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The Properties of Planets Around Giant Stars⋆⋆⋆

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The Properties of Planets Around Giant Stars⋆⋆⋆ A&A 566, A113 (2014) Astronomy DOI: 10.1051/0004-6361/201323345 & c ESO 2014 Astrophysics The properties of planets around giant stars, M. I. Jones1,2, J. S. Jenkins2,P.Bluhm3,P.Rojo2,4, and C. H. F. Melo5 1 Department of Electrical Engineering and Center of Astro-Engineering UC, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile e-mail: [email protected] 2 Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Casilla 36-D Las Condes, Santiago, Chile 3 Departamento de Astronomía, Universidad de Concepción, Casilla 160-C Concepción, Chile 4 Department of Astronomy, Cornell University, 610 Space Sciences Building, Ithaca NY14853-6801, USA 5 European Southern Observatory, Casilla 19001 Santiago, Chile Received 27 December 2013 / Accepted 22 April 2014 ABSTRACT Context. More than 50 exoplanets have been found around giant stars, revealing different properties when compared to planets orbiting solar-type stars. In particular, they are super-Jupiters and are not found orbiting interior to ∼0.5 AU. Aims. We are conducting a radial velocity study of a sample of 166 giant stars aimed at studying the population of close-in planets orbiting giant stars and how their orbital and physical properties are influenced by the post-MS evolution of the host star. Methods. We have collected multiepoch spectra for all of the targets in our sample. We have computed precision radial velocities from FECH/CHIRON and FEROS spectra, using the I2 cell technique and the simultaneous calibration method, respectively. Results. We present the discovery of a massive planet around the giant star HIP 105854. The best Keplerian fit to the data leads to an orbital distance of 0.81 ± 0.03 AU, an eccentricity of 0.02 ± 0.03 and a projected mass of 8.2 ± 0.2 MJ. With the addition of this new planet discovery, we performed a detailed analysis of the orbital properties and mass distribution of the planets orbiting giant stars. We show that there is an overabundance of planets around giant stars with a ∼ 0.5−0.9 AU, which might be attributed to tidal decay. Additionally, these planets are significantly more massive than those around MS and subgiant stars, suggesting that they grow via accretion either from the stellar wind or by mass transfer from the host star. Finally, we show that planets around evolved stars have lower orbital eccentricities than those orbiting solar-type stars, which suggests that they are either formed in different conditions or that their orbits are efficiently circularized by interactions with the host star. Key words. planetary systems – techniques: radial velocities – stars: evolution 1. Introduction giant phase. Unfortunately, giant stars present a high level of jit- ff ter (e.g., Sato et al. 2005), limiting the detection only to giant For the past two decades, di erent methods have been devel- planets (K ∼> 30 m s−1). oped to detect and characterize extrasolar planets, giving rise So far, more than 50 giant planets have been found around to the discovery of more than 1000 planetary systems. Among giant stars, revealing interesting properties that seem to contrast them, the radial velocity (RV) technique has led to the detec- the properties of giant planets discovered around dwarf stars. tion of a significant fraction of these systems. Unfortunately, First, there is no planet orbiting any giant star in orbits inte- this method is strongly sensitive to the spectral properties of rior to a ∼ 0.5 AU, in contrast to what is observed in solar- the object, and hence it is mainly restricted to solar-type stars. type stars, where there is a large number of discovered close- On the one hand, very low-mass stars (such as mid and late in gas giants. This observational result suggests that the inner- M stars) are faint and their spectral distribution peaks in the most planets are destroyed by the host star during the expansion infrared (IR) region, hence they have been mostly excluded in phase of the stellar envelope. Moreover, several theoretical stud- optical RV surveys. On the other hand, intermediate- and high- ies have shown that the tidal interaction between the planet and ∼> mass stars (M 1.5 M ) are too hot and thus they have an the stellar convective envelope leads to the loss of orbital an- optical spectrum that is characterized by few and broad lines. gular momentum. As a result, close-in planets are expected to However, after the main-sequence (MS), they expand becom- spiral inward and hence are subsequently engulfed by the host ing cooler, and also their rotational velocity is strongly reduced. star (e.g., Livio & Soker 1983; Kunitomo et al. 2011). However, As a result, their spectra present thousands of narrow absorp- Johnson et al. (2007) showed that close-in planets are also absent tion lines, becoming suitable targets for precision RV