Population Synthesis in the Blue. I. Synthesis of the Integrated Spectrum of 47 Tucanae from Its Color‐Magnitude Diagram

Abstract

We perform an empirical synthesis of the blue integrated spectrum of the metal-rich globular cluster 47 Tucanae, based directly on the color-magnitude diagram of the cluster coupled to a moderately high-resolution spectral library. Freed from any significant dependence on theory, we are able to perform a fundamental test of the adequacy of the spectral library and its associated stellar parameters. Excellent fits are achieved for almost all absorption-line indices studied, provided the computations are corrected for two limitations of the spectral library, namely, the lack of a representative set of metal-poor giants and the absence of CN-strong stars. The latter effect is corrected by means of spectrum synthesis from model photospheres, considering the abundance pattern of CNstrong and CN-normal stars. We also need to perform a slight correction of the metallicity of the cluster (by -0.05 dex in relation to the standard value [Fe/H]=-0.7) in order to match the metal-line index measurements in the cluster spectrum.After these relatively small adjustments, the overall spectral agreement is good. Good fits are achieved for Hβ, Hγ, Mg b, < F e >, Ca4227 and Fe4383, and only Hδ F is overpredicted. Thus, ages inferred from Hδ F are slightly older than the ages based on the other Balmer lines, by ∼ 3 Gyrs. The success of this exercise suggests that previous failures to synthesize the spectrum of 47 Tuc must have arisen from inadequacies in the theoretical evolutionary isochrones and/or luminosity functions. Such a possibility is considered in a companion paper.

The impact ofα/Fe enhanced stellar evolutionary tracks on the ages of elliptical galaxies

“…The typical error is given in the lower-left corner. Black squares are the Bulge clusters NGC 6528 and NGC 6553 (Puzia et al 2002). suggests that the Red Giant Branch of the Salasnich et al (2000) α/Fe enhanced tracks is indeed too warm (Schiavon et al 2002). The integrated absorption indices of the two Bulge clusters NGC 6528 and NGC 6553 are consistent with both α/Fe enhanced and solar-scaled stellar tracks, as discussed before.…”

A grid of synthetic spectra and indices Fe5270, Fe5335, Mgb and Mg$_\mathsf{2}$ as a function of stellar parameters and [ $\mathsf{\alpha}$/Fe]

“…9 of Schiavon et al (2002) shows a clear separation between dwarfs and giants, therefore a clear dependence on gravity, and a well behaved function of temperature. The same is obtained with the Mgb indices computed from the synthetic spectra, if we select the same ranges of parameters as shown in Schiavon et al (2002). More complicated dependences on parameters appear however when the whole space of stellar parameters is used, which is possible with the grid of synthetic spectra.…”

“…Note that the difficulties in finding fitting functions from the whole grid of synthetic spectra does not appear when using observed indices: Fig. 9 of Schiavon et al (2002) shows a clear separation between dwarfs and giants, therefore a clear dependence on gravity, and a well behaved function of temperature. The same is obtained with the Mgb indices computed from the synthetic spectra, if we select the same ranges of parameters as shown in Schiavon et al (2002).…”

Structure of Disk‐dominated Galaxies. II. Color Gradients and Stellar Population Models

“…In their most basic form, commonly referred to as simple stellar populations (SSPs), these models provide evolutionary information for a coeval population of stars born with a given composition and initial mass function (IMF). The closest physical analog to such SSPs are globular cluster systems from which the SSP models are calibrated (e.g., Schiavon et al 2002). Several such SSP models have been produced by a number of independent groups and are in a constant state of modification as improvements to many of the input parameters (e.g., stellar libraries, model atmospheres, convection, mass loss, mixing) come to light.…”