Parallax measurements of the Mira-Type star UX Cygni with phase-referenced VLBI

Abstract

We have measured the annual parallax of the Mira-Type star UX Cygni of 0.54±0.06 mas using phase-referenced VLBI observations of its circumstellar water masers with the VLBA. The corresponding distance is 1.85+0.25 -0.18 kpc. This is the first measurement of star's annual parallax based on the observations of water masers. Some kinds of variable stars, includingMira variables, have period-luminosity relations, and so they are used as distance indicators. However, they are only relative distance indicators because we do not know the relationships between periods and absolute luminosities. If we could measure this relationship for Mira variables, it would become an absolute distance indicator, allowing for an independent measurement of the distance to Large Magellanic Cloud (LMC), which is important for the calibration of period-luminosity relations for other kinds of variable stars. Our result is consistent with the period-luminosity relation of Mira variables in LMC and its distance from present observations.