KW Sagittarii

Did You Know?

• Henrietta Hill Swope listed KW Sagittarii as a variable star in 1942.
• Its peculiar cool spectrum once led to comparisons with symbiotic variables, but it is no longer considered a cataclysmic binary.
• The star varies erratically in brightness over a range of about two magnitudes.

Distance

A distance of 2,420 parsecs assumes membership in the Sagittarius OB5 association. The Hipparcos parallax is negative, indicating a large distance. Gaia Data Release 2 parallax carries a significant margin of error and excessive astrometric noise, making it unreliable. A 2021 study gave a photogeometric distance of 2,159 pc using Gaia DR3.

Characteristics

KW Sagittarii is a dust-enshrouded luminous cool supergiant with a spectral type varying between M0 and M4. It is sometimes described as a cool hypergiant and loses mass at a certain rate per year. It is a confirmed maser source of H₂O masers, while SiO and OH masers have been proposed but not detected. A 2005 study using a MARCS model calculated a high luminosity and a very large radius based on an assumed effective temperature and distance, placing it among the four largest and most luminous galactic red supergiants, alongside V354 Cephei, KY Cygni, and Mu Cephei. A 2011 re-measurement gave a bolometric luminosity around a certain value based on the same distance. More recent calculations placing the Sagittarius OB5 association closer suggest lower estimates for radius and luminosity; based on a newer distance, luminosity and radius were derived from a measured angular diameter, which was re-measured in 2023.