Searching for Li-rich giants in a sample of 12 open clusters: Li enhancement in two stars with substellar companions

The aim of this work is to search for Li-rich giants in a sample of clusters where planets have been searched, thus we can study the planet engulfment scenario to explain Li replenishment using a proper comparison sample of stars without detected giant planets. We derived Li abundances for a sample of 67 red giant stars in 12 different open clusters using standard spectral synthesis techniques and high resolution spectra (from HARPS and UVES). We also determined masses, ages and radius from PARSEC stellar isochrones to constrain the evolutionary stage of these stars. We find three stars in different clusters with clearly enhanced Li abundances compared to other stars within the cluster. Interestingly, the only two stars with a detected substellar companion in our sample belong to that group. One of the planet hosts, NGC2423No3, might lie close to the luminosity bump on the HR diagram, a phase where Li production by the Cameron-Fowler process is supported by extra-mixing to bring fresh Li up to the surface. On the other hand, NGC4349No127 is a more massive and more evolved giant that does not seem to be in the evolutionary phase where other Li-rich stars are found. We discuss the possibility that the Li enhancement of this star is triggered by the engulfment of a planet, considering that close-in planets hardly survive the RGB-tip and the early-AGB phases.

[1]  S. G. Sousa,et al.  ARES v2 - new features and improved performance , 2015, 1504.02725.

[2]  M. Tsantaki,et al.  Chemical abundances and kinematics of 257 G-, K-type field giants. Setting a base for further analysis of giant-planet properties orbiting evolved stars , 2015, 1503.08346.

[3]  S. Lucatello,et al.  ON THE SERENDIPITOUS DISCOVERY OF A Li-RICH GIANT IN THE GLOBULAR CLUSTER NGC 362 , 2015, 1502.01341.

[4]  T. Granzer,et al.  Evidence for enhanced mixing on the super-meteoritic Li-rich red giant HD 233517 , 2015 .

[5]  C. Prieto,et al.  THE PUZZLING Li-RICH RED GIANT ASSOCIATED WITH NGC 6819 , 2015, 1501.05625.

[6]  M. Tsantaki,et al.  Li abundances in F stars: planets, rotation, and Galactic evolution , 2014, 1412.4618.

[7]  M. Tsantaki,et al.  Spectroscopic parameters for solar-type stars with moderate/high rotation. New parameters for 10 planet-hosts , 2014, 1407.6765.

[8]  G. Carraro,et al.  A super lithium-rich red-clump star in the open cluster Trumpler 5 , 2014, 1403.6461.

[9]  J. De Ridder,et al.  OLD PUZZLE, NEW INSIGHTS: A LITHIUM-RICH GIANT QUIETLY BURNING HELIUM IN ITS CORE , 2014, 1402.6339.

[10]  R. Rebolo,et al.  Li depletion in solar analogues with exoplanets - Extending the sample , 2013, 1311.6414.

[11]  M. Tsantaki,et al.  Deriving precise parameters for cool solar-type stars Optimizing the iron line list ?;??;??? , 2013, 1304.6639.

[12]  B. Anthony-Twarog,et al.  A LITHIUM-RICH RED GIANT BELOW THE CLUMP IN THE KEPLER CLUSTER, NGC 6819 , 2013, 1303.2984.

[13]  L. Girardi,et al.  parsec: stellar tracks and isochrones with the PAdova and TRieste Stellar Evolution Code , 2012, 1208.4498.

[14]  K. Cunha,et al.  OBSERVABLE SIGNATURES OF PLANET ACCRETION IN RED GIANT STARS. I. RAPID ROTATION AND LIGHT ELEMENT REPLENISHMENT , 2012, 1208.1775.

[15]  G. Nowak,et al.  BD+48 740—Li OVERABUNDANT GIANT STAR WITH A PLANET: A CASE OF RECENT ENGULFMENT? , 2012, 1206.4938.

[16]  N. Santos,et al.  Metallicities for 6 nearby open clusters from high-resolution spectra of giant stars. [Fe/H] values for a planet search sample , 2012, 1201.1108.

[17]  Padova,et al.  Lithium abundances along the RGB: FLAMES-GIRAFFE spectra of a large sample of low-mass Bulge stars , 2011, 1111.3572.

