There is a special group of red tiny beasts wandering across the whole Milky Way Galaxy. They are old enough to be called "living fossils", and rarely to be seen not only because of faint, but also of quite sparse in the solar neighborhood.

"They are red subdwarf stars who are 'aborigines' of the Milky Way and considered to be born in the Galactic halo. Learning more about them can help us know the history of our Galaxy. But people usually mixed them with some metal-poor disk born red dwarfs," said Prof. A-Li Luo from National Astronomical Observatories of Chinese Academy of Sciences (NAOC), who had led his team making in-depth research on the M-type subdwarfs.

The research team studied the properties of thousands of subdwarfs to understand the intrinsic difference between these scarce M subdwarfs and the normal M dwarfs who are the most numerous stellar member in the Milky Way. The work results have been published in The Astrophysical Journal.

"We have revised the spectroscopic classification system and measured the atmospheric parameters of all the dwarfs/subdwarf samples, the results show that the new system behaves better on finding the red tiny beasts, which are the genuine subdwarfs," said Shuo Zhang, the first author of this research paper.

By studying the effects of the revision in the classification system, which were applied to the spectroscopic subdwarf samples selected from LAMOST and SDSS, the researchers find that the "genuine" subdwarfs identified by luminosity and motion are the ones with the lowest metal abundances, and they also have the largest gravities which is an important new discovery.

"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", said Shuo Zhang. "We found that gravity value is a great criterion to separate them from the genuine subdwarfs."

"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." said Prof. Luo.

Figure: Comparison between an M subdwarf and the Sun. (Credit: LAMOST)

The paper can be accessed at https://iopscience.iop.org/article/10.3847/1538-4357/abcfc5.