WOH G64 is an unusual red supergiant ( RSG ) in the Large Magellanic Cloud ( LMC ) , with a number of properties that set it apart from the rest of the LMC RSG population , including a thick circumstellar dust torus , an unusually late spectral type , maser activity , and nebular emission lines .
Its reported physical properties are also extreme , including the largest radius for any star known and an effective temperature that is much cooler than other RSGs in the LMC , both of which are at variance with stellar evolutionary theory .
We fit moderate-resolution optical spectrophotometry of WOH G64 with the MARCS stellar atmosphere models , determining an effective temperature of 3400 \pm 25 K. We obtain a similar result from the star ’ s broadband V - K colors .
With this effective temperature , and taking into account the flux contribution from the aysmmetric circumstellar dust envelope , we calculate log ( L / L _ { \odot } ) = 5.45 \pm 0.05 for WOH G64 , quite similar to the luminosity reported by Ohnaka and collaborators based on their radiative transfer modeling of the star ’ s dust torus .
We determine a radius of R / R _ { \odot } = 1540 , bringing the size of WOH G64 and its position on the H-R diagram into agreement with the largest known Galactic RSGs , although it is still extreme for the LMC .
In addition , we use the Ca ii triplet absorption feature to determine a radial velocity of 294 \pm 2 km s ^ { -1 } for the star ; this is the same radial velocity as the rotating gas in the LMC ’ s disk , which confirms its membership in the LMC and precludes it from being an unusual Galactic halo giant .
Finally , we describe the star ’ s unusual nebula emission spectrum ; the gas is nitrogen-rich and shock-heated , and displays a radial velocity that is significantly more positive than the star itself by 50 km s ^ { -1 } .