Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters

Abstract

We conduct a series of comparisons between spectroscopic and photometric observations of globular clusters and stellar models to examine their predictive power. Data from medium-to-high resolution spectroscopic surveys of lithium allow us to investigate first dredge-up and extra mixing in two clusters well separated in metallicity. Abundances at first dredge-up are satisfactorily reproduced but there is preliminary evidence to suggest that the models overestimate the luminosity at which the surface composition first changes in the lowest-metallicity system. Our models also begin extra mixing at luminosities that are too high, demonstrating a significant discrepancy with observations at low metallicity. We model the abundance changes during extra mixing as a thermohaline process and determine that the usual diffusive form of this mechanism cannot simultaneously reproduce both the carbon and lithium observations. Hubble Space Telescope photometry provides turnoff and bump magnitudes in a large number of globular clusters and offers the opportunity to better test stellar modelling as function of metallicity. We directly compare the predicted main-sequence turn-off and bump magnitudes as well as the distance-independent parameter ∆M V MSTO bump . We require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some clusters and cannot match the bump in low-metallicity systems. Changes to the distance modulus, metallicity scale and bolometric corrections may impact on the direct comparisons but ∆M V MSTO bump , which is also underestimated from the models, can only be improved through changes to the input physics. Overshooting at the base of the convective envelope with an efficiency that is metallicity dependent is required to reproduce the empirically determined value of ∆M V MSTO bump .

Carbon Isotope Ratios in M10 Giants

“…We note photometric observations of M10 place the location of the LFB at M V ∼ 0.7 (Nataf et al 2013). In order to match the location of the luminosity function bump, our models were shifted by +0.40 mags, comparable to shifts found for other clusters with similar metallicities (Zoccali et al 2000;Meissner & Weiss 2006;Cassisi et al 2011;Angelou et al 2015).…”

Convective boundary mixing in low- and intermediate-mass stars – I. Core properties from pressure-mode asteroseismology

“…Stellar models systematically fail to reproduce the magnitude of the bump in the luminosity function of globular clusters. The typical discrepancies of approximately 0.3 magnitudes (Cassisi & Salaris 1997;Salaris et al 2002;Angelou et al 2012Angelou et al , 2015 can straightforwardly be reconciled though a deeper first dredge-up (i.e., overshooting during the deepest penetration of the convective envelope). During the core-helium burning phase, canonical stellar models are currently unable to reproduce the observed gravity-mode period spacing.…”

Modelling of Red Giant Stars: The state-of-the-art

“…Several spectroscopic observations of metal poor Galactic halo stars provide however compelling evidence of an additional mixing process occurring when RGB stars reach the luminosity of the RGB bump: a sudden drop of the isotopic ratio 12 C/ 13 C, a decrease of the Li and C abundances and an increase of the N abundance is clearly shown by the empirical data. This 'non-canonical' mixing seems to be a universal process as it affects ∼ 95% of low-mass stars, regardless of whether they populate the halo field or clusters (see [51] and references therein). The mechanism that has been proposed as a possible source for this mixing episode should be associated with the molecular weight inversion due to the 3 He burning through the 3 He( 3 He, 2p) 4 He nuclear reaction, in the outer wing of the H-burning shell, and the associated thermohaline mixing.…”

“…This notwithstanding, there are still several shortcomings in the inclusion of this physical process in stellar model computations. At first, as it has been shown by [51], values of C th ∼1000 are required to reproduce the 12 C/ 13 C abundance pattern in halo RGB stars, but in order to reproduce the same trend for 7 Li abundances a quite smaller value is necessary (C th ∼200). In addition, hydrodynamical simulations predict a different expression for the diffusion coefficient, that is reasonably reproduced by Eq.…”