Spectroscopic Metallicities for Planet-Host Stars: Extending the Samples
Total Page：16
File Type：pdf, Size：1020Kb
A&A 437, 1127–1133 (2005) Astronomy DOI: 10.1051/0004-6361:20052895 & c ESO 2005 Astrophysics Spectroscopic metallicities for planet-host stars: Extending the samples N. C. Santos1,2, G. Israelian3, M. Mayor2,J.P.Bento1,P.C.Almeida1,S.G.Sousa1, and A. Ecuvillon3 1 Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal e-mail: [email protected] 2 Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland 3 Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain Received 17 February 2005 / Accepted 23 March 2005 Abstract. We present stellar parameters and metallicities for 29 planet-host stars, as well as for a large volume-limited sample of 53 stars not known to be orbited by any planetary-mass companion. These stars add to the results presented in our previous series of papers, providing two large and uniform samples of 119 planet-hosts and 94 “single” stars with accurate stellar param- eters and [Fe/H] estimates. The analysis of the results further confirms that stars with planets are metal-rich when compared with average field dwarfs. Important biases that may compromise future studies are also discussed. Finally, we compare the metallicity distributions for single planet-hosts and planet-hosts in multiple stellar systems. The results show that a small differ- ence cannot be excluded, in the sense that the latter sample is slighly overmetallic. However, more data are needed to confirm this correlation. Key words. stars: abundances – stars: fundamental parameters – stars: chemically peculiar – stars: planetary systems – stars: planetary systems: formation 1. Introduction the metal content of the Sun have an orbiting planetary-mass companion. The discovery that stars with giant planet companions are par- Interestingly, these observational results are now being cor- ticularly metal-rich when compared to single field dwarfs is roborated by theoretical models of planet formation, and give providing crucial clues to our understanding of the processes support to the core-accretion model to explain the origin of of planetary formation and evolution. This fact, first discussed the now discovered giant planets (Pollack et al. 1996; Alibert by Gonzalez (1997) and Gonzalez (1998), was later proved by et al. 2004). Current models can even reasonably explain the the uniform analysis of large samples of planet-host and field observed [Fe/H] distribution of planet-host stars (Ida & Lin “single” stars (Santos et al. 2001). 2004a,b). However, and as discussed in Santos et al. (2004b), Further refinements to these studies have shown that the current data still cannot exclude that giant planets may also be probability of finding (and presumably forming) a planet, at formed by the disk instability model (Boss 1997, 2002; Mayer least of the kind now discovered by radial-velocity surveys, et al. 2002), in particular planets orbiting low metallicity stars. is a strong function of the stellar metal content (Santos et al. In the last few years we have published a series of pa- 2001, 2003, 2004b; Reid 2002; Fischer & Valenti 2005). pers regarding the study of the stellar metallicity-giant planet Although the exact dependence is still not completely settled connection (Santos et al. 2000, 2001, 2003, 2004b) (hereafter (see Santos et al. 2004b, and references therein), it was shown Papers I, II, III and IV, respectively). In the last three of these that about 25% of solar type stars with twice the solar metal- papers we have presented a spectroscopic analysis of a sample licity are orbited by a planet, while less than 5% of stars with of 41 “single” field stars, used as a reference to study the excess metallicity observed in planet hosts. In Paper IV we presented Based on observations collected at La Silla Observatory, ESO, a uniform spectroscopic analysis of 98 extra-solar planet-host Chile, with the FEROS spectrograph at the 2.2-m MPI/ESO tele- scope (programs 72.C-0033 and 74.C-0135) and the CORALIE spec- stars, including most of the known objects. The continuation of trograph at the 1.2-m Euler Swiss Telescope, with the UVES spec- this work is extremely important, since the search for new pos- trograph at the VLT 8.2-m Kueyen telescope (programs 074.C-0134), sible correlations between the presence (and properties) of ex- and with the SARG spectrograph at the TNG telescope, operated at oplanets and the chemical abundances of their host stars needs the island of La Palma. increased samples. Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20052895 1128 N. C. Santos et al.: Spectroscopic metallicities for planet-host stars In this paper we add 53 new stars to our “comparison” in December 2004, using the UVES spectrograph at the sample, as well as precise and uniform metallicity estimates VLT 8.2-m Kueyen telescope (run ID 074.C-0134). These lat- for a further 29 known planet-hosts, most of which have no ter spectra were reduced using the UVES pipeline and have a previous spectroscopic metallicity estimate. In Sects. 2 and 3 resolution R ∼ 75 000. we present the observed samples, and in Sect. 4 we review For HD 104985 and HD 219449 we further obtained spec- the metallicity distribution of planet-host stars, as well as of tra with a resolution of 57 000 using the SARG spectrograph, the whole volume-limited comparison sample of 94 solar-type at the TNG telescope (La Palma Observatory, Spain). For a few F-G-K dwarfs. The analysis confirms, as expected, that stars other recently-announced planet-hosts, stellar parameters were with planets are clearly metal-rich when compared to average derived using the HARPS spectrograph (R ∼ 100 000 – for de- field dwarfs. In Sect. 4.1 we review a few possible biases that tails see e.g. Pepe et al. 2004). These parameters were also pre- should be considered when studying the metallicity of planet- sented in the papers announcing the discovery of the exoplanets host stars. In Sect. 5 we compare the metallicity distributions (see Table 1). for “single” planet-host stars with that found for planet-hosts in The list of planet-host stars presented in this paper is shown multiple stellar systems. We conclude in Sect. 6. in Table 1. For planet-discovery references see the Geneva Planet-Search web pages1. / 2. The samples Adding the 97 planet-host stars whose [Fe H] values were listed in Paper IV2 to the results listed in Table 1, we now 2.1. New comparison sample have a uniform sample of metallicities and stellar parameter for 119 stars with orbiting planets. From the whole known In Papers II through IV we compared the metallicity distribu- planet-host star sample, the only 4 objects for which we could tion for stars with planets with a volume-limited sample of h h not obtain metallicity estimates are GJ 876 and GJ 436 (both stars having right-ascensions (RA) between 20 and 9 ,and M-dwarfs – see Bonfils et al. 2005), HD 45350 and HD 13189 a distance below 20 pc as derived from Hipparcos parallaxes (a giant star). To these we add the transiting planets detected (ESA 1997). All these stars belong to the CORALIE southern by the OGLE survey (Konacki et al. 2003; Bouchy et al. 2004; planet search sample (Udry et al. 2000), have spectral types be- Pont et al. 2004), three currently with poor estimates given tween F8 and M1, and were not found to harbor any planetary the low quality of the now available spectra, as well as TrEs-1 companion. (Alonso et al. 2004). In order to extend the number of comparison stars in our samples, we have now observed a complementary sample of CORALIE stars with RA between 9h and 20h (and d < 20 pc). 3. Spectroscopic analysis These stars were observed using the FEROS spectrograph at the 2.2-m ESO/MPI telescope (ESO, La Silla) in March 2004 The spectroscopic analysis was done in LTE using the 2002 3 (observing run ID 072.C-0033). Data reduction was done us- version of the code MOOG (Sneden 1973) and a grid of ing the FEROS pipeline, and the wavelength calibration was Kurucz Atlas plane-parallel model atmospheres (Kurucz 1993). done using the spectrum of a ThAr lamp. The final spectra have Stellar parameters and metallicities were derived as in a resolution R =∆λ/λ = 48 000, and the S/N between ∼200 Santos et al. (2004b), based on the Equivalent Widths (EW) and 400. of 39 Fe and12Fe week lines, and by imposing excitation A few stars from the former comparison sample for which and ionization equilibrium. The line EW were measured using 4 we could not obtain a spectrum before were also observed, IRAF “splot” and “bplot” routines within the echelle pack- both using FEROS and the CORALIE spectrograph at the age. A Fortran 77 code that uses a Downhill Simplex Method 1.2-m Euler Swiss telescope, also at La Silla observatory (R = (Press et al. 1992) was then used to find the best solution in the 50 000). (stellar) parameter space once the line EWs are measured. The comparison sample presented in this paper is listed in The results of this analysis are presented in Tables 1 and 2. Table 2. Together with Table 5 of Paper IV, this constitutes As shown in Paper IV, this methodology gives excellent results a large volume-limited sample of 94 “single” stars (with no for the derived stellar parameters, compatible with other spec- known planetary-mass companions).