First VLBI Imaging of SiO $v=0$, $J=1\to 0$ Masers in VY Canis Majoris

Abstract

We achieved the first VLBI detections of the ground vibrational state ($v=0$) ${}^{28}$SiO (hereafter, SiO) and ${}^{29}$SiO masers of the $J=1\to 0$ rotational transitions, towards the 25 \Msun ~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 $J=1\to 0$ transition in the $v=3$ state towards VY CMa for the first time with VLBI. The SiO $J=1\to 0$ maser spot in $v=0$ 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${}^{\mathrm{h}}$ 22${}^{\mathrm{m}}$ 58.${}^{\mathrm{s}}$32, $-$25${}^{\circ }$ 46${}^{\prime }$ 3.${}^{\prime \prime }$4) in J2000 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 southwest 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.

10 pages, 4 figures, accepted for the publication of the Publication of Astronomical Society of Japan (PASJ)