A&A 422, 217-223 (2004)
DOI: 10.1051/0004-6361:20040248
A. Weiss1 - H. Schlattl1,2 - M. Salaris2 - S. Cassisi3
1 - Max-Planck-Institut für Astrophysik,
Karl-Schwarzschild-Str. 1, 85748 Garching,
Federal Republic of Germany
2 -
Astrophysics Research Institute, Liverpool John Moores University,
Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
3 -
INAF - Osservatorio Astronomico Collurania,
via Mentore Maggini, 64100 Teramo, Italy
Received 11 February 2004 / Accepted 2 April 2004
Abstract
Two alternative scenarios concerning the origin and
evolution of extremely metal-poor halo stars are investigated. The
first one assumes that the stars have been completely metal-free
initially and produced observed carbon and nitrogen overabundances
during the peculiar core helium flash typical of low-mass
Population III stars. The second scenario assumes that the initial
composition resulted from a mixture of primordial material with
ejecta from a single primordial supernova. Both scenarios are shown
to have problems in reproducing C, N, and O abundances
simultaneously, and both disagree with observed
-ratios, although in different directions. We
concentrate on the most iron-poor, carbon-rich object of this class,
HE 0107-5240, and conclude that the second scenario presently offers the
more promising approach to understand these objects, in particular
because evolutionary tracks match observations very well.
Key words: stars: low mass, brown dwarfs - stars: interiors - stars: abundances - stars: evolution - stars: individual: HE 0107-5240
The extremely (or ultra) metal-poor stars (UMPS) of the galactic halo are believed to be the closest links of the Galaxy to the first generation of stars, to Population III. We therefore hope to learn about the first epoch of star formation and the end of the "Dark Ages'' because they either are members of Population III themselves or because they carry the immediate imprint of massive Pop. III stars or primordial supernovae. They have received considerable attention in the recent past because of the fact that they are at the crossroads of stellar evolution, star formation, galactic chemical evolution and cosmology, recent CMB results and the question of reionization by Pop. III stars.
The discovery of HE 0107-5240, with a record low abundance of heavy
elements of
,
which is about a factor of 10
below the previously known lowest value, raised our interest to link
this star to Pop. III. Remarkably, the total "metallicity'' of HE 0107-5240
is not metal-poor due to a carbon and nitrogen overabundance of
and
,
putting it in a large
subgroup of UMPS with similar peculiar composition.
From the point of view of stellar evolution theory, metal-free stars are interesting in themselves because of some aspects of their structure and evolution which differ drastically from those of ordinary Pop. II or I stars. One of these peculiarities is the fact that during the core helium flash, mixing between the helium and hydrogen shell and the convective envelope can take place, resulting in a carbon- and nitrogen-rich envelope and a second red giant branch phase. Therefore, it is a plausible assumption that the UMPS are true Pop. III stars and that the carbon-rich subgroup consists of stars that experienced the first, peculiar helium flash. We want to emphasize that in this paper we are concerned only with the abundances of the CNO-elements, since heavier elements are unaffected by the nuclear processes in low-mass stars.
Table 1: Stellar parameters and chemical composition of HE 0107-5240 (Christlieb et al. 2004), and two theoretical models as explained in Sects. 2 (M 1) and 3 (M 2). The column "M 1 (initial)'' denotes the composition of the polluting SN ejecta; the interior of model M 1 has Z=0 throughout.
In our previous papers, we investigated the scenario
mentioned above to explain the observed chemical composition of
carbon-rich UMPS qualitatively and thus to link them
to Population III. The main problem one faces is that the
flash-induced mixing appears to result always, independent of the
details and assumptions of the calculations, in the same amount of
carbon and nitrogen, such that for
the
predicted carbon overabundance is
,
which is at the upper limit of observed
values. Since the C overabundance of HE 0107-5240 is as high as
the value found in our previous calculations, it appeared to be
particularly worthwhile to apply our approach to this star. Similar
calculations have recently been performed independently by
Picardi et al. (2004)
.
