The Role of Carbon in Red Giant Spectro-Seismology

Although red clump stars function as reliable standard candles, their surfacecharacteristics (i.e. T_eff, log g, and [Fe/H]) overlap with thoseof red giant branch stars, which are not standard candles. Recent results haverevealed that spectral features containing carbon (e.g. CN molecular bands)carry information correlating with the ”gold-standard” asteroseismicclassifiers that distinguish red clump from red giant branch stars. However,the underlying astrophysical processes driving the correlation between thesespectroscopic and asteroseismic quantities in red giants remain inadequatelyexplored. This study aims to enhance our understanding of this”spectro-seismic” effect, by refining the list of key spectral featurespredicting red giant evolutionary state. In addition, we conduct furtherinvestigation into those key spectral features to probe the astrophysicalprocesses driving this connection. We employ the data-driven The Cannonalgorithm to analyse high-resolution (R∼80,000) Veloce spectra from theAnglo-Australian Telescope for 301 red giant stars (where asteroseismicclassifications from the TESS mission are known for 123 of the stars). Theresults highlight molecular spectroscopic features, particularly thosecontaining carbon (e.g. CN), as the primary indicators of the evolutionarystates of red giant stars. Furthermore, by investigating CN isotopic pairs(that is, ^12C^14N and ^13C^14N) we find statisticallysignificant differences in the reduced equivalent widths of such lines,suggesting that physical processes that change the surface abundances andisotopic ratios in red giant stars, such as deep mixing, are the driving forcesof the ”spectro-seismic” connection of red giants.