The magnetic propeller accretion regime of LkCa 15

Alécian

et al. 2020

A&A

Context. Classical T Tauri stars are pre-main sequence stars surrounded by an accretion disk. They host a strong magnetic field, and both magnetospheric accretion and ejection processes develop as the young magnetic star interacts with its disk. Studying this interaction is a major goal toward understanding the properties of young stars and their evolution. Aims. The goal of this study is to investigate the accretion process in the young stellar system HQ Tau, an intermediate-mass T Tauri star (1.9 M⊙). Methods. The time variability of the system is investigated both photometrically, using Kepler-K2 and complementary light curves, and from a high-resolution spectropolarimetric time series obtained with ESPaDOnS at CFHT. Results. The quasi-sinusoidal Kepler-K2 light curve exhibits a period of 2.424 d, which we ascribe to the rotational period of the star. The radial velocity of the system shows the same periodicity, as expected from the modulation of the photospheric line profiles by surface spots. A similar period is found in the red wing of several emission lines (e.g., HI, CaII, NaI), due to the appearance of inverse P Cygni components, indicative of accretion funnel flows. Signatures of outflows are also seen in the line profiles, some being periodic, others transient. The polarimetric analysis indicates a complex, moderately strong magnetic field which is possibly sufficient to truncate the inner disk close to the corotation radius, rcor ∼ 3.5 R⋆. Additionally, we report HQ Tau to be a spectroscopic binary candidate whose orbit remains to be determined. Conclusions. The results of this study expand upon those previously reported for low-mass T Tauri stars, as they indicate that the magnetospheric accretion process may still operate in intermediate-mass pre-main sequence stars, such as HQ Tauri.

Section: Discussioncontrasting

confidence: 62%

“…several such studies (e.g., Bouvier et al 2007b;Donati et al 2007Donati et al , 2019Alencar et al 2012Alencar et al , 2018.…”

Section: Introductionmentioning

confidence: 99%

Magnetospheric accretion in the intermediate-mass T Tauri star HQ Tauri

Pouilly

Alécian

et al. 2020

A&A

“…The longitudinal component of the magnetic field projected onto the LoS is still modulated at the stellar rotation period (see Fig. 17) and now reaches a maximum value of 800 G, which is about four times larger than the intensity deduced from the LSD profiles, as is often the case for young accreting stars (e.g., Donati et al 2019). It is worth mentioning here that the intensity of B l , as well as its sign, are widely different when using LSD (photospheric) profiles and the CaII IRT (chromospheric) lines.…”

Section: Zeeman-doppler Analysismentioning

confidence: 79%

“…The rotational modulation over a timescale of weeks of emission line profiles, veiling, and Zeeman signatures allows one to draw magnetic maps of the stellar surface and to reconstruct the structure of the magnetospheric accretion region and investigate its dynamics. Previous monitoring campaigns have been quite successful in interpreting the observed properties and variability of young stellar objects in the framework of the magnetospheric accretion model (e.g., Bouvier et al 2007a;Alencar et al 2012Alencar et al , 2018Donati et al 2019). These provided new insights into the physics of the interaction region between the inner disk edge and the stellar surface, which impacts both early stellar evolution, and, potentially, planet formation and/or migration at the inner disk edge.…”

Section: Introductionmentioning

confidence: 99%

Investigating the magnetospheric accretion process in the young pre-transitional disk system DoAr 44 (V2062 Oph)

