3.1. Dust sources and relative contributions
It is clear that there is no universal dust model that can be applied to
a galaxy as a whole, or to galaxies with different evolutionary
histories. Consider the production of dust from an evolving single
stellar population (SSP) with masses between ~ 0.8 and 40
M
. The first
dust that will be injected into the ISM will probably be formed in
late-type Wolf-Rayet (WR) stars. Dust formation has only been observed
to occur in the coolest C-rich stars of type WC8 and WC9. At least in a
few cases, the formation of dust in these objects was induced by the
interaction of the WR ejecta with the wind from a companion O-star
[53].
WR stars are however minor sources of dust that overall contribute less
than 1% of the total mass of dust injected by supernovae and AGB stars
into the ISM
[35,
19].
After about 5 Myr the first stars of the SSP will undergo core collapse
giving rise to Type II supernova (SN) events. SN ejecta contain layers
that are C- and O-rich, which are not intermixed on a molecular
level. Consequently, SNe can produce both carbon and silicate type dust
particles
[42].
Stars with masses below ~ 8
M will undergo
the AGB phase, lose mass, and evolve into white dwarfs.
Figure 2 depicts
the carbon and silicate yields from AGB stars with an initial solar
metallicity. Stellar yields were taken from Marigo
[49].
Stars with a C/O > 1 ratio in their ejecta were assumed to condense
only carbon dust, whereas stars with a C/O < 1 ratio were assumed to
condense only silicate type dust. The yields were calculated assuming a
condensation efficiency of unity in the ejecta. The mass range of carbon
producing AGB stars is between ~ 1.6 and 4
M, a range that
widens at lower stellar metallicities
[19].
So carbon dust from AGB stars will first be injected into the ISM after
about 200 Myr, when ~ 4
M stars evolve
off the main sequence.
 |
Figure 2. The carbon and silicate yield from AGB stars, based on the Marigo [49] yields. |
The AGB yields depicted in Figure 2 represent an idealized situation. In reality, AGB stars undergo thermal pulsations (the explosive ignition of the He-rich shell), that cause the convective mixing of C-rich gas with the outer stellar envelope. After repeated thermal pulses a star can evolve from an O-rich giant to a C-rich star. The changing composition of the stellar envelope will affect the chemistry of dust formation. Detailed kinetic nucleation calculations (Ferrarotti & Gail [25]) show that AGB stars of a given mass can indeed form both, carbonaceous and silicate, type dust particles. Including the effect of radiation pressure on the newly-formed dust on the dynamics of the envelope, they find AGB yields that are significantly lower, by factors between 3 and 10, from those presented in Figure 2. A similar conclusion was reached by Morgan & Edmunds [54] using a simpler model for dust formation in AGB stars.
Figure 3 shows the carbon and silicate yields
from both, AGB stars and SN II, weighted by the stellar initial
mass function (IMF), taken to be the Salpeter IMF between 0.7 and 40
M. SNe yields
were taken from Woosley & Weaver
[67],
and a condensation efficiency of unity was adopted in calculating the
dust yield. The figure shows that the main contributors to the
interstellar carbon abundance are low mass AGB stars, whereas SN II
are the main contributors to the silicate abundance in the ISM. We
emphasize however, that the yields presented in the figure are ideal
ones, and that the actual yield of dust in SNe and AGB stars is still
highly uncertain (see Section 4 below).
 |
Figure 3. IMF-weighted carbon and silicate yields from AGB stars and Type II supernovae. SN yields were taken from Woosley & Weaver [67]. |