Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userGiant planets and brown dwarfs on wide orbits: a code comparison project
71
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Gas clumps formed within massive gravitationally unstable circumstellar discs are potential seeds of gas giant planets, brown dwarfs and companion stars. Simulations show that competition between three processes -- migration, gas accretion and tidal disruption -- establishes what grows from a given seed. Here we investigate the robustness of numerical modelling of clump migration and accretion with the codes PHANTOM, GADGET, SPHINX, SEREN, GIZMO-MFM, SPHNG and FARGO. The test problem comprises a clump embedded in a massive disc at an initial separation of 120 AU. There is a general qualitative agreement between the codes, but the quantitative agreement in the planet migration rate ranges from $sim 10$% to $sim 50$%, depending on the numerical setup. We find that the artificial viscosity treatment and the sink particle prescription may account for much of the differences between the codes. In order to understand the wider implications of our work, we also attempt to reproduce the planet evolution tracks from our hydrodynamical simulations with prescriptions from three previous population synthesis studies. We find that the disagreement amongst the population synthesis models is far greater than that between our hydrodynamical simulations. The results of our code comparison project are therefore encouraging in that uncertainties in the given problem are probably dominated by the physics not yet included in the codes rather than by how hydrodynamics is modelled in them.
rate research
Read More
Context: We studied numerically the formation of giant planet (GP) and brown dwarf (BD) embryos in gravitationally unstable protostellar disks and compared our findings with directly-imaged, wide-orbit (>= 50 AU) companions known to-date. The viability of the disk fragmentation scenario for the formation of wide-orbit companions in protostellar disks around (sub-)solar mass stars was investigated. Methods: We used numerical hydrodynamics simulations of disk formation and evolution with an accurate treatment of disk thermodynamics. The use of the thin-disk limit allowed us to probe the long-term evolution of protostellar disks. We focused on models that produced wide-orbit GP/BD embryos, which opened a gap in the disk and showed radial migration timescales similar to or longer than the typical disk lifetime. Results: While disk fragmentation was seen in the majority of our models, only 6 models out of 60 revealed the formation of quasi-stable, wide-orbit GP/BD embryos. Disk fragmentation produced GP/BD embryos with masses in the 3.5-43 M_J range, covering the whole mass spectrum of directly-imaged, wide-orbit companions to (sub-)solar mass stars. On the other hand, our modelling failed to produce embryos on orbital distances <= 170 AU, whereas several directly-imaged companions were found at smaller orbits down to a few AU. Disk fragmentation also failed to produce wide-orbit companions around stars with mass <= 0.7 Msun, in disagreement with observations. Conclusions: Disk fragmentation is unlikely to explain the whole observed spectrum of wide-orbit companions to (sub-)solar-mass stars and other formation mechanisms, e.g., dynamical scattering of closely-packed companions onto wide orbits, should be invoked to account for companions at orbital distance from a few tens to approx 150 AU and wide-orbit companions with masses of the host star <= 0.7 Msun. (abridged)
Understanding the formation and evolution of giant planets ($ge$1 $M_{Jup}$) at wide orbital separation ($ge$5 AU) is one of the goals of direct imaging. Over the past 15 years, many surveys have placed strong constraints on the occurrence rate of wide-orbit giants, mostly based on non-detections, but very few have tried to make a direct link with planet formation theories. In the present work, we combine the results of our previously published VLT/NaCo large program with the results of 12 past imaging surveys to constitute a statistical sample of 199 FGK stars within 100 pc, including 3 stars with sub-stellar companions. Using Monte Carlo simulations and assuming linear flat distributions for the mass and semi-major axis of planets, we estimate the sub-stellar companion frequency to be within 0.75-5.7% at the 68% confidence level (CL) within 20-300 AU and 0.5-75 $M_{Jup}$, which is compatible with previously published results. We also compare our results with the predictions of state-of-the-art population synthesis models based on the gravitational instability (GI) formation scenario by Forgan & Rice (2013), with and without scattering. We estimate that in both the scattered and non-scattered populations, we would be able to detect more than 30% of companions in the 1-75 $M_{Jup}$ range (95% CL). With the 3 sub-stellar detections in our sample, we estimate the fraction of stars that host a planetary system formed by GI to be within 1.0-8.6% (95% CL). We also conclude that even though GI is not common, it predicts a mass distribution of wide-orbit massive companions that is much closer to what is observed than what the core accretion scenario predicts. Finally, we associate the present paper with the release of the Direct Imaging Virtual Archive (DIVA, http://cesam.lam.fr/diva/), a public database that aims at gathering the results of past, present, and future direct imaging surveys.
