A&A 428, L9-L12 (2004)
DOI: 10.1051/0004-6361:200400094
B. Plez1 - V. Hill2 - R. Cayrel3 - M. Spite 2 - B. Barbuy4 - T. C. Beers5 - P. Bonifacio6 - F. Primas7 - B. Nordström8,9
1 - GRAAL, CNRS UMR 5024, Université de Montpellier 2,
34095 Montpellier Cedex 5, France
2 -
Observatoire de Paris, GEPI, CNRS UMR 8111, 5 place Jules Janssen,
92195 Meudon Cedex, France
3 -
Observatoire de Paris, GEPI, CNRS UMR 8111, 61 av. de l'Observatoire,
75014 Paris, France
4 -
IAG Universidade de São Paolo, Dep. de Astronomia CP 3386,
Rua do Matão 1226, São Paolo 05508-900, Brazil
5 -
Department of Physics & Astronomy and JINA, Michigan State
University, East Lansing, MI 48824, USA
6 -
Istituto Nazionale per l'Astrofisica - Osservatorio Astronomico
di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
7 -
ESO, Karl-Schwarzschild-Str. 2, 85749 Garching bei München,
Germany
8 -
Lund Observatory, Box 43, 22100 Lund, Sweden
9 -
Niels Bohr Institute, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
Received 21 July 2004 / Accepted 21 October 2004
Abstract
In a previous paper we were able to measure the abundance of uranium
and thorium in the very-metal poor halo giant BPS CS 31082-001, but only obtained an upper limit for the
abundance of lead (Pb). We have got from ESO 17 h of additional
exposure on this star in order to secure a detection of the minimum
amount of lead expected to be present in CS 31082-001, the
amount arising from the decay of the original content of Th and U in
the star. We report here this successful detection. We find an LTE
abundance
dex, one dex below the
upper limits given by other authors for the similar stars CS 22892-052 and BD +17
3248, also enhanced
in r-process elements. From the observed present abundances of
Th and U in the star, the expected amount of Pb produced by the decay
of 232Th, and 238U alone, over 12-15 Gyr is
dex. The decay of 235U is more difficult to estimate, but
is probably slightly below the contribution of 238U, making the
contribution of the 3 actinides only slightly below, or even equal
to, the measured abundance.
The contribution from the decay of 234U has was not included,
for lack of published data. In this sense our determination is
a lower limit to the contribution of actinides to lead production.
We comment this result, and we note that
if a NLTE analysis, not yet possible, doubles our observed abundance,
the decay of the 3 actinides will still represent 50 per cent of the
total lead, a proportion higher than the values considered so far in
the literature.
Key words: stars: abundances - physical data and processes: nuclear reactions, nucleosynthesis, abundances - atomic data
The detection of uranium in an old, very metal-poor star of the
galactic halo, BPS CS 31082-001, was first reported in
Cayrel et al. (2001). A greatly improved analysis, Hill et al. (2002), (quoted as Paper I) was made possible by a
redetermination of crucial atomic data by Nilsson et al. (2002a,b). Hill et al. have determined the abundance of U (
)
and of Th (
), in the usual scale
,
in CS 31082-001. These abundances have
been used as cosmo-chronometers, comparing them to theoretical
estimates of the initial production ratio. The time
in Gyr
elapsed from the formation of the two actinides to now, is linked to
the production ratio
and the ratio measured in the
star
by the simple relation:
where the coefficient 21.76 is derived
from the half-lives of 232Th and 238U. The superiority of
the pair U/Th over the pair Th/Eu has been amply demonstrated, for
example in Goriely & Clerbaux (1999), Goriely & Arnould
(2001), or Wanajo et al. (2002, see their Fig. 7). As
both U and Th decay to the stable element lead, it is of great
interest to know the abundance of lead in the star. In Hill et al. (2002), only an upper limit to the lead abundance was
given, and we report here the result of a new observation obtained at
ESO Paranal to get this abundance. The time requested was 17 h,
enough to detect the minimum amount of lead coming from the decay of
the observed elements 238U and 232Th into 206Pb, and
208Pb, respectively. In addition, lead may come from other
channels, in particular from the decay of 235U into 207Pb,
and from more unstable nuclides decaying very quickly to lead, such as 234U.
