THE STELLAR CONTENT OF 30 DORADUS DERIVED FROM SPATIALLY INTEGRATED ULTRAVIOLET SPECTRA: A TEST OF SPECTRAL SYNTHESIS MODELS

摘要

Using the IUE satellite, we have obtained spatially integrated ultraviolet spectra of three areas within the giant H II region 30 Dor in the Large Magellanic Cloud. The spectra correspond to spatial regions with sizes of 20″ x 20″, 1′ x 1′, and 3′ x 3′, all of which are approximately centered on R136. We have performed a spectral synthesis analysis of the spectra of the two larger regions and compared the results with the known stellar content in these regions. The spectral synthesis models are sensitive to the ultraviolet continuum level, the P Cygni profile of the C IV λ1550 line, the absorption strength of the Si IV λ1400 line, and the emission strength of the He II λ1640 line. The intrinsic continuum levels and the profiles of these stellar wind lines provide constraints on the age and duration of the starburst episode within a region, as well as on the upper cutoff mass of the initial mass function. From our analysis we find that the present-day value of the upper cutoff mass in the 1′ x 1′ and 3′ x 3′ regions has a lower limit of ～50 solar mass, a result which is in good agreement with several other recent determinations. The age of the starburst episode must be less than ～3 Myr, also in agreement with other estimates. Comparison of the observed total numbers of O and W-R stars with those predicted from the various models favors an instantaneous burst of star formation in the regions. However, the differences between the two burst scenarios we investigated (instantaneous and continuous) are small at such a young age, and distinguishing between the two is difficult. We are now confident that these spectral synthesis models can be used to determine the stellar content of more distant star-forming regions.