Annual parallax measurements of a semi-regular variable star SV Pegasus with VERA

Abstract

Many studies have shown that there are clear sequences in the period-luminosity relationship (PLR) for Mira variables and semi-regular variables (SRVs) in the Large Magellanic Cloud (LMC). To investigate the PLR for SRVs in our galaxy, we examined the annual parallax measurement and conducted K'-band photometric monitoring of an SRV star SV Pegasus (SV Peg). We measured the position change of the associated H₂O maser spots by phase-referencing Very Long Baseline Interferometry (VLBI) observations with VERA at 22 GHz, spanning approximately 3 yr, and detected an annual parallax of π = 3.00 ± 0.06 mas, corresponding to a distance of D = 333 ± 7 pc. This result is in good agreement with the Hipparcos parallax and improves the accuracy of the distance from 35% to 2%. However, the Gaia DR2 catalog gave a parallax of π = 1.12 ± 0.28 mas for SV Peg. This indicates that the Gaia result might be blurred by the effect of the stellar size because the estimated stellar radius was ∼5 mas, which is comparable to the parallax. We obtained a K'-band mean magnitude of m_{K^' }} = -0.48mag and a period of P = 177 d from our photometric monitoring with a 1 m telescope. Using the trigonometric distance, we derived an absolute magnitude of M_{K^' }}=-8.09 ± 0.05mag. This result shows that the position of SV Peg in the PLR falls on the C' sequence found in the PLR in the LMC, which is similar to other SRVs in our galaxy.