180 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
Dispersal of protoplanetary discs: How stellar properties and the local environment determine the pathway of evolution
We study the evolution and final dispersal of protoplanetary discs that
evolve under the action of internal and external photoevaporation, and
different degrees of viscous transport. We identify five distinct dispersal
pathways, which are i) very long lived discs (Myr), ii) inside-out
dispersal where internal photoevaporation dominates and opens inner holes, iii)
outside-in dispersal where external photoevaporation dominates through disc
truncation and two intermediate regimes characterised by lingering material in
the inner disc with the outer disc dispersed predominantly by either internal
or external photoevaporation. We determine how the lifetime, relative impact of
internal and external winds and clearing pathway varies over a wide, plausible,
parameter space of stellar/disc/radiation properties. There are a number of
implications, for example in high UV environments because the outer disc
lifetime is shorter than the time-scale for clearing the inner disc we do not
expect transition discs to be common, which appears to be reflected in the
location of transition disc populations towards the Orion Nebular Cluster.
Irrespective of environment, we find that ongoing star formation is required to
reproduce observed disc fractions as a function of stellar cluster age. This
work demonstrates the importance of including both internal and external winds
for understanding protoplanetary disc evolution.Comment: Submitted to MNRAS. 19 pages, 15 figure
Planet formation via pebble accretion in externally photoevaporating discs
We demonstrate that planet formation via pebble accretion is sensitive to
external photoevaporation of the outer disc. In pebble accretion, planets grow
by accreting from a flux of solids (pebbles) that radially drift inwards from
the pebble production front. If external photoevaporation truncates the outer
disc fast enough, it can shorten the time before the pebble production front
reaches the disc outer edge, cutting off the supply of pebble flux for
accretion, hence limiting the pebble mass reservoir for planet growth.
Conversely, cloud shielding can protect the disc from strong external
photoevaporation and preserve the pebble reservoir. Because grain growth and
drift can occur quickly, shielding even on a short time-scale (<1 Myr) can have
a non-linear impact on the properties of planets growing by pebble accretion.
For example a planetary seed at 25 au stays at 25 au with a
lunar mass if the disc is immediately irradiated by a G field, but
grows and migrates to be approximately Earth-like in both mass and orbital
radius if the disc is shielded for just 1 Myr. In NGC 2024, external
photoevaporation is thought to happen to discs that are <0.5 Myr old, which
coupled with the results here suggests that the exact planetary parameters can
be very sensitive to the star forming environment. Universal shielding for
time-scales of at least Myr would be required to completely nullify
the environmental impact on planetary architectures.Comment: Accepted for publication in mnras, 12 pages, 8 figure
Photoevaporation obfuscates the distinction between wind and viscous angular momentum transport in protoplanetary discs
How protoplanetary discs evolve remains an unanswered question. Competing
theories of viscosity and magnetohydrodynamic disc winds have been put forward
as the drivers of angular momentum transport in protoplanetary discs. These two
models predict distinct differences in the disc mass, radius and accretion
rates over time, that could be used to distinguish them. However that
expectation is built on models that do not include another important process -
photoevaporation, both internally by the host star and externally by
neighbouring stars. In this work we produce numerical models of protoplanetary
discs including viscosity, magnetohydrodynamic disc winds, and internal and
external photoevaporation. We find that even weak levels of external
photoevaporation can significantly affect the evolution of protoplanetary
discs, influencing the observable features such as disc radii, that might
otherwise distinguish between viscous and wind driven discs. Including internal
photoevaporation further suppresses differences in evolution between viscous
and wind driven discs. This makes it much more difficult than previously
anticipated, to use observations of nearby star forming regions to determine
whether discs are viscous or wind driven. Interestingly we find that evolved
protoplanetary discs in intermediate FUV environments may be the best cases for
differentiating whether they evolve through viscosity or magnetohydrodynamic
disc winds. Ultimately this work demonstrates the importance of understanding
what are the key evolutionary processes and including as many of those as
possible when exploring the evolution of protoplanetary discs.Comment: Accepted for publication in MNRAS. 16 pages, 14 figure
Dusty circumbinary discs: inner cavity structures and stopping locations of migrating planets
We present the results of two-fluid hydrodynamical simulations of
circumbinary discs consisting of gas and dust, with and without embedded
planets, to examine the influence of the dust on the structure of the tidally
truncated inner cavity and on the parking locations of migrating planets. In
this proof-of-concept study, we consider Kepler-16 and -34 analogues, and
examine dust fluids with Stokes numbers in the range and dust-to-gas ratios of 0.01 and 1. For the canonical dust-to-gas
ratio of 0.01, we find the inclusion of the dust has only a minor effect on the
cavity and stopping locations of embedded planets compared to dust-free
simulations. However, for the enhanced dust-to-gas ratio of unity, assumed to
arise because of significant dust drift and accumulation, we find that the dust
can have a dramatic effect by shrinking and circularising the inner cavity,
which brings the parking locations of planets closer to the central binary.
