Spectroscopic differentiation of stars within the Red Clump

Abstract

Although Red Clump (RC) stars have been increasingly used as standard candles to measure stellar extinctions, distances, chemistry and kinematics both in and out of the Milky Way due to the small variation within their properties, this ever growing interest and appreciation in RC methods seems to not have been followed by the development of easier ways to quickly identify RC stars, for the RC and a part of the Red Giant Branch (RGB) are superimposed in the Hertzsprung-Russell (HR) diagram and the only way to differentiate them has so far been by resorting to complicated chemical and asteroseismological methods. After APOKASC "a project devoted to asteroseismology" revealed an offset between the surface gravities determined with spectroscopy and those determined with asteroseismology, the idea that there could exist a missing spectroscopic variable that allows to distinguish between RC and RGB stars by spectroscopic methods alone soon came out and recent studies have found that such variable could be related to the CN-cycle of stars, being the CN I absorption spectral lines stronger in RC stars than in similar RGB stars. In this dissertation, that line of work is resumed by measuring the equivalent width (EW) of 9 absorption spectral lines (4 Fe I, 1 Ti I and 4 CN I lines) within the wavelength range [7400 Å, 7500 Å] for a sample of 31 stars within the RC's neighborhood in the HR diagram. Although the sample of stars used was too small to really "prove" something, no counterexample to the hypothesis could be found; at least at first glance, the results seem to indicate that the limit EWTi/EWCN = 2.5 is of great significance since all RGB stars but one fell above it and all RC stars fell below it, forming a remarkably small clump in the (Absolute visual magnitude, EWTi/EWCN) plane. Furthermore, it was found that adding the EWTi/EWCN dimension to the classical 2D HR diagram results in a "3D HR space" that seems to be able to differentiate the 2 evolutionary stages (RC and RGB) as distinct 3D clumps and also suggests that some of the stars whose evolutionary status could not be found in literature are actually carbon stars traveling up the Asymptotic Giant Branch (AGB), for they also manifest as a distinct 3D clump. In any case, however, the fact that the strength of the lines of the CN I molecule seems to be able to differentiate RC and RGB stars reveals a puzzling difference between their photosphere's C-content that does not find any explanation within classical low-mass stellar evolution theory. This phenomenon could be caused by some still-to-be-explained non-canonical convection mechanism related to the helium flash, but far more evidence is required to assert this with strong conviction.