Investigating the Magnetospheric Accretion Process in the Young Pre
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A&A 643, A99 (2020) Astronomy https://doi.org/10.1051/0004-6361/202038892 & c J. Bouvier et al. 2020 Astrophysics Investigating the magnetospheric accretion process in the young pre-transitional disk system DoAr 44 (V2062 Oph) A multiwavelength interferometric, spectropolarimetric, and photometric observing campaign?,?? J. Bouvier1, E. Alecian1, S. H. P. Alencar2, A. Sousa1, J.-F. Donati3, K. Perraut1, A. Bayo4,5, L. M. Rebull6, C. Dougados1, G. Duvert1, J.-P. Berger1, M. Benisty1,7, K. Pouilly1, C. Folsom3, C. Moutou3, and the SPIRou consortium 1 Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France e-mail: [email protected] 2 Departamento de Fisica – ICEx – UFMG, Av. Antonio Carlos 6627, 30270-901 Belo Horizonte, MG, Brazil 3 Univ. de Toulouse, CNRS, IRAP, 14 avenue Belin, 31400 Toulouse, France 4 Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile 5 Núcleo Milenio de Formación Planetaria – NPF, Universidad de Valparaíso, Valparaíso, Chile 6 Infrared Science Archive (IRSA), IPAC, 1200 E. California Blvd., California Institute of Technology, Pasadena, CA 91125, USA 7 Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS, UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile Received 10 July 2020 / Accepted 10 September 2020 ABSTRACT 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 char- acterize the central star’s properties from the high signal-to-noise, high-resolution optical spectra we obtained during the campaign. −9 −1 ◦ DoAr 44 is a young 1.2 M star, moderately accreting from its disk (M˙ acc = 6.5 10 M yr ), 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 4.6 R? (0.043 au), which is consistent with the 5 R? (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. Key words. stars: pre-main sequence – stars: variables: T Tauri, Herbig Ae/Be – stars: magnetic field – stars: formation – accretion, accretion disks 1. Introduction material from their circumstellar disks, a remnant of the pro- tostellar collapse and the birthplace of planetary systems. The T Tauri stars are low-mass pre-main sequence stars at an early so-called classical T Tauri stars (cTTSs) have specific proper- stage of their evolution. For a few million years, they accrete ties, such as a rich emission-line spectrum, large photometric variability, and a strong continuum excess flux superimposed ? Full Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. onto a late-type photospheric spectrum (Herbig 1962). The u-strasbg.fr/viz-bin/cat/J/A+A/643/A99 unique properties of this class of objects eventually vanish ?? Based on observations obtained at the Canada-France-Hawaii as the disk dissipates on a timescale of a few million years, Telescope (CFHT), at the European Organisation for Astronomical and the young stellar object transitions to a weak-line T Tauri Research in the Southern Hemisphere under ESO programme 0103.C- star (wTTS, Ménard et al. 1999). Indeed, many of the spe- 0097, and at the Las Cumbres Observatory global telescope network cific properties of cTTSs are thought to derive from the accre- (LCOGT). tion process (Kenyon & Hartmann 1987; Bertout et al. 1988; A99, page 1 of 18 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A&A 643, A99 (2020) Basri & Bertout 1989) and, more specifically, to arise from the June 2019 interaction region between the inner disk and the star. The currently accepted paradigm that describes this interac- tion is the magnetospheric accretion scenario where the strong 0.9 stellar magnetic field controls, at least partly, the accretion flow from the inner disk edge to the stellar surface (Camenzind 1990; Koenigl 1991, see reviews by Bouvier et al. 2007a; 1.0 Hartmann et al. 2016). With large-scale surface magnetic fields g' (mag) of a few hundred to a few kG (e.g., Donati 2020; Sokal et al. 2020), the stellar magnetosphere truncates the inner gaseous 1.1 disk at a distance of a few stellar radii (typically 3–8 R?) above E E E E E S S S S S S S S G G the photosphere. From there, the accreted material is channeled 8640 8645 8650 8655 8660 along magnetic funnel flows, and hits the stellar surface almost HJD-2,450,000.0 at free-fall velocity to create a localized accretion shock where Fig. 1. Visual summary of observations obtained in June 2019, as the kinetic energy of the flow is dissipated. In this scenario, the part of the DoAr 44 coordinated campaign. The differential g0-band emission line spectrum of cTTSs is partly formed in the accre- light curve (see Sect. 3.2) is shown as colored circles, with the color tion funnel flows (Hartmann et al. 1994), and the accretion shock scale reflecting the Julian Date, while the epochs of CFHT/ESPaDOnS, is responsible for the continuum excess flux from the X-ray CFHT/SPIRou, and ESO VLTI/Gravity observations are indicated by range to the optical wavelengths, which is referred to as veiling triangles labeled with “E”, “S”, and “G”, respectively. Note that the (Calvet & Gullbring 1998). March 2019 ESPaDOnS run, which is part of the campaign reported The magnetospheric process extends over a scale less than here, is not shown in this figure. The average g-band magnitude of the 0.1 au from the star, which corresponds to ≤1 mas angular sepa- system during the campaign was g = 13:35 according to the publicly ration on the sky at the distance of nearby star forming regions. available Zwicky Transient Facility light curve (Masci et al. 2019). Therefore, most constraints on the physics of magnetospheric accretion come from observational campaigns monitoring the spectral variability of selected cTTSs over successive rotation addition, below we derive a photometric period of 2.96 d from periods. The rotational modulation over a timescale of weeks a publicly available ASAS-SN light curve (see Sect. 3.2), which of emission line profiles, veiling, and Zeeman signatures allows we ascribe to the stellar rotation period. These properties made one to draw magnetic maps of the stellar surface and to recon- it a suitable target for a dedicated large-scale monitoring cam- struct the structure of the magnetospheric accretion region and paign, with the additional interest of probing the magnetospheric investigate its dynamics. Previous monitoring campaigns have accretion process in a pre-transitional disk system. This sys- been quite successful in interpreting the observed properties tem featured a large dust-depleted cavity in its circumstellar and variability of young stellar objects in the framework of disk, extending up to a distance of 30 au from the central star the magnetospheric accretion model (e.g., Bouvier et al. 2007b; (Andrews et al. 2011). The goal of this campaign was to con- Alencar et al. 2012, 2018; Donati et al. 2019). These provided strain the structure and dynamics of the magnetospheric accre- new insights into the physics of the interaction region between tion region between the inner edge of the compact inner disk the inner disk edge and the stellar surface, which impacts both and the central star by measuring the magnetic field strength at early stellar evolution, and, potentially, planet formation and/or the stellar surface, reconstructing the geometry of the magne- migration at the inner disk edge. tospheric funnel flows and accretion shocks from the analysis We present here the results of a new campaign target- of line profile variability and photometric variations, and, ulti- ing the young star DoAr 44 (also known as V2062 Oph, mately, attempting to resolve the interaction region over scales Haro 1–16, ROXs 44, HBC 268) located at a distance of of a few stellar radii above the stellar surface using long-baseline 146 ± 1 pc (Gaia Collaboration 2016, 2018) in the L1688 cloud interferometry.