Sub-millimeter Detection of a Galactic Center Cool Star IRS 7 by ALMA

IRS 7 is an M red supergiant star which is located at 5'5 north of Sagittarius A'. We detected firstly the continuum emission at 340 GHz of IRS 7 using the Atacama Large Millimeter/submillimeter Array (ALMA). The total flux density of IRS 7 is S-nu = 448 +/- 45 mu Jy. The flux density indicates that IRS 7 has a photosphere radius of R = 1170 +/- 60 R-circle dot, which is roughly consistent with the previous Very Large Telescope Interferometer measurement. We also detected a shell-like feature with a northern extension in the H30 alpha recombination line using ALMA. The electron temperature and electron density of the shell-like structure are estimated to be (T) over bar (e)* = 4650 +/- 500 K and (n) over bar (e) = (6.1 +/- 0.6) x 10(4) cm(-3), respectively. The mass loss rate is estimated to be <(m)over dot> similar to 1 x 10(-4) M-circle dot yr(-1), which is consistent with a typical mass loss rate of a pulsating red supergiant star with M= 20-25 M-circle dot. The kinematics of the ionized gas would support the hypothesis that the shell-like structure made by the mass loss of IRS 7 is supersonically traveling in the ambient matter towards the south. The brightened southern half of the structure and the northern extension would be a bow shock and a cometary-like tail structure, respectively.