223 research outputs found

### Numerical simulations of the type III migration:I. Disc model and convergence tests

We investigate the fast (type III) migration regime of high-mass protoplanets
orbiting in protoplanetary disks. This type of migration is dominated by
corotational torques. We study the details of flow structure in the planet's
vicinity, the dependence of migration rate on the adopted disc model, and the
numerical convergence of models (independence of certain numerical parameters
such as gravitational softening). We use two-dimensional hydrodynamical
simulations with adaptive mesh refinement,based on the FLASH code with improved
time-stepping scheme. We perform global disk simulations with sufficient
resolution close to the planet, which is allowed to freely move throughout the
grid. We employ a new type of equation of state in which the gas temperature
depends on both the distance to the star and planet, and a simplified
correction for self-gravity of the circumplanetary gas. We find that the
migration rate in the type III migration regime depends strongly on the gas
dynamics inside the Hill sphere (Roche lobe of the planet) which, in turn, is
sensitive to the aspect ratio of the circumplanetary disc. Furthermore,
corrections due to the gas self-gravity are necessary to reduce numerical
artifacts that act against rapid planet migration. Reliable numerical studies
of Type III migration thus require consideration of both the thermal andthe
self-gravity corrections, as well as a sufficient spatial resolution and the
calculation of disk-planet attraction both inside and outside the Hill sphere.
With this proviso, we find Type III migration to be a robust mode of migration,
astrophysically promising because of a speed much faster than in the previously
studied modes of migration.Comment: 17 pages, 15 figures, submitted to MNRAS. Comments welcom

### The evolution of a supermassive binary caused by an accretion disc

The interaction of a massive binary and a non-self-gravitating circumbinary
accretion disc is considered. The shape of the stationary twisted disc produced
by the binary is calculated. It is shown that the inner part of the disc must
lie in the binary orbital plane for any value of viscosity.
When the inner disc midplane is aligned with the binary orbital plane on the
scales of interest and it rotates in the same sense as the binary, the
modification of the disc structure and the rate of decay of the binary orbit,
assumed circular, due to tidal exchange of angular momentum with the disc, are
calculated. It is shown that the modified disc structure is well described by a
self-similar solution of the non-linear diffusion equation governing the
evolution of the disc surface density. The calculated time scale for decay of
the binary orbit is always smaller than the "accretion" time $t_{acc}=m/{\dot
M}$ ($m$ is the mass of the secondary component, and $\dot M$ is the disc
accretion rate), and is determined by ratio of secondary mass $m$, assumed to
be much smaller than the primary mass, the disc mass inside the initial binary
orbit, and the form of viscosity in the disc.Comment: to be published in MNRA

### Collisional dust avalanches in debris discs

We quantitatively investigate how collisional avalanches may developin debris
discs as the result of the initial break-up of a planetesimal or comet-like
object, triggering a collisional chain reaction due to outward escaping small
dust grains. We use a specifically developed numerical code that follows both
the spatial distribution of the dust grains and the evolution of their
size-frequency distribution due to collisions. We investigate how strongly
avalanche propagation depends on different parameters (e.g., amount of dust
released in the initial break-up, collisional properties of dust grains and
their distribution in the disc). Our simulations show that avalanches evolve on
timescales of ~1000 years, propagating outwards following a spiral-like
pattern, and that their amplitude exponentially depends on the number density
of dust grains in the system. We estimate a probability for witnessing an
avalanche event as a function of disc densities, for a gas-free case around an
A-type star, and find that features created by avalanche propagation can lead
to observable asymmetries for dusty systems with a beta Pictoris-like dust
content or higher. Characteristic observable features include: (i) a brightness
asymmetry of the two sides for a disc viewed edge-on, and (ii) a one-armed open
spiral or a lumpy structure in the case of face-on orientation. A possible
system in which avalanche-induced structures might have been observed is the
edge-on seen debris disc around HD32297, which displays a strong luminosity
difference between its two sides.Comment: 18 pages, 19 figures; has been accepted for publication in Astronomy
and Astrophysics, section 6. Interstellar and circumstellar matter. The
official date of acceptance is 29/08/200

