A&A 474, 221-228 (2007)
DOI: 10.1051/0004-6361:20066735
D. M. Allen
-
G. F. Porto de Mello
Observatório do Valongo/UFRJ, Ladeira do Pedro Antonio 43, 20080-090 Rio de Janeiro, RJ, Brazil
Received 10 November 2006 / Accepted 6 July 2007
Abstract
Aims. We present abundances for Ru and Hf, compare them to abundances of other heavy elements, and discuss the problems found in determining Ru and Hf abundances with laboratory gf-values in the spectra of barium stars.
Methods. We determined Ru and Hf abundances in a sample of giant and dwarf barium stars, by the spectral synthesis of two Ru I (4080.574 and
4757.856) and two Hf II (
4080.437 and
4093.155) transitions. The stellar spectra were observed with FEROS/ESO, and the stellar atmospheric parameters lie in the range 4300 <
/K < 6500, -1.2 < [Fe/H]
0 and
.
Results. The Hf II 4080 and the Ru I
4758 observed transitions result in a unreasonably high solar abundance, given certain known uncertainties, when fitted with laboratory gf-values. For these two transitions we determined empirical gf-values by fitting the observed line profiles of the spectra of the Sun and Arcturus. For the sample stars, this procedure resulted in a good agreement of Ru and Hf abundances given by the two available lines. The resulting Ru and Hf abundances were compared to those of Y, Nd, Sm and Eu. In the solar system Ru, Sm and Eu are dominated by the r-process and Hf, Nd and Y by the s-process, and all of these elements are enhanced in barium stars since they lie inside the s-process path. Ru abundances show large scatter when compared to other heavy elements, whereas Hf abundances show less scatter and closely follow the abundances of Sm and Nd, in good agreement with theoretical expectations. We also suggest a possible, unexpected, correlation of Ru and Sm abundances. The observed behaviour in abundances is probably due to variations in the 13C pocket efficiency in AGB stars, and, though masked by high uncertainties, hint at a more complex scenario than proposed by theory.
Key words: stars: abundances - stars: chemically peculiar - stars: late-type
Barium stars are chemically peculiar objects which present large excesses of the elements due to the neutron capture s-process. These stars are not evolved enough to self-enrich during the thermal pulses in the AGB phase. The standard explanation for their peculiarities is a binary status. The former primary, more massive, evolves faster and goes into the AGB phase, whereby it convectively enriches its atmosphere with s-process products. After a phase of strong mass loss through stellar wind, it becomes a white dwarf and is detected in the ultraviolet only with difficulty (Böhm-Vitense et al. 2000), if at all. The former secondary is enriched by mass accretion from the stellar wind of its companion, and presents in its atmosphere vestiges of the nucleosynthesis of the former AGB star, being presently observed as the barium star.
Such stars usually have been studied by their strong excesses of the heavy elements chiefly synthesized by the s-process, Sr, Y, Zr, Ba, La, Ce and Nd being the most spectroscopically accessible. Ruthenium and hafnium, in their turn, have been very little studied even for this chemically peculiar class of stars. In fact, the literature on abundances of Ru and Hf, for any class of star, is very scarce.
These elements were previously detected in the very metal-poor
stars CS 22892-052 (Sneden et al. 2003, and references therein) and
CS 31082-001 (Hill et al. 2002), thanks to their large enhancement of
neutron-capture elements. The lines of Hf and Ru are weak and most of
them lie
blended in the crowded near-UV region of the spectrum, generally
hampering their detection in normal stars.
Sneden et al. found [Ru/Fe] = +1.34 and [Hf/Fe] = +1.24
for CS 22892-052, at [Fe/H] = -3.1 and Hill et al. found
[Ru/Fe] = +1.42 and [Hf/Fe] = +1.43 for CS 31082-001, at [Fe/H] = -2.9. Both these works only employed spectral lines with
.
For another very metal poor star, HD 122563
with [Fe/H] = -2.7, Honda et al. (2006) found [Ru/Fe] = 0.07 from two
lines with
.
