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The precise mechanisms that provide the non-radiative energy for heating thechromosphere and the corona of the Sun and those of other stars constitute anactive field of research. By studying stellar chromospheres one aims atidentifying the relevant physical processes. Defining the permittable extent ofthe parameter space can also serve as a template for the Sun-as-a-star. Earlierobservations with Herschel and APEX have revealed the temperature minimum ofalpha Cen, but these were unable to spatially resolve the binary intoindividual components. With the data reported here, we aim at remedying thisshortcoming. Furthermore, these earlier data were limited to the wavelengthregion between 100 and 870mu. In the present context, we intend to extend thespectral mapping to longer wavelengths, where the contrast between stellarphotospheric and chromospheric emission becomes increasingly evident. ALMA isparticularly suited to point sources, such as unresolved stars. ALMA providesthe means to achieve our objectives with both its high sensitivity of thecollecting area for the detection of weak signals and the high spatialresolving power of its adaptable interferometer for imaging close multiplestars. This is the first detection of main-sequence stars at a wavelength of3mm. Furthermore, the individual components of the binary alpha CenAB areclearly detected and spatially well resolved at all ALMA wavelengths. The highS/N of these data permit accurate determination of their relative flux ratios.The previously obtained flux ratio of 0.44, which was based on measurements inthe optical and at 70mu, is consistent with the present ALMA results, albeitwith a large error bar. Given the distinct difference in their cyclic activity,the similarity of their submm SEDs appears surprising.

ALMA observations of alpha Centauri: First detection of main-sequence stars at 3mm wavelength