We therefore present a model for HE 0107-5240, based on the flash-induced mixing (FIM) in Sect. 2, repeating the basic features of this event. In Sect. 3, we will then show calculations for an alternative scenario, which assumes that this star (and other UMPS) are formed directly from the ejecta of Pop. III supernovae, i.e. that UMPS are the immediate successors of true, massive Pop. III objects. This scenario is also applied to other extremely metal-poor halo stars. Sect. 4 summarizes our conclusions.
The stellar parameters and chemical abundances of HE 0107-5240 (see
Table 1) are taken from Christlieb et al. (2002), Christlieb et al. (2004), and Bessell et al. (2004).
The value for oxygen is probably an upper limit and the error given is one-sided according to
Bessell et al. (2004). The final value might be close to
.
For the calculations of all our models we used our Garching stellar evolution code; details of the codes and of the calculations of metal-free stars were given in Schlattl et al. (2001). Mass and initial composition of the models will be specified below for each calculation. We use a constant mixing length parameter of 1.77, which results from a solar model calibration (see Schlattl & Weiss 1999). We neglect mass loss, but take into account the relativistic mass defect in nuclear burning; this is the reason for a slight reduction of stellar mass (see Table 1).
In our previous papers (Weiss et al. 2000; Schlattl et al. 2001,2002) we
followed the idea that the UMPS are proper Pop. III stars, i.e. their
initial composition was completely metal-free (Z=0), and that the
observed surface metal abundances, in particular that of heavy elements and
iron, are due to an external pollution or accretion event which took
place in the early phases of the approximately 12 Gyr of main-sequence
lifetime. Technically, we added a specific amount of solar
metallicity material on top of the initial, zero-age model such
that during the Red Giant phase, after dredge-up and dilution the
observed
values were reached
(see Weiss et al. 2000, Paper I). With this approach, the interior,
nuclear evolution of the model is that of a metal-free star.
In this scenario the peculiar C and N
abundances are then produced by the star itself during the core helium
flash. This flash-induced mixing (FIM) has been described extensively
by various authors (Fujimoto et al. 2000; Schlattl et al. 2001; Fujimoto et al. 1990, Paper II), therefore
it suffices to repeat that the convective region caused by the helium
flash is able to extend well into the hydrogen burning shell and to
mix protons into the carbon-helium intershell region, where the protons
are immediately captured on carbon nuclei to form nitrogen.
The additional energy from this CN-flash leads to further expansion of
the intershell layer such that the hydrogen and helium shells are
extinguished at their previous locations and instead a rejuvenated
hydrogen shell establishes itself within the previous intershell
layer. The timescale for this event is of the order of less than one day,
such that the
reaction is too slow
to strongly affect the oxygen abundance. Additionally, there are not
even enough protons to allow the
/
equilibrium value of roughly 0.1 at this temperature to be reached.
Later on, the convective envelope is able to penetrate into the
CN-rich layers, mixing these elements to the surface. Figure 1
shows the whole sequence of events for a
model of
Zi=0 during the first 104 years after the flash; at this time the mixing
to the surface has taken place and the nuclear shells have reached
their final structure.
Fujimoto et al. (2000) already specified conditions under which the FIM could
occur: broadly speaking, a (total) metallicity of
(where the M stands for the global metal abundance) and mass below
are required. In Paper II we added the influence of such parameters as initial helium abundance, the amount of polluting material, and the effect of
sedimentation on the main sequence. We found that the total
metallicity might be slightly higher (
)
than
quoted by Fujimoto et al. (2000), also due to the use of updated plasma neutrino
emission rates. The reader should also refer to
Picardi et al. (2004) for a detailed description of the FIM and further
tests concerning the conditions under which it can occur.
