280 research outputs found
Planetesimal collisions in binary systems
We study the collisional evolution of km-sized planetesimals in tight binary
star systems to investigate whether accretion towards protoplanets can proceed
despite the strong gravitational perturbations from the secondary star. The
orbits of planetesimals are numerically integrated in two dimensions under the
influence of the two stars and gas drag. The masses and orbits of the
planetesimals are allowed to evolve due to collisions with other planetesimals
and accretion of collisional debris. In addition, the mass in debris can evolve
due to planetesimal-planetesimal collisions and the creation of new
planetesimals. We show that it is possible in principle for km-sized
planetesimals to grow by two orders of magnitude in size if the efficiency of
planetesimal formation is relatively low. We discuss the limitations of our
two-dimensional approach.Comment: 5 pages, 5 figures, accepted for publication in MNRA
Low-mass planets in nearly inviscid disks: Numerical treatment
Embedded planets disturb the density structure of the ambient disk and
gravitational back-reaction will induce possibly a change in the planet's
orbital elements. The accurate determination of the forces acting on the planet
requires careful numerical analysis. Recently, the validity of the often used
fast orbital advection algorithm (FARGO) has been put into question, and
special numerical resolution and stability requirements have been suggested. In
this paper we study the process of planet-disk interaction for small mass
planets of a few Earth masses, and reanalyze the numerical requirements to
obtain converged and stable results. One focus lies on the applicability of the
FARGO-algorithm. Additionally, we study the difference of two and
three-dimensional simulations, compare global with local setups, as well as
isothermal and adiabatic conditions. We study the influence of the planet on
the disk through two- and three-dimensional hydrodynamical simulations. To
strengthen our conclusions we perform a detailed numerical comparison where
several upwind and Riemann-solver based codes are used with and without the
FARGO-algorithm.
With respect to the wake structure and the torque density acting on the
planet we demonstrate that the FARGO-algorithm yields correct results, and that
at a fraction of the regular cpu-time. We find that the resolution requirements
for achieving convergent results in unshocked regions are rather modest and
depend on the pressure scale height of the disk. By comparing the torque
densities of 2D and 3D simulations we show that a suitable vertical averaging
procedure for the force gives an excellent agreement between the two. We show
that isothermal and adiabatic runs can differ considerably, even for adiabatic
indices very close to unity.Comment: accepted by Astronomy & Astrophysic
Laboratory Photo-chemistry of PAHs: Ionization versus Fragmentation
Interstellar polycyclic aromatic hydrocarbons (PAHs) are expected to be
strongly processed by vacuum ultraviolet photons. Here, we report experimental
studies on the ionization and fragmentation of coronene (C24H12), ovalene
(C32H14) and hexa-peri-hexabenzocoronene (HBC; C42H18) cations by exposure to
synchrotron radiation in the range of 8--40 eV. The results show that for small
PAH cations such as coronene, fragmentation (H-loss) is more important than
ionization. However, as the size increases, ionization becomes more and more
important and for the HBC cation, ionization dominates. These results are
discussed and it is concluded that, for large PAHs, fragmentation only becomes
important when the photon energy has reached the highest ionization potential
accessible. This implies that PAHs are even more photo-stable than previously
thought. The implications of this experimental study for the photo-chemical
evolution of PAHs in the interstellar medium are briefly discussed
On the corotation torque for low-mass eccentric planets
SMF acknowledges the support of an STFC PhD studentship. The simulations presented in this paper were performed on the QMUL HPC facility purchased under the SRIF initiatives
Planetesimal Formation In Self-Gravitating Discs
We study particle dynamics in local two-dimensional simulations of
self-gravitating accretion discs with a simple cooling law. It is well known
that the structure which arises in the gaseous component of the disc due to a
gravitational instability can have a significant effect on the evolution of
dust particles. Previous results using global simulations indicate that spiral
density waves are highly efficient at collecting dust particles, creating
significant local over-densities which may be able to undergo gravitational
collapse. We expand on these findings, using a range of cooling times to mimic