The observations were carried out with the ESO VLT using the UVES spectrograph with image slicer #2,
leading to a spectral resolution of 80 000.
A total of 13 exposures were collected in service mode,
reaching a total exposure time of 17 h. The signal-to-noise ratio
of the combined spectrum is around 600 per pixel. The data were
reduced using the standard UVES pipeline (Ballester et al.
2000).
The signal we were looking for is very weak: a depression of only
0.5 per cent expected at 4057.807 Å, in the red wing of a
weak CH line located at 4057.718 Å. After correcting each spectrum
for radial-velocity shifts, several methods for combining the 13 spectra were tested, including (i) a straight average of the best 10 spectra (those with no cosmic hits in that wavelength region); (ii) averaging the spectra after clipping points further away than
2.5
from the median of the distribution for each pixel; and
(iii) averaging the 9 spectra closest to the median of the
distribution for each pixel. All methods yielded a very similar result
in the Pb region, and we display only one of them (average of the nine
spectra closest to the median) in Fig. 1, where the error
bars represent the photon noise for each pixel.
![]() |
Figure 1:
The observed Pb I 4057.8 Å line in CS 31082-001.
Dots: observed (combined) spectrum, with the photon noise
plotted as error bars; lines: synthetic spectra computed for the
abundances indicated in the figure. The left panel shows the
surrounding wavelength region, including the two continuum windows
that were used to normalize the observed spectrum, while on the
right panel, a zoom on the Pb line is presented.
The best fit is found for
![]() |
Open with DEXTER |
We used the same model atmosphere and spectrum synthesis code
(turbospectrum: Plez et al. 1993; Alvarez & Plez 1998) as in Paper I, achieving
complete self-consistency
between the model and the spectra computations. Our spectrum synthesis
of the Pb 4057 Å region is displayed in Fig. 1, where it is
compared to the observations. In addition to the photon noise itself
(error bars in the observed points in Fig. 1), various sources
of uncertainties on the Pb abundance determination were
examined:
In the next section we discuss this result with respect to former attempts to measure Pb abundance in other very old stars, and with respect to the amount of lead expected from the decay of the actinides Th and U. We do not attempt to explain our result by theoretical arguments, considering that it is the observational result that justify this letter. A more complete discussion of the impact of this new measurement on nuclear astrophysics will be included in a forthcoming paper also dealing with the analysis of our newly completed HST/STIS observations of CS 31082-001.
Other r-process enhanced stars have been
searched for Pb, leading only to upper limits or very uncertain
detections, whether using the very weak 4058 Å line or the
intrinsically stronger (but observable only from space) UV line
2833 Å.
We report in Table 1 the
(Pb/Th) ratios
for CS 31082-001 and the two other stars with secure upper
limits: CS 22892-052 (Th from Sneden et al. 2003, Pb from Hill et al. 2002) and
BD +17
3248 (Cowan et al. 2002).
For completeness, we note that Sneden et al. (1998) detected Pb
from the UV line in HD 115444, but the line was affected by a
spike and the authors themselves regarded the Pb abundance
in this star as very uncertain.
Th is taken as reference element for the Pb abundance, because
of the direct connection between these elements. In the 3 stars, which
are as old or older than globular clusters, about half of the initial
Th content has decayed into 208Pb, and half has survived.
The Pb/Th ratio detected in CS 31082-001 clearly stands out as an
extremely low value compared to any upper limit so far placed on an
r-process enriched star.
Clear channels of production of lead are the decay of 232Th into 208Pb , of 238U into 206Pb, and of 235U into 207Pb. The amount of 208Pb is fixed by the observation of 232Th now, and the knowledge of the decayed fraction after the matter has been isolated in the atmosphere of the star, at a known rate.
Table 1: Pb in CS 314082-001 and two other very metal-poor stars.
The epoch of the nucleosynthesis of the photospheric matter of
CS 31082-001, cannot be more than
Gyr ago (Big Bang
epoch according to WMAP results, Spergel et al. 2003), and
should be at least as much as the age of globular clusters (
Gyr according to Chaboyer 2001), taking into
consideration the very low metallicity,
of the star. The median is 13.5, also the age of first stellar
formation according to Kogut et al. (2003).
