Stellar Dipole Field Research Articles

Close-in giant-planet formation via in-situ gas accretion and their natal disk properties

Aims. The origin of close-in Jovian planets is still elusive. We examine the in-situ gas accretion scenario as a formation mechanism of these planets. Methods. We reconstruct natal disk properties from the occurrence rate distribution of close-in giant planets, under the assumption that the occurrence rate may reflect the gas accretion efficiency onto cores of these planets. Results. We find that the resulting gas surface density profile becomes an increasing function of the distance from the central star with some structure at r ≃ 0.1 au. This profile is quite different from the standard minimum-mass solar nebula model, while our profile leads to better reproduction of the population of observed close-in super-Earths based on previous studies. We compute the resulting magnetic field profiles and find that our profiles can be fitted by stellar dipole fields (∝r−3) in the vicinity of the central star and large-scale fields (∝r−2) at the inner disk regions, either if the isothermal assumption breaks down or if nonideal magnetohydrodynamic effects become important. For both cases, the transition between these two profiles occurs at r ≃ 0.1 au, which corresponds to the period valley of giant exoplanets. Conclusions. Our work provides an opportunity to test the in-situ gas accretion scenario against disk quantities, which may constrain the gas distribution of the minimum-mass extrasolar nebula.

Disruption of circumstellar discs by large-scale stellar magnetic fields

Spectropolarimetric surveys reveal that 8-10\% of OBA stars harbor large-scale magnetic fields, but thus far no such fields have been detected in any classical Be stars. Motivated by this, we present here MHD simulations for how a pre-existing Keplerian disc -- like that inferred to form from decretion of material from rapidly rotating Be stars -- can be disrupted by a rotation-aligned stellar dipole field. For characteristic stellar and disc parameters of a near-critically rotating B2e star, we find that a polar surface field strength of just 10 G can significantly disrupt the disc, while a field of 100 G, near the observational upper limit inferred for most Be stars, completely destroys the disc over just a few days. Our parameter study shows that the efficacy of this magnetic disruption of a disc scales with the characteristic plasma beta {(defined as the ratio between thermal and magnetic pressure)} in the disc, but is surprisingly insensitive to other variations, e.g. in stellar rotation speed, or the mass loss rate of the star's radiatively driven wind. The disc disruption seen here for even a modest field strength suggests that the presumed formation of such Be discs by decretion of material from the star would likely be strongly inhibited by such fields; this provides an attractive explanation for why no large-scale fields are detected from such Be stars.

Asymmetric MHD outflows/jets from accreting T Tauri stars

Observations of jets from young stellar objects reveal the asymmetric outflows from some sources. A large set of $2.5$D MHD simulations has been carried out for axisymmetric viscous/diffusive disc accretion to rotating magnetized stars for the purpose of assessing the conditions where the outflows or jets are asymmetric relative to the equatorial plane. We consider initial magnetic fields that are symmetric about the equatorial plane and consist of a radially distributed field threading the disc (disc-field) and a stellar dipole field.({\bf 1}). For pure disc-fields the symmetry or asymmetry of the outflows is affected by the midplane plasma $\beta$ of the disc (where $\beta$ is the ratio of the plasma pressure to the magnetic pressure). For the low density discs with small plasma $\beta$ values, outflows are observed to be symmetric to within $10\%$ over timescales of hundreds of inner disc orbits. For the denser higher $\beta$ discs, the coupling of the upper and lower coronal plasmas is broken, and quasi-periodic field motion in the two hemispheres becomes different. This asymmetry leads to asymmetric episodic outflows. ({\bf 2.}) Accreting stars with a stellar dipole field and no disc-field exhibit episodic, two component outflows - a magnetospheric wind and an inner disc wind from somewhat larger radial distances. Both are characterized by similar velocity profiles but the magnetospheric wind has densities $\gtrsim 10$ times that of the disc wind. ({\bf 3}.)Adding a disc-field parallel to the stellar dipole field acts to enhance the magnetospheric winds but suppress the disc wind. ({\bf 4}.) In contrast, adding a disc-field which is anti-parallel to the stellar dipole field in the disc acts to suppress the magnetospheric and disc winds. Our simulations reproduce some key features of observations of asymmetric outflows of T Tauri stars.

