2 Observational data

Spectra were obtained in the northern hemisphere with the AURÉLIE spectrograph (Gillet et al. 1994) associated with the 1.52 m telescope at Observatoire de Haute-Provence (OHP), in order to acquire complementary data to HIPPARCOS observations (Grenier & Burnage 1995).

The initial programme gathers early-type stars for which measurement is needed. More than 820 spectra have been collected for 249 early-type stars from January 1991 to May 1994. As shown in Fig. 1, B9 to A2-type stars represent the major part of the sample (70%). Most of the stars are on the main sequence and only about one fourth are classified as more evolved than the luminosity class III-IV.

These northern stars are brighter than the magnitude V=7. Nevertheless, three stars are fainter than this limit and do not belong to the HIPPARCOS Catalogue (ESA 1997). Derivation of their magnitude from TYCHO observations turned out to be: HD 23643 V=7.79, HD 73576 V=7.65 and HD 73763 V=7.80. These additional stars are special targets known to be $\delta$ Scuti stars.

Figure 1: Distribution of the spectral type for the 249 programme stars.

AURÉLIE spectra were obtained in three different spectral ranges (Fig. 2):

• $\Lambda _1$: 4110-4310 Å. This domain was initially chosen to detect chemically peculiar stars by measuring equivalent widths of the lines Si II 4128, Si II 4131, Sr II 4216, and Sc II 4247.
• $\Lambda _2$: 4226-4432 Å or 4270-4475 Å. Centered around H$\gamma$, this spectral range allows the determination of the effective temperature.
• $\Lambda _3$: 4390-4600 Å, or 4400-4610 Å. This domain is dedicated to determination, although all the three ranges are used to measure the rotational velocity.

Two thirds of the sample have observations in each of the three ranges. The $\Lambda _3$ range is particularly aimed at measurement, and contains the largest number of lines selected for this purpose (twice as much as $\Lambda _1$ and $\Lambda _2$). Besides, it is the only one which covers the magnesium doublet at 4481 Å. This line remains often alone for measurement in fast rotators. It is thus significant to note that among the 249 stars of the sample, three were observed only in the $\Lambda _1$ range, eight only in $\Lambda _2$ and eleven in both $\Lambda _1$ and $\Lambda _2$ only. Overall 22 stars have no observation in $\Lambda _3$. The reason is that these stars have already known . Moreover they are effective temperature standard stars or reference stars for chemical abundances. Some changes in the configuration of the instrument meant the central wavelengths of the $\Lambda _2$ and $\Lambda _3$ domains have been slightly modified during mission period. The entrance of the slit is a 600 $\mu$m hole, i.e. 3 $^{\prime\prime}$ on the sky, dedicated to the 1.52 m Coudé telescope. The dispersion of the collected spectra is 8.1 Å  ${\rm mm}^{-1}$ and the resolving power is about 16 000. The barrette detector is a double linear array TH 7832, made of 2048 photo-diodes. Reduction of the data has been processed using MIDAS procedures. Flat field correction with a tungsten lamp and wavelength calibration with a Th-Ar lamp have been made with classical procedures. Nevertheless, a problem occurred when applying the flat-field correction with the tungsten calibration lamp. The division by the W lamp spectrum produced a spurious effect in the resulting spectrum at a given position in the pixels axis. This effect distorts the continuum, as it can be seen on the spectra of Vega (Fig. 2). This problem triggered the decision to change the instrumental configuration and central wavelengths of the spectral ranges.

Figure 2:  Observed spectra of Vega are displayed for the different spectral ranges: top panel, range $\Lambda _1$; middle panel, range $\Lambda _2$; bottom panel, range $\Lambda _3$. Each domain covers nearly 200 Å. $\Lambda _1$) the first range extends from the red wing of H$\delta$ to the blue wing of H$\gamma$ which restricts the reliable normalization area. It contains seven of the selected lines. $\Lambda _2$) centered around H$\gamma$, this range only contains five selected lines. $\Lambda _3$) this range contains the largest number of selected lines, 16 in total, among which the doublet line Mg II 4481. The 23 selected lines (listed in Table 2) are indicated, and show up twice in the overlap areas. The instrumental feature coming from flat-fielding lamp is noticeable in the three spectra ($\sim$4280 Å in $\Lambda _1$, $\sim$4390 Å in $\Lambda _2$, $\sim$4560 Å in $\Lambda _3$).