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Magnetic Fields of Intermediate Mass T Tauri Stars A&A 608, A77 (2017) Astronomy DOI: 10.1051/0004-6361/201731889 & c ESO 2017 Astrophysics Magnetic fields of intermediate mass T Tauri stars A. Lavail1; 2, O. Kochukhov1, G. A. J. Hussain2, E. Alecian3, G. J. Herczeg4, and C. Johns-Krull5 1 Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden e-mail: [email protected] 2 ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany 3 UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France 4 The Kavli Institute for Astronomy and Astrophysics, Peking University, 100871 Beijing, PR China 5 Department of Physics and Astronomy, Rice University, 6100 Main Street, Houston, TX 77005, USA Received 4 September 2017 / Accepted 11 November 2017 ABSTRACT Aims. In this paper, we aim to measure the strength of the surface magnetic fields for a sample of five intermediate mass T Tauri stars and one low mass T Tauri star from late-F to mid-K spectral types. While magnetic fields of T Tauri stars at the low mass range have been extensively characterized, our work complements previous studies towards the intermediate mass range; this complementary study is key to evaluate how magnetic fields evolve during the transition from a convective to a radiative core. Methods. We studied the Zeeman broadening of magnetically sensitive spectral lines in the H-band spectra obtained with the CRIRES high-resolution near-infrared spectrometer. These data are modelled using magnetic spectral synthesis and model atmospheres. Addi- tional constraints on non-magnetic line broadening mechanisms are obtained from modelling molecular lines in the K band or atomic lines in the optical wavelength region. Results. We detect and measure mean surface magnetic fields for five of the six stars in our sample: CHXR 28, COUP 107, V2062 Oph, V1149 Sco, and Par 2441. Magnetic field strengths inferred from the most magnetically sensitive diagnostic line range from 0.8 to 1.8 kG. We also estimate a magnetic field strength of 1.9 kG for COUP 107 from an alternative diagnostic. The magnetic field on YLW 19 is the weakest in our sample and is marginally detected, with a strength of 0.8 kG. Conclusions. We populate an uncharted area of the pre-main-sequence HR diagram with mean magnetic field measurements from high-resolution near-infrared spectra. Our sample of intermediate mass T Tauri stars in general exhibits weaker magnetic fields than their lower mass counterparts. Our measurements will be used in combination with other spectropolarimetric studies of intermediate mass and lower mass T Tauri stars to provide input into pre-main-sequence stellar evolutionary models. Key words. stars: pre-main sequence – stars: magnetic field – line: profiles 1. Introduction and therefore must be produced at an earlier phase of evolution. As IMTTS are precursors of the Herbig Ae/Be stars, the study Magnetic fields influence physical processes in stars at all of their magnetic fields can shed new light on the origin of mag- stages of their evolution. In the pre-main-sequence (PMS) phase netic fields in intermediate mass stars. in particular magnetic fields govern accretion processes (see Bouvier et al. 2007; Hartmann et al. 2016 for a review of magne- Magnetic fields in T Tauri stars have been investigated ei- tospheric accretion in classical T Tauri stars), which affect stellar ther by measuring the magnetic broadening of the intensity pro- formation and angular momentum evolution. However, the evo- files of infrared Ti i lines around 22 000 Å (Johns-Krull 2007– lution of stellar magnetic fields from the birth line, when all stars hereafter JK07; Yang et al. 2008; Yang & Johns-Krull 2011) or are fully convective, and along the PMS, when stars may develop using the Zeeman Doppler imaging modelling of circular polar- a radiative core, is still not thoroughly understood. ization spectra (Donati et al. 2007, 2008; Hussain et al. 2009, re- Intermediate mass T Tauri stars (IMTTS; defined here to viewed in Hussain 2012). The former method is the only method have mass 1 M 6 M 6 4 M ) are key targets to study the mag- that is capable of providing an unbiased estimate of the to- netic behaviour of PMS stars, test theories of magnetic field for- tal magnetic flux since polarimetric observables are severely mation, and understand the incidence and properties of magnetic affected by cancellation of opposite polarities at small spatial fields in more evolved stars. These IMTTS are the precursors to scales. For their sample of 14 classical T Tauri stars, JK07 mea- the PMS Herbig Ae/Be stars and eventually to the A/B type stars sured mean magnetic field strengths from 1.12 kG to 2.90 kG on the main sequence. Less than 10% of these stars host a mag- and found no significant correlation between the observed field netic field (Power et al. 2007; Donati & Landstreet 2009), which strengths and field strengths predicted by simple magnetospheric is generally strong, of simple structure (dipole-like), and stable accretion models. Yang & Johns-Krull(2011) measured field over at least several decades. strengths ranging from 1.30 to 3.45 kG in their sample of 14 A recent large spectropolarimetric survey aimed at study- additional T Tauri stars and reported that there is no correlation ing the incidence of magnetism among the precursors of A- between field strengths hBi and stellar age for the entire sample and B-type stars, Herbig Ae/Be stars, revealed similar properties of 33 T Tauri stars studied using magnetic line broadening. How- 2 (Alecian et al. 2013), indicating that the global magnetic field is ever, they found that magnetic fluxes 4πR∗hBi decrease steadily likely conserved between the PMS and main-sequence phases, with stellar age. Article published by EDP Sciences A77, page 1 of8 A&A 608, A77 (2017) In this paper, we report magnetic field strengths for a sam- ple of six additional T Tauri stars obtained using magnetic line broadening measurements. Our targets are mostly in the interme- diate mass range, with masses ranging from 1.1 M to 2.0 M , which has not been investigated as well as the lower mass range of the classical T Tauri star regime. One target (CHXR 28) is however in the low mass end at 0.6 M . Considering the range of spectral types of our sample, well-established cool stars mag- netic diagnostics such as Ti i lines around 22 200 Å and FeH lines around 10 000 Å are unsuitable. We instead use Fe i lines around 15 650 Å – used by Valenti et al.(1995) to analyse the spectra of the active K2V star Eridani – which are deep enough in our spectra. This paper is organized as follows. In Sect.2, we discuss the acquisition and reduction of CRIRES spectra in the near-infrared H and K bands used in our study. We describe our methods to es- timate projected rotational velocities v sin i, analyse the Zeeman broadening of magnetically sensitive spectral lines, and estimate mean magnetic field modulus hBi in Sect.3. We report our mea- surements for each star in the sample in