Bringing VY Canis Majoris down to size: an improved determination of its effective temperature.
MASSEY P., LEVESQUE E.M. and PLEZ B.
Abstract (from CDS):
The star VY CMa is a late-type M supergiant with many peculiarities, mostly related to the intense circumstellar environment due to the star's high mass-loss rate. Claims have been made that would imply that this star is considerably more luminous (L∼5x105 L☉) and larger (R∼2800 R☉) than other Galactic red supergiants (RSGs). Indeed, such a location in the H-R diagram would be well within the ``Hayashi forbidden zone'', where stars cannot be in hydrostatic equilibrium. These extraordinary properties, however, rest on an assumed effective temperature of 2800-3000 K, far cooler than recent work has shown RSGs to be. To obtain a better estimate, we fit newly obtained spectrophotometry in the optical and NIR with the same MARCS models used for our recent determination of the physical properties of other RSGs; we also use V-K and V-J from the literature to derive an effective temperature. We find that the star likely has a temperature of 3650 K, a luminosity L∼6x104 L☉, and a radius of ∼600 R☉. These values are consistent with VY CMa being an ordinary, evolved 15 M☉ RSG and agree well with the Geneva evolutionary tracks. We find that the circumstellar dust region has a temperature of 760 K, and an effective radius of ∼130 AU, if spherical geometry is assumed for the latter. What causes this star to have such a high mass loss and large variations in brightness (but with little change in color) remains a mystery at present, although we speculate that perhaps this star and NML Cyg are simply normal RSGs caught during an unusually unstable time.
Stars: Evolution - Stars: Late-Type - Stars: Mass Loss - Stars: Supergiants