MNRAS 450, 1900–1915 (2015) doi:10.1093/mnras/stv716
Chemical abundances and kinematics of 257 G-, K-type field giants.
Setting a base for further analysis of giant-planet properties
orbiting evolved stars
V. Zh. Adibekyan,1† L. Benamati,1,2 N. C. Santos,1,2 S. Alves,3,4 C. Lovis,5 S. Udry,5
G. Israelian,6,7 S. G. Sousa,1 M. Tsantaki,1,2 A. Mortier,8 A. Sozzetti9
and J. R. De Medeiros10
1Instituto de Astrofı́sica e Ciência do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, PT4150-762 Porto, Portugal
2Departamento de Fı́sica e Astronomia, Faculdade de Ciências da Universidade do Porto, PT4150-762 Porto, Portugal
3Instituto de Astrofı́sica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
4CAPES Foundation, Ministry of Education of Brazil, 70040-020 Brası́lia/DF, Brazil
5Observatoire de Genève, Universit de Genve, 51 ch. des Maillettes, CH-1290 Sauverny, Switzerland
6Instituto de Astrofı́sica de Canarias, E-38200 La Laguna, Tenerife, Spain
7Departamento de Astrofı́sica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
8SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, UK
9INAF – Osservatorio Astrofisico di Torino, I-10025 Pino Torinese, Italy
10Departamento de Fı́sica Teórica e Experimental, Universidade Federal do Rio Grande do Norte, Campus Universitário Lagoa Nova,
59072-970 Natal, RN, Brazil
Accepted 2015 March 27. Received 2015 March 25; in original form 2015 January 26
ABSTRACT
We performed a uniform and detailed abundance analysis of 12 refractory elements (Na, Mg,
Al, Si, Ca, Ti, Cr, Ni, Co, Sc, Mn, and V) for a sample of 257 G- and K-type evolved stars from
the CORALIE planet search programme. To date, only one of these stars is known to harbour
a planetary companion. We aimed to characterize this large sample of evolved stars in terms of
chemical abundances and kinematics, thus setting a solid base for further analysis of planetary
properties around giant stars. This sample, being homogeneously analysed, can be used as
a comparison sample for other planet-related studies, as well as for different type of studies
related to stellar and Galaxy astrophysics. The abundances of the chemical elements were
determined using an local thermodynamic equilibrium (LTE) abundance analysis relative to
the Sun, with the spectral synthesis code MOOG and a grid of Kurucz ATLAS9 atmospheres. To
separate the Galactic stellar populations, both a purely kinematical approach and a chemical
method were applied. We confirm the overabundance of Na in giant stars compared to the field
FGKdwarfs. This enhancementmight have a stellar evolutionary character, but departures from
LTE may also produce a similar enhancement. Our chemical separation of stellar populations
also suggests a ‘gap’ in metallicity between the thick-disc and high-α metal-rich stars, as
previously observed in dwarfs sample from HARPS. The present sample, as most of the giant
star samples, also suffers from the B − V colour cut-off, which excludes low-log g stars with
high metallicities, and high-log g star with low [Fe/H]. For future studies of planet occurrence
dependence on stellar metallicity around these evolved stars, we suggest to use a subsample of
stars in a ‘cut-rectangle’ in the log g–[Fe/H] diagram to overcome the aforementioned issue.
Key words: methods: observational – techniques: spectroscopic – stars: abundances –
planetary systems.
1 IN T RO D U C T I O N
Based on observations collected at the Paranal Observatory, ESO (Chile) The precise chemical and kinematic characterization of
with the Ultra-violet and Visible Echelle Spectrograph (UVES) of the VLT, intermediate-mass, evolved stars is very important for different
under programmes 085.C-0062 and 086.C-0098. fields of both Galactic and stellar astronomy, and the emerging
†E-mail: vadibekyan@astro.up.pt field of planetary sciences.
©C 2015 The Authors
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Abundances and kinematics of G and K giants 1901
Many studies observed significant differences in chemical abun- et al. 2000) extrasolar planet search programme. High-resolution
dances between main-sequence dwarf and evolved stars (e.g. Friel and high signal-to-noise (S/N) spectra were obtained using the
et al. 2003; Jacobson, Friel & Pilachowski 2007; Villanova, Carraro UVES spectrograph. Precise stellar parameters for the entire sample
& Saviane 2009; Santrich, Pereira & Drake 2013). While these dif- were determined in Alves et al. (2015) by using the same spectra
ferences for some elements might by astrophysical, having a stellar as we did for this study. The spectroscopic stellar parameters and
evolutionary character (e.g. Tautvaišiene et al. 2005, for sodium), metallicities were derived by imposing excitation and ionization
several authors however suggested that the differences may arise equilibrium. The spectroscopic analysis was completed assuming
also in the analysis, being dependent on the particular method and LTE with a grid of Kurucz atmosphere models (Kurucz 1993),
line-list used (e.g. Santos et al. 2009). Along the same line, one and the 2002 version of the MOOG2 radiative transfer code (Sneden
should consider also non-local thermodynamic equilibrium (non- 1973). We refer the reader to Alves et al. (2015) and Sousa (2014)
LTE) effects which are stronger for giants than for dwarfs and may for details.
have a strong influence on the analysis (e.g. Bergemann et al. 2013; Alves et al. (2015) derived the atmospheric parameters by using
Alexeeva, Pakhomov & Mashonkina 2014; Bergemann, Kudritzki three different line-lists of Fe I and Fe II (Hekker & Meléndez 2007;
& Davies 2014). Sousa et al. 2008; Tsantaki et al. 2013). Whilst showing that the
Understanding the mentioned issues, will not only allow us to use of different line-lists gives compatible results, the parameters
improve of stellar atmosphere models, but also will have very im- derived following Tsantaki et al. (2013) were adopted, so we also
portant implications in several fields of astrophysics. For instance, do for the rest of the present paper.
it would help us shed light on the statistical and evolutionary prop- The stars in the sample have effective temperatures 4700  Teff
erties of planetary systems around giant stars, e.g. on the possible  5600 K, surface gravities 2.2  log g  3.7 dex, microturbulence
absence of the correlation between stellar metallicity and formation 1  ξ t  3.2 km s−1and they lie in the metallicity range of −0.75
efficiency of giant planets (e.g. Pasquini et al. 2007; Takeda, Sato  [Fe/H]  0.3 dex.
