A mystery in the evolution of solar-like low-mass stars is the existence
of red giant stars that show anomalously large abundances of lithium
(Li). A team of astronomers from NAOJ and NAOC (National Astronomical
Observatories of China) has obtained clear evidence, based on
measurements of surface chemical abundances and internal structure, that
red giants with extremely high Li abundances are at the stage where
they are burning helium at their centers. This result will be a clue to
help solve long-standing problems in the stellar evolution theories.
Stars
like the Sun can stably shine for a very long time, powered by hydrogen
nuclear burning at their centers. However, when most of the hydrogen at
the center has been converted to helium, the energy needed to support
the structure of the star and keep it luminous starts to be supplied by
hydrogen nuclear burning in the area around the central core (Figure 1).
Such stars evolve into the stage known as red giant stars with
increased luminosity. The convection at the surface of these stars
becomes more active and mixes the material at the surface with the
internal material. Lithium (Li), the third lightest elements following
hydrogen and helium, is a fragile element that is destroyed at the high
temperatures found inside of stars, and hence, becomes less abundant at
the surface of red giants in which convection is very active. However, a
small fraction of red giants are known to have orders of magnitude
larger Li abundances compared to usual red giants (see Note 1). This is a
mystery in the study of stellar structure and evolution.
Recent observations have suggested that red giants showing anomalously
high Li excesses are so-called clump stars, which are a kind of red
giant in which helium nuclear burning occurs at the central part. This
could be a useful hint to solve the question about the origin of
Li-enhanced stars. However, it is not easy to distinguish the
evolutionary status of a red giant, and no definitive conclusion had
been obtained.
This team has identified a number of red giants
that exhibit excess Li from the spectra obtained with the spectroscopic
survey telescope LAMOST in China (Figure 2), and distinguished the
evolutionary stages of these objects through asteroseismology using the
photometry data from the space telescope Kepler (see Note 2). This
reveals that most of the Li-enhanced red giants are clump stars, and
objects that show particularly high Li abundances are only found in this
evolutionary stage. In this study, high-resolution spectra of 26 stars
were obtained with the Subaru Telescope to derive reliable Li
abundances, corroborating the above results (Figure 2, 3).
Low-mass stars like the Sun have been studied relatively well in the
history of astronomy. The existence of Li-enhanced red giants, however,
indicates that there remains an essential problem in our understanding
of stellar structure and evolution. Although the mechanism that enhances
Li is unknown, the result that the evolutionary status of such stars
has been successfully identified is a big step toward solving the
question. The high-resolution spectra obtained with the Subaru Telescope
should contain rich information about the chemical abundances of many
other elements in addition to Li and will be useful for further
understanding of the nature of Li-enhanced stars.
This result
was published on October 5, 2020 in Nature Astronomy (Hong-Liang Yan,
Yu-Tao Zhou, Xianfei Zhang, Yaguang, Qi Gao, Jian-Rong Shi, Gang Zhao,
Wako Aoki, Tadafumi Matsuno, Yan Li, Xiao-Dong Xu, Haining Li, Ya-Qian
Wu, Meng-Qi Jin, Benoit Mosser, Shao-Lan Bi, Jian-Ning Fu, Kaike Pan,
Takuma Suda, Yu-Juan Liu, Jing-Kun Zhao and Xi-Long Liang, "Most lithium-rich low-mass evolved stars revealed as red clump stars by asteroseismology and spectroscopy").
This research has been supported by the JSPS-Chinese Academy of Science
joint program since 2016. The observations with the Subaru Telescope
were conducted in an intensive program in 2016-2017.
Note 1:
The frequency of distinctively Li-enhanced stars like those studied in
this work is estimated to be about 1% of all red giants.
Note 2:
The internal structures of stars can be probed by observing the
oscillation of the stellar surface; similar to how the structure of the
Earth can be investigated by monitoring earthquakes. This approach is
called asteroseismology, enabling us to distinguish clump stars from
other red giants having different internal structures. The oscillation
of the stellar surface is measured as small, periodic variations in
brightness. The precise and long-term measurements of brightness for a
large number of stars with Kepler enable us to apply asteroseismology to
many red giants.
Source: Subaru Telescope