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Astron. Astrophys. 320, 181-184 (1997)

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4. Recommended fitting coefficients

The resulting 20 sets of all 4 fit parameters were grouped in order to establish useful relations involving two parameters of a bursting star: [FORMULA], and [FORMULA], vs. observables. Perhaps the best method is defined by the following steps.

1. For each [FORMULA], values of [FORMULA] corresponding to all listed [FORMULA] were averaged to establish a unique relation [FORMULA], independent of gravity.

2. Parameters [FORMULA] were averaged in all analysed spectra, which yielded the average value of [FORMULA] (standard deviation).

Unfortunately, normalization factors [FORMULA] are much less useful for averaging purposes. They exhibited significant scatter due to numerical roundoff errors in the fitting procedure, and will not be used in the following sections.

Table 2 gives four averaged values of [FORMULA], corresponding to each considered [FORMULA]. The averaged spectral hardening ratio, [FORMULA]. Such a value is fully consistent with earlier estimates, [FORMULA] (cf. Lewin et al. 1993). Note, however, that [FORMULA] 's discussed in this paper were determined form fits, which are not strictly blackbodies.


Table 2. Recommended fitting parameters. The average values [FORMULA], and [FORMULA] = [FORMULA] (standard deviation).

Now the whole fitting procedure has been repeated for each model atmosphere, but with fixed [FORMULA] and values of [FORMULA] given in Table 2. This has given the values of chemical potential µ, which are also listed in Table 2.

It is interesting to note, that the parameter µ is a function of surface gravity [FORMULA] in models of each considered [FORMULA]. Only in some range of model parameters, determination of chemical potential µ does not provide unique determination of [FORMULA] (e.g. at [FORMULA] K).

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998