1,340 research outputs found
Exploring the formation by core accretion and the luminosity evolution of directly imaged planets: The case of HIP 65426 b
A low-mass companion to the two-solar mass star HIP65426 has recently been
detected by SPHERE at around 100 au from its host. Explaining the presence of
super-Jovian planets at large separations, as revealed by direct imaging, is
currently an open question.
We want to derive statistical constraints on the mass and initial entropy of
HIP65426b and to explore possible formation pathways of directly imaged objects
within the core-accretion paradigm, focusing on HIP65426b.
Constraints on the planet's mass and post-formation entropy are derived from
its age and luminosity combined with cooling models. For the first time, the
results of population synthesis are also used to inform the results. Then, a
formation model that includes N-body dynamics with several embryos per disc is
used to study possible formation histories and the properties of possible
additional companions. Finally, the outcomes of two- and three-planet
scattering in the post-disc phase are analysed, taking tides into account.
The mass of HIP65426b is found to be Mp = 9.9 +1.1 -1.8 MJ using the hot
population and Mp = 10.9 +1.4 -2.0 MJ with the cold-nominal population. Core
formation at small separations from the star followed by outward scattering and
runaway accretion at a few hundred AU succeeds in reproducing the mass and
separation of HIP65426b. Alternatively, systems having two or more giant
planets close enough to be on an unstable orbit at disc dispersal are likely to
end up with one planet on a wide HIP65426b-like orbit with a relatively high
eccentricity (>~ 0.5).
If this scattering scenario explains its formation, HIP65426b is predicted to
have a high eccentricity and to be accompanied by one or several roughly
Jovian-mass planets at smaller semi-major axes, which also could have a high
eccentricity. This could be tested by further direct-imaging as well as
radial-velocity observations.Comment: 17 pages, 11 figures. A&A in press. Bern EXoplanet cooling curves
(BEX) available upon request. v2: Language and other minor changes; Fig. 4
now has labels summarising a possible formation pathway discussed in the tex
Theoretical models of planetary system formation: mass vs semi-major axis
Planet formation models have been developed during the last years in order to
try to reproduce the observations of both the solar system, and the extrasolar
planets. Some of these models have partially succeeded, focussing however on
massive planets, and for the sake of simplicity excluding planets belonging to
planetary systems. However, more and more planets are now found in planetary
systems. This tendency, which is a result of both radial velocity, transit and
direct imaging surveys, seems to be even more pronounced for low mass planets.
These new observations require the improvement of planet formation models,
including new physics, and considering the formation of systems. In a recent
series of papers, we have presented some improvements in the physics of our
models, focussing in particular on the internal structure of forming planets,
and on the computation of the excitation state of planetesimals, and their
resulting accretion rate. In this paper, we focus on the concurrent effect of
the formation of more than one planet in the same protoplanetary disc, and show
the effect, in terms of global architecture and composition of this
multiplicity. We use a N-body calculation including collision detection to
compute the orbital evolution of a planetary system. Moreover, we describe the
effect of competition for accretion of gas and solids, as well as the effect of
gravitational interactions between planets. We show that the masses and
semi-major axis of planets are modified by both the effect of competition and
gravitational interactions. We also present the effect of the assumed number of
forming planets in the same system (a free parameter of the model), as well as
the effect of the inclination and eccentricity damping.Comment: accepted in Astronomy and Astrophysic
From stellar to planetary composition: Galactic chemical evolution of Mg/Si mineralogical ratio
The main goal of this work is to study element ratios that are important for
the formation of planets of different masses. We study potential correlations
between the existence of planetary companions and the relative elemental
abundances of their host stars. We use a large sample of FGK-type dwarf stars
for which precise Mg, Si, and Fe abundances have been derived using HARPS
high-resolution and high-quality data. A first analysis of the data suggests
that low-mass planet host stars show higher [Mg/Si] ratios, while giant planet
hosts present [Mg/Si] that is lower than field stars. However, we found that
the [Mg/Si] ratio significantly depends on metallicity through Galactic
chemical evolution. After removing the Galactic evolution trend only the
difference in the [Mg/Si] elemental ratio between low-mass planet hosts and
non-hosts was present in a significant way. These results suggests that
low-mass planets are more prevalent around stars with high [Mg/Si]. Our results
demonstrate the importance of Galactic chemical evolution and indicate that it
may play an important role in the planetary internal structure and composition.Comment: Accepted by A&A (Letter to the Editor
Planet Formation: Statistics of spin rates and obliquities of extrasolar planets
We develop a simple model of planetary formation, focusing our attention on
those planets with masses less than 10 Earth masses and studying particularly
the primordial spin parameters of planets resulting from the accretion of
planetesimals and produced by the collisions between the embryos. As initial
conditions, we adopt the oligarchic growth regime of protoplanets in a disc
where several embryos are allowed to form. We take different initial planetary
system parameters and for each initial condition, we consider an evolution of
20 millon of years of the system. We perform simulations for 1000 different
discs, and from their results we derive the statistical properties of the
assembled planets. We have taken special attention to the planetary obliquities
and rotation periods, such as the information obtained from the mass and semi
major axis diagram, which reflects the process of planetary formation. The
distribution of obliquities was found to be isotropic, which means that planets
can rotate in direct or indirect sense, regardless of their mass. Our results
regarding the primordial rotation periods show that they are dependent on the
region where the embryo was formed and evolved. According to our results, most
of the planets have rotation periods between 10 and 10000 hours and there are
also a large population of planets similar to terrestrial planets in the Solar
System.Comment: 10 pages, 16 figures, accepted for publication in MNRA
Planetary mass-radius relations across the galaxy
Planet formation theory suggests that planet bulk compositions are likely to
reflect the chemical abundance ratios of their host star's photosphere.
