The Distance, Absolute Magnitude and Space Motion of Alpha Orionis

Extract

Alpha Orionis (Betelgeuse) is the brightest, and presumably the nearest, of the M-type supergiants. It is also one of the least reddened. These features make it important for the calibration of the absolute magnitudes and intrinsic colors of M supergiants. However, its distance has been difficultto establish, and there has been considerable debate concerning its reddening.

The Hipparcos parallax for α Ori is 7.63 ± 1.64 mas, corresponding to a distance of 131 ± 30 pc and a true distance modulus (m — M)_o of 5.59 ± 0.48 mag. If we adopt <V> = 0.50 and E(B - V) = 0.05, the mean absolute visual magnitude becomes <Mv> = -5.24, a result consistent with a luminosity classification of lab.

The CN strength of α Ori measured in the near infrared also indicates luminosity class Iab. Furthermore, the star’s absolute magnitude at 1.04 μm, for the Hipparcos distance, is -8.34, in agreement with the M2 Iab supergiants in h &. χ Per for a distance modulus of (m — M)_o = 12.0.

From the Hipparcos proper-motion components and distance and a published radial velocity, we derive a space motion of 27.8 km s^-1 with components U = +19.7, V = -7.3, and W = -1.5 km s^-1 with respect to the Sun. This motion is not very different from the local standard of rest. It seems unlikely that μ Ori, at a distance only one-third that of the Trapezium, is physically associated with the Orion Complex.

At 131 pc, the observed optical angular diameter of μ Ori, 0.055 arcsec, translates to a linear diameter of 7.2 AU. Centered on the Sun, itsphotosphere would be in the asteroid belt. Its angular diameter measured bythe Hubble Space Telescope in the UV (corresponding to the size of the chromosphere) is a factor of 2 larger. Surface features detected interferometrically are believed to be ~ 10% of the diameter in extent, corresponding to 0.7 AU or 10⁸ km.