All Figures

$\begin{figure} \par\includegraphics[width=17.25cm,clip]{3768fig1.eps} \end{figure}$ Figure 1: Blue spectra of 4U 0614+091 and 4U 1626-67. H and He lines are lacking. The emission lines are identified as O II-III and C II-III (see also Table 2). All absorption features are of interstellar (i.s.) origin: Ca II 3934/3968 Å, the 4300 Å CH band, and two diffuse interstellar bands (DIB). Plotted over the two observed spectra is a synthetic accretion disk spectrum reddened with E(B-V) = 0.55 and E(B-V) = 0.40, respectively.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=17.25cm,clip]{3768fig2.eps} \end{figure}$ Figure 2: Red spectra; 4U 0614+091 shows emission lines from C II-IV and O II-III, while the 4U 1626-67 spectrum is virtually featureless. All absorption features are either telluric (marked by horizontal bars) or of interstellar origin (Na I 5890/5896 Å and a DIB at 5780 Å). The features at 5575 Å are artifacts ("x'') due to an O I sky emission line. Overplotted is the synthetic accretion disk spectrum like in Fig. 1.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=17.25cm,clip]{3768fig3.eps} \end{figure}$ Figure 3: Complete VLT spectrum of 4U 1626-67 compared to a spectrum taken with HST. The VLT spectrum, scaled to the HST spectrum at 5600 Å, is flatter possibly due to problems in absolute flux calibration. The HST spectrum has a poorer resolution and S/N-ratio, although some emission features at $\lambda <5000$ Å can be recognized in both datasets. Artificial emission spikes are marked by "x''. Overplotted is the synthetic accretion disk spectrum, like in previous figures, but now reddened with E(B-V) = 0.20 to fit the continuum shape of the HST spectrum.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=17.25cm,clip]{3768fig4.eps} \end{figure}$ Figure 4: Detail from three single spectra of 4U 0614+091. Thick line: spectrum taken on Nov. 03, 2003. Two thin lines: spectra taken consecutively on Dec. 15, 2003 (see Table 1). The strength of the two strongest emission features, the C II multiplets at 6580 Å and 7235 Å, show an increase over a time interval of several weeks.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{3768fig5.eps} \end{figure}$ Figure 5: Vertical structure of the C/O-disk model at a distance of 20 000 km from the neutron star. The emergent disk flux at this location corresponds to $T_{{\rm eff}}$ = 28 000 K. The physical variables are plotted against the column mass measured from the surface towards the mid-plane.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{3768fig6.eps} \end{figure}$ Figure 6: Vertical ionisation stratification of chemical elements in the disk models at a distance of 20 000 km from the neutron star. The vertical line drawn at $\log m = -0.9$ indicates $\tau _{\rm Ross}=1$ .
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{3768fig7.eps} \end{figure}$ Figure 7: Comparison of models with zero and 10% hydrogen and helium content to the observations. From the lack of H $_\beta$ and He II emission lines in the observed spectra, we conclude that the accretion disks are strongly H and He deficient. The model spectra were normalised to the local continuum flux.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{3768fig8.eps} \end{figure}$ Figure 8: Comparison of models with zero and 10% neon content to the observations. The model including neon exhibits weak Ne II lines in the region 4350-4450 Å, which cannot be detected in the observed spectra.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=17.25cm,clip]{3768fig4.eps} \end{figure}$	Figure 4: Detail from three single spectra of 4U 0614+091. Thick line: spectrum taken on Nov. 03, 2003. Two thin lines: spectra taken consecutively on Dec. 15, 2003 (see Table 1). The strength of the two strongest emission features, the C II multiplets at 6580 Å and 7235 Å, show an increase over a time interval of several weeks.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{3768fig5.eps} \end{figure}$	Figure 5: Vertical structure of the C/O-disk model at a distance of 20 000 km from the neutron star. The emergent disk flux at this location corresponds to $T_{{\rm eff}}$ = 28 000 K. The physical variables are plotted against the column mass measured from the surface towards the mid-plane.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{3768fig6.eps} \end{figure}$	Figure 6: Vertical ionisation stratification of chemical elements in the disk models at a distance of 20 000 km from the neutron star. The vertical line drawn at $\log m = -0.9$ indicates $\tau _{\rm Ross}=1$ .
Open with DEXTER

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{3768fig7.eps} \end{figure}$	Figure 7: Comparison of models with zero and 10% hydrogen and helium content to the observations. From the lack of H $_\beta$ and He II emission lines in the observed spectra, we conclude that the accretion disks are strongly H and He deficient. The model spectra were normalised to the local continuum flux.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{3768fig8.eps} \end{figure}$	Figure 8: Comparison of models with zero and 10% neon content to the observations. The model including neon exhibits weak Ne II lines in the region 4350-4450 Å, which cannot be detected in the observed spectra.
Open with DEXTER