I-band Asymptotic Giant Branch (IAGB) Stars. II. A First Estimate of Their Precision and a Differential Zero Point

Abstract

Hubble Space Telescope observations of 92 galaxies that have a strong showing of I -band asymptotic giant branch (IAGB) stars in their color–magnitude diagrams (CMDs) are used to measure the relative offset between the mean apparent I -band magnitudes of the IAGB population and the corresponding apparent I -band magnitudes of the TRGB as measured in the same frames (and CMDs) of those individual galaxies. This first exploratory, large-sample comparison is independent of any extinction (foreground or internal) that may be shared by these two populations. The marginalized luminosity functions used to determine the modal value of the IAGB population are well fit by a single, symmetric Gaussian. The difference in the two apparent magnitudes (in the sense IAGB minus TRGB) is −0.589 mag, with a combined standard deviation of ±0.119 mag. Adopting M _I = −4.05 mag for the TRGB stars, the modal absolute magnitude of the IAGB is then calculated to be M _I (IAGB) = −4.64 ± 0.12 mag. The ensemble dispersion quoted above gives a standard error on the mean of ±0.012 mag (based on the full sample of 92 galaxies). Independently, the three geometry-based zero-points for I -band AGB stars are found (in Paper I) to be M _I = −4.49 ± 0.003 mag in the LMC (4204 stars), M _I = −4.67 ± 0.008 mag for the SMC (916 stars), and M _I = −4.78 ± 0.030 mag for NGC 4258 (62 stars), leading to a global zero-point (weighted) average of = −4.64 ± 0.15 mag (stat). The scatter found in the anchors is comparable to the scatter in the field sample discussed here, but the calibration sample is small. The application of this method to galaxies well outside of the Local Group shows that these standard candles can readily be found and measured out to at least 9 Mpc, using already available archival data.

Keywords

• Carbon stars
• Distance indicators
• Asymptotic giant branch stars