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Register a new userMass loss on the red giant branch: plasmoid-driven winds above the RGB bump
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The onset of cool massive winds in evolved giants is correlated with an evolutionary feature on the red giant branch known as the bump. Also at the bump, shear instability in the star leads to magnetic fields that occur preferentially on small length scales. Pneuman (1983) has suggested that the emergence of small scale flux tubes in the Sun can give rise to enhanced acceleration of the solar wind as a result of plasmoid acceleration (the melon seed mechanism). In this paper, we examine the Pneuman formalism to determine if it may shed some light on the process that drives mass loss from stars above the bump. Because we do not currently have detailed information for some of the relevant physical parameters, we are not yet able to derive a detailed model. Instead, our goal in this paper is to explore a proof of concept. Using parameters that are known to be plausible in cool giants, we find that the total mass loss rate from such stars can be replicated. Moreover, we find that the radial profile of the wind speed in such stars can be steep or shallow depending on the fraction of the mass loss which is contained in the plasmoids. This is consistent with empirical data which indicate that the velocity profiles of winds from cool giants range from shallow to steep.
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We present a comparison between theoretical models and the observed magnitude difference between the horizontal branch and the red giant branch bump for a sample of 53 clusters. We find a general agreement, though some discrepancy is still present at the two extremes of the metallicity range of globular clusters.
We obtain stringent constraints on the actual efficiency of mass loss for red giant branch stars in the Galactic globular cluster 47 Tuc, by comparing synthetic modeling based on stellar evolution tracks with the observed distribution of stars along the horizontal branch in the colour-magnitude-diagram. We confirm that the observed, wedge-shaped distribution of the horizontal branch can be reproduced only by accounting for a range of initial He abundances --in agreement with inferences from the analysis of the main sequence-- and a red giant branch mass loss with a small dispersion. We have carefully investigated several possible sources of uncertainty that could affect the results of the horizontal branch modeling, stemming from uncertainties in both stellar model computations and the cluster properties such as heavy element abundances, reddening and age. We determine a firm lower limit of ~0.17$Mo for the mass lost by red giant branch stars, corresponding to horizontal branch stellar masses between ~0.65Mo and ~0.73Mo (the range driven by the range of initial helium abundances). We also derive that in this cluster the amount of mass lost along the asymptotic giant branch stars is comparable to the mass lost during the previous red giant branch phase. These results confirm for this cluster the disagreement between colour-magnitude-diagram analyses and inferences from recent studies of the dynamics of the cluster stars, that predict a much less efficient red giant branch mass loss. A comparison between the results from these two techniques applied to other clusters is required, to gain more insights about the origin of this disagreement.
We performed a detailed study of the evolution of the luminosity of He-ignition stage and of the red giant branch bump luminosity during the red giant branch phase transition for various metallicities. To this purpose we calculated a grid of stellar models that sample the mass range of the transition with a fine mass step equal to ${rm 0.01M_odot}$. We find that for a stellar population with a given initial chemical composition, there is a critical age (of 1.1-1.2~Gyr) around which a decrease in age of just 20-30 million years causes a drastic drop in the red giant branch tip brightness. We also find a narrow age range (a few $10^7$ yr) around the transition, characterized by the luminosity of the red giant branch bump being brighter than the luminosity of He ignition. We discuss a possible link between this occurrence and observations of Li-rich core He-burning stars.
We compare model predictions to observations of star counts in the red giant branch bump (RGBB) relative to the number density of first-ascent red giant branch at the magnitude of the RGBB, $EW_{RGBB}$. The predictions are shown to exceed the data by $(5.2 pm 4.3)$% for the BaSTI models and by $(17.1 pm 4.3)$% for the Dartmouth models, where the listed errors are purely statistical. These two offsets are brought to zero if the Galactic globular cluster metallicity scale is assumed to be overestimated by a linear shift of $sim 0.11$ dex and $sim 0.36$ dex respectively. This inference based on RGBB star counts goes in the opposite direction to the increase in metallicities of ${Delta}$[M/H]$approx$0.20 dex that would be required to fix the offset between predicted and observed RGBB luminosities. This comparison is a constraint on deep mixing models of stellar interiors, which predict decreased rather than increased RGBB star counts. We tabulate the predictions for RGBB star counts as a function of [Fe/H], [$alpha$/Fe], CNONa, initial helium abundance, and age. Though our study suggests a small zero-point calibration issue, RGBB star counts should nonetheless be an actionable parameter with which to constrain stellar populations in the differential sense. The most significant outliers are toward the clusters NGC 5025 (M53), NGC 6723, and NGC 7089 (M2), each of which shows a $sim 2 sigma$ deficit in their RGBB star counts.
The location of Galactic Globular Clusters (GC) stars on the horizontal branch (HB) should mainly depend on GC metallicity, the first parameter, but it is actually the result of complex interactions between the red giant branch (RGB) mass loss, the coexistence of multiple stellar populations with different helium content, and the presence of a second parameter which produces dramatic differences in HB morphology of GCs of similar metallicity and ages (like the pair M3--M13). In this work, we combine the entire dataset from the Hubble Space Telescope Treasury survey and stellar evolutionary models, to analyse the HBs of 46 GCs. For the first time in a large sample of GCs, we generate population synthesis models, where the helium abundances for the first and the extreme second generations are constrained using independent measurements based on RGB stars. The main results are: 1) the mass loss of first generation stars is tightly correlated to cluster metallicity. 2) the location of helium enriched stars on the HB is reproduced only by adopting a higher RGB mass loss than for the first generation. The difference in mass loss correlates with helium enhancement and cluster mass. 3) A model of pre-main sequence disc early loss, previously developed by the authors, explains such a mass loss increase and is consistent with the findings of multiple-population formation models predicting that populations more enhanced in helium tend to form with higher stellar densities and concentrations. 4) Helium-enhancement and mass-loss both contribute to the second parameter.
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