Zero-metallicity stars - I. Evolution at constant mass
A&A
Zero-metallicity stars
P. Marigo 1 2
L. Girardi 1 2
C. Chiosi 1 2
P. R. Wood 0 1
0 Mount Stromlo and Siding Spring Observatories, Australian National University , Private Bag, Weston Creek PO, ACT 2611 , Australia
1 Send o print requests to: P. Marigo
2 Dipartimento di Astronomia, Universita di Padova , Vicolo dell'Osservatorio 2, 35122 Padova , Italia
We present extensive evolutionary models of stars with initial zero-metallicity, covering a large range of initial masses (i.e. 0:7 M M 100 M ). Calculations are carried out at constant mass, with updated input physics, and applying an overshooting scheme to convective boundaries. The nuclear network includes all the important reactions of the p-p chain, CNO-cycle and -captures, and is solved by means of a suitable semi-implicit method. The evolution is followed up to the thermally pulsing AGB in the case of low- and intermediate-mass stars, or to the onset of carbon burning in massive stars. The main evolutionary features of these models are discussed, also in comparison with models of non-zero metallicity. Among several interesting aspects, particular attention has been paid to describe: i) the rst synthesis of 12C inside the stars, that may suddenly trigger the CNO-cycle causing particular evolutionary features; ii) the pollution of the stellar surface by the dredge-up events, that are e ective only within particular mass ranges; iii) the mass limits which conventionally de ne the classes of low-, intermediate-, and high-mass stars on the basis of common evolutionary properties, including the upper mass limit for the achievement of super-Eddington luminosities before C-ignition in the high-mass regime; and iv) the expected pulsational properties of zero-metallicity stars. All relevant information referring to the evolutionary tracks and isochrones is made available in computer-readable format.
stars; evolution { stars; interiors { stars; Hertzsprung{Russell (HR) diagram { stars; low-mass
1. Introduction
The standard Big Bang Nucleosynthesis
(e.g. Olive 1999)
predicts that essentially no elements heavier than 7Li
should exist in the gas mixture in which the rst stars
are generated. Only in the framework of inhomogeneous
Big Bang nucleosynthesis (see Jedamzik 2000 and
references therein) we can expect some non-negligible
primordial metal production.
The stars with negligible, if not zero, initial metal
abundance { named population III objects (Pop-III for
short) { are expected to have structure and evolution
distinct from those in which traces of heavier elements, like
those of the CNO group, are present.
The major di erence between zero-metal stars and
those of normal metal content (even down to very low
values of Z) lies in the mechanism of nuclear energy
generation. In fact, owing to the lack of CNO nuclei, the
premain sequence gravitational contraction cannot stop until
the central temperature and density are high enough to
allow the p-p chain to provide the energy budget. Since the
p-p chain is a poor thermostat as compared to the
CNOcycle, very high temperatures can be reached in the
central regions. This may eventually lead (depending on the
mass of the star) to the rst synthesis of primary carbon
through the 3- reaction, while the star is still on the main
sequence. As a consequence, the CNO-cycle is activated at
the very high temperatures characterising the 3- process,
possibly causing a dramatic change in the dominant
energy source (from p-p chain to CNO-cycle), hence a ecting
the stellar structure amid the main sequence evolution. As
demonstrated below, similar changes in the evolutionary
behaviour may also occur at later stages, whenever carbon
is rst produced (or transported) into di erent regions of
the star.
The threshold abundance of 12C at which the
distinct behaviour of zero metal stars appears is as low as
Z XC 10−10−10−9
(see Cassisi & Castellani 1993)
.
Starting from these values and higher, the e ciency of
CNO-cycle is su cient to recover the standard behaviour.
Needless to say, the distinct evolutionary behaviour of
these stars may also imply a very distinct nucleosynthesis
and chemical pollution of the interstellar medium, if
compared to stars with initial Z > 10−10. Also their properties
in the HR-diagram should be quite peculiar. Both aspects However, over the years, interest has turned towards
are particularly important in the modelling of more \normal" stars. In fact, various fragmentation
models of primordial gas have suggested that the rst stars
1. the initial chemical enrichment of galaxies, when the might have formed with low/intermediate masses, e.g.
rst metals and additional light elements have been Yoshi & Saio (1986) nd the peak of the mass function
made available to the second generation of stars, and at roughly 4−10 M ;
Nakamura & Umemura (1999)
nd
probably also to the intergalactic medium; a the typical mass of 3 M , which may grow to 16 M by
2. the initial spectrophotometric evolution of galaxies, (...truncated)