The variable mass-loss in low-mass red giant stars: Chromospheric motion across stellar evolution in 47 Tuc

Abstract

Mass-Loss (ML) is a critical process that remains poorly understood despite its broad implications across astrophysics. Our current knowledge is insufficient to reproduce ML consistently across stars, particularly in cool, low-mass giants, where the time-variable nature of chromospheres (CHR) ---the region where most physical phenomena powering ML in this regime are expected to occur--- is poorly constrained. We characterize the relevant timescales of CHR variability in red giants and explore correlations with intrinsic stellar properties. We analyze the first large temporal spectroscopic monitoring of CHR variability using the H$α$ line in red giants belonging to the globular cluster NGC 104 (47 Tuc) using FLAMES/GIRAFFE data. We found that after $\sim$4 days, for $\log(L/L_\odot)\gtrsim2.4$, variations in H$α$ on average increase and widen their range of values with increasing luminosity, together with a systematic blueshift of the line core ---indicative of outflows--- and deeper cores ---indicating an increased chromospheric optical depth. The characteristic timescale required to observe variations in the H$α$ profile is approximately one day, showing a flat trend with luminosity. While shorter-timescale variations remain consistent with statistical uncertainties, a possible increase toward dimmer stars is found. These trends appear similarly correlated with luminosity and effective temperature. No further correlation is found with different cluster sub-populations, nor are significant differences observed between AGB and RGB stars. The outflow trends closely match previous observations across a wide metallicity range. We discuss whether these observations reflect changes in H$α$ sensitivity to the winds, a genuine atmospheric transition driven by changes in heating efficiency, or continued CHR expansion with geometrically dominated H$α$ formation.