scholarly journals Jets, Disk Winds, and Warm Disk Coronae in Classical T Tauri Stars

1997 ◽  
Vol 182 ◽  
pp. 443-454
Author(s):  
John Kwan
Keyword(s):  
Accretion Disk ◽  
High Speed ◽  
Line Emission ◽  
Disk Surface ◽  
T Tauri Stars ◽  
Additional Indication ◽  
Low Speed ◽  
T Tauri ◽  
Speed Flow ◽  
Balmer Lines

Arguments, based on analysis of the forbidden line emission, are summarized that point to two components of mass ejection in classical T Tauri stars. A low-speed wind originates from the accretion disk at between ∼ 0.05 AU and ≳3 AU from the star, and a high-speed flow, which becomes collimated into a jet within ≲ 100 AU, originates as a wind from either the star or the very inner part of the accretion disk. The [OI] λ5577 emission in the low-speed component also requires the presence of a warm disk corona, with an electron density of ∼ 107 cm–3 and a temperature of ∼ 8000 K. Additional indication of a warm disk corona comes from analysis of the central absorptions seen in the profiles of the hydrogen Balmer lines. The need to heat the disk corona implies that a substantial fraction of the energy released in the accretion of matter through the disk may be dissipated at the disk surface.

New Infrared Veiling Measurements and Constraints on Accretion Disk Models for Classical T Tauri Stars

2001 ◽  
Vol 561 (2) ◽  
pp. 1060-1073 ◽  
Author(s):  
Christopher M. Johns‐Krull ◽  
Jeff A. Valenti
Keyword(s):  
Accretion Disk ◽  
T Tauri Stars ◽  
T Tauri

scholarly journals The accretion disk paradigm for young stars

2007 ◽  
Vol 3 (S243) ◽  
pp. 1-12 ◽  
Author(s):  
Claude Bertout
Keyword(s):  
Accretion Disk ◽  
Indirect Evidence ◽  
Circumstellar Disks ◽  
T Tauri Stars ◽  
Young Stars ◽  
Young Stellar Objects ◽  
Recent Attempt ◽  
Disk Model ◽  
Stellar Objects ◽  
T Tauri

AbstractAccretion and magnetic fields play major roles in several of the many models put forward to explain the properties of T Tauri stars since their discovery by Alfred Joy in the 1940s. Early investigators already recognized in the 1950s that a source of energy external to the star was needed to account for the emission properties of these stars in the optical range.The opening of new spectral windows from the infrared to the ultraviolet in the 1970s and 1980s showed that the excess emission of T Tauri stars and related objects extends into all wavelength domains, while evidence of outflow and/or infall in their circumstellar medium was accumulating.Although the disk hypothesis had been put forward by Merle Walker as early as 1972 to explain properties of YY Orionis stars and although Lynden-Bell and Pringle worked out the accretion disk model and applied it specifically to T Tauri stars in 1974, the prevailing model for young stellar objects until the mid-1980s assumed that they experienced extreme solar-type activity. It then took until the late 1980s before the indirect evidence of disks presented by several teams of researchers became so compelling that a paradigm shift occurred, leading to the current consensual picture.I briefly review the various models proposed for explaining the properties of young stellar objects, from their discovery to the direct observations of circumstellar disks that have so elegantly confirmed the nature of young stars. I will go on to discuss more modern issues concerning their accretion disk properties and conclude with some results obtained in a recent attempt to better understand the evolution of Taurus-Auriga young stellar objects.

scholarly journals Outflows and particle acceleration in the accretion disks of young stars

2019 ◽  
Vol 201 ◽  
pp. 09004
Author(s):  
Sergey Khaibrakhmanov ◽  
Alexander Dudorov
Keyword(s):  
Particle Acceleration ◽  
Accretion Disks ◽  
High Speed ◽  
Large Scale ◽  
T Tauri Stars ◽  
Radiative Heat Exchange ◽  
Current Layer ◽  
X Rays ◽  
Disk Structure ◽  
T Tauri

Magneto-gas-dynamic (MGD) outflows from the accretion disks of T Tauri stars with fossil large-scale magnetic fileld are investigated. We consider two mechanisms of the outflows: rise of the magnetic flux tubes (MFT) formed in the regions of efficient generation of the toroidal magnetic fileld in the disk due to Parker instability, and acceleration of particles in the current layer formed near the boundary between stellar magnetosphere and the accretion disk. Structure of the disk is calculated using our MGD model of the accretion disks. We simulate dynamics of the MFT in frame of slender flux tube approximation taking into account aerodynamic and turbulent drags, and radiative heat exchange with external gas. Particle acceleration in the current layer is investigated on the basis of Sweet-Parker model of magnetic reconnection. Our calculations show that the MFT can accelerate to velocities up to 50 km s-1 causing periodic outflows from the accretion disks. Estimations of the particle acceleration in the current layer are applied to interpret high-speed jets and X-rays observed in T Tauri stars with the accretion disks.

Forbidden-line emission and infrared excesses in T Tauri stars - Evidence for accretion-driven mass loss?