[18]  K. Biazzo,et al.  IRAS12556-7731: a "chamaeleonic" lithium-rich M-giant , 2011, 1106.3471.

[19]  S. Ida,et al.  PLANET ENGULFMENT BY ∼1.5–3 M☉ RED GIANTS , 2011, 1106.2251.

[20]  Brno,et al.  Lithium-rich giants in the Galactic thick disk , 2011, 1103.1658.

[21]  B. E. Reddy,et al.  ORIGIN OF LITHIUM ENRICHMENT IN K GIANTS , 2011, 1102.2299.

[22]  K. Cunha,et al.  THE SUPER LITHIUM-RICH RED GIANT RAPID ROTATOR G0928+73.2600: A CASE FOR PLANET ACCRETION? , 2010, 1010.2954.

[23]  Frank Timmes,et al.  MODULES FOR EXPERIMENTS IN STELLAR ASTROPHYSICS (MESA) , 2010, 1009.1622.

[24]  M. Livio,et al.  THE ORBITAL EVOLUTION OF GAS GIANT PLANETS AROUND GIANT STARS , 2009, 0910.2396.

[25]  S. Ortolani,et al.  Li-rich red giant branch stars in the Galactic bulge , 2009, 0907.4372.

[26]  M. Asplund,et al.  Departures from LTE for neutral Li in late-type stars , 2009, 0906.0899.

[27]  K. Cunha,et al.  MEASUREMENTS OF THE ISOTOPIC RATIO 6Li/7Li IN STARS WITH PLANETS , 2009, 0903.4873.

[28]  N. Santos,et al.  Metallicities for 13 nearby open clusters from high-resolution spectroscopy of dwarf and giant stars Stellar metallicity, stellar mass, and giant planets , 2008, 0811.2392.

[29]  M. Mayor,et al.  Planets around evolved intermediate-mass stars - I. Two substellar companions in the open clusters NGC 2423 and NGC 4349 , 2007, 0706.2174.

[30]  J. Lattanzio,et al.  Compulsory Deep Mixing of 3He and CNO Isotopes in the Envelopes of Low-Mass Red Giants , 2007, 0706.2710.

[31]  Sofia Randich,et al.  Time scales of Li evolution: A Homogeneous analysis of open clusters from ZAMS to late-MS , 2005 .

[32]  N. Santos,et al.  On the nature of lithium-rich giant stars. Constraints from beryllium abundances , 2005, astro-ph/0504133.

[33]  F. Herwig,et al.  Enhanced Extra Mixing in Low-Mass Red Giants: Lithium Production and Thermal Stability , 2004 .

[34]  R. Rebolo,et al.  New measurement of the 6 Li= 7 Li isotopic ratio in the extra-solar planet host star HD 82943 and line blending in the Li 6708 Å region ? , 2003, astro-ph/0304358.

[35]  M. Livio,et al.  The accretion of brown dwarfs and planets by giant stars — II. Solar-mass stars on the red giant branch , 1999, astro-ph/9905235.

[36]  J. Valenti,et al.  Spectroscopy Made Easy: A New Tool for Fitting Observations with Synthetic Spectra , 1996 .

[37]  A. I. Boothroyd,et al.  Creation of 7Li and Destruction of 3He, 9Be, 10B, and 11B in Low-Mass Red Giants, Due to Deep Circulation , 1995, astro-ph/9512122.

[38]  S. Woosley,et al.  The Evolution and Explosion of Massive Stars. II. Explosive Hydrodynamics and Nucleosynthesis , 1995 .

[39]  R. Kurucz.  ATLAS9 Stellar Atmosphere Programs and 2 km/s grid. , 1993 .

[40]  A. Boothroyd,et al.  The Creation of Superrich Lithium Giants , 1992 .

[41]  A. Boesgaard,et al.  Lithium in the Hyades Cluster , 1986 .

[42]  A. G. W. Cameron,et al.  Lithium and the s-process in red-giant stars , 1971 .

[43]  J. Alexander.  A possible source of lithium in the atmospheres of some red giants , 1967 .

[44]  K. Schwarzschild,et al.  The Observatory , 1886 .