On Using the Color-Magnitude Diagram Morphology of M67 to Test Solar Abundances

Abstract

The open cluster M67 has solar metallicity and an age of about 4 Gyr. The turn-off mass is close to the minimum mass for which solar metallicity stars develop a convective core during main sequence evolution as a result of the development of hydrogen burning through the CNO-cycle. The morphology of the color-magnitude-diagram (CMD) of M67 around the turn-off shows a clear hook-like feature, direct sign that stars close to the turn-off have convective cores. VandenBerg et al. investigated the possibility of using the morphology of the M67 turn-off to put constraints on the solar metallicity, particularly CNO elements, for which solar abundances have been revised downwards by more than 30% over the last few years. Here, we extend their work filling in the gaps in their analysis. To this aim, we compute isochrones appropriate for M67 using new (low metallicity) and old (high metallicity) solar abundances and study whether the characteristic turn-off in the CMD of M67 can be reproduced or not. We also study the importance of other constitutive physics on determining the presence of such a hook, particularly element diffusion, overshooting and nuclear reaction rates. We find that using the new solar abundance determinations, with low CNO abundances, makes it more difficult to reproduce the characteristic CMD of M67. This result is in agreement with results by VandenBerg et al. However, changes in the constitutive physics of the models, particularly overshooting, can influence and alter this result to the extent that isochrones constructed with models using low CNO solar abundances can also reproduce the turn-off morphology in M67. We conclude that only if all factors affecting the turn-off morphology are completely under control (and this is not the case), M67 could be used to put constraints on solar abundances.

Constraining mixing processes in stellar cores using asteroseismology

“…This has relevant consequences when isochrones are used to reproduce the shape of the CMD of a stellar population given a chemical composition and a set of input physics (VandenBerg et al 2007;Magic et al 2010). Previous methods of analysis do not allow us to ensure the existence or absence of a semiconvective region in stellar interiors.…”

“…The size of the overshooting region is limited further in the case of small convective cores. This is done in GARSTEC using a geometrical cutoff factor, allowing the overshooting region to extend only to a fraction of the convective zone (see Weiss & Schlattl 2008;Magic et al 2010, for details).…”

S-process in extremely metal-poor, low-mass stars

“…To test the influence and associated uncertainties of this poorly-understood phenomenon, two values have been chosen of the free parameter f that determines the extension of the overshooting regions: f = 0.016 and 0.07 (models M4 and M5 respectively - Table 1). The first value is a standard choice for this prescription of overshooting, because its results in the main sequence are comparable to the canonical choice of 0.2 pressure scale heights (Magic et al 2010), a value known to reproduce different observational constraints such as the width of the main sequence observed in different open clusters. The higher value is intended to represent a case where overshooting is much larger than in convective cores and deep stellar envelopes.…”

The Dependence of Convective Core Overshooting on Stellar Mass: A Semi-empirical Determination Using the Diffusive Approach with Two Different Element Mixtures

“…Comparisons between models using both approximations to overshooting over limited ranges in mass indicate the resulting internal structures are rather similar (e.g., Herwig et al 1997;Noels et al 2010;Magic et al 2010), suggesting perhaps a rough scaling between α ov and f ov . However, some asteroseismic studies claim to be able to distinguish the internal structures from both types of models, implying they are in fact not quite the same.…”