Neutron-Capture Elements in the Double-Enhanced Star HE 1305-0007: a New
  s- and r-Process Paradigm
[1] [2] (Received 12 January 2007)
Neutron-Capture Elements in the Double-Enhanced Star HE 1305-0007: a New s- and
r-Process Paradigm∗
CUI Wen-Yuan()1,2,3, CUI Dong-Nuan()1, DU Yun-Shuang()1, ZHANG Bo()1,2
Department of Physics, Hebei Normal University, Shijiazhuang 050016
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012
Graduate School of the Chinese Academy of Sciences, Beijing 100049
The star HE 1305-0007 is a metal-poor double-enhanced star with metallicity [Fe/H] = −2.0,
which is just at the upper limit of the metallicity for the observed double-enhanced stars. Using
a parametric model, we find that almost all s-elements were made in a single neutron exposure.
This star should be a member of a post-common-envelope binary. After the s-process material
has experienced only one neutron exposure in the nucleosynthesis region and is dredged-up to its
envelope, the AGB evolution is terminated by the onset of common-envelope evolution. Based on
the high radial-velocity of HE 1305-0007, we speculate that the star could be a runaway star from
a binary system, in which the AIC event has occurred and produced the r-process elements.
PACS numbers: 97.10.Cv,26.45.+h,97.10.Tk
The discovery that several stars show enhancements
of both r-process and s-process elements (s+r stars
hereafter)[1,2] is puzzling, as they require pollution from
both an AGB star and a supernova. In 2003, Qian and
Wasserburg[3] proposed a theory, i.e. accretion-induced
collapse(AIC), for the possible creation of s+r-process
stars. Another possible s+r scenario is that the AGB
star transfers s-rich matter to the observed star but not
suffer from a large mass loss and at the end of the AGB
phase, the degenerate core of low-metallicity, high-mass
AGB star may reach the Chandresekhar mass, leading to
type-1.5 supernova.[4] Because the initial-final-mass re-
lation flats at higher metallicity,[4] the degenerate cores
of high-metallicity AGB stars are smaller than those of
the low-metallicity stars, the formation of AIC or SN1.5
is more difficult in the high-metallicity binary system,
which can explain the upper limit of the metallicity
([Fe/H] < −2.0) for the observed r+s stars.[5] Recently,
Barbuy et al.[6] and Wanajo et al.[7] suggested massive
AGB stars (M = 8 ∼ 12M⊙) to be the origin of these
double enhancements. Such a large mass AGB star could
possibly provide the observed enhancement of s-process
elements in the first phase, and explode or collapse pro-
viding the r-process elements. However, the modeling of
the evolution of such a large mass metal-poor star is a
difficult task, an amount of the s-process material is pro-
duced and its abundance distribution is still uncertain.[7]
The generally favoured s-process model till now is as-
sociated with the partial mixing of protons (PMP here-
after) into the radiative C-rich layers during thermal
pulses.[8−11] PMP activates the chain of reactions 12C(p,
γ)13N(β)13C(α, n)16O, which likely occurs in a narrow
mass region of the He intershell (i.e. 13C-pocket) during
the interpulse phases of an AGB star. The nucleosynthe-
sis of neutron-capture elements in the carbon-enhanced
metal-poor stars (CEMP stars hereafter)[12] can be in-
vestigated by abundance studies of s-rich or r-rich stars.
In 2006, Goswami et al.[13] analysed the spectra of the s-
and r-rich metal-poor star HE 1305-0007, and concluded
that the observed abundances could not be well fit by a
scaled solar system r-process pattern nor by the s-process
pattern of an AGB model. This star shows that the en-
hancements of the neutron-capture elements Sr and Y
are much lower than the enhancement of Ba and the
abundances ratio [Pb/Ba] is only about 0.05. Because
of the Na overabundance, which is believed to be formed
through deep CNO-burning, Goswami et al.[13] have also
speculated that this star should be polluted by a massive
AGB star. Clearly, the restudy of elemental abundances
in this object is still very important for well understand-
ing the nucleosynthesis of neutron-capture elements in
metal-poor stars.
The chemical abundance distributions of the very
metal-poor double-enhanced stars are excellent informa-
tion to set new constraints on models of neutron-capture
processes at low metallicity. The metallicity of HE 1305-
0007 is [Fe/H] = −2.0, which is just at the upper limit of
the metallicity for the observed double-enhanced stars.
