Astronomy & Astrophysics

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$\begin{figure} \par\includegraphics[width=8.1cm,clip]{aah4158f1a.eps}\hspace*{4mm} \includegraphics[width=8.1cm,clip]{aah4158f1b.eps}\end{figure}$ Figure 1: Left panel: K vs. (J-K) diagram for the LMC region of sky ( $101^\circ \le \alpha \le 61^\circ$ and $-77^\circ \le \delta \le -63^\circ$ ). The data are taken from the 2MASS Second Incremental Data Release. Note the red tail of visible C-stars departing from the sequence of red giants at $(J-K)\sim 1.4$ and extending up to $(J-K)\sim 2$ . Right panel: The same from DENIS Point Source Catalogue towards the LMC (Cioni et al. 2000).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f2.eps} \end{figure}$ Figure 2: A subset of theoretical isochrones described in Marigo & Girardi (2001), and transformed to JK photometry by means of Girardi et al. (2002) tables. TP-AGB models are computed with the $\kappa _{\rm fix}$ prescription. The metallicity is Z=0.008, and isochrones are presented at equally-spaced age intervals of $\Delta \log t = 0.1$ . The dotted lines mark the isochrone sections corresponding to C-stars (with C/O >1). The plot limits correpond closely to those of Fig. 1 for a LMC distance modulus of 18.5 mag. In these and other models in literature, C stars do not form any red tail.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f3.eps} %\end{figure}$ Figure 3: Distribution of point sources from the 2MASS Second Incremental Data Release in the LMC region of the sky, projected on the $(\alpha ,\delta )$ plane. To reduce confusion, we plot only the objects with $K_{\rm s}<12.5$ . The circle delimits the area selected for our analysis.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f4.eps} %\end{figure}$ Figure 4: 2MASS data for the innermost $4^\circ$ of the LMC. Note the red tail of visible C-stars extending up to $(J-K_{\rm s})\sim 2$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{aah4158f5a.eps}\hspace*{3mm} \includegraphics[width=8.1cm,clip]{aah4158f5b.eps} \end{figure}$ Figure 5: Simulated CMDs for both the Galaxy foreground and the LMC population. Panel a) The Galaxy foreground stars. The coloured dots mark the stars belonging to the disk (green) and halo (black) populations. Panel b) The LMC population, marked in blue for all stars before the TP-AGB phase, and separated into O-rich (cyan) and C-rich stars (red) during the TP-AGB phase. Note that the C-star population distribute almost vertically over the red giant branch.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f6.eps} %\end{figure}$ Figure 6: The same data as in Fig. 5a, but in the M_K vs. (J-K) plane.
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{aah4158f7a.eps}\hspace*{4m... ...ps}\hspace*{4mm} \includegraphics[width=8.1cm,clip]{aah4158f7d.eps} \end{figure}$ Figure 7: Simulated CMDs based on TP-AGB tracks with $\kappa _{\rm fix}$ prescription for molecular opacities. The simulations includes both the Galaxy foreground and the LMC population. The coloured dots mark several kinds of stars: foreground stars in the Galaxy disk (green) and halo (black); LMC giants before the TP-AGB phase (blue), and separated into O-rich (cyan) and C-rich stars (red) during the TP-AGB phase. Note that the C-star population distribute almost vertically over the red giant branch. Different (J-K) vs. $T_{\rm eff}$ transformations are tested in the simulations of the C stars: Panel a) The same as for M stars (based on Fluks et al. 1994); Panel b) Bergeat et al. (2001) one; Panel c) A fitting relation involving C/O (Eq. (1)). A modest inclination appears in the C-star sequence. Panel d) The same as in panel c), but plotting only the data with a maximum C/O value of 2.0.
