J/MNRAS/523/1297ivo://CDS.VizieR/J/MNRAS/523/1297doi:10.26093/cds/vizier/bg-gb-18hCDSJones H.R.A.Pavlenko Y.Lyubchik Y.Bessell M.Allard N.,Pinfield D.J.2026-08-03T07:32:55Z2023-12-28T13:47:17ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frm-starsvisible-astronomyspectroscopyA blue depression is found in the spectra of M dwarfs from 4000 to 4500{AA}. 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 4227{AA} and leads to nearby features being weaker by about two orders of magnitude than predicted. We consider a variety of possible causes for the depression including temperature, gravity, metallicity, dust, damping constants and atmospheric stratification. We also consider relevant molecular opacities which might be the cause identifying AlH, SiH, and NaH in the spectral region. However, none of these solutions are satisfactory. In the absence of a more accurate determination of the broadening of the calcium line perturbed by molecular hydrogen, we find a promising empirical fit using a modified Lorentzian line profile for the calcium resonance line. Such fits provide a simplistic line-broadening description for this calcium resonance line and potentially other un-modelled resonance lines in cool high pressure atmospheres. Thus we claim the most plausible cause of the blue depression in the optical spectra of M dwarfs is a lack of appropriate treatment of line broadening for atomic calcium. The broad wings of the calcium resonance line develop at temperatures below about 4000K and are analogous to the neutral sodium and potassium features which dominate the red optical spectra of L dwarfs.2023MNRAS.523.1297Jhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/523/1297CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/MNRAS/523/1297https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/MNRAS/523/1297http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/MNRAS/523/1297https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/MNRAS/523/1297https://vizier.iucaa.in/viz-bin/votable?-source=J/MNRAS/523/1297http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/MNRAS/523/1297GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/MNRAS/523/1297/stars (List of studied M dwarfs)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/MNRAS/523/1297/stars?https://vizier.iucaa.in/viz-bin/conesearch/J/MNRAS/523/1297/stars?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/MNRAS/523/1297/stars?GETtext/xml+votable180.050000true192.1989719108929-11.2075791635502780.0055555555555555566/5420 17648 22014 24719 25079 27462 31781 41591https://cdsarc.cds.unistra.fr/viz-bin/moc/J/MNRAS/523/1297?format=asciiOpticaldefaultJ/MNRAS/523/1297/starsList of studied M dwarfsNameStar namemeta.id;meta.maincharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right ascension (J2000)pos.eq.ra;meta.mainDEJ2000Declination (J2000)pos.eq.dec;meta.mainspShow the spectrummeta.ref.urlcharFileName1Name of the first spectrum file in subdirectory spmeta.id;meta.filecharsp2Show the spectrummeta.ref.urlcharFileName2Name of the second spectrum file in subdirectory spmeta.id;meta.filechar