Variations in the abundance of particular chemical species in stellar
photospheres between different galactic stellar populations demonstrate that
there are differences among the expected solid planet bulk compositions. We aim
to present planetary mass-radius relations of solid planets for kinematically
differentiated stellar populations, namely, the thin disc, thick disc, and
halo. Using two separate internal structure models, we generated synthetic
planets using bulk composition inputs derived from stellar abundances. We
explored two scenarios, specifically iron-silicate planets at 0.1 AU and
silicate-iron-water planets at 4 AU. We show that there is a persistent
statistical difference in the expected mass-radius relations of solid planets
among the different galactic stellar populations. At 0.1 AU for silicate-iron
planets, there is a 1.51 to 2.04\% mean planetary radius difference between the
thick and thin disc stellar populations, whilst for silicate-iron-water planets
past the ice line at 4 AU, we calculate a 2.93 to 3.26\% difference depending
on the models. Between the halo and thick disc, we retrieve at 0.1 AU a 0.53 to
0.69\% mean planetary radius difference, and at 4 AU we find a 1.24 to 1.49\%
difference depending on the model. Future telescopes (such as PLATO) will be
able to precisely characterize solid exoplanets and demonstrate the possible
existence of planetary mass-radius relationship variability between galactic
stellar populations.Comment: 11 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Detection of Neptune-size planetary candidates with CoRoT data. Comparison with the planet occurrence rate derived from Kepler
[Abridged] Context. The CoRoT space mission has been searching for transiting
planets since the end of December 2006. Aims. We aim to investigate the
capability of CoRoT to detect small-size transiting planets in short-period
orbits, and to compare the number of CoRoT planets with 2 \leq R_p \leq 4
Rearth with the occurrence rate of small-size planets provided by the
distribution of Kepler planetary candidates (Howard et al. 2012). Methods. We
performed a test that simulates transits of super-Earths and Neptunes in real
CoRoT light curves and searches for them blindly by using the LAM transit
detection pipeline. Results. The CoRoT detection rate of planets with radius
between 2 and 4 Rearth and orbital period P \leq 20 days is 59% (31%) around
stars brighter than r'=14.0 (15.5). By properly taking the CoRoT detection rate
for Neptune-size planets and the transit probability into account, we found
that according to the Kepler planet occurrence rate, CoRoT should have
discovered 12 \pm 2 Neptunes orbiting G and K dwarfs with P \leq 17 days in six
observational runs. This estimate must be compared with the validated Neptune
CoRoT-24b and five CoRoT planetary candidates in the considered range of
planetary radii. We thus found a disagreement with expectations from Kepler at
3 \sigma or 5 \sigma, assuming a blend fraction of 0% (six Neptunes) and 100%
(one Neptune) for these candidates. Conclusions. This underabundance of CoRoT
Neptunes with respect to Kepler may be due to several reasons. Regardless of
the origin of the disagreement, which needs to be investigated in more detail,
the noticeable deficiency of CoRoT Neptunes at short orbital periods seems to
indirectly support the general trend found in Kepler data, i.e. that the
frequency of small-size planets increases with increasing orbital periods and
decreasing planet radii.Comment: 10 pages, 7 figures. Accepted for publication in A&
Orbital and physical properties of planets and their hosts: new insights on planet formation and evolution
We explore the relations between physical and orbital properties of planets
and properties of their host stars to identify the main observable signatures
of the formation and evolution processes of planetary systems. We use a large
sample of FGK dwarf planet hosts with stellar parameters derived in a
homogeneous way from the SWEET-Cat database to study the relation between
stellar metallicity and position of planets in the period-mass diagram. In the
second part we use all the RV-detected planets orbiting FGK stars to explore
the role of planet-disk and planet-planet interaction on the evolution of
orbital properties of planets with masses above 1MJup. We show that planets
orbiting metal-poor stars have longer periods than those in metal-rich systems.