There have been many theoretical studies of s-process
nucleosynthesis in low-mass AGB stars. Unfortunately,
the precise mechanism for chemical mixing of protons
from the hydrogen-rich envelope into the 12C-rich layer
to form a 13C-pocket is still unknown.[14] It is interest-
ing to adopt the parametric model for metal-poor stars
presented by Aoki et al.[15] and developed by Zhang et
al.[5] to study the physical conditions which could repro-
duce the observed abundance pattern found in this star.
In this Letter, we investigate the characteristics of the
nucleosynthesis pathway that produces the special abun-
dance ratios of s- and r-rich object HE 1305-0007 using
the s-process parametric model.[5] The calculated results
are presented. We also discuss the characteristics of the
s-process nucleosynthesis at low metallicity.
We explored the origin of the neutron-capture elements
in HE 1305-0007 by comparing the observed abundances
with predicted s- and r-process contribution. For this
http://arxiv.org/abs/0704.0576v1
purpose, we adopt the parametric model for metal-poor
stars presented by Zhang et al.[5] The ith element abun-
dance in the envelope of the star can be calculated by
Ni(Z) = CsNi,s + CrNi,r10
[Fe/H], (1)
where Z is the metallicity of the star, Ni,s is the abun-
dance of the i-th element produced by the s-process in the
AGB star and Ni,r is the abundance of the ith element
produced by the r-process (per Si = 106 at Z = Z⊙), Cs
and Cr are the component coefficients that correspond
to contributions from the s-process and r-process respec-
tively.
There are four parameters in the parametric model of
s- and r-rich stars. They are the neutron exposure per
thermal pulse ∆τ , the overlap factor r, the component co-
efficient of the s-process Cs and the component coefficient
of the r-process Cr. The adopted initial abundances of
seed nuclei lighter than the iron peak elements were taken
to be the solar-system abundances, scaled to the value of
[Fe/H] of the star. Because the neutron-capture-element
component of the interstellar gas to form very mental-
deficient stars is expected to consist of mostly pure r-
process elements, for the other heavier nuclei we use the
r-process abundances of the solar system,[16] normalized
to the value of [Fe/H]. The abundances of r-process nu-
clei in Eq. (1) are taken to be the solar-system r-process
abundances[16] for the elements heavier than Ba, for the
other lighter nuclei we use solar-system r-process abun-
dances multiplied by a factor of 0.4266.[5,17] Using the
observed data in the sample star HE 1305-0007, the pa-
rameters in the model can be obtained from the para-
metric approach.
Figure 1 shows our calculated best-fit result. For this
star, the curves produced by the model are consistent
with the observed abundances within the error limits.
The agreement of the model results with the observations
provides strong support to the validity of the paramet-
ric model. In the AGB model, the overlap factor r is a
fundamental parameter. In 1998, Gallino et al.[8] (G98
hereafter) have found an overlap factor of r ≃ 0.4−0.7 in
their standard evolution model of low-mass (1.5−3.0M⊙)
AGB stars at solar metallicity. The overlap factor calcu-
lated for other s-enhanced metal-poor stars lies between
0.1 and 0.81.[5] The overlap factor deduced for HE 1305-
0007 is about r = 0+0.17
−0.00, which is much smaller than
the range presented by G98. This just implies that iron
seeds could experience only one neutron exposure in the
nucleosynthesis region.[18]
For the third dredge-up and the AGB model, sev-
eral important properties depend primarily on the core
mass.[19−21] In the core-mass range 0.6 ≤ Mc ≤ 1.36,
an analytical formula for the AGB stars was given by
Iben[19] showing that the overlap factor increases with
decreasing core mass. Combing the formula and the
initial-final mass relations,[4] Cui and Zhang [22] obtained
the overlap factor as a function of the initial mass and
metallicity. In an evolution model of AGB stars, a
small r may be realized if the third dredge-up is deep
40 50 60 70 80
 = 0.71(mbarn 1)
 r  = 0.00
Cr = 67.43
Cs= 0.0047
 = 1.290
FIG. 1: Best fit to observational result of HE 1305-0007.
The black circles with appropriate error bars denote the ob-
served element abundances, the solid line represents predic-
tions from s-process calculations considering r-process contri-
bution (taken from Ref. [13]).
enough for the s-processed material to be diluted by ex-
tensive admixture of unprocessed material. Karakas[21]
and Herwig[23,24] have found that the third dredge-up is
more efficient for the AGB stars with larger core masses,
confirming the low values of r obtained by Iben[19] in
these cases. In AGB stars with initial mass in the range
M = 1.0−4.0M⊙, the core mass Mc lies between 0.6 and
1.2M⊙ at [Fe/H] = −2.0. According to the formula pre-
sented by Iben,[19] the corresponding values of r would
range between 0.76 and 0.26. Obviously, the overlap fac-
tor of HE 1305-0007 is smaller than this range.
