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We present a study of the massive star content of 30 Dor using a population synthesis model in the ultraviolet. The relatively small distance to the LMC allows one to spatially resolve the individual components in 30 Dor. This property offers an important test for theoretical models.
UV spectra of the central region of 30 Dor have been collected with IUE by trailing the entrance aperture across the central $1' \times 1'$ and $3' \times 3'$ region. Using our model, we synthesize as a function of time the monochromatic luminosity from 912 to 10000\AA, the number of ionizing photons emitted shortward of 912\AA ($N_{Ly}$), the UV lines of SiIV, CIV and HeII, the spectral type following from the stellar parameters and the evolutionary status. We generated a series of starburst models for the two limiting cases of a continuous and an instantaneous burst. A Salpeter-type IMF with $\alpha = 2.35$ is adopted and two metallicities, $Z_\odot$ and 0.25$Z_\odot$, are considered.
The modeling of the UV spectrum provides a stringent limit on the age and the IMF upper cut-off mass for the central regions of 30 Dor. The flux level depends on the age and the upper cut-off mass. Combining the results for the instantaneous and the continuous scenarios, we find that the age of the observed region is less than 5 Myr. The observations cannot be matched for an upper cut-off mass less then 50 $M_\odot$. The conclusions reached from the synthesis of the UV lines are very consistent considering the fact that we are here limited by a spectral library at solar metallicity. The SiIV, CIV and HeII lines offer characteristics which are very sensitive to the massive star content. For example, evolved massive stars display a strong P Cygni profile of SiIV with a large velocity shift to the blue due to Doppler effect in their fast and dense stellar winds. Based on the synthesis of the lines alone (at $Z_\odot$), an age of 3 Myr is found for the central regions of 30 Dor, again with an upper cut-off mass larger than 50 $M_\odot$. The number of O and Wolf-Rayet stars observed and classified in the central region of 30 Dor are well reproduced.
