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A blue depression is found in the spectra of M dwarfs from 400 to 450nm. This depression shows an increase toward lower temperatures though is particularly sensitive to gravity and metallicity. It is the single strongest and most sensitive feature in the optical spectra of M dwarfs. The depression appears as centred on the neutral calcium resonance line at 423nm and leads to nearby features being weaker by about two orders of magnitude than predicted in stars like Proxima Centauri. We consider a variety of possible causes for the depression including temperature, gravity, metallicity, dust, damping constants, atmospheric stratification as well as variety of molecular species. We find a simplistic line-broadening description can describe this calcium resonance line and potentially other un-modelled resonance lines in cool high pressure atmospheres. However, the derivation of atmospheric parameters for cool stars is significantly compromised until a general solution is implemented in model atmospheres.
Professor Hugh Jones is an ANSO visiting fellow working in the LAMOST group during the summer of 2023. He is a professor at the University of Hertfordshire in the UK with a career in the discovery and characterisation of nearby low-mass stars, brown dwarfs and exoplanets including Proxima Centauri which is the subject of this talk. Previous positions in Liverpool and Tokyo enabled him to build up instrumentation and model atmosphere experience. He started his studies of “The Coolest Dwarfs” as the title of his 1995 PhD thesis from the University of Edinburgh partly conducted at the Space Telescope Institute, Baltimore.