We have extensively explored the convergence of the
abundance distribution of s-process elements through re-
current neutron exposures. All elements, including Pb,
were found to be made in the first neutron exposure.
This is consistent with the small overlap factor r ≃ 0
deduced in our best-fit model. Thus the possibility that
the s-process material has experienced only one neutron
exposure in the nucleosynthesis region is existent.
In 2000, Fujimoto, Ikeda and Iben[25] have proposed
a scenario for the extra-metal-poor AGB stars with
[Fe/H]< −2.5 in which the convective shell triggered by
the thermal runaway develops inside the helium layer.
Once this occurs,12C captures proton to synthesize13C
and other neutron-source nuclei. The thermal runaway
continues to heat material in the thermal pulse so that
neutrons produced by the 22Ne(α, n)25Mg reaction as
well as the 13C(α, n)16O reaction may contribute. In this
case, only one episode of proton mixing into He intershell
layer occurs in metal-poor stars.[25,15,45] After the first
two pulses no more proton mixing occurs although the
third dredge-up events continue to repeat, so the abun-
dances of the s-rich metal-poor stars can be characterized
by only one neutron exposure. Obviously, the metallicity
of HE 1305-0007 is higher than the range of metallicity
for this scenario.
0.1 1
FIG. 2: Best fit to observational result of metal-deficient
star HE 1305-0007 shows the calculated abundances logε(Pb),
logε(Ba) and logε(Sr) and reduced χ2 (bottom)as a function
of the neutron exposure ∆τ in a model with Cr = 67.4,
Cs = 0.0047 and r = 0. These are compared with the ob-
served abundances of HE 1305-0007.
0.0 0.2 0.4 0.6 0.8 1.0
FIG. 3: The same as those in Fig. 2 but as a function of the
overlap factor r in a model with ∆τ = 0.71.
One major goal of this work is to explore the charac-
teristics of the binary system that HE 1305-0007 origin
belongs to. The enhancement of the neutron-capture ele-
ments Ba and Pb suggests that in a binary system a mass-
transfer episode from a former AGB star took place. The
radial-velocity measurement indicates that HE 1305-0007
is a high-velocity star, with a radial-velocity of 217.8 km
s−1. From the high velocity of HE 1305-0007, we could
speculate that the star could be a runaway star from a bi-
nary system, which has experienced the AIC event. The
strong overabundance of r-process elements for HE 1305-
0007 (Cr = 67.4) should be a significant evidence for the
AIC scenario. In this case, the orbital separation must be
small enough to allow for capture of a sufficient amount
of material to create the formation of this event. As-
suming that HE 1305-0007 is formed in a binary system,
the AGB connection strongly suggests that this star is a
member of a post-common-envelope binary. This must
be the case if the overabundances of s-process elements
are attributed to mass-transfer from an AGB star. We
can only speculate about the effects of common-envelope
phase on the nuclear signatures in a metal-poor star that
was formed from this mechanism. One case could in-
volve several thermal pulses with dredge-up causing the
observed abundance distribution corresponding to larger
overlap factor. However, after the s-process material has
experienced only one neutron exposure in the nucleosyn-
thesis region and is dredged-up to its envelope, the AGB
evolution is terminated by the onset of common-envelope
evolution. This could explain the characteristic of single
neutron exposure in this star. In addition, based on the
Na overabundance, Goswami et al.[13] have speculated
that HE 1305-0007 should be polluted by a massive AGB
star, which has a large core-mass and favours the forma-
tion of AIC. Clearly, a detailed theoretical investigation
of this scenario is highly desirable.
