Vl.Tta$1n Astronomy, Vol. 31, pp. 611--614, 1988 0083-6656/88 $0.00+ .50

Copyright © 1988 Science Press & Pergamon Journals Lid,

SUPERNOVAE AND THE MASS LOSS FROM PRIMORDIAL CLOUDS

Ma Er Beijing Astronomical Observatory, China

Yu Yunqiang Dept. of Physics, Peking University, China

Chen Shi Institute of Theoretical Physics, Beijing, China

The formation of the dwarf galaxies is of special importance. Their

low density and low metallicity suggest a large amount of mass was

lost in early stage of their evolution. Larson (]974) was the first

to propose that the supernova-driven wind caused this mass loss.

This idea met two main difficulties. Firstly, the time scale of SN

heating is very short comparing with that of the dynamical evolution

of the system, therefore a large mass loss would have disrupted the

system. Sec~idly, large mass loss needs a high SN explosion rate

which implies a high star formation rate (SFR). Since the life time

of a star and the dynamical time scale of a low mass cloud are not

very different, high SFR will turn most gas into stars and there

will not be much gas left to be driven. Low SFR can keep much gas

in the system, but there will not be enough SN explosions. In this

paper a new mechanism for the influence of supernova (SN) explosions

on the evolution of low mass primordial clouds is suggested. Then

the difficulties of SN-driven wind model mentioned above can be

avoided.

The volume density of galaxies is shown in Fig. I as a function

of galaxy mass. From these curves we can see the densities of dE

galaxies and the densities extrapolated from that of the normal

galaxies can differ by almost 3 orders of magnitude. Some observa-

tions indicate that dE galaxies may have lost much of their mass

during early stages of evolution and that their progenitors may have

been much denser. Assuming that the low mass progenitors of dE

galaxies have a simil~ar density-mass relation for normal ellipticals

extrapolated to the appropriate lower masses, we term these

6|I

612 Ma Er et al.

primordial objects high density clouds ("HD clouds"). We may also

take the emporical relation for dE galaxies as the appropriate

relation for dE's progenitors and we will call such progenitors

hereafter as low density clouds("LD clouds").

Becayse the virial velocity of a low mass cloud is low for

reasonable assumed densities, the SN-driven gas loss processes will

mainly occur in the radiative phase rather than in the adiabatic

phase.

It is therfore not a good approximation to consider the shock

process as an adiabatic one, and radiative dissipation should be

taken into account. The relevant quantities such as M , R and T V V V

the mass, the range of heated gas and the time scale of the heating

process are calculated for both HD and LD models.

Assuming the initial mass function (IMF) and the star formation

rate (SFR), the rate of SN explosions and the gas content in a

cloud have been calculated numerically.

For the-evolution of low mass galaxies, SN explosions will cause

a considerable amount of mass loss from the cloud by two main

mechanisms: sudden and complete ga~-driving by multiple SN explo-

sions and the gradual boundary effect. The r~lative importance of

these two. depends critically on the cloud model and the star

formation'rate. Generally speaking, the slower the star formation,

the more important is the boundary effect.

Fig. 2 and Fig. 3 shows the dependence of mass loss from a LD

cloud and a HD cloud as functions of their total mass respectively.

The subscripts of ],2 refer to different star formation rates. (]

refers to a high rate) LX, L B and L T refer to mass loss caused by

the sudden and complete gas-driven process, the gradual boundary

effect and the combination of the two. The result shows the

following:

Though the dE galaxies have much lower densities than might

be expected form the extrapolated empirical relation for normal E

galaxies, their progenitors cannot be so rare, otherwise the

feedback effect of SN explosions would be very severe. During their

early evolution, whether the star formation rate is high or low,

up to 90% of the original gas will be lost. If a cloud loses half

Supernovae and the Mass Loss 613

of the mass in a period of time much shorter than its dynamical

time, the cloud will be destroyed. But the gas loss from a cloud

boundary is much slower than its dynamical evolution, so the cloud

can still survive even if it loses 90% of its gas.

According to high density models, dE's progenitors would lose

part of their mass and decrease their density accordingly. Take a

107 solar mass cloud as an example. If the star formation rate is

high, the 25% of the gas will be lost, while with a low star

formation rate 43% of the gas will be lost. These results would

account for the low density and low metallicity observed in dE

galaxies at present.

REFERENCES

Binggeli, B., Sandage, A., and Tarenghi, M., (1984) Astron. J.

89, 64. Dekel, A. & Silk, J. (1986) Astrophys. J., 303, 39. Efatathiou, G. & Silk, J., (1983) Fundamentals of cosmic physics 9.

Fall, S.M., Rees, M.J. (1985) Astrophys. J. 298, 18. Larson, R.B. (1974) Mon. Not. R. Astron. Soc., 169, 229. Larson, R.B. (1986), Mon. Not. R. Astron. Soc., 218, 409.

* Full paper is in preprint form and submitted to Scientia Sinica.

2

0 o

-1

- 2

- 3

p~ -- 1M®(pc) -3

6 7 8 9 I

o ~

log M/M®

Fig. l

614 Ma Er et al.

100

,J 50

..J

0

100

50

0

6 7 8 9

log M/Mo

I-,B2 = LT 2

1 I

6 7 8 9 10

log M/M®

Fig.2

Fig.3