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Advances in Astronomy and Space Physics, 4, 20-24 (2014) Abundances in the atmosphere of the metal-rich planet-host star HD 77338 I. O. Kushniruk1∗, Ya. V. Pavlenko2,3, J. S. Jenkins4, H. R. A. Jones3 1Taras Shevchenko National University of Kyiv, Glushkova ave., 2, 03127 Kyiv, Ukraine 2Main Astronomical Observatory of the NAS of Ukraine, Akademika Zabolotnoho str., 27, 03680 Kyiv, Ukraine 3Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hateld, Hertfordshire AL10 9AB, UK 4Departamento de Astronomía, Universidad de Chile, Camino el Observatorio 1515, Las Condes, Santiago, Chile Abundances of Fe, Si, Ni, Ti, Na, Mg, Cu, Zn, Mn, Cr and Ca in the atmosphere of the K-dwarf HD 77338 are determined and discussed. HD 77338 hosts a hot Uranus-like planet and is currently the most metal-rich single star to host any planet. Determination of abundances was carried out in the framework of a self-consistent approach developed by Pavlenko et al. (2012). Abundances were computed iteratively by the ABEL8 code, and the process converged after 4 iterations. We nd that most elements follow the iron abundance, however some of the iron peak elements are found to be over-abundant in this star.
Key words: stars: abundances, stars: atmospheres, stars: individual (HD 77338), line: proles a Bayesian analysis and a periodogram analysis.
According to modern theory, the formation of the Determining the chemical composition of stars is nucleus of chemical elements from carbon to iron is one of the primary goals of astrophysics. Such in- the result of thermonuclear reactions involving He, vestigations help us to better understand the chem- C, O, Ne and Si in stars. After the depletion of ical enrichment of the Galaxy and to make some hydrogen reserves, a star's core starts running a 3α assumptions about the mechanisms involved in el- reaction, where it produces a number of elements as ement evolution in the interstellar medium, and in a result of the following transformations: stellar atmospheres in particular [12]. While study- ing the Sun, the problem of the abundances of cer- 3 4He −→ 12C, 12C + 4He −→ 16O + γ, tain atoms necessitated a model to explain this. It was nally explained with the introduction of the pp- 16O + 4He −→ 20Ne + γ. and CNO- cycles in the interior of the Sun. However, this was not sucient to explain the presence of large After reaching a specic threshold temperature, car- amounts of Helium. The next step in studying the bon begins fusing with the formation of Ne, Na and evolution of elements was the introduction of nucle- osynthesis theory. Modern scientic understanding is that chemical elements were formed as a result of C + 12C −→ 20Ne + 4He + 4.62 MeV, the processes occurring in stars, leading to evolution- 12C + 12C −→ 23Na + p + 2.24 MeV, ary changes of their physical conditions. Therefore, the problem of nuclide formation is also closely re- 12C + 12C −→ 24Mg + γ − 2.60 MeV. lated to the issue of the evolution of stars and plane- Aluminium can then be produced by: tary systems. Recently Jenkins et al. [5] announced the discovery of a low-mass planet orbiting the super 24Mg + p −→ 25Al + γ. HD 77338 as part of our ongoing Calan-Hertfordshire Extrasolar Planet Search [6]. The best-t planet The combustion reaction of oxygen is a dual-channel solution has an orbital period of 5.7361 ± 0.0015 process and causes the presence of Al, S, P, Si and days and with a radial velocity semi-amplitude of Mg. One of the channels is: only 5.96 ± 1.74 ms−1, giving a minimum mass of O + 16O −→ 30Si + 1H + 1H + 0.39 MeV, ⊕. The best-t eccentricity from this solution , and is in the agreement with results of O + 16O −→ 24Mg + 4He + 4He − 0.39 MeV, I. O. Kushniruk, Ya. V. Pavlenko, J. S. Jenkins, H. R. A. Jones, 2014 Advances in Astronomy and Space Physics I. O. Kushniruk, Ya. V. Pavlenko, J. S. Jenkins, H. R. A. Jones 16O + 16O −→ 27Al + 4He + 1H − 1.99 MeV, the star, i. e. the iron abundance. In turn, we recom- With continuous temperature growth, silicon burn- puted the abundances of many elements which show ing is initiated. This process is described by a num- signicant absorption lines in the observed spectrum ber of reactions. As a result we can receive, for ex- of HD 77338.