the conditions at a large range of radii within the disc. Here we use the
Pencil Code to solve the 2D local shearing sheet equations for gas on a fixed
grid together with the equations of motion for solids coupled to the gas solely
through aerodynamic drag force. We find that spiral density waves can create
significant enhancements in the surface density of solids, equivalent to 1-10cm
sized particles in a disc following the profiles of Clarke (2009) around a
solar mass star, causing it to reach concentrations several orders of magnitude
larger than the particles mean surface density. We also study the velocity
dispersion of the particles, finding that the spiral structure can result in
the particle velocities becoming highly ordered, having a narrow velocity
dispersion. This implies low relative velocities between particles, which in
turn suggests that collisions are typically low energy, lessening the
likelihood of grain destruction. Both these findings suggest that the density
waves that arise due to gravitational instabilities in the early stages of star
formation provide excellent sites for the formation of large,
planetesimal-sized objects.Comment: 11 pages, 8 figures, accepted for publication in MNRA
Porosity measurements of interstellar ice mixtures using optical laser interference and extended effective medium approximations
Aims. This article aims to provide an alternative method of measuring the
porosity of multi-phase composite ices from their refractive indices and of
characterising how the abundance of a premixed contaminant (e.g., CO2) affects
the porosity of water-rich ice mixtures during omni-directional deposition.
Methods. We combine optical laser interference and extended effective medium
approximations (EMAs) to measure the porosity of three astrophysically relevant
ice mixtures: H2O:CO2=10:1, 4:1, and 2:1. Infrared spectroscopy is used as a
benchmarking test of this new laboratory-based method. Results. By
independently monitoring the O-H dangling modes of the different water-rich ice
mixtures, we confirm the porosities predicted by the extended EMAs. We also
demonstrate that CO2 premixed with water in the gas phase does not
significantly affect the ice morphology during omni-directional deposition, as
long as the physical conditions favourable to segregation are not reached. We
propose a mechanism in which CO2 molecules diffuse on the surface of the
growing ice sample prior to being incorporated into the bulk and then fill the
pores partly or completely, depending on the relative abundance and the growth
temperature.Comment: 9 pages, 6 figures, 1 table. Accepted for publication in A&
On the dynamics and collisional growth of planetesimals in misaligned binary systems
Context. Abridged. Many stars are members of binary systems. During early
phases when the stars are surrounded by discs, the binary orbit and disc
midplane may be mutually inclined. The discs around T Tauri stars will become
mildly warped and undergo solid body precession around the angular momentum
vector of the binary system. It is unclear how planetesimals in such a disc
will evolve and affect planet formation. Aims. We investigate the dynamics of
planetesimals embedded in discs that are perturbed by a binary companion on a
circular, inclined orbit. We examine collisional velocities of the
planetesimals to determine when they can grow through accretion. We vary the
binary inclination, binary separation, D, disc mass, and planetesimal radius.
Our standard model has D=60 AU, inclination=45 deg, and a disc mass equivalent
to the MMSN. Methods. We use a 3D hydrodynamics code to model the disc.
Planetesimals are test particles which experience gas drag, the gravitational
force of the disc, the companion star gravity. Planetesimal orbit crossing
events are detected and used to estimate collisional velocities. Results. For
binary systems with modest inclination (25 deg), disc gravity prevents
planetesimal orbits from undergoing strong differential nodal precession (which
occurs in absence of the disc), and forces planetesimals to precess with the
disc on average. For bodies of different size the orbit planes become modestly
mutually inclined, leading to collisional velocities that inhibit growth. For
larger inclinations (45 degrees), the Kozai effect operates, leading to
destructively large relative velocities. Conclusions. Planet formation via
planetesimal accretion is difficult in an inclined binary system with
parameters similar to those considered in this paper. For systems in which the
Kozai mechanism operates, the prospects for forming planets are very remote.Comment: 24 pages, 16 figures, recently published in Astronomy and
Astrophysic
Spiral arms in scattered light images of protoplanetary discs: Are they the signposts of planets?
