The red giant branch bump in rotating low-mass stars: Implication for the theory of stellar evolution

The red giant branch bump (RGB bump) is a key diagnostic of an internal stellar structure, yet the impact of stellar rotation— which is a fundamental parameter altering chemical stratification—on its luminosity, amplitude, and morphology remains unquantified across the physically relevant mass range (0.8--2.2,M_⊙). We present the first systematic investigation of how rotation modifies RGB bump properties (luminosity, amplitude, and duration) across 0.8--2.2,M_⊙, with the goal of quantifying the physical link between rotation-induced mixing, the mean molecular weight (μ) discontinuity, and the bump's observational signatures. Using the Modules for Experiments in Stellar Astrophysics ( ) code, we computed a grid of stellar models at solar metallicity (Z=0.014) with initial masses of 0.8--2.2,M_⊙ and initial rotational velocities ranging from 0 to 250, MESA Our models include a comprehensive treatment of angular momentum transport and rotation-induced chemical mixing. The RGB bump arises from structural readjustment when the hydrogen-burning shell (HBS) encounters the μ discontinuity left by the first dredge-up: a steeper μ gradient drives a more pronounced luminosity dip (larger bump amplitude) in the HBS, creating a distinct 'zig-zag' in the Hertzsprung-Russell (HR) diagram. Rotation-induced mixing modifies the RGB bump by smoothing the sharp μ gradient (hydrogen-helium discontinuity) and altering the depth of the first dredge-up. Rotation slightly reduces the dredge-up depth for the star with M<1.8 M_⊙, while it significantly shallows the convective envelope for M>1.8,M_⊙, flattening the μ gradient. Rotation-induced mixing depletes surface 12 C (lowering ^12C/^13C) and enriches nitrogen (raising ^14N/^12C). More critically, it affects the RGB bump by smoothing the sharpness and diminishing the amplitude of the hydrogen-helium discontinuity. This effect reduces the amplitude of the luminosity dip in the HBS. Slowly rotating stars (v_ ̊m ini km/s ) exhibit an RGB bump at a lower luminosity and a higher effective temperature than non-rotating models, a trend most pronounced in low-mass stars; whereas faster rotation delays the bump's onset and further weakens its amplitude. Our results demonstrate that rotation is a critical, previously unquantified driver of RGB bump diversity in stellar populations.