This work demonstrates the importance of considering both gas and dust in
studies of circumbinary discs and planets, and provides a potential means of
explaining the orbital properties of circumbinary planets such as Kepler-34b,
which have hitherto been difficult to explain using gas-only hydrodynamical
simulations.Comment: Accepted for publication in MNRAS, 20 pages, 17 figure
Global N-body simulations of circumbinary planet formation around Kepler-16 and -34 analogues I: Exploring the pebble accretion scenario
Numerous circumbinary planets have been discovered in surveys of transiting
planets. Often, these planets are found to orbit near to the zone of dynamical
instability, close to the central binary. The existence of these planets has
been explained by hydrodynamical simulations that show that migrating
circumbinary planets, embedded in circumbinary discs, halt at the central
cavity that is formed by the central binary. Transit surveys are naturally most
sensitive to finding circumbinary planets with the shortest orbital periods.
The future promise of detecting longer period systems using radial-velocity
searches, combined with the anticipated detection of numerous circumbinary
planets by ESA's PLATO mission, points to the need to model and understand the
formation and evolution of circumbinary planets in a more general sense than
has been considered before. With this goal in mind, we present a newly
developed global model of circumbinary planet formation that is based on the
mercury6 symplectic N-body integrator, combined with a model for the
circumbinary disc and prescriptions for a range of processes involved in planet
formation such as pebble accretion, gas envelope accretion and migration. Our
results show that under reasonable assumptions, the pebble accretion scenario
can produce circumbinary systems that are similar to those observed, and in
particular is able to produce planets akin to Kepler-16b and Kepler-34b.
Comparing our results to other systems, we find that our models also adequately
reproduce such systems, including multi-planet systems. Resonances between
neighbouring planets are frequently obtained, whilst ejections of planets by
the central binary acts as an effective source of free floating planets.Comment: Accepted for publication in MNRAS, 23 pages, 16 figure
Constraining the formation history of the TOI-1338/BEBOP-1 circumbinary planetary system
The recent discovery of multiple planets in the circumbinary system
TOI-1338/BEBOP-1 raises questions about how such a system formed. The formation
of the system was briefly explored in the discovery paper, but only to answer
the question do current pebble accretion models have the potential to explain
the origin of the system? We use a global model of circumbinary planet
formation that utilises N-body simulations, including prescriptions for planet
migration, gas and pebble accretion, and interactions with a circumbinary disc,
to explore the disc parameters that could have led to the formation of the
TOI-1338/BEBOP-1 system. With the disc lifetime being the main factor in
determining how planets form, we limit our parameter space to those that
determine the disc lifetime. These are: the strength of turbulence in the disc,
the initial disc mass, and the strength of the external radiation field that
launches photoevaporative winds. When comparing the simulated systems to
TOI-1338/BEBOP-1, we find that only discs with low levels of turbulence are
able to produce similar systems. The radiation environment has a large effect
on the types of planetary systems that form, whilst the initial disc mass only
has limited impact since the majority of planetary growth occurs early in the
disc lifetime. With the most TOI-1338/BEBOP-1 like systems all occupying
similar regions of parameter space, our study shows that observed circumbinary
planetary systems can potentially constrain the properties of planet forming
discs.Comment: Accepted for publication in MNRAS, 15 pages, 10 figure
Pion Breather States in QCD
We describe a class of pionic breather solutions (PBS) which appear in the
chiral lagrangian description of low-energy QCD. These configurations are
long-lived, with lifetimes greater than fm/c, and could arise as
remnants of disoriented chiral condensate (DCC) formation at RHIC. We show that
the chiral lagrangian equations of motion for a uniformly isospin-polarized
domain reduce to those of the sine-gordon model. Consequently, our solutions
are directly related to the breather solutions of sine-gordon theory in 3+1
dimensions. We investigate the possibility of PBS formation from multiple
domains of DCC, and show that the probability of formation is non-negligible.Comment: 9 pages, 4 figure
Time evolution of the chiral phase transition during a spherical expansion
We examine the non-equilibrium time evolution of the hadronic plasma produced
in a relativistic heavy ion collision, assuming a spherical expansion into the
vacuum. We study the linear sigma model to leading order in a large-
expansion. Starting at a temperature above the phase transition, the system
expands and cools, finally settling into the broken symmetry vacuum state. We
consider the proper time evolution of the effective pion mass, the order
parameter , and the particle number distribution. We
examine several different initial conditions and look for instabilities
(exponentially growing long wavelength modes) which can lead to the formation
of disoriented chiral condensates (DCCs). We find that instabilities exist for
proper times which are less than 3 fm/c. We also show that an experimental
signature of domain growth is an increase in the low momentum spectrum of
outgoing pions when compared to an expansion in thermal equilibrium. In
comparison to particle production during a longitudinal expansion, we find that
in a spherical expansion the system reaches the ``out'' regime much faster and
more particles get produced. However the size of the unstable region, which is
related to the domain size of DCCs, is not enhanced.Comment: REVTex, 20 pages, 8 postscript figures embedded with eps
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