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Register a new userPossible Signatures of Magnetospheric Accretion onto Young Giant Planets
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Richard V. E. Lovelace
Publication date
2010
fields
Physics
and research's language is
English
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Magnetospheric accretion is an important process for a wide range of astrophysical systems, and may play a role in the formation of gas giant planets. Extending the formalism describing stellar magnetospheric accretion into the planetary regime, we demonstrate that magnetospheric processes may govern accretion onto young gas giants in the isolation phase of their development. Planets in the isolation phase have cleared out large gaps in their surrounding circumstellar disks, and settled into a quasi-static equilibrium with radii only modestly larger than their final sizes (i.e., $ r sim 1.4 r_{rm final}$). Magnetospheric accretion is less likely to play a role in a young gas giants main accretion phase, when the planets envelope is predicted to be much larger than the planets Alfven radius. For a fiducial 1 M$_J$ gas giant planet with a remnant isolation phase accretion rate of $dot{M}_{odot} =$ 10$^{-10} M_{odot}{rm yr}^{-1}=10^{-7}M_{J}{rm yr}^{-1}$, the disk accretion will be truncated at $sim 2.7r_J$ (with $r_J$ is Jupiters radius) and drive the planet to rotate with a period of $sim$7 hours. Thermal emission from planetary magnetospheric accretion will be difficult to observe; the most promising observational signatures may be non-thermal, such as gyrosynchrotron radiation that is clearly modulated at a period much shorter than the rotation period of the host star.
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We study the accretion of dust particles of various sizes onto embedded massive gas giant planets, where we take into account the structure of the gas disk due to the presence of the planet. The accretion rate of solids is important for the structure of giant planets: it determines the growth rate of the solid core that may be present as well as their final enrichment in solids. We use the RODEO hydrodynamics solver to solve the flow equations for the gas, together with a particle approach for the dust. The solver for the particles equations of motion is implicit with respect to the drag force, which allows us to treat the whole dust size spectrum. We find that dust accretion is limited to the smallest particle sizes. The largest particles get trapped in outer mean-motion resonances with the planet, while particles of intermediate size are pushed away from the orbit of the planet by the density structure in the gas disk. Only particles smaller than approximately s_max =10 micron may accrete on a planet with the mass of Jupiter. For a ten times less massive planet s_max=100 micron. The strongly reduced accretion of dust makes it very hard to enrich a newly formed giant planet in solids.
Models of magnetically-driven accretion and outflows reproduce many observational properties of T Tauri stars. This concept is not well established for the more massive Herbig Ae/Be stars. We intend to examine the magnetospheric accretion in Herbig Ae/Be stars and search for rotational modulation using spectroscopic signatures, in this first paper concentrating on the well-studied Herbig Ae star HD101412. We used near-infrared spectroscopic observations of the magnetic Herbig Ae star HD101412 to test the magnetospheric character of its accretion disk/star interaction. We reduced and analyzed 30 spectra of HD101412, acquired with the CRIRES and X-shooter spectrographs installed at the VLT (ESO, Chile). The spectroscopic analysis was based on the He I lambda 10,830 and Pa gamma lines, formed in the accretion region. We found that the temporal behavior of these diagnostic lines in the near-infrared spectra of HD101412 can be explained by rotational modulation of line profiles generated by accreting gas with a period P = 20.53+-1.68 d. The discovery of this period, about half of the magnetic rotation period P_m = 42.076 d previously determined from measurements of the mean longitudinal magnetic field, indicates that the accreted matter falls onto the star in regions close to the magnetic poles intersecting the line-of-sight two times during the rotation cycle. We intend to apply this method to a larger sample of Herbig Ae/Be stars.