One of the striking discoveries of protoplanetary disc research in recent years are the spiral arms seen in several transitional discs in polarized scattered light. An interesting interpretation of the observed spiral features is that they are density waves launched by one or more embedded (proto)planets in the disc. In this paper, we investigate whether planets can be held responsible for the excitation mechanism of the observed spirals. We use locally isothermal hydrodynamic simulations as well as analytic formulae to model the spiral waves launched by planets. Then H-band scattered light images are calculated using a 3D continuum radiative transfer code to study the effect of surface density and pressure scaleheight perturbation on the detectability of the spirals. We find that a relative change of âŒ3.5 in the surface density (ÎŽÎŁ/ÎŁ) is required for the spirals to be detected with current telescopes in the near-infrared for sources at the distance of typical star-forming regions (140 pc). This value is a factor of 8 higher than what is seen in hydrodynamic simulations. We also find that a relative change of only 0.2 in pressure scaleheight is sufficient to create detectable signatures under the same conditions. Therefore, we suggest that the spiral arms observed to date in protoplanetary discs are the results of changes in the vertical structure of the disc (e.g. pressure scaleheight perturbation) instead of surface density perturbations.This work has been supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stv104
Modelling circumbinary protoplanetary disks II. Gas disk feedback on planetesimal dynamical and collisional evolution in the circumbinary systems Kepler-16 and 34
Aims. We investigate the feasibility of planetesimal growth in circumbinary
protoplanetary disks around the observed systems Kepler- 16 and Kepler-34 under
the gravitational influence of a precessing eccentric gas disk. Methods. We
embed the results of our previous hydrodynamical simulations of protoplanetary
disks around binaries into an N-body code to perform 3D, high-resolution,
inter-particle gravity-enabled simulations of planetesimal growth and dynamics
that include the gravitational force imparted by the gas. Results. Including
the full, precessing asymmetric gas disk generates high eccentricity orbits for
planetesimals orbiting at the edge of the circumbinary cavity, where the gas
surface density and eccentricity have their largest values. The gas disk is
able to efficiently align planetesimal pericenters in some regions leading to
phased, non-interacting orbits. Outside of these areas eccentric planetesimal
orbits become misaligned and overlap leading to crossing orbits and high
relative velocities during planetesimal collisions. This can lead to an
increase in the number of erosive collisions that far outweighs the number of
collisions that result in growth. Gravitational focusing from the static
axisymmetric gas disk is weak and does not significantly alter collision
outcomes from the gas free case. Conclusions. Due to asymmetries in the gas
disk, planetesimals are strongly perturbed onto highly eccentric orbits. Where
planetesimals orbits are not well aligned, orbit crossings lead to an increase
in the number of erosive collisions. This makes it difficult for sustained
planetesimal accretion to occur at the location of Kepler-16b and Kepler-34b
and we therefore rule out in-situ growth. This adds further support to our
initial suggestions that most circumbinary planets should form further out in
the disk and migrate inwards.Comment: 12 pages and 12 figure
Forming Circumbinary Planets: N-body Simulations of Kepler-34
Observations of circumbinary planets orbiting very close to the central stars
have shown that planet formation may occur in a very hostile environment, where
the gravitational pull from the binary should be very strong on the primordial
protoplanetary disk. Elevated impact velocities and orbit crossings from
eccentricity oscillations are the primary contributors towards high energy,
potentially destructive collisions that inhibit the growth of aspiring planets.
In this work, we conduct high resolution, inter-particle gravity enabled N-body
simulations to investigate the feasibility of planetesimal growth in the
Kepler-34 system. We improve upon previous work by including planetesimal disk
self-gravity and an extensive collision model to accurately handle
inter-planetesimal interactions. We find that super-catastrophic erosion events
are the dominant mechanism up to and including the orbital radius of
Kepler-34(AB)b, making in-situ growth unlikely. It is more plausible that
Kepler-34(AB)b migrated from a region beyond 1.5 AU. Based on the conclusions
that we have made for Kepler-34 it seems likely that all of the currently known
circumbinary planets have also migrated significantly from their formation
location with the possible exception of Kepler-47(AB)c.Comment: 6 pages, 5 figures, accepted for publication in ApJ
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