All Figures

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{2356fig1.eps} \end{figure}$ Figure 1: FTS spectra (summer and winter, in grey) compared to our synthesized spectrum (black) at the ¹²CO 2-0 bandhead near 2.29 $\mu$ m. The good match confirmes the quality of our model in the FLUOR wavelength region. The discrepancy in the Titanium line is due to a poorly known $\log{gf}$ value.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{2356fig2.eps} \end{figure}$ Figure 2: The transmission curves of the 4 narrow band filters used in the FLUOR instrument on IOTA together with a synthetic K-band spectrum of Arcturus. For cool late-type stars, the filter centered at 2.03 $\mu$ m probes a H₂O band in the spectrum (not present in Arcturus). For a K giant, the filters at 2.15 and 2.22 $\mu$ m both probe the continuum, and the 4th filter at 2.39 $\mu$ m covers the CO first overtone band (and possibly also H₂O).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{2356fig3.eps} \end{figure}$ Figure 3: Intensity profiles for a plane parallel model at $T_{\rm {eff}} = 4250$ K and $\log{g} = 1.0$ , normalized to the maximum intensity in the K-band at the center of the stellar disk. Each gray curve was computed for an OS wavelength within the K band. Clearly there is a wide range of shapes over the stellar disk, but integration over the bandpasses reduces the differences. Because the model is semi-infinite for all viewing angles, $\mu =0$ corresponds to a singularity.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{2356fig4.eps} \end{figure}$ Figure 4: Intensity profiles for a spherical model at $T_{\rm {eff}} = 4250$ K and $\log{g} = 1.0$ , normalized to the maximum intensity in the K-band at the center in the stellar disk. Each gray curve was computed for an OS wavelength within the K band. We now see even more variety than in Fig. 3, mainly toward the stellar limb. The 2.39 $\mu$ m filter differs significantly from the others in this outer region¹.
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In the text

$\begin{figure} \par\includegraphics[width=8.35cm,clip]{2356fig5.eps} \end{figure}$ Figure 5: A comparison between the FLUOR visibility measurements of Arcturus and a plane-parallel MARCS visibility model ( $T_{\rm {eff}} = 4250$ K, $\log{g} = 1.50$ ; $\chi ^2_{{\rm r}} = 5.63$ for $\phi = 21.32 \pm 0.19$ mas). The four theoretical curves coincide, while there is a significant dispersion of the data points.
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In the text