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Figure 1:
Upper panel: evolution of the nuclear shells (hatched) and convective
layers (shaded) in an initially metal-free model for HE 0107-5240 of
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Schlattl et al. (2002, Paper III) investigated in detail whether
and how the resulting envelope composition after the FIM is influenced by a very
small, but non-zero initial metallicity, or how it depends on details
of convection theory, such as, for example, the inclusion of
overshooting. We found that final abundances of
and
(starting
with an
-element, i.e. oxygen, enhancement of +0.4) are very
insensitive to all these variations with the exception of an artifical
reduction of the convective mixing velocity by a significant factor of 104 or
more. This could be the typical mixing velocity in semiconvective
layers. For example, Merryfield (1995) found in numerical
simulations of semiconvection mixing velocities of up to
,
while Achatz, Müller & Weiss (2004, in preparation) quote fully convective velocities of
in their multidimensional hydro-simulations of
the core helium flash. Interestingly, Herwig (2002) states that
in order to model "Sakurai's Object'' (V3443 Sgr), where a similar
mixing-and-burning might have taken place, mixing velocities reduced
by a factor of 104 are needed, too, in order to reproduce the
peculiar surface composition. Nevertheless, it remains to be
investigated whether the inclusion of
semiconvection would modify our results in the desired direction.
Similarly, a final
ratio of less than 6 was
the rule, slightly lower than the quoted numbers in the literature,
which were around 10. Since hardly any
oxygen is produced during the FIM we concluded that
-values would be necessary to decide whether the
FIM-scenario could be the explanation for the observed anomalies.
In the past we had assumed that the additional envelope material from
pollution is of solar composition, both for simplicity and because we
were interested rather in the interior evolution than in detailed
surface abundances. Modelling a particular star, of course, warrants a
realistic composition of the polluting material.
The
isotope ratio of HE 0107-5240
(>30; Christlieb et al. 2002) is
definitely higher than the values in our published FIM-models. In them,
it is
5, somewhat larger than the CN-equilibrium values found in
the intershell layers. This slight enhancement is due to the
assumption that the additional surface material had solar
composition with a carbon isotope ratio of 90.
Obviously, a still higher value will also raise the final ratio of the
model, bringing it in better agreement with observations.
Umeda & Nomoto (2003) showed that the pattern of elements heavier than Mg in
HE 0107-5240 agrees
with that predicted from nucleosynthesis in zero-metallicity type II
supernovae of initial mass
(Umeda & Nomoto 2002), under the assumption of severe fall-back to a
massive black hole and complete mixing of the ejected material.
Specifically they use a
model with explosion energy of
,
complete mixing of material within the
helium core of
and severe fallback of matter interior to
,
such that only
of
(or Fe, after the nuclear decay) are ejected. The latter condition comes from the requirement that
is achieved.
We therefore took the composition for the polluting material to be that of
a theoretical Pop. III supernova explosion from Chieffi & Limongi (2002),
selecting the
model. The explosion energy in this model
had been set to
.
The results of
Chieffi & Limongi (2002) do not
differ significantly from those of Umeda & Nomoto (2002) for comparable
parameters (progenitor mass, mass cut, explosion energy), at least with respect
to the elements of interest for us. In this SN model, a total of
is ejected, of which there are
of
hydrogen,
of helium,
of carbon,
of nitrogen, and
of oxygen. This matter
is assumed to be completely mixed.
The amount of polluting material actually dumped onto the initial model
and the mass cut have to be selected such that
the final iron abundance on the RGB after dredge-up and helium flash
mixing matches that of HE 0107-5240. Note that we do not have to care about
in the polluting material, since carbon production
during the helium flash and subsequent mixing will dominate the final
abundances. In model "M 1'' presented here (see
Table 1 and Fig. 1), the
mass was
,
and the amount
of polluting material
.
Its
initial composition is given in
Table 1, column "M 1 (initial)''.
While
in the SN ejecta, this is reduced to -3.6 already in the
initial main sequence model, which has a convective envelope of
.
After the convective envelope has reached is deepest
extent of about
,
the correct final
of -5.3 is reached. We have also
run models with a
mass increased respectively reduced
by a factor of 10, implying opposite factors for the additional
polluting mass.
