2025PASJ...77..459S

We have achieved the first VLBI detections of the ground vibrational state (v = 0) ²⁸SiO (hereafter, SiO) and ²⁹SiO masers of the J = 1→0 rotational transitions, towards the 25 $M_{\odot }$ red supergiant (RSG) star, VY Canis Majoris (VY CMa), taking advantage of the high sensitivity of the VLBI Exploration of Radio Astrometry (VERA) telescopes that coordinate with the Nobeyama 45 m telescope. In addition, we successfully detected the SiO J = 1→0 transition in the v = 3) state towards VY CMa for the first time with VLBI. The SiO J = 1→0 maser spot in the v = 0 state was detected in the cross-power spectra taken with the baselines involving the Nobeyama 45 m telescope. The combination of previously reported absolute astrometry and the relative astrometry technique allowed us to derive the location of the SiO v = 0 maser spot, (RA, Dec) = (${7^{\rm h}22^{\rm m}58{_{.}^{\rm s}}32}$, $-25^{\circ }$${46^{\prime }3{^{\prime \prime }_{.}}4}$) in J2000.0 at an absolute positional accuracy of $\sim$100 milliarcseconds (mas). The SiO v = 0 maser spot is offset by the amount of ($\Delta$RA, $\Delta$Dec) = (-150, -300) (mas) to the south-west of the stellar position, suggesting that the v = 0) maser spot is associated with its outflow activity. This observational study demonstrates that the brightest SiO v = 0 maser spot is compact (3 mas), producing an extremely high brightness of $\sim 10^7$ K. This indicates that the SiO v = 0 maser action may originate from strong shocks in the stellar wind emanating from this extreme RSG that leads to its intense mass ejection.