The Pistol Star and star

To model the "Pistol Star" and star #362 we have assumed the atmosphere to be composed of H, He, C, N, O, Si, and Fe. The new blanketed models provide a significant improvement in our knowledge of the physical properties of the "Pistol Star" compared with the results we obtained in Figer et al. (1998) using non-blanketed models. The new model solves the dichotomy between the "high"- and "low"-luminosity (Teff) solutions in Figer et al. (1998) through the analysis of the metal lines (see the excellent fits in Figure 13.1). The Si ii, Mg ii, and Fe ii lines "choose" the low-luminosity model. We find a luminosity of around 1.75 x 106L© and an effective temperature of Teff ~ 11,000 K. This result, which reduces the previous estimate of the star luminosity by a factor of two, shows the importance of the new generation of models. Given the Teff and high wind density of the object, we do find a degeneracy in the H/He ratio; see also Hillier et al. (1998b). In principle, fits of virtually equal quality may be obtained with H/He number ratios varying from 10 to 0.05. The only line that may help to break this degeneracy is the He i 2.112-vm absorption line, which seems to favor H/He number ratios between 3 and 0.05. An important consequence of this degeneracy is the mass fractions derived for Fe, Si, and Mg. Our models show that, once the H/He ratio falls below unity, the resulting metal abundances have to be scaled down. In other words, if H/He < 1 then we will obtain only an upper limit on the metal abundances. We obtain Solar iron abundance as an upper limit for the "Pistol Star" (see the line fits in Figure 13.1). This estimate is rather robust since there is a large number of Fe ii diagnostic lines. Further, if we assume that the object displays He enrichment consistent with an LBV evolutionary phase, H/He > 1, we may conclude that the "Pistol Star" has Solar Fe abundance, in agreement with previous estimates from differential analysis of cool stars in the GC (Carr et al. 2000; Ramirez et al. 2000). The silicon abundance is obtained from the three Si ii lines in the H band. The lines at 1.69 |vm are extremely sensitive to the effective temperature of the star as well as to the transition zone between the star's photosphere and wind, while the line at 1.718 |vm constitutes a more robust abundance diagnostic. In any case, our Si ~ 1.4Si© result should be regarded with some caution. Magnesium, on the other hand, provides more diagnostic lines through Mg ii both in the H band and in the K band. We regard Mg ~ 1.6Mg0 as our current best estimate. This slight enrichment in a-elements versus Fe seems to favor the situation of an IMF dominated by massive stars in the GC.

For star #362 (Geballe et al. 2000), nearly a twin of the "Pistol Star," we also obtain a luminosity of around 1.7 x 106L© and an effective temperature of Teff ~ 105,00 K (see the line fits in Figure 13.2). We derive a Solar iron abundance as upper limit as well and obtain Si ~ 1.8Si© and Mg ~ 2.2Mg0, in agreement with the slight enrichment in a-elements versus Fe found for the "Pistol Star."

Wavelength (nm)

Figure 13.2. Some UKIRT-CGS4 (R - 5,000) H-, K-, and L-band and Bra observations of the LBVc "#362" in the Quintuplet cluster and model fits.

Wavelength (nm)

Figure 13.2. Some UKIRT-CGS4 (R - 5,000) H-, K-, and L-band and Bra observations of the LBVc "#362" in the Quintuplet cluster and model fits.

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