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 19 M sun ) 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 ± 409 pc 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 0.8 M sun . It is not a 30 M sun red supergiant. The progenitor mass of this post-AGB star may be in the range of 4 M sun but may be a bit more.

High-resolution spectroscopy of the high-velocity hot post-AGB star IRAS 18379–1707 (LS 5112)

“…Among post-AGB supergiants there are few high velocity stars. Among the cooler post-AGB supergiants HD 56126 (+105 km s −1 ) (Parthasarathy et al 1992), HD 179821 (+81.8 km s −1 ) (Parthasarathy et al 2019), were found and among hotter post-AGB stars LS III +52 • 24 (IRAS 22023+5249) (-148 km s −1 ) (Sarkar et al 2012), and BD+33 • 2642 (-100 km s −1 ) (Napiwotzki et al 1994).…”