A&A 484, L27-L30 (2008)
DOI: 10.1051/0004-6361:20079169
LETTER TO THE EDITOR
M. Lugaro1,2,
-
S. E. de Mink1,
-
R. G. Izzard1 -
S. W. Campbell3,2 -
A. I. Karakas4 -
S. Cristallo5 -
O. R. Pols1 -
J. C. Lattanzio2 -
O. Straniero5 -
R. Gallino6,2 -
T. C. Beers7
1 - Sterrekundig Instituut, Universiteit Utrecht, PO Box 80000,
3508 TA Utrecht, The Netherlands
2 -
Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences,
Monash University, Victoria 3800, Australia
3 -
Academia Sinica Institute of Astronomy & Astrophysics, Taipei, Taiwan
4 -
Research School of Astronomy and Astrophysics, Mt. Stromlo Observatory,
Cotter Rd., Weston, ACT 2611, Australia
5 -
INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy
6 -
Dipartimento di Fisica Generale, Universitá di Torino, Torino, Italy
7 -
Department of Physics and Astronomy, Center for the Study of Cosmic
Evolution, and Joint Institute for Nuclear Astrophysics, Michigan State
University, East Lansing, MI, USA
Received 30 November 2007 / Accepted 25 April 2008
Abstract
Aims. A super-solar fluorine abundance was observed in the carbon-enhanced metal-poor (CEMP) star HE 1305+0132 ([F/Fe] = +2.90, [Fe/H] = -2.5). We propose that this observation can be explained using a binary model that involve mass transfer from an asymptotic giant branch (AGB) star companion and, based on this model, we predict F abundances in CEMP stars in general. We discuss wether F can be used to discriminate between the formation histories of most CEMP stars: via binary mass transfer or from the ejecta of fast-rotating massive stars.
Methods. We compute AGB yields using different stellar evolution and nucleosynthesis codes to evaluate stellar model uncertainties. We use a simple dilution model to determine the factor by which the AGB yields should be diluted to match the abundances observed in HE 1305+0132. We further employ a binary population synthesis tool to estimate the probability of F-rich CEMP stars.
Results. The abundances observed in HE 1305+0132 can be explained if this star accreted 3-11% of the mass lost by its former AGB companion. The primary AGB star should have dredged-up at least 0.2
of material from its He-rich region into the convective envelope via third dredge-up, which corresponds to AGB models of
and mass
2
.
Many AGB model uncertainties, such as the treatment of convective borders and mass loss, require further investigation. We find that in the binary scenario most CEMP stars should also be FEMP stars, that is, have [F/Fe] > +1, while fast-rotating massive stars do not appear to produce fluorine. We conclude that fluorine is a signature of low-mass AGB pollution in CEMP stars, together with elements associated with the
neutron-capture process.
Key words: stars: individual: HE 1305+0132 - stars: AGB and post-AGB - stars: abundances - nuclear reactions, nucleosynthesis, abundances
Carbon-enhanced metal-poor (CEMP) stars are chemically peculiar
objects, which represent 10-20% of all halo stars
(Cohen et al. 2005; Lucatello et al. 2006; Beers & Christlieb 2005). Most of CEMP stars exhibit radial velocity
variations, which imply the presence of a binary companion
(Lucatello et al. 2005). A significant fraction of CEMP
stars (70-80%, according
to Aoki et al. 2007, CEMP-s) also exhibit enhancements in heavy elements such as Ba
and Pb, which are produced by slow neutron captures (s process) in
asymptotic giant branch (AGB) stars
(e.g., Gallino et al. 1998). One scenario to explain the
abundance patterns in CEMP stars is therefore mass transfer from a
former AGB companion in which the carbon and heavy neutron-capture
elements were produced (e.g., Ivans et al. 2005; Thompson et al. 2008). However, a certain
fraction of CEMP stars, which have typically [Fe/H]
< -2.7,
exhibit low or no neutron-capture element
abundances (CEMP-no). These stars might have formed instead from material ejected
by rapidly rotating massive stars (Meynet et al. 2006) or faint type II
supernovae (Umeda & Nomoto 2005). At extremely low metallicities, [Fe/H] < -4,
giant CEMP stars could have enriched themselves in carbon via a ``dual
core flash'' - where mixing of protons during the core helium flash
induces a hydrogen flash - while in the early phases of AGB stars of masses
1.5
and [Fe/H
2.3, a ``dual
shell flash'' may occur, where protons
are ingested into the convective pulse
(Cristallo et al. 2007; Fujimoto et al. 1990; Hollowell et al. 1990; Fujimoto et al. 2000; Picardi et al. 2004).