This trend is valid for masses at least from 10MEarth to 4MJup. Earth-like
planets orbiting metal-rich stars always show shorter periods (fewer than 20
days) than those orbiting metal-poor stars. We also found statistically
significant evidence that very high mass giants have on average more eccentric
orbits than giant planets with lower mass.Finally, we show that the
eccentricity of planets with masses higher than 4MJup tends to be lower for
planets with shorter periods. Our results suggest that the planets in the P-MP
diagram are evolving differently because of a mechanism that operates over a
wide range of planetary masses. This mechanism is stronger or weaker depending
on the metallicity of the respective system. One possibility is that planets in
metal-poor disks form farther out from their central star and/or they form
later and do not have time to migrate as far as the planets in metal-rich
systems. The trends and dependencies obtained for very high mass planetary
systems suggest that planet-disk interaction is a very important and
orbit-shaping mechanism for planets in the high-mass domain. Shortened.Comment: 8 pages, 4 figures and 1 table. Accepted for publication in A&
The near-infrared spectral energy distribution of {\beta} Pictoris b
A gas giant planet has previously been directly seen orbiting at 8-10 AU
within the debris disk of the ~12 Myr old star {\beta} Pictoris. The {\beta}
Pictoris system offers the rare opportunity to study the physical and
atmospheric properties of an exoplanet placed on a wide orbit and to establish
its formation scenario. We obtained J (1.265 {\mu}m), H (1.66 {\mu}m), and M'
(4.78 {\mu}m) band angular differential imaging of the system between 2011 and
2012. We detect the planetary companion in our four-epoch observations. We
estimate J = 14.0 +- 0.3, H = 13.5 +- 0.2, and M' = 11.0 +- 0.3 mag. Our new
astrometry consolidates previous semi-major axis (sma=8-10 AU) and excentricity
(e <= 0.15) estimates of the planet. These constraints, and those derived from
radial velocities of the star provides independent upper limits on the mass of
{\beta} Pictoris b of 12 and 15.5 MJup for semi-major axis of 9 and 10 AU. The
location of {\beta} Pictoris b in color-magnitude diagrams suggests it has
spectroscopic properties similar to L0-L4 dwarfs. This enables to derive
Log10(L/Lsun) = -3.87 +- 0.08 for the companion. The analysis with 7
PHOENIX-based atmospheric models reveals the planet has a dusty atmosphere with
Teff = 1700 +- 100 K and log g = 4.0+- 0.5. "Hot-start" evolutionary models
give a new mass of 10+3-2 MJup from Teff and 9+3-2 MJup from luminosity.
Predictions of "cold-start" models are inconsistent with independent
constraints on the planet mass. "Warm-start" models constrain the mass to M >=
6MJup and the initial entropies to values (Sinit >= 9.3Kb/baryon), intermediate
between those considered for cold/hot-start models, but likely closer to those
of hot-start models.Comment: 19 pages, accepted in Astronomy and Astrophysic
A survey of young, nearby, and dusty stars to understand the formation of wide-orbit giant planets
Direct imaging has confirmed the existence of substellar companions on wide
orbits. To understand the formation and evolution mechanisms of these
companions, the full population properties must be characterized. We aim at
detecting giant planet and/or brown dwarf companions around young, nearby, and
dusty stars. Our goal is also to provide statistics on the population of giant
planets at wide-orbits and discuss planet formation models. We report a deep
survey of 59 stars, members of young stellar associations. The observations
were conducted with VLT/NaCo at L'-band (3.8 micron). We used angular
differential imaging to reach optimal detection performance. A statistical
analysis of about 60 % of the young and southern A-F stars closer than 65 pc
allows us to derive the fraction of giant planets on wide orbits. We use
gravitational instability models and planet population synthesis models
following the core-accretion scenario to discuss the occurrence of these
companions. We resolve and characterize new visual binaries and do not detect
any new substellar companion. The survey's median detection performance reaches
contrasts of 10 mag at 0.5as and 11.5 mag at 1as. We find the occurrence of
planets to be between 10.8-24.8 % at 68 % confidence level assuming a uniform
distribution of planets in the interval 1-13 Mj and 1-1000 AU. Considering the
predictions of formation models, we set important constraints on the occurrence
of massive planets and brown dwarf companions that would have formed by GI. We
show that this mechanism favors the formation of rather massive clump (Mclump >
30 Mj) at wide (a > 40 AU) orbits which might evolve dynamically and/or
fragment. For the population of close-in giant planets that would have formed
by CA, our survey marginally explore physical separations (<20 AU) and cannot
constrain this population
Constraining planet structure from stellar chemistry: the cases of CoRoT-7, Kepler-10, and Kepler-93
We explore the possibility that the stellar relative abundances of different
species can be used to constrain the bulk abundances of known transiting rocky
planets. We use high resolution spectra to derive stellar parameters and
chemical abundances for Fe, Si, Mg, O, and C in three stars hosting low mass,
rocky planets: CoRoT-7, Kepler-10, and Kepler-93. These planets follow the same
line along the mass-radius diagram, pointing toward a similar composition. The
derived abundance ratios are compared with the solar values. With a simple
stoichiometric model, we estimate the iron mass fraction in each planet,
assuming stellar composition. We show that in all cases, the iron mass fraction
inferred from the mass-radius relationship seems to be in good agreement with
the iron abundance derived from the host star's photospheric composition. The
results suggest that stellar abundances can be used to add constraints on the
composition of orbiting rocky planets.Comment: A&A Letters, in pres
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