Quasi-periodic Eruptions from Helium Envelope of Hydrogen-deficient
  Stars Stripped by Supermassive Black Holes
  Quasi-periodic eruptions (QPEs), which are a new kind of X-ray bursts with a
recurrence time of several hours, have been detected from supermassive black
holes (SMBHs) in galactic nuclei. Recently, the two QPEs discovered by the
\textit{eROSITA} show asymmetric light curves with a fast rise and a slow
decline. Current models cannot explain the observational characteristics of
QPEs. Here we show that QPEs can be generated from the Roche lobe overflows at
each periapsis passage of an evolved star orbiting an SMBH. The properties of
the companion stars are constrained via analytic estimations. We find that
hydrogen-deficient post-AGB stars are promising candidates. Modules for
Experiments in Stellar Astrophysics (MESA) stellar evolution code is used to
construct the hydrogen-deficient stars which can fulfill the requirements, as
obtained through analytical estimates, to produce the properties of QPEs,
including the fast-rise and slow-decay light curves, periods, energetics, and
rates. Furthermore, the extreme mass ratio $\sim 10^5$ between the SMBH and the
donor will lead to a phenomenon called extreme mass-ratio inspiral (EMRI),
producing millihertz gravitational waves. These QPEs would be detected as EMRI
sources with electromagnetic counterparts for space-based GW detectors, such as
Laser Interferometer Space Antenna (LISA) and Tianqin. They would provide a new
way to measure the Hubble constant and further test the Hubble constant
tension.
