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Astron. Astrophys. 318, 535-542 (1997)

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2. Observations, reductions and results

The spectroscopic observations were carried out at four different runs, in February and September 1989, in August 1990, and in August 1994, at the ESO 1.4 m CAT and the CES spectrometer. The slit width was set to 0.47 mm, corresponding to [FORMULA] on the sky. The spectral resolution was 53,000. A number of different slit positions were tried, with the star in the slit, with the star outside the slit in the slit direction, with the star on either side of the slit in the cross direction at different distances, from a few seconds of arc to more than [FORMULA]. Exposure times ranged from a few minutes at on-star positions, to 3 hours. Typical observing times for the off-star positions were 1 [FORMULA] hours. One should note that with the alt-alt mounting of the CAT this means that the spectrometer slit changes its position angle about 30 degrees during the exposure, so that the observed envelope spectrum is a mean of a fairly extended section of the envelope.

17 bright carbon stars with CO detections were chosen as targets - spectroscopic evidence of envelope emission was found for three of them, R Scl, V Aql and X TrA, and only observations of these are discussed subsequently. (It should be remarked that the first positive discovery of envelope emission for R Scl was made in remote observing mode, with the observer operating from Garching near Munich.) For calibration purposes we also observed some early-type stars. The observed spectrum frames were, after subtraction of bias, divided by a lamp flat field. Next, the spectra were wavelength calibrated by means of Th lamp spectra. After that, the off-star spectra were subtracted by the on-star spectrum, scaled to the same overall "continuum level", in an attempt to subtract the stellar component that is scattered into the envelope spectrum by the Earth's atmosphere and the telescope/spectrometer. This was made individually for each pixel row on the detector perpendicular to the slit direction.

The results of this procedure are illustrated in Fig. 1 and 2. It is obvious from Fig. 1 that already in the raw off-star spectrum of R Scl with no subtraction made one may trace an emission component, although the complex line spectrum does not admit any safe conclusions on the strength of the emission component until the subtraction is made. In Fig. 2 the subtracted spectra of the envelope of R Scl are displayed for different distances from the star.

[FIGURE] Fig. 1. An off-star spectrum ([FORMULA]) for R Scl at the K I line position. The dotted line is an on-star spectrum, which was scaled by about a factor of 0.0016.

[FIGURE] Fig. 2a and b. The subtracted K I spectra (off-star minus scaled on-star) for different distances from R Scl. In this case the star was located outside the slit in its direction, about [FORMULA] away from the slit end. The top spectrum represents the emission about [FORMULA] from the star, the following at a distance of [FORMULA] further away, the next spectrum with another increment of [FORMULA] in the distance, etc. The two figures are for two different position angles.

The calibration of the spectra in absolute fluxes (on Earth) was made by means of the observations of early-type stars. The fluxes were estimated from the observed count rates, the differences in apparent magnitudes, in exposure times and zenith distances between the programme stars and the calibration stars, the colour differences and using the absolute calibration for Vega by Hayes & Latham (1975). This only results in rough fluxes; they may be in error by about a factor of 3. However, considering the other uncertainties in the estimates to follow, this is satisfactory. Also, the relative fluxes at different locations in the envelope are far more accurate. Moreover, for the mass-loss estimates to be described below, we are only dependent on the measured emission-line fluxes relative to the stellar fluxes at the corresponding wavelengths; quantities which are not dependent on the absolute calibration.

In Fig. 2 we display spectra from two different slit positions on the envelope around R Scl and it is seen that the line profiles are significantly different in the two different slit orientations, albeit fluxes come from regions at roughly the same distance from the star. In Fig. 3 spectra of the envelope of X TrA are shown, in Fig. 4 those for V Aql, and in Fig. 5 the sodium D-lines of R Scl are shown without subtraction of the photospheric spectrum. Clearly, the envelope emission is also visible in these lines. Fig. 6, finally, displays the K I line region for a star, S Sct, where no emission could be detected spectroscopically.

[FIGURE] Fig. 3. K I spectrum of the envelope around X TrA at a distance from the star of [FORMULA]. The dotted line denotes a scaled on-star spectrum.
[FIGURE] Fig. 4. K I spectrum of the envelope around V Aql at a distance from the star of [FORMULA]. The dotted line denotes a scaled on-star spectrum.
[FIGURE] Fig. 5. Na I spectra of R Scl. Solid line is the off-star spectrum at a distance from the star of [FORMULA], and the dotted line the scaled on-star spectrum.
[FIGURE] Fig. 6. K I spectrum of the envelope around S Sct at a distance from the star of [FORMULA]. The dotted line denotes a scaled on-star spectrum.

Our observations of the Na D line regions are less complete, less homogeneous and more affected by blends than the K I 769.9 nm observations. Therefore, in the present discussion we concentrate on the K I data.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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