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Figure 2: The classification diagram of the sample targets. Sources with  values above 4 are definied to be embedded sources (grey squares). Objects on the left side of the empirical separation line (dashed line) are group II members, on the right side group I. A classification solely based on the 60 micron excess gives almost equal results. Objects with a 60 micron excess exceeding 10 mag (black diamonds) are almost all group I sources, while targets with an excess below 10 mag (grey diamonds) belong mostly to group II. Some targets have been marked with their sequence number (see Table 5); those are the (tentative) nanodiamond sources (see Sect. 3.4).

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In the text

Figure 3: The ISAAC 3.5 micron spectra of GU CMa, HD 76534, HD 130437, V921 Sco and MWC 297. The Humphreys series of the H I atom are indicated with vertical lines. The spectra in the top panels are clearly different from the spectra in the bottom panels. The first 3 targets are most likely normal Be stars, with the 3 micron continuum flux emanating from the stellar photosphere. The latter two are early-type accretion-disk systems. The near-IR continuum is due to hot circumstellar dust.

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In the text

Figure 4: The infrared spectrum of V921 Sco (CD-4111721). The black curve is the ISO spectrum (R=150). The grey curve around 10 micron is the long-slit TIMMI2 spectrum (courtesy of R. van Boekel). This spectrum is scaled to the IRAS 12 micron flux. For clarity, a blow-up of the 3 micron wavelength region is shown in the inset. The grey curve in this panel is the ISAAC spectrum (R=750, not scaled); the black curve is the ISO spectrum at a resolution of 500. The slope of the ISAAC spectrum clearly deviates from the slope of the ISO spectrum at 3 micron, most likely due to slit losses. The ISO spectrum is dominated by strong PAH features. No PAH emission is detected in the ground-based spectra. The ISAAC spectrum shows the Pfund  line, the Humphreys series and possibly a 3.5 m feature. In the high-resolution ISO spectrum, the Pf  line is visible on top of the PAH 3.3 m feature. The TIMMI2 spectrum reveals that beneath the overwhelming PAH emission in the ISO spectrum, a shallow silicate absorption band resides.

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In the text

	Figure 5: Comparison of the peak-over-continuum ratio of the PAH 3.3 m feature, measured in the ISO and ISAAC spectrum. Filled symbols indicate targets for which we have detected the feature in at least one of the infrared spectra. The numbers refer to the target numbers listed in Table 5. Upper limits are indicated by arrows. The dashed line represents ratio ISAAC = ratio ISO.
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In the text

Figure 6: The line flux ratio of the PAH 3.3 m and the nanodiamond 3.53 m feature versus the NANO 3.53 m line flux. The dashed line indicates where both line fluxes are equal. The black square indicates the ISO measurements of Elias 3-1. The black diamonds represent HD 97048 and MWC 297. The grey diamonds are the nanodiamond candidates V921 Sco, HD 163296 and T CrA. The black dots represent the targets with a detected PAH 3.3, and undetected NANO 3.53 feature. Only in HD 97048 and Elias 3-1, both features are detected. The line flux ratio of the PAH 3.3 and NANO 3.53 features is smaller than one in these targets, while the upper limits on this ratio are larger than unity in the other (candidate) nanodiamond emitters.

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In the text

	Figure 7: The line flux ratio of Pf  and NANO 3.53 m versus the NANO 3.53 m line flux. Legend see Fig. 6.
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In the text

Figure 8: The X-ray luminosity versus the NANO 3.53 m line luminosity L3.5. The latter quantity is the line flux rescaled by a factor , with d the distance to the source as listed in Skinner et al. (2004), Hamaguchi et al. (2005) or Damiani et al. (2006). We apply the same distance factor to the 3.53 m line flux as was applied for the cited , to avoid systematic errors. The plotting symbols are the same as in Fig. 6. The (candidate) nanodiamond emitting sources have X-ray luminosities which are not uncommon in the group of sample stars observed in the X-ray wavelength range.

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In the text

	Figure 1: The ISAAC spectra of the sample stars, rebinned to a resolution of 750. When available, ISO-SWS (R = 150) or PHT-S () spectra are overplotted in grey. The part of the ISAAC spectrum between 3.3 and 3.32 m has been excluded. This part is unreliable due to the high atmospheric opacity in this wavelength range.
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In the text

	Figure 1: continued.
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In the text

	Figure 1: continued.
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In the text

Figure A.1: The normalized spectra of the candidate nanodiamond emitters. The bottom two spectra are a standard-star spectrum, and the inverse of it. They represent the spectral shape of an undercorrected continuum  (a) and an overcorrected continuum   (b). The peak position of the two curves are indicated by the edges of the grey box. The target spectra are flagged with a and b, based on the apparent shape of the continuum. The real detected nanodiamond features in MWC 297 and HD 97048 are plotted at the top of the figure. The latter is drawn in a separate box because of its impressive strength compared to the features or artefacts in the other spectra. The peak position of the 3.53 and 3.43 m features in this spectrum have been indicated by the dashed line. Most of the detected features can be brought back to data reduction artefacts. Only the features in V921 Sco, HD 163296 and T CrA may have deviating shapes.


In the text