Schuler et al. (2007) derived a super-solar fluorine abundance
of A(F) =
for the halo star
HE 1305+0132, which corresponds to
[F/Fe] = +2.9. This is the
most Fe-deficient star, [Fe/H
,
for which the fluorine
abundance has been measured to date. HE 1305+0132 also exhibits
overabundances of C and N ([C/Fe
;
[N/Fe
)
and an O abundance typical of halo stars ([O/Fe
).
Lines of Ba and Sr are observed
in its spectra (Goswami 2005), which place HE 1305+0132 in the
group of CEMP-s stars.
Fluorine abundances were first determined in AGB stars by Jorissen et al. (1992).
Enhancements of up to 30 times the
solar value were reported, demonstrating that these stars produce
fluorine. Observations of post-AGB stars and planetary nebulae,
the progeny of AGB stars, confirm that these objects are also enriched
in fluorine (Zhang & Liu 2005; Werner et al. 2005).
type II supernovae (Woosley & Haxton 1988) and Wolf-Rayet stars during helium burning
(Meynet & Arnould 2000) have been theoretically identified as F production sites,
but they are not observationally confirmed. In contrast to that observed
in CEMP stars and in particular HE 1305+0132, type II
supernovae typically produce more O than C, a part from a narrow range of initial mass around 80
(Woosley et al. 2002) or in
the case of faint supernovae at Z = 0 (Umeda & Nomoto 2005), while
models of Wolf-Rayet stars show that
fluorine production in these stars scales with stellar metallicity
and decreases when rapid rotation is included (Palacios et al. 2005).
The aims of this paper are to discuss the yields of C, N, and F from AGB stars at the metallicities relevant to CEMP stars (Sect. 2); to determine if the abundances measured for HE 1305+0132 can be explained using the AGB binary scenario (Sect. 3); and to evaluate the consequences of fluorine production in AGB stars on the CEMP stellar population (Sect. 4). We evaluate AGB modelling uncertainties related to different physics prescriptions and nuclear reaction rates in Sect. 5 and present our conclusions in Sect. 6.
Fluorine can be produced in AGB stars via the
18O(p, )15N(
F reaction chain
during thermal pulses (TPs) associated with the periodic activation of
the He-burning shell. While 18O is provided by the
14N(
F(
O chain, protons are
produced via 14N(n, p)14C reactions, and neutrons originate
from 13C(
, n)16O reactions. The 13C and 14N
nuclei are mixed down into the convective He shell from the ashes of
the H-burning shell. After each TP quenches, third dredge-up (TDU) may
occur, which trasports 19F to the convective envelope, together with
12C produced by partial He burning in the TPs.
Yields for C, N, and F at [Fe/H] = -2.3 from the models of
Lugaro et al. (2004) and Karakas & Lattanzio (2007) are shown in Fig. 1. The
reader is
referred to these papers for details of the computational methods. The
profiles of the C and the F yields as a function of the initial stellar
mass closely follow each other. This is because the TDU
carries primary 12C to the stellar envelope. This is
converted into primary 13C and 14N in the H-burning ashes,
whose abundances drive the synthesis of F.
Hence, fluorine production in AGB stars of low metallicity is of
a primary nature (Lugaro et al. 2004).
When the stellar mass is higher than 3
,
proton captures at the base of the convective envelope (hot bottom
burning, HBB) lead to the conversion of C into N and to the
destruction of F. Hence, we expect that the high F abundance
([F/Fe
4) predicted by Meynet et al. (2006) in the envelope of
a low-metallicity rotating 7
model at the beginning of the AGB phase
will be completely destroyed by HBB during the subsequent AGB phase.
![]() |
Figure 1: C, N, and F yields from AGB models of different masses and [Fe/H] = -2.3. Note that the F yields are plotted on a different scale. |
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In Fig. 2 we show the abundances of F and C+N,
relative to H, observed in the star and compare these values
to the abundance ratios in the
material lost by AGB stars according to the models presented in
Fig. 1.
To reproduce the observed abundances we employ two free parameters: (1) the initial mass of the polluting AGB star, which determines the chemical
composition of the accreted material; and (2) the amount by which the
accreted material is diluted into the envelope of the polluted
star. In Fig. 2 the composition that is produced by the
mixing of material before accretion with
material from the AGB star lies on a straight line connecting
the two components; the amount of dilution
is represented by the position of the point on the mixing line.
To explain the measured abundances, we require an AGB initial mass
of between 1.7 and 2.3
,
and dilution of the accreted material
by a factor of between six and nine.
