Stellar [Fe/H] and the frequency of exo-Neptunes

Spectroscopic parameters for 451 stars in the HARPS GTO planet search program: Stellar [Fe/H] and the frequency of exo-Neptunes S. G. Sousa∗,† , N. C. Santos∗,∗∗ , M. Mayor∗∗ , S. Udry∗∗ , L. Casagrande‡, G. Israelian§ , F. Pepe∗∗ , D. Queloz∗∗ and M. J. P. F. G. Monteiro∗,† Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal Departamento de Matemática Aplicada, Faculdade de Ciências da Universidade do Porto, Portugal ∗∗ Observatoire de Genève, 51 Ch. des Mailletes, 1290 Sauverny, Switzerland ‡ University of Turku - Tuorla Astronomical Observatory, Väisäläntie 20, FI-21500 Piikkiö, Finland § Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain ∗

†

Abstract. We present a catalogue with accurate stellar parameters for the 451 stars that compose the HARPS Guaranteed Time Observations (GTO) “high precision” sample. We use our results, together with the others found in the literature, to study the metallicity-planet correlation, namely for very low mass planets. The results presented here suggests that contrarily to their jovian couterparts, neptune-like planets do not form preferentially around metal-rich stars. The ratio of jupiter-toneptunes is also an increasing function of stellar metallicity. These results are discussed in the context of the core-accretion model for planet formation. Keywords: stars: fundamental parameters – stars: planetary systems – stars: planetary systems: formation PACS: 97.10.Tk

THE SPECTROSCOPIC CATALOGUE OF THE HARPS GTO “HIGH PRECISION” SAMPLE Extra-solar planets have continued to be discovered, since the first detection of a giant planet orbiting a solar-like star [1], including several planets in the Neptune-mass regime [e.g. 2, 3, 4]. A by-product of these planet searches are the high-quality spectra, of high S/N and high-resolution. We present a catalogue of spectroscopic stellar parameters for the HARPS Guaranteed Time Observations (GTO) “high precision” sample. Figure 1 presents some characteristics of the sample. This plot shows that the sample is composed mainly of main-sequence solar-type stars. We also present the metallicity distribution that has a mean value of about −0.09 dex. This is compatible with the comparison samples presented in the work of Santos et al. [2]. The catalog is electronically available on CDS and also at the author webpage1 . 1

http://www.astro.up.pt/∼sousasag/harps_gto_catalogue.html

CREDIT LINE (BELOW) TO BE INSERTED ON THE FIRST PAGE OF EACH PAPER CP1094, Cool Stars, Stellar Systems and the Sun: 15th Cambridge Workshop, edited by E. Stempels © 2009 American Institute of Physics 978-0-7354-0627-8/09/$25.00

477 Downloaded 17 Jul 2009 to 193.136.48.52. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp

FIGURE 1. In the left panel, we show the distribution of the sample stars in the H-R diagram. The filled circles represent the planet hosts in our sample. We also plot some evolutionary tracks computed with CESAM for a 1.0, 1.1 and 1.2 solar masses. In the right panel, we present the metallicity distribution of the sample.

PLANET-HOSTS IN THE SAMPLE There are 66 planet hosts belonging to our sample, three host neptunian planets (HD4308, HD69830 and HD160691). The first two of these only harbor neptunian planets [5, 6], while the third also has Jupiter-like planets [7]. In Fig.2, we present the metallicity distribution of these two types of host stars. Althought the numbers are small, we find a wider spread of metallicities for stars hosting Neptunian-like planets.

FIGURE 2. Metallicity distribution of jovian planet hosts and neptunian planet hosts within the HARPS GTO “high precision” spectroscopic catalogue.


FIGURE 3. Metallicity distribution for the sample presented in this work (dotted line), for stars hosting neptunian planets (dashed line), and for stars exclusively hosting neptunian planets (full line). The mean metallicity of each distribution is indicated on the plot.

NEPTUNIAN PLANET HOSTS To increase the number of neptunian-planet hosts, we added all those we found in the literature. This includes M stars where the metallicity determination can be difficult; we therefore use the mean of the [Fe/H] values quoted in the literature. star Gl 581 Gl 876 HD69830 HD160691 Gl 674 55 Cnc e HD4308 HD190360 HD219828 GJ 436 Gl 176

[Fe/H]

jov.?