& Murata 2008; Ghezzi et al. 2010; Maldonado, Villaver & Eiroa
2013; Mortier et al. 2013c; Jofré et al. 2015)1 which was found for
3 C H E M I C A L A BU N DA N C E S
main-sequence dwarf stars (e.g. Gonzalez 1997; Santos, Israelian
& Mayor 2001, 2004; Fischer & Valenti 2005; Johnson et al. 2010; For the abundance derivation, we closely followed the method de-
Sousa et al. 2011; Mortier et al. 2013b). scribed in Adibekyan et al. (2012a).
Several explanations have been suggested for the aforementioned
lack ofmetallicity enhancement for giant stars hosting a giant planet.
Higher stellar mass of giants may compensate the lack of metals 3.1 Selection of the lines and abundance derivation
(e.g. Ghezzi et al. 2010), possible spectroscopic analysis issues in The initial line-list and the atomic data were taken from Adibekyan
giant stars (e.g. Hekker&Meléndez 2007; Santos et al. 2009), selec- et al. (2012a) and Neves et al. (2009). Neves et al. (2009) provided
tion biases in giant star samples (Mortier et al. 2013c). However, one the astrophysical (calibrated) oscillator strength and solar equivalent
should note that some studies reported an enhancedmetallicity of gi- widths of the lines. Since the spectra of cool evolved stars are
ant starswith planets, butwith small samples of planet hosts (Hekker more line crowded (which cause strong blending) compared to their
& Meléndez 2007; Quirrenbach, Reffert & Bergmann 2011). We unevolved hotter counterparts, we aimed to carefully select a subset
refer the reader to Alves et al. (2015, and reference therein) for more of unblended lines from Adibekyan et al. (2012a). For this purpose,
detailed review on the topic. as a reference we used a very high S/N and high-resolution archival
In this paper, we focus on the chemical and kinematic properties spectrum of the K-type giant Arcturus observed with the NARVAL
of a sample of 257 field giant stars which are observed within the spectrograph (Mortier et al. 2013c). We measured the equivalent
context of the CORALIE extrasolar planet search programme. The widths (EWs) of the selected lines both manually, using a Gaussian-
main characteristics of the sample along with the homogeneously fitting procedure within the IRAF3 splot task, and automatically, by
derived stellar atmospheric parameters are presented in a parallel using the ARES4 code (Sousa et al. 2007). We calculated the mean
paper (Alves et al. 2015). The uniform chemical analysis of these relative difference ((EWARES − EWIRAF)/ EWIRAF) and standard
giant stars is very important to explore the specific chemical re- deviation of the relative difference of the EW measurements and
quirements for the formation and evolution of planetary systems applied 2σ -clipping.We repeated this procedure a second time after
around them. The paper is organized as follows: in Section 2, we the outliers were excluded. Finally, 118 lines out of 164 were left
briefly introduce the sample used in this work. The method of the that show a relative difference in EW of less than 15 per cent. These
chemical abundance determination and analysis will be explained in lines were once again checked by eye within IRAF to make sure that
Section 3. The distinction of different Galactic stellar populations they are not blended and hence the correspondence between the EW
and kinematic properties of the stars are presented in Section 4. measurements is not by chance.5
Then, after discussing the metallicity distribution of the stars in After selecting the isolated lines, the abundances for 12 elements
Section 5, we summarize our main results in Section 6. (Na, Mg, Al, Si, Ca, Ti, Cr, Ni, Co, Sc, Mn, and V) were determined
2 SAMPLE D ESCRIPTION AND STELLAR 2 The source code of MOOG can be downloaded at http://www.
PA R A M E T E R S as.utexas.edu/∼chris/moog.html
3 IRAF is distributed by National Optical Astronomy Observatories, operated
Our sample comprises 257 G- and K-type evolved stars that are by the Association of Universities for Research in Astronomy, Inc., under
being surveyed for planets in the context of the CORALIE (Udry contract with the National Science Foundation, USA.
4 The ARES code can be downloaded at http://www.astro.up.pt/
∼sousasag/ares
1 Indeed, Reffert et al. (2015) claims a strong evidence for a planet– 5 The line-list is available at the CDS: http://cdsarc.u-strasbg.
metallicity correlation for giant planet host stars. fr/viz-bin/qcat?J/MNRAS/
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1902 V. Zh. Adibekyan et al.
Figure 1. [Cr I/Cr II], [Sc I/Sc II], and [Ti I/Ti II] as a function of atmospheric parameters for our sample of evolved stars (black points) and for the sample of
FGK dwarf stars from Adibekyan et al. (2012a, grey dots).
using an LTE analysis relative to the Sun with the 2010 version of line scatter errors and errors induced by uncertainties in the model
the MOOG (Sneden 1973) and a grid of Kurucz ATLAS9 plane- atmosphere parameters. In cases when only one line used to derive
parallel model atmospheres with no α-enhancement. The reference the abundances, a typical 0.1 dex error for line-to-line scatter was
abundances used in the abundance analysis were taken fromAnders assumed. For our sample stars, the errors induced by uncertainties
& Grevesse (1989). We note that our analysis is differential and the in the parameters of model atmosphere varies from about 0.02 dex
source of the reference abundances is not crucial. For the automatic (for Si I) to ≈0.06 dex (for V I) and in general are smaller than the
EW measurements, we used ARES for which the input parameters line-to-line scatter errors. The final errors for the studied elements
were the same as in Sousa et al. (2008) and the rejt parameter are smallest for Al I (≈0.04 dex) and largest for V I (≈0.14 dex).
is calculated following the procedure discussed in Mortier et al.
