89 Herculis (also catalogued as HD 163506) is a bright, evolved binary star system located in the constellation Hercules. It is classified as a post‑asymptotic‑giant‑branch (post‑AGB) object, meaning the primary component is an intermediate‑mass star that has left the AGB phase and is evolving toward the planetary‑nebula stage.
Basic characteristics
- Position (J2000.0): Right ascension ≈ 17 h 47 m 02 s, Declination ≈ +33° 01′ 18″.
- Apparent magnitude: V ≈ 5.5, making it faintly visible to the naked eye under dark skies.
- Spectral type: The primary star is typically listed as F2 Ib, indicating a luminous supergiant‑like spectrum.
- Distance: Gaia EDR3 parallax measurements place the system at roughly 1.5 kiloparsecs (∼ 5 000 light‑years), though the exact distance carries a modest uncertainty due to the star’s variability and circumstellar dust.
- Luminosity: Estimates based on spectral energy distribution modeling give a bolometric luminosity of several thousand L☉, consistent with a post‑AGB star.
Binary nature
- 89 Her is a spectroscopic binary. Radial‑velocity monitoring reveals an orbital period of about 288 days with a low orbital eccentricity (e < 0.1).
- The companion is not directly observed but is inferred to be a low‑mass main‑sequence star (≈ 0.5–1 M☉) based on the mass function derived from the orbit.
Circumstellar environment
- The system exhibits a strong infrared excess, indicative of a stable, dusty circumbinary disc. High‑resolution interferometric observations in the near‑ and mid‑infrared have resolved the disc, showing a compact, optically thick structure with a radius of a few astronomical units.
- Molecular line studies (e.g., CO J=2–1) reveal a slow, bipolar outflow superimposed on the rotating disc, suggesting ongoing mass loss and shaping of the eventual planetary nebula.
Variability
- Photometric monitoring shows low‑amplitude variability (ΔV ≈ 0.1 mag) on timescales of days to weeks, attributed to pulsations typical of post‑AGB stars. The variability does not match the large‑amplitude, alternating‑deep‑shallow minima characteristic of classical RV Tauri variables, and the star is not formally classified as an RV Tauri type.
Scientific significance
89 Her is often cited as a prototype for binary post‑AGB systems possessing circumbinary discs. Its well‑characterized orbital parameters and resolved disc make it a valuable laboratory for studying mass transfer, disc formation, and the shaping mechanisms that influence the morphology of planetary nebulae.
References
- Studies utilizing high‑resolution spectroscopy and interferometry (e.g., Bujarrabal et al., 2007; Hillen et al., 2015) provide detailed measurements of the disc geometry and outflow kinematics.
- Gaia Collaboration (2021) supplies the parallax and proper‑motion data employed for distance estimation.
All information presented is drawn from peer‑reviewed astronomical literature and standard astrophysical databases.