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Studying M Subdwarfs can Help Unravel the History of Milky Way Galaxy

Throughout the entire Milky Way Galaxy, there is a unique group of red tiny beasts that are wandering. They are red subdwarf stars considered the “aborigines” of the Milky Way and believed to be born in the Galactic halo and thick disk.

Comparison between an M subdwarf and the Sun. Image Credit: LAMOST.

Learning more about these M subdwarfs can help us know the history of our Galaxy. They are indeed quite rare in the solar neighborhood, but people usually mixed them with some metal-poor disk-born red dwarfs.

Ali Luo, Professor, National Astronomical Observatories, Chinese Academy of Sciences

Professor Luo’s group has been performing detailed research on M subdwarfs.

The properties of thousands of subdwarfs were investigated by the research group to comprehend the inherent variation between these scarce M subdwarfs and the normal M dwarfs, which are the largest category of stellar members in the Milky Way.

The study was reported in The Astrophysical Journal.

Image Credit: Geoffrey Villaume/ 

We have revised the spectroscopic classification system and measured the atmospheric parameters of all the dwarf/subdwarf samples. The results show that the new system behaves better on finding the red tiny beasts, which are the genuine subdwarfs.

Shuo Zhang, Study First Author, Chinese Academy of Sciences

The impacts of the review in the classification system were studied by scientists and applied to the spectroscopic subdwarf samples chosen from SDSS and LAMOST. They discovered that the “genuine” subdwarfs determined by luminosity and motion are the ones with the least metal abundances, and also exhibit the greatest gravities, which were not taken into account as an iconic feature of subdwarf stars.

For decades, people have collectively referred to the real subdwarfs and a group of metal-poor dwarfs as ‘subdwarfs’ based on some specific spectral features, but these misclassified stellar objects behave like classical disk dwarfs on all aspects. We found that gravity value is a great criterion to separate them from the genuine subdwarfs.

Shuo Zhang, Study First Author, Chinese Academy of Sciences

The largest sample of M subdwarf stars we built ever had helped us to make this important discovery. More importantly, the kinematic and chemical analysis shows that the sample contains a mix of objects belonging to various stellar populations and having different origins, which deserves further study,” stated Professor Luo.

Journal Reference:

Zhang, S., et al. (2021) M-subdwarf Research. II. Atmospheric Parameters and Kinematics. The Astrophysical Journal.

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