An updated 2017 astrometric solution for betelgeuse

Abstract

We provide an update for the astrometric solution for the Type II supernova progenitor Betelgeuse using the revised Hipparcos Intermediate Astrometric Data (HIAD) of van Leeuwen, combined with existing VLA and new e-MERLIN and ALMA positions. The 2007 Hipparcos refined abscissa measurements required the addition of so-called Cosmic Noise of 2.4 mas to find an acceptable 5-parameter stochastic solution. We find that a measure of radio Cosmic Noise should also be included for the radio positions because surface inhomogeneities exist at a level significant enough to introduce additional intensity centroid uncertainty. Combining the 2007 HIAD with the proper motions based solely on the radio positions leads to a parallax of pi = 5.27 +/- 0.78 mas (190(-25)(+33) pc), smaller than the Hipparcos 2007 value of 6.56 +/- 0.83 mas (152(-17)(+22) pc). Furthermore, combining the VLA and new e-MERLIN and ALMA radio positions with the 2007 HIAD, and including radio Cosmic Noise of 2.4 mas, leads to a nominal parallax solution of 4.51 +/- 0.80 mas (222(-34)(+48) pc), which, while only 0.7 sigma different from the 2008 solution of Harper et al., is 2.6 sigma different from the solution of van Leeuwen. An accurate and precise parallax for Betelgeuse is always going to be difficult to obtain because it is small compared to the stellar angular diameter (theta = 44 mas). We outline an observing strategy utilizing future mm and sub-mm high-spatial resolution interferometry that must be used if substantial improvements in the precision and accuracy of the parallax and distance are to be achieved.