For the very metal poor ([Fe/H] = -3.5) TP-AGB star,
CS 30322-023, Masseron et al. (2006) found [Ru/Fe] = 1.05 and [Hf/Fe] = 0.53.
Ruthenium and hafnium were
also observed in some chemically peculiar stars. Tomkin & Lambert (1983)
found [Fe/H] = -0.32, [Ru/Fe] = +0.78 and [Hf/Fe] = +1.09 for the barium
star HR 774, from the
4584 and
5309 Ru I
lines, and the
7132 Hf II line. For the symbiotic
star AG Draconis, Smith et al. (1996) found [Hf/Fe] = +0.86 and
[M/Fe] = -1.5 by using the
7132 line. Yushchenko et al. (2002) found
for the barium star
Cyg [Ru/Fe]
-0.04 and
[Hf/Fe]
0.47, for which they also found
[Fe I/H] = +0.02
0.10 and
[Fe II/H] = +0.06
0.08, employing the
4584,
4869 and
5026 Ru I lines, and the
4093 Hf II line. We note that the lines of
Ru I used by Yushchenko et al. are not visible in our
spectra.
Ruthenium lies close to Nb and Mo in the periodic table. This element could be called an r-process element, since, according to Arlandini et al. (1999), the contributions from the s-, r-, and p-processes for its abundance are respectively, 32.3%, 59.7%, and 8%. Hafnium is a heavy s-process element, lying close to La and Ba, and the abundance contributions from the s-, r-, and p-processes are, respectively, 55.5%, 44.16% and 0.34%, according to Arlandini et al. (1999). Abundance determinations of Ru and Hf for a statistically significant sample of barium stars might help increase the number of elements studied in these objects, thereby shedding further light on the detailed abundance distribution of heavy elements in these stars. The abundance ratio of the heavy group of the s-process elements, Ba to Nd, to the lighter group, Sr to Zr, has been traditionally used as a neutron exposure parameter (Allen & Barbuy 2006b; Smiljanic et al. 2007; North et al. 1994; Luck & Bond 1991). These data are necessary to clarify the mechanism that accounts for the large spread in abundance excesses of neutron capture elements between the mild barium and barium stars. The notion that these differences could be accounted for by differing metallicities, which in its turn results in different neutron exposure levels due to a higher ratio of neutron to seed nuclei density for lower metallicity stars, is not corroborated by the data (Smiljanic et al. 2007; Boyarchuk et al. 2002). A possible correlation with orbital parameters, in the sense that the mild barium stars have smaller s-process excesses due to longer orbital periods, has been claimed (Böhm-Vitense et al. 1984) but does not seem to explain the observations either (Jorissen et al. 1998). This has led Smiljanic et al. (2007) to suggest that a possible explanation might lie in a different mass range of the progenitors of mild barium and barium stars, or in different mixing properties in the two classes of stars. In order to test these and other possible scenarios, detailed abundance data are mandatory.
In this work, we present Ru and Hf abundances for a large sample of dwarf barium stars, mild barium stars and classical barium stars, based on high resolution and high signal-to-noise spectra. We also discuss discrepancies in the Ru and Hf oscillator strengths available in the literature, and their effect on the derived abundances. This paper is organized as follows. Section 2 briefly presents the data and the determination of the stellar atmospheric parameters; Sect. 3 describes the choice of spectral lines and the evaluation of published laboratory oscillator strengths; in Sect. 4 the uncertainty calculations are detailed; in Sect. 5 the derivation of abundances is described, and in Sect. 6 our conclusions are drawn.
All spectra for the sample stars were obtained with the 1.52 m
telescope at ESO, La Silla, using the Fiber Fed Extended Range
Optical Spectrograph (FEROS, Kaufer et al. 2000). FEROS spectra
have a constant resolving power of
R = 48 000 from 3600 to 9200
.
The target sample for the present study
includes 25 dwarf, subgiant and giant barium stars from Allen & Barbuy (2006a):
the S/N of the spectra of this sample ranges from 100 to 250.