The core helium flash and the induced mixing that takes place are
illustrated in Fig. 1. The resulting abundances after the
flash can be found in Table 1, column "M 1
(final)''. Obviously, there is again too much C and N produced, and
is much too close to the equilibrium value,
because of the small amount of polluting matter added to the star. As
stated before, we have modified this parameter, but found only
marginal variations in the resulting abundances, except in the most
extreme case of
of polluting material, with
only
of
.
In this case, the final C and N abundances were lower by one order of magnitude due to the large
amount of polluting matter and the carbon isotope ratio increased
slightly to 5.0. However, in this case
is too
high. While one could try to find better suited initial SN-yields, for
example by choosing another SN-progenitor mass, inspection of the
corresponding tables in the papers quoted reveal that in order to
achieve the very low Fe abundance one always has to add such small
amounts of SN-ejecta that the dilution of the carbon-rich intershell
matter of approximately
by the carbon-poorer envelope is
almost negligible, such that the final C and N abundances can hardly
be reduced
to the level of HE 0107-5240. Additionally, the
is
always too low, even if the SNe is basically
-free. We
therefore conclude that - as in the case of other, more iron-rich
UMPS - the amount of carbon and nitrogen relative to iron is too
high. In fact it appears that in the observed stars
and
are constant within a factor of 10, and that this value
is lower by a factor of 10-100 than that of the models. In addition,
the carbon isotope ratio reflects that the observed material has been
exposed to CN-burning to a much lower degree than our models
predict. The oxygen abundance in M 1, which nicely fits that of HE 0107-5240 is
solely due to the initial SN-composition, since hardly any oxygen is
produced in the model.
Umeda & Nomoto (2003) and Nomoto et al. (2003) advocate the idea, based on
the heavy element abundances, that HE 0107-5240 and other UMPS are second
generation stars, forming immediately after only one Pop. III
supernova has exploded and thus carrying the immediate imprint of
it. Such objects have also been termed "Pop. II.5'', to discriminate
them from Pop. II, where the heavy metal composition is the result of
many, well mixed SNe. We add that Limongi et al. (2003) criticized
this idea on grounds of incompatible Ni and C abundances, which
require very different mass cuts and because of the relative
abundances among lighter elements like Na and Mg. Instead, they suggest
the superposition of two primordial supernovae of 15 and
.
Since we are concerned here with only a few elements (C, N, O, Fe), this alternative scenario, which predicts a too large
oxygen abundance of
,
does not differ
qualitatively for our purposes, such that we follow the simpler
suggestion of Umeda & Nomoto (2003). Both scenarios imply that HE 0107-5240 had
a homogeneous initial composition with a heavy metal abundance
throughout the interior as is observed today, in contrast to the model
of the last section, which had this only in the polluted envelope
layers. We therefore calculated the straightforward evolution of such
a model (M 2) up the RGB and through the helium core flash.
Table 2: Observational data for our selection of extremely metal-poor halo stars; HE 0107-5240 is repeated for completeness.
The initial homogeneous composition for our model was obtained as
follows:
We chose the same SN model of
by
Chieffi & Limongi (2002) as before. This yields the total amount of mass for
each element as a function of mass cut. All lighter elements are
present only outside the Ni core. Thus, in order to obtain the observed
,
the mass cut must be set accordingly. For HE 0107-5240 a Ni mass
of
is needed (Ni will decay
into Fe). Furthermore, the SN material is assumed to be completely
mixed. To set the Fe abundance for the model's initial
composition correctly, a dilution with primordial H/He-matter is
needed (the SN ejecta being hydrogen-poor). In the present case, a
dilution factor of 1:360 leads to
.
All other element abundances result from the SN model (see
Table 1). In particular, the initial nitrogen abundance is
approximately solar, and
very high.
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Figure 2:
Evolution of model M 2 (![]() ![]() ![]() ![]() |
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The main sequence and red giant evolution is that of an ordinary,
moderately metal-poor low-mass star. The mass was chosen as
to obtain an age of 13.1 Gyr at the RGB tip.
