230 research outputs found
Evolved stars and the origin of abundance trends in planet hosts
Tentative evidence that the properties of evolved stars with planets may be
different from what we know for MS hosts has been recently reported. We aim to
test whether evolved stars with planets show any chemical peculiarity that
could be related to the planet formation process. We determine in a consistent
way the metallicity and individual abundances of a large sample of evolved
(subgiants and red giants) and MS stars with and without known planetary
companions. No differences in the vs. condensation temperature (Tc)
slopes are found between the samples of planet and non-planet hosts when all
elements are considered. However, if the analysis is restricted to only
refractory elements, differences in the Tc-slopes between stars with and
without known planets are found. This result is found to be dependent on the
stellar evolutionary stage, as it holds for MS and subgiant stars, while there
seem to be no difference between planet and non-planet hosts among the sample
of giants. A search for correlations between the Tc-slope and the stellar
properties reveals significant correlations with the stellar mass and the
stellar age. The data also suggest that differences in terms of mass and age
between MS planet and non-planet hosts may be present. Our results are well
explained by radial mixing in the Galaxy. The sample of giant contains stars
more massive and younger than their MS counterparts. This leads to a sample of
stars possibly less contaminated by stars not born in the solar neighbourhood,
leading to no chemical differences between planet and non planet hosts. The
sample of MS stars may contain more stars from the outer disc (specially the
non-planet host sample) which might led to the differences observed in the
chemical trends.Comment: Accepted for publication by Astronomy and Astrophysic
Chemical fingerprints of hot Jupiter planet formation
The current paradigm to explain the presence of Jupiters with small orbital
periods (P 10 days; hot Jupiters) that involves their formation beyond the
snow line following inward migration, has been challenged by recent works that
explored the possibility of in situ formation. We aim to test whether stars
harbouring hot Jupiters and stars with more distant gas-giant planets show any
chemical peculiarity that could be related to different formation processes.
Our results show that stars with hot Jupiters have higher metallicities than
stars with cool distant gas-giant planets in the metallicity range +0.00/+0.20
dex. The data also shows a tendency of stars with cool Jupiters to show larger
abundances of elements. No abundance differences between stars with
cool and hot Jupiters are found when considering iron peak, volatile elements
or the C/O, and Mg/Si ratios. The corresponding -values from the statistical
tests comparing the cumulative distributions of cool and hot planet hosts are
0.20, 0.01, 0.81, and 0.16 for metallicity, , iron-peak, and
volatile elements, respectively. We confirm previous works suggesting that more
distant planets show higher planetary masses as well as larger eccentricities.
We note differences in age and spectral type between the hot and cool planet
hosts samples that might affect the abundance comparison. The differences in
the distribution of planetary mass, period, eccentricity, and stellar host
metallicity suggest a different formation mechanism for hot and cool Jupiters.
The slightly larger abundances found in stars harbouring cool Jupiters
might compensate their lower metallicities allowing the formation of gas-giant
planets.Comment: Accepted by Astronomy & Astrophysic
Connecting substellar and stellar formation. The role of the host star's metallicity
Most of our current understanding of the planet formation mechanism is based
on the planet metallicity correlation derived mostly from solar-type stars
harbouring gas-giant planets. To achieve a far more reaching grasp on the
substellar formation process we aim to analyse in terms of their metallicity a
diverse sample of stars (in terms of mass and spectral type) covering the whole
range of possible outcomes of the planet formation process (from planetesimals
to brown dwarfs and low-mass binaries). Our methodology is based on the use of
high-precision stellar parameters derived by our own group in previous works
from high-resolution spectra by using the iron ionisation and equilibrium
conditions. All values are derived in an homogeneous way, except for the M
dwarfs where a methodology based on the use of pseudo equivalent widths of
spectral features was used. Our results show that as the mass of the substellar
companion increases the metallicity of the host star tendency is to lower
values. The same trend is maintained when analysing stars with low-mass stellar
companions and a tendency towards a wide range of host star's metallicity is
found for systems with low mass planets. We also confirm that more massive
planets tend to orbit around more massive stars. The core-accretion formation
mechanism for planet formation achieves its maximum efficiency for planets with
masses in the range 0.2 and 2 M. Substellar objects with higher
masses have higher probabilities of being formed as stars. Low-mass planets and
planetesimals might be formed by core-accretion even around low-metallicity
stars.Comment: Accepted by A&
The Penn State - Torun Centre for Astronomy Planet Search stars. II. Lithium abundance analysis of the Red Giant Clump sample
Using the sample of 348 stars from the PennState-Torun Centre for Astronomy
Planet Search, for which uniformly determined atmospheric parameters are
available, with chemical abundances and rotational velocities presented here,
we investigate various channels of Li enrichment in giants. Our work is based
on the HET/HRS spectra. The A(Li) was determined from the 670.8nm line, while
we use a more extended set of lines for alpha-elements abundances. In a series
of K-S tests, we compare Li-rich giants with other stars in the sample. We also
use available IR photometric and kinematical data in search for evidence of
mass-loss. We investigate properties of the most Li-abundant giants in more
detail by using multi-epoch precise radial velocities. We present Li and
alpha-elements abundances, as well as vsini for 348 stars. We detected Li in 92
stars, of which 82 are giants. 11 of them show significant Li abundance
A(Li)>1.4 and 7 of them are Li-overabundant objects, according to criterion of
A(Li)>1.5 and their location on HR diagram, including two giants with Li
abundances close to meteoritic level. For another 271 stars, upper limits of
A(Li) are presented. We show that Li-rich giants are among the most massive
stars from our sample and show larger than average effective temperatures. They
are indistinguishable from the complete sample in terms of their distribution
of luminosity, [Fe/H], vsini, and alpha-elements abundances. Our results do not
point out to one specific Li enrichment mechanism operating in our sample of
giants. On the contrary, in some cases, we cannot identify fingerprints of any
of known scenarios. We show, however, that the 4 most Li-rich giant in our
sample either have low-mass companions or have RV variations at the level of
~100 m/s, which strongly suggests that the presence of companions is an
important factor in the Li-enrichment processes in giants.Comment: Accepted for publication in A&A, 13 figures, 11 tables, 26 page
Searching for signatures of planet formation in stars with circumstellar debris discs
(Abridged) Tentative correlations between the presence of dusty debris discs
and low-mass planets have been presented. In parallel, detailed chemical
abundance studies have reported different trends between samples of planet and
non-planet hosts. We determine in a homogeneous way the metallicity, and
abundances of a sample of 251 stars including stars with known debris discs,
with debris discs and planets, and only with planets. Stars with debris discs
and planets have the same [Fe/H] behaviour as stars hosting planets, and they
also show a similar -Tc trend. Different behaviour in the -Tc
trend is found between the samples of stars without planets and the samples of
planet hosts. In particular, when considering only refractory elements,
negative slopes are shown in cool giant planet hosts, whilst positive ones are
shown in stars hosting low-mass planets. Stars hosting exclusively close-in
giant planets show higher metallicities and positive -Tc slope. A
search for correlations between the -Tc slopes and the stellar
properties reveals a moderate but significant correlation with the stellar
radius and as well as a weak correlation with the stellar age. The fact that
stars with debris discs and stars with low-mass planets do not show neither
metal enhancement nor a different -Tc trend might indicate a
correlation between the presence of debris discs and the presence of low-mass
planets. We extend results from previous works which reported differences in
the -Tc trends between planet hosts and non hosts. However, these
differences tend to be present only when the star hosts a cool distant planet
and not in stars hosting exclusively low-mass planets.Comment: Accepted for publication in Astronomy and Astrophysic
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