To this we added the sample described in detail by
Smiljanic et al. (2007), involving 8 mild and classical barium
stars, plus 6 normal giants, with spectral S/N ratio ranging from 500 to 600. The journal of observations and derivation of
atmospheric parameters have been described in detail by
these authors.
Allen & Barbuy (2006a) determined effective temperatures
from
photometry, surface gravities
from the stellar positions in
theoretical HR diagrams, and iron abundances from equivalent widths of
approximately 150 Fe I lines and 30 Fe II lines.
Smiljanic et al. (2007) determined
,
and metallicities
from the simultaneous excitation and ionization equilibria of the
equivalent widths of an average number of 120 Fe I and 12 Fe II lines. Surface gravities were also computed from the
stellar luminosities and theoretical HR diagrams, and a very good
agreement was found for the two sets of gravities.
The LTE abundance analysis and the spectrum synthesis calculations
for Ru and Hf were performed by employing the codes by Spite (1967,
and subsequent improvements in the past thirty years),
described in Cayrel et al. (1991) and Barbuy et al. (2003). The adopted model
atmospheres (NMARCS) were computed with a version of the MARCS
code, initially developed by Gustafsson et al. (1975) and subsequently
updated by Plez et al. (1992), used here for stars with gravities
,
and Edvardsson et al. (1993), here used for less evolved
stars with
.
Abundances for Y, Nd, Sm, and Eu
were taken from Allen & Barbuy (2006a) and for stars of Smiljanic et al. (2007),
abundances based on spectrum synthesis calculations were performed
and will be detailed in a forthcoming paper.
Ruthenium and hafnium present spectral lines ranging from the UV
to the IR: for the lines that appear for
,
the FEROS spectra do not allow good fits to synthetic ones,
this region being too crowded for cool stars. So, in this work we
looked for lines with larger wavelengths. The following lines,
detected in the spectra of the Sun (Kurucz et al. 1984) and Arcturus
(Hinkle et al. 2000), were considered for abundance determinations in
the barium star spectra:
4080.574,
4144.1968,
4381.272, and
4757.856 for Ru I and
4080.437 and
4093.155 for Hf II. The
lines
4144.1968 and
4381.272 are not visible in
our barium star spectra, and so were discarded. Table 1
shows the oscillator strengths (
)
and excitation
potential (
)
for the Ru I and Hf II lines
used in this work, as well as the references for the
values. The main
source was the Vienna Atomic Line
Database (VALD, Piskunov et al. 1995). The
for Hf II
4080.437 line is given by Lundqvist et al. (2006) and Lawler et al. (2007).
If the
value of -1.596 is used, the solar abundance of Hf is 0.7 dex
higher than the value given by Grevesse & Sauval (1998), as shown in Fig. 1. For Arcturus
(Fig. 2), the
resulting abundance is
(Hf) = 0.98 and
[Hf/Fe] = +0.64, much higher than expected for a normal, slightly
metal-poor giant like Arcturus. Similarly, if the VALD
value (-0.890) for the Ru I
4757.856 line is used,
the solar abundance of Ru is 0.35 dex higher than in
Grevesse & Sauval, as shown in Fig. 3. These results
are deemed as unreasonable in the face of the known uncertainties.
For these two transitions we determined gf-values by fitting the
observed line profiles of the spectra of the Sun and Arcturus,
shown in Table 1 and Figs. 1, 2, and 3. We used for the Sun
K,
(cgs) and a microturbulent
velocity of
km s-1, and an atmospheric model from
Edvardsson et al. (1993). For Arcturus, we used atmospheric parameters from
Meléndez et al. (2003),
K,
(cgs),
[Fe/H] = -0.54, excepting
km s-1 which was
fitted in this work. The Plez et al. (1992) grid was used to
derive the atmospheric model.
It is conceivable that an unidentified transition, blended with
the Ru and Hf lines, explains these large discrepancies. The
region around 4080 contains molecular lines of CN, so one
must be careful in fitting synthetic spectra there. One
possibility is that a CN feature, missing in our molecular lines
database, might be blended with the Hf II
4080.574
transition. Using the gf-values given by Lundqvist et al. (2006) or Lawler et al. (2007)
could make the synthetic line weaker
than the observed one, consequently increasing the resulting
abundance from this line.
