The puzzling high velocity G5 supergiant star HD 179821: new insight from Gaia DR2 data

Abstract

HD 179821 is classified as G5 Ia star. From the IRAS colors and spectral energy distributions it was classified as a post-AGB star. But some studies classify it as a massive (30 to 19M⊙) post-red supergiant evolving to become a Type II supernova. Its mass and evolutionary status remained a hotly debated question even after several detailed spectroscopic studies as the distance was not known. We use the parallax of HD 179821 from the second Gaia data release, and deduce its distance 2959±409pc and its absolute magnitude MV=‑5.7±0.3. Using the absolute magnitude determined in this paper, we show that HD 179821 fits very well with post-AGB tracks in the H‑R diagram. Our results clearly confirm that HD 179821 is a post-AGB star of mass in the range of 0.70 to 0.75 solar mass. It is not a 30M⊙ red supergiant. The progenitor mass of this post-AGB star may be in the range of 4M⊙ but may be a bit more.