(2013c). The EWs of all the lines used in the derivation of the
abundances for all studied stars is available at CDS. 3.3 Testing the validity of the stellar parameters
The final abundance for each star and element was calculated to
be the average value of the abundances given by all lines detected Although Alves et al. (2015) have shown that the stellar parameters
in a given star and element. Individual lines for a given star and in general agree very well with the literature data, the consistency
element with a line dispersion more than a factor of 2 higher than does not always imply correctness. Moreover, the stellar parameters
the rms were excluded. In this way we avoided the errors caused were derived by completing an LTE abundance analysis and by
by bad pixels, cosmic rays, or other unknown effects. A sample of using only iron lines. To check the validity limit of the adopted
our results for three stars is presented in Table 2 and the complete methodology in terms of stellar parameter ranges, we tested our
results are available at the CDS. results in two ways (see also Adibekyan et al. 2012a).
First, in Fig. 1, we plot the [Cr I/Cr II], [Sc I/Sc II], and [Ti I/Ti II]
as a function of the stellar parameters to ensure that the ionization
equilibrium enforced on the Fe II lines (Alves et al. 2015) is accept-
3.2 Uncertainties
able to other elements. For comparison, the field FGK dwarf stars
Since the abundances were determined via the measurement of from Adibekyan et al. (2012a) are presented. Most of the trends are
EWs and using already determined stellar parameters, the errors nearly flat around zero in contradiction to their unevolved counter-
might come from the EW measurements, from the errors in the parts forwhich a gradual increasewith decreasingTeff was observed.
atomic parameters, and from the uncertainties of the atmospheric For our stars, only an increase of [Sc I/Sc II] ratio can be seen with
parameters that were used tomake an atmospheremodel. In addition the decrease of Teff. However, the results obtained for Sc I and Cr II
to the above-mentioned errors, one should add systematic errors that should be considered with caution since their abundances have been
can occur due to non-LTE or granulation (3D) effects. To minimize derived by using only one line. From the figure, one can notice a
the errors, it is very important to use high-quality data and as many small offset from zero for [Ti I/Ti II] ratio and [Sc I/Sc II]. These pos-
lines as possible for each element. itive offsets probably do not have relation to the non-LTE effects
We followed Adibekyan et al. (2012a) for the calculation of as discussed in Bergemann (2011) for [Ti I/Ti II] and still need to be
the errors. In short, we first varied the model parameters by an understood.
amount of their 1σ errors available for each star and calculated Tsantaki et al. (2013) showed that by correcting stellar parameters
the abundance differences between the values obtained with and (mainly Teff), using carefully selected line-list especially designed
without varying the parameter. Then we evaluated the errors in the for cool stars, the observed trends of [X I/X II] with stellar parameters
abundances of all elements [X/H], adding quadratically the line-to- get flatter. For example, an overestimation ofTeff for cool starsmight
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Abundances and kinematics of G and K giants 1903
Figure 2. [X/Fe] versus Teff plots. The black dots represent the stars of the sample and the grey small dots represent stars from Adibekyan et al. (2012a). The
blue and red solid lines depict the linear fits of the current data and the data from Adibekyan et al. (2012a), respectively. The blue dashed line is the fit of our
data after correcting for the trend with Teff. Each element is identified in the upper-right corner of the respective plot.
cause of the trends. The dependence of the abundances of ionized first, we obtained the zero-centred distribution of the correlation
and neutral species on the surface gravity is also discussed inMortier coefficient by randomly bootstrapping (building random samples
et al. (2013a). by shuffling the parameters among the observed set of parameters)
Another way of testing the stellar parameters is to plot [X/Fe] the observed data pairs 104 times. Then we calculated the corre-
against Teff (Fig. 2). For the comparison, the dwarf sample of lation coefficient for each of these uncorrelated data sets and then
Adibekyan et al. (2012a) is also presented. Stellar evolutionary the average and standard deviation of these values. By assuming a
models do not suggest significant trends of these ratios with Teff. Gaussian distribution for R (correlation coefficient), we calculated
However, for several elements we detected significant trends. To the probability that the R of the original data set was obtained by
evaluate the significance of the trends, we performed a linear fit and pure chance. The significance of the trends and the slopes are pre-
followed the procedure described in Figueira et al. (2013). In short, sented in Table 1. From the figure, one can see that for most cases
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1904 V. Zh. Adibekyan et al.
Table 1. The slope, correlation coefficient, and the significance of The [X/H] abundances of all the stars before and after correction
the [X/Fe] linear trends with the Teff. (if the significance of the correlation is above 3σ ) for the Teff trends
are available at the CDS (see also Table 2).
Elem Slope R2 N z-score
Na I 1.62 ± 0.36 × 10−4 0.72 × 10−1 256 4.3
− 3.4 [X/Fe] versus [Fe/H] relation. Galactic chemical evolutionMg I − 2.51 ± 0.38 × 10 4 0.14 × 100 257 6.1
A II − 2.69 ± 0.29 × 10−4 0.24 × 100 257 7.9 The [X/Fe] versus [Fe/H] relation plot is traditionally used to study
Si I − 2.39 ± 0.16 × 10−4 0.45 × 100 257 11.0 theGalactic chemical evolution because iron is a good chronological
Ca I 0.61 ± 1.80 × 10−5 0.44 × 10−3 257 0.3 indicator of nucleosynthesis. In Fig. 3, we present the dependence
Sc I − 3.00 ± 0.34 × 10−4 0.23 × 100 255 7.6
− − of [X/Fe] on metallicity for our sample of giant stars and for FGKSc II − 0.78 ± 0.23 × 10 4 0.43 × 10 1 257 3.2
± × − × − dwarf stars from Adibekyan et al. (2012a).