If we assume that the mass of HE 1305+0132 is 0.8
,
which is the
typical mass of halo
stars, and calculate the evolutionary track of a star of this
mass and metallicity
Z = 10-4 (using the stellar evolution code STARS, see,
e.g., Pols et al. 1995), we find that the observed effective temperature
kK (Schuler et al. 2007) indicates that the star is a
giant with a convective envelope. When the envelope reaches its
maximum depth, the outermost 60% of the mass of the star is
convective
With these assumptions, we find that the star should have accreted
0.05-0.12
from a former AGB companion. Given that the total mass
lost during AGB evolution is in the range 1.0-1.5
,
this corresponds to
the accretion of 3-11% of the mass lost by the AGB star.
We implicitly assume that the composition of the accreted material can be represented well by the average composition of the total material ejected by the AGB star. In reality, however, the binary orbit is altered during mass transfer, so the composition of the accreted material varies because the surface AGB composition varies. Moreover, the evolution of the AGB star itself may be altered by the presence of a binary companion. Since there are no models presently available that describe both binary and detailed AGB evolution simultaneously, we have defaulted to using the AGB average composition.
In principle, it is possible to constrain the initial orbital period
of the binary system.
The system must have been sufficiently wide to allow the primary star to
evolve into an AGB star before filling its Roche lobe; this
requirement sets a lower limit to the initial orbital period.
On the other
hand, the efficiency of wind accretion decreases with the distance
between the two stars, which implies that the required amount of
accreted material
may set an upper limit on the initial orbital period. Using the binary
evolution code described in Izzard et al. (2006), we find that the range of
initial orbital periods that corresponds to an accretion efficiency >
in
a system with initial masses 0.8 and 2
is between 7 and 27 years.
![]() |
Figure 2:
Abundances by number of F and C+N with respect to H as
observed in HE 1305+0132 with 1![]() |
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With the binary evolution code described in Izzard et al. (2006), we simulated a population of binaries assuming the Kroupa et al. (1993) initial mass function, a uniform distribution of initial mass ratios, and the Duquennoy & Mayor (1991) initial period distribution.
We find that, of all turn-off and giant CEMP stars formed by mass
transfer from an AGB companion, 81% are expected to show an enhanced
fluorine abundance of [F/Fe] > +1, 12% have [F/Fe] >
+2, and 0.005% have [F/Fe] >+3. This can be understood qualitatively
from the results presented in Fig. 1 because
AGB stars that produce carbon also produce
fluorine. To date, fluorine enhancements have not been reported for other
CEMP stars because high-resolution spectra in the 2.3-2.4 m band
are required, which are not yet
available. We predict that fluorine should be found in most CEMP stars
that formed via the AGB binary scenario, thus representing a tracer of low-mass
AGB pollution in addition to s-process element enhancements.
The fluorine abundance in the particular case of HE 1305+0132 appears to
be exceptionally high, since we expect
only 0.04% of turn-off and giant CEMP stars to have A(19F
,
when considering the observational error bar.
However, we emphasize that many uncertainties play a role in this estimate,
both in the assumed distribution functions (which are reasonable for
stars in the solar neighbourhood, but not necessarily for halo stars),
and in the assumed wind accretion efficiency. The latter is based on
the Bondi & Hoyle (1944) prescription, whereas hydrodynamical simulations
(Nagae et al. 2004; Theuns et al. 1996) predict typically lower accretion efficiencies.
These uncertainties deserve more attention in a follow-up
study (Izzard et al., in preparation). Moreover,
the theoretical understanding of
F production in
AGB stars is itself still affected by many uncertainties, as discussed below.
Table 1:
Number of TPs with TDU, total
TDU mass, and C, N, and F yields (all in
)
for different AGB
models of 2
at [Fe/H] = -2.3.
To evaluate AGB model uncertainties we discuss a set of models of
2
and [Fe/H] = -2.3, computed using different physics and nuclear
reaction rate assumptions (Table 1). The
first four models in Table 1 are
computed using the codes described
in Karakas & Lattanzio (2007). Model 1 is that used in
the previous section for comparison with HE 1305+0132. Model 2 includes a region in
which protons from the envelope are mixed
down into the top layer of the He- and C-rich intershell (the region
between the H and He shells) at the end of each TDU. Proton captures
on 12C generate a ``pocket'' rich in 13C, the main
neutron source for the s-process in these stars, and 14N
(Herwig 2005). Extra 15N is
produced in the 13C-14N pocket, which is then converted to
19F in the following convective TP (see
also Goriely & Mowlavi 2000). The introduction of a 13C-14N pocket of
0.002
increases the 19F yield by 60%, while the C and N
yields are unaffected. In Model 3 of Table 1
we consider a model computed using the upper limit
(UL) of the 18F(
, p)21Ne rate. This increases the
19F yield by a factor of
2.5 (see details and
discussion in Karakas et al. 2008). When both a 13C-14N pocket and
the upper limit of the 18F(
, p)21Ne reaction rate is
used (Model 4 of Table 1), the 19F yield
increases by a factor of three. The other
main nuclear uncertainties originate from the
14C(
)18O
and the 19F(
, p)22Ne reaction rates (see discussion
in Lugaro et al. 2004). For
the latter, the latest evaluation by Ugalde et al. (2008) needs
to be tested in AGB models.
