The Acs Nearby Galaxy Survey Treasury. Ix. Constraining Asymptotic Giant Branch Evolution With Old Metal-Poor Galaxies

Abstract

In an attempt to constrain evolutionary models of the asymptotic giant branch (AGB) phase at the limit of low masses and low metallicities, we have examined the luminosity functions and number ratios between AGB and red giant branch (RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy Survey Treasury. This database provides Hubble Space Telescope optical photometry together with maps of completeness, photometric errors, and star formation histories for dozens of galaxies within 4 Mpc. We select 12 galaxies characterized by predominantly metal-poor populations as indicated by a very steep and blue RGB, and which do not present any indication of recent star formation in their color-magnitude diagrams. Thousands of AGB stars brighter than the tip of the RGB (TRGB) are present in the sample (between 60 and 400 per galaxy), hence, the Poisson noise has little impact in our measurements of the AGB/RGB ratio. We model the photometric data with a few sets of thermally pulsing AGB (TP-AGB) evolutionary models with different prescriptions for the mass loss. This technique allows us to set stringent constraints on the TP-AGB models of low-mass, metalpoor stars (with M < 1.5 M , [Fe/H] −1.0). Indeed, those which satisfactorily reproduce the observed AGB/RGB ratios have TP-AGB lifetimes between 1.2 and 1.8 Myr, and finish their nuclear burning lives with masses between 0.51 and 0.55 M . This is also in good agreement with recent observations of white dwarf masses in the M4 old globular cluster. These constraints can be added to those already derived from Magellanic Cloud star clusters as important mileposts in the arduous process of calibrating AGB evolutionary models.

Near-infrared photometric properties of asymptotic giant branch stars in the dwarf irregular galaxy IC 1613

“…It is evident that the straight gray line of g = 22.2 is located at the plume of AGB stars. We describe the isochrones (Girardi et al 2010) for the metallicity of Z = 0.001 with age of log(t yr ) = 9.0, 9.4 in the (g − K s , g) CMD. The isochrones of AGB stars also demonstrate that our assignment of separating magnitude between supergiant and AGB stars is appropriate.…”

“…The (J − K s , K s ) CMD of the bright stellar populations in IC 1613 was examined with theoretical isochrones of the AGB stars (Girardi et al 2010;Bressan et al 2012). Figure 5 shows the (J − K s , K s ) CMD of stars brighter than K s = 18.2 compared with the isochrones of two metallicities of A51, page 4 of 12 Z = 0.001, 0.008 and several ages.…”

“…The green and red dots correspond to the M-giant and C stars. The red curves in the (g − Ks, g) CMD are isochrones (Girardi et al 2010) for the metallicity of Z = 0.001 with an age of log(t yr ) = 9.0, 9.4. log (t yr ) = 8.1, 9.0, and 9.8. We also compared the color distribution of the M-giant stars with the theoretical isochrone.…”

The Expected Stellar Populations in the Kepler and CoRoT Fields

“…More recently, the code has become a fundamental tool to test stellar evolutionary tracks from the Padova-Trieste group (e.g. Girardi et al, 2009Girardi et al, , 2010Bianchi et al, 2011;Rosenfield & al., 2014). With the recent/ongoing addition of new quantities in the code, like the surface chemical composition and asteroseismic parameters, it is ready to be applied in the simulation and interpretation of large databases like those provided by CoRoT and Kepler.…”

Infrared spectroscopy of asymptotic giant branch stars in the Galactic bulge

“…1 effectively correspond to the evolutionary track on the AGB for a 1.5 M star, characterized by varying luminosities and mass-loss rates (see e.g. Girardi et al 2010). These fields therefore offer unique opportunities to study the evolution of 1.5 M stars and their circumstellar material as they evolve on the AGB.…”