LI ABUNDANCES IN PTPS RED CLUMP GIANTS

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1 Dig Sites of Stellar Archeology: Giant Stars in the Milky Way Ege Uni. J. of Faculty of Sci., Special Issue, 2014, LI ABUNDANCES IN PTPS RED CLUMP GIANTS Monika Adamów 1, *, Andrzej Niedzielski 1, Grzegorz Nowak 1, Beata Deka 1, Michalina Górecka 1, Aleksander Wolszczan 2,3 1 Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, Torun, Poland 2 Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA 3 Center for Exoplanets and Habitable Worlds, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA Abstract: Lithium is a very intriguing element. While built into the stars it may be easily destroyed at temperatures of only 2.5 mln K, which makes it a very sensitive indicator of processes in stellar interiors, i.e. stellar age. Hence, investigation of lithium abundances requires understanding of various mechanisms occurring during stellar evolution. We therefore pay much attention to lithium within the ongoing PennState - Toruń Planet Search (PTPS) with the Hobby Eberly-Telescope. Here we present preliminary results of spectral analysis focused on Li and alpha-element abundances for 348 most evolved PTPS giants. Keywords: stars: abundances; late-type stars; planetary systems. 1. INTRODUCTION According to both observations and stellar evolution theory Lithium is destroyed during stellar evolution on the Red Giant Branch (RGB), initially due to the so-called first dredge-up and later on due to slow extra mixing mechanism. Nevertheless, some giants reveal high surface lithium enhancement. This phenomenon is usually explained with some internal lithium production process, supported by fast, non-standard mixing or alternatively by accretion of Li-rich material from a supernova remnants, or from a Lirich companion - brown dwarf or planet. More extensive description of those processes may be found in e.g. [1]. The PennState - Toruń Planet Search (PTPS) program is a long term, ongoing project dedicated to search for planets around evolved stars with the radial velocity technique using the Hobby-Eberly Telescope. The ultimate goal of PTPS is analysis of evolution of planetary systems. Within PTPS in parallel to a planet search we * Corresponding Author: Tel: +48 (56) Monika.Adamow@astri.uni.torun.pl 19

2 also perform a detailed in-depth spectral analysis of all observed stars with the aim to better constrain their evolutionary status. Details of our survey (observing strategy, motivation, and data analysis) have been presented in detail elsewhere [2]. The richness of the PTPS sample in stars at various stages of stellar evolution allows for discussion of lithium abundance evolution in relation to evolutionary status of stars. As we have data from a precise radial velocity survey we may discuss lithium abundances in the context of stellar binarity or hosting low-mass companions. Here we present preliminary results of the A(Li) investigation of red giants from PTPS sample. 2. OBSERVATIONS AND DATA ANALYSIS 2.1. Sample The total PTPS sample that we have been monitoring since early 2004 is composed of field solar- and intermediate-mass stars at various stages of stellar evolution. Here we discuss a subsample of 348 stars, selected to be presumably RGC stars. They constitute the so-called RGC subsample of the PTPS and fall in the clump giant region of the HR-diagram, which contains stars of various masses over a range of evolutionary stages. Detailed description of this sample and selection criteria can be found in paper [3] Observations PTPS is conducted with the 9.2 m Hobby-Eberly Telescope (HET) [4], operated in the queue-scheduled mode [5] and equipped with the High Resolution Spectrograph (HRS, [6]). For PTPS the HRS is used in its R= resolving power mode with a I 2 gas cell. The spectra consist of 46 echelle orders recorded on the blue CCD chip ( nm) and 24 orders on the red one ( nm). The signal-to-noise ratio was typically better than per resolution element at 590 nm. The blue spectra are used for precise radial velocity determinations with the gas cell. For detailed spectroscopic analysis both red' spectrum, unaffected by the I 2 lines and blue template (obtained with no gas cell inserted into the optical path) are used. 20

3 In the Li abundance analysis only one order of the red spectra containing the 7 Li 670,8 nm line was used. Since the HET/HRS flat-field spectra are occasionally contaminated with a spectral feature near the 7 Li line, which (depending on the actual RV of a star) may mimic the Li line and influence the abundance analysis, all flat field spectra were checked and those affected by this feature were omitted to avoid the contamination Data analysis Spectral analysis was performed with Spectroscopy Made Easy package (SME) [7]. SME assumes LTE and plane-parallel geometry, but ignores magnetic field, molecular lines and mass loss. Spectrum synthesis is based on Kurucz s models of stellar atmospheres [8]. In SME analysis the observed spectrum is considered as a model constraint. As an input SME requires a set of lines from Vienna Atomic Line Database (VALD) [9] identified in the spectrum, stellar data (including radial velocity) and the instrumental profile. Although SME requires the metallicity ratio [M/H] rather than [Fe/H], we used [Fe/H] determined in [3] as the closest approximation to metallicity. We used SME in the nm ranges and fitted the 7 Li line at nm as well as several lines of Al, Ti, Si and Ca, rotation velocity and macroturbulent velocity together. In the first attempt only template spectra (with no I 2 gas cell inserted) were used for the analysis. However, I 2 molecular absorption affects the spectrum up to 660 nm only, so it does not influence 7 Li line region. Hence, in the final analysis it was also possible to use the red spectra obtained with the gas-cell inserted, which allowed for A(Li) uncertainty estimates (rms). We analyzed 4750 spectra in total - from 2 to about 100 spectra per star depending of its status in PTPS, 14 for one object on average. 3. RESULTS The collection of lithium abundances in combination with an existing PTPS database of radial velocities is in position to give a new insight into possible lithium enhancement mechanisms for giants. Fig. 1 presents the Hertzsprung-Russell diagram for stars from the PTPS giant sample. We have identified 17 Li-rich stars, defined here as those with A(Li)>1.3. Not all of them may be considered as Li-overabundant as their evolutionary status is not 21

4 certain. It cannot be excluded that some of such stars are objects undergoing first dredge up. However, we have found also two objects with A(Li) exceeding the so-called Figure 1. The HR diagram for the PTPS giant-star sample. Lithium abundances are color-coded (vertical bar on the right). meteoritic value of A(Li)=3.3 [10], therefore they definitely have undergone some lithium production process. One of the most intriguing stars in this sample is BD [11]. It is the first case of an exceptionally Li-overabundant giant (A(Li)=2.2) that is very likely accompanied by a giant planet. BD b travels in a highly eccentric orbit. One possible interpretation of this detection is that a migration episode in the original system left behind a close-in planet, which has subsequently merged with the star, and a more distant, surviving planet in an eccentric orbit. Hence enhanced lithium abundance might be a consequence of a recent planet engulfment episode by the parent star. 22

5 BD is not the only Li-rich star exhibiting changes in radial velocities that might be interpreted as presence of stellar or low mass companion (planets or brown dwarfs). Among 17 red giant stars with enhanced Li abundance only a few do not show significant variations in radial velocities, which makes them single, non-active stars within the precision maintained within PTPS. Ten stars in the Li-rich giants sample are being monitored as probable planet hosts and the other six objects are binaries. In addition to RV measurements, we have determined basic atmospheric parameters (effective temperature, surface gravity, microturbulent velocity, chemical composition), luminosities, masses, radii [3] and rotational velocities for all stars in the sample. Therefore the data collected within PTPS represent also a great tool for verifying connection of lithium abundance with stellar parameters. We have found that stars with easily detectable Li lines tend to be slightly more massive when compared to the rest of the sample. Li-rich stars reveal also faster rotational velocities, however not all of them might be classified as fast rotators. A more detailed study of PTPS giants will be presented elsewhere [12]. ACKNOWLEDGMENT We thank Dr. Nikolai Piskunov and Dr. Jeff Valenti for making SME available for us. We thank the HET resident astronomers and telescope operators for obtaining the PTPS spectra..a., G.N., B.D., M.G. and A.N. were supported in part by the Polish Ministry of Science and Higher Education grant N N and by the Polish National Science Centre grant no. UMO-2012/07/B/ST9/ M.A. is also supported by the Polish National Science Centre grant no. UMO-2012/05/N/ST9/ A.W. was supported by the NASA grant NNX09AB36G. The HET is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians- Universität München, and Georg-August- Universität Göttingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly. The Center for Exoplanets and Habitable Worlds is supported by the Pennsylvania State University, the Eberly College of Science, and the Pennsylvania Space Grant Consortium. 23

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