Very little is known about magnetic fields of extrasolar planets and brown dwarfs. We use the energy flux scaling law presented by Christensen et al. (2009) to calculate the evolution of average magnetic fields in extrasolar planets and brown dwarfs under the assumption of fast rotation, which is probably the case for most of them. We find that massive brown dwarfs of about 70 M_Jup can have fields of a few kilo-Gauss during the first few hundred Million years. These fields can grow by a factor of two before they weaken after deuterium burning has stopped. Brown dwarfs with weak deuterium burning and extrasolar giant planets start with magnetic fields between ~100G and ~1kG at the age of a few Myr, depending on their mass. Their magnetic field weakens steadily until after 10Gyr it has shrunk by about a factor of 10. We use observed X-ray luminosities to estimate the age of the known extrasolar giant planets that are more massive than 0.3M_Jup and closer than 20pc. Taking into account the age estimate, and assuming sun-like wind-properties and radio emission processes similar to those at Jupiter, we calculate their radio flux and its frequency. The highest radio flux we predict comes out as 700mJy at a frequency around 150MHz for $tau$Boob, but the flux is below 60mJy for the rest. Most planets are expected to emit radiation between a few Mhz and up to 100MHz, well above the ionospheric cutoff frequency.
In recent years there have been many attempts to characterize the occurrence of stellar, BD and planetary-mass companions to solar-type stars, with the aim of constraining formation mechanisms. From RV observations a dearth of companions with masses between 10-40 MJup has been noticed at close separations, suggesting the possibility of a distinct formation mechanism for objects above and below this range. We present a model for the substellar companion mass function (CMF). It consists of the superposition of the planet and BD companion mass distributions, assuming that we can extrapolate the RV measured companion mass function for planets to larger separations and the stellar companion mass-ratio distribution over all separations into the BD mass regime. By using both the results of the VLT/NaCo large program and the complementary archive datasets that probe the occurrence of planets and BDs on wide orbits around solar-type stars, we place some constraints on the planet and BD distributions. We developed a MC simulation tool to predict the outcome of a given survey, depending on the shape of the orbital parameter distributions. Comparing the predictions with the results of the observations, we calculate how likely different models are and which can be ruled out. Current observations are consistent with the proposed model for the CMF, as long as a sufficiently small outer truncation radius is introduced for the planet separation distribution. The results of the direct imaging surveys searching for substellar companions around Sun-like stars are consistent with a combined substellar mass spectrum of planets and BDs. This mass distribution has a minimum between 10 and 50 MJup, in agreement with RV measurements. The dearth of objects in this mass range would naturally arise from the shape of the mass distribution, without the introduction of any distinct formation mechanism for BDs.
There is no universally acknowledged criterion to distinguish brown dwarfs from planets. Numerous studies have used or suggested a definition based on an objects mass, taking the ~13-Jupiter mass (M_J) limit for the ignition of deuterium. Here, we investigate various deuterium-burning masses for a range of models. We find that, while 13 M_J is generally a reasonable rule of thumb, the deuterium fusion mass depends on the helium abundance, the initial deuterium abundance, the metallicity of the model, and on what fraction of an objects initial deuterium abundance must combust in order for the object to qualify as having burned deuterium. Even though, for most proto-brown dwarf conditions, 50% of the initial deuterium will burn if the objects mass is ~(13.0 +/- 0.8)M_J, the full range of possibilities is significantly broader. For models ranging from zero-metallicity to more than three times solar metallicity, the deuterium burning mass ranges from ~11.0 M_J (for 3-times solar metallicity, 10% of initial deuterium burned) to ~16.3 M_J (for zero metallicity, 90% of initial deuterium burned).
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
