IMPORTANT INFORMATION FOR ALL BASTI USERS

last modification: 2 Apr 2007

We have recomputed the library of α-enhanced models and isochrones from Z=0.001 to Z=0.04, employing α-enhanced low-T opacities obtained with the new opacity code by Ferguson et al. (2005, ApJ, 623, 585 - F05) .

The metal mixture and the grid of Z and Y values are the same as before.

The need for this recomputation has arisen from the results presented by Weiss et al. (2006, astro-ph/0605666). These authors have revealed that the old α-enhanced low-temperature (low-T) opacities by Alexander & Ferguson (1994, ApJ, 437, 879 - AF94) that we employed in our α-enhanced calculations, are affected by an error related to a series of typos in the input abundances to the opacity tables run at that time (this problem exists only for the α-enhanced AF94 data, not for the scaled solar ones). These erroneous opacities affect mainly the Teff of Red Giant Branch (RGB) models for metallicities above Z~0.001-0.002.

Due to the updated opacity code, the use of the F05 scaled solar opacities causes a change of the solar mixing length, that has to be increased by 0.12 compared to the value we obtained with the old AF94 scaled solar data. With the appropriate recalibration of the mixing length, 1Mo tracks of solar composition computed with the AF94 and F05 opacities are essentially indistinguishable (see Fig. 1).

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Figure 1: Comparison of the scaled solar evolutionary tracks for our standard solar model (from the pre-Main Sequence to the tip of the RGB) computed with the AF94 and F05 scaled solar opacities, and the appropriate solar mixing length calibrations.
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Figure 2: Comparison of the alpha-enhanced evolutionary tracks for Z=0.001, 1Mo models (from the pre-Main Sequence to the tip of the RGB) computed with the AF94 and F05 scaled solar opacities, and the appropriate solar mixing length calibrations.

This is true also for models of different masses and different metallicities. We have therefore employed the recalibrated solar mixing length when computing the new α-enhanced models; in this way they are completely consistent with our scaled solar database (that was computed with the AF94 opacities and the appropriate solar mixing length). Figures 2,3 and 4 display a comparison between the new and old 1.0Mo α-enhanced tracks, for the labelled metallicities.

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Figure 3: As in Fig2 but for Z=0.004.
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Figure 4: As in Fig2 but for Z=0.0198.

For Z<0.001 we have verified that α-enhanced models computed with the F05 opacities (and recalibrated mixing length) are identical to the existing models computed with the (erroneous) α-enhanced AF94 opacities. Therefore we did not recomputed the model database for Z<0.001.

We strongly recommend to use α-enhanced models computed with the F05 opacities.

Our best regards!!!