ample, Ar, Ni, S, etc. 56Ni, after two β decays, turns Table 1: Simbad's list of previous assessments of abun- into 56Fe. It is the nal stage of the fusion of nuclides in massive stars, which forms the nucleus of the iron dances in the atmosphere of HD 77338 [Fe/H] Comp. star Ref.
The production of heavy elements is provided by other mechanisms. They are called s- and r- processes. An s-process, or slow neutron capture, is a formation of heavier nuclei by lighter nuclei through successive neutron capture. The original element in the s-process is 56Fe. The reaction chain ends with 209Bi. It is thought that s-processes occur mostly in stars on the asymptotic giant branch. For the The observations of HD 77338 were carried out s-process to run, an important condition is the abil- as part of the Calan-Hertfordshire Extrasolar Planet ity to produce neutrons. The main neutron source Search (CHEPS) program [4]. The main aim of the program is monitoring a sample of metal-rich stars in the southern hemisphere to search for short pe- 13C + 4He −→ 16O + n, riod planets that have a high probability to transit their host stars, along with improving the existing 22Ne + 4He −→ 25Mg + n. statistics for planets orbiting solar-type and metal- An examples of an s-process reaction is: rich stars. The high-S/N (> 50) and high-resolution (R = 100 000) spectrum of HD 77338, observed with the HARPS spectrograph [10], was reduced using the 56Fe + n −→ 57Fe + n −→ 58Fe + n −→ 59Fe β− standard automated HARPS pipeline and analysed in this work in order to determine the chemical abun- −→59Co + n −→ 60Co β− −→ 60Ni + n−→ . .
dances and other physical parameters of the stellar Elements heavier than H and He are usually called metals in astrophysics. Their concentration is signicantly lower relative to hydrogen and helium, however, they are the source of thousands of spec- Firstly, we selected good absorption lines for tral lines originating from a star's atmosphere. The all elements of interest that are present in spec- abundance of iron depends on a star's age and its tra of the Sun and HD 77338. These lines should position in the galaxy [9]. Metal-rich stars are also not be blended (see [3]) and be intense enough in known to be rich in orbiting giant exoplanets. High both spectra. We selected lists of lines of each ele- metallicity appears to be a major ingredient in the ment that were to be used for the abundance inves- formation of planets through core accretion [5].
tigations. We used line list data, which was taken HD 77338 is one of the most metal-rich stars in from a database of atomic absorption spectra VALD the sample of [3] and in the local Solar neighbour- [7], to compute synthetic spectra of the Sun for hood in general. Its spectral type is given as K0IV a plane-parallel model atmosphere with parameters in the Hipparcos Main Catalogue [17]. However, Teff / log g/[Fe/H] = 5777/4.44/0.0 [15]. The model HD 77338 is not a sub-giant, as labelled in Hipparcos atmosphere was used to compute the synthetic spec- [5], its mass and radius are smaller than those of the tra using WITA6 [13], building a grid of models with Sun: M = 0.93 ± 0.05M, R = 0.88 ± 0.04R. A dierent microturbulent velocities Vt = 0 − 3 km/s parallax of 24.54 ± 1.06 mas for HD 77338 means the with a step size of 0.25 km/s. The shapes of the line star is located at a distance of 40.75 ± 1.76 pc. Its absorption proles were constructed as Voigt func- eective temperature and surface gravity were found tion proles H(a, v), and a classical approach was Teff = 5370 ± 80 K, log g = 4.52 ± 0.06 [5]. More used to compute the damping eects [20]. To com- stellar parameters for HD 77338 and detailed infor- pute the rotational prole we followed the procedure mation about its planetary system are in [5].
described in Gray [2].