9 In the figure, we also
Ti I 0.81 2.94 10 5 0.29 10 3 257 0.2
− ± × −4 × −2 showed the average value of [X/Fe] for stars in the metallicity rangeTi II 0.44 0.34 10 0.64 10 257 1.2
− ± × −4 × 0 of 0.0 ± 0.1 dex, where the Galactic chemical evolution effects areV I 1.79 0.29 10 0.13 10 256 5.9
Cr 0.56 ± 0.10 × 10−4 0.98 × 10−1 257 4.9 small. As one can see, for all the elements the general behaviour ofI
Cr II 1.33 ± 0.34 × 10−4 0.57 × 10−1 246 3.6 [X/Fe] with the metallicity is similar for giant and dwarf stars, and
Mn I 1.33 ± 0.34 × 10−4 0.57 × 10−1 247 2.3 reflects the Galactic chemical evolution in the solar neighbourhood.
Co I − 0.32 ± 0.03 × 10−3 0.33 × 100 257 9.2 However, one can also clearly notice that, for some elements (Co,
Ni I − 0.78 ± 0.11 × 10−4 0.16 × 100 257 6.3 Na, V, Mn, Al, and Si) while the Galactic chemical evolution trends
are similar, they are shifted: for giant stars having higher [X/Fe]
values at a fixed metallicity. The largest offset is seen for Na and
the trends are less steep compared to those observed for the dwarfs,6 Mn, and a bit less in Si and Al. In general, Na and Al are not
which in turn speaks about the correctness of the stellar parameters good tracers of chemical evolution and affected by internal mixing
used to derive the abundances. processes in the stars. The Mn abundance was obtained by using
Adibekyan et al. (2012a) already discussed several possible rea- only one line and it should be considered with caution. Moreover,
sons for the observed trends of [X I/X II] with stellar parameters, the scatter in [Mn/Fe] is very high, indicating unrealistic abundances
and [X/Fe] with T 7eff and concluded that the observed trends are for some fraction of the stars.
probably not an effect of stellar evolution, and uncertainties in at- Overabundances of sodium and aluminium (also silicon in some
mospheric models are the dominant effect in the measurements. cases) in open cluster giants (compared to the abundances of dwarfs)
The authors afterwards removed the T trend as it was done also in were already observed by several authors (e.g. Friel et al. 2003; Friel,eff
other works (e.g. Valenti & Fischer 2005; Petigura & Marcy 2011). Jacobson & Pilachowski 2005; Tautvaišiene et al. 2005; Jacobson
Since by fitting the data and simply subtracting the fit would et al. 2007; Villanova et al. 2009; Santrich et al. 2013). In most
force the mean [X/Fe] to zero (which is an non-physical situation), of these studies, the trends were explained as a stellar evolutionary
Adibekyan et al. (2012a) added a constant term chosen so that the effect, due to the deep mixing produced by the hydrogen burning
correction is zero at solar temperature. In our case, the stars are cycle, after stars have left the main sequence. For a complete pic-
cooler and their temperatures do not reach the solar temperature so ture, one should perform thorough analysis taking into account the
we decided to apply another approach. In this case, the constant non-LTE effects which are stronger for giants stars and also the
term was chosen so that the correction is zero at T = 4960 K, systematic errors which might arise due to particular spectroscopiceff
which is the mean temperature of our sample stars. However, we analysis method used. For example, it is well known that sodium
appreciate the fact that this approach and the choice of the constant lines suffer from non-LTE effects which lead to an overestimation
term is arbitrary. For this reason, we decide to use the original of the Na abundances (e.g. Alexeeva et al. 2014). In our analysis,
(before detrending) chemical abundances for the rest of our study. we used two sodium lines (at 6154.23 and 6160.75 Å) which were
The dependence of [X/Fe] on stellar gravity and microturbulence studied for non-LTE effects in Alexeeva et al. (2014). The average
andmetallicity is shown in Fig. B1, Fig. B2, and Fig. 3, respectively. EWs of these lines were ∼70 mÅ for 6154.23 Å, and ∼80 mÅ
For most of the species, we did not observe a trend with ξ and log g, for the 6160.75Å line. According to Alexeeva et al., the non-LTEt
and some of the observed trends probably reflect the correlation of correction for our stars should be from −0.1 to −0.15 dex, which
T with other stellar parameters (see Fig. A1). is close to the difference in [Na/Fe] between giants and dwarfseff
As a final check, we compare our derived abundances with those observed in this study.
obtained by Liu et al. (2007). We note that this is the only literature The difference in Al abundances ([Al/Fe]) between giants and
sourcewherewe find enough stars (14 stars) in common to compare. dwarfs obtained for solar-metallicity stars is not large (0.07 dex
We found very good agreement for all the species except vanadium: – about 1σ scatter), but seems to increase at lower metallicities.
[Na/H] = 0.02 ± 0.12, [Mg/H] = −0.06 ± 0.12, [Al/H] = However, it should be noted that direct comparison of the abundance
0.05 ± 0.04, [Ca/H] = 0.03 ± 0.06, [Si/H] = 0.03 ± 0.03, ratios at lower metallicities is not straightforward, since the Galactic
[Ti/H] = 0.04 ± 0.08, [Ni/H] = −0.01 ± 0.05, and [V/H] chemical evolution effects and the relative fraction of thin- and
= 0.19 ± 0.05.8 thick-disc stars can be dominant. Several authors studied the non-
LTE effects on the formation of Al lines (e.g. Baumueller & Gehren
1996, 1997;Menzhevitski, Shimansky& Shimanskaya 2012). They
showed that the non-LTE correction of the Al abundances, derived
6 Note that the stellar parameters for the dwarfs were not derived by using from the subordinate doublet λλ 6696.03, 6698.68 Å, is very small
the Tsantaki et al. (2013) line-list which is especially designed for cool stars.
