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 $S_\nu=448\pm45 \mu$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 $\bar{T}^\ast_{\mathrm
e}=4650\pm500$ K and $\bar{n}_{\mathrm e}=(6.1\pm0.6)\times10^4$ 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 $M=20-25 M_\odot$. 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.
