Abstract
The age measurement of the stellar halo component of the Galaxy is based mainly on the comparison of the main sequence turn-off luminosity of the globular cluster (GC) stars with theoretical isochrones. The standard procedure includes a vertical shift, in order to account for the distance and extinction to the cluster, and a horizontal one, to compensate the reddening. However, the photometry is typically performed with broad-band filters where the shape of the stellar spectra introduces a shift of the effective wavelength response of the system, dependent on the effective temperature (or color index) of the star. The result is an increasing distortion—actually a rotation and a progressive compression with the temperature—of the color-magnitude diagrams relatively to the standard unreddened isochrones, with increasing reddening. This effect is usually negligible for reddening E ( B - V ) on the order of or smaller than 0.15, but it can be quite relevant at larger extinction values. While the ratio of the absorption to the reddening is widely discussed in the literature, the importance of the latter effect is often overlooked. In this contribution, we present isochron simulations and discuss the expected effects on age dating of high-reddening globular clusters.
Highlights
Interstellar reddening causes a reduction of the flux received from the stars following the well-known Whitford law [1,2]
In practical cases of wide-band photometry, the interstellar extinction convolution with the spectra of stars having different Teff causes a shift in the effective wavelength of the bands, whose effects are not much considered in the literature
One of the main results of the paper is an increase of the RV ratio for the spectral type of the stars most used in the extinction determination for globular clusters (GCs), and a consequent shortening of the Galaxies 2017, 5, 28; doi:10.3390/galaxies5030028
Summary
Interstellar reddening causes a reduction of the flux received from the stars following the well-known Whitford law [1,2]. [6] published detailed results on the selective extinctions for stars with temperatures in the range 3500 ≤ Teff (K) ≤ 40, 000, metallicities equal to [Fe/H] = −2.0, −1.0, and +0.5, and luminosity classes I, III, and V, based on Kurucz synthetic spectra and Scheffer interstellar extinction. The basis of the stellar temperature dependence is well established, none of these papers clearly addressed the issue of the systematic effects of the reddening on GC ages derived from isochrone fitting in color-magnitude diagrams (CMDs) In this contribution, we show how even a relatively moderate extinction AV = 1 can produce a significant bias when the age is derived from main sequence (MS) turn off (TO) fitting using isochrones not corrected for temperature-dependent extinction. Our simulations include the Johnson BVI and ACS@HST F606W/F775W/F814W photometric bands
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