7 Note that the Adibekyan et al. (2012a) sample essentially consists of
dwarfs. 9 Only stars with Teff = T ± 500 K are presented, because of the highest
8 [X/H] = [X/H]our − [X/H]theirs. accuracy in the parameters and chemical abundances in these stars.
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Abundances and kinematics of G and K giants 1905
Figure 3. [X/Fe] versus [Fe/H] plots. The black dots represent the stars of the sample and the grey small dots represent stars from Adibekyan et al. (2012a)
with Teff = T ± 500 K. The red circle and blue square show the average [X/Fe] value of stars with [Fe/H] = 0.0 ± 0.1 dex. Each element is identified in the
upper-right corner of the respective plot.
Table 2. Sample table of the derived abundances of the elements, rms, total error, and number of measured
lines for each star.
Star . . . [Si I/H] rms err [Si I/H]∗corr n [Ca I/H] rms err n . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HD47001 . . . −0.20 0.09 0.09 −0.26 14 −0.25 0.05 0.06 11 . . .
HD73898 . . . −0.30 0.03 0.03 −0.28 14 −0.32 0.04 0.05 11 . . .
HD16815 . . . −0.16 0.07 0.07 −0.20 15 −0.25 0.04 0.06 12 . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Notes. ∗The [X/H] abundances after correction for the Teff trends.
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Figure 4. [Si/Fe] versus [Fe/H] plots for Si lines of different EP. The black dots represent the stars of the sample and the grey small dots represent stars
from Adibekyan et al. (2012a) with Teff = T ± 500 K. The wavelength of each line and excitation energy of the lower energy level (χ1) is identified in the
lower-left corner of the respective plot.
at solarmetallicities, does not depend strongly on the surface gravity be seen in the middle panel of the plot the three lines with exactly
and only significant at temperatures above 6500 K (Menzhevitski the same χ1 show different behaviour, for λ 5753.64 Å showing the
et al. 2012). largest difference.
The next element for which we obtained small, but systematic Fig. 4 shows that the picture is complex and probably several
difference between giants and dwarfs is Si. The abundance of Si is process (non-LTE, unresolved blends) are acting and affecting the
not expected to be affected by extra mixing processes in the stars. abundances at the same time. To select the ‘best’ lines i.e. lines
The few studies of the Si abundances taking into account the non- which give similar average abundances to that obtained for the
LTE deviations showed that the effect is significant only at very low dwarfs, for all the lines we calculated the average [Si/Fe] abundance
metallicities (e.g. Bergemann et al. 2013) and the non-LTE correc- ratio obtained for all the giants with solar metallicity ± 0.1 dex
tion for Si of the Sun is about−0.05 dex (Sukhorukov & Shchukina and compared that with the average [Si/Fe] obtained for dwarfs
2012). Since in this study for the Al abundance derivation, we used (Adibekyan et al. 2012a) with metallicities in the range of 0.0 ±
several lines with different excitation potentials (EP), which means 0.1 dex. Then we used the rms (scatter) of the [Si/Fe] calculated
different atmospheric layers of formation and hence different sensi- for the dwarfs10 to quantify the observed differences (in n× σ ). We
tivities to non-LTE deviations, we decided to analyse [Si/Fe] against found 5 (out of 15) Si lines which give [Si/Fe] abundance similar to
[Fe/H] for each individual line. In Fig. 4, we plot [Si/Fe] against
[Fe/H] for nine Si lines with the lowest, intermediate and highest
excitation energy of the lower energy level (χ1). At the first glance, 10 The scatter obtained for the dwarfs by averaging the abundance of many
it looks like the lines with the highestχ1 show the highest deviations lines is more realistic than the scatter obtained from one spectral line for
from the abundances derived for the FGK dwarfs. However, as can giant stars.
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Abundances and kinematics of G and K giants 1907
Figure 5. Toomre diagram for the entire sample. The left- and right-hand panels show the separation of the stellar groups according to the Bensby, Feltzing
& Lundström (2003, B03) and Robin et al. (2003, R03) prescription, respectively. The symbols are explained in the figure.
that obtained for the dwarfs (less than 2σ difference). The [Si/Fe] Database.11 Combining the measurement errors in the parallaxes,
abundance obtained by averaging the abundances of the mentioned proper motions, and radial velocities, the resulting average errors in
five lines against [Fe/H] is presented in the Fig. B3. We note that we the U, V, and W velocities are of about 2–3 km s−1.
do not claim that these selected lines are not affected by non-LTE To assess the likelihood of the stars being a member of different
effects of unresolved blends, but they provide abundances similar to Galactic populations,we followedReddy, Lambert&AllendePrieto
that obtained for dwarfs, which probably means that they are more (2006). The probabilities that the stars belong to different stellar
realistic. populations were calculated, having adopted both the Bensby et al.
We repeated the aforementioned procedure for all the lines for (2003) and Robin et al. (2003) population fractions. We refer the
each element and calculated the difference in [Xline/Fe] between gi- reader to Adibekyan et al. (2012a) for the details of the computation.
ants and dwarfs for each line. This differences, in n× σ , is presented The Galactic space velocity components and the probabilities to
in the last column of the line-list table (available at CDS). The [X/Fe] assign the stellar population to which the stars belong are available
versus [Fe/H] relation obtained by using only the ‘best’ lines (less at the CDS.
than 2σ difference) is shown in Fig. B3. The corresponding [X/H] According to the Bensby et al. (2003) criteria, among the 183
abundances are available at CDS. stars, we have 176 (96 per cent) stars from the thin disc, 5 from
the thick disc, and 2 stars are considered to be transition stars
that do not belong to any group. Adopting the criteria from Robin
4 K I N E M AT I C S A N D S T E L L A R PO P U L AT I O N S et al. (2003) gives 177 (97 per cent) thin-disc stars, 5 star with
kinematics suggesting a thick/thin-disc transition, and one star with
It is becoming increasingly clear that a separation of the Galactic a classification of thick-disc/halo transition object. The distribution
stellar components based only on stellar abundances is superior to of the stars in the Tommre diagram is shown in Fig. 5.
