Abundance pattern of the chemically peculiar star Feh-Duf
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Головна астрономічна обсерваторія НАН України
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| Цитувати: | Abundance pattern of the chemically peculiar star Feh-Duf / N.A. Drake, C.B. Pereira // Кинематика и физика небесных тел. — 2009. — Т. 25, № 6-додаток. — С. 202-206. — Бібліогр.: 23 назв. — англ. |
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irk-123456789-729582015-01-02T03:01:47Z Abundance pattern of the chemically peculiar star Feh-Duf Drake, N.A. Pereira, C.B. Физические характеристики и химический состав звезд 2009 Article Abundance pattern of the chemically peculiar star Feh-Duf / N.A. Drake, C.B. Pereira // Кинематика и физика небесных тел. — 2009. — Т. 25, № 6-додаток. — С. 202-206. — Бібліогр.: 23 назв. — англ. 0233-7665 http://dspace.nbuv.gov.ua/handle/123456789/72958 en Кинематика и физика небесных тел Головна астрономічна обсерваторія НАН України |
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Физические характеристики и химический состав звезд Физические характеристики и химический состав звезд |
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Физические характеристики и химический состав звезд Физические характеристики и химический состав звезд Drake, N.A. Pereira, C.B. Abundance pattern of the chemically peculiar star Feh-Duf Кинематика и физика небесных тел |
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Article |
| author |
Drake, N.A. Pereira, C.B. |
| author_facet |
Drake, N.A. Pereira, C.B. |
| author_sort |
Drake, N.A. |
| title |
Abundance pattern of the chemically peculiar star Feh-Duf |
| title_short |
Abundance pattern of the chemically peculiar star Feh-Duf |
| title_full |
Abundance pattern of the chemically peculiar star Feh-Duf |
| title_fullStr |
Abundance pattern of the chemically peculiar star Feh-Duf |
| title_full_unstemmed |
Abundance pattern of the chemically peculiar star Feh-Duf |
| title_sort |
abundance pattern of the chemically peculiar star feh-duf |
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Головна астрономічна обсерваторія НАН України |
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2009 |
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Физические характеристики и химический состав звезд |
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http://dspace.nbuv.gov.ua/handle/123456789/72958 |
| citation_txt |
Abundance pattern of the chemically peculiar star Feh-Duf / N.A. Drake, C.B. Pereira // Кинематика и физика небесных тел. — 2009. — Т. 25, № 6-додаток. — С. 202-206. — Бібліогр.: 23 назв. — англ. |
| series |
Кинематика и физика небесных тел |
| work_keys_str_mv |
AT drakena abundancepatternofthechemicallypeculiarstarfehduf AT pereiracb abundancepatternofthechemicallypeculiarstarfehduf |
| first_indexed |
2025-07-05T21:37:29Z |
| last_indexed |
2025-07-05T21:37:29Z |
| _version_ |
1836844528225484800 |
| fulltext |
ABUNDANCE PATTERN OF THE CHEMICALLY PECULIAR STAR FEH-DUF
N. A. Drake1, C. B. Pereira2
c© 2009
1Astronomical Institute, St. Petersburg State University
Universitetski pr. 28, St. Petersburg 198504, Russia
e-mail: drake@on.br
2Observatório Nacional/MCT
Rua José Cristino, 77, CEP 20921-400, São Cristóvão, Rio de Janeiro, Brazil
e-mail: claudio@on.br
We present the results of a study of the very high velocity (vrad = 448.0 ± 1.0 km · s−1) low-
metallicity ([Fe/H]= −1.93) star Feh-Duf (Fehrenbach & Duflot, 1981) showing peculiar chemical
abundance. Using a high-resolution spectrum obtained at ESO 2.2-m telescope with the FEROS
echelle spectrograph we determined atmospheric parameters of this star and performed the detailed
analysis of its abundance pattern. We showed that this star has enhanced carbon and heavy
s-process element abundance ([C/Fe] = +0.58, [hs/Fe] = +0.88 dex). The carbon isotopic ratio is
low (12C/13C= 8). We found that the α-element abundance is reduced as compared to Galactic
field stars of a similar metallicity. The evolution state of this star and its possible extragalactic
origin are discussed.
INTRODUCTION
The star Feh-Duf was discovered in 1981 by Fehrenbach & Duflot [6] during their measurements of the radial
velocities of the stars in the region of the Large Magellanic Cloud. They found that this star has a very high radial
velocity (+440 km · s−1) and also that exceedingly strong CH absorption bands are observed in the spectrum of
this star. This spectral characteristic is typical for CH stars, a group of peculiar late-type population II giants
whose overabundance of carbon, as well as of the s-process elements, is explained by mass-transfer in a binary
system from a former asymptotic giant branch (AGB) star, now a white dwarf.
In this work we present the results of the high-resolution spectroscopic study of Feh-Duf, determine its
abundance pattern and discuss the evolutionary state as well as a possible extragalactic origin of this star.