During core hydrogen burning
CN-conversion takes place and the nuclear results become evident after
the first dredge-up. The final nitrogen abundance as well as the
carbon isotope ratio are quite similar to those observed in HE 0107-5240 (Table 1). Oxygen is hardly changed during the
evolution, and reflects completely the choice of the SN model. A survey of the quoted Pop. III SNe model literature reveals
that
varies between -0.2 and 1.1; in our selected model
it is close to the solar value. The very high carbon
abundance of HE 0107-5240 thus implies a similar oxygen enrichment, which is
therefore too abundant by more than one order of
magnitude and poses one of the problems of the present scenario for
the nature of HE 0107-5240. Figure 2 shows the
evolutionary track of M 2 (in the
vs.
diagram), on top of which the observed position of HE 0107-5240 and error
bars (Christlieb et al. 2004) are plotted. Obviously, there is a very good
agreement, and indeed HE 0107-5240 is in a post-dredge-up phase. The assumption that
HE 0107-5240 is a star on the first red giant branch, which has evolved
as a single star, is supported by the result of Bessell et al. (2004).
Due to the rather high total metallicity, which is one of the basic parameters influencing the appearance of the FIM event, the core helium flash happens at high luminosity and without any non-canonical or extended mixing.
We close this part with a comment on opacities. So far we have used
tables with appropriate H, He, and metal abundances, where the
internal metal distribution is assumed to be that of -element
rich Pop. II stars. Indeed, in the present models a large part of the
metals is actually carbon and nitrogen. Since the total metallicity is
Z=7.1
10-4 =
,
one can no longer assume that
the individual metal abundances are unimportant (see Salaris & Weiss 1998, for a
discussion of this issue). As in Schlattl et al. (2001) we have
therefore also computed a model with opacities for C- and N-enhanced
matter (see this paper for detail), and found no significant
differences, except that the model is younger by 0.7 Gyr after the main
sequence. Due to lack of appropriate tables we could not account for
the increased oxygen abundance.
In addition to HE 0107-5240 we calculated additional models for several
other objects with abundances taken from the literature (see
Table 2), which are believed to be first ascent giants on the
grounds of their surface gravity and effective temperature. The
composition of the initial, homogeneous model
was always obtained by mixing ejected material of the
supernova model by Chieffi & Limongi (2002) with pristine matter. The mass cut
and mixing factor are the free parameters chosen in such a way as to
obtain approximately the observed iron and carbon
abundance.
Figure 3 shows the resulting evolutionary track of the model
for CS22957-027 together with the Pop. III model of Sect. 2 (dashed
line) and a solar-type Pop. I star to emphasize the effect of the strong C and N enrichment after the FIM. Since
and
of HE 0107-5240 are very similar to that of CS 22957-027, this plot also illustrates the fact that the post-flash Pop. III model is slightly
too cool for HE 0107-5240.
Table 3 contains the summary of these
calculations. Since the evolutionary tracks cross the observationally
allowed range in
and
several times, we
offer various possible choices along the RGB and AGB evolution. We
note in particular that some stars have high enough surface gravities
to be still on the lower RGB, before the hydrogen shell encounters the
composition discontinuity left behind by the convective envelope when
it had reached its deepest extension. This phase is commonly referred
to as the "bump'', because of the slower evolutionary speed of red
giants when they reach the suddenly increasing hydrogen supply above
the discontinuity (see Salaris et al. 2002, for a review on low-mass red giant
evolution).
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Figure 3:
Evolution of our model for CS22957-027 (Table 3;
solid line) compared to that of a Pop. III model similar to M1 (
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Table 3:
Pop. II.5 models using the
SN yields of
Chieffi & Limongi (2002). The mass cut has been found by inter- or extrapolating
between SN models with different
to obtain the correct
.
is the mixing proportion between primordial and SN matter,
needed to reach (approximately) the observed
-value.
The various parameters of the models start at Col. 5 (age).
Evolutionary models that could represent the
objects are marked by the following labels:
E-RGB: on RGB, but before bump;
B-RGB: at bump; AB-RGB: after bump; L-RGB: close to tip; E-AGB: on
early AGB, after 2nd dredge-up; L-AGB: immediately before
thermal pulses commence; TP-AGB: thermal pulse phase on AGB.