kinematic separation (e.g. Adibekyan et al. 2011; Lee et al. 2011; As mentioned above, in addition to the difference in their kine-
Navarro et al. 2011; Liu & van de Ven 2012; Recio-Blanco et al. matics, the thin- and thick-disc stars are also different in their α
2014), because chemistry is a relatively more stable property of a content at a given metallicity (e.g. Fuhrmann 1998, 2008). This di-
star than its spatial positions and kinematics. However, asmentioned chotomy in the chemical evolution allows one to separate different
above, some changes in abundances of some elements are expected stellar populations.
when the stars are evolving and leaving the main sequence. In this The [α/Fe] versus [Fe/H] plot for the sample stars along with
analysis, to separate the thin- and thick-disc stellar components, we the dwarf stars from Adibekyan et al. (2012a) with Teff = T
used the position of the stars in the [α/Fe]–[Fe/H] plane (here α ± 500 K is depicted in Fig. 6.12 As one can see from the figure
refers to the average abundance of Mg, Si, and Ti), but separately the two samples show similar trends, with giant stars having on
also a kinematics approach is applied. average higher [α/Fe] values at a fixed (low) [Fe/H]. This observed
The space velocity components for 183 stars out of 257 were difference might arise from our assumption of LTE line formation.
derived with respect to the local standard of rest, adopting the The non-LTE effects are stronger for metal-poor stars, but these
standard solar motion (U, V, W) = (11.1, 12.24, 7.25)
− effects depends also on other stellar parameters (e.g. gravity andkm s 1 of Schönrich, Binney & Dehnen (2010). For the remain-
ing 73 stars, we did not calculate the velocities because of the
deficit of astrometric literature data. The radial velocities, parallaxes 11 http://simbad.u-strasbg.fr/simbad/
and proper motions were taken from the SIMBAD Astronomical 12 The chemical dissection of the discs is presented in the appendix.
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1908 V. Zh. Adibekyan et al.
Figure 6. [α/Fe] versus [Fe/H] for the current sample (black dots) and for Figure 7. Left-hand panel: [Fe/H] versus log g for the current sample (black
the stars from Adibekyan et al. (2012a) with T = T ± 500 K (grey dots) and for the stars from Adibekyan et al. (2012a, grey dots). The twoeff
small dots). The separation between the thick- and thin-disc stars for the black dashed lines were drawn by eye and show the biases in the samples
two samples are presented in black and grey dashed lines. due to the B − V cut-off. Right-hand panel: the metallicity distribution of
the two aforementioned samples. The distribution of the giants stars (grey
line) was multiplied by 2 for the better visual comparison. The median and
temperature) and also they are different for different elements and
its standard deviation are also presented for metallicity distributions of both
they differ from line to line. Thus, for fully understanding of the giants and dwarfs.
main reason of the observed abundance difference between giants
and dwarfs, a complete non-LTE analysis is needed. a possible reason might be a selection bias due to B − V colour
Our chemical separation of the Galactic discs suggests that 23 cut-off.
stars (9 per cent) in the sample show enhanced α-abundances. In In Fig. 7, we plotted the relation between stellar metallicity and
Adibekyan et al. (2011, 2013), the high-α stars were separated into surface gravity. For the comparison, the dwarf stars sample from
two groups with a gap in both [α/Fe] and metallicity. It is interesting Adibekyan et al. (2012a) is also presented. From the figure, one can
to see that the gap in [Fe/H] for high-α stars can be also seen in our easily see that the giant stars sample lacks high-metallicity and low-
sample at the same metallicity (≈−0.2/−0.3 dex). Following the gravity stars, and also low-metallicity and high-gravity stars. This
same logic and definitions as in Adibekyan et al. (2013), the 10 stars
− is again probably because of the selection criteria used to define thewith enhanced [α/Fe] and [Fe/H] below 0.3 dex can be classified sample.
as thick-disc stars, and the remaining 13 stars as high-α metal-rich To avoid the issues related to the selection effects, an unbi-
stars (hαmr). With this definition, we see that 4 per cent of the stars ased giant sample with no colour cut-off and homogeneously
belong to the Galactic thick disc, as the kinematic separation was derived parameters is needed that is systematically searched for
suggesting. planetary companions. However, it is still possible to overcome
We note that the current sample is small and we will avoid of the effect of the B − V colour cut-off if one considers, for
a definite conclusion about the existence of the mentioned ‘gap’
≈ − example, only stars in the ‘cut rectangle’ shown in Fig. 7 (redat [Fe/H] 0.3 dex and the distinction of the two α-enhanced rectangle), where the stars are equally distributed. However, these
metal-poor and metal-rich populations. However, the fact that the ‘cut rectangles’ will consist of stars with narrower ranges of
two different homogeneously analysed samples (the current one metallicities (from −0.25 to 0.15 dex in the example of Fig. 7),
and the one from Adibekyan et al. 2011) show quite similar fea- which is also an issue since the giant-planet–metallicity correla-
tures probably is more than just a hint about the existence of the tion is more pronounced at high metallicities (at least for dwarf
hαmr stars as a distinct stellar family. Moreover, recent study of stars).
an inner disc metal-rich open cluster, Berkeley 81, shows that the In the right-hand panel of Fig. 7, we show the metallicity distri-
stars are enhanced in α-element Magrini et al. (2015), thus con- bution of giant and dwarf stars where narrower [Fe/H] distribution
firming that the hαmr stars have inner disc origin as suggested by of giants is apparent. The figure also shows that the two distribu-
Adibekyan et al. (2013). However, we want to note that no simi- tions are peaked at similar metallicities, close to the solar value. The
lar ‘gap’ was found in Bensby, Feltzing & Oey (2014) where the median (and its standard deviation) of the metallicity distributions
authors suggested that the hαmr stars present the metal-rich tail of of giant and dwarf stars are −0.05 (0.18) and −0.10 (0.33) dex,
the thick disc. As mentioned in Bensby et al. (2014), a large sample respectively.13 Several studies have already observed this tendency
with well-controlled selection function (e.g. Gaia-ESO survey – of evolved stars lacking the metal-rich and very metal-poor tails
Gilmore et al. 2012) would help us to understand the real nature of (e.g. Taylor & Croxall 2005; Luck & Heiter 2007; Takeda et al.
the hαmr stars. 2008; Ghezzi et al. 2010).
