CHASE Spectral Survey I: Measuring Contamination in the Color-selected J-region Asymptotic Giant Branch Distance Indicator

Abstract

The J-region asymptotic giant branch (JAGB) method for distance measurements uses the near uniform luminosity of carbon-rich AGB stars as a standard candle. Its application rests on an assumed but rarely examined premise: the stars contained in the JAGB color-magnitude selection box are primarily carbon-rich AGB stars. We investigate the purity of the photometric selection in the Milky Way with an optical spectroscopic survey (~3590-7150 $\mathring{A}$) called CHASE (Carbon stars, Hubble tension, And Spectroscopic Exploration of JAGB populations) using the Shane 3 m telescope at Lick Observatory. These stars are classified using molecular bandhead step indices, which recover 100% of known classifications. We report the observations of 76 stars (54 within the standard JAGB selection box of 1.5 < J - K < 2.0 mag) and find only 13 of 54 stars are carbon-rich, a 76$\pm$6 oxygen-rich contamination under blind photometric selection. This contamination remains high (71%) even within $\pm$0.4 mag of the JAGB peak itself, showing it is not confined to the edges of the selection box. This inhomogeneity may explain why recent literature find field-to-field and methodological variations of 0.1-0.2 mag that now dominate its systematic error budget. These variations could be caused, in part, by inhomogeneous stellar populations that consist of differing spectral types, and hence differing mean magnitudes.