The last two lines refer to models for CS 22943-037, for which we have
used alternative SN-yields from a
model by Umeda & Nomoto 2002 (see
text).
Comparing the final abundances with those observed it becomes obvious
that no star can easily be modelled. For all of them the carbon isotope
ratios are much too high; we are therefore facing the opposite problem
we have with the FIM-scenario. In addition, nitrogen abundances for
most stars, but in particular for
CS 22943-037 and CS 31082-001 are too low, indicating a lack of
mixing. Both problems could be cured if one assumes that these stars
experience at or after the bump additional mixing between the hydrogen
shell and the convective envelope. Such extra-mixing is now known to
take place in basically all low-mass metal-poor stars, both in the
field and in clusters, and leads to reduced carbon and increased
nitrogen abundances as well as to reduced carbon isotope ratios. We
refer the reader to Gratton et al. (2000) for an observational overview and to
Denissenkov et al. (1998) for a comprehensive theoretical paper.
The bottom two lines of Table 3 refer to a case, for which we
used a
model by Umeda & Nomoto (2002) instead of our standard
SN-model, to show the influence of the initial composition. The iron
abundance of these ejecta is too high (
;
)
for the required
,
such that we multiplied it by a factor 0.067 (given
in Col. 4). Nitrogen and oxygen have abundances of -0.6 and -0.1 in
the standard spectroscopic scale. The explosion energy was
.
In spite of the different initial composition
there is hardly a change in the final one, with the exception of the
nitrogen abundance on the RGB.
In earlier papers (Papers II and III) we have applied our scenario for
the evolution of initially
metal-free Pop. III stars with additional surface pollution to some
known objects of the galactic halo, investigating the possibility that
the observed severe carbon and nitrogen enhancements are due to
internal production and mixing in the course of the first core helium
flash. These models indicated that both the abundances of C and N in
the models are too high, and that the carbon isotope ratio is too
close to equilibrium values. In addition, we noticed at that time that
a statistically representative sample of UMPS is needed to verify that
the number of carbon-enhanced objects is in agreement with the
predictions from the models, which would be post-flash, and therefore
short-lived compared to lower RGB stars. The detection of HE 0107-5240 with
the lowest iron abundance of
and highest carbon
enhancement of
,
allowed us to investigate this
scenario more closely. We thus computed specific models using
realistic SN yields from the literature.
The low iron content of HE 0107-5240 can only be achieved by adding tiny
amounts of SN-ejecta matter or to impose rigid mass cuts for the SN explosion. We find that, independent of the particular choice of
polluting matter, the final carbon and nitrogen abundances, which
result from the core helium flash and the subsequent
mixing, exceed the observed abundances by orders of magnitude,
similar to the case of the less extreme cases we modelled in our previous
papers. It appears that both in nature, and in our models, the amount of overabundant C and N is rather constant, such that with
decreasing Fe-abundance the relative overabundance is increasing. This
effect can actually been seen in Fig. 2 of Rossi et al. (1999) by looking at
the upper envelope of the
vs.
distribution. However, the models appear to produce 10-100 times too
much carbon and nitrogen.
Additionally, the carbon isotope ratio in HE 0107-5240 is definitely
above equilibrium values, which are always obtained in our
simulations. These results, with the exception of the oxygen
abundance, are widely independent on whether we use solar or early SNe
material for the pollution.
Therefore, HE 0107-5240 contradicts most strongly our flash-induced mixing
models. This conclusion has been reached independently by
Picardi et al. (2004) on the grounds of very similar results, a fact which
is encouraging for the reliability of the models. In particular,
and
are 6.1 and 5.8, respectively, to
be compared to our values of 6.0 and 6.2. The fact that we have a
higher nitrogen abundance we ascribe to the sensitivity of nitrogen
production to the competition between mixing and nuclear
processing. The carbon isotope ratio in both cases is close to 4. For
a more detailed comparison of the internal processes we refer the
reader to Picardi et al. (2004). In both cases the post-FIM evolution up
to a second, more classical core helium flash (see Schlattl et al. 2001, for
details) is by a factor of 10 or more shorter than
that before the FIM, such that a much larger number of C-normal UMP giants are to be expected. This is in contradiction with the statistics of UMPS so far. As a final argument against the
FIM-scenario for HE 0107-5240, Picardi et al. also stress that this
scenario predicts a very high Li abundance after the FIM of
,
which is above the (conservative) upper limit
of 5.3 obtained by Christlieb et al. (2004).
