Abstract

Context. Due to their wide wavelength coverage across the millimetre to centimetre (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 (EUV) to centimetre range as compiled from various catalogues and observatory archives. We compare 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) are quantified in terms of a “spectral flux excess” parameter (∆S/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 type from A1 to M3.5 and the Sunas-a-star as reference. The stars with Teff = 3000 − 7000 K (F – M type) showed a systematically higher S obs(ν) than S mod(ν) in the mm – cm range. Their ∆S/S mod exhibit a monotonic rise with decreasing frequency. The steepness of this rise is higher for cooler stars in the Teff = 3000 – 7000 K range, though the lone fully convective star (Teff ∼ 3000 K) in the sample deviated from this trend. Meanwhile, S obs(ν) of the A type stars agreed with S mod(ν) within errors. Conclusions. The systematically high ∆S/S mod in F – M stars points to the presence of hotter upper atmospheric layers, i.e. a chromosphere and corona, in these stars like for the Sun. The mm – cm ∆S/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.