Tests of stellar model atmospheres by optical interferometry

Abstract

Context. Optical interferometry allows a measurement of the intensity profile across a stellar disc, leading to a direct test and calibration of theoretical model atmospheres as well as to a precise determination of fundamental stellar parameters. Aims. We present a comparison of the visual and near-infrared intensity profile of the M0 giant γ Sagittae to plane-parallel ATLAS 9 as well as to plane-parallel and spherical PHOENIX model atmospheres. Methods. We use previously described visual interferometric data obtained with the Navy Prototype Optical Interferometer (NPOI) in July 2000. We apply the recently developed technique of coherent integration, and thereby obtain visibility data of more spectral channels (526-852 nm) and with higher precision than before. In addition, we employ new measurements of the near-infrared K-band (∼2200 nm) diameter of γ Sagittae obtained with the instrument VINCI at the ESO VLT Interferometer (VLTI) in 2002. Results. The spherical PHOENIX model leads to a precise definition of the Rosseland angular diameter and a consistent high-precision diameter value for our NPOI and VLTI/VINCI data sets of Θ Ross = 6.06 ± 0.02 mas, with the Hipparcos parallax corresponding to R Ross = 55 ± 4 R , and with the bolometric flux corresponding to an effective temperature T eff = 3805 ± 55 K. Our visual visibility data close to the first minimum and in the second lobe constrain the limb-darkening effect and are generally consistent with the model atmosphere predictions. The visual closure phases exhibit a smooth transition between 0 and π. Conclusions. The agreement between the NPOI and VINCI diameter values increases the confidence in the model atmosphere predictions from optical to near-infrared wavelengths as well as in the calibration and accuracy of both interferometric facilities. The consistent night-by-night diameter values of VINCI give additional confidence in the given uncertainties. The closure phases suggest a slight deviation from circular symmetry, which may be due to surface features, an asymmetric extended layer, or a faint unknown companion.

ULySS: a full spectrum fitting package

“…The LSF was determined by using several stars in common between CFLIB and the ELODIE library. The results, presented in (Hekker & Meléndez 2007); interferometric measurements tend to indicate a lower T eff : 3650-3800 K, and hence probably lower gravity log(g) = 0.9 (Neilson & Lester 2008;Wittkowski et al 2006). This discrepancy is not inconsistent with ULySS, but the ELODIE interpolator surely deserves to be improved in this region of the HR diagram.…”

GaiaFGK benchmark stars: Effective temperatures and surface gravities

“…Mozurkewich et al (2003) integrated over the SED derived from broadband photometry alone, while Wittkowski et al (2006b) used Table 7. Masses M, parallaxes π, and luminosities L, and their uncertainties (absolute, u, and in percent, %u), for Gaia FGK benchmark stars.…”

Surface convection and red-giant radius measurements

“…IV, for details). We then searched the literature for more recent observations, and added the measurement of γ Sge by Wittkowski et al (2006), δ Eri and ξ Hya by Thévenin et al (2005), and the recent high accuracy CHARA/FLUOR measurements of Oph (Mazumdar et al 2009) and η Ser (Mérand et al 2010). When several independent angular diameter measurements were available for the same star, we combined them into a single value taking into account their original error bars and the consistency of the different measurements.…”

“…The bolometric luminosity was estimated using the K band photometry and the corresponding bolometric corrections BC(K) by Houdashelt et al (2000). For γ Sge, we extrapolated the BC(K) value for T eff = 3805 K (Wittkowski et al 2006). For Oph and δ Eri, we adopted the T eff metallicity and luminosity from De Ridder et al (2006) and Thevenin et al (2005), respectively.…”