On the thermal instability around the boundary of H II region in the Galactic center

The excitation of oscillation in the rotating disk of the Galactic center is considered. HII regions are usually thermally stable, because of the heating due to photoelectrons produced by ionizations of the UV radiation from central OB stars. The different point from the usual HII regions is that the Galactic center is the rotational HII region. The coupling between the rotational and non‐adiabatic effects brings the instabilities of nonradial oscillations in the HII region of the Galactic center. Hence, it is possible to say that the spiral and bar‐like structure is the trapped nonradial oscillations of gravity mode, excited by the heating of photoelectrons. Furthermore, the oscillation in the outer region surrounding the HII region is generated by the nonadiabatic resonance, where the epicyclic frequency is near to the Lamb frequency. This oscillation is expected to be the warp motion of the molecular ring.