SUPERNOVA
EFFICIENCY IN HEATING THE
INTERSTELLAR MEDIUM OF STARBURST GALAXIES
Claudio Melioli
(IAG/USP), Elisabete M. de Gouveia Dal
Pino (IAG/USP)
Annibale
D'Ercole
(Oss. Astr. di BO, Italy)
The interstellar medium
heated by supernova explosions (SN) may acquire an expansion velocity larger
than the escape velocity and leave the galaxy through a supersonic wind.
Galactic winds are effectively observed in many local starburst -galaxies. SN
ejecta are transported out of the galaxies by such winds which thus affect the
chemical evolution of the galaxies. The effectiveness of the processes
mentioned above depends on the heating efficiency (HE) of the SNs. In a
starburst region several SN explosions occur at high rate inside a relatively
small volume. A superbubble of high temperature and low density takes place,
and in this environment the successive generations of SNRs do not reach high
density during their expansion, their radiative losses remain negligible and it
is common to assume a value of HE close to unity. But this assumption fails in
reproducing both the chemical and dynamical characteristics of starburst
galaxy. In order to solve this problem, we have constructed a simple
semi-analytic model able to give us insights on the thermalisation of the ISM
inside a starburst region. The most important physical phenomena are studied,
assuming a three-phase medium composed by hot gas, SNR and clouds. The most
important result is a very low SN efficiency value in the first 10 Myrs, which
gets closer to 1 only after about 15-20 Myrs. On the whole, we can conclude
that the HE has a depending-time trend as it results from initial conditions
and parameter assumptions. This model allows to scale down typical HE values
and explains the low values assumed in some chemical models (D'Ercole &
Melioli, MNRAS, 2002). Presently, we are implementing a 3D, gasdynamical code
aiming to check the SN HE estimates obtained from the analytical model above,
by including all the SB environment contents and fully solving the
chemo-dynamical equations of the three-phase system.