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CAUP Researchers: Nuno C. Santos, Sérgio A. G. Sousa
Team at CAUP: Origin and Evolution of Stars and Planets
Other Researchers: Michel Mayor (CH), W. Benz (CH), François Bouchy (FR), Pedro Figueira (CH), Gaspare Lo Curto (ESO), Christophe Lovis (CH), Claudio H. F. Melo (ESO), Claire Moutou (FR), Dominique Naef (CH), Francesco Pepe (CH), Didier Queloz (CH), Stéphane Udry (CH)

The HARPS search for southern extra-solar planets
XXI. Three new giant planets orbiting the metal-poor stars HD5388, HD181720, and HD190984
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Astronomy and Astrophysics, Volume 512, pp. A47 (2010)

We present the discovery of three new giant planets around three metal-deficient stars: HD5388 b (1.96 MJup), HD181720 b (0.37 MJup), and HD190984 b (3.1 MJup). All the planets have moderately eccentric orbits (ranging from 0.26 to 0.57) and long orbital periods (from 777 to 4885 days). Two of the stars (HD181720 and HD190984) were part of a program searching for giant planets around a sample of ∼100 moderately metal-poor stars, while HD5388 was part of the volume-limited sample of the HARPS GTO program. Our discoveries suggest that giant planets in long period orbits are not uncommon around moderately metal-poor stars.


Figure 1 | Radial velocity measurements for HD 181720 as a function of time and best Keplerian fit to the data, which reveals the presence of a giant planet orbiting around this star.The upper panel shows the radial velocity time series while the lower plot presents the residuals of the fit for the orbit derived [2].

Soon after the discovery of the first extra-solar planets, astronomers found evidence for a positive correlation between the metallicity of the host stars (reflecting the amount of solid material in the proto-planetary disk) and the presence of a giant planet. This observational result suggests that these planets are formed more easily around metal-rich stars, a result that is consistent with the core accretion model for planet formation.

To better constrain the frequency of planets around metal-poor stars, a radial velocity survey for planets orbiting a sample of 104 metal-poor dwarfs started in 2003 making use of the HARPS spectrograph (in the context of the HARPS GTO program). Preliminary results of this survey indicate that at least three of these stars host a planet (including the one previously reported in [1]). Further results for this survey, which revealed a total of 3 giant planets and another promising candidate, were published in early 2011, in two separate papers, [2] and [3]. In addition to this sample, in this highlighted paper, we also present the results for a star from a large volume-limited survey for giant planets, part of another HARPS GTO program.

The radial-velocity time series of each of the stars (example shown in Figure 1), obtained between September 2003 and September 2009, was studied to search for any signal present in the data that could indicate the presence of orbiting planets. This study detected the presence of three giant planets orbiting the stars HD 5388, HD 181720, and HD 190984 (Table 1).

Parameter HD 5388 b HD 181720 b HD190984 b
Minimum mass (MJup) 1.96 0.37 3.1
Period (days) 777 956 4885
Semi-major axis (AU) 1.76 1.78 5.5
Eccentricity 0.40 0.26 0.57
Table 1 | Parameters of the 3 planets detected.

These results indicate that the frequency of planets around stars with about 1/4 the metal content of the Sun should be at least as high as ∼3% and further analysis of the data allowed us to take this estimate to ∼11% [2]. This suggests that overall, giant planetary systems around solar type stars may be more frequent that previously thought.

Interestingly, no short period giant planets were found orbiting stars of the sample. All giants have long period orbits. Whether this trend reflects a true physical correlation or is simply due to the small number statistics is still to be confirmed, but if real it may provide new constraints for the models of planet formation and evolution.

This project was led by the CAUP member Nuno C. Santos and the spectroscopic analysis of the stars in this survey was performed by Sérgio G. Sousa, also a CAUP member; both of them took part in the observation of the target stars.

[1] N. C. Santos, M. Mayor, F. Bouchy, F. Pepe, D. Queloz, S. Udry, The HARPS search for southern extra-solar planets. XII. A giant planet orbiting the metal-poor star HD 171028, Astronomy and Astrophysics, Volume 474, Issue 2, pp. 647-651 (2007)

[2] N. C. Santos, M. Mayor, X. Bonfils, X. Dumusque, F. Bouchy, P. Figueira, C. Lovis, C. Melo, F. Pepe, D. Queloz, D. Ségransan, S. G. Sousa, S. Udry, The HARPS search for southern extrasolar planets XXV. Results from the metal-poor sample, Astronomy and Astrophysics (accepted)

[3] S. G. Sousa, N. C. Santos, G. Israelian, C. Lovis, M. Mayor, P. B. Silva and S. Udry, Spectroscopic characterization of a sample of metal-poor solar-type stars from the HARPS planet search program, Astronomy and Astrophysics (accepted)

Instituto de Astrofísica e Ciências do Espaço

O Instituto de Astrofísica e Ciências do Espaço é (IA) é uma nova, mas muito aguardada, estrutura de investigação com uma dimensão nacional. Ele concretiza uma visão ousada, mas realizável para o desenvolvimento da Astronomia, Astrofísica e Ciências Espaciais em Portugal, aproveitando ao máximo e realizando plenamente o potencial criado pela participação nacional na Agência Espacial Europeia (ESA) e no Observatório Europeu do Sul (ESO). O IA é o resultado da fusão entre as duas unidades de investigação mais proeminentes no campo em Portugal: o Centro de Astrofísica da Universidade do Porto (CAUP) e o Centro de Astronomia e Astrofísica da Universidade de Lisboa (CAAUL). Atualmente, engloba mais de dois terços de todos os investigadores ativos em Ciências Espaciais em Portugal, e é responsável por uma fração ainda maior da produtividade nacional em revistas internacionais ISI na área de Ciências Espaciais. Esta é a área científica com maior fator de impacto relativo (1,65 vezes acima da média internacional) e o campo com o maior número médio de citações por artigo para Portugal.

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