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 Sν = 448 ± 45 μJy. The flux density indicates that IRS 7 has a photosphere radius of R = 1170 ± 60 R☉, 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 T¯e∗ = 4650 ± 500 K and n¯ e = (6.1 ± 0.6) × 104 cm−3, respectively. The mass loss rate is estimated to be m̊ ∼ 1 × 10−4 M☉ yr−1, which is consistent with a typical mass loss rate of a pulsating red supergiant star with M = 20-25 M☉. 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. © The Author(s) 2020. Published by Oxford University Press on behalf of the Astronomical Society of Japan.