6. Toward the history of LMC 4
LMC 4 contains a substantial population of young stars of age 10 Myr, while a quite older ( Myr) background population may exist. A huge gas cloud must have been present in which the conditions were favourable for the burst of star formation Myr ago. The consequences of that burst are observed today. These can be summarized as follows. A volume of which we see the projected surface area of about kpc2 is filled with essentially coeval stars. The volume contains little neutral gas in numerous low column density filaments (Domgörgen et al. 1995), and at the same time clearly contains ionzed gas (see Bomans et al. 1996).
Inside LMC 4 we have a large number of young main sequence stars. From the present day mass function, extended with the same slope to the mass rich end to also include the original mass rich stars, we can calculate the number of stars in each mass range initially present in our 'J'-shaped region:
A fit to the Geneva evolutionary tracks (Schaerer et al. 1993) for an initial mass yields a stellar lifetime relation of:
For star populations one would expect all stars with to have exploded into SNe in the first 10 Myr. Thus we get , for roughly 5% of the LMC 4 area. Extrapolating from the 'J'-shaped region to the entire interior of LMC 4 means that in this SGS of 10 Myr age about 5-7 supernovae have exploded. With an average supernova energy output of 10 erg the total number of past SNe will have dumped at least 10 erg into LMC 4.
On the other hand, the supernova rate right after starformation is very small and Eq. (8) indicates that after 5 Myr only 200 SNe have exploded inside LMC 4, or less than 10% of the total in 10 Myr.
At present there is little neutral gas inside LMC 4, whereas the structure must have had lots of relatively dense neutral gas for the star formation. We can estimate the minimum energy input required to dissolve the birth cloud. Assuming a thickness of originally 500 pc with a gas density in the volume V, the energy needed for total ionization is eV = erg cm3. A further calculation shows that roughly erg cm3 can accelerate all particles of the entire birth cloud to 100 km s-1, enough to disrupt the cloud. This energy is easily provided by the supernovae. In fact, a substantial fraction of the energy needed is already released between 5 and 8 Myr.
Summarizing, based on the recognition that all young stars of LMC 4 are nearly coeval at 10 Myr, and on the sequel that supernovae will go off everywhere inside LMC 4 at a fair rate dumping energy rather evenly in the birth cloud, we can explain the structure of LMC 4 as we see it today.
In consequence of that, once the first supernovae occur, their individual (but soon the collective) blast waves may trigger star formation at the edges. This fits with the age derived for LH 72 south. Starformation at the edges of LMC 4 will have taken place at very recent times as secondary process. We expect that an age determination for the stars in any of the H II regions to the east of LMC 4 will show very young ( 5 Myr) ages, like the ones at the NW (Will et al. 1996) and the SW (Petr et al. 1994).
The present supernova rate inside LMC 4 as calculated from our equations is about 1 per 670 yr which is comparable to the SN rate of the whole LMC derived from counting supernova remnants (Chu & Kennicutt 1988). Our mass function and main sequence lifetime indicate an increase of the SN rate to 1 per 120 yr in 25 Myr from now, at a time when the stars of originally 8 will explode.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998