2. The observations and reduction procedure

We carried out two observing runs during the months of December 1992 and August 1993, both with the Cassegrain spectrograph attached to the 2.2 m telescope at the Calar Alto Observatory, Almerıa, Spain. The detector used in both runs was the GEC#15 CCD. The two instrumental arrangements were different, giving slightly different resolutions. For the first configuration, we used the f/3 camera, at 0.66 Å/pixel spectral dispersion, giving a resolution of about 1.5 Å at 6000 Å. For the latter arrangement we used the f/1.5 camera which produces a spectral dispersion 1.36 Å giving a resolution of about 2.7 Å, at 6000 Å. The red spectra were centered on the H line at 6563 Å. The blue spectra included only H (4861 Å) and H (4340 Å) in the first run, while in the second one we extended the range from H to the Balmer discontinuity ( 3647 Å).

The spectra were reduced by means of the Starlink FIGARO software package (Shortridge 1991). Several bias frames were taken during the night, their stability was checked and then they were averaged, before subtraction from the program frames. No drift in dark current was found after several hours of operation, and at different time exposures. The flat fields were recorded at the end of the night. A smooth function was fitted to the flat field spectrum with which each program frame was normalized. Finally, wavelength calibration was done by comparison with available He-Ar calibration lamps at the telescope using the corresponding subroutine provided by FIGARO. As much as twenty different lines were identified and used for calibration purposes.

The measurement of equivalent widths, full widths at half maximum, and velocity peak separation for the Balmer lines was done with the Starlink DIPSO package (Howarth & Murray 1991). Sufficient continuum was taken into account to allow the wings to effectively reach the continuum level. For stars where the Balmer jump was recorded, we measured the EW only up to H. Beyond this point the continuum was poorly defined by line blocking and the merging of the wings, although it can be seen up to H15 or more. Full widths at half maximum were measured only for emission lines.

An accurate treatment of the errors in the estimation of equivalent widths is very difficult. The main source of uncertainty is the exact location of the continuum, which for a poor signal-to-noise ratio can reach an uncertainty of up to 10% (Gray 1992). In order to estimate a mean error for our measurements, we decided to accurately study some selected spectra following the method outlined in Chalabaev & Maillard (1983). The result was that the estimation by this method is completely consistent with the statistical dispersion when several measurements, assuming several continuum levels, are made with DIPSO. As a general rule we can assign an absolute error of about 1 Å. On the other hand, large emission often implies very extended wings which can cause an overestimation of the continuum level of up to 10% and hence underestimation of the EW. These contributions to measurement errors have to be added quadratically so that, roughly speaking, a maximum error of about 10% can be assigned to all the measurements.

No correction has been applied to equivalent widths to account for photospheric absorption since our goal in the present paper is just to present the spectroscopic observations and extract some conclusions without the assumption of any model. In fact, the correction will be done in the future using the photometric data of Paper I. However, corrections for instrumental broadening have been applied to full widths by means of the usual relation

where ( Å, for the first run and around 2.7 Å, for the second. Mean estimated errors are probably around km/s.

Rotational velocities have been taken from Slettebak (1982, 1985). All these results are shown in Tables 1 (click here) to 3 (click here), while in Table 4 (click here) we present the observed equivalent widths for regular B type stars also included in our survey.

 

Cluster Star FWHM rem.

NGC 457 14 -5.9 0.6 2.1 148 175

91 -9.5 1.6 2.7 571

153 -44.1 -4.7 -1.1 286 288 213

198 -2.0 2.0 3.1 455

NGC 663 2 -43.0 392

6 -33.5 157

8 -11.0 401

10 -10.7 250 350

21 -4.0 2.4 2.8 100 314

51 -8.7 572

84 -42.7 325

92 -1.9 2.7 3.0 200 450

93 -48.0 150 246

107 2.4 100

110 2.5 80

121 -21.0 400

124 -14.6 200 380

130 -32.4 399

141 -44.0 -3.3 1.5 250 305 274 173

194 -16.5 -1.1 1.5 250 400 334 112 1

222 -16.3 80 181

224 -37.0 380

243 -8.6 1.6 3.0 100 324 261

297 -37.0 250 349

146 3.1 2.8 3.3 200 2

309 -34.3 -3.5 0.4 270 268 219 227

566 1.8 250 2

717 -1.4 1.8 3.1 166 112

1261 -58.4 -4.7 0.4 300 375 272 301

1268 2.3 2.5 2.8

1702 -20.9 -1.4 2.3 150 194 168 141

2088 -10.6 0.3 2.6 180 441 438

2138 -2.1 1.4 2.6 100 268

2165 -27.2 -2.6 1.7 100 147 92 90

2284 -66.4 -5.3 -0.1 300 263 218 235

2371 1.1 2.2 2.8 100

2402 -8.1 -0.1 1.9 150 404 530

Pleiades 486 4.2 6.3 6.0 180

(Dec. 92) 980 -11.7 3.7 5.0 280 340 424

1432 -4.0 4.7 4.9 140 211 3

2181 -29.8 2.8 5.8 320 327 269 4

Pleiades 486 4.0 6.0 6.7 180

(Aug. 93) 980 -11.2 3.9 5.9 280 342 357

1432 4.6 5.8 140 3

2181 -36.5 2.1 6.4 320 314 306 4

NGC 2323 157 -3.7 6.1 5.8 175 134 5

NGC 2422 42 -5.8 6.4 7.5 98 118

45 -26.2 -1.2 1.9 230 452 337 5

125 -6.4 5.0 5.6 275 269

 

 

Cluster Star FWHM rem.

NGC 7654 778 3.9 4.7 417 5

782 -2.2 4.9 4.4 213 5

928 -8.3 5.4 6.7 434 5

989 -27.5 343 5

Cep OB3 6 10.3 2

9 4.6 2

15 -17.6 1.7 3.4 384 324 6

40 2.8 3.4 4.0

51 9.6 11.4 10.0 2

64 9.0 2

65 8.9 2

69 -1.8 2.6 3.4 279 7

80 10.5 2