The evolution of carbon, sulphur and titanium isotopes from high redshift to the local Universe
G. L. Hughes
2
B. K. Gibson
2
L. Carigi
1
2
P. S anchez-Bl azquez
2
J. M. Chavez
0
D. L. Lambert
0
0
University Of Texas, Department of Astronomy
,
Austin, Texas 78712, USA
1
Instituto de Astronoma, Universidad Nacional Autonoma de Mexico
, A.P. 70-264,
Mexico
, 04510 D.F.,
Mexico
2
University Of Central Lancashire, Centre for Astrophysics
, Preston, PR1 2HE
Recent observations of carbon, sulphur and titanium isotopes at redshifts z1 and in the local stellar disc and halo have opened a new window into the study of isotopic abundance patterns and the origin of the chemical elements. Using our Galactic chemical evolution code GETOOL, we have examined the evolution of these isotopes within the framework of a Milky Way-like system. We have three aims in this work: first, to test the claim that novae are required, in order to explain the carbon isotope patterns in the Milky Way; secondly, to test the claim that sulphur isotope patterns at high redshift require an initial mass function (IMF) biased towards massive stars; and thirdly, to test extant chemical evolution models against new observations of titanium isotopes that suggest an anti-correlation between trace-to-dominant isotopes with metallicity. Based upon our dual-infall galactic chemical evolution modelling of a Milky Way-like system and the subsequent comparison with these new and unique data sets, we conclude the following: novae are not required to understand the evolution of 12C/13C in the solar neighbourhood; a massive star-biased IMF is consistent with the low ratios of 12C/13C and 32S/34S seen in one high-redshift late-type spiral, but the consequent super-solar metallicity prediction for the interstellar medium in this system seems highly unlikely; and deficient isotopes of titanium are predicted to correlate positively with metallicity, in apparent disagreement with the new data sets; if confirmed, classical chemical evolution models of the Milky Way (and the associated supernovae nucleosynthetic yields) may need a substantial overhaul to be made consistent.
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Galactic chemical evolution models are employed to study the
spatial and temporal evolution of elements and isotopes
throughout the Universe. When coupled to phenomenological
representations of galaxy assembly, such models can be compared directly
with exquisite elemental and isotopic abundance patterns observed
locally in the Milky Way. From these comparisons, conclusions can
be drawn regarding the veracity of the underlying micro-physics
governing stellar evolution and nucleosynthesis, in addition to the
macro-physics governing the assembly of galaxies, the
redistribution of the interstellar medium (ISM) over galactic time-scales,
and the relative birthrate of stars of various masses (the so-called
initial mass function, IMF). While most galactic chemical
evolution models to date have concentrated on predictions related to the
total elemental abundance patterns, in unique circumstances, the
availability of detailed isotopic patterns can enhance the
predictive power of these models; such isotopic patterns afford additional
leverage for discriminating between the various origin sites for the
chemical elements. A full literature review of the field would be
unwieldy, but we refer the reader to any number of comprehensive
reviews and the many important references therein for example
Timmes, Woosley & Weaver (1995) (hereafter TWW95), Prantzos,
Aubert & Audouze (1996), Fenner & Gibson (2003), Romano &
Matteucci (2003) (hereafter RM03) and Chiappini et al. (2008).
Recent observational work has opened a new window into the
study of isotopic abundance patterns specifically, the identification
(for the first time) of carbon and sulphur isotopes at redshifts z1
(Muller et al. 2006, hereafter M06; Levshakov et al. 2006), coupled
with the recent determination of titanium isotopic abundances in the
local stellar disc and halo (Chavez 2008). The abundances of each
of these isotopes, and their evolution with redshift, hold clues as
to the relative importance of supernovae (SNe) versus asymptotic
giant branch (AGB) stars versus novae in seeding the Universe with
these important elements.
In this work, we present predictions for isotopic ratios of carbon,
sulphur and titanium within the framework of a classical Milky
Way-like disc galaxy model. The paper is organised as follows:
the fundamental nucleosynthetic origins of the relevant carbon,
sulphur and titanium isotopes are first (briefly) reviewed in Section 2;
in Section 3, we introduce the chemical evolution code (GETOOL)
and the four stellar yield compilations employed in our modelling;
finally, the results are presented and summarized in Sections 4 and
5, respectively.
2 O R I G I N O F C A R B O N , S U L P H U R A N D
T I TA N I U M I S O T O P E S
Because we in the astronomical community are generally more
accustomed to discussing elemental abundance patterns, rather than
isotopic patterns, we felt it would be useful to provide an over (...truncated)