All models presented do not take account of the effects
induced by carbon enhancement on the opacities of the cool external
layers of AGB stars. When
,
C-bearing molecules, most
notably C2and CN, increase the opacity of the external layers, causing the
envelope to expand and the star to become larger and
cooler (Marigo 2002). Models of AGB stars of low mass and
metallicity with C- and N-enhanced low-temperature opacities have been
calculated using the Frascati Raphson Newton evolutionary code (FRANEC)
code (Cristallo et al. 2007; Straniero et al. 2006). In these models the
mass-loss rate strongly increases with respect to models in which
opacities are always calculated using the initial
Z = 10-4solar-scaled composition. The resulting yields (Model 5 of
Table 1) are
5 times smaller than
in the Karakas models. For comparison, the results obtained with the
same code, but using opacities calculated for the initial solar-scaled
composition (Cristallo 2006) are also reported in
Table 1 (Model 6), and are in good agreement
with the Karakas model, in spite of the different choices of
mass-loss rate and treatment of the convective borders
(see Straniero et al. 2006, for details).
It is evident that further work is required to address the uncertainties in the AGB fluorine yields at low metallicities. In particular, the inclusion of low-temperature carbon-enhanced opacities in the Karakas models (Karakas et al., in preparation) will provide an independent comparison to the results obtained by the FRANEC code and by Marigo (2002). Since a clear dependence of the mass-loss rate on the metallicity has still not be identified, different mass-loss prescriptions should be tested (Cristallo et al., in preparation). Finally, we note that the possible occurrence of the dual shell flash at the beginning of the AGB phase may also affect fluorine production and needs to be investigated in detail (e.g, Campbell & Lattanzio 2008).
We have shown that it is possible to reproduce the C and F abundances observed in
the CEMP-s star HE 1305+0132 via binary mass transfer from
a companion by accretion
of 3-11
of the mass lost by the primary star during its AGB phase. The AGB
star should have dredged-up at least
0.2
of its intershell material
into the convective envelope by means of the TDU.
While rapidly rotating massive stars
produce enough carbon and nitrogen to form CEMP stars, they do not appear
to produce fluorine (Palacios et al. 2005). The binary formation
scenario is thus favoured in the case of HE 1305+0132. In general, we
predict that most CEMP
stars formed by mass transfer from an AGB companion
should also be FEMP stars, i.e., have [F/Fe] > +1.
Hence, fluorine appears to be a useful discriminant between the
different scenarios proposed for the formation of CEMP stars.
During the preparation of this manuscript, another
halo object highly enriched in fluorine was discovered, the
planetary nebula BoBn 1
(Otsuka et al. 2008). The metallicity, as well as all the C and N abundances
observed in
this object are the same, within errors, as those of HE 1305+0132, which
suggests that BoBn 1 has a close connection to CEMP stars, perhaps
representing the evolutionary outcome of a CEMP star. On the other hand, the
derived F abundance in this object is roughly a factor of three higher than that
obtained for HE 1305+0132. This observation can be explained via the binary
scenario only if we consider
the F yields we computed including the 13C-14N pocket or
the upper limit of the 18F(, p)21Ne reaction.
Following this indication, we multiplied the F yields by a
factor of three in our stellar population model and calculated a probability of
12% for CEMP stars to have A(19F
.
This result provides us with
a possibility to alleviate the problem of the
extremely small probability of the high F abundance assessed for HE 1305+0132.
Acknowledgements
We thank an anonimous referee for her/his criticisms, which have much helped improving the clarity and focus of this paper. M.L. and R.G.I. gratefully acknowledge the support of NWO. S.C. acknowledges the APAC national supercomputing facility. A.I.K. acknowledges support from the Australian Research Council. R.G. acknowledges support by the Italian MIUR-PRIN06 Project ``Late phases of Stellar Evolution: Nucleosynthesis in Supernovae, AGB stars, Planetary Nebulae''. T.C.B. acknowledges partial support for this work from the National Science Foundation under grants AST 04-06784, AST 07-07776, and PHY 02-16783, Physics Frontier Center/Joint Institute for Nuclear Astrophysics (JINA).