Angular Diameters: Vis, IR, indirect (Chair: P. Borde)
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1. How to use/combine existing measurements?

Use caution with the literature! Do not average all existing measurements without a second thought: interferometric groups have sometimes reanalyze their data (e.g. Mark III). Prefer homogeneous sets of measurements.



2. Brief review of indirect method for estimating angular diameters

- Infrared Flux Method (IRFM): e.g. Blackwell et al. 1994, A&A, 282, 899-910;

- Surface Brightness Method: e.g. Di Benedetto 1998, A&A, 339, 858-871;

- Spectrophotometric method: Cohen et al. 1999, AJ, 117, 1864-1889. Already reviewed on the first day of the workshop. For an application to interferometry, see Borde et al. 2002, A&A, 393, 183-193;

- The overall agreement between all these methods is better than 5%. However, for stars with typical diameters of 2-3 mas, this is still too large for the needs of interferometers with scales like the VLTI.



3. Discussions around the definition of UD/LD diameters:

- The limb-darkened diameter (LD) is usually defined by the Stefan-Boltzmann law: flux (W/m^2) = LD^2*sigma*Teff^4/4 (sigma is Boltzmann's constant). Therefore, LD does not dependent on the wavelength;

- Interferometers measure uniform disk diameters (UD) that depend on the wavelength. LD can be converted into UD provided assuming some kind of limb-darkening law. With linear limb-darkening coefficients, the conversion is straightforward using the formula from Hanbury Brown 1974, MNRAS, 167, 475-483;

- Refined limb-darkening laws abound in the literature: e.g. Hestroffer 1997, A&A, 327, 199-206 or Claret 2000, A&A, 363, 1081-1190;

- For a review on the different definitions of the radius in stellar models, see Baschek et al. 1991, A&A, 246, 374-382.



4. Achieving a very precise calibration of interferometers (non-exhaustive list)

- The effect of the variation of the baseline vector during the acquisition of a sequence of interferograms has to be evaluated;

- Wide-band filters induce a wavelength smearing effect (the resolution varies from one end of the fliter to the other) that has to be taken into account; 

- A direct computation of the expected V^2 in a wide-band filter from a model atmosphere is probably the key to high precision calibration. In this regard, the notion of UD diameter (valid at a unique wavelength) becomes useless.