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Chemical Abundances of Planet-Host Stars?

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Chemical Abundances of Planet-Host Stars? A&A 404, 715–727 (2003) Astronomy DOI: 10.1051/0004-6361:20030543 & c ESO 2003 Astrophysics Chemical abundances of planet-host stars? Results for alpha and Fe-group elements A. Bodaghee1,N.C.Santos1;2, G. Israelian3, and M. Mayor1 1 Observatoire de Gen`eve, 51 ch. des Maillettes, 1290 Sauverny, Switzerland 2 Centro de Astronomia e Astrof´ısica da Universidade de Lisboa, Observat´orio Astron´omico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal 3 Instituto de Astrof´ısica de Canarias, 38200 La Laguna, Tenerife, Spain Received 3 February 2003 / Accepted 7 April 2003 Abstract. In this paper, we present a study of the abundances of Si, Ca, Sc, Ti, V, Cr, Mn, Co, and Ni in a large set of stars known to harbor giant planets, as well as in a comparison sample of stars not known to have any planetary-mass companions. We have checked for possible chemical differences between planet hosts and field stars without known planets. Our results show that overall, and for a given value of [Fe/H], the abundance trends for the planet hosts are nearly indistinguishable from those of the field stars. In general, the trends show no discontinuities, and the abundance distributions of stars with giant planets are high [Fe/H] extensions to the curves traced by the field dwarfs without planets. The only elements that might present slight differences between the two groups of stars are V, Mn, and to a lesser extent Ti and Co. We also use the available data to describe galactic chemical evolution trends for the elements studied. When comparing the results with former studies, a few differences emerge for the high [Fe/H] tail of the distribution, a region that is sampled with unprecedented detail in our analysis. Key words. stars: abundances – stars: fundamental parameters – stars: chemically peculiar – stars: evolution – planetary systems – solar neighborhood 1. Introduction not clear) anomalies. The situation concerning the light ele- ments (in particular Li and Be – Garc´ıa L´opez&PerezdeTaoro Stars with planetary companions have been shown to be, on 1998; Deliyannis et al. 2000; Gonzalez & Laws 2000; Ryan average, considerably metal-rich when compared with stars in 2000; Gonzalez et al. 2001; Israelian et al. 2001; Santos et al. the solar neighborhood (e.g. Gonzalez 1997; Fuhrmann et al. 2002; Reddy et al. 2002; Israelian et al. 2003) is not very differ- 1997; Gonzalez 1998; Sadakane et al. 1999; Santos et al. 2000; ent; the debate is just beginning to heat up and many questions Gonzalez et al. 2001; Santos et al. 2001, 2003). The most re- remain open. cent results suggest indeed that the efficiency of planetary for- In almost every case1, the authors have been constrained to mation seems to be strongly dependent on the metal content of compare the results for the star-with-planet samples with other the cloud that gave origin to the star and planetary system. studies in the literature concerning stars without known plane- Until now, however, most chemical studies of the planet tary companions. This might have introduced undesirable sys- hosts used iron as the reference element. The few system- tematic errors, given that the different studies have not used atic studies in the literature concerning other metals (Gonzalez the same set of spectral lines and model atmospheres to derive 1997; Sadakane et al. 1999; Gonzalez & Laws 2000; Santos the stellar parameters and abundances. How important are the et al. 2000; Sadakane et al. 2001; Gonzalez et al. 2001; Smith systematic differences between the various studies? And how et al. 2001; Sadakane et al. 2003) revealed a few possible (but can these differences lead to mistakes? A systematic compar- ison between two groups of stars (with and without planetary Send offprint requests to:N.C.Santos, e-mail: [email protected] companions) is thus needed. ? Based on observations collected at the La Silla Observatory, ESO To try to fill (at least in part) this gap, we present in this pa- (Chile), with the CORALIE spectrograph at the 1.2-m Euler Swiss per a detailed and uniform study of the elements Si, Ca, Sc, Ti, telescope and the FEROS spectrograph at the 1.52-m ESO telescope, V, Cr, Mn, Co, and Ni in a large sample of planet-host stars, and with the VLT/UT2 Kueyen telescope (Paranal Observatory, ESO, in a “comparison” volume-limited sample of stars not known Chile) using the UVES spectrograph (Observing run 67.C-0206, in to have any planetary-mass companions. In Sect. 2, we present service mode), with the TNG and William Herschel Telescopes, both operated at the island of La Palma, and with the ELODIE spectrograph 1 With probably the only exception being the studies of beryllium at the 1.93-m telescope at the Observatoire de Haute Provence. by Santos et al. (2002). Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20030543 716 A. Bodaghee et al.: Chemical abundances of planet-host stars our samples as well as the chemical analysis, and in Sect. 3 we a set of (weak) lines was chosen from the literature, with wave- compare the abundances. The results seem to indicate that no lengths between about 5000 and 6800 Å (the usual spectral do- clearly significant differences exist between the two groups of main of our data). Then, using the Kurucz Solar Atlas (Kurucz stars. In Sect. 4, we use the current data to explore the galac- et al. 1984), we verified each line to check for possible blends. tic chemical evolution trends. Given the high metal content of Only isolated lines were taken; for these, the solar equivalent many stars in our sample, we could access the high [Fe/H] tail widths (EW) were measured. of the distributions with unprecedented detail. We conclude in Using these measured EW values and a solar atmosphere 1 Sect. 5. with (Teff,logg, ξt) = (5770 K, 4.44 dex, 1.00 km s− ), we have derived log g f values for the individual lines from an inverted solar analysis (using the ewfind driver in MOOG). The solar 2. Data, atmospheric parameters, and chemical abundances were taken from Anders & Grevesse (1989). In or- analysis der to do the subsequent chemical analysis for our targets in a reasonable amount of time, we reduced the number of lines per 2.1. The data element to a maximum of 10. The final list of lines used is presented in Table 1. ∼ In a series of recent papers (Santos et al. 2000, 2001, 2003), we For each line in our targets, the EW was measured using have been gathering spectra for most of the planet-host stars the IRAF2 splot tool, and the abundances were computed us- known today. This data has been used to derive precise and uni- ing MOOG with the abfind driver. The average of the abun- form stellar parameters for the target stars, as well as accurate dances for the lines of a given element was then considered. iron abundances. The results published to this point have been In Tables 3, 4, 5, and 6 we summarize the derived abundances used to show that stars with planets are substantially metal-rich for all stars with and without planetary-mass companions. The when compared with average field dwarfs. uncertainties in the tables denote the rms around the mean. The The current paper employs the same spectra and stellar pa- number of lines used in each case is also noted. rameters derived in these works. This allowed access to el- We remark that in particular cases, some lines were elim- emental abundances for 77 stars with low-mass companions inated from the analysis when the quality of the spectrum in (planetary or brown-dwarf candidates). the region of interest was not good enough to permit a reliable Spectra for the comparison sample used in this work were EW measurement (e.g. lower than usual S/N or the presence of obtained with the main goal of deriving the metallicities for a cosmic-rays). large sample of stars in a limited volume around the Sun and not known to harbor any planetary-mass companions (Santos et al. 2001). With the exception of HD 39091 (that was, in the 2.3. Uncertainties meanwhile, found to have a brown-dwarf candidate compan- ion, Jones et al. 2002), the comparison sample used here con- Uncertainties can sway the abundances in various ways. For sists of the remaining 42 objects in Table 1 of Santos et al. example, errors can affect individual lines with random er- (2001). This sample is indeed perfect and appropriate for the rors in the EWs, oscillator strengths and damping constants. current work, as its stellar parameters and iron abundances have Systematic errors in the EWs can arise from unnoticed blends been derived using the same methods as those used for all the or a poor location of the continuum. These errors are hard to planet hosts analyzed in this paper. spot, but they are minimized thanks to the normally high qual- ity of our data. Atmospheric parameter uncertainties should be We should caution that this comparison sample is built the primary source of abundance error in a species with many from a list of stars that are surveyed for planets, but for which lines, whereas inaccuracies in the EWs are eventually more im- none have yet been found. Of course, this does not mean that portant when only a few lines are available. these stars do not have any planetary mass companions at all (they might have e.g.
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