4. Comparison with direct measurements of stellar parameters
The first most obvious step is to compare the photometric estimates of stellar parameters with the direct measurements available for the calibrating stars of Table 3. Table 5 summarizes the results for eight of the most relevant comparison stars. The errors of each indirect determination are omitted, being not of relevance for the present purposes. Indeed, it can be seen that all photometric estimates show a very close agreement with the corresponding direct measurements at less than two sigmas, except for the Vega's flux. Of course, this good agreement is to be expected for angular diameters, all stars being primary calibrators of the correlation of Fig. 3. However, this is not the case for the predicted fluxes and then for the final temperatures of these stars. Notably, it should be appreciated the indirect determination of the Sun's temperature according to its well-determined near-infrared colour in the Johnson magnitude system (Campins et al. 1985). There are also somewhat discrepant data which include the angular diameter of CMi (1.6 SD) and the bolometric fluxes of Lyr (2.4 SD) and Boo (1.4 SD). The corresponding photometric temperatures differ by no more than 1 % from the measured ones, except the temperature of Lyr which shows a discrepancy as large as 2 % mainly due to the difference of about 10 % between predicted and measured bolometric fluxes. In fact, the integrated flux of Lyr is not correctly represented by its V, K photometry, in contrast with its photometric angular size nearly close to the interferometric measurement. Notice that, by using the TCS colour of = - 0.025 mag, the photometric diameter would decrease to the value 3.13 mas now discrepant at 1.6 SD with the measured one, but the flux would increase by only 1 % leaving the significant flux discrepancy unsolved.
Table 5. Comparison between measured and predicted stellar parameters by broadband photometry
On the other hand, this flux discrepancy seems to affect only the star Lyr, as it can be seen from the additional flux data listed in Table 6. Here, all the calibrating A-type stars of Table 3 are reported with their integrated fluxes. For all stars, but not for Lyr, the agreement is remarkably good at the 4 % level of accuracy quoted for each determination. This comparison enables us to estimate as well the amount of systematic error due to independent absolute calibrations which might affect flux determinations. With the exception of Lyr, the average difference is found to be as small as 0.5 %. This remarkable consistency for the critical field of A-type stars gives a further valuable support to the reliability of photometric temperatures for the ISO standard stars at the target accuracies quoted in Table 4, but not for Lyr whose flux discrepancy remains an unsolved problem.
Table 6. Comparison between measured and predicted integrated fluxes of A-type stars
© European Southern Observatory (ESO) 1998
Online publication: October 22, 1998