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Astron. Astrophys. 328, 167-174

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No stellar age gradient inside supergiant shell LMC4*

Jochen M. Braun1, Dominik J. Bomans2,1,**, Jean-Marie Will1,***, and Klaas S. de Boer1

1Sternwarte der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
2University of Illinois at Urbana-Champaign, Department of Astronomy, 1002 West Green Street, Urbana, IL 61801, USA

Received 2 May 1997 / Accepted 21 July 1997


The youngest stellar populations of a 'J'-shaped region inside the supergiant shell (SGS) LMC4 have been analysed with CCD photometry in B, V passbands. This region consists of 2 coherent strips, one from the east to the west reaching about $400\;$pc across the OB superassociation LH77 and another extending about $850\;$pc from south to north.

The standard photometric methods yield 25 colour-magnitude diagrams (CMDs) which were used for age determination of the youngest star population by isochrone fitting. The resultant ages lie in the range from $9\;$Myrto $16\;$Myrwithout correlation with the distance to the LMC4 centre. We therefore conclude that there must have been one triggering event for star formation inside this great LMC SGS with a diameter of $1.4\;$kpc.

We construct the luminosity function and the mass function of five regions consisting of 5 fields to ensure that projection effects don't mask the results. The slopes lie in the expected range ($\gamma \in [0.22;0.41]$ and $\Gamma \in [-1.3;-2.4]$ respectively). The greatest values of the slope occur in the north, which is caused by the absence of a young, number-dominating star population.

We have calculated the rate with which supernovae (SNe) have exploded in LMC4, based on the finding that all stars are essentially coeval. A total of 5-7$\cdot10^3$ supernovae has dumped the energy of $10^{54.5}\;$erg over the past 10Myr into LMC4, in fact enough to tear the original star-forming cloud apart in the time span between 5 and 8Myr after the starformation burst. We conclude that LMC4 can have been formed without a contribution from stochastic self-propagating star formation (SSPSF), although the ring of young associations and HIIregions around the edge have been triggered by the events inside LMC4.

Key words: stars: early-type - Hertzsprung-Russell (HR) diagram - stars: luminosity function, mass function - ISM: bubbles - ISM: individual objects: LMC4 - Magellanic Clouds

*Based on observations collected at the European Southern Observatory (ESO), La Silla, Chile.
**Feodor Lynen-Fellow of the Alexander von Humboldt-Foundation
***Present address: Hewlett-Packard GmbH, Herrenberger Str. 130, D-71034 Böblingen

Send offprint requests to: J.M. Braun ('jbraun@astro.uni-bonn.de')

© European Southern Observatory (ESO) 1997

Online publication: October 30, 1997
Last change: March 24, 1998