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Dig Sites of Stellar Archeology: Giant Stars in the Milky Way
Ege Uni. J. of Faculty of Sci., Special Issue, 2014, 19-24
LI ABUNDANCES IN PTPS RED CLUMP GIANTS
Monika Adamów1,*, Andrzej Niedzielski1, Grzegorz Nowak1, Beata Deka1, Michalina
Górecka1, Aleksander Wolszczan2,3
1
Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University,
Grudziadzka 5, 87-100 Torun, Poland
2
3
Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory,
University Park, PA 16802, USA
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) 611 3058
E-mail: [email protected]
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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].
2.2. 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=60 000 resolving power mode with a I2 gas cell.
The spectra consist of 46 echelle orders recorded on the “blue” CCD chip (407-592 nm)
and 24 orders on the “red” one (602-784 nm). The signal-to-noise ratio was typically
better than 200-250 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 I2 lines and “blue” template (obtained
with no gas cell inserted into the optical path) are used.
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M. ADAMÓW et al. / Ege Uni. J. of Faculty of Sci., Special Issue, 2014, 19-24
In the Li abundance analysis only one order of the red spectra containing the 7Li 670,8
nm line was used. Since the HET/HRS flat-field spectra are occasionally contaminated
with a spectral feature near the 7Li 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.
2.3. 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 669.5-672.5 nm ranges and fitted the 7Li line at 670.8 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 I2 gas cell inserted) were used for the
analysis. However, I2 molecular absorption affects the spectrum up to 660 nm only, so it
does not influence 7Li 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
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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+48 740 [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+48 740b 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.
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BD+48 740 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 N203 510938 and by the Polish National Science
Centre grant no. UMO-2012/07/B/ST9/04415. M.A. is also supported by the Polish
National Science Centre grant no. UMO-2012/05/N/ST9/03836. 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-MaximiliansUniversität 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.
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[12]
M. Adamów et al. 2013, PhD Thesis, Nicolaus Copernicus Univerity, Toruń -
unpublished.
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