All Figures

$\begin{figure} \par\mbox{\includegraphics[width=8cm,height=9cm]{5763fig1.ps} \hspace{1cm} \includegraphics[width=8cm,height=9cm]{5763fig2.ps} } \end{figure}$ Figure 1: Left panel: Grotrian term diagram for our 217-level (207 for C I, 9 for C II and one for C III) carbon model. For clarity, only the 453 bound-bound radiative transitions for C I are shown here. Right panel: Grotrian term diagram for the smaller carbon model atom used to compute the non-LTE abundance corrections for the C I lines around 830.0-960.0 nm of particular interest here. The model includes 10 C I levels and 1 C II level. The bound-bound transitions in the triplet system generate from splitted levels in this model, producing three and six lines around $963.0 {\rm ~and~} 910.0$ nm respectively.
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In the text

$\begin{figure} \par\includegraphics[width=14cm,height=14cm]{5763fig3.ps} \end{figure}$ Figure 2: Departure coefficients from LTE ( left panels), line source and Planck function comparison ( middle panels) and resulting line profile ( right panels) for the C I line at $\sim$ 910.0 nm shown for different [Fe/H] for runs with our larger atomic model. The atmospheric models all have $T_{\rm {eff}}$ = 6000 K, log g = 4.50 [cgs] and [C/Fe] = 0, with [Fe/H] going from 0.0 ( top) to -3.0 ( bottom). The hydrogen collision efficiency has been set to $S_{\rm {H}}=10^{-3}$ . For the departure coefficients the solid curve represents the lower level and the dotted one the upper level for the transition of interest. In the middle panels the line source function (solid line) and Planck function (dashed line) are normalized to the continuum intensity. Location of $\tau _{\rm continuum}=1$ (solid line at log $_{10}~\tau_{500}\sim 0$ ) and $\tau_{\rm line~ center}=1$ (in LTE and non-LTE, dashed and solid line respectively) are shown by vertical lines in the left and middle panels. In all cases, the non-LTE line profile (solid line) is stronger than in LTE (dashed line).
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=8.6cm,height=11cm,clip]{5763fig4... ...5mm}\includegraphics[width=8.6cm,height=11cm,clip]{5763fig5.ps} } \end{figure}$ Figure 3: Results from multi- MULTI runs for a star with $T_{\rm {eff}}$ = 6000 K and log g = 4.50 and solar metallicity are shown for the C I line around 910.0 nm. The effect on the line strength of multiplying each of the different rates for radiative transitions in turn by a factor of 2 is shown in the left panel. The right panel shows the effect when varying the collisional rates individually by the same amount. Solid lines indicate positive contribution (strengthening), dashed lines mean negative contribution (weakening) from that particular transition to the strength of the line studied. The thickness indicates the relative change in the non-LTE equivalent width ( $W^{\rm pert}-W^{\rm ref}$ )/ $W^{\rm ref}$ between the perturbed and reference case, as in the figure legend, while the arrows indicate the net flow (evaluated at $\tau_{\rm line~ center}=1.0$ ) for the transitions.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=8.6cm,height=11cm,clip]{5763fig6... ...5mm}\includegraphics[width=8.6cm,height=11cm,clip]{5763fig7.ps} } \end{figure}$ Figure 4: Same as Fig. 3, but for [Fe/H]=-3.
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In the text

$\begin{figure} \par\includegraphics[angle=90,width=16cm,clip]{5763fig8.ps} \end{figure}$ Figure 5: The net rates for radiative (no markers) and collisional (with markers) transitions are shown in the upper left plot for the lower level of the transition at $\sim$ 910.0 nm in a model with $T_{\rm {eff}}$ = 6000 K, log g = 4.50 and solar metallicity. Only the rates for the transitions important for the level balance are shown, marked with different symbols. Their sum is also marked with crosses and confirms that the rest of the levels play a minor role here. The horizontal line at ${\rm d}n/{\rm d}t=0$ marks the expectation that the total sum of all rates is equal to zero. The levels in the legends are numbered accordingly to our atomic model (see Fig. 1), i.e. increasing with excitation potential. The middle left plot similarly shows the rates for the upper level of the transition, while the bottom left plot shows the total sum of the net radiative and collisional rates respectively (positive = upwards). The plots on the right show the same quantities, but for a model with [Fe/H] = -3. Note the different scale for the vertical axes in the plots with different metallicity.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=8.5cm]{5763fig9.ps} \hspace{0.25cm} \includegraphics[width=8.5cm]{5763fig10.ps} } \end{figure}$ Figure 6: Non-LTE abundance corrections over the grid of temperatures and gravities explored, with varying metallicity (as indicated in the insets), for the neutral carbon absorption line at 909.4834 nm (used e.g. by Akerman et al. 2004, as one of the abundance indicators for their determination of C in halo stars). The corrections refer to the case with [C/Fe] $_{\rm LTE}=+0.4$ , typical of metal-poor halo stars and have been obtained from calculations with fine splitting included in the atomic model. The four leftmost panels show the corrections we obtained when neglecting hydrogen collisions, while the rightmost panels represent the results with $S_{\rm {H}}=1$ . Large non-LTE corrections are present at high temperature and low gravity, in the various cases.
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In the text