Using the Simbad database one can nd informa- All abundance determinations were performed by tion on the previous assessments of abundances in the ABEL8 code [14]. Details of the full procedure the atmosphere of HD 77338 (see Table 1). In most used are described in [16], see [3], also for more de- cases the authors only determine the metallicity of tails on the line selection and tting procedure.
Advances in Astronomy and Space Physics I. O. Kushniruk, Ya. V. Pavlenko, J. S. Jenkins, H. R. A. Jones Fig. 1: The dotted line represents the observed spec- Fig. 2: Cr i absorption line proles computed for a model trum of the Sun, the solid line is the observed spectrum atmosphere of 5315/4.39 and log N(Cr) = −6.09 using of HD 77338, the dashed line shows the synthetic spec- ABEL8 and tted to the observed spectrum of HD 77338 trum computed by Wita618 with log N(Cr) = −6.11 for shown by dashed and solid lines, respectively. The verti- a model atmosphere of 5315/4.39. This plot was used cal lines show the tted parts of Cr sc i line proles.
to detect clean parts of Cr line proles marked here by The model atmosphere for HD 77338 was com- The solar spectrum is well-studied, and abun- puted using the parameters determined by Jenkins et dances for the Sun are known to very high accuracy; al. [5] using the SAM12 code [15]. Again, as the rst therefore it represents a very good template. Fig. 1 step of our analysis, we determined the microturbu- and Fig. 2 illustrate the presence of spectral lines of lent velocity in the atmosphere of HD 77338. The Cr in the observed spectrum of the Sun as a star [8].
minimum of the slope of Ea provides Vt = 0.75 km/s Arrows on the plot show the spectral range which for log N(Fe) = −4.120 ± 0.07 or [Fe/H] = 0.281 was selected to compute proles of two Cri lines to (iteration 1). For other elements we used the same be used later by ABEL8 [14] in the determination of value (Vt = 0.75 km/s).
the abundance of chromium. We employ a similar We carried out 4 iterations to determine all abun- selection in the solar spectrum Sun, and in the spec- dances. In each next step the abundances from trum of HD 77338 for lines of Fe i, Si i, Ni i, Ti i, Na i, the previous determination were used to recompute Mg i, Cu i, Zn i, Mn i, Cr i, Al i, and Ca i.
the model atmosphere by SAM12 [15] and the syn- We veried whether our input data were of suf- thetic spectra. Each time we are approaching self- cient quality and quantity to reproduce the abun- consistency by computing the model atmosphere dances in the atmosphere of the Sun. We computed that relates to the nal metallicity of the star.
abundances for the Sun using the ts of the theoret- In Table 2 we present our results for 12 dierent ical spectra to the proles of the selected lines. In ionic species. We compare our abundances with the that way we can test our method and estimate the solar values, obtained using a model atmosphere of accuracy of our abundance determination. Then, we 5777/4.44/0.00. They are in good agreement with investigated the dependence E each other. Fig. 2 and Fig. 3 show the line proles of a = ∂a/∂E00, where a and E00 are the iron abundance and excitation poten- Cr and Mg calculated using a Vt = 0.75 km/s.
tial of the corresponding radiative transition forming In Fig. 4 we show the dependence of [X/H] on the absorption line. Best ts of the selected lines of atomic number of each element for every iteration.
Fe i in the computed spectra, when compared to their The presence of errors can be explained by the pres- observed proles in the solar spectrum, provide the ence of noise in the selected spectral lines, along with only having a small number of lines to work with a of Vt = 0.75 km/s. The abundances of iron and other elements were then obtained using this for some elements. In Fig. 5 we present the depen- adopted value for the microturbulence; the results dence of [X/Fe] for the nal iteration, where dierent are shown in Table 2. It is worth noting that our elements are shown using dierent plotting shapes, abundances agree with the reference values within depending on their mechanism of formation.
an accuracy of ±0.1 dex.