We then investigated an alternative scenario, assuming that HE 0107-5240 and other extremely metal-poor stars are representing a kind of "early Pop. II'' or Pop. II.5 class of objects. Their homogeneous initial composition is of low, but finite metallicity. However, contrary to standard Pop. II stars, the material still carries the imprint of one or few individual SNe of Pop. III. We restricted ourselves to one particular SN model, since in terms of CNO-elements the various models available (mass, explosion energy, author) do not vary drastically. The choice was made according to inferences based on reproducing the heavy elements in HE 0107-5240.
We find that, after the mass cut and dilution with pristine interstellar
material have been fixed, the stars are carbon and oxygen rich already
on the main sequence, and produce large amounts of nitrogen in
CN-cycling; as a consequence of standard first dredge-up the nitrogen
will be transported to the surface after main-sequence termination. The
evolution is quite standard, as for Pop. II models. C and N abundances
agree naturally very well with the observations, but carbon isotope
ratios are in this scenario definitely higher than for most stars
under consideration (with the exception of HE 0107-5240). Therefore, for
we face the opposite problem of the one
we have for the FIM-models. Oxygen is always enriched, due to the composition of
SN ejecta. It would therefore be
necessary to obtain results for oxygen for UMP stars
to put further constraints on the various possibilities for the nature
of the UMPS.
In case of the objects we tried to model and for which we had oxygen
abundances, it appears that we can roughly reproduce the
observations. However, in the case of HE 0107-5240 the SN yields predict too
much oxygen. This problem can also be noticed in the models by
Limongi et al. (2003), while it seems to be less severe in Umeda & Nomoto (2002)
using a less massive SN progenitor with only
explosion energy. The composition of one of the
objects we investigated could be matched satisfactorily.
The observed stars lie very nicely on the tracks for our
Pop. II.5 scenario, although the errors are too large to allow
excluding the FIM-possibility on those grounds. It is possible to
identify different evolutionary stages for the observed
objects. Generally, the later the evolutionary phase, the higher the N
abundance and the lower the
ratio, reducing
some of the problems. If the star would be on the AGB,
a better agreement is possible, in particular, if the 3rd dredge-up
happens during the thermal pulses, because in this case, the carbon
isotope ratio would be reduced strongly. However, we have not
followed the evolution of the models this far.
Again, statistically significant samples of UMPs would be
necessary to further look into this question.
In spite of the remaining problems, we presently favor the idea that the observed
extremely metal-poor stars of the galactic halo are stars formed
directly from the ejecta of one or a few Pop. III SNe of intermediate mass
(
), which are diluted with metal-free primordial
gas. Overall, the agreement between model and observations appears to
be better, and there is still a large uncertainty concerning the
SN yield composition. Also, whether one or two or a few SNe have
contributed to the initial composition of an UMP (see the
discussion in Limongi et al. 2003), allows further fine-tuning of
models. Nevertheless, solid statistical samples are clearly needed for
further progress. Finally, we point out that all SN models favored
indicate progenitor masses of
.
Currently, the
primordial star formation scenario is favouring much higher initial
masses for Pop. III stars. This question, too, remains to be
resolved. The extremely metal-poor stars with their particular
composition may guide us in this.
Acknowledgements
A.W. wishes to thank the participants of the "First Stars II'' meeting held at Pennsylvania State University in June 2003 for encouraging him to publish the results, and the organizers and the DFG for substantial travel support. S.C. warmly acknowledeges financial support by MURST (PRIN2002, PRIN2003).