All Figures

$\begin{figure} \par\includegraphics[width=15.3cm,clip]{3832f1.eps} \end{figure}$ Figure 1: Spectra of the sample stars around H $\gamma$ and H $\alpha$ . The major spectral features are identified, short vertical marks indicate Fe II lines. Luminosity increases from bottom to top. Likewise, H $\alpha$ develops from a pure absorption line into a P-Cygni-profile because of the strengthening stellar wind. Note the marked incoherent electron scattering wings of H $\alpha$ in the most luminous object.
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In the text

$\begin{figure} \par\mbox{\resizebox{8.5cm}{!}{\includegraphics{3832f2a.eps}}\hsp... ...\hspace*{5mm} \resizebox{8.5cm}{!}{\includegraphics{3832f2d.eps}} } \end{figure}$ Figure 2: Comparison of different model atmospheres: temperature structure ( top) and electron density ( bottom) for two supergiant models as a function of the Rosseland optical depth. On the left for the LC Ib with stellar parameters according to our model for $\eta$ Leo, on the right for the LC Iae (HD 92207), corresponding to our sample objects at lowest and highest luminosity. Full line: A TLAS9 line-blanketed LTE model (adopted for the analysis), dashed: A TLAS9 H+He LTE model without line blanketing, dashed-dotted: T LUSTY H+He non-LTE model without line blanketing, boxes: grey atmosphere. A second A TLAS9 line-blanketed LTE model is displayed in the case of the Iae object (dotted), calculated with background opacities corresponding to solar metallicity. In our final model for HD 92207 the background opacities are reduced by an empirical factor of two in metallicity, correcting at least qualitatively for neglected non-LTE effects. In the inset the formation region of weak lines is displayed enlarged.
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In the text

$\begin{figure} \par\mbox{\resizebox{8.5cm}{!}{\includegraphics{3832f3a.eps}}\hsp... ...\hspace*{2mm} \resizebox{8.5cm}{!}{\includegraphics{3832f3f.eps}} } \end{figure}$ Figure 3: Comparison of theoretical non-LTE profiles for several diagnostic lines, calculated on the basis of different model atmospheres for A0 supergiants of LC Ib ( $\eta$ Leo, left) and Iae (HD 92207, right). Ordinate is normalised flux. The same line designations as in Fig. 2 are used, and the profiles are broadened accounting for instrumental profile, rotation and macroturbulence (see Table 2). Note that the He I and Mg I lines react sensitively to the detailed structure of the models while for the Balmer lines and the Mg II lines only the profiles of the line-blanketed and the unblanketed models can be discriminated well.
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In the text

$\begin{figure} \par\mbox{\resizebox{8.5cm}{!}{\includegraphics{3832f4a.eps}}\hsp... ...\hspace*{5mm} \resizebox{8.5cm}{!}{\includegraphics{3832f4d.eps}} } \end{figure}$ Figure 4: Influence of the helium abundance on the atmospheric structure ( bottom) and effects of metallicity ( middle) and microturbulence (top set of curves) on the atmospheric line blanketing: temperature structure ( upper panels) and electron density ( lower panels). On the left the comparison is made for a LC Ib object with stellar parameters matching those of our model for $\eta$ Leo, on the right for the LC Iae (HD 92207). Only one parameter is modified each time: atmospheric helium abundance of y = 0.089 (solar value, full line) and y = 0.15 (dotted line); using ODFs with [M/H] = 0.0, -0.3, -0.7 and -1.0 dex (full, dotted, dashed, dashed-dotted lines); using ODFs with $\xi$ = 8, 4 and 2 km s^-1 (full, dotted, dashed lines). In the inset, the formation region for weak lines is enlarged. For convenience, the individual sets of curves have been shifted by the factors indicated.
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In the text

