* Astronomy

Title: Bringing VY Canis Majoris Down to Size: An Improved Determination of Its Effective Tempeature
Authors: Philip Massey, Emily M. Levesque, Bertrand Plez


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 suggest this star is considerably more luminous (L = 5 x 105 solar luminosity) and larger (R=2800 solar radii) than other Galactic red supergiants (RSGs). Indeed, such a location in the H-R diagram would be well in the "Hayashi forbidden zone" where stars cannot be in hydrostatic equilibrium. These extraordinary properties, however, rest upon an assumed effective temperature of 2800-3000 K, far cooler than recent work have shown Red Super Giants to be.
To obtain a better estimate, the researchers 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 Red Super Giants; they also use V-K and V-J from the literature to derive an effective temperatures.
They find that the star likely has a temperature of 3650 K, a luminosity L = 6 x 104 solar luminosity, and a radius of 600 solar radii.
These values are consistent with VY CMa being an ordinary evolved 15 solar mass Red super giant, and agree well with the Geneva evolutionary tracks. They find that the circumstellar dust region has a temperature of 760 K, and an effective radius of approximately 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 colour), remains a mystery at present, although they speculate that perhaps this star (and NML Cyg) are simply normal Red Super Giants caught during an unusually unstable time.

VY Canis Majoris is about 5,000 light years away, shining on average between visual magnitudes of 7.4 and 9.6, it is bright enough to be seen through small telescopes or even large binoculars when bright.
VY CMa is a producer of natural MASERS (Microwave Amplification by the Stimulated Emission of Radiation). For a MASER to naturally form there needs to be a dense source (such as a thick nebula) and the right kind of molecule (OH, H2O, SiO, CH, CH3OH, etc)


Position(2000): RA = 07h22m59s Dec = -25°46'02"

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