OBSERVATIONS
The high-resolution spectrum of Feh-Duf analyzed in this work was obtained with the FEROS echelle spec-
trograph at the 2.2-m telescope of ESO at la Silla, Chile, on November 20, 2008. The spectral resolution is
R = λ/Δλ = 48 000, corresponding to 2.2 pixels of 15 μm, and the spectral range coverage is from 3800 Å to
9200 Å. Two expositions, each of 3600 s, were obtained. The S/N ratio was evaluated by measuring the rms flux
fluctuation in selected continuum windows, and the typical value was S/N = 100. The spectra were reduced
with the MIDAS pipeline reduction package consisting of the following standard steps: CCD bias correction,
flat-fielding, spectrum extraction, wavelength calibration, correction of barycentric velocity, and spectrum rec-
tification. Figure 1 shows a section of the Feh-Duf spectrum in the 6120–6150 Å region.
ATMOSPHERIC PARAMETERS
The determination of atmospheric parameters, effective temperature (Teff), surface gravity (log g), microturbu-
lent velocity (ξm), and metallicity ([Fe/H]1) was carried out using the local thermodynamic equilibrium (LTE)
atmosphere models of Kurucz [11] and the current version of the LTE spectral synthesis software moog [18].
The unblended Fe i and Fe ii lines were used in the analysis. The log gf values of iron lines were taken from
Lambert et al. [13]. Following the usual iterative procedure, we derived the effective temperature and micro-
turbulence by requiring the iron abundances to be independent of the excitation potential and of the equivalent
width (see Fig. 2). The surface gravity was determined through an ionization balance by forcing Fe ii to
yield the same total iron abundance as Fe i. We derived the following atmospheric parameters for Feh-Duf:
Teff = 4500± 120 K, log g = 0.9 ± 0.1, ξm = 1.9 ± 0.2, and [Fe/H] =–1.93 ± 0.10.
1In this work, we use the standard spectroscopic abundance notation log ε(X) = log(NX/NH) + 12 and [X/H]= log(NX/NH)∗−
− log(NX/NH)�. The solar abundances are from Anders & Grevesse [2].
202
Figure 1. Spectrum of Feh-Duf in the 6120–6150 Å spectral region containing the line of Ba ii 6141.73 Å
Figure 2. Star Feh-Duf. Top: Iron abundances from Fe i lines vs. equivalent width. Bottom: Iron abundances from Fe i
lines vs. excitation potential. 45 Fe i and 9 Fe ii lines were used for the atmospheric parameters determination
Since the parallax of Feh-Duf has not been measured, we estimated its luminosity and distance using theo-
retical evolutionary tracks. The value of the bolometric correction was estimated using the formula from Alonso
et al. [1] BC(V )=–0.495. Assuming stellar mass of 0.8 M� and using the evolution tracks from Fagotto et al. [5]
and Girardi et al. [7], we estimated the luminosity of the star MV = −2.3 and log L/L� = 3.01 which results
in a distance of about 5.9 kpc. We adopted the reddening to be EB−V = 0.06 (Grieve & Madore [9], Pompeia
et al. [16]). This value of the colour excess agrees well with the intensity of the interstellar Na i D1 and D2 lines
of 126.9 mÅ and 176.1 mÅ, respectively.
The radial velocity of Feh-Duf was determined to be vrad = +448.3 ± 1.0 km · s−1. This value is in good
agreement with the value of vrad = +440 km · s−1 obtained by Fehrenbach & Duflot [6].
CHEMICAL ABUNDANCE ANALYSIS
The abundance analysis was performed using the local thermodynamic equilibrium (LTE) model-atmosphere
techniques and the current version of the moog program. The abundances of the elements which have unblended
lines and are not affected by the hyperfine splitting were determined using the equivalent widths measured by
203
Figure 3. Observed (dots) and synthetic (lines) spectra of Feh-Duf in the region around the C2 5635 Å band head.
The synthetic spectra are shown for the carbon abundances of log ε(C) =none, 7.16, 7.21, 7.26
means of the splot task of the IRAF package. For other elements we used the synthetic spectrum method.
Carbon and nitrogen abundances as well as the 12C/13C isotopic ratio were determined with spectrum synthesis
technique by means of successive iterations using the following molecular spectral features: C2 (0, 1) band head
of the Swan system A3Πg −X3Πu at 5635 Å, C2 feature at 5086 Å, CH lines of the A2Δ−X2Π system, 12CN
and 13CN lines of the (2, 0) red system A2Π − X2Σ in the 7994–8020 Å wave range. The oxygen abundance
was determined by synthesis of the [O i] 6300.304 Å line. The detailed description of the CNO and 12C/13C
determinations may be found in Drake & Pereira [4] and Pereira & Drake [15]. The lead abundance was obtained
from the Pb i line at λ4057.81 Å. The line data which include isotopic shifts and hyperfine splitting have been
taken from van Eck et al. [21]. Figures 3 and 4 show the observed and synthetic spectra in the regions around
the C2 molecule lines at 5635 Å and the Pb i 4057.81 Å line, respectively. The obtained elemental abundances
are presented in Fig. 5.