surveys. around intermediate-mass subgiant stars. Additionally, Bowler Therefore, enough RV information can be obtained to search et al. (2010) confirmed the Johnson et al. findings and also they for planets around intermediate-mass stars and to study the dy- showed that planets around intermediate-mass stars tend to re- namical interaction between them and the host star during the side at a larger orbital distance (a ∼> 1 AU) than planets around solar-type stars. According to these results, planets around gi- Based on observations collected at La Silla – Paranal Observatory ff under programs IDs 085.C-0557, 087.C.0476, 089.C-0524 and ant stars are not found in close-in orbits because of a di erent 090.C-0345. formation scenario around intermediate-mass stars instead of a The RV Table is only available at the CDS via anonymous ftp to dynamical evolutionary process, such as tidal orbital decay. cdsarc.u-strasbg.fr (130.79.128.5)orvia Second, the mass distribution of planets around giant stars is http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/566/A113 different than for planets around dwarf stars. In particular, their Article published by EDP Sciences A113, page 1 of 9 A&A 566, A113 (2014) occurrence rate is higher and they are on average more massive barycentric correction performed by the FEROS pipeline was (Döllinger et al. 2009). This observational result is supported by disabled. theoretical studies that predict an increase in the frequency of Additionally, we obtained 36 spectra using the old fiber giant planets with the mass of the host star (e.g., Kennedy & echelle spectrograph (FECH) and CHIRON (Tokovinin et al. Kenyon 2008). 2013), both of them installed in the 1.5 m telescope, at The Cerro Last, planet-hosting giant stars are on average metal-poor, Tololo Inter-American Observatory. The two spectrographs are in direct contrast to the well known planet-metallicity con- equipped with an I2 cell that is placed in the light beam, which nection that is observed in MS stars (Gonzalez 1997;Santos superimposes thousands of narrow absorption lines in the spec- et al. 2001; Fischer & Valenti 2005), which seems to be also tral region between ∼5000−6000 Å. The I2 absorption spectrum present in the low-mass planetary regime (Jenkins et al. 2013a). was used as a precise wavelength reference (see Sect. 2.2). We Following the discussion presented in Schuler et al. (2005), used the narrow slit with FECH (R ∼ 45 000) and the slit mode Pasquini et al. (2007) argued that the enhanced metallicity of with CHIRON (R ∼ 90 000). The typical observing time was MS host stars is explained by pollution, instead of being a pri- 700−800 s, leading to a S /N ∼> 50. The extraction of the data mordial feature of the protoplanetary disk. In this scenario, a sig- was done in a similar fashion as that for the FEROS spectra, us- nificant amount of material, including planetesimals and planet ing the automatic reduction pipeline that is offered to CHIRON cores, fall into the star, hence increasing the metal abundance users. For more details see Jones (2013). in its atmosphere. However, after these polluted stars evolve ff o the main-sequence, they develop large convective envelopes, 2.2. Radial velocity computations where the metal excess is diluted. This idea might explain the aforementioned discrepancy between MS and giant host stars The radial velocities from the FEROS spectra were computed metallicity distribution. However, more recently, Mortier et al. by applying the simultaneous calibration method (Queloz et al. (2013) showed that RV surveys of giant stars are biased toward 1999). The details of the calculations are explained in Jones et al. metal-poor stars, which explains why planets are preferentially (2013) and Jones & Jenkins (2014), but the basic ideas of the found around metal-poor giants. On the other hand, giant plan- procedure are also described below. ets around intermediate-mass stars might be efficiently formed First, we computed the cross-correlation function (CCF) be- by the disk instability mechanism (Boss 1997). In this scenario tween the observed spectrum and a template. In this case the the metal content of the protoplanetary disk does not play a sig- template corresponds to a high S/N observation of HIP 105854. nificant role on the formation of planets, hence it might explain We repeated this procedure on chunks of ∼50 Å width (four why we do not see a trend toward metal richness in host giant chunks per order, from 35 different orders). We then used an it- stars. erative rejection code to compute the mean radial velocity from In this paper, we present the discovery of a massive planet all of the chunk velocities. In a similar fashion, we also applied around the red giant branch (RGB) star HIP 105854, as part this procedure to the sky fiber, which is illuminated during the of the EXoPlanets aRound Evolved StarS (EXPRESS) project science observation by a ThArNe lamp.
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