Red Giant Branch Stars: The Theoretical Framework

Abstract

ABSTRACT. Theoretical predictions of red giant branch stars' effective temperatures, colors, luminosities, and surface chemical abundances are a necessary tool for the astrophysical interpretation of the visible-near-infrared integrated light from unresolved stellar populations, the color-magnitude diagrams of resolved stellar clusters and galaxies, and spectroscopic determinations of red giant chemical abundances. On the other hand, the comparison with empirical constraints provides a stringent test for the accuracy of present generations of red giant models.We review the current status of red giant star modeling, discussing in detail the still-existing uncertainties affecting the model input physics (e.g., electron conduction opacity, treatment of the superadiabatic convection) and the adequacy of the physical assumptions built into the model computations.We compare theory with several observational features of the red giant branch in Galactic globular clusters, such as the luminosity function "bump," the luminosity of the red giant branch tip, and the envelope chemical abundance patterns, to show the level of agreement between current stellar models and empirical data concerning the stellar luminosities, star counts, and surface chemical abundances.

Evolutionary implications of the new triple-α nuclear reaction rate for low mass stars

“…Given the excellent agreement between stellar evolution models and observations of the RGB (Salaris et al 2002) when the models employ the NACRE (or relatively similar) triple-α reaction rates, such as Caughlan & Fowler (1988) or Fynbo et al (2005), it is difficult to envision how such a dramatic change as that proposed by OKK could lead to improved agreement with observations. Indeed, the triple-α reaction rate determined by OKK is in stark disagreement with fundamental observational evidence of stellar evolution, namely: (1) the existence of extended red giant branches in old stellar systems; (2) the location of core He-burning stars in the H-R or color-magnitude diagram; and (3) the lifetime ratio of the core He-burning phase to the red giant phase as manifested by the number ratio of horizontal 3.5 3.6 3. branch stars to RGB stars, also known as the R parameter (e.g.…”

Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters

“…The bolometric corrections are functions of metallicity, T eff and surface gravity and thus vary throughout evolution. Salaris, Cassisi & Weiss (2002) Lind et al (2008) in their study of NGC 6397. Their method, similar to ours described above, converts visual magnitude to luminosity by applying a calibration from Alonso, Arribas & Martínez-Roger (1999).…”

“…(i) By comparing the parameter ∆V bump HB = V bump − VHB; the V magnitude difference between the RGB bump and the horizontal branch at the RR Lyrae instability strip magnitude (Fusi Pecci et al 1990;Zoccali et al 1999;Salaris, Cassisi & Weiss 2002;Riello et al 2003;Bjork & Chaboyer 2006;Meissner & Weiss 2006;Di Cecco et al 2010). …”

Surface convection and red-giant radius measurements

“…i) The opacities: the low temperature opacities dominated by metallicity effects are critical in determining the effective temperature of giant stars at a given luminosity (Salaris et al 2002). Below log T = 3.75, we use the data from Ferguson et al (2005) 3 .…”

“…Although the effect of the EOS is somewhat less important than the effects of opacities or atmosphere boundary conditions (Salaris et al 2002), it still determines the temperature gradient in the convection zone and therefore affects the radius.…”