scholarly journals The Observational Evidence for Accretion

1997 ◽  
Vol 182 ◽  
pp. 391-405 ◽  
Author(s):  
Lee Hartmann
Keyword(s):  
Large Range ◽  
Observational Evidence ◽  
Young Stars ◽  
Young Stellar Objects ◽  
Stellar Objects ◽  
T Tauri ◽  
Accretion Rates ◽  
Low Mass ◽  
Stellar Magnetospheres ◽  
Mass Ejection

Outflows from low-mass young stellar objects are thought to draw upon the energy released by accretion onto T Tauri stars. I briefly summarize the evidence for this accretion and outline present estimates of mass accretion rates. Young stars show a very large range of accretion rates, and this has important implications for both mass ejection and for the structure of stellar magnetospheres which may truncate T Tauri disks.

scholarly journals Models of Accretion Disks Around Young Stars

2004 ◽  
Vol 221 ◽  
pp. 403-410 ◽  
Author(s):  
Paola D'Alessio ◽  
Nuria Calvet ◽  
Lee Hartmann ◽  
James Muzerolle ◽  
Michael Sitko
Keyword(s):  
Accretion Disks ◽  
Young Stars ◽  
Stellar Surface ◽  
Young Stellar Objects ◽  
Stellar Objects ◽  
Disk Structure ◽  
Accretion Rates ◽  
Accretion Shocks

We discuss the importance of accretion in calculating disk models for young stellar objects. In particular, we show that a disk inner rim, irradiated by both the star and the accretion shocks at the stellar surface, can naturally explain recent observations of DG Tau with the Keck interferometer. We present models for two objects, with mass accretion rates differing by almost two orders of magnitude, to illustrate the effects of accretion on the overall disk structure and emission.

scholarly journals The Infrared Companions of T Tauri Stars: Clues to the Formation and Early Evolution of Binaries

2001 ◽  
Vol 200 ◽  
pp. 265-274 ◽  
Author(s):  
Chris D. Koresko ◽  
Christoph Leinert
Keyword(s):  
T Tauri Stars ◽  
Young Stars ◽  
Early Evolution ◽  
Young Stellar Objects ◽  
Physical Status ◽  
Stellar Objects ◽  
T Tauri ◽  
Mode Of Development ◽  
Source Of Information

Infrared companions are young stellar objects with unusual properties gravitationally bound to more or less typical T Tauri stars. As such they promise to be the source of information on either a particular phase in the development of young stars or on a particular mode of development. We discuss the observed properties of infrared companions as well as attempts to explain their physical status with the aim to see how much of solid conclusion has been obtained so far.

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 The bipolar-flow phenomenon

1991 ◽  
Vol 147 ◽  
pp. 447-448
Author(s):  
Wolfgang Kundt
Keyword(s):  
Active Galactic Nucleus ◽  
Lorentz Factor ◽  
T Tauri Stars ◽  
Young Stars ◽  
Young Stellar Objects ◽  
Radio Sources ◽  
Stellar Objects ◽  
Pair Plasma ◽  
T Tauri ◽  
Relativistic Pair

Very young stars, known as ‘pre-T-Tauri stars’ (PTTS) or ‘young stellar objects’ (YSO), tend to be surrounded by elongated outflow regions involving ‘Herbig-Haro objects’ (HH). Such ‘bipolar flows’ (BF) are reminiscent of the extragalactic radio sources for which a consensus has formed in 1986 that their jets consist of extremely relativistic pair plasma, of typical Lorentz factor 104±1, generated by an ‘active galactic nucleus’ (AGN); [refs 6,7]. Here I collect new circumstantial evidence for the relativistic nature of BFs.

scholarly journals Recent results on polarization of T Tauri stars and other young stellar objects

1990 ◽  
Vol 137 ◽  
pp. 179-184
Author(s):  
Pierre Bastien ◽  
François Ménard
Keyword(s):  
Rotation Period ◽  
T Tauri Stars ◽  
Young Stars ◽  
Young Stellar Objects ◽  
Current Evidence ◽  
Line Of Sight ◽  
Stellar Objects ◽  
T Tauri ◽  
Thick Part ◽  
The Difference

Recent results on observations and models of polarization of T Tauri stars (TTS) and other young stellar objects (YSO's) are presented. In particular, the difference in polarization properties between the classical T Tauri stars (CTTS) and the weak-line T Tauri stars (WTTS) is made. A correlation between polarization and rotation period is searched for but not found. Observations of polarization maps for many young stars are considered. Two types of models have been proposed to explain these observations. Current evidence favors single and multiple scattering in flattened, optically thick, structures, i.e. disks, around many TTS. In particular, the size of the optically thick part of the disks, and their inclination to our line of sight can determined for published polarization maps.

scholarly journals The ASAS-SN catalogue of variable stars – VIII. ‘Dipper’ stars in the Lupus star-forming region

2020 ◽  
Vol 496 (3) ◽  
pp. 3257-3269 ◽  
Author(s):  
J W Bredall ◽  
B J Shappee ◽  
E Gaidos ◽  
T Jayasinghe ◽  
P Vallely ◽  
...  
Keyword(s):  
Main Sequence ◽  
Statistical Significance ◽  
Variable Stars ◽  
Young Stars ◽  
Light Curves ◽  
Young Stellar Objects ◽  
Circumstellar Dust ◽  
Stellar Objects ◽  
Star Forming ◽  
T Tauri

