Добірка наукової літератури з теми "Magnetocentrifugal launching"

Abstract Jets can facilitate the mass accretion onto the protostars in star formation. They are believed to be launched from accretion disks around the protostars by magnetocentrifugal force, as supported by the detections of rotation and magnetic fields in some of them. Here we report a radial flow of the textbook-case protostellar jet HH 212 at the base to further support this jet-launching scenario. This radial flow validates a central prediction of the magnetocentrifugal theory of jet formation and collimation, namely, the jet is the densest part of a wide-angle wind that flows radially outward at distances far from the (small, sub-au) launching region. Additional evidence for the radially flowing wide-angle component comes from its ability to reproduce the structure and kinematics of the shells detected around the HH 212 jet. This component, which can transport material from the inner to outer disk, could account for the chondrules and Ca–Al-rich inclusions detected in the solar system at large distances.

Abstract Understanding the origin of accretion and dispersal of protoplanetary disks is fundamental for investigating planet formation. Recent numerical simulations show that launching winds are unavoidable when disks undergo magnetically driven accretion and/or are exposed to external UV radiation. Observations also hint that disk winds are common. We explore how the resulting wind mass loss rate can be used as a probe of both disk accretion and dispersal. As a proof-of-concept study, we focus on magnetocentrifugal winds, magnetorotational instability turbulence, and external photoevapotaion. By developing a simple yet physically motivated disk model and coupling it with simulation results available in the literature, we compute the wind mass loss rate as a function of external UV flux for each mechanism. We find that different mechanisms lead to different levels of mass loss rate, indicating that the origin of disk accretion and dispersal can be determined, by observing the wind mass loss rate resulting from each mechanism. This determination provides important implications for planet formation. This work thus shows that the ongoing and future observations of the wind mass loss rate for protoplanetary disks are paramount to reliably constrain how protoplanetary disks evolve with time and how planet formation takes place in the disks.

ABSTRACT Protoplanetary discs should exhibit a weak vertical variation in their rotation profiles. Typically, this ‘vertical shear’ issues from a baroclinic effect driven by the central star’s radiation field, but it might also arise during the launching of a magnetocentrifugal wind. As a consequence, protoplanetary discs are subject to a hydrodynamical instability, the ‘vertical shear instability’ (VSI), whose breakdown into turbulence could transport a moderate amount of angular momentum and facilitate, or interfere with, the process of planet formation. Magnetic fields may suppress the VSI, however, either directly via magnetic tension or indirectly through magnetorotational turbulence. On the other hand, protoplanetary discs exhibit notoriously low ionization fractions, and non-ideal effects, if sufficiently dominant, may come to the VSI’s rescue. In this paper, we develop a local linear theory that explores how non-ideal magnetohydrodynamics influences the VSI, while exciting additional diffusive shear instabilities. We derive a set of analytical criteria that establish when the VSI prevails, and then show how it can be applied to a representative global model of a protoplanetary disc. Our calculations suggest that within ∼10 au the VSI should have little trouble emerging in the main body of the disc, but beyond that, and in the upper regions of the disc, its onset depends sensitively on the size of the preponderant dust grains.

Abstract BP Psc is an active late-type (sp:G9) star with unclear evolutionary status lying at high galactic latitude b = −57○. It is also the source of the well collimated bipolar jet. We present results of the proper motion and radial velocity study of BP Psc outflow based on the archival Hα imaging with the GMOS camera at 8.1-m Gemini-North telescope as well as recent imaging and long-slit spectroscopy with the SCORPIO multi-mode focal reducer at 6-m BTA telescope of SAO RAS. The 3D kinematics of the jet revealed the full spatial velocity up to ∼140 km·s−1 and allows us to estimate the distance to BP Psc system as D = 135 ± 40 pc. This distance leads to an estimation of the central source luminosity L* ≈ 1.2L⊙, indicating that it is the ≈1.3 M⊙ T Tauri star with an age t ≲ 7 Myr. We measured the electron density of order Ne ∼ 102 cm−3 and mean ionization fraction f ≈ 0.04 within the jet knots and estimated upper limit of the mass-loss rate in NE lobe as $\dot{M}_{out}\approx 1.2\cdot 10^{-8}M_{\odot }\cdot yr^{-1}$. The physical characteristics of the outflow are typical for the low-excitation YSO jets and consistent with the magnetocentrifugal mechanism of its launching and collimation. Prominent wiggling pattern revealed in Hα images allowed us to suppose the existence of a secondary substellar companion in a non-coplanar orbit and estimate its most plausible mass as Mp ≈ 30MJup. We conclude that BP Psc is one of the closest to the Sun young jet-driving systems and its origin is possibly related to the episode of star formation triggered by expanding supershells in Second Galactic quadrant.

Protostellar jets and winds play a crucial role in the dynamics and evolution of the starformation process. They may effectively regulate mass accretion by removing angular momentum from the circumstellar disc. Despite their importance, the physical processes driving the outflow phenomena remain poorly understood. This thesis presents a consistent model for the outflow structure and dynamics of the young stellar object DG Tauri, using data of unprecedented spatial and spectral resolution from the Near-infrared Integral Field Spectrograph (NIFS) on Gemini North. The approaching outflow shows two components in [Fe II] 1.644 m emission. A stationary recollimation shock is observed in the high-velocity jet, in agreement with previous Xray and FUV observations. The pre-shock jet velocity, and inferred jet launch point (400-700 km s-1 and 0.02-0.07 AU, respectively), are signifcantly different from previous estimates. Jet `acceleration' beyond the shock is interpreted as intrinsic velocity variability. Careful analysis reveals no evidence of jet rotation, contrary to previous work. A wide-angle, low-velocity blueshifted molecular out ow is observed in H2 1-0 S(1) 2.1218 m emission. Both outflows are consistent with a magnetocentrifugal disc wind origin, although an X-wind origin for the jet cannot be excluded. The lower-velocity [Fe II] component surrounds the jet, and is interpreted as a turbulent mixing layer generated by lateral jet entrainment of molecular wind material. An analytical model of an entrainment layer is constructed, based on Riemann decomposition of directly observable outflow parameters. The model reproduces the velocity field of the entrained material without invoking an arbitrary `entrainment efficiency' parameter. The luminosity and mass entrainment rate estimated using the model are in agreement with observations. Such lateral entrainment requires a magnetic field strength of order a few mG at hundreds of AU above the disc surface; independent arguments are advanced to support this conclusion. The receding outflow of DG Tau takes on a bubble-shaped morphology. Kinetic models indicate this structure is a quasi-static bubble with an internal velocity field describing expansion. It is proposed that this bubble forms because the receding counterjet from DG Tau is obstructed by a clumpy ambient medium. There is evidence of interaction between the counterjet and ambient material, which is attributed to the large molecular envelope around the DG Tau system. An analytical model of a momentum-driven bubble is shown to be consistent with observations. It is concluded that the bipolar outflow from DG Tau is intrinsically symmetric; the observed asymmetries are due to environmental effects. The observational interpretations and comprehensive modelling of the DG Tau outflows presented in this thesis constitute a significant step forward in gaining a full physical understanding of how stars accrete their mass. The complex nature of the approaching jet provides the first clear indications of the diverse phenomena associated with protostellar mass loss. The different morphology of the receding out ow has highlighted the role of environmental factors in defining outflow characteristics. Together this work presents a new and more detailed view of the complex mechanisms associated with the formation of a low-mass star.