Extrasolar Planets

Recent papers on astro-ph

Wind Accretion to Dipole
- Bondi accretion
- Isolated old NS
- Propeller stage
- Magneto t a i l s

Disk Accretion to Dipole
- Inclined   rotator
- F u n n e l flows
- Propeller   stage
- Hot spots on star
- Radiative   shock

The Origin of Jets

Accretion Disks Theory
- Counterrotating
- ADAF theory

Extrasolar Planets

EXTRASOLAR PLANETS

MAGNETOSPHERIC GAP AND ACCUMULATION OF GIANT PLANETS CLOSE TO THE STAR

[abstract] [full text] [plots from the paper] [animation]

We performed a set of 3D simulations using our code based on the “cubed sphere” grid (Koldoba et al. 2002) with the main goal of analyzing the density distribution in the magnetospheric gap. Simulations were set up in a way similar to those of Romanova et al. (2003, 2004). Namely, quasi-stationary initial conditions were used which permitted slow viscous accretion from the disk to a star. An a −viscosity was incorporated to the code with typical values of a−parameter: a= 0.02 and 0.04. The magnetic axis μ is misaligned relative to rotational axis of the star W_* by an angle Q. The rotational axis of the star coincides with that of the disk.

Simulations were done for parameters typical for T Tauri type stars: M_* = 0.8M_o, R_* = 2.5R_o, B_* = 10³G, dM/dt ≈ 3 10⁻⁸ M_o/yr. Compared to our previous 3D runs, we changed parameters so as to increase the size of magnetospheric gap to r_A = (4 − 5)R_* versus r_A = (2 − 3)R_* in our previous papers. The low-density magnetospheric gap can be quite large. Inside the magnetospheric radius (which corresponds approximately to the edge of the disk), the magnetic field lines are closed, while outside of this radius they are carried by the matter of the disk or corona. Our simulations show that the density inside the magnetospheric gap is about 100 − 300 times smaller than the density in the nearby disk.

Magnetospheric Gap at Different Q. We performed 3D simulations for different misalignment angles from Q= 0^◦ to 90^◦. We investigated the magnetospheric gaps in the equatorial plane. Simulations have shown that matter flow is different at small and large misalignment angles. For angles Q< 45◦, matter flows to the star along funnel streams which are above and below the equatorial plane. Thus within the magnetospheric gap r < r_A the matter density in the equatorial plane is greatly reduced. For larger angles, matter also accretes to a star through the funnel streams. However, part of the funnel streams is located in the equatorial plane and the magnetospheric gap is not empty. Thus at large , the planets orbiting in the equatorial plane will interact with the dense gas of the streams and may continue to migrate inward to the star.

Accretion through Equatorial Funnels at Low g. There is another possible reason why the magnetospheric gap may have some matter density. There are possible instabilities which may lead to the direct accretion of matter through the magnetosphere in the equatorial plane. To investigate such instabilities, we took the almost aligned case, Q= 5^◦, and decreased the adiabatic index from g= 5/3 to = 1.1. The adiabatic index may be significantly lower than its ideal value in the case of high electron heat conductivity which may occur in a highly ionized plasma. In our simulations, the low value of g acts to give a low temperature in the disk and the funnel flow. We observed that matter partially accretes in the equatorial plane. Matter accreted through funnels which are located inside the magnetosphere. They penetrate inwards through the Rayleigh-Taylor type instability (e.g. Arons & Lea 1976) up to some distance r₁, and then form regular funnel streams along the field lines. The distance of penetration depends on the ratio r_A/R_*. At relatively small values r_A/R_*, the equatorial funnels may penetrate almost to the surface of the star. At larger values of r_A/R_*, the funnels move inward only part of the way. Thus, in the case of a weak magnetic field and/or high accretion rate, the magnetospheric radius r_A ≈ (1 − 3)R_*, and a small adiabatic indexes g, the gap will not be empty and planets will continue to migrate inward unless the tidal interaction or some other force will prevent them against falling to the star. In the opposite case of a larger magnetosphere, the equatorial funnels will occupy only a part of the gap and planet may survive longer inside the innermost gap.