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 ${5{^{\prime \prime}_{.}}5}$ north of Sagittarius A$^\ast$. 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 $S_{\, \nu} =448\pm 45\, \mu$Jy. The flux density indicates that IRS 7 has a photosphere radius of $R=1170\pm 60\, R_{\odot }$, 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 H30α recombination line using ALMA. The electron temperature and electron density of the shell-like structure are estimated to be $\bar{T}^\ast _\mathrm{e}=4650\pm 500\:$K and $\bar{n}_\mathrm{e}=(6.1\pm 0.6)\times 10^4\:$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 $M = 20$–$25\, M_{\odot }$. 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 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.