Alécian

Alencar

et al. 2020

A&A

Context. Young stars interact with their accretion disk through their strong magnetosphere. Aims. We aim to investigate the magnetospheric accretion/ejection process in the young stellar system DoAr 44 (V2062 Oph). Methods. We monitored the system over several rotational cycles, combining high-resolution spectropolarimetry at both optical and near-IR wavelengths with long-baseline near-IR inteferometry and multicolor photometry. Results. We derive a rotational period of 2.96 d from the system's light curve, which is dominated by stellar spots. We fully characterize the central star's properties from the high signal-to-noise, high-resolution optical spectra we obtained during the campaign. DoAr 44 is a young 1.2M star, moderately accreting from its disk (Ṁ acc = 6.5 10 −9 M yr −1), and seen at a low inclination (i 30 •). Several optical and near-IR line profiles probing the accretion funnel flows (Hα, Hβ, HeI 1083 nm, Paβ) and the accretion shock (HeI 587.6 nm) are modulated at the stellar rotation period. The most variable line profile is HeI 1083 nm, which exhibits modulated redshifted wings that are a signature of accretion funnel flows, as well as deep blueshifted absorptions indicative of transient outflows. The Zeeman-Doppler analysis suggests the star hosts a mainly dipolar magnetic field, inclined by about 20 • onto the spin axis, with an intensity reaching about 800 G at the photosphere, and up to 2±0.8 kG close to the accretion shock. The magnetic field appears strong enough to disrupt the inner disk close to the corotation radius, at a distance of about 4.6R (0.043 au), which is consistent with the 5R (0.047 au) upper limit we derived for the size of the magnetosphere in our Paper I from long baseline interferometry. Conclusions. DoAr 44 is a pre-transitional disk system, exhibiting a 25-30 au gap in its circumstellar disk, with the inner and outer disks being misaligned. On a scale of 0.1 au or less, our results indicate that the system is steadily accreting from its inner disk through its tilted dipolar magnetosphere. We conclude that in spite of a highly structured disk on the large scale, perhaps the signature of ongoing planetary formation, the magnetospheric accretion process proceeds unimpeded at the star-disk interaction level.

“…Classical T Tauri stars (CTTs) are young stellar objects (a few Myr old) with M * 2 M that are surrounded by accretion disks (see e.g., the review by Hartmann et al 2016). These stars are magnetically active, exhibiting multipolar fields that are typically ∼kG strong (e.g., Johns-Krull 2007;Donati et al 2008Donati et al , 2019Donati et al , 2020Gregory et al 2008;Johnstone et al 2014) and show signatures of mass accretion, withṀ acc varying from 10 −8 to 10 −10 M yr −1 (e.g., Gullbring et al 1998;Hartmann et al 1998;Herczeg & Hillenbrand 2008;Ingleby et al 2014;Venuti et al 2014;Alcalá et al 2017). The measured stellar magnetic fields are strong enough to disrupt the disk and channel the accretion flow into funnels that impact the stellar surface at near free-fall speed, forming hot accretion spots.…”

Section: Introductionmentioning

confidence: 99%

Magnetic torques on T Tauri stars: Accreting versus non-accreting systems

Pantolmos

Zanni

2020

A&A

Context. Classical T Tauri stars (CTTs) magnetically interact with their surrounding disks, a process that is thought to regulate their rotational evolution. Aims. We compute torques acting on the stellar surface of CTTs that arise from different accreting (accretion funnels) and ejecting (stellar winds and magnetospheric ejections) flow components. Furthermore, we compare the magnetic braking due to stellar winds in two different systems: isolated (i.e., weak-line T Tauri and main-sequence) and accreting (i.e., classical T Tauri) stars. Methods. We use 2.5D magnetohydrodynamic, time-dependent, axisymmetric simulations that were computed with the PLUTO code. For both systems, the stellar wind is thermally driven. In the star-disk-interaction (SDI) simulations, the accretion disk is Keplerian, viscous, and resistive, and is modeled with an alpha prescription. Two series of simulations are presented, one for each system (i.e., isolated and accreting stars). Results. In classical T Tauri systems, the presence of magnetospheric ejections confines the stellar-wind expansion, resulting in an hourglass-shaped geometry of the outflow, and the formation of the accretion columns modifies the amount of open magnetic flux exploited by the stellar wind. These effects have a strong impact on the stellar-wind properties, and we show that the stellar-wind braking is more efficient in the SDI systems than in the isolated ones. We further derive torque scalings over a wide range of magnetic field strengths for each flow component in an SDI system (i.e., magnetospheric accretion and ejections, and stellar winds), which directly applies a torque on the stellar surface. Conclusions. In all the performed SDI simulations, the stellar wind extracts less than 2% of the mass accretion rate and the disk is truncated by up to 66% of the corotation radius. All simulations show a net spin-up torque. We conclude that in order to achieve a stellar-spin equilibrium, we need either more massive stellar winds or disks that are truncated closer to the corotation radius, which increases the torque efficiency of the magnetospheric ejections.