1990 ◽  
Vol 354 ◽  
pp. 687 ◽  
Author(s):  
Sylvie Cabrit ◽  
Suzan Edwards ◽  
Stephen E. Strom ◽  
Karen M. Strom
Keyword(s):  
Mass Loss ◽  
Line Emission ◽  
T Tauri Stars ◽  
T Tauri

A Magnetic Accretion Disk Model for the Infrared Excesses of T Tauri Stars

1996 ◽  
Vol 462 ◽  
pp. 439 ◽  
Author(s):  
Scott J. Kenyon ◽  
Insu Yi ◽  
Lee Hartmann
Keyword(s):  
Accretion Disk ◽  
T Tauri Stars ◽  
Disk Model ◽  
T Tauri

Study on Performance Measurement of Beijing Urban Expressway Based on Microwave Data

2014 ◽  
Vol 505-506 ◽  
pp. 1014-1022
Author(s):  
Yao Wang ◽  
Chang Qiao Shao
Keyword(s):  
Performance Measurement ◽  
Traffic Control ◽  
High Speed ◽  
Transition Probability ◽  
Traffic Data ◽  
High Speed Flow ◽  
Low Speed ◽  
Traffic Conditions ◽  
Urban Expressway ◽  
Speed Flow

The research provides an occupancy-based performance measurement for Beijing urban expressway traffic that would be beneficial for further improvement of traffic control. An analysis of the field traffic data shows that the phenomenon of speed transition happens frequently once occupancy reaches to the critical occupancy (30%). Analyzed with speed transition probability and state stability at different occupancy and speed, four traffic states could be defined as stable high-speed flow, unstable high-speed flow, unstable low-speed flow and stable low-speed flow. The performance of each traffic state is measured by transportation efficiency. The result shows that once occupancy changes from 30% to 31%, transportation efficiency drop 27.8%, representing an extra 1/4 time cost for all vehicles on road. Therefore lane occupancy should be controlled under 30% to avoid a deteriorating traffic conditions.

scholarly journals Counter-hairpin vortices in the turbulent wake of a sharp trailing edge

2011 ◽  
Vol 689 ◽  
pp. 317-356 ◽  
Author(s):  
Sina Ghaemi ◽  
Fulvio Scarano
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
High Speed ◽  
High Speed Flow ◽  
Trailing Edge ◽  
Hairpin Vortices ◽  
Low Speed ◽  
Speed Flow ◽  
Low Speed Streaks

AbstractThe unsteady organization and evolution of coherent structures within the turbulent boundary layer and subsequent wake of the sharp symmetric trailing edge of a NACA0012 aerofoil are investigated. The experiments are conducted in an open test-section wind tunnel at ${\mathit{Re}}_{c} = \text{386\hspace{0.167em}000} $ based on the aerofoil chord and ${\mathit{Re}}_{\theta } = 1300$ based on the boundary layer momentum thickness. An initial characterization of the flow field using two-component particle image velocimetry (PIV) is followed by the investigation of the unsteady organization and evolution of coherent structures by time-resolved three-dimensional PIV based on a tomographic approach (Tomo-PIV). The inspection of the turbulent boundary layer prior to the trailing edge in the region between 0.15 and $0. 8\hspace{0.167em} {\delta }_{99} $ demonstrated streaks of low- and high-speed flow, while the low-speed streaks are observed to be more coherent along with strong interaction with hairpin-type vortical structures similar to a turbulent boundary layer at zero pressure gradient. The wake region demonstrated gradual deterioration of both the low- and the high-speed streaks with downstream progress. However, the low-speed streaks are observed to lose their coherence at a faster rate relative to the high-speed streaks as the turbulent flow develops towards the far wake. The weakening of the low-speed streaks is due to the disappearance of the viscous sublayer after the trailing edge and gradual mixing through the transport of the remaining low-speed flow towards the free stream. This transport of low-speed flow is performed by the ejection events induced by the hairpin vortices as they also persist into the developing wake. The higher persistence of the high-speed streaks is associated with counter-hairpin vortical activities as they oppose the deterioration of the high-speed streaks by frequently sweeping the high-speed flow towards the wake centreline. These vortical structures are regarded as counter-hairpin vortices as they exhibit opposite characteristics relative to the hairpin vortices of a turbulent boundary layer. They are topologically similar to the hairpins as they appear to be U-shaped but with inverted orientation, as the spanwise portion is in the vicinity of the wake centreline and the legs are inclined at an approximately $6{0}^{\ensuremath{\circ} } $ to the wake axis in the downstream direction demonstrating a strain-dominated topology. The counter-hairpin vortices are partially wrapped around the high-speed streaks and contribute to the wake development by transporting high-speed flow towards the wake centreline. Similar to the hairpin vortices of a turbulent boundary layer, the occurrence of a complete counter-hairpin vortex is occasional while its derivatives (portions of spanwise or quasi-streamwise vortices) are more frequently observed. Therefore, a pattern recognition algorithm is applied to establish characterization based on an ensemble-averaged counter-hairpin vortex. The formation of the counter-hairpin vortices is due to an additional degree of interaction between the low- and high-speed streaks after the trailing edge across the wake centreline. The shear layer produced along the wake centreline by neighbouring low- and high-speed streaks promotes the formation of spanwise vortices that form the counter-hairpin vortices by connection to quasi-streamwise vortices. Finally, a conceptual model is proposed to depict the three-dimensional unsteady organization and evolution of coherent structures in the wake region based on the hairpin and counter-hairpin vortex signatures.

Sign in / Sign up

Export Citation Format

Share Document