Astronomy & Astrophysics

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$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics[angle=-90,clip]{0185fig1.eps}}\end{figure}$ Figure 1: The difference between the observed ( $M_V^{\rm H}$ ) and predicted ( $M_V^{\rm SK}$ ) absolute magnitude of O stars with $0 < \sigma _{\pi _{\rm H}}/\pi _{\rm H} < 1$ is not correlated with the projected stellar rotational velocity (Howarth et al. 1997). The typical error in $v \sin i$ is 20 km s^-1. The dotted line is the relation determined by Lamers et al. (1997).
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In the text

$\begin{figure} \resizebox{14cm}{!}{\includegraphics[angle=-90,clip]{0185fig2.eps}}\end{figure}$ Figure 2: The absolute magnitude difference calculated for O stars compared to that expected for stars uniformly distributed in a disk. The magnitude difference for real O stars (big dots) is defined as $\Delta M_V = M_V^{\rm H} - M_V^{\rm SK}$ , with $M_V^{\rm SK}$ from the Schmidt-Kaler et al. (1982) calibration, and $\lambda = \sigma _{\pi _{\rm H}} / \pi _{\rm H}$ . For the simulated stars (small dots) $\Delta M_{\rm V}$ is the difference between the true absolute magnitude and that calculated from the "observed'' parallax $\pi ^\prime \sim {\rm N}(\pi , \sigma _\pi )$ , and $\lambda = \sigma _\pi / \pi ^\prime$ with $\sigma _\pi = 0.82$ mas. The dotted lines indicate the radial distance at which simulated stars are located; the dashed line denotes the edge of the disk. The good agreement between the observations and the simulation indicates that large magnitude differences are to be expected when using the relatively uncertain Hipparcos parallax measurements of distant O stars.
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In the text

$\begin{figure} \resizebox{8.8cm}{!}{\includegraphics[angle=-90,clip]{0185fig3.eps}}\end{figure}$ Figure 3: A close-up of Fig. 2 for low values of $\lambda$ . It shows that absolute magnitudes inferred from the Hipparcos parallax are unreliable for $\sigma _{\pi _{\rm H}} / \pi _{\rm H} \gtrsim 0.15$ . The names of two O stars with the most accurate parallaxes are indicated.
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In the text

$\begin{figure} \resizebox{14cm}{!}{\includegraphics[angle=-90,clip]{0185fig4.eps}}\end{figure}$ Figure 4: The difference between the parallax observed by Hipparcos ( $\pi _{\rm H}$ ) and that expected from the absolute magnitude calibration by Schmidt-Kaler et al. (1982) ( $\pi _{\rm SK}$ ), calculated for all bright O stars in the sky. The average difference is not significantly different from zero if the absolute magnitude calibration is left unchanged ( $\Delta M_{V,{\rm cal}} = 0$ ). The consequences of adjusting the calibration by +2 and -4 mag are illustrated. The $\vartriangle$ point associated with $\zeta$ Oph ( $\pi _{\rm H} = 7.12$ ) is offscale at $\Delta \pi = -10.9$ mas.
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In the text

$\begin{figure} \resizebox{8.8cm}{!}{\includegraphics[angle=-90,clip]{0185fig5.eps}}\end{figure}$ Figure 5: Some B stars in the sample of Lamers et al. (1997) do not compare well to the simulation at small values of $\lambda$ (compare Fig. 3), indicating that their predicted absolute magnitude may be incorrect.
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In the text

$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics[angle=-90,clip]{0185fig1.eps}}\end{figure}$	Figure 1: The difference between the observed ( $M_V^{\rm H}$ ) and predicted ( $M_V^{\rm SK}$ ) absolute magnitude of O stars with $0 < \sigma _{\pi _{\rm H}}/\pi _{\rm H} < 1$ is not correlated with the projected stellar rotational velocity (Howarth et al. 1997). The typical error in $v \sin i$ is 20 km s^-1. The dotted line is the relation determined by Lamers et al. (1997).
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$\begin{figure} \resizebox{8.8cm}{!}{\includegraphics[angle=-90,clip]{0185fig3.eps}}\end{figure}$	Figure 3: A close-up of Fig. 2 for low values of $\lambda$ . It shows that absolute magnitudes inferred from the Hipparcos parallax are unreliable for $\sigma _{\pi _{\rm H}} / \pi _{\rm H} \gtrsim 0.15$ . The names of two O stars with the most accurate parallaxes are indicated.
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$\begin{figure} \resizebox{8.8cm}{!}{\includegraphics[angle=-90,clip]{0185fig5.eps}}\end{figure}$	Figure 5: Some B stars in the sample of Lamers et al. (1997) do not compare well to the simulation at small values of $\lambda$ (compare Fig. 3), indicating that their predicted absolute magnitude may be incorrect.
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