Does the Masing Gas Fall Inward VY CMa?

The water maser emission from the unusual supergiant, VY CMa, has been observed with the Urumqi 25 m radio telescope. A simple red-shifted maser spectrum was detected in the 1.6 Jy noise level during our monitoring observations. All of the H₂O maser features are red-shifted with respect to the VY CMa stellar velocity of 17.6 km.s^-1. The spectrum appears to be consisted of two striking like groups and each group has three features. The appearances and the ratio of these three features in the two maser groups are very similar. The H₂O maser emission arises from a region close to the supergiant, about 10 stellar radii of the star. The simple red-shifted spectrum could be due to infall masing gas on the near side of the disk. Pulsation of the central star causes oscillations in this portion of the envelope, with shock wave propagating outward and with material falling inward. The masing gas motions are very complex, by which the dramatic changes or the quasi-sinusoidal fluctuation in flux may be explained.