7 Low-resolution spectroscopy

   
7.1 Observations and reductions

Low-resolution spectra ( $\Delta\lambda\sim3$ Å), covering the spectral range 4400 - 8200 Å, have been obtained at the European Southern Observatory (ESO, La Silla, Chile) on the 1.52 m telescope equipped with the Boller & Chivens spectrograph (grating $\char93$23 + filter GG 420; dispersion of 114 Å mm^-1) and a Loral/Lesser thinned, UV flooded $2048\times 2048$ CCD (CCD $\char93$39; 15 $\mu$m pixels).

The CCD frames were corrected for the electronic offset (bias), for the relative pixel-to-pixel response variation (flat-field) and for the sky foreground lines. Wavelength calibration was performed from helium-argon lamp spectra taken at least every two spectra. An optimal extraction of the spectra was performed according to the method of Horne (1986). The extracted spectra were multiplied by the instrumental response function, obtained from the spectra of flux-calibrated standard stars (namely CD $-32^{\circ}9927$, LTT 3218, LTT 4816). The whole reduction sequence was performed within the "long'' context of the MIDAS software package.

The signal-to-noise (S/N) ratio was estimated for each spectrum in the following way: three S/N values were computed for the three best exposed CCD lines (along the dispersion axis), in the neighbourhood of three spectral region of interest ($\sim 6000$, 7000 and 7500 Å). These S/N values were then combined according to Eq. (17) of Newberry (1991). When the exposure time on a given star has been split in two, the final S/N ratio was computed using Eq. (18) of Newberry (1991). The degradation of the S/N ratio due to flat-field correction has not been taken into account. The average of these three S/N ratio values are listed in Table 6 for each target star.

Such low-resolution spectra have been obtained for 158 stars out of the 205 Henize S stars. Some spectral standards K stars, M stars and non-Henize S stars were also observed.

   
7.2 Construction of band-strength indices

    Table 2: Wavelength boundaries (in Å) used in the computation of the band indices defined in Sect. 7.2

name	$\lambda_{\rm B,i}$	$\lambda_{\rm B,f}$	$\lambda_{\rm C,i}$	$\lambda_{\rm C,f}$
Na D	5883	5903	5800	5847
ZrO	4640	4657	4600	4625
ZrO	5718	5735	5680	5720
ZrO	5748	5757	5680	5720
ZrO	6378	6382	6310	6345
ZrO	6412	6441	6310	6345
ZrO	6505	6530	6452	6475
ZrO	6541	6560	6452	6475
TiO	5448	5454	5410	5448
TiO	5591	5600	5500	5550
TiO	5615	5620	5500	5550
TiO	5759	5767	5680	5720
TiO	5810	5820	5800	5847
TiO	5847	5869	5800	5847
TiO	6159	6180	6067	6130
TiO	6187	6198	6067	6130
TiO	6651	6674	6452	6475
TiO	6681	6706	6452	6475
TiO	6714	6735	6452	6475
TiO	7054	7069	7014	7057
TiO	7125	7144	7014	7057
LaO	7380	7390	7362	7403
LaO	7403	7410	7362	7403

These low-resolution spectra allow to distinguish subclasses within the S family. To set this classification on a quantitative basis, band-strength indices have been constructed that indicate the strength of a specific band (or line) with respect to a nearby pseudo-continuum. More precisely, the index characterizing the band/line $X_{\lambda}$ is defined as $I_{{\rm X}, \lambda} = (C_{\rm max} -B_{\rm min})/C_{\rm max}$, where $C_{\rm max}$ is the maximum "pseudo-continuum'' flux inside the wavelength interval [ $\lambda_{\rm C,i}$, $\lambda_{\rm C,f}$], and $B_{\rm min}$ is the minimum "band'' flux inside the wavelength interval [ $\lambda_{\rm B,i}$, $\lambda_{\rm B,f}$]. The adopted values of $\lambda_{\rm B,i}$, $\lambda_{\rm B,f}$, $\lambda_{\rm C,i}$ and $\lambda_{\rm C,f}$ are listed in Table 2. An average index $I_{\rm X}$ for a given oxide is then computed as the mean strength of all the bands listed in Table 2 for that oxide.

The ZrO bands used in the computation of the mean ZrO index were taken from Table 1 of Ake (1979), but only the 7 bands not too strongly contaminated by TiO bands have finally been retained. In practice, Pearson's correlation coefficients between each ZrO index and the average TiO index were computed for all non-pure S stars. If they were larger than 0.6, the corresponding ZrO band was rejected.