Large scale magnetic fields in discs: jets and reconnection X-winds

AbstractIn this contribution we first briefly review our current knowledge on the physics of accretion discs driving self-confined jets. It will be shown that a large scale magnetic field is expected to thread the innermost disc regions, giving rise to a transition from an outer standard accretion disc to an inner jet emitting disc. We then report new progresses on the theory of star-disc interaction, allowing to explain the formation of accretion funnel flows with stellar dipole fields consistent with observational constraints. Such a connection is now not only probed by modern observations but it is also requested for spinning down protostars, which are known to be both actively accreting and contracting. This spin down most probably relies on the angular momentum removal by ejection. Two such scenarios will be addressed here, namely “accretion-powered stellar winds” (Matt & Pudritz 2005) and “Reconnection X-winds” (Ferreira, Pelletier & Appl 2000). The latter can slow down a protostar on time scales shorter or comparable to the embedded phase. It will be shown that these two scenarios are not incompatible and that transitions from one to another may even occur as they mainly depend on the stellar dynamo.

Stellar dynamo driven wind braking versus disc coupling

AbstractStar‐disc coupling is considered in numerical models where the stellar field is not an imposed perfect dipole, but instead a more irregular self‐adjusting dynamo‐generated field. Using axisymmetric simulations of the hydromagnetic mean‐field equations, it is shown that the resulting stellar field configuration is more complex, but significantly better suited for driving a stellar wind. In agreement with recent findings by a number of people, star‐disc coupling is less efficient in braking the star than previously thought. Moreover, stellar wind braking becomes equally important. In contrast to a perfect stellar dipole field, dynamo‐generated stellar fields favor field‐aligned accretion with considerably higher velocity at low latitudes, where the field is weaker and originating in the disc. Accretion is no longer nearly periodic (as it is in the case of a stellar dipole), but it is more irregular and episodic. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Generation of Magnetic Fields in Dusty Protoplanetary Disks

The remanent magnetization of meteorite material in the solar system indicates that magnetic fields of some Gauss were present in the early protoplanetary disk. It is shown that such relatively strong magnetic fields can be generated in dusty protoplanetary disks by relative shear motions of the charged dust and the neutral gas components. The ultimate energy source is the gravitational field of the stellar object that results in a sheared accretion flow. Self-consistent multi-fluid simulations show that for plasma parameters typical for protoplanetary disks shear flows with collisional momentum transfer between the different components of the dusty plasma result in a very efficient generation of magnetic fields with strengths of about 0.1–1 Gauss on spatial scales of two astronomical units in about two years. Such strong magnetic fields should influence the coupling of the stellar dipole field and the disk, the accretion flow, stellar winds and jets and the formation of protoplanetary objects.

Collimation of a central wind by a disc-associated magnetic field

Studies of jets from young stellar objects (YSOs) suggest that material is launched from a small central region at wide opening angles and collimated by an interaction with the surrounding environment. Using time-dependent, numerical magnetohydrodynamic simulations, we follow the detailed launching of a central wind via the coupling of a stellar dipole field to the inner edge of an accretion disc. Our method employs a series of nested computational grids, which allows the simulations to follow the central wind out to scales of tens of astronomical units (au), where it may interact with its surroundings. The coupling between the stellar magnetosphere and the inner edge of the disc has been known to produce an outflow containing both a highly collimated jet plus a wide-angle flow. The jet and wide-angle wind flow at roughly the same speed (100–200 km s−1), and most of the energy and mass is carried off at relatively wide angles. We show that the addition of a weak disc-associated field (≪ 0.1 G) has little effect on the wind launching, but it collimates the entire flow (jet + wide wind) at a distance of several au. The collimation is inevitable, regardless of the relative polarity of the disc field and stellar dipole, and the result is a more powerful and physically broader collimated flow than from the star–disc interaction alone. Within the collimation region, the morphology of the large-scale flow resembles a pitchfork, in projection. We compare these results with observations of outflows from YSOs and discuss the possible origin of the disc-associated field.

Location of the Inner Radius of a Magnetically Threaded Accretion Disk

In models for disk accretion onto magnetized objects, the inner radius R0 of the Keplerian disk is conventionally expressed in the form R0 = ξrA, where rA is the Alfven radius for spherical accretion and the parameter ξ is usually taken to be 0.5. We point out that the value of ξ in fact depends on the fraction of the star's magnetic flux threading the disk, which is poorly known from theory: in general, ξ 1.35η4/7, where η ≤ 1 denotes the threading coefficient. Application of the beat frequency model to binary X-ray pulsars showing quasi-periodic oscillations suggests strongly that η ~ 1 for these objects: the stellar dipole field essentially fully threads the disk and ξ 1. When combined with improved accretion torque models characterized by critical fastness parameters near unity, the corrected values of ξ ≡ R0/rA should allow more reliable determinations of the accreting star's dipole moment and mass-radius relationship.