EMISSA (Exploring Millimeter Indicators of Solar-Stellar Activity). I. The initial millimeter-centimeter main-sequence star sample

Context. Due to their wide wavelength coverage across the millimeter to centimeter (mm-cm) range and their increased sensitivity, modern interferometric arrays facilitate observations of the thermal and non-thermal radiation that is emitted from different layers in the outer atmospheres of stars.
Aims: We study the spectral energy distribution (S_obs(ν)) of main-sequence stars based on archival observations in the mm-cm range with the aim to study their atmospheric stratification as a function of stellar type.
Methods: The main-sequence stars with significant detection in mm bands were identified in the ALMA Science Archive. These data were then complemented with spectral flux data in the extreme ultraviolet to cm range as compiled from various catalogues and observatory archives. We compared the resultant S_obs(ν) of each star with a photospheric emission model (S_mod(ν)) calculated with the PHOENIX code. The departures of S_obs(ν) from S_mod(ν) were quantified in terms of a spectral flux excess parameter (ΔS∕S_mod) and studied as a function of stellar type.
Results: The initial sample consists of 12 main-sequence stars across a broad range of spectral types from A1 to M3.5 and the Sun-as-a-star as reference. The stars with T_eff = 3000-7000 K (F-M type) showed a systematically higher S_obs(ν) than S_mod(ν) in the mm-cm range. Their ΔS∕S_mod exhibits a monotonic rise with decreasing frequency. The steepness of this rise is higher for cooler stars in the T_eff = 3000-7000 K range, although the single fully convective star (T_eff ~ 3000 K) in the sample deviates from this trend. Meanwhile, S_obs(ν) of the A-type stars agrees with S_mod(ν) within errors.
Conclusions: The systematically high ΔS∕S_mod in F-M stars indicates hotter upper atmospheric layers, that is, a chromosphere and corona in these stars, like for the Sun. The mm-cm ΔS∕S_mod spectrum offers a way to estimate the efficiency of the heating mechanisms across various outer atmospheric layers in main-sequence stars, and thereby to understand their structure and activity. We emphasise the need for dedicated surveys of main-sequence stars in the mm-cm range.