Evolution of Chromospheric Activity: M67 Red Giants

Abstract

Echelle spectra of the Ca II H- and K-line region (λ3950) of 15 red giant stars in the open cluster M67 reveal atmospheric dynamics and determine chromospheric radiative losses in order to assess chromospheric heating requirements and to follow the evolution of chromospheric activity. M67 red giants in conjunction with giants in younger clusters create a continuous group of red giants in the color-magnitude diagram, with 0.1 ≤ (B-V) ≤ 1.65 along the red giant branch. M67 contains the more evolved clump giants as well. Asymmetric line emission cores, indicative of outflowing material, are found in a majority of the M67 giants on the red giant branch and occur over the complete sample, from MV = -0.8 to MV = 1.9, suggesting that outward mass motions are well established at these luminosities. Radiative losses, as measured by emission strengths from Ca II, decrease smoothly with decreasing stellar effective temperature in M67 and connect well to a combined sample of warmer cluster giants (NGC 2477, IC 4756, and the Hyades) with M ≤ 3 M☉ studied by Beasley & Cram. Stellar effective temperature predominantly determines the level of chromospheric Ca II losses for giants with M ≤ 3 M☉. No evidence is found for a sharp decline in the flux of Ca II predicted by the Rutten & Pylyser magnetic model for chromospheric heating. However, emission in field giants (which tend to be younger) suggests that sharp declines in surface flux with decreasing effective temperature characteristic of pure magnetic activity occur for ion species more highly excited than Ca II. Although acoustic models for chromospheric heating apparently agree with the measured Ca II flux levels for the coolest giants, additional heating processes must be present in warmer giants and clump stars. Clump giants exhibit Ca II fluxes consistent with stars of similar colors on the red giant branch, suggesting a renaissance in chromospheric heating occurs after evolution beyond the red giant branch. Chromospheric heating in the giants of solar metallicity is likely to be derived from multiple sources, both hydrodynamic causes as well as magnetically related processes. The relative contribution of each heating mechanism changes with temperature in the stellar atmosphere, and these proportions may change during a star's evolution from the main sequence.