Japanese VLBI Network Mapping of SiO v = 3 J = 1–0 Maser Emission in W Hydrae

We report first on two-epoch mapping observations of SiO $v =$ 3 $J =$ 1–0 maser emission in the semiregular variable W Hydrae using the Japanese VLBI Network. The flux density of the $v =$ 3 $J =$ 1–0 emission detected on 2009 February 28 was two orders of magnitude smaller than those of the $v =$ 1 and $v =$ 2 emissions, while a month and half later the $v =$ 3 flux density suddenly increased by a factor of ~25. In contrast, the $v =$ 1 and $v =$ 2 flux densities decreased during this period, as expected from the optical light curve. At the first epoch, the $v =$ 3 maser features were located inside of a ring composed of the $v =$ 1 and $v =$ 2 features by 6 mas (0.5 AU) toward the central star. These offsets are meaningfully larger than the error of the fitted ring radius and the difference in the ring sizes of $v =$ 1 and $v =$ 2 masers (0.2 AU). The present result at the first epoch suggests that $v =$ 3 $J =$ 1–0 SiO masers are predominantly excited in a pumping mechanism (e.g., collisional pumping) different from that recently proposed on the basis of the line overlap with infrared H₂O lines. Interestingly, the second-epoch observation revealed that the $v =$ 3 features were located on almost the same ring as the $v =$ 2 ring, which is consistent with what line-overlap theory suggests.