Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AA Tau

Donati, J.‐F.; Skelly, M. B.; Bouvier, J.; Gregory, S. G.; Grankin, K. N.; Jardine, M.; Hussain, G. A. J.; Ménard, F.; Dougados, C.; Unruh, Y. C.; Mohanty, Subhanjoy; Aurière, M.; Morin, J.; Farès, R.

doi:10.1111/j.1365-2966.2010.17409.x

Cited by 128 publications

(224 citation statements)

References 34 publications

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“…In some other cTTS, the Zeeman-Doppler imaging technique has been used to reconstruct the large-scale magnetic topology and to outline Ca ii line emission from the shocked gas near the stellar surface: V2129 Oph (Donati et al 2007, and, BP Tau (Donati et al 2008), CV Cha and CR Cha (Hussain et al 2009), and AA Tau (Donati et al 2010). While the recovered structures are sometimes complex, it is not uncommon to see hot spots, as indicated by the enhanced Ca ii emission.…”

Section: Modelling the Rv Variationsmentioning

confidence: 99%

Accretion-powered chromospheres in classical T Tauri stars

Петров

Gahm

Stempels

et al. 2011

A&A

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Context. Optical spectra of classical T Tauri stars (cTTS) are rich in emission lines of low-excitation species that are composed of narrow and broad components, which indicates the existence of two emitting regions with different kinematics, densities, and temperatures. The photospheric spectrum is often veiled by an excess continuous emission. This veiling is usually attributed to radiation from a heated region beneath the accretion shock. The broad emission lines of H i, He ii, Ca ii, Fe ii, and other species are thought to form in a larger volume of gas. Aims. The aim of this research is to clarify the nature of the veiling, and whether the narrow chromospheric lines of Fe i and other metals represent a standard chromosphere of a late-type star, or are induced by mass accretion. Methods. We carried out high-resolution spectroscopy of selected cTTS with a special focus on DR Tauri and followed variations of chromospheric features, such as narrow Fe i emission lines, and accretion signatures such as the veiling continuum and the He ii line emission. Results. We found that the amount of veiling in DR Tau varies from practically nothing to factors more than 10 times the stellar continuum intensity, and that the veiling is caused by both a non-photospheric continuum and chromospheric line emission filling in the photospheric absorption lines. The latter causes differential veiling because stronger lines are more veiled. We developed methods to separate the two sources of veiling. Several veiled T Tauri stars show a common effect: the radial velocities of photospheric and chromospheric lines vary in anti-phase. This is caused by an area with enhanced chromospheric emission, which is offset from the pole of rotation and is associated with the hot spot formed at the footprint of the magnetic funnel of mass accretion. Conclusions. The enhanced chromospheric emission in cTTS is linked not only to solar-like magnetic activity, but is powered to a greater extent by the accreting gas. We suggest that the area of enhanced chromospheric emission is induced by mass accretion, which modifies the local structure of stellar atmosphere in an area that is more extended than the hot accretion spot. The narrow emission lines from this extended area are responsible for the extra component in the veiling through line-filling of photospheric absorption lines.

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Section: Modelling the Rv Variationsmentioning

confidence: 99%

Accretion-powered chromospheres in classical T Tauri stars

Петров

Gahm

Stempels

et al. 2011

A&A

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“…Johns-Krull 2007; Yang & JohnsKrull 2011) and spectropolarimetric (e.g. BP Tau, AA Tau, V2129 Oph, TW Hya, GQ Lup; Donati et al 2008Donati et al , 2010Donati et al , 2011aDonati et al ,b, 2012 observations, the inferred magnetic field strength |B| of CTTSs is on the order of a few kG. Given the stream density adopted in our simulations (n str0 = 10 11 cm −3 ), we considered cases with a minimum value of magnetic field strength for which β < 1 in the shocked column (namely |B| = 500 G).…”

Section: Mhd Modelingmentioning

confidence: 99%

Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

Orlando

Bonito

Argiroffi

et al. 2013

A&A

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Context. According to the magnetospheric accretion model, hot spots form on the surface of classical T Tauri stars (CTTSs) in regions where accreting disk material impacts the stellar surface at supersonic velocity, generating a shock. Aims. We investigate the dynamics and stability of postshock plasma that streams along nonuniform stellar magnetic fields at the impact region of accretion columns. We study how the magnetic field configuration and strength determine the structure, geometry, and location of the shock-heated plasma. Methods. We model the impact of an accretion stream onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our model considers the gravity, the radiative cooling, and the magnetic-field-oriented thermal conduction (including the effects of heat flux saturation). We explore different configurations and strengths of the magnetic field. Results. The structure, stability, and location of the shocked plasma strongly depend on the configuration and strength of the magnetic field. In the case of weak magnetic fields (plasma β > ∼ 1 in the postshock region), a large component of B may develop perpendicular to the stream at the base of the accretion column, which limits the sinking of the shocked plasma into the chromosphere and perturbs the overstable shock oscillations induced by radiative cooling. An envelope of dense and cold chromospheric material may also develop around the shocked column. For strong magnetic fields (β < 1 in the postshock region close to the chromosphere), the field configuration determines the position of the shock and its stand-off height. If the field is strongly tapered close to the chromosphere, an oblique shock may form well above the stellar surface at the height where the plasma β ≈ 1. In general, we find that a nonuniform magnetic field makes the distribution of emission measure vs. temperature of the postshock plasma at T > 10 6 K lower than when there is uniform magnetic field. Conclusions. The initial magnetic field strength and configuration in the region of impact of the stream are expected to influence the chromospheric absorption and, therefore, the observability of the shock-heated plasma in the X-ray band. In addition, the field strength and configuration also influence the energy balance of the shocked plasma with its emission measure at T > 10 6 K, which is lower than expected for a uniform field. The above effects contribute to underestimating the mass accretion rates derived in the X-ray band.

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“…The magnetic field topology of AA Tau was measured twice (2007 and 2009) with spectropolarimetry [17] and was found to vary but not by much between the two epochs. It is composed of a 08001-p.4 simple axisymmetric field inclined with respect to the rotation axis of the star and presents a strong kG dipole and a weaker octupole component.…”

Section: Aa Taumentioning

confidence: 99%

“…The intensity and topology of the stellar surface magnetic field can be obtained through the broadening of Zeeman sensitive lines [29,43] and Zeeman Doppler Imaging (ZDI) [16,17]. Ultraviolet (UV) and optical excess, that veils the photospheric lines, are produced at the hot spot [5,26].…”

Section: Introductionmentioning

confidence: 99%

Observational clues to the physics at the magnetosphere in young stellar objects

Alencar

2014

EPJ Web of Conferences

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Abstract. Classical T Tauri stars offer an excellent laboratory to study the accretion process and star-disk interaction, which define the transfer of mass and angular momentum in the system. The interaction takes place very close to the star, in a region of about a few stellar radii, and it is therefore challenging to study in detail the physics of the magnetosphere in young stellar objects. We will discuss, through observational examples, how our description of a static and axisymmetric stellar magnetosphere from the early 90s has evolved to a very dynamical multipolar region that takes into account non-steady accretion and time-dependent star-disk interactions.

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Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AA Tau

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Cited by 128 publications

References 34 publications

Accretion-powered chromospheres in classical T Tauri stars

Accretion-powered chromospheres in classical T Tauri stars

Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

Observational clues to the physics at the magnetosphere in young stellar objects

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