Bringing VY Canis Majoris Down to Size: An Improved Determination of Its Effective Temperature

Abstract

The star VY CMa is a late-type M supergiant with many peculiarities,mostly related to the intense circumstellar environment due to thestar's high mass-loss rate. Claims have been made that would imply thatthis star is considerably more luminous (L~5{\times}10^{5}L_{solar}) and larger (R~2800 R_{solar}) than otherGalactic red supergiants (RSGs). Indeed, such a location in the H-Rdiagram would be well within the ``Hayashi forbidden zone,'' where starscannot be in hydrostatic equilibrium. These extraordinary properties,however, rest on an assumed effective temperature of 2800-3000 K, farcooler than recent work has shown RSGs to be. To obtain a betterestimate, we fit newly obtained spectrophotometry in the optical and NIRwith the same MARCS models used for our recent determination of thephysical properties of other RSGs; we also use V-K and V-J from theliterature to derive an effective temperature. We find that the starlikely has a temperature of 3650 K, a luminosityL~6{\times}10^{4} L_{solar}, and a radius of ~600R_{solar}. These values are consistent with VY CMa being anordinary, evolved 15 M_{solar} RSG and agree well with theGeneva evolutionary tracks. We find that the circumstellar dust regionhas a temperature of 760 K, and an effective radius of ~130 AU, ifspherical geometry is assumed for the latter. What causes this star tohave such a high mass loss and large variations in brightness (but withlittle change in color) remains a mystery at present, although wespeculate that perhaps this star and NML Cyg are simply normal RSGscaught during an unusually unstable time.