$\begin{figure} \par\includegraphics[width=8.35cm,clip]{2356fig6.eps} \end{figure}$ Figure 6: For the spherical model with slightly lower surface gravity ( $T_{\rm {eff}} = 4250$ K, $\log{g} = 1.00$ ; $\chi ^2_{{\rm r}} = 5.38$ for $\phi = 22.37 \pm 0.19$ mas), the visibility curve in the CO-probing filter no longer coincides with the others, corresponding to a slightly larger diameter, but it still cannot match the measured visibilities.
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{2356fig7.eps} \end{figure}$ Figure 7: Narrow-band data on Arcturus obtained in 2001 with different calibrators. The best fitting LD disk model (full line) shown here ( $\chi ^2_{{\rm r}} = 64.5$ ) leaves the same kind of residuals as seen in the 2002 data.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{2356fig8.eps} \end{figure}$ Figure 8: Squared visibility as a function of time, and the best fitting binary model.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{2356fig9.eps} \end{figure}$ Figure 9: Squared visibility as a function of time, and the best fitting single-star SPH model.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{2356fig10.eps} \end{figure}$ Figure 10: Marginal $\chi ^2$ for separation $\rho$ , position angle $\theta$ , contrast ratio r and primary angular diameter $\phi _1$ . Global minima are marked by adotted line.
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{2356fig11.eps} \end{figure}$ Figure 11: Confidence intervals as a function of separation $\rho$ and position angle $\theta$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{2356fig12.eps} \end{figure}$ Figure 12: Confidence intervals as a function of contrast ratio r and primary angular diameter $\phi _1$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{2356fig13.eps} \end{figure}$ Figure 13: Upper panel: comparison of new UBVG (Geneva), JHKL (SAAO) and IRAS fluxes with a synthetic SED computed from our MARCS model atmosphere for Arcturus (A). There is no clear indication of a composite SED. Lower panel: the companion we suggest here (Kurucz model shown) would not be detectable in the residuals after subtraction of the primary.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{2356fig14.eps}\vspace*{2mm} \includegraphics[width=8.5cm,clip]{2356fig15.eps} \end{figure}$ Figure 14: Luminosity as a function of initial mass for a system of 10^9.85 and 10^10.25 years old. For the companion, we indicate the possible range in luminosity The two peaks in luminosity on the right hand side correspond to the RGB and AGB phase respectively.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{2356fig1.eps} \end{figure}$	Figure 1: FTS spectra (summer and winter, in grey) compared to our synthesized spectrum (black) at the ¹²CO 2-0 bandhead near 2.29 $\mu$ m. The good match confirmes the quality of our model in the FLUOR wavelength region. The discrepancy in the Titanium line is due to a poorly known $\log{gf}$ value.
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$\begin{figure} \par\includegraphics[width=8.35cm,clip]{2356fig5.eps} \end{figure}$	Figure 5: A comparison between the FLUOR visibility measurements of Arcturus and a plane-parallel MARCS visibility model ( $T_{\rm {eff}} = 4250$ K, $\log{g} = 1.50$ ; $\chi ^2_{{\rm r}} = 5.63$ for $\phi = 21.32 \pm 0.19$ mas). The four theoretical curves coincide, while there is a significant dispersion of the data points.
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$\begin{figure} \par\includegraphics[width=8.35cm,clip]{2356fig6.eps} \end{figure}$	Figure 6: For the spherical model with slightly lower surface gravity ( $T_{\rm {eff}} = 4250$ K, $\log{g} = 1.00$ ; $\chi ^2_{{\rm r}} = 5.38$ for $\phi = 22.37 \pm 0.19$ mas), the visibility curve in the CO-probing filter no longer coincides with the others, corresponding to a slightly larger diameter, but it still cannot match the measured visibilities.
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$\begin{figure} \par\includegraphics[width=8.1cm,clip]{2356fig7.eps} \end{figure}$	Figure 7: Narrow-band data on Arcturus obtained in 2001 with different calibrators. The best fitting LD disk model (full line) shown here ( $\chi ^2_{{\rm r}} = 64.5$ ) leaves the same kind of residuals as seen in the 2002 data.
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$\begin{figure} \par\includegraphics[width=8.2cm,clip]{2356fig8.eps} \end{figure}$	Figure 8: Squared visibility as a function of time, and the best fitting binary model.
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$\begin{figure} \par\includegraphics[width=8.2cm,clip]{2356fig9.eps} \end{figure}$	Figure 9: Squared visibility as a function of time, and the best fitting single-star SPH model.
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$\begin{figure} \par\includegraphics[width=8.2cm,clip]{2356fig10.eps} \end{figure}$	Figure 10: Marginal $\chi ^2$ for separation $\rho$ , position angle $\theta$ , contrast ratio r and primary angular diameter $\phi _1$ . Global minima are marked by adotted line.
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$\begin{figure} \par\includegraphics[width=8.1cm,clip]{2356fig11.eps} \end{figure}$	Figure 11: Confidence intervals as a function of separation $\rho$ and position angle $\theta$ .
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{2356fig12.eps} \end{figure}$	Figure 12: Confidence intervals as a function of contrast ratio r and primary angular diameter $\phi _1$ .
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$\begin{figure} \par\includegraphics[width=8.2cm,clip]{2356fig13.eps} \end{figure}$	Figure 13: Upper panel: comparison of new UBVG (Geneva), JHKL (SAAO) and IRAS fluxes with a synthetic SED computed from our MARCS model atmosphere for Arcturus (A). There is no clear indication of a composite SED. Lower panel: the companion we suggest here (Kurucz model shown) would not be detectable in the residuals after subtraction of the primary.
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$\begin{figure} \par\includegraphics[width=8.5cm,clip]{2356fig14.eps}\vspace*{2mm} \includegraphics[width=8.5cm,clip]{2356fig15.eps} \end{figure}$	Figure 14: Luminosity as a function of initial mass for a system of 10^9.85 and 10^10.25 years old. For the companion, we indicate the possible range in luminosity The two peaks in luminosity on the right hand side correspond to the RGB and AGB phase respectively.
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