Calibrating and standardizing the Tip of the Red Giant Branch in the Small Magellanic Cloud using small-amplitude red giants

TL;DR

This study calibrates the Tip of the Red Giant Branch in the Small Magellanic Cloud using small amplitude red giants, revealing population effects on distance measurements and proposing improved standardization methods.

Contribution

It introduces a new calibration of the TRGB using variability data from SARGs, accounting for population differences between the SMC and LMC.

Findings

The B-sequence provides the most accurate TRGB calibration.

Variability-based calibration reduces systematic uncertainties.

Population diversity affects TRGB distance estimates.

Abstract

We investigate the absolute calibration of the Tip of the Red Giant Branch (TRGB) in the Small Magellanic Cloud (SMC) using small amplitude red giant stars (SARGs) classified by the Optical Gravitational Lensing Experiment (OGLE). We show that all stars near the SMC's TRGB are SARGs. Distinguishing older and younger RGs near the Tip according to two period-luminosity sequences labeled A and B, we show many similarities among SARG populations of the LMC and the SMC, along with notable differences. Specifically, SMC SARGs have shorter periods due to lower metallicity and smaller amplitudes due to younger ages than LMC SARGs. We discover two period-color relations near the TRGB that span all A- and B-sequence stars in the OGLE-III footprints of the SMC and LMC, and we investigate using periods instead of color for TRGB standardization. Using variability derived information only, we trace the SMC's age and metallicity gradients and show the core to be populated by younger, more metal rich RGs. The B-sequence yields the brightest and most accurate calibration ($M_{\mathrm{F814W,syn}} = -4.057 \pm 0.019 (\mathrm{stat.}) \pm 0.029 (\mathrm{syst.})$ mag), which we use to measure the distance modulus difference between the Clouds and investigate metallicity effects. Distance measurements not informed by variability should employ the SARGs-based calibration based on all stars near the tip ($M_{\mathrm{F814W,syn}} = -4.024 \pm 0.041 (\mathrm{stat.}) \pm 0.029 (\mathrm{syst.})$ mag). Our work highlights the impact of RG population diversity on TRGB distance measurements. Further study is needed unravel these effects and improve TRGB standardization.