Astron. Astrophys. 327, 337-341 (1997)

3. Results

3.1. The 9600Å window

In the 9600 Å window, we expected to find two strong DIBs at 9577 and 9632 Å. Indeed, the spectra of reddened stars show two strong absorptions near these positions (Fig. 2).

Fig. 2. Spectra of reddened stars divided by the spectra of unreddened stars from which the stellar lines were removed. Top: HD 43384 (B3Ia) corrected using HD 47105. Middle: HD 63804 (A0Iap) corrected with HD 81188. Bottom: HD 80077 (B2Iape) corrected with HD 81188. The last two spectra are displaced by -0.1 and -0.2 units.

The DIB 9577 has a rest wavelength (in air) at 9577.4 0.2 Å. The band profile is symmetric, with strong Lorenz-like wings. In HD 80077, the DIB has a Full-Width-at-Half-Maximum of FWHM = 3.0 0.2 Å. The equivalent width per unit E is 0.072 0.028, 0.105 0.011 and 0.122 0.013 respectively for HD 43384, HD 63804 and HD 80077. From the figure shown in Foing & Ehrenfreund (1994), we measure W/E = 0.19 (not 0.39 as given in their paper).

The absorption feature at 9632 Å is a blend of a DIB and the MgII stellar lines. The presence of the DIB 9632 is indicated by the high relative intensity of the 9632 Å blend in the reddened stars as compared to that in non-reddened stars (Tabel 1). Fortunately, there is not much telluric contamination at this wavelength, which allows us to do the following analysis. Assuming that there is an underlaying DIB in the 9632 Å feature with a similar profile as 9577, we substracted a 9577 profile from the 9632 Å feature after proper translation and scaling. When the subtraction is applied to the spectrum of HD 80077, we obtained a residual absorption which has the correct Doppler shift for a MgII stellar line (see Fig. 3 and Tab.1). When the procedure is applied to the spectrum of HD 63804, the same residual is now shifted by +1.8 Å, again consistent with a MgII line for this star (see Fig. 3 and Table 1). The best fit is obtained for a scaling factor of of 0.85 (0.7-0.9) (HD 80077) and 0.92 (0.8-0.95) (HD 63804) times the depth of 9577, which measures the relative intensity of both bands. We find that 9632 has W/E 0.10 0.02 and rest wavelength 9632.6 0.2 Å. In HD 43384, the DIB is weak compared to the stellar lines and the result is uncertain.

Table 1. Equivalent width (in mÅ) of absorption features in the 9600 Å window. *-shift: offset position stellar lines as observed from rest wavelength in air (in Å).

Fig. 3a and b. Overlay of DIB 9577 (dashed line) on the 9632 feature (dots). The difference spectrum (solid line) reveils the MgII stellar doublet.

3.2. The 9400 Å window

In the 9400 window, we searched for two weak features at 9366 and 9419 Å (taking into account the matrix shift of the C60 bands).

There are four strong stellar lines in the spectra (Fig. 4). The two strong lines at 9387 and 9394 Å are due to N I (9386.79 and 9392.80). The CI line at 9405.77 is present in the standard star HD 52089. In the spectrum of HD 63804 is an emission feature at 9429 Å, which remains unidentified. A strong emission band in HD 52089 at 9413 Å (see the middle spectrum in Fig. 4) is probably a "ghost" from internal reflections in the spectrograph. The 9413.46 line of SI is not responsible for the feature, as there should also be similarly strong lines at 9421.93 and 9437.11 (Moore 1972).

Fig. 4. As Fig. 2 - 9400 Å window. From top to bottom: HD 43384 (B3Ia) corrected using HD 34085 (B8Ia); HD 63804 (A0Iap) corrected with HD 52089 (B2II); HD 80077 (B2Iape) corrrected with HD 34085 (B8Ia). The last two spectra are displaced by -0.15 and -0.30 units.

We are able to put a strong upper limit for DIB absorption near 9419 Å in the spectrum of HD 80077 in this region, by using HD 52089 as a telluric standard. In that case, the baseline is not affected by the stellar line at about 9419 Å in the spectrum of HD 34085 (Fig. 4). The expected DIB at 9419 9 Å is not detected with certainty. There is a depression near 9428 Å, however, that would suggest W/E 0.013 Å per magnitude, or about 9% of the intensity of 9632, assuming a FWHM of 3.0 Å as expected if it is a C60 vibronic transition (Maier 1994). The reported ratio from laboratory measurements is about 15% (Fulara et al. 1993). However, that depression may well be caused by residual telluric lines.

For the expected position at 9366 Å, there are some candidate wiggles, with W/E = 0.017 Å in HD 63804 and 0.021 Å per magnitude in HD 80077. These values should be considered upper limits, because the features may well be flatfield or atmosphere line remnants. Only in HD 80077 reduced with HD 52089 is there a suggestion of a DIB. If this is a DIB, then the peak intensity is about 16 % of that of 9577. The expected ratio from laboratory measurements is about 30%.

© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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