M p sini

-0.33/-0.25/-0.29 -0.12/-0.03/-0.08 -0.06 0.30 -0.28 0.33 -0.34 0.24 0.19 -0.32/0.02/-0.15 -0.10

no yes no yes no yes no yes yes no no

5.09 5.72 10.49 13.99 11.76 10.81 14.94 18.12 20.98 22.89 24.16

[Fe/H] reference Bean et al. [8] / Bonfils et al. [9] / Mean Bean et al. [8] / Bonfils et al. [9] / Mean This work This work Bonfils et al. [10] Santos et al. [2] This work Santos et al. [2] Melo et al. [11] Bean et al. [8] / Bonfils et al. [9] / Mean Endl et al. [12]

Using the values of [Fe/H] listed in the table, we compared the metallicity distribution of the spectroscopic catalogue presented in this paper, which corresponds to the [Fe/H] distribution of the solar neighborhood, with two different distributions. The first, composed of all stars presented in the table, and the second, composed of stars that host only neptunian planets (“no” in the table). The result can be observed


in Fig. 3. The metallicity distribution of the catalogue presented in this work has a mean metallicity of < [Fe/H] >= −0.09, while the metallicity distribution of stars known to harbor at least one neptunian planet corresponds to a mean metallicity of < [Fe/H] >= −0.03. Finally, stars known to host only neptunian planets have a mean metallicity of < [Fe/H] >= −0.21. These numbers imply that the few stars with low-mass planets found appear to follow a trend in metallicity that differ from well-established relations for giant planets as first pointed out by Udry et al. [5]. This result is supported by planet-formation models based on the core-accretion paradigm [13, 14]. These works suggest that neptunian planets should be found in a wider range of stellar metallicities. Lower-mass planets could even be preferentially found orbiting metal-poorer stars.

ACKNOWLEDGMENTS S.G.S and N.C.S. would like to acknowledge the support from the Fundação para a Ciência e Tecnologia (Portugal) in the form of fellowships and grants SFRH/BD/17952/ 2004, POCI/CTE-AST/56453/2004, PPCDT/CTE-AST/56453/2004 and PTDC/CTEAST/66181/2006, with funds from the European program FEDER. We thank the anonymous referee for the useful comments and suggestions.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

M. Mayor, and D. Queloz, Nature 378, 355–+ (1995). N. C. Santos, G. Israelian, and M. Mayor, A&A 415 (2004). B. E. McArthur, M. Endl, W. D. Cochran, G. F. Benedict, D. A. Fischer, G. W. Marcy, R. P. Butler, D. Naef, M. Mayor, D. Queloz, S. Udry, and T. E. Harrison, APJ 614, L81–L84 (2004). S. Udry, X. Bonfils, X. Delfosse, T. Forveille, M. Mayor, C. Perrier, F. Bouchy, C. Lovis, F. Pepe, D. Queloz, and J.-L. Bertaux, A&A 469, L43–L47 (2007). S. Udry, M. Mayor, W. Benz, J.-L. Bertaux, F. Bouchy, C. Lovis, C. Mordasini, F. Pepe, D. Queloz, and J.-P. Sivan, A&A 447, 361–367 (2006). C. Lovis, M. Mayor, F. Pepe, Y. Alibert, W. Benz, F. Bouchy, A. C. M. Correia, J. Laskar, C. Mordasini, D. Queloz, N. C. Santos, S. Udry, J. Bertaux, and J. Sivan, Nature 441, 305–309 (2006). F. Pepe, A. C. M. Correia, M. Mayor, O. Tamuz, J. Couetdic, W. Benz, J.-L. Bertaux, F. Bouchy, J. Laskar, C. Lovis, D. Naef, D. Queloz, N. C. Santos, J.-P. Sivan, D. Sosnowska, and S. Udry, A&A 462, 769–776 (2007). J. L. Bean, G. F. Benedict, and M. Endl, APJ 653, L65–L68 (2006). X. Bonfils, T. Forveille, X. Delfosse, S. Udry, M. Mayor, C. Perrier, F. Bouchy, F. Pepe, D. Queloz, and J.-L. Bertaux, A&A 443, L15–L18 (2005). X. Bonfils, M. Mayor, X. Delfosse, T. Forveille, M. Gillon, C. Perrier, S. Udry, F. Bouchy, C. Lovis, F. Pepe, D. Queloz, N. C. Santos, and J.-L. Bertaux, A&A 474, 293–299 (2007). C. Melo, N. C. Santos, W. Gieren, G. Pietrzynski, M. T. Ruiz, S. G. Sousa, F. Bouchy, C. Lovis, M. Mayor, F. Pepe, D. Queloz, R. da Silva, and S. Udry, A&A 467, 721–727 (2007). M. Endl, W. D. Cochran, R. A. Wittenmyer, and A. P. Boss 709 (2007), 0709.0944. S. Ida, and D. N. C. Lin, APJ 616, 567–572 (2004). W. Benz, C. Mordasini, Y. Alibert, and D. Naef, “Extrasolar giant planet formation: Monte Carlo simulations,” in Tenth Anniversary of 51 Peg-b: Status of and prospects for hot Jupiter studies, edited by L. Arnold, F. Bouchy, and C. Moutou, 2006, pp. 24–34.
