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Astron. Astrophys. 317, 889-897 (1997)

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2. Observations

The observations were obtained in July 1995, and details of individual exposures are given in Table 1. The detector was a Tektronix CCD ([FORMULA] 24-µm pixels). The spectrograph was used in conjunction with an image-slicer (Diego 1993), and the output was binned by a factor of eight perpendicular to the dispersion direction in order to reduce the readout noise associated with the broad spectrum which results. At the wavelength of the Ca K line (3933.663 Å) the dispersion was 0.086 Å mm-1, giving a spectral coverage of 2.1 Å. The resolution, measured with the aid of a stabilized He-Ne laser, was [FORMULA] km s-1 (FWHM), corresponding to a resolving power of [FORMULA].


Table 1. List of the nearby stars observed for the interstellar Ca K line using the UHRF. [FORMULA] and [FORMULA] are the projected heliocentric velocities of the Local Interstellar and G clouds towards each of these stars (Lallement et al. 1995). The last column gives the total number of counts (electrons) obtained in the local continuum for each 24-µm spectral element adjacent to the interstellar lines (generally this is formed by the core of the stellar photospheric Ca K line.)

The CCD images were divided by a flatfield, and the spectra extracted using the FIGARO data-reduction package (Shortridge 1988). Background light was measured from the inter-order region and subtracted. Wavelength calibration was performed using a Th-Ar comparison lamp. Second-order polynomial fits to between 5 and 7 comparison lines yielded typical rms residuals of [FORMULA] Å (0.04 km s-1); in no case were the rms residual s greater than [FORMULA] Å (0.06 km s-1). Once wavelength calibrated the spectra were converted to the heliocentric velocity frame, and multiple exposures (Table 1) were co-added. Most of these stars exhibit strong photospheric Ca K lines, the cores of which act as the local continua for the interstellar lines. These were fitted by low-order polynomials and divided out, to give the normalised spectra shown in Fig. 1.

[FIGURE] Fig. 1. The interstellar Ca K lines observed with the UHRF. The observed data are plotted as histograms. The smooth curves are theoretical line profiles with the parameters given in Table 2. The velocities of individual absorption components are indicated by vertical tick marks.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998