$\begin{figure} \par\resizebox{8.5cm}{!}{\includegraphics{3832f5a.eps}}\hspace*{5... ...s}}\hspace*{4mm} \resizebox{8.5cm}{!}{\includegraphics{3832f5f.eps}}\end{figure}$ Figure 5: Influence of the helium abundance ( bottom), metallicity ( middle) and microturbulence ( top set of curves) on the profiles of diagnostic lines for our models for $\eta$ Leo and HD 92207. The same designations as in Fig. 4 are used, and the profiles are broadened accounting for instrumental profile, rotation and macroturbulence. Note the strong sensitivity of the lines to these "secondary'' parameters at LC Iae.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{3832f6.eps} \end{figure}$ Figure 6: Example for the occurrence of pressure inversion in supergiants of spectral types later than $\sim$ A4. The pressure stratification as a function of mass scale is displayed for two A TLAS9 models for $T_{\rm eff} = 7700$ K and $\log g = 0.42$ (the computed Eddington limit, dotted line) and 0.47 (full line). The inset compares the respective H $\delta$ profiles. A change in $\log g$ by 0.05 dex results in equivalent widths differing by a factor of 2 - an artefact of the pressure bump in the line-formation depth and its impact on the Stark broadening.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{3832f7a.eps}\par\vspace*{2mm} \includegraphics[width=7.8cm,clip]{3832f7b.eps}\end{figure}$ Figure 7: Comparison of non-LTE departure coefficients b_i for O I ( top) and Fe II ( bottom) in HD 92207 as a function of $\tau _{\rm R}$ . Displayed are thedeparture coefficients of the ground state (thick-dotted line), ofthe ground state of the next higher ion (long-dashed line) and those of the levels from which the diagnostic spectral lines in the optical/near-IR arise (O I). In the case of Fe II the departure coefficients of all non-LTE levels are shown. The two ionic species are representative for the non-LTE behaviour of the light (including the $\alpha$ -process elements) and iron group elements, see the text for further discussion.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3832f8a.eps}\hspace*{4mm} \includegraphics[width=8cm,clip]{3832f8b.eps} \end{figure}$ Figure 8: Impact of stellar parameter variations on non-LTE line profile fits for H $\delta$ in $\eta$ Leo ( left) and HD 92207 ( right). Spectrum synthesis for the adopted parameters (see Table 2, thick red line) and varied parameters, as indicated (thin lines), are compared to observation. A vertical shift by 0.5 units has been applied to the upper profiles. Note the decrease of the Balmer line strength in the progression from LC Ib ( $\eta$ Leo) to Iae (HD 92207), and the increased sensitivity to the surface gravity parameter. In fact, the comparison indicates that for the more luminous supergiants uncertainties in the surface gravity determination become as low as 0.05 dex (dotted lines). The sensitivity to reasonable $T_{\rm eff}$ -changes is almost negligible. Wind emission slightly contaminates the red wing of H $\delta$ in HD 92207.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{3832f9a.eps}\hspace*{4mm} \includegraphics[width=7.8cm,clip]{3832f9b.eps}\end{figure}$ Figure 9: Temperature determination for $\eta$ Leo using the Mg I/II non-LTE ionization equilibrium. Displayed are the observed line profiles for some of the strategic lines, Mg I $\lambda$ 5183 ( left panel) and Mg II $\lambda$ 4390 and He I $\lambda$ 4387 ( right panel), and the best non-LTE fit for stellar parameters as given in Table 2 (thick red line); theoretical profiles for varied parameters are also shown (thin lines, as indicated). A vertical shift by 0.3 units has been applied to the upper set of profiles. Note the strong sensitivity of the minor ionic species, Mg I, to parameter changes, while Mg II is virtually unaffected.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3832f10a.eps}\hspace*{4mm} \includegraphics[width=8cm,clip]{3832f10b.eps}\end{figure}$ Figure 10: Fit diagrams of temperature- and gravity-sensitive indicators for $\eta$ Leo and $\beta$ Ori in the $T_{\rm eff}$ - $\log g$ -plane. The curves are parameterised by surface helium abundance y. The intersection determines $T_{\rm eff}$ and $\log g$ , with uncertainties as indicated.
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In the text

$\begin{figure} \includegraphics[width=12cm,clip]{3832f11.eps}\end{figure}$ Figure 11: Determination of projected rotational and macroturbulent velocities, exemplarily shown for three spectral regions of $\eta$ Leo including discrete and blended absorption features. The upper comparison between observation (black line) and spectrum synthesis (red line, for finally derived mean abundances) is made for $\zeta$ = 16.5 km s^-1 and zero $v~\sin i$ (our finally adopted optimum values), the lower comparison is made assuming a pure rotational profile with $v~\sin i$ = 13.3 km s^-1. See the text for further discussion. Line identifiers are given. The markers are vertically extended by 0.05 units.
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{3832f12.eps}\end{figure}$ Figure 12: Computed (red full lines) and measured spectral energy distributions for the sample stars from the UV to the near-IR in the K-band. Overall, good to excellent agreement is obtained. Displayed are IUE spectrophotometry (full lines), Johnson (boxes), Strömgren (triangles), Walraven (filled circles) and Geneva photometry (open circles). The observations have been dereddened using colour excesses according to Table 2. A ratio R_V = A_V/E(B-V) = 3.1, as typical for the local ISM, is assumed for $\beta$ Ori and $\eta$ Leo. In the case of the notably reddened HD 92207 both, color excess and R_V, can be determined, implying R_V = 3.9. For clarity, the data for $\eta$ Leo has been shifted vertically by -3 units. IUE spectrophotometry for HD 111613 is not available. Consequently, this star has been omitted from the comparison.
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{3832f13.eps}\end{figure}$ Figure 13: Elemental abundances in $\eta$ Leo from individual spectral lines of elements with non-LTE calculations, plotted as a function of equivalent width: non-LTE (solid) and LTE results (open symbols) for neutral (boxes) and single-ionized species (circles). The grey bands cover the 1 $\sigma$ -uncertainty ranges around the non-LTE mean values (see Table 4). Note that in the determination of these some additional spectral lines have been used in some cases, which are accessible to spectrum synthesis only. Data for the strongest O I lines has been adopted from Przybilla et al. (2000). Proper non-LTE calculations reduce the line-to-line scatter and remove systematic trends. Note that even weak lines can show considerable departures from LTE.
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In the text

