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The precise mechanisms that provide the non-radiative energy for heating thechromosphere and the corona of the Sun and other stars are at the focus ofintense contemporary research. Observations at submm/mm wavelengths areparticularly useful to obtain information about the run of the temperature inthe upper atmosphere of Sun-like stars. We used ALMA to study the chromosphericemission of the aCen binary system in all six available frequency bands duringCycle2 in 2014/15. Since ALMA is an interferometer, the multi-telescope arrayis particularly suited for the observation of point sources. With its largecollecting area, the sensitivity is high enough to allow the observation ofnearby main-sequence stars at submm/mm wavelengths for the first time. Thecomparison of the observed spectral energy distributions with theoretical modelcomputations provides the chromospheric structure in terms of temperature anddensity above the stellar photosphere and the quantitative understanding of theprimary emission processes. Both stars were detected and resolved at all ALMAfrequencies. For both aCenA and B, the existence and location of thetemperature minima, firstly detected from space with Herschel, are wellreproduced by the theoretical models of this paper. For aCenB, the temperatureminimum is deeper than for A and occurs at a lower height in the atmosphere,but for both stars, Tmin/Teff is consistently lower than what is derived fromoptical and UV data. The submm/mm emission of the aCen stars is indeed verywell reproduced by modified chromospheric models of the Quiet Sun. This mostlikely means that the non-radiative heating mechanisms of the upper atmospherethat are at work in the Sun are operating also in other solar-type stars.

ALMA's view of the Sun's nearest neighbours: The submm/mm SEDs of the alpha Centauri binary and a new source