  The absorption signals of metastable He in HD 209458b and several other
exoplanets can be explained via escaping atmosphere model with a subsolar He/H
ratio. The low abundance of helium can be a result of planet formation if there
is a small amount of helium in their primordial atmosphere. However, another
possibility is that the low He/H ratio is caused by the process of mass
fractionation of helium in the atmosphere. In order to investigate the effect
of the fractionation in the hydrogen-helium atmosphere, we developed a
self-consistent multi-fluid 1D hydrodynamic model based on the well-known
open-source MHD code PLUTO. Our simulations show that a lower He/H ratio can be
produced spontaneously in the multi-fluid model. We further modeled the
transmission spectra of He 10830 lines for HD 209458b in a broad parameter
space. The transmission spectrum of the observation can be fitted in the
condition of 1.80 times the X-ray and extreme-ultraviolet flux of the quiet
Sun. Meanwhile, the ratio of the escaping flux of helium to hydrogen,
$F_{He}/F_{H}$, is 0.039. Our results indicate that the mass fractionation of
helium to hydrogen can naturally interpret the low He/H ratio required by the
observation. Thus, in the escaping atmosphere of HD 209458b, decreasing the
abundance of helium in the atmosphere is not needed even if its He abundance is
similar to that of the Sun. The simulation presented in this work hints that in
the escaping atmosphere, mass fractionation can also occur on other exoplanets,
which needs to be explored further.