### Survival of icy grains in debris discs. The role of photosputtering

We put theoretical constraints on the presence and survival of icy grains in
debris discs. Particular attention is paid to UV sputtering of water ice, which
has so far not been studied in detail in this context. We present a
photosputtering model based on available experimental and theoretical studies.
We quantitatively estimate the erosion rate of icy and ice-silicate grains,
under the influence of both sublimation and photosputtering, as a function of
grain size, composition and distance from the star. The effect of erosion on
the grain's location is investigated through numerical simulations coupling the
grain size to its dynamical evolution. Our model predicts that photodesorption
efficiently destroy ice in optically thin discs, even far beyond the
sublimation snow line. For the reference case of beta Pictoris, we find that
only > 5mm grains can keep their icy component for the age of the system in the
50-150AU region. When taking into account the collisional reprocessing of
grains, we show that the water ice survival on grains improves (grains down to
~ 20 um might be partially icy). However, estimates of the amount of gas
photosputtering would produce on such a hypothetical population of big icy
grains lead to values for the OI column density that strongly exceed
observational constraints for beta Pic, thus ruling out the presence of a
significant amount of icy grains in this system. Erosion rates and icy grains
survival timescales are also given for a set of 11 other debris disc systems.
We show that, with the possible exception of M stars, photosputtering cannot be
neglected in calculations of icy grain lifetimes.Comment: 12 pages, 9 figures. accepted by A&

### Eccentricity Evolution of Extrasolar Multiple Planetary Systems due to the Depletion of Nascent Protostellar Disks

Most extrasolar planets are observed to have eccentricities much larger than
those in the solar system. Some of these planets have sibling planets, with
comparable masses, orbiting around the same host stars. In these multiple
planetary systems, eccentricity is modulated by the planets' mutual secular
interaction as a consequence of angular momentum exchange between them. For
mature planets, the eigenfrequencies of this modulation are determined by their
mass and semi-major axis ratios. But, prior to the disk depletion, self gravity
of the planets' nascent disks dominates the precession eigenfrequencies. We
examine here the initial evolution of young planets' eccentricity due to the
apsidal libration or circulation induced by both the secular interaction
between them and the self gravity of their nascent disks. We show that as the
latter effect declines adiabatically with disk depletion, the modulation
amplitude of the planets' relative phase of periapse is approximately invariant
despite the time-asymmetrical exchange of angular momentum between planets.
However, as the young planets' orbits pass through a state of secular
resonance, their mean eccentricities undergo systematic quantitative changes.
For applications, we analyze the eccentricity evolution of planets around
Upsilon Andromedae and HD168443 during the epoch of protostellar disk
depletion. We find that the disk depletion can change the planets' eccentricity
ratio. However, the relatively large amplitude of the planets' eccentricity
cannot be excited if all the planets had small initial eccentricities.Comment: 50 pages including 11 figures, submitted to Ap

### The 1.2 Millimeter Image of the beta Pictoris Disk

We present millimeter imaging observations in the 1200 micron continuum of
the disk around beta Pictoris. With the 25 arcsec beam, the beta Pic disk is
unresolved perpendicularly to the disk plane (< 10 arcsec), but slightly
resolved in the northeast-southwest direction (26 arcsec). Peak emission is
observed at the stellar position. A secondary maximum is found 1000 AU along
the disk plane in the southwest, which does not positionally coincide with a
similar feature reported earlier at 850 micron. Arguments are presented which
could be seen in support of the reality of these features. The observed
submm/mm emission is consistent with thermal emission from dust grains, which
are significantly larger than those generally found in the interstellar medium,
including mm-size particles, and thus more reminiscent of the dust observed in
protostellar disks. Modelling the observed scattered light in the visible and
the emission in the submm/mm provides evidence for the particles dominating the
scattering in the visible/NIR and those primarily responsible for the thermal
emission at longer wavelengths belonging to different populations.Comment: 6 pages, 3 postscript figures, accepted for publication in Astronomy
and Astrophysic

### Narrow-Angle Astrometry with the Space Interferometry Mission: The Search for Extra-Solar Planets. II. Detection and Characterization of Planetary Systems