$\begin{figure} \par\includegraphics[width=9cm,height=6cm]{5763fig11.ps} \end{figure}$ Figure 7: The non-LTE abundance corrections are plotted versus line strength for the C I lines around $941 {\rm ~and~} 910$ nm. The models have $T_{\rm {eff}}$ = 6000 K , log g = 4.0 and [Fe/H] = $-3,-2,-1~ {\rm and} ~0$ (for which different symbols are used in the figure: filled squares, diamonds, triangles and circles respectively). Here [C/Fe] = 0 and $S_{\rm {H}}=10^{-3}$ have been assumed. For each absorption line, the non-LTE corrections at different metallicities are connected to underline the effect of the metallicity change.
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In the text

$\begin{figure} \par\includegraphics[width=6.3cm,clip]{5763fig12.ps} \end{figure}$ Figure 8: The non-LTE abundance corrections we found in this work (vertical axis scale) for the 909.4834 nm absorption line in a range of atmospheric models are compared to the corresponding results in Takeda & Honda (2005), for the case $S_{\rm {H}}=1$ . The solid line marks the one to one relation, while the dotted lines include a region where the difference between the corrections in the two studies is limited to $\le$ 0.15 dex.
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In the text

$\begin{figure} \par\includegraphics[width=14cm,clip]{5763fig13.ps} \end{figure}$ Figure 9: The non-LTE abundance corrections (empty triangles: $S_{\rm H}=0$ ; filled triangles: $S_{\rm {H}}=1$ ) for the 34 halo stars in the sample by Akerman et al. (2004) as a function of $T_{\rm {eff}}$ a), log g b) and [Fe/H] c).
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{5763fig14.ps}\hspace*{4mm} \includegraphics[width=7.4cm,clip]{5763fig15.ps} \end{figure}$ Figure 10: Trends of [C/Fe] vs. [Fe/H] ( left panel) and [O/Fe] vs. [Fe/H] ( right panel) in Milky Way halo (squares) and disk stars (circles, Bensby & Feltzing 2006, from [C I] and [O I] lines free from non-LTE effects). The LTE values from Akerman et al. (2004), corrected for our different choice of carbon and oxygen solar abundances, are represented as empty squares in the two panels, while the non-LTE abundances are shown as filled squares. The non-LTE results are those obtained when neglecting collisions with hydrogen. Note that the [O/Fe] non-LTE values in the right plot for the five most metal-poor stars already include the new O I non-LTE abundance corrections we present here (see text).
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{5763fig16.ps}\par\vspace*{4mm} \includegraphics[width=12cm,clip]{5763fig17.ps} \end{figure}$ Figure 11: Trend of [C/O] vs. [O/H] in Milky Way halo and disk stars. The upper panel represents the new observational trends obtained when our non-LTE calculations are applied to the data in Akerman et al. (2004) (using our different choice of carbon and oxygen solar abundances and assuming negligible collisions with hydrogen). The [O/H] values already take into account our more negative non-LTE corrections for oxygen at low metallicity. The empty and filled squares represent the [C/O] LTE and non-LTE values respectively. The lower panel uses the same notation, with results now calculated accounting for hydrogen collisions for carbon (with a choice of $S_{\rm {H}}=1$ ). In both panels, the starred symbol indicates the [C/O] value we believe is more realistic for LP815-43 (see discussion in text).
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=8.6cm,height=11cm,clip]{5763fig6... ...5mm}\includegraphics[width=8.6cm,height=11cm,clip]{5763fig7.ps} } \end{figure}$	Figure 4: Same as Fig. 3, but for [Fe/H]=-3.
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$\begin{figure} \par\includegraphics[width=14cm,clip]{5763fig13.ps} \end{figure}$	Figure 9: The non-LTE abundance corrections (empty triangles: $S_{\rm H}=0$ ; filled triangles: $S_{\rm {H}}=1$ ) for the 34 halo stars in the sample by Akerman et al. (2004) as a function of $T_{\rm {eff}}$ a), log g b) and [Fe/H] c).
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