Advances in Astronomy and Space Physics I. O. Kushniruk, Ya. V. Pavlenko, J. S. Jenkins, H. R. A. Jones Table 2: Abundances in the atmosphere of HD 77338, iteration 4.
log N(X), ABEL8 log N(X) v sin i, km/s −5.403 ± 0.000 −5.767 ± 0.033 2.33 ± 0.44 −5.376 ± 0.029 −5.588 ± 0.023 1.25 ± 0.13 −6.085 ± 0.032 −6.345 ± 0.026 1.91 ± 0.14 −7.670 ± 0.058 −8.133 ± 0.067 2.17 ± 0.44 −4.158 ± 0.038 −4.439 ± 0.023 2.06 ± 0.11 Mg i −4.228 ± 0.058 −4.367 ± 0.095 1.50 ± 0.50 Mn i −5.957 ± 0.097 −6.600 ± 0.046 2.69 ± 0.21 −5.387 ± 0.048 −5.789 ± 0.054 1.94 ± 0.31 −5.367 ± 0.033 −5.756 ± 0.027 2.29 ± 0.15 −4.111 ± 0.054 −4.469 ± 0.058 2.39 ± 0.13 −6.897 ± 0.040 −7.064 ± 0.028 1.88 ± 0.13 −7.028 ± 0.065 −7.375 ± 0.048 2.25 ± 0.25 existence of a planetary system around metal-rich We also computed v sin i for both stars. It is worth noting that we determined all parameters in the framework of a fully self-consistent approach (see [15] for more details). In general, lines of Mn, Cu can not be used to obtain v sin i because these lines usu- ally have several close components, but in our case the parameter v sin i was used to adjust theoretical proles to get the proper ts to the observed special features. We believe that ts to Fe i lines provide reasonable measures of the rotational velocity.
Our results show that the abundances of most el- ements in the atmosphere can be described well by 5710.7 5710.8 5710.9 5711 5711.1 5711.2 5711.3 5711.4 5711.5 the overall metallicity. However, we found an over- abundance of some of the iron peak elements (e. g.
Mn, Cu). Interestingly, Cu is an element formed Fig. 3: Computed by ABEL8 and observed line pro- through the s-process and its abundance follows that les of Mg in the spectrum of HD 77338 with a model of Fe, whereas Zn and elements formed through the atmosphere of 5315/4.39, logN(Mg) = −4.23 shown by p-process, e. g. Ni1, show a noticeable overabun- dashed and solid lines, respectively. The vertical lines dance compared to iron. It would be interesting to show the tted part of Mgi line prole.
compare these results for HD 77338 with other metal rich stars to see if this is a common trend for super metal-rich stars. We plan to investigate this issue in a following paper (Ivanyuk et al. 2015, in prepara- We determined abundances for 12 ionic species in the atmosphere of the Sun and the metal-rich exo- planet host star HD 77338. Our values for the solar abundances are in good agreement with results from JSJ acknowledges the support of the Basal-CATA previous authors, proving the validity of our method.
grant. YP's work has been supported by an FP7 We used the solar spectrum as a reference to select POSTAGBinGALAXIES grant (No. 269193; Inter- the proper list of absorption lines to be used later in national Research Sta Exchange Scheme). Authors the analysis of the HD 77338 spectrum.
thank the compilers of the international databases Our [X/H] correlates well with the condensation used in our study: SIMBAD (France, Strasbourg), temperature of the ions (Tcond), see discussion in [11].
VALD (Austria, Vienna), and the authors of the at- This may indicate the presence of a common shell (in las of the spectrum of the Sun as a star. We thank the past) and can be an additional criterion for the the anonymous Referee for some reasonable remarks and helpful comments.
Advances in Astronomy and Space Physics I. O. Kushniruk, Ya. V. Pavlenko, J. S. Jenkins, H. R. A. Jones Fig. 4: Dependence of [X/H] on atomic number of each Fig. 5: Dependence of [X/Fe] on the atomic number of element for HD 77338. Open circles show values found in each element. The dierent plotting shapes represent the the rst iteration, lled circles are for iteration 2, open dierent formation mechanism of each element. Open squares are for iteration 3, and nally the stars show the circles are for α-elements, diamonds show the thermonu- results for iteration 4.
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