As there is a difficulty in understanding the canonical scheme of the Galactic chemical evolution of the alpha-rich giant stars with theoretical young ages, it has been categorized as an abnormal population.
A new study headed by Ph.D. student Meng Zhang and Professor Huawei Zhang from Peking University as well as Dr. Maosheng Xiang from Max-Planck Institute for Astronomy has revealed the nature of these stellar anomalies.
The study result has been reported in The Astrophysical Journal.
The current observations disclosed a population of alpha-element enhanced giant stars with surprising high masses. Having an assumption about the single-star evolution, their masses indicate young ages. But as far as the context of the Galactic chemical evolution was concerned, stars formed at early epochs were enhanced in alpha-elements. Hence, alpha-rich stars are usually believed to be old.
Various scenarios have been suggested to describe the origins of such alpha-rich “young” stars. Few try to utilize unique Galactic chemo-dynamic events, for example, star formation events next to the edge of the Galactic bar; and certain attributes to binary evolution.
In this work, taking advantage of the big sample of stars from LAMOST spectroscopic surveys, the scientists discovered over 1000 alpha-rich “young” stars. Integrating with the astrometric data offered by the Gaia satellite, they learned the kinematics and chemistry of such stars.
They discovered that these stars tend to share the same kinematics as that of the canonical alpha-rich old stars. But their chemical properties vary from the old stars. Generally, the so-called alpha-rich “young” stars consist of more nitrogen and carbon, and the content of barium is considerably high in almost 15% of these stars than the majority of those old stars.
It could not be described by their single-star evolution, so external sources of such additional elements are required.
As the element barium is majorly produced by asymptotic giant branch (AGB) stars who have evolved to the late stage of their life, AGB stars become the candidate donors. This can be accomplished if they are in a binary system; in particular, an alpha-rich “young” star can “eat” the materials having rich barium and carbon from its AGB companion.
When the single stellar evolution is considered, these stars look young, because they are massive. However, our results support the previous suggestion that these stellar anomalies are products of binary evolution.
Meng Zhang, Study First Author, Peking University
Meng Zhang added, “Their high mass is the result of accreting materials from their companions in the binary systems. Our study confirms that most, if not all, alpha-rich stars in the Galactic disk seem to be old.”
It is known to be the first-ever time to study the kinematics and chemistry of the alpha-rich massive stars by utilizing such a large sample.
The Large Sky Area Multi-Object Fabre Spectroscopic Telescope (LAMOST) at the Xinglong Station in China has been functioned by the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC).
Zhang, M., et al. (2021) Most “Young” α-rich Stars Have High Masses but are Actually Old. The Astrophysical Journal. doi.org/10.3847/1538-4357/ac22a5.