high resolution made by the Sloan Foundation 2.5-meter telescope located in New Mexico, and the Working Group on red giants of CoRoT The Space Telescope French space agency (CNES) mainly dedicated to the studies of asteroseismology and exoplanet.
Credit: F. Anders
“This work,” says Josefina Montalban, associated INAF Department of Physics and Astronomy, University of Padua, one of the authors of the research, “shows how promising collaborations that bring together the skills and the classical methods that observe from the ground surface of the stars, with new techniques such as asteroseismology, which thanks to space missions like CoRoT and future Plato, allow access even inside the stars.”
To the study also participated Thaise Rodrigues and Leo Girardi, INAF, Astronomical Observatory of Padua, Andrea Miglio, associated INAF University of Birmingham, and Marica Valentini, that is now a researcher at Leibniz Institute for Astrophysics in Potsdam.