Parallax of a Mira variable R Ursae Majoris studied with astrometric VLBI

Abstract

We have measured an annual parallax of the Mira variable R Ursae Majoris (R UMa) with the VLBI Exploration for Radio Astronomy (VERA). From the monitoring VLBI observations over a span of about two years, we detected H₂O maser spots in the LSR velocity range from 37 to 42 km s^-1. We derived an annual parallax of 1.97 ± 0.05 mas, and this gives a corresponding distance of 508 ± 13 pc. The VLBI maps revealed 72 maser spots distributed in an ∼110 au area around the expected stellar position. Circumstellar kinematics of the maser spots were also revealed by subtracting a systemic motion in the Hipparcos catalog from proper motions of each maser spot derived from our VLBI observations. Infrared photometry was also conducted to measure a K-band apparent magnitude, and we obtained a mean magnitude of m_K = 1.19 ± 0.02 mag. Using the trigonometric distance, m_K is converted to a K-band absolute magnitude of M_K = -7.34 ± 0.06 mag. This result gives a much more accurate absolute magnitude for R UMa than previously provided. We solved a zero-point of the M_K-log P relation for the Galactic Mira variables and obtained a relation of M_K = -3.52 log P + (1.09 ± 0.14). Other long-period variables, including red supergiants, whose distances were determined with astrometric VLBI, were also compiled to explore the different sequences of the M_K-log P relation.