  H4-1 is a planetary nebula (PN) located in the Galactic halo, and is notably
carbon-rich and one of the most metal-deficient PNe in the Milky Way. To unveil
its progenitor evolution through the accurate measurement of the gas mass, we
conducted a comprehensive investigation of H4-1, using the newly obtained
Seimei/KOOLS-IFU spectra and multiwavelength spectro-photometry data. The
emission line images generated from the KOOLS-IFU datacube successfully resolve
the ellipsoidal nebula and the equatorial flattened disk that are frequently
seen in bipolar PNe evolved from massive progenitors. By a fully data-driven
method, we directly derived the seven elemental abundances, the gas-to-dust
mass ratio, and the gas and dust masses based on our own distance scale. By
comparing the observed quantities with both the photoionization model and the
binary nucleosynthesis model, we conclude that the progenitors of an initial
mass of 1.87 Msun and 0.82 Msun are second generation stars formed ~4 Gyrs
after the Big Bang, and underwent mass-transfers, binary merger, and ultimately
evolved into a PN showing unique chemical abundances. Our binary model
successfully reproduces the observed abundances and also explains evolutionary
time scale of H4-1.
