The Galactic halo, bulge and disk(s) are all relevant to early times, only the thin disk being younger than the bulge and thick disk. The respective roles of hierarchical clustering, mergers and monolithic collapse are still not very clear; probably all play a role, but the halo and bulge share a low specific angular momentum while the thick and thin disks share a high one and may result from later accretion of gas by the bulge, which would then resemble an E-galaxy. However, it is also possible that the bulge evolved from the disk by way of a bar.
![]() |
Figure 2. Schematic cross-section through the Galaxy. |
In any case, the stellar dynamics of the halo favour what
Thomas, Greggio & Bender
(1999)
refer to as a "fast clumpy collapse", basically the old idea of
Eggen, Lynden-Bell &
Sandage (1962)
placed in the context of modern hierarchical clustering
scenarios. One point of interest is the metallicity distribution
function (MDF), recently extended to very low metallicities by
Beers et al. (1998).
The MDF is essentially the modified Simple-model type distribution
originally noted by
Hartwick (1976),
with a peak at about 1/10 of the true yield, down to [Fe/H]
- 3. Below that it begins
to fall off and there are virtually no stars
(compared to a predicted number of about 10) below -4, which could
represent enrichment either from a hypothetical Population III or from
contamination of low-mass stars by a nearby supernova.
A significant clue to early Galactic chemical evolution comes from the
relation between oxygen and
-particle elements, thought
to come exclusively or mainly
from type II supernovae, and iron, more than half of which in the Solar
System comes
from Type Ia. Fig 3 suggests that there is a
plateau in O,
/ Fe at low
metallicities (assumed to represent early times), but there is currently a
controversy in the case of oxygen. Abundances derived from the forbidden
[OI] line, which is probably the most reliable source when it is not too
weak, suggest a plateau, but from measurements of the near UV OH bands
in dwarfs and subgiants, both
Israelian, GarcĂa Lopez
& Rebolo (1998)
and Boesgaard et al. (1999)
have derived a rising trend with diminishing [Fe/H] more or less
following the
open squares in the top panel of Figure 3. In
contrast,
Fulbright & Kraft
(1999)
have studied the [OI] spectral region in two of the extreme cases and
find lower O/Fe ratios fitting the plateau. There are technical
difficulties in both methods:
the OH bands are subject to uncertainties in UV continuum absorption (cf.
Balachandran & Bell 1998
on solar beryllium abundance) and effective temperature,
while the forbidden line in the relevant cases is so very weak that the
definition of the continuum becomes a crucial source of uncertainty.
![]() |
Figure 3. Abundance ratios of oxygen and
|
However this controversy comes out, the
O, enhancement is not
universal, as has been shown, e.g. by
Nissen & Schuster
(1997);
there are "anomalous" halo
stars which have more solar-like element ratios even at quite low
metallicities, a feature that is also found in the Magellanic Clouds and
can be explained on the
basis of slower star formation rates and effective yields diminished by
outflows (e.g.
Pagel & Tautvaisiene
1998).
However, within the halo the presence
of "anomalies" shows no obvious relation with extreme kinematic
properties that might be signatures of a captured satellite
(Stephens 1999).
Within the thick disk, the
/ Fe ratio is remarkably
uniform, even up to
quite high metallicities, indicating an old "get rich quick"
population. This is well brought out by the work of
Fuhrmann (1998)
on Mg, shown in Figure 4, and
in a still unpublished study of oxygen by
Gratton et al. (1996),
and it may be that this trend is continued in the bulge (cf.
Rich 1999).
The data cast an interesting light on the formation of the thick disk,
since they indicate a hiatus in star
formation during which Fe/
increased but overall metallicity diminished,
maybe from inflow of relatively unprocessed material, e.g. in a merger,
before the stars now belonging to the thin disk were formed.
![]() |
Figure 4. [Mg/Fe] vs [Fe/H] and [Fe/Mg] vs [Mg/H] for stars of the Galactic halo, thick disk and thin disk, after Fuhrmann (1998). Courtesy Klaus Fuhrmann. |
Returning to the earliest stage of evolution of the Population II halo,
when we consider a regime in which [Fe/H] < - 2.5 or so, we reach a
stage where pollution by a single supernova becomes significant over a
region the size of a globular cluster or superbubble of the order of
105
M. Metallicity
(however defined) then becomes a poor clock and strange patterns appear,
accompanied by significant scatter
(McWilliam 1997).
There are marked changes within the iron group, with Cr, Mn (and Cu)
going down relative to iron and Co going up.
Ryan, Norris & Beers
(1996)
suggest that at these low levels [Fe/H]
is an increasing function of the mass of an individual supernova, and
Tsujimoto & Shigeyama
(1998)
have estimated revised stellar yields as a function of progenitor
mass on this basis. Most yields increase, with the conspicuous exception
of the r-process, whose representative Eu/Fe has a large scatter and may be
anti-correlated
with [Fe/H]. Ba and Sr also mainly come from the r-process at these low
metallicities and have even more scatter because the s-process can also
contribute in evolved
stars or stars with evolved companions. In a model recently put forward by
Tsujimoto, Shigeyama &
Yoshii (1999),
stars form in superbubbles dominated by a
single supernova, so that their composition is a weighted mean of the
interstellar medium (with [Eu/Fe]
[
/Fe] = constant) and
supernova ejecta. Fe/H
increases with the mass of the supernova while Eu/Fe decreases, leading
to an
anti-correlation with scatter superimposed until the ISM is sufficiently
enriched to take over and normal Galactic chemical evolution proceeds.
Further evidence for inhomogeneity comes from the abundances of the
light elements
6Li, beryllium and boron, which show an unexpected "primary"
behaviour - at least relative to iron - down to
very low metallicities. This cannot be understood on the basis of
spallation of interstellar CNO nuclei by primary cosmic ray protons and
-particles; these
give a reasonable explanation for their abundances in the Sun and
Population I stars
in general but led to an expectation of secondary behaviour (Be,B/O
O/H)
with diminishing metallicity.
(1)
There are also
energetic problems with the production by interstellar spallation at low
metallicity
(Ramaty et al. 1997).
Thus various inhomogeneous processes have been proposed,
beginning with the hypothesis of
Duncan, Lambert & Lemke
(1992)
that fast CNO nuclei in primary cosmic rays are reponsible, and that their
abundance is dominated
by supernova ejecta rather than the interstellar medium. A more detailed
model by
Ramaty & Lingenfelter
(1999)
postulates an origin of of cosmic rays from acceleration
of ions sputtered off dust grains in supernova ejecta by shocks within a
superbubble.
Thus the composition of cosmic rays is more or less constant and they
dominate light element production at early times in the way suggested by
Duncan, Lambert & Lemke.
1 With the large increase in O/Fe claimed by Israelian et al. and Boesgaard et al. there could be some semblance of secondary behaviour of the light elements after all, along with iron, magnesium, calcium etc; the likelihood of this depends on how the oxygen debate comes out. Back.