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 $5\mathrm{"}.5$ north of Sagittarius A${}^{\ast }$. We detected firstly the continuum emission at 340 GHz of IRS 7 using ALMA. The total flux density of IRS 7 is $S_{\nu }=448\pm 45\mu$Jy. The flux density indicates that IRS 7 has a photosphere radius of $R=1170\pm60 ~R_\odot$, which is roughly consistent with the previous VLTI measurement. We also detected a shell like feature with north extension in the H30$\alpha$ recombination line by ALMA. The electron temperature and electron density of the shell like structure are estimated to be $\bar{T}^\ast_{\mathrm e}=4650\pm500$Kand$\bar{n}{\mathrm e}=(6.1\pm0.6)\times10^4$cm$^{-3}$, respectively. The mass loss rate is estimated to be $\dot{m} \sim 1\times 10^{-4} M\odot$yr$^{-1}$, which is consistent with a typical mass loss rate of a pulsating red supergiant star with $M=20-25M_{\odot }$. 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 toward the south. The brightened southern half of the structure and the north extension would be a bow shock and a cometary-like tail structure, respectively.