$\begin{figure} \par\mbox{\resizebox{8.5cm}{!}{\includegraphics{3832f14a.eps}}\hs... ...space*{2mm} \resizebox{8.5cm}{!}{\includegraphics{3832f14d.eps}} }\end{figure}$ Figure 14: Results from the elemental abundance analysis (relative to the solar composition) for our sample stars. Filled symbols denote non-LTE, open symbols LTE results. The symbol size codes the number of spectral lines analysed - small: 1 to 5, medium: 6 to 10, large: more than 10. Boxes: neutral, circles: single-ionized, diamonds: double-ionized species. The error bars represent 1 $\sigma$ -uncertainties from the line-to-line scatter. The grey shaded area marks the deduced metallicity of the objects within 1 $\sigma$ -errors. Non-LTE abundance analyses reveal a striking similarity to the solar abundance distribution, except for the light elements which have been affected by mixing with nuclear-processed matter.
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In the text

$\begin{figure} \par\includegraphics[width=16.5cm,clip]{3832f15.eps}\end{figure}$ Figure 15: Comparison of our spectrum synthesis (thin red line) with the high-resolution spectrum of HD 92207 (full line). The major spectral features are identified, short vertical marks designate Fe II lines. The location of the diffuse interstellar band around 4430 Å is indicated. With few exceptions, excellent agreement between theory and observation is found (H $\gamma$ is affected by the stellar wind).
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In the text

$\begin{figure} \par\includegraphics[width=12cm]{3832f16.eps} %\end{figure}$ Figure 16: The evolutionary status of the sample stars and mixing with nuclear-processed matter. Evolutionary tracks accounting for mass-loss and rotation from Meynet & Maeder (2003) are displayed, for solar metallicity at an initial rotational velocity of 300 km s^-1 (full line) and the non-rotating case (dotted line). Numbers along the tracks give the ratio of N/C (by mass, (N/C)₀ is the initial value). For the sample objects (dots) the observed ratios are given. The three more massive supergiants have most likely evolved directly from the main sequence, whereas $\eta$ Leo appears to have already undergone the first dredge-up (as inferred from its high N/C ratio and also the helium enrichment) and is currently in a blue-loop phase. Uncertainties in $\log T_{\rm eff}$ are of the order of the symbol size.
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In the text

$\begin{figure} \par\includegraphics[width=16.5cm,clip]{3832f17.eps} \end{figure}$ Figure 17: Same as Fig. 15, however artifically degraded to 5 Å resolution. The excellent agreement between theory and observation is preserved. Deviations are found only for H $\gamma$ , Mg II $\lambda$ 4481 (see text) and in the regions of the interstellar band around 4430 Å and near Ti II $\lambda$ 4501 (due to a CCD defect, cf. also Fig. 15.
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In the text

$\begin{figure} \par\includegraphics[width=16.5cm,clip]{3832f18.eps} \end{figure}$ Figure 18: Same as Fig. 17, but artifically degraded to a $S/N \simeq 50$ . The comparison with synthetic spectra for scaled abundances and modified temperatures (thin red lines, as indicated) shows that the models react sensitively to changes of the metal abundances only. Consequently, stellar parameters estimated from the spectral classification ( $T_{\rm eff}$ ) and from fitting of the higher Balmer lines ( $\log g$ ) suffice to constrain the stellar metallicity to $\sim$ 0.2 dex. The lower set of spectra in each panel has been shifted by -0.2 units.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3832f10a.eps}\hspace*{4mm} \includegraphics[width=8cm,clip]{3832f10b.eps}\end{figure}$	Figure 10: Fit diagrams of temperature- and gravity-sensitive indicators for $\eta$ Leo and $\beta$ Ori in the $T_{\rm eff}$ - $\log g$ -plane. The curves are parameterised by surface helium abundance y. The intersection determines $T_{\rm eff}$ and $\log g$ , with uncertainties as indicated.
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$\begin{figure} \par\includegraphics[width=16.5cm,clip]{3832f17.eps} \end{figure}$	Figure 17: Same as Fig. 15, however artifically degraded to 5 Å resolution. The excellent agreement between theory and observation is preserved. Deviations are found only for H $\gamma$ , Mg II $\lambda$ 4481 (see text) and in the regions of the interstellar band around 4430 Å and near Ti II $\lambda$ 4501 (due to a CCD defect, cf. also Fig. 15.
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