Red giant branch bump brightness in 7 metal-poor globular clusters obtained with GAIA DR2

Abstract

The identification of the red giant branch bump brightness in metal-poor globular clusters is important for low-mass stellar evolution. The release of Gaia DR2 prompted us to revisit the red giant branch bump (RGBB) in galactic globular clusters. We apply a popular nonparametric density estimation approach, kernel density estimation (KDE), to explore the position of RGBB in 7 metal-poor globular clusters (GCs). The <inline-formula id="IEq1"><mml:math><mml:mi>G</mml:mi></mml:math></inline-formula> and <inline-formula id="IEq2"><mml:math><mml:mi>V</mml:mi></mml:math></inline-formula> magnitudes of the RGBB according to our clustering algorithm, <inline-formula id="IEq3"><mml:math><mml:msub><mml:mi>G</mml:mi><mml:mrow><mml:mi>B</mml:mi><mml:mo>,</mml:mo><mml:mi>K</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula> and <inline-formula id="IEq4"><mml:math><mml:msub><mml:mi>V</mml:mi><mml:mrow><mml:mi>B</mml:mi><mml:mo>,</mml:mo><mml:mi>K</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula>, respectively show the RGB bump magnitude detected by the KDE method in <inline-formula id="IEq5"><mml:math><mml:mi>G</mml:mi></mml:math></inline-formula> band and <inline-formula id="IEq6"><mml:math><mml:mi>V</mml:mi></mml:math></inline-formula> band. They show the overdensity location in the luminosity function of the RGB stars. Based on the results derived by KDE, a maximum-likelihood analysis via a Markov Chain Monte Carlo (MCMC) approach is adopted to detect the RGB bump feature and obtain more accurate RGBB brightnesses in <inline-formula id="IEq7"><mml:math><mml:mi>G</mml:mi></mml:math></inline-formula> band and <inline-formula id="IEq8"><mml:math><mml:mi>V</mml:mi></mml:math></inline-formula> band for the samples. We find that the red giant branch bump brightness becomes fainter as the global metallicity increases in clusters with <inline-formula id="IEq9"><mml:math><mml:mo stretchy="false">[</mml:mo><mml:mtext>M</mml:mtext><mml:mo stretchy="false">/</mml:mo><mml:mtext>H</mml:mtext><mml:mo stretchy="false">]</mml:mo><mml:mo>≤</mml:mo><mml:mo>‑</mml:mo><mml:mn>1.4</mml:mn></mml:math></inline-formula>. We present the empirical relation between the global metallicity <inline-formula id="IEq10"><mml:math><mml:mo stretchy="false">[</mml:mo><mml:mtext>M</mml:mtext><mml:mo stretchy="false">/</mml:mo><mml:mtext>H</mml:mtext><mml:mo stretchy="false">]</mml:mo></mml:math></inline-formula> and absolute magnitude <inline-formula id="IEq11"><mml:math><mml:msub><mml:mi>M</mml:mi><mml:mi>V</mml:mi></mml:msub></mml:math></inline-formula> of the red giant branch bump for clusters with <inline-formula id="IEq12"><mml:math><mml:mo stretchy="false">[</mml:mo><mml:mtext>M</mml:mtext><mml:mo stretchy="false">/</mml:mo><mml:mtext>H</mml:mtext><mml:mo stretchy="false">]</mml:mo><mml:mo>≤</mml:mo><mml:mo>‑</mml:mo><mml:mn>1.4</mml:mn></mml:math></inline-formula>. We verify that discrepancies between observations and theory for metal-poor globular clusters with <inline-formula id="IEq13"><mml:math><mml:mo stretchy="false">[</mml:mo><mml:mtext>M</mml:mtext><mml:mo stretchy="false">/</mml:mo><mml:mtext>H</mml:mtext><mml:mo stretchy="false">]</mml:mo><mml:mo>≤</mml:mo><mml:mo>‑</mml:mo><mml:mn>1.4</mml:mn></mml:math></inline-formula>....