| We present a study of the star-formation history (SFH), and the initial mass function (IMF) in the 30 Doradus super-association. The study is divided in six natural stages: (1) profile fitting photometry; (2) characterization of the instrument; (3) calibration using stars with spectroscopy; (4) visualization of the stellar properties using the color-magnitude stereogram; (5) Bayesian analysis to obtain physical quantities; and (6) the construction of the SFH and IMF. The reduction and characterization of systematic errors are the most important steps of any IMF study: we note the following sources of systematic errors: (a) the upper magnitude cut-off, used to filter out saturated and non-linear stars, results in a false steepening of the high-mass end of the IMF, particularly affecting older systems; (b) Be stars and blue B-type supergiants mimic luminosity class V stars of higher effective temperatures, thus flattening the IMF; (c) the magnitude limit effect introduced by variable reddening, that flattens the low mass end of the derived IMF. For IMF determination we have identified the mass window 10 M⊙ ≤ M ≤ 40 M⊙ , that is free of effects (a) and (c) in our photometry. We have found that the SFH of the region is characterized by a 7--15 My old burst, across the whole area studied, followed by a period of reduced, nearly constant, star-formation activity. This activity has been punctuated by clustered, burst-like, star-formation episodes of varying intensity in several places. For NGC2070, the OB association LH104, and the field, the derived IMFs are consistent with a power law with Salpeter slope only if they have different SFH: a young and almost instantaneous burst for NGC2070, and nearly constant star formation, after the 7--15 My burst for the field and LH104. Other studies reveal star-formation episodes across the LMC, starting 15--30 My ago. We propose that the origin of such an apparently synchronized, large-scale, activity, is the recent entry of the LMC into a thick disk of ionized gas, analogous to that proposed by Moore and Davis (1994) to explain the origin of the Magellanic Stream. |