The stars in this sample have stellar masses between 1.5 and
5 METALLICITY DISTRIBU TION 4.0M (Alves et al. 2015), and hence should be on average younger
As mentioned above, several authors tried to understand the reason
why the apparent giant-planet–metallicity correlation does not exist 13 We note that the standard deviation of the median is calculated as 1.25*σ ,
for evolved stars. As recently suggested by Mortier et al. (2013c), where σ is the standard deviation of the distribution.
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Abundances and kinematics of G and K giants 1909
than the dwarfs from Adibekyan et al. (2012a). The younger age metallicity. As discussed in Mortier et al. (2013c), this selection
together with the age–metallicity dispersion relation (e.g. da Silva bias might be the reason of the absence of the correlation between
et al. 2006; Haywood 2008; Casagrande et al. 2011; Maldonado occurrence of giant-planet planets and stellar metallicity. We sug-
et al. 2013) might explain the narrower [Fe/H] distribution of the gest to use stars in a ‘cut-rectangle’ in the log g–[Fe/H] diagram to
giants. Young stars are mostly local since they do not have time to overcome the aforementioned issue, if an unbiased sample is not
migrate within theGalaxy (Wang&Zhao 2013;Minchev, Chiappini available on hand.
& Martig 2013). Radial migration in the disc is expected to make Although the current sample still contains only one star known
the metallicity distribution wider, but does not change the mean to orbit a planetary companion (HD 11977 – Setiawan et al. 2005),
abundance (Wang&Zhao 2013), as we see in Fig. 7. This is because most of the stars have already been periodically observed over
mostly massive stars contribute to the chemical enrichment of the the last years. Before a significant number of planets are detected,
interstellar medium and they contribute mainly around their birth this sample can be used as a homogeneous comparison sample to
places because of their very short lifetime. The lack of very metal- study planet occurrence around giant stars. However, when explor-
rich giants can be understood along the same migration process, ing chemical peculiarities of planet-hosting giant stars, one should
most of the old stars which migrate would come from the inner, bear inmind the chemical properties of these evolved stars discussed
metal-rich disc (Minchev et al. 2013; Wang & Zhao 2013). in this paper (e.g. enhancement in Na, Al, etc.).
In addition to the aforementioned astrophysical explanation, we
would like to note again the selection effects which may arise in
evolved star samples due to B − V colour cut-off. This selection AC K N OW L E D G E M E N T S
bias may also make the metallicity distribution narrower.
This work was supported by the European Research Coun-
cil/European Community under the FP7 through Starting Grant
6 SU M M A RY O F T H E R E S U LT S agreement number 239953. This work was also supported by the
We have carried out a uniform abundance analysis for 12 refrac- Gaia Research for European Astronomy Training (GREATITN)
tory elements for a sample of 257 field G-, K-type evolved stars Marie Curie network, funded through the European Union Sev-
that are being surveyed for planets using precise radial–velocity enth Framework Programme ([FP7/2007-2013]) under grant agree-
measurements with the CORALIE spectrograph. The abundances ment number 264895. V.Zh.A. and S.G.S acknowledge the support
were derived using a carefully selected line-list and are based on from the Fundação para a Ciência e a Tecnologia, FCT (Portu-
the precise spectroscopic parameters derived by Alves et al. (2015) gal) in the form of the fellowships SFRH/BPD/70574/2010 and
using the same spectra as were used in the present study. SFRH/BPD/47611/2008, respectively. NCS was supported by FCT
We found that for all the elements Galactic chemical evolution through the Investigator FCT contract reference IF/00169/2012 and
trends are similar for giant and dwarf stars, while for some species POPH/FSE (EC) by FEDER funding through the programme ‘Pro-
[X/Fe] values are shifted towards higher values at a fixed metal- grama Operacional de Factores de Competitividade’ – COMPETE.
licity. Our LTE analysis confirms the overabundance of Na in gi- Research activities of the Observational Stellar Board of the Fed-
ant stars compared to the field FGK dwarf stars from Adibekyan eral University of Rio Grande do Norte are supported by contin-
et al. (2012a). This overabundance may have a stellar evolutionary uous grants of CNPq and FAPERN Brazilian agencies and by the
character, even though the possible departures from non-LTE may INCT-INEspao. SA acknowledges Post-Doctoral Fellowship from
produce an enhancement of a similar degree (Alexeeva et al. 2014). the CAPES brazilian agency (BEX-2077140), and also support by
We showed that an observed small overabundance of Si compared Iniciativa Cientı́fica Milenio through grant IC120009, awarded to
to the field FGK dwarf vanishes when a shorter, carefully select The Millennium Institute of Astrophysics. GI acknowledges finan-
line-list is used. cial support from the SpanishMinistry projectMINECOAYA2011-
To separate Galactic stellar populations, we applied both a purely 29060. AM received funding from the European Union Seventh
kinematical approach and chemical method. Our chemical separa- Framework Programme (FP7/2007-2013) under grant agreement
tion suggests that 91 per cent of the stars, being α-poor, belong to number 313014 (ETAEARTH). This research has made use of
the thin disc and the remaining 9 per cent of the stars show enhanced the SIMBAD database operated at CDS, Strasbourg, France. We
α-element abundances at a fixed [Fe/H]. This sample (while being thank Mahmoudreza Oshagh for his interesting comments related
not very large) also suggests a ‘gap’ in [Fe/H] for high-α stars as to Fig. 6, and Elisa Delgado Mena for a very constructive discus-
observed in Adibekyan et al. (2011). Following the definition of sion. We would also like to thank the anonymous referee for useful
the last authors, 4 per cent of the stars were classified as thick-disc comments that helped to improve the paper.
members (being metal-poor) and 5 per cent as hαmr stars.