The neutron exposure per pulse, ∆τ , is another funda-
mental parameter in the AGB model. In 2006, Zhang
et al.[5] have deduced the neutron exposure per pulse
for other s-enhanced metal-poor stars which lies between
0.45 and 0.88 mbarn−1. The neutron exposure deduced
for HE 1305-0007 is about ∆τ = 0.71+0.06
−0.04 mbarn
−1. Fig-
ures 2 and 3 show the calculated abundances logε(Pb),
logε(Ba) and logε(Sr) as versus the neutron exposure ∆τ
in a model with Cr = 67.4, Cs = 0.0047 and r = 0
and versus overlap r with ∆τ = 0.71 mbarn−1, respec-
tively. These are compared with the observed abun-
dances of HE 1305-0007. There is only one region in
Fig. 2, ∆τ = 0.71+0.06
−0.04 mbarn
−1, in which all the ob-
served ratios of three representative elements can be ac-
counted for within the error limits. The bottom panel
in Fig. 2 displays the reduced χ2 value calculated in
our model with all detected elemental abundances be-
ing taken into account and there is a minimum, with
χ2 = 1.290, at ∆τ = 0.71 mbarn−1. From Fig. 3,
we find that the abundances logε(Pb), logε(Ba) and
logε(Sr)are insensitive to the overlap factor r in a wider
range, 0 ≤ r ≤ 0.17. The uncertainties of the parameters
for the star HE 1305-0007 are similar to those for metal-
poor stars LP 625-44 and LP 706-7 obtained by Aoki et
al.[15]
In addition, it is worth further commenting on the be-
haviour of logε(Sr), logε(Ba) and logε(Pb) as a function
of the neutron exposure ∆τ seen in Fig. 2. The non-
linear trends displayed in the plot reveal the complex
dependence on the neutron exposure. The trends can be
illustrated as follows. Starting from low neutron expo-
sure and moving toward higher neutron exposure values,
they show how the Sr peak elements are preferentially
produced at nearly ∆τ∼ 0.4mbarn−1. At larger neutron
exposure (e.g., ∆τ∼ 0.7mbarn−1), the Ba-peak elements
become dominant. In fact, the higher neutron exposure
favors large amounts of production of the heavier ele-
ments such as Ba, La, etc. and less Sr, Y, etc.,[22] which
is the reason of the abundance pattern of the s-process
elements in HE 1305-0007, i.e. the enhancements of the
neutron-capture elements Sr and Y are much lower than
the enhancement of Ba and the abundances ratio [Pb/Ba]
is only about 0.05. Then a higher value of logε(Pb)∼ 4
follows at ∆τ = 1.5 mbarn−1. In this case, the s-process
flow extends beyond the Sr-peak and Ba-peak nuclei to
cause an accumulation at 208Pb. Clearly, logε(Pb) is very
sensitive to the neutron exposure.
The r- and s-process component coefficients of HE
1305-0007 are about 67.4 and 0.0047, which implies that
this star belongs to s+r stars. Recently, Zhang et al.[5]
have calculated 12 s+r stars with 0.0005 ≤ Cs ≤ 0.0060.
The s-process component coefficient of HE 1305-0007 lies
in this range. The Ba and Eu abundances are most use-
ful for unraveling the sites and nuclear parameters asso-
ciated with the s- and r-process corresponding to those
in extremely metal-poor stars, polluted by material with
a few times of nucleosynthesis processing. In the Sun,
the elemental abundances of Ba and Eu consist of signif-
icantly different combinations of s- and r-process isotope
contributions, with s:r ratios for Ba and Eu of 81:19 and
6:94, respectively.[16] From Eq. (1), we can obtain the s:r
ratios for Ba and Eu are 95.7:4.3 and 30.1:69.9, which
are obviously larger than the ratios in the solar system.
From Fig. 1 we find that our model cannot explain the
larger errors of some neutron-capture elements, such as
Y and Zr in HE 1305-0007. This implies that our un-
derstanding of the true nature of s-process or r-process
is incomplete for at least some of these elements.[27]
In conclusion, the star HE 1305-0007 is an s+r star
with metallicity [Fe/H] = −2.0, which is just at the upper
limit of the metallicity for the observed double-enhanced
stars. Theoretical predictions for abundances starting
with Sr fit well the observed data for the sample star,
providing an estimation for neutron exposure occurred
in AGB star. The calculated results indicated that al-
most all s-elements were made in the first neutron expo-
sure. Once this happens, after only one time dredge-up,
the observed abundance profile of the s-rich stars may
be reproduced in a single neutron exposure. From the
high radial-velocity of HE 1305-0007, we speculate that
the star could be a runaway star from a binary system,
which has experienced the AIC event. The r-process el-
ements in HE 1305-0007 (Cr = 67.4) should come from
the AIC event. Because the orbital separation must be
small enough to allow for capture of a sufficient amount of
material to create the formation of AIC, this star should
be a member of a post-common-envelope binary. After
the s-process material has experienced only one neutron
exposure in the nucleosynthesis region and is dredged-up
to its envelope, the AGB evolution is terminated by the
onset of common-envelope evolution. Clearly, such an
idea requires a more detailed high-resolution study and
long-term radial-velocity monitoring in order to reach a
definitive conclusion. More in-depth theoretical and ob-
servational studies of this scenario is highly desirable.
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[1] ∗Supported by the National Natural Science Foundation of
China under Grant Nos 10373005, 10673002 and 10778616.
[2] ∗∗To whom correspondence should be addressed. Email:
zhangbo@hebtu.edu.cn
	References