The detection of a dust disc around G29-38 and transits from debris orbiting WD1145+017 confirmed that the photospheric trace metals found in many white dwarfs arise from the accretion of tidally disrupted planetesimals. The composition of these planetesimals is similar to that of rocky bodies in the inner solar system. Gravitationally scattering planetesimals towards the white dwarf requires the presence of more massive bodies, yet no planet has so far been detected at a white dwarf. Here we report optical spectroscopy of a $simeq27,750$K hot white dwarf that is accreting from a circumstellar gaseous disc composed of hydrogen, oxygen, and sulphur at a rate of $simeq3.3times10^9,mathrm{g,s^{-1}}$. The composition of this disc is unlike all other known planetary debris around white dwarfs, but resembles predictions for the makeup of deeper atmospheric layers of icy giant planets, with H$_2$O and H$_2$S being major constituents. A giant planet orbiting a hot white dwarf with a semi-major axis of $simeq15$ solar radii will undergo significant evaporation with expected mass loss rates comparable to the accretion rate onto the white dwarf. The orbit of the planet is most likely the result of gravitational interactions, indicating the presence of additional planets in the system. We infer an occurrence rate of spectroscopically detectable giant planets in close orbits around white dwarfs of $simeq10^{-4}$.
The growth process of proto-planets can be sped-up by accreting a large number of solid, pebble-sized objects that are still present in the protoplanetary disc. It is still an open question on how efficient this process works in realistic turbulent discs. Here, we investigate the accretion of pebbles in turbulent discs that are driven by the purely hydrodynamical vertical shear instability (VSI). For this purpose, we perform global three-dimensional simulations of locally isothermal, VSI turbulent discs with embedded protoplanetary cores from 5 to 100 $M_oplus$ that are placed at 5.2 au distance from the star. In addition, we follow the evolution of a swarm of embedded pebbles of different size under the action of drag forces between gas and particles in this turbulent flow. Simultaneously, we perform a set of comparison simulations for laminar viscous discs where the particles experience stochastic kicks. For both cases, we measure the accretion rate onto the cores as a function of core mass and Stokes number ($tau_s$) of the particles and compare it to recent MRI turbulence simulations. Overall the dynamic is very similar for the particles in the VSI turbulent disc and the laminar case with stochastic kicks. For the small mass planets (i.e. 5 and 10 $M_oplus$), well-coupled particles with $tau_s = 1$, which have a size of about one meter at this location, we find an accretion efficiency (rate of particles accreted over drifting inward) of about 1.6-3%. For smaller and larger particles this efficiency is higher. However, the fast inward drift for $tau_s = 1$ particles makes them the most effective for rapid growth, leading to mass doubling times of about 20,000 yr. For masses between 10 and 30 $M_oplus$ the core reaches the pebble isolation mass and the particles are trapped at the pressure maximum just outside of the planet, shutting off further particle accretion.
To investigate the inner regions of protoplanetary disks, we performed near-infrared interferometric observations of the classical TTauri binary system S CrA. We present the first VLTI-GRAVITY high spectral resolution ($Rsim$4000) observations of a classical TTauri binary, S CrA (composed of S CrA N and S CrA S and separated by $sim$1.4), combining the four 8-m telescopes in dual-field mode. Our observations in the near-infrared K-band continuum reveal a disk around each binary component, with similar half-flux radii of about 0.1 au at d$sim$130 pc, inclinations ($i=$28$pm$3$^o$ and $i=$22$pm$6$^o$), and position angles (PA=0$^opm$6$^o$ and PA=-2$^opm$12$^o$), suggesting that they formed from the fragmentation of a common disk. The S CrA N spectrum shows bright HeI and Br$gamma$ line emission exhibiting inverse P-Cygni profiles, typically associated with infalling gas. The continuum-compensated Br$gamma$ line visibilities of S CrA N show the presence of a compact Br$gamma$ emitting region the radius of which is about $sim$0.06 au, which is twice as big as the truncation radius. This component is mostly tracing a wind. Moreover, a slight radius change between the blue- and red-shifted Br$gamma$ line components is marginally detected. The presence of an inverse P-Cygni profile in the HeI and Br$gamma$ lines, along with the tentative detection of a slightly larger size of the blue-shifted Br$gamma$ line component, hint at the simultaneous presence of a wind and magnetospheric accretion in S CrA N.
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