2 Observations and data reduction

This paper describes results obtained with the Cooled Grating Spectrometer (CGS4) of the United Kingdom Infrared Telescope (UKIRT), Mauna Kea, Hawaii. The observations were carried out during the periods of 1996 June 28 - 29 and 1996 September 30 - October 2 (see Tables 1 and 2). CGS4 provides spectral coverage from 1 to 5 $\mu$m and the observations described here were made using the 256 $\times$ 256 pixel infrared array as a detector. The observations were made using the short focal length camera plus the 150 line/mm grating, giving coverage from 2.05 to 2.22 $\mu$m with a velocity resolution of $\sim 70$ km s^-1.

Initial data reduction was carried out at the telescope using the CGS4DR software (Puxley et al. 1992). This removes bad pixels, debiases, flat-fields, linearity corrects and interleaves oversampled scan positions. The subsequent stages of data reduction were carried out using the Starlink-supported package FIGARO. For each target and standard this comprised correcting slit rotation, sky subtraction, extraction, derippling and wavelength calibration using observations of a CuAr lamp.

In order to ensure accurate removal of telluric features from the spectra we followed a procedure similar to that outlined by Hanson et al. (1996, henceforth HCR96). An A0 - A3 III-V star was observed after each target at an airmass within 0.1 of the target. Once per hour observations were also taken of a G2-3V star. The only non-telluric feature in the A star spectra is Br$\gamma$. A simple interpolation over this feature is not appropriate however, as there is also a telluric feature at this wavelength that would lead to spurious emission features contaminating the strength and profile of the Br$\gamma$ emission lines we expect from our targets. Instead we used the G star observations divided by the solar spectrum to calculate the telluric features in the region of Br$\gamma$. These were then patched into the A star spectra. Note that in order to ensure the A star, G star, target and solar spectra were all properly aligned in wavelength space cross correlations of various telluric and stellar photospheric features were derived for each spectrum and the appropriate offsets applied.

Figure 1: Histograms showing the membership of each of the spectral groupings by spectral type (solid areas). The distribution of spectral types for the total data set is also indicated on each plot (hollow areas). The data have been binned into n bins containing objects in the range B(n-1).5 to B(n).4. Therefore a B0.2 object will appear in the B0 bin, however a B0.5 object appears in the B1 bin

Figure 2: Plot of Br$\gamma$ equivalent width (EW) in -Å against spectral type. Triangular symbols represent luminosity class III, squares luminosity class IV and circles luminosity class V. Star symbols represent those objects with only a historical classification. Filled squares represent data for normal B III-V stars taken from HCR96

Figure 3: Histogram showing the number of stars which have undergone a phase change (solid areas) and the total distribution of stars as a function of spectral type (hollow plus solid areas)

Figure 4: Plot of Br$\gamma$ equivalent width (EW) in -Å against He I 2.058 $\mu$m (EW)