The metallicity distribution of the giant stars is shown to be
narrower than that of their non-evolved dwarf counterparts (see R E F E R E N C E S
also Taylor & Croxall 2005; Takeda et al. 2008), but peaked at
almost solar metallicity as in case of the dwarfs. The lack of very Adibekyan V. Zh., Santos N. C., Sousa S. G., Israelian G., 2011, A&A, 535,
metal-rich and metal-poor stars can be explained by the fact that L11
most of the stars are originated in the solar vicinity. Evolved stellar Adibekyan V. Zh., Sousa S. G., Santos N. C., Delgado Mena E., González
samples mostly consist of massive stars, which have shorter lifetime Hernández J. I., Israelian G., Mayor M., Khachatryan G., 2012a, A&A,
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Abundances and kinematics of G and K giants 1911
Figure A1. Interdependence of the stellar atmospheric parameters of the sample stars. The blue dotted lines depict the linear fits of the full data, and the sold
lines are the fits after removing five ‘outliers’. The 2σ intervals of the linear fit of the ξ t–log g relation are shown in blue dashed lines. The black crosses
indicate the five outliers.
Table A1. The coefficients of the linear fits (y= a× X+b) of the relations were responsible for the ‘strong’ relation observed between ξ t and
between the stellar parameters, along with the correlation coefficient and Teff.
the significance. The number of stars is 251. After this test, we decided to present the relation of microturbu-
lence only with log g and [Fe/H], which has the following functional
Elem a b R2 z-score form:
ξ t–Teff 0.120 ± 0.065 0.825 ± 0.326 0.013 1.7
± − ± ξt = 2.72(±0.08)− 0.457(±0.031)× log glog g–Teff 0.847 0.089 1.356 0.441 0.266 7.9
[Fe/H]–Teff 0.280 ± 0.069 − 1.472 ± 0.343 0.061 3.9 + 0.072(±0.044)× [Fe/H] (A1)
[Fe/H]–log g 0.234 ± 0.041 − 0.751 ± 0.116 0.116 5.4
− ± ± We note that this empirical relation is valid only for the range ofξ t–log g 0.440 0.029 2.673 0.083 0.476 10.8
ξ t–[Fe/H] − 0.154 ± 0.057 1.407 ± 0.010 0.027 2.6 stellar parameters that the stars in our sample cover.
it shows the strongest correlation), by applying 2σ -clipping (two
APPENDI X B: [X/ F E ] D E P E N D E N C E O N
times of residual standard deviation). Then, after cleaning the data
STELLAR PARAMETERS
from outliers we again fitted the data and again evaluated the signif-
icance of the relations. We found that microturbulence significantly In this section, we present [X/Fe] versus log g (Fig. B1), [X/Fe] ver-
correlated with the surface gravity (at about 11σ level), and with sus microturbulence (Fig. B2), and [X/Fe] versus [Fe/H] (Fig. B3)
the metallicity but with less degree of significance. The five outliers plots derived from the ‘best’ lines as discussed in the main text.
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1912 V. Zh. Adibekyan et al.
Figure B1. [X/Fe] versus log g plots. Each element is identified in the upper-right corner of the respective plot. The black dots represent the stars of the current
sample.
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Abundances and kinematics of G and K giants 1913
Figure B2. [X/Fe] versus microturbulence plots. Each element is identified in the upper-right corner of the respective plot. The black dots represent the stars
of the sample and the grey small dots represent stars from Adibekyan et al. (2012a).
MNRAS 450, 1900–1915 (2015)
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1914 V. Zh. Adibekyan et al.
Figure B3. [X/Fe] versus [Fe/H] plots derived from the ‘best’ lines. For Na I, Cr II, and Mn I there was no ‘best’ line(s) found. The black dots represent the
stars of the sample and the grey small dots represent stars from Adibekyan et al. (2012a) with Teff = T ± 500 K. The red circle and blue square show the
average [X/Fe] value of stars with [Fe/H] = 0.0 ± 0.1 dex. Each element is identified in the upper-right corner of the respective plot.
MNRAS 450, 1900–1915 (2015)
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Abundances and kinematics of G and K giants 1915
A P P E N D I X C : SE PA R AT I O N O F T H E
GALACTI C DI SCS BY α- E N H A N C E M E N T
For the separation of Galactic stellar population by the chemical
properties of the stars was done following the method presented
in Adibekyan et al. (2011). We first divided the sample into three
metallicity bins: [Fe/H] < −0.3 dex, [Fe/H] > 0.0 dex, and stars
in between. For the lowest and highest metallicity bins, we easily
identified the minima in the [α/Fe] histograms. For the interme-
diate metallicity, stars just plotting the [α/Fe] histogram will not
reveal the minima, because the stars at these metallicities show
a decrease of [α/Fe] with [Fe/H] (see Fig. C1). Thus, we first
detrended the [α/Fe] by applying a liner fit and subtracting it.
Then in the [α/Fe] histogram we identified the minima and by
adding it to the previously applied liner fit we obtained the line
which separates the high- and low-α stars at −0.3 ≤ [Fe/H] ≤
0.0 dex. The separation lines for each metallicity bin presented in
Fig. C1.
Figure C1. High-α and low-α separation histograms for the stars with
metallicities < −0.3 dex (bottom), −0.3 ≤ [Fe/H] ≤ 0.0 dex (middle), and
[Fe/H] > 0.0 dex (top). The dotted lines are the separation curves between
the thin and thick discs. This paper has been typeset from a TEX/LATEX file prepared by the author.
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