Our analysis shows the large overabundance of carbon and nitrogen in Feh-Duf ([C/Fe] =+0.58,
[N/Fe] =+0.66). The carbon isotope ratio is low, 12C/13C =8, above, however, the equilibrium isotope ratio
reached in the CN cycle (12C/13C =3.6). The enhancement of the 12C/13C ratio may be caused by the mass
transfer from the AGB donor – a carbon star which may have had a high value of the carbon isotope ratio
(Lambert et al. [12]). The oxygen abundance is [O/Fe]= +0.10. Comparison of the oxygen-to-iron ratio
in Feh-Duf and Galactic halo stars indicate that the [O/Fe] ratio in the Feh-Duf is about 0.3 dex below
the corresponding value for the stars of the same metallicity in the Galaxy (Carretta et al. [3], Ryan, Norris &
Bessel [17]). Other α-elements, such as Ca and Ti, also have lower abundances ([Mg/Fe]= 0.42, [Ca/Fe] =0.29,
[Ti/Fe] =–0.02, [Mg+Ca+Ti/3Fe]= 0.23).
The abundances of the heavy s-process elements are enhanced ([Ba/Fe]=0.95, [La/Fe]=0.75, [Ce/Fe] =0.88,
[Nd/Fe] =0.92, and [Pb/Fe] =1.58) whereas the abundance of the light s-process element Y is low
([Y/Fe] = –0.07). According to the theories, the neutron-capture nucleosynthesis in AGB stars is metallicity-
dependent. The first-peak elements (such as Y) are bypassed in favor of second-peak elements and those from
the third peak. The high [Ba/Y] and [Pb/Ce] ratios are consistent with the expectations from metal-poor AGB
s-process yields (Travaglio et al. [20], Venn et al. [22]).
KINEMATICS
Assuming the distance to the star to be d = 5.9 kpc, we calculated Galactic space-velocity components (U, V, W )
for Feh-Duf using the algorithm proposed by Johnson & Soderblom [10]. Proper motions were taken from
NOMAD catalog (Zacharias [23]). The obtained heliocentric space velocities are (U, V, W )= (–116, –491,
–106) km · s−1. We transformed the V component of the space velocity of Feh-Duf to the Galactic Reference
Frame using the (U, V, W )� = (9, 232, 7) (Venn et al. [22]) which results in VGRF = −259 km · s−1. This value
of VGRF shows that Feh-Duf has extreme retrograde motion. As pointed out by Marsakov & Borkova [14], a star
born in a monotonically collapsing single protogalactic cloud could not be in a retrograde orbit. The extreme
retrograde motion of Feh-Duf may be a sign that this star was accreted by the Milky Way from a dwarf satellite
galaxy.
204
Figure 4. Observed (dots) and synthetic (lines) spectra of Feh-Duf in the region around the Pb i line at 4057.8 Å.
The synthetic spectra are shown for the lead abundances of log ε(Pb) = 0.10, 1.40, and 1.75. The upper line shows
a synthesis without contribution from the CH molecule lines
Figure 5. Abundance pattern of Feh-Duf. Error bars represent the estimated uncertainties
CONCLUSIONS
We assumed that Feh-Duf has a mass of 0.8 M� to be compatible with the low metallicity determined for this
star of [Fe/H] = –1.93. Even though the luminosity of the Feh-Duf is compatible with the star at the early AGB
phase, the binary nature of the carbon and s-element enrichment to be preferred, since the envelope mass of
a 0.8 M� star is too small for the third dredge up phenomenon to occur (Staniero et al. [19]). That is, Feh-Duf
is a CH star. Feh-Duf is also a “lead star” since its lead-to-cerium ratio is high, [Pb/Ce]=+0.70.
Recently, Venn et al. [22] carried out an analysis of the chemical abundances of the stars in the Galaxy and
in the Milky Way dwarf spheroidal (dSph) satellite galaxies. They confirmed that the [α/Fe] ratios of most
stars in the dSph galaxies are lower than Galactic stars of similar metallicity. Figure 2 from Venn et al. [22]
shows that dSph stars are well separated from the majority of Galactic disk and halo stars. They also found
that there is overlap in the low [α/Fe] ratios between dSphs stars and Galactic halo stars on extreme retrograde
orbits (V < −420 km · s−1).
Almost all of the nuclei of α-elements are synthesized during shell burning in Type II supernovae (SNe II),
while the iron-peak elements are ejected to the interstellar medium during the SN Ia explosions. Therefore,
a lower [α/Fe] ratio means that a star has originated within a dSph galaxy having the SN Ia/SN II ratio enhanced
as compared to the Galaxy.
The large retrograde velocity and chemical abundances distinct from Galactic halo stars of same metallicity
may suggest that Feh-Duf has an extragalactic origin and was captured by the Milky Way.
205
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