(Abridged) The probability of detecting additional companions is essentially
unchanged with respect to the single-planet configurations, but after fitting
and subtraction of orbits with astrometric signal-to-noise ratio
$\alpha/\sigma_d\to 1$ the false detection rates can be enhanced by up to a
factor 2; the periodogram approach results in robust multiple-planet detection
for systems with periods shorter than the SIM mission length, even at low
values of $\alpha/\sigma_d$, while the least squares technique combined with
Fourier series expansions is arguably preferable in the long-period regime. The
accuracy on multiple-planet orbit reconstruction and mass determination suffers
a typical degradation of 30-40% with respect to single-planet solutions; mass
and orbital inclination can be measured to better than 10% for periods as short
as 0.1 yr, and for $\alpha/\sigma_d$ as low as $\sim 5$, while
$\alpha/\sigma_d\simeq 100$ is required in order to measure with similar
accuracy systems harboring objects with periods as long as three times the
mission duration. For systems with all components producing
$\alpha/\sigma_d\simeq 10$ or greater, quasi-coplanarity can be reliably
established with uncertainties of a few degrees, for periods in the range
$0.1\leq T\leq 15$ yr; in systems where at least one component has
$\alpha/\sigma_d\to 1$, coplanarity measurements are compromised, with typical
uncertainties on the mutual inclinations of order of $30^\circ-40^\circ$. Our
findings are illustrative of the importance of the contribution SIM will make
to the fields of formation and evolution of planetary systems.Comment: 61 pages, 14 figures, 5 tables, to appear in the September 2003 Issue
of the Publications of the Astronomical Society of the Pacifi

### Bimodal Brightness Oscillations in Models of Young Binary Systems

We consider a model for the cyclic activity of young binary stars that
accrete matter from the remnants of a protostellar cloud. If the orbit of such
a binary system is inclined at a small angle to the line of sight, then the
streams of matter and the density waves excited in the circumbinary disk can
screen the primary component of the binary from the observer. To study these
phenomena by the SPH (smoothed particle hydrodynamics) method, we have computed
grids of hydrodynamic models for binary systems based on which we have
constructed the light curves as a function of the orbital phase.The main
emphasis is on investigating the properties of the brightness oscillations.
Therefore, the model parameters were varied within the following ranges: the
component mass ratio q = M2 : M1 = 0.2 - 0.5 and the eccentricity e = 0 - 0.7.
The parameter that defined the binary viscosity was also varied. We adopted
optical grain characteristics typical of circumstellar dust. Our computations
have shown that bimodal oscillations are excited in binaries with eccentric
orbits, provided that the binary components do not differ too much in mass. In
this case, the ratios of the periods and amplitudes of the bimodal oscillations
and their shape depend strongly on the inclination of the binary plane and its
orientation relative to the observer. Our analysis shows that the computed
light curves can be used in interpreting the cyclic activity of UX Ori stars.Comment: 14 pages, 5 figures, 1 tabl

### Hydrodynamic Processes in Young Binary Systems as a Source of Cyclic Variations of Circumstellar Extinction

Hydrodynamic models of a young binary system accreting matter from the
remnants of a protostellar cloud have been calculated by the SPH method. It is
shown that periodic variations in column density in projection onto the primary
component take place at low inclinations of the binary plane to the line of
sight. They can result in periodic extinction variations. Three periodic
components can exist in general case. The first component has a period equal to
the orbital one and is attributable to the streams of matter penetrating into
the inner regions of the binary. The second component has a period that is a
factor of 5-8 longer than the orbital one and is related to the density waves
generated in a circumbinary (CB) disk. The third, longest period is
attributable to the precession of the inner asymmetric region of CB disk. The
relationship between the amplitudes of these cycles depends on the model
parameters as well as on the inclination and orientation of the binary in
space. We show that at a dust-to-gas ratio of 1:100 and and a mass extinction
coefficient of 250 cm$^2$ g$^{-1}$, the amplitude of the brightness variations
of the primary component in the V-band can reach $1^m$ at a mass accretion rate
onto the binary components of $10^{-8} M_{\odot}$ yr$^{-1}$ and a $10^{\rm o}$
inclination of the binary plane to the line of sight. We discuss possible
applications of the model to pre-main-sequence stars.Comment: 13 pages, 6 figures, published in Astronomy Letters (v.33, 2007

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