Ultraviolet Stellar and Cluster Spectra Compared to Theoretical Templates
As part of NASA and Hubble Treasury programs, we are generating theoretical spectra for globular clusters and galaxies from first principles. To check their validity, we are assessing how well they match high-resolution echelle spectra of the Sun and nearby stars. Once agreement is reasonable, spectra at fixed metallicity and light-element enhancement are coadded with weights representing the relative contribution of various types of stars in old stellar systems, to generate theoretical composite spectra for single stellar populations. These are compared to globular cluster spectra, notably in M31, to derive properties such as age and metallicity.
These 1D LTE spectral calculations fit high-resolution spectra reasonably well over 2200–9000 Å for stars from A to K types, once gf values are optimized accordingly. Weighted coadditions of spectral calculations can match M31 globular clusters over this entire wavelength range. The age-metallicity degeneracy can be broken, but only with high-quality data, and only if rare stages of stellar evolution are incorporated where necessary.
Our theoretical composite spectra are limited to old systems, ≥ 1Gyr, by the range of stellar standards for calibration. Calculations run from 2200–9000 Å, as Castelli models stop at 3500 K, and line-list improvements stop at 9000 Å. We cannot establish yet whether fits are unique, for no goodness-of-fit metrics exist yet. The coaddition weights remain highly uncertain, as they are for empirical templates also.
KeywordsGlobular Cluster Theoretical Spectrum Spectral Calculation Stellar Spectrum Nearby Star
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