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$^ast$. We detected firstly the continuum emission at 340 GHz of IRS 7 using ALMA. The total flux density of IRS 7 is $S_ u=448pm45 mu$Jy. The flux density indicates that IRS 7 has a photosphere radius of $R=1170pm60 ~R_odot$, 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 $bar{T}^ast_{mathrm e}=4650pm500$ K and $bar{n}_{mathrm e}=(6.1pm0.6)times10^4$ cm$^{-3}$, respectively. The mass loss rate is estimated to be $dot{m} sim 1times 10^{-4} M_odot$ yr$^{-1}$, which is consistent with a typical mass loss rate of a pulsating red supergiant star with $M=20-25 M_odot$. 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.