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Towards a fundamental calibration of stellar parameters of A, F, G, K dwarfs and giants *
G.P. Di Benedetto
Received 25 May 1998 / Accepted 21 August 1998
I report on the implementation of the empirical surface brightness technique using the near-infrared Johnson broadband colour as suitable sampling observable aimed at providing accurate effective temperatures of 537 dwarfs and giants of A-F-G-K spectral-type selected for a flux calibration of the Infrared Space Observatory (ISO). The surface brightness-colour correlation is carefully calibrated using a set of high-precision angular diameters measured by modern interferometry techniques. The stellar sizes predicted by this correlation are then combined with the bolometric flux measurements available for a subset of 327 ISO standard stars in order to determine one-dimensional temperature scales of dwarfs and giants. The resulting very tight relationships show an intrinsic scatter induced by observational photometry and bolometric flux measurements well below the target accuracy of 1 % required for temperature determinations of the ISO standards. Major improvements related to the actual direct calibration are the high-precision broadband K magnitudes obtained for this purpose and the use of Hipparcos parallaxes for dereddening photometric data.
The temperature scale of F-G-K dwarfs shows the smallest random errors closely consistent with those affecting the observational photometry alone, indicating a negligible contribution from the component due to the bolometric flux measurements despite the wide range in metallicity for these stars. A more detailed analysis using a subset of selected dwarfs with large metallicity gradients strongly supports the actual bolometric fluxes as being practically unaffected by the metallicity of field stars, in contrast with recent results claiming somewhat significant effects. The temperature scale of F-G-K giants is affected by random errors much larger than those of dwarfs, indicating that most of the relevant component of the scatter comes from the bolometric flux measurements. Since the giants have small metallicities, only gravity effects become likely responsible for the increased level of scatter.
The empirical stellar temperatures with small model-dependent corrections are compared with the semiempirical data by the Infrared Flux Method (IRFM) using the large sample of 327 comparison stars. One major achievement is that all empirical and semiempirical temperature estimates of F-G-K giants and dwarfs are found to be closely consistent between each other to within 1 %. However, there is also evidence for somewhat significant differential effects. These include an average systematic shift of % affecting the A-type stars, the semiempirical estimates being too low by this amount, and an additional component of scatter as significant as 1 % affecting all the comparison stars. The systematic effect confirms the results from other investigations and indicates that previous discrepancies in applying the IRFM to A-type stars are not yet removed by using new LTE line-blanketed model atmospheres along with the updated absolute flux calibration, whereas the additional random component is found to disappear in a broadband version of the IRFM using an infrared reference flux derived from wide rather than narrow band photometric data.
Key words: stars: atmospheres stars: fundamental parameters
Send offprint requests to: G. P. Di Benedetto
Online publication: October 22, 1998