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{aah4158f8a.eps}\hspace*{4mm} \includegraphics[width=8.1cm,clip]{aah4158f8b.eps}\end{figure}$ Figure 8: Left panels: (J-K) vs. $T_{\rm eff}$ and (J-K) vs. C/O relations for galactic C-stars. Observed data (filled squares) are from Bergeat et al. (2001, objects common to their tables 4 and 13). In the upper panel, lines show the predictions of the fit relation given by Eq. (1), assuming a constant C/O ratio, equal to 1.1 (solid) and 3.0 (dashed). Right panels: The residuals of fitting Eq. (1) to the data, plotted as a function of both $T_{\rm eff}$ and C/O. The data is well reproduced by this relation to within 0.1 mag.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f9.eps}\end{figure}$ Figure 9: Final C/O ratio (at the end of the TP-AGB phase) as a function of the stellar mass (at the onset of the TP-AGB phase), predicted by synthetic models with variable molecular opacities $\kappa _{\rm var}$ , and parameter set ( $\lambda =0.50$ , $\log T_{\rm b}^{\rm dred} = 6.4$ ). Results for solar metallicity (Z=0.019) are compared to those for LMC composition (Z=0.008).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f10.eps} \end{figure}$ Figure 10: Z=0.008 theoretical isochrones for TP-AGB models computed with variable molecular opacities. The dredge-up parameters are the same as in the models of Fig. 2. The dotted lines mark the isochrone sections corresponding to C-stars. The conversion to the (J-K) photometry is now based on Eq. (1).
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{aah4158f11a.eps}\hspace*{2... ...ps}\hspace*{2mm} \includegraphics[width=8.1cm,clip]{aah4158f11d.eps}\end{figure}$ Figure 11: Simulated H-R diagram based on TP-AGB tracks with variable molecular opacities (i.e. Marigo 2002). Note the departure of the C-star population away from the red giant branch towards redder colours. The mass loss prescriptions is taken from Vassiliadis & Wood (1993), assuming that AGB stars are either fundamental-mode or first overtone pulsators. Panel a) case $\log T_{\rm b}^{\rm dred} = 6.4$ , fundamental mode. Panel b) case $\log T_{\rm b}^{\rm dred}=6.45$ , fundamental mode. Panel c) case $\log T_{\rm b}^{\rm dred} = 6.4$ , first overtone. Panel d) case $\log T_{\rm b}^{\rm dred}=6.45$ , first overtone.
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{aah4158f12a.eps}\hspace*{3mm} \includegraphics[width=8.1cm,clip]{aah4158f12b.eps}\end{figure}$ Figure 12: Left panel: Simulated CMD based on TP-AGB tracks with variable molecular opacities (i.e. Marigo 2002) and $\log T_{\rm b}^{\rm dred}=6.45$ , assuming an half-and-half mixture of fundamental-mode and first-overtone pulsators for the stars which evolve into the C-star phase. Right panel: The same diagram for the selected 2MASS data. Notice the very good description of all CMD features, including the C-star red tail (in red) and the plume of O-rich low-mass TP-AGB stars (in cyan).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f3.eps} %\end{figure}$	Figure 3: Distribution of point sources from the 2MASS Second Incremental Data Release in the LMC region of the sky, projected on the $(\alpha ,\delta )$ plane. To reduce confusion, we plot only the objects with $K_{\rm s}<12.5$ . The circle delimits the area selected for our analysis.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f4.eps} %\end{figure}$	Figure 4: 2MASS data for the innermost $4^\circ$ of the LMC. Note the red tail of visible C-stars extending up to $(J-K_{\rm s})\sim 2$ .
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f6.eps} %\end{figure}$	Figure 6: The same data as in Fig. 5a, but in the M_K vs. (J-K) plane.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f9.eps}\end{figure}$	Figure 9: Final C/O ratio (at the end of the TP-AGB phase) as a function of the stellar mass (at the onset of the TP-AGB phase), predicted by synthetic models with variable molecular opacities $\kappa _{\rm var}$ , and parameter set ( $\lambda =0.50$ , $\log T_{\rm b}^{\rm dred} = 6.4$ ). Results for solar metallicity (Z=0.019) are compared to those for LMC composition (Z=0.008).
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{aah4158f10.eps} \end{figure}$	Figure 10: Z=0.008 theoretical isochrones for TP-AGB models computed with variable molecular opacities. The dredge-up parameters are the same as in the models of Fig. 2. The dotted lines mark the isochrone sections corresponding to C-stars. The conversion to the (J-K) photometry is now based on Eq. (1).
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