Japanese VLBI network mapping of sio v = 3 J = 1-0 maser emission in W hydrae

Abstract

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. © 2010. Astronomical Society of Japan.