ABSTRACT Some young stellar objects such as T Tauri-like ‘dipper’ stars vary due to transient partial occultation by circumstellar dust, and observations of this phenomenon inform us of conditions in the planet-forming zones close to these stars. Although many dipper stars have been identified with space missions such as Kepler/K2, ground-based telescopes offer longer term and multiwavelength perspectives. We identified 11 dipper stars in the Lupus star-forming region in data from the All-Sky Automated Survey for SuperNovae (ASAS-SN), and further characterized these using observations by the Las Cumbres Global Observatory Telescope (LCOGT) and the Transiting Exoplanet Survey Satellite (TESS), as well as archival data from other missions. Dipper stars were identified from a catalogue of nearby young stars and selected based on the statistical significance, asymmetry, and quasi-periodicity or aperiodicity of variability in their ASAS-SN light curves. All 11 stars lie above or redwards of the zero-age main sequence and have infrared (IR) excesses indicating the presence of full circumstellar discs. We obtain reddening–extinction relations for the variability of seven stars using our combined ASAS-SN-TESS and LCOGT photometry. In all cases, the slopes are below the ISM value, suggesting larger grains, and we find a tentative relation between the slope (grain size) and the $K_\text{s}-[22 \, \mu \text{m}]$ IR colour regarded as a proxy for disc evolutionary state.

scholarly journals Outflows and Accretion in Young Stellar Objects

2004 ◽  
Vol 219 ◽  
pp. 599-610 ◽  
Author(s):  
Nuria Calvet
Keyword(s):  
Observational Evidence ◽  
Young Stellar Objects ◽  
Opening Angle ◽  
Wide Angle ◽  
Wind Model ◽  
Stellar Objects ◽  
Disk Wind ◽  
T Tauri ◽  
Fu Ori ◽  
Temperature Enhancement

Outflows in young stellar objects are powered by accretion, and ∼ 0.1 of the accreted material is lost in the outflow. Observational evidence is analyzed in the context of models for the origin of the wind. Winds in FU Ori objects are clear examples of disk winds. In Classical T Tauri stars, there is evidence for the existence of a wide angle wind at scales < 100 AU, which supports the X-wind model prediction that narrow jets are the result of density/temperature enhancement towards the axis of the system. However, recent HST observations of the DG Tau jet indicate that the opening angle of the wind is more confined than predicted by the X-wind model, in better agreement with disk wind theories.

scholarly journals The bipolar-flow phenomenon

1991 ◽  
Vol 147 ◽  
pp. 447-448
Author(s):  
Wolfgang Kundt
Keyword(s):  
Active Galactic Nucleus ◽  
Lorentz Factor ◽  
T Tauri Stars ◽  
Young Stars ◽  
Young Stellar Objects ◽  
Radio Sources ◽  
Stellar Objects ◽  
Pair Plasma ◽  
T Tauri ◽  
Relativistic Pair

Very young stars, known as ‘pre-T-Tauri stars’ (PTTS) or ‘young stellar objects’ (YSO), tend to be surrounded by elongated outflow regions involving ‘Herbig-Haro objects’ (HH). Such ‘bipolar flows’ (BF) are reminiscent of the extragalactic radio sources for which a consensus has formed in 1986 that their jets consist of extremely relativistic pair plasma, of typical Lorentz factor 104±1, generated by an ‘active galactic nucleus’ (AGN); [refs 6,7]. Here I collect new circumstantial evidence for the relativistic nature of BFs.

scholarly journals On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

Galaxies ◽  
2020 ◽  
Vol 8 (2) ◽  
pp. 39
Author(s):  
Ignacio Mendigutía
Keyword(s):  
Boundary Layer ◽  
Magnetic Fields ◽  
Young Stellar Objects ◽  
Driving Mechanism ◽  
Be Stars ◽  
Stellar Objects ◽  
Star Forming ◽  
T Tauri ◽  
Accretion Rates ◽  
Magnetospheric Accretion

Understanding how young stars gain their masses through disk-to-star accretion is of paramount importance in astrophysics. It affects our knowledge about the early stellar evolution, the disk lifetime and dissipation processes, the way the planets form on the smallest scales, or the connection to macroscopic parameters characterizing star-forming regions on the largest ones, among others. In turn, mass accretion rate estimates depend on the accretion paradigm assumed. For low-mass T Tauri stars with strong magnetic fields there is consensus that magnetospheric accretion (MA) is the driving mechanism, but the transfer of mass in massive young stellar objects with weak or negligible magnetic fields probably occurs directly from the disk to the star through a hot boundary layer (BL). The intermediate-mass Herbig Ae/Be (HAeBe) stars bridge the gap between both previous regimes and are still optically visible during the pre-main sequence phase, thus constituting a unique opportunity to test a possible change of accretion mode from MA to BL. This review deals with our estimates of accretion rates in HAeBes, critically discussing the different accretion paradigms. It shows that although mounting evidence supports that MA may extend to late-type HAes but not to early-type HBes, there is not yet a consensus on the validity of this scenario versus the BL one. Based on MA and BL shock modeling, it is argued that the ultraviolet regime could significantly contribute in the future to discriminating between these competing accretion scenarios.

scholarly journals Circumstellar maher of young low-mass stars: Observations versus theory

1987 ◽  
Vol 122 ◽  
pp. 23-38 ◽  
Author(s):  
Claude Bertout
Keyword(s):  
T Tauri Stars ◽  
Young Stellar Objects ◽  
Low Mass Stars ◽  
Stellar Objects ◽  
Star Forming ◽  
T Tauri ◽  
Region I ◽  
Low Mass ◽  
Infrared Sources

After presenting NGC 7129 as a prototypical star-forming region, I discuss what can be learned from the radio spectra of embedded infrared sources. I then review available observational evidence for disks around young stellar objects, with emphasis on accretion disks around T Tauri stars. Finally, new results on the role of magnetic fields in the circumstellar activity of T Tauri stars are presented.

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