Emission-line stars discovered in the UKST Hα survey of the Large Magellanic Cloud - I. Hot stars

We present new, accurate positions, spectral classifications, radial and rotational velocities, Hα fluxes, equivalent widths and B, V, I, R magnitudes for 579 hot emission-line stars (classes B0-F9) in the Large Magellanic Cloud (LMC) which include 469 new discoveries. Candidate emission-line stars were discovered using a deep, high-resolution Hα map of the central 25 degree² of the LMC obtained by median stacking a dozen 2 h Hα exposures taken with the UK Schmidt Telescope (UKST). Spectroscopic follow-up observations on the Anglo-Australian Telescope, the UKST, the Very Large Telescope, the South African Astronomical Observatory 1.9 m and the 2.3-m telescope at Siding Spring Observatory have established the identity of these faint sources down to magnitude R_equiv ∼ 23 for Hα (4.5 × 10^-17 ergs cm^-2 s^-1 Å^-1).

Confirmed emission-line stars have been assigned an underlying spectral classification through cross-correlation against 131 absorption line template spectra covering the range O1-F8. We confirm 111 previously identified emission-line stars and 64 previously known variable stars with spectral types hotter than F8. The majority of hot stars identified (518 stars or 89 per cent) are of class B. Of all the hot emission-line stars in classes B-F, 130 or 22 per cent are type B[e], characterized by the presence of forbidden emission lines such as [S II], [N II] and [O II]. We report on the physical location of these stars with reference to possible contamination from ambient H II emission. Only 13 of the emission-line stars require additional high-resolution spectroscopic observations in order to assign a spectroscopic classification. They have nonetheless been added to the catalogue.

Along with flux calibration of the Hα emission, we provide the first Hα luminosity function for selected sub-samples after correction for any possible nebula or ambient contamination. We find a moderate correlation between the intensity of Hα emission and the V magnitude of the central star based on SuperCOSMOS magnitudes and Optical Gravitational Lensing Experiment (OGLE)-II photometry where possible. Cool stars from classes G-S, with and without strong Hα emission, will be the focus of Part II in this series.