J/A+A/699/A262 Abundances of very metal-poor stars (Sitnova+, 2025)
Unlocking the mystery of Sr synthesis in the early Galaxy through analysis of
barium isotopes in very metal-poor stars.
Sitnova T.M., Lombardo L., Mashonkina L.I., Rizzuti F., Cescutti G.,
Hansen C.J., Bonifacio P., Caffau E., Koch-Hansen A., Meynet G.,
Fernandes de Melo R.
<Astron. Astrophys. 699, A262 (2025)>
=2025A&A...699A.262S 2025A&A...699A.262S
ADC_Keywords: Stars, metal-deficient ; Abundances ; Models
Keywords: stars: abundances - stars: atmospheres = Galaxy: evolution -
Galaxy: halo
Abstract:
We determine the contributions of the rapid (r) and slow (s) neutron
capture processes to the Ba isotope mixture, along with Ba, Eu, and Sr
NLTE abundances, in a sample of very metal-poor stars. The selected
stars formed before the contribution from the main s-process in low-
and intermediate-mass stars became significant. Some of our sample
stars are enhanced in Sr, with [Sr/Ba] reaching up to 0.7. These stars
gained their high Sr abundance from a poorly understood process,
sometimes referred to in the literature as a light element primary
process, which may appear to be a weak s-process or a weak r-process.
Our aim is to uncover the nature of this additional Sr source. The
abundances derived from the resonance BaII 4554 and 4934 A lines are
influenced by the adopted Ba isotope mixture. We compute Ba isotope
mixtures corresponding to different r- to s-process contributions
(pure r-process, 80%/20%, 50%/50% and 12%/88%, i. e. solar ratio) and
determine the corresponding abundances from the BaII resonance lines
in each sample star. Additionally, we determine Ba abundances from
weak subordinate BaII lines, which are unaffected by the adopted Ba
isotope mixture. We then compare the Ba abundances derived from the
subordinate lines with those from the BaII resonance lines. We find a
higher s-process contribution to Ba isotopes in stars with greater
[Sr/Eu] and [Sr/Ba] overabundances, suggesting that the additional Sr
synthesis was due to the early s-process occurring in massive stars.
Using Sr-enhanced stars, we estimate the [Sr/Ba] ratio produced by the
early s-process and obtain [Sr/Ba]earlyS=1.1±0.2. The derived
value should be regarded as an upper limit, as we cannot definitively
exclude the possibility of a contribution to Sr from the weak
r-process, which produces Sr but not Ba. Regarding the potential
synthesis of Sr and Ba in the i-process in massive stars, our results
for Ba isotopes and element abundances argues that there was no
detectable contribution from this process within the error bars in our
sample stars. In the early Galaxy, before significant main s-process
enrichment, barium and strontium were produced primarily by the main
r-process and the early s-process, which occurred in rapidly rotating
massive stars.
Description:
We present a spectroscopic analysis of 16 VMP stars and determine
their Sr, Ba, and Eu NLTE abundances, along with the contributions of
the r- and s-processes to their Ba isotope mixture.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
stars.dat 54 16 List of studied stars
table2.txt 60 881 NLTE abundance corrections and equivalent widths
in mÅ for TiII lines as a function of Teff,
logg, and [Fe/H]
table3.dat 135 48 NLTE and LTE abundances and EWs (mÅ) of BaII
lines in the sample stars
table4.dat 63 48 NLTE and LTE abundances and EWs (mÅ) from
individual SrII and EuII lines in the sample
stars
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Description of file: table2.txt
If EW = -1 and {DELTA}NLTE = -1, this means that EW is either <3mÅ or
not computed in a model atmosphere with given parameters.
Byte-by-byte Description of file: stars.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- Name Star name
14- 15 I2 h RAh Simbad right ascension (J2000)
17- 18 I2 min RAm Simbad right ascension (J2000)
20- 24 F5.2 s RAs Simbad right ascension (J2000)
26 A1 --- DE- Simbad declination sign (J2000)
27- 28 I2 deg DEd Simbad declination (J2000)
30- 31 I2 arcmin DEm Simbad declination (J2000)
33- 36 F4.1 arcsec DEs Simbad declination (J2000)
38- 54 A17 --- SName Simbad name
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- Name Star name
15- 18 A4 --- Param Parameter name (NLTE, LTE, EW) (G1)
22- 27 F6.2 --- Value1 BaII 4554Å, r/s=100/0 value for Param
31- 36 F6.2 --- Value2 BaII 4554Å, r/s=80/20 value for Param
40- 45 F6.2 --- Value3 BaII 4554Å, r/s=50/50 value for Param
49- 54 F6.2 --- Value4 BaII 4554Å, r/s=15/85 value for Param
58- 63 F6.2 --- Value5 BaII 4554Å, r/s=0/100 value for Param
67- 72 F6.2 --- Value6 BaII 4934Å, r/s=100/0 value for Param
76- 81 F6.2 --- Value7 BaII 4934Å, r/s=80/20 value for Param
85- 90 F6.2 --- Value8 BaII 4934Å, r/s=50/50 value for Param
94- 99 F6.2 --- Value9 BaII 4934Å, r/s=12/88 value for Param
103-108 F6.2 --- Value10 BaII 4934Å, r/s=0/100 value for Param
112-117 F6.2 --- Value11 ?=-99.9 BaII 5853Å value for Param
121-126 F6.2 --- Value12 ?=-99.9 BaII 6141Å value for Param
130-135 F6.2 --- Value13 ?=-99.9 BaII 6496Å value for Param
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- Name Star name
15- 18 A4 --- Param Param name (NLTE, LTE, EW) (G1)
22- 27 F6.2 --- Value1 SrII 4077Å value for Param
31- 36 F6.2 --- Value2 SrII 4215Å value for Param
40- 45 F6.2 --- Value3 EuII 3819Å value for Param
49- 54 F6.2 --- Value4 EuII 4129Å value for Param
58- 63 F6.2 --- Value5 EuII 4205Å value for Param
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Note (G1): NLTE = NLTE abundance, LTE = LTE abundance,
EW = equivalent width in mÅ.
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Acknowledgements:
T.M. Sitnova, sitamih(at)gmail.com
(End) Patricia Vannier [CDS] 11-Jun-2025