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Figure 1:
Fits for two oscillator strengths
of the Hf II ![]() ![]() ![]() ![]() |
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Figure 2:
Fits for two oscillator strengths
of the Hf II ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Figure 3:
Fits for two oscillator strengths
in the solar spectrum for the Ru I ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Two stars were used to compute the abundance uncertainties: HD
181053 from Smiljanic et al. (2007) with
K,
,
[Fe/H] = -
,
and
,
and HD 87080 from Allen & Barbuy (2006a) with
K,
,
,
and
.
The abundance uncertainties
were calculated by verifying how much the variation of 1
on the atmospheric parameters affects the output value of the
synthesis program, here
.
Table 2 shows the values taken into account in this
calculation and the resulting uncertainties.
Under the simplifying hypothesis of independent errors, the
uncertainty of the output value is given by
The average value of Ap (Apm) is obtained by averaging the
individual abundances of two lines and not from several
measurements of the same line. In the latter case, the standard
deviation could be used to calculate the uncertainty on Apm.
Considering this, we found it more suitable to apply a
propagation of errors taking into account the uncertainty
calculated with Eq. (1). Thus, the uncertainty on Apm is
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(2) |
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(3) |
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(4) |
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(5) |
For [Hf/Ru] the uncertainties are determined by
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(6) |
Uncertainties on elements other than Ru and Hf were described in Allen & Barbuy (2006a).
Table 1:
Atomic constants for the Ru I and Hf II
lines used in this work, and references for .
"Sun''
denotes the gf-values fitted on the solar and Arcturus spectra.
Other gf-values were taken from VALD, Lundqvist et al. (2006) (L06),
and Lawler et al. (2007) (L07).
Table 2:
Uncertainties on abundances.
:
output with
the atmospheric parameters adopted;
:
output by
altering 1
on adopted
;
:
output by altering 1
on adopted metallicity;
:
output by altering 1
on adopted
;
:
output by altering 1
on adopted
microturbulent velocity
;
:
,
the uncertainty on
(X) from
Table 4;
:
,
the
uncertainty on [Ru/Fe] or [Hf/Fe] from Table 4.
The Ru and Hf transitions in the spectra of the barium stars of
our sample are generally weak, and this makes the abundance
determinations from them difficult. Figure 4 shows
the synthetic spectrum fit in the star HD 181053 for the
4080.437 (Hf II), and
4080.574
(Ru I) lines. Figure 5 shows the fit for the
4757.856 Ru I line in the star HD 87080.
The results for each line of our sample barium stars used for the
abundance calculations are shown in Table 3, and the
average values are given in Table 4. The latter shows that the
average values are mainly in the ranges
+0.18 [Ru/Fe]
+2.00 and
+0.20
[Hf/Fe]
+1.71. Only for the star HD
210910 were the values lower than these ranges. Some stars
indicated in Tables 3 and 4 were considered
normal rather than
barium stars by Smiljanic et al. (2007) and their abundances were found to
be in the ranges
-0.05
[Ru/Fe]
+0.43 and
-0.20
[Hf/Fe]
+0.37. The upper values of these
ranges in the
normal stars, when compared with those of the barium stars,
while showing the extent of the uncertainties, also highlight the
large overabundance of Ru and Hf in most barium stars of the
sample. In Fig. 6 we plot the run of [Ru/Fe] and
[Hf/Fe] with [Fe/H] for all stars we analyzed. They show a
distinctive decreasing trend of both [Ru/Fe] and [Hf/Fe] towards
increasing [Fe/H] for the sample barium stars. It is noteworthy,
however, that the normal stars present no trend in
their [Ru/Fe] and [Hf/Fe] abundances, having [Ru, Hf/Fe]
0 over almost an order of magnitude variation in metallicity.
In some cases only an upper limit for the abundance could be derived, as
indicated in Tables 3 and 4. If only one of the
two lines has
an upper limit for the Ru abundance while the other has a good
fit, the adopted abundance was that resulting from the best line
and only this line was taken into account to compute the average
of the abundance, as seen comparing Tables 3 and 4.
For some stars the
fit at 4080 is very uncertain and we chose not to derive
any abundance from it. The fit for BD+18 5215 at
4757 could not be carried out, and there is a spike of
noise at
4093 for HD 147609, so there are no results for
these transitions in these stars. For HD 20894, considered a
normal star by Smiljanic et al. (2007), and the barium stars HD 48565
and HD 76225, we found a large difference (>0.3 dex) between
the abundance results of the two Ru (barium stars) or Hf (HD 20894) lines. For all the other stars, the two lines used led to
similar abundance values for Ru as well as Hf. In Table 5
the stars are ordered by increasing temperature, where
the problems of the abundance derivation seem to be
related to higher temperatures. Although the temperatures of HD 210910 and HD 204075 are not very high, their broad lines cause
some difficulties in deriving the abundances. HD 106191 has an
upper limit for Hf, and no fit for the
4080 line, yet its
temperature is not very high either. However, its S/N is lower
(S/N
100) than for other stars (see Smiljanic et al. 2007; Allen & Barbuy 2006a).
As a counter-example, the S/N
250 spectrum of HD 89948
allowed good fits for all lines, despite the high temperature of
this object. Regarding the lines of Ru I, the fit of the
4757.856 line for the sample barium stars had higher
quality than for the
4080.574 line due to its
freedom from neighboring perturbing lines.
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Figure 4:
Fitting of the Hf II
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Figure 5:
Fitting of the Ru I line
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Figure 6: [X/Fe] vs. [Fe/H] and [Hf/Ru] vs. [[Fe/H] for the sample stars. Symbols: squares: barium stars; triangles: stars considered normal rather than barium stars by Smiljanic et al. (2007). The arrows in the hafnium panel indicate an upper limit for HR 107, HD 106191, HD 147609, and HD 210910. The error bars indicate the maximum value of uncertainties on each axis. |
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Figure 7: Comparison of the [Hf/Fe] and [Ru/Fe] behavior with [Y/Fe], [Nd/Fe], [Sm/Fe], and [Eu/Fe]. Filled squares are the barium stars and open triangles are those considered normal rather than barium stars by Smiljanic et al. (2007). The least-square fits for [Hf/Fe] vs. [Nd/Fe] and [Hf/Fe] vs. [Sm/Fe] are explained in the text. The full line indicates the fit for all sample stars and the dashed line, that excluding the normal stars. |
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From Fig. 6, the run of [Hf/Ru] with Fe
shows very large scatter, up to 2 dex, with an apparent advantage for
Ru abundances over those of Hf, since most of the values of [Hf/Ru] lie below
zero. According to the Table 4, among 33 barium stars, 17 have
[Hf/Ru] < 0, 2 have [Hf/Ru] = 0, and 14 have [Hf/Ru] > 0. Yet
for normal stars of this table, 5 among 6 stars have [Hf/Ru] < 0.
However, the highest values for [Ru/Fe]
correspond mainly to stars that have only one line available and
also when for one of the lines only an upper limit is available,
which is much lower than the other estimate.
As shown in Sect. 4, uncertainties for [Ru/Fe] are
much larger than ones for [Hf/Fe]. Furthermore, in cases where only one line
was available, the error must be larger than for other stars
when the two lines gave a result. The highest value of Ru abundance was
found for HD 107574. The spectral lines of this star are broader
than those of most other stars, and, although the broadening is not
as strong as for HD 210910 and HD 204075, this line may be blended
with another line that does not exist in our linelists, and it
may be enough to give such high result. In fact, the line
4757.856
usually gives higher results than
4080.574. Note that if the
gf-value from VALD was used, the result would be higher. Hence, this
spectral region merits further spectroscopic analysis. A similar dificulty in
determining abundances was found for the star HD 2454, with quite similar
atmospheric parameters, in particular, microturbulent velocity.
According to the values of [X/Fe] for other neutron capture elements
found for this star, these very low values of [Hf/Ru] may not be real.
Table 6: Contributions of s- and r-processes for the abundances of Hf, Ru, Y, Eu, Sm, and Nd in solar system, following Arlandini et al. (1999). The last column is the sum of s- and r-processes.
Table 7: Least-square fits, [X1/Fe] = A[X2/Fe] + B, where X1 is Ru or Hf and X2 can be Y, Nd, Eu or Sm; "cov'' is the covariance between A and B; "d.o.f.'' is the number of degrees of freedom. Numbers in parenthesis are errors in last decimals.
Figure 7 shows the Ru and Hf abundances compared to those of Y, Nd, Sm, and Eu for all stars of the sample. Yttrium can properly represent the s-process, given that, according to Arlandini et al. (1999), 92% and 8% of its abundance is due to s- and r-processes in the solar system, respectively, as shown in Table 6. In this table, the missing abundance fractions are due to processes other than s(main component) and r, and are seen to be of little significance. Europium, compared to yttrium, has almost opposite behavior, with 5.78% and 94.25% of its contribution from the s- and r-processes, respectively. In their turn, neodymium and hafnium may be considered as mild s-elements since the s-process forms the bulk of their abundances but does not entirely dominate their production, as it does for yttrium. Ruthenium and samarium, on the other hand, are mainly contributed, but not entirely dominated, by the r-process. These elements were chosen for a comparison since they span a wide range of contributions from the s- and r-processes, from a strong dominance of the s-process for yttrium, through a more or less balanced contribution to a high dominance of the r-process for europium.
To study the correlations between abundances shown in
Fig. 7, least-square fits are shown in Table 7. A remarkable feature of Fig. 7 is
the large scatter of Ru abundances, not found in the Hf abundances. Also, Ru abundances seem to present little
correlation with those of other elements, as shown by the values
of
in Table 7.
The results for Hf have much lower scatter and are no less
remarkable: high scatter is found only in the [Hf/Fe] run with
[Y/Fe]. Rather tight positive correlations are found between the
Hf abundances and those of Nd and Sm, as can be judged by the
value of the
of Table 7. For Nd and
Sm, two fits for each are represented in Fig. 7: if all
sample stars are included in the fit, the results are:
If those objects considered normal rather than barium stars by Smiljanic et al. (2007)
are withdrawn from the sample, the results are:
The main component of the s-process is believed to occur as a
chain from Fe seed nuclei up to Bi. The
neutron fluency may be enough to feed the first s-process
peak (near magic neutron number N = 50, in our discussion
represented by Y and Ru), then the second peak (near magic neutron
number N = 82, here Nd and Sm) and then on to the third
peak (near magic neutron number N = 126), as a function of the
so-called 13C pocket efficiency (still a free parameter in
current modelling), providing the bulk of the neutron flux (at
least for low fluxes) through the
C(
, n)
O reaction (Busso et al. 1999). It is thus possible that, in a
round of s-processing, not all peaks are equally fed by the
neutron fluency, generating scatter on the abundance ratios
involving elements from different peaks. Figure 16 of
Busso et al. (1999) illustrates this scatter on a diagram
of [hs/ls] vs. [Fe/H], as a function of the
efficiency of the 13C pocket. For the [Ru/Fe] vs.
[Y/Fe] ratios we could in principle expect a good correlation,
since they both belong to the first peak, but Y is a magic neutron
element, for which an abundance enhancement is expected,
partially masking this correlation. In fact, according to the
in Table 7, the abundance correlation
between Ru and Y is only just as good as for Ru and Sm, and not
very clear. The [Ru/Fe] and [Eu/Fe] run demonstrates a very poor
correlation, not unexpected, these being elements from different
peaks, and with dissimilar fractional contributions from the
s- and r-processes. The correlation between Ru (a first peak element)
and Nd or Sm is also expected to be worse under this reasoning,
since the latter are both near the second peak. Indeed, the run of
[Ru/Fe] and [Nd/Fe] does not show a good correlation, but that of
[Ru/Fe] and [Sm/Fe], somewhat unexpectedly, shows the hint of one,
as judged by the
value. On the other hand, Hf, Nd,
and Sm are all near the second s-process peak, so one would expect
lower scatter in the abundance ratios shown in Fig. 7,
also confirmed by our data. Some of the theoretical expectations
of AGB s-process nucleosynthesis are therefore borne out by our
results, but these, taken together, suggest a more complex
behavior of the abundance ratios of Ru and Hf with Y, Nd, Eu and Sm than established by the current state of theory.
Clearly, more data on
the abundances of these two little-studied, spectroscopically not
very accessible, elements, are desirable to better
constrain theoretical scenarios of s-process nucleosynthesis.
We present abundances of Ru and Hf determined through the spectrum
synthesis of two lines for each element available in the
spectra of dwarf and giant barium stars. Good agreement was
obtained for each pair of Ru and Hf lines for most
sample stars. For a few stars, the abundance
difference derived from the two lines was higher than 0.3 dex. We found that for the 4080.437 of
Hf II, and
4757.856 of Ru I lines,
published gf-values do not fit well to the observed solar spectrum.
We offer tentative explanations for these discrepancies,
which merit further study. New experiments determining the
for these lines are needed. Further, reliable abundance
determinations of these elements can contribute
considerably to our knowledge of heavy element abundances
in this class of chemically peculiar stars, besides helping
better constrain theoretical scenarios of AGB s-process
nucleosynthesis, still prone to important uncertainties.
We compared the run of [Hf/Fe] and [Ru/Fe] abundance ratios with other heavy elements, chosen to represent different fractional contributions from the s- and r-process, as judged by the solar system isotopic composition. The abundance of Hf is closely correlated with that of Sm and Nd, in reasonable agreement with theoretical expectations. It is noteworthy that, although Sm is an r-process dominated element, and Nd presents abundance fractions from s- and r-process nucleosynthesis similar to Hf in the solar system abundance pattern, in our barium star data both elements are well correlated with Hf, probably because all three elements lie near the second s-process peak. Ru is not clearly correlated with the other heavy elements, excepting possibly Y and Sm. A correlation of Ru and Y abundances may be masked partially by the magic neutron number nature of the latter. The possibility of a correlation in the Ru and Sm abundances, in the light of a similar fractional contribution from the s- and r-processes even though these elements belong to different s-processing peaks, deserves further investigation. The stars should span a larger metallicity interval than the one studied here. These results suggest a more complex relationship between the excesses of the various heavy elements in barium stars than implied by theoretical considerations.
Acknowledgements
D.M.A. acknowledges a FAPERJ post-doctoral fellowship N152.680/2004, as well as CAPES, for the post-doctoral fellowship N
BEX 3448/06-1. We are also grateful to Licio da Silva, Luciana Pompéia, Paula Coelho, and Jorge Meléndez for carrying out some observations of our sample spectra. We are grateful to Beatriz Barbuy for make available part of the spectra and the spectrum synthesis code. GFPM acknowledges financial support by CNPq/Conteúdos Digitais (grant 552331/01-5), CNPq/MEGALIT/Institutos do Milênio program, and a FAPERJ (grant APQ1/26/170.687/2004). We thank the referee, Dr. Roberto Gallino, for his criticism and comments, which very considerably improved this paper.
Table 3: Equivalent width and abundance results for the sample stars, line by line. The symbol "<'' indicates an upper limit. The stars signaled with "*'' were considered normal rather than barium stars by Smiljanic et al. (2007).
Table 4:
[Fe/H] and the mean values for
(X) and [X/Fe] for all stars of the
sample. The symbol "<'' indicates an upper limit. The stars signaled with "*'' were
considered normal rather than barium stars by Smiljanic et al. (2007). The number of lines used to
compute the medium is shown in brackets.
Table 5:
This table was organized in increasing order of
,
with the
comments about the results shown in the last column of each series of 4 columns.