An international team of astrophysicists, led by Cristina Chiappini from the Leibniz Institute for Astrophysics

Potsdam, discovered the group of red giant stars, according to a press release. The stars' existence cannot be explained by standard chemical evolution models of the Milky Way, suggesting that the chemical enrichment history of the galactic disc is more complex than originally assumed.

The term "galactic archaeology" was coined to describe the fact that the Milky Way's history is not just based in the quantities of various chemical elements seen in stellar atmospheres (abundances), but also in stellar motions. One of the pillars of galactic archaeology is the use of stellar abundance ratios as an indirect estimator of age. The time delay between interstellar medium enrichment in alpha elements and iron can be used as a chemical clock. The chemical clock has been shown to work for many stars.

However, the authors of the new study demonstrate that alpha/iron enhancement is no guarantee that a star is in fact old. It has only recently become possible to determine precise ages for these stars, thanks to asteroseismology. The new method measures pulsation frequencies.

"Although there were similar stars in previous surveys, they were not identified as such and only very few of them," said co-author Friedrich Anders. "This may explain why these stars have received little attention so far."

"Future observations will provide more clues as to the origin of these stars and the complex chemical evolution of the Milky Way," concluded Cristina Chiappini.

The new CoRoT-APOGEE (CoRoGEE) sample is the result of collaboration between APOGEE (a high-resolution infrared survey) -- part of the Sloan Digital Sky Survey III (SDSS III) -- and the CoRoT red giant working group. This collaboration enabled hundreds of red giant stars to be followed up spectroscopically, providing seismic information in the CoRoT fields. At present, only CoRoGEE can explore the inner-disc regions and determine the age of its field stars.

Reference:
"Young [alpha/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?" Chiappini et al. 2015 April 10, A&A, 576, L12 [https://www.aanda.org/10.1051/0004-6361/201525865, preprint: https://arxiv.org/abs/1503.06990].