Adopting
Gyr for the age of the actinides in CS 31082-001, we easily derive both the original content in 232Th
and 238U, and the fraction of them transformed into 208Pb
and 206Pb. For example:
But 235U cannot be treated the same way, as there is not enough
235U left to have an observed value. We must then rely on
theoretical works, usually done for reproducing the solar system
isotopic abundances, but not necessarily adequate for
CS 31082-001 which has a clear overabundance of the
actinides with respect to the lighter r-elements, compared to
the solar system. However we can hope that in the restricted mass
range under consideration , 232Th to 238U, neutron exposures
producing the right ratio 238U/232Th may also produce the
right ratio 235U/238U. In Tables 1 and 2 of Goriely &
Arnould (2001), several neutron exposures are considered with a
wide set of mass models. Forgetting the solar system, we keep the
exposures giving the right 238U/232Th production ratio for
CS 31082-001, compatible with an age
Gyr. With this
constraint, the production ratio R of 235U/238U lies
between 0.67 and 0.87. Taking the median 0.77 seems a reasonable
estimate. The full amount of produced 235U is converted into
207Pb in CS31082-001, because of the fast decay of this
isotope. Table 2 summarizes our findings. Interestingly,
the case R=1.0 gives an amount of total lead equal to the observed
one, leaving no other channel for the production of r-lead. We
have not included the 234U decay to 206Pb channel here,
because of the lack of published estimates of the 234U/238U
production ratio.
This channel (which could
be as high as the 235U contribution) can only
increase the contribution of the actinide-path to the total production
of Pb, thereby reducing even further any other production
channel.
A warning is appropriate here: our analysis is based on the LTE approximation, which must be questioned, especially in the blue and the UV in very metal-poor giants, where the continuum is in a large part due to Rayleigh scattering. We examine this in the next subsection.
Table 2:
Three abundance patterns examined for the lead feature.
The abundances of 207Pb are computed for 3 assumed values of the
production ratio
,
the first
considered as the most probable, and the other two chosen
30 per cent below and above. All
abundances are with respect to 1012 hydrogen atoms.
It would be very useful to have a NLT analysis of Pb, as the lower
level of the measured line is very deep, part of the ground-level
term, and is of Pb I when most of lead is in the Pb II stage. In
metal-poor stars these deep levels tend to be over-ionized, but not
always. If the deep levels are indeed over-ionized, the LTE assumption
predicts too large a population of the lower level, and an
underestimated abundance. In an attempt to estimate the size of
possible NLTE effects, we checked the /
ratio at the
position of the Pb I line, as well as at 4076 Å and 2035 Å,
corresponding to the ionization limit of the upper and lower levels of
the transition, respectively. In these layers, the ratio
/
of the mean intensity to the local Planck function is
larger than one, but remains on the order of two. It is unlikely that
the NLTE correction is larger than this factor of two, so the
production of Pb, outside the decay of 232Th, 235U, and
238U is bound to be less or equal to the actinide production.
Our result clearly concerns a particular class of objects: very metal-poor stars born in the early days of the Galaxy, and strongly enhanced in r-process elements. Is the observed low ratio Pb/Th particular to this class of objects, or is it a more general property of the r-process in the mass range 206-238? There is now a general agreement that the fraction of Pb produced by the r-process in the solar system it practically unknown, after the discovery that zero-metal stars can produce a lot of s-lead (Goriely & Siess 2001; Van Eck et al. 2003). This has modified the estimates of the amount of s-lead produced in the Galaxy before the birth of the Sun (Gallino et al 1998), and of the r-lead, obtained by subtracting the s-lead from the total lead. If the r-lead of the solar system were mainly produced by the decay of the actinides, as for CS 31082-001, it is easy to verify that the r-lead in the solar system would be of the order of only 1 to 3% of the total lead.
Acknowledgements
We are indebted to Prof. R. Gallino for informations on the production of lead by the s-process. T.C.B. acknowledges partial funding from NSF grants AST 00-98508 and 00-98549, and PHY 02-16783. B.N. acknowledges support from the Carlsberg Foundation and the Nordic Academy for Advanced Studies.