708 research outputs found
Feedback-limited Accretion: Luminous Signatures from Growing Planets
Planets form in discs of gas and dust around stars, and keep growing by
accretion of disc material while available. Massive planets clear a gap in that
protoplanetary disc, but still accrete through spiral wakes. On its way to the
planet, the gas will settle on a \emph{circumplanetary} disc around the planet
and slowly accrete on to it. The energy of the accreted gas will be released,
heating the planet surroundings in a feedback process. For high enough
accretion rates the planet should be detectable at infrared wavelengths. We aim
to find whether detectable planet luminosities, , can occur when considering that the planet luminosity is
coupled to the accretion, and also to study which other effects has the
feedback on the dynamics of the circumplanetary and the gap regions. We model a
planet with mass ratio , orbiting at 10 AU from a solar mass star,
using a modified version of the 2D code FARGO-AD, which includes a prescription
for the accretion and feedback luminosity of the planet. We find that the
planetary feedback is able to partially deplete the circumplanetary disc, and
to reduce the accretion rate onto the planet. However, detectable luminosities
of are still produced. The
feedback also contributes to partially refilling the gap, to heat up the
coorbital region, and to perturb the orbital velocity of the gas.Comment: Submitted to MNRA
Dense Deformation Field Estimation for Atlas Registration using the Active Contour Framework
In this paper, we propose a new paradigm to carry outthe registration task with a dense deformation fieldderived from the optical flow model and the activecontour method. The proposed framework merges differenttasks such as segmentation, regularization, incorporationof prior knowledge and registration into a singleframework. The active contour model is at the core of ourframework even if it is used in a different way than thestandard approaches. Indeed, active contours are awell-known technique for image segmentation. Thistechnique consists in finding the curve which minimizesan energy functional designed to be minimal when thecurve has reached the object contours. That way, we getaccurate and smooth segmentation results. So far, theactive contour model has been used to segment objectslying in images from boundary-based, region-based orshape-based information. Our registration technique willprofit of all these families of active contours todetermine a dense deformation field defined on the wholeimage. A well-suited application of our model is theatlas registration in medical imaging which consists inautomatically delineating anatomical structures. Wepresent results on 2D synthetic images to show theperformances of our non rigid deformation field based ona natural registration term. We also present registrationresults on real 3D medical data with a large spaceoccupying tumor substantially deforming surroundingstructures, which constitutes a high challenging problem
Limiting eccentricity of sub-parsec massive black hole binaries surrounded by self-gravitating gas discs
We study the dynamics of supermassive black hole binaries embedded in
circumbinary gaseous discs, with the SPH code Gadget-2. The sub-parsec binary
(of total mass M and mass ratio q=1/3) has excavated a gap and transfers its
angular momentum to the self--gravitating disc (M_disc=0.2 M). We explore the
changes of the binary eccentricity e, by simulating a sequence of binary models
that differ in the initial eccentricity e_0, only. In initially low-eccentric
binaries, the eccentricity increases with time, while in high-eccentric
binaries e declines, indicating the existence of a limiting eccentricity e_crit
that is found to fall in the interval [0.6,0.8]. We also present an analytical
interpretation for this saturation limit. An important consequence of the
existence of e_crit is the detectability of a significant residual eccentricity
e_LISA} by the proposed gravitational wave detector LISA. It is found that at
the moment of entering the LISA frequency domain e_LISA ~ 10^{-3}-10^{-2}; a
signature of its earlier coupling with the massive circumbinary disc. We also
observe large periodic inflows across the gap, occurring on the binary and disc
dynamical time scales rather than on the viscous time. These periodic changes
in the accretion rate (with amplitudes up to ~100%, depending on the binary
eccentricity) can be considered a fingerprint of eccentric sub-parsec binaries
migrating inside a circumbinary disc.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Optical and Infrared Photometry of the Unusual Type Ia Supernova 2000cx
We present optical and infrared photometry of the unusual Type Ia supernova
2000cx. With the data of Li et al. (2001) and Jha (2002), this comprises the
largest dataset ever assembled for a Type Ia SN, more than 600 points in
UBVRIJHK. We confirm the finding of Li et al. regarding the unusually blue B-V
colors as SN 2000cx entered the nebular phase. Its I-band secondary hump was
extremely weak given its B-band decline rate. The V minus near infrared colors
likewise do not match loci based on other slowly declining Type Ia SNe, though
V-K is the least ``abnormal''. In several ways SN 2000cx resembles other slow
decliners, given its B-band decline rate (Delta m_15(B) = 0.93), the appearance
of Fe III lines and weakness of Si II in its pre-maximum spectrum, the V-K
colors and post-maximum V-H colors. If the distance modulus derived from
Surface Brightness Fluctuations of the host galaxy is correct, we find that the
rate of light increase prior to maximum, the characteristics of the bolometric
light curve, and the implied absolute magnitude at maximum are all consistent
with a sub-luminous object with Delta m_15(B) ~ 1.6-1.7 having a higher than
normal kinetic energy.Comment: 46 pages, 17 figures, to be published in Publications of the
Astronomical Society of the Pacifi
Is there really a debris disc around ?
The presence of a debris disc around the Gyr-old solar-type star
was suggested by the infrared
excess detection. Follow-up observations with /PACS revealed
a double-lobed feature, that displayed asymmetries both in brightness and
position. Therefore, the disc was thought to be edge-on and significantly
eccentric. Here we present ALMA/ACA observations in Band 6 and 7 which
unambiguously reveal that these lobes show no common proper motion with
. In these observations, no flux has been detected
around that exceeds the levels. We
conclude that surface brightness upper limits of a debris disc around
are at 1.3 mm, and
at 870 microns. Our results overall demonstrate
the capability of the ALMA/ACA to follow-up observations of
debris discs and clarify the effects of background confusion.Comment: 6 pages, 2 figures, 2 table
Atlas-based Segmentation using a Model of Lesion Growth
We propose a method for brain atlas deformation in presence of large space-occupying tumors or lesions, based on an a priori model of lesion growth that assumes radial expansion of the lesion from its central point. Atlas-based methods have been of limited use for segmenting brains that have been drastically altered by the presence of large space-occupying lesions. Our approach involves four steps. First, an affine registration brings the atlas and the patient into global correspondence. Secondly, a local registration warps the atlas onto the patient volume. Then, the seeding of a synthetic tumor into the brain atlas provides a template for the lesion. The last step is the deformation of the seeded atlas, combining a method derived from optical flow principles and a model of lesion growth. Results show that a good registration is performed and that method can be applied to automatic segmentation of structures and substructures in brains with gross deformation, with important medical applications in neurosurgery, radiosurgery and radiotherapy
Evolution of massive stars with new hydrodynamic wind models
Here we present evolutionary models for a set of massive stars, introducing a
new prescription for the mass-loss rate obtained from hydrodynamical
calculations in which the wind velocity profile, , and the
line-acceleration, , are obtained in a self consistently way.
Replacing mass-loss rates at the Main Sequence stage from the standard Vink's
formula by our new recipe, we generate a new set of evolutionary tracks for
and and metallicities
(Galactic), (LMC), and (SMC).
Our new derived formula for mass-loss rate predicts a dependence , where is not longer constant but dependent on the stellar
mass: ranging from when , to when
.
We found that models adopting the new recipe for retain more mass
during their evolution, which is expressed in larger radii and consequently
more luminous tracks over the Hertzsprung-Russell diagram. These differences
are more prominent for the cases of and 120 at
solar metallicity, where we found self-consistent tracks are dex
brighter and keep extra mass up to 20 , compared with the classical
models using the previous formulation for mass-loss rate.
Moreover, we observed remarkable differences for the evolution of the
radionuclide isotope Al in the core and the surface of the star. Since
are weaker than the commonly adopted values for evolutionary
tracks, self-consistent tracks predict a later modification in the abundance
number of Al in the stellar winds. This new behaviour could provide
useful information about the real contribution of this isotope from massive
stars to the Galactic interstellar medium.Comment: Accepted for publication in Astronomy & Astrophysic
Evolution of binary black holes in self gravitating discs: dissecting the torques
We study the interplay between gas accretion and gravity torques in changing a binary elements and its total angular momentum (L) budget. Especially, we analyse the physical origin of the gravity torques (T_g) and their location within the disc. We analyse 3D SPH simulations of the evolution of initially quasi-circular massive black hole binaries (BHBs) residing in the central hollow of massive self-gravitating circumbinary discs. We use different thermodynamics within the cavity and for the numerical size of the black holes to show that (i) the BHB eccentricity growth found previously is a general result, independent of the accretion and the adopted thermodynamics; (ii) the semi-major axis decay depends both on the T_g and on the interplay with the disc-binary L-transfer due to accretion; (iii) the spectral structure of the T_g is predominately caused by disc edge overdensities and spiral arms developing in the body of the disc and, in general, does not reflect directly the period of the binary; (iv) the net T_g changes sign across the BHB corotation radius. We quantify the relative importance of the two, which appear to depend on the thermodynamical properties of the instreaming gas, and which is crucial in assessing the disc-binary L-transfer; (v) the net torque manifests as a purely kinematic (non-resonant) effect as it stems from the cavity, where the material flows in and out in highly eccentric orbits. Both accretion onto the black holes and the interaction with gas streams inside the cavity must be taken into account to assess the fate of the BHB. Moreover, the total torque exerted by the disc affects L(BHB) by changing all the elements (mass, mass ratio, eccentricity, semimajor axis) of the BHB. Common prescriptions equating tidal torque to semi-major axis shrinking might therefore be poor approximations for real astrophysical systems
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