460 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
The Effect of Tides on the Population of PN from Interacting Binaries
We have used the tidal equations of Zahn to determine the maximum orbital
distance at which companions are brought into Roche lobe contact with their
giant primary, when the primary expands during the giant phases. This is a key
step when determining the rates of interaction between giants and their
companions. Our stellar structure calculations are presented as maximum radii
reached during the red and asymptotic giant branch (RGB and AGB, respectively)
stages of evolution for masses between 0.8 and 4.0 Mo (Z=0.001 - 0.04) and
compared with other models to gauge the uncertainty on radii deriving from
details of these calculations. We find overall tidal capture distances that are
typically 1-4 times the maximum radial extent of the giant star, where
companions are in the mass range from 1 Jupiter mass to a mass slightly smaller
than the mass of the primary. We find that only companions at initial orbital
separations between ~320 and ~630 Ro will be typically captured into a Roche
lobe-filling interaction or a common envelope on the AGB. Comparing these
limits with the period distribution for binaries that will make PN, we deduce
that in the standard scenario where all ~1-8 Mo stars make a PN, at most 2.5
per cent of all PN should have a post-common envelope central star binary, at
odds with the observational lower limit of 15-20 per cent. The observed
over-abundance of post-interaction central stars of PN cannot be easily
explained considering the uncertainties. We examine a range of explanations for
this discrepancy.Comment: 19 pages, 16 figures, accepted by Monthly Notices of the Royal
Astronomical Societ
Can solid body destruction explain abundance discrepancies in planetary nebulae?
In planetary nebulae, abundances of oxygen and other heavy elements derived
from optical recombination lines are systematically higher than those derived
from collisionally excited lines. We investigate the hypothesis that the
destruction of solid bodies may produce pockets of cool, high-metallicity gas
that could explain these abundance discrepancies. Under the assumption of
maximally efficient radiative ablation, we derive two fundamental constraints
that the solid bodies must satisfy in order that their evaporation during the
planetary nebula phase should generate a high enough gas phase metallicity. A
local constraint implies that the bodies must be larger than tens of meters,
while a global constraint implies that the total mass of the solid body
reservoir must exceed a few hundredths of a solar mass. This mass greatly
exceeds the mass of any population of comets or large debris particles expected
to be found orbiting evolved low- to intermediate-mass stars. We therefore
conclude that contemporaneous solid body destruction cannot explain the
observed abundance discrepancies in planetary nebulae. However, similar
arguments applied to the sublimation of solid bodies during the preceding
asymptotic giant branch (AGB) phase do not lead to such a clear-cut conclusion.
In this case, the required reservoir of volatile solids is only one
ten-thousandth of a solar mass, which is comparable to the most massive debris
disks observed around solar-type stars, implying that this mechanism may
contribute to abundance discrepancies in at least some planetary nebulae, so
long as mixing of the high metallicity gas is inefficient.Comment: 8 pages, no figures, ApJ in pres
The Interaction of Asymptotic Giant Branch Stars with the Interstellar Medium
We study the hydrodynamical behavior of the gas expelled by moving Asymptotic
Giant Branch Stars interacting with the ISM. Our models follow the wind
modulations prescribed by stellar evolution calculations, and we cover a range
of expected relative velocities (10 to 100 km/s), ISM densities (between 0.01
and 1 cm-3), and stellar progenitor masses (1 and 3.5 Msun). We show how and
when bow-shocks, and cometary-like structures form, and in which regime the
shells are subject to instabilities. Finally, we analyze the results of the
simulations in terms of the different kinematical stellar populations expected
in the Galaxy.Comment: ApJ in press, 42 pages, 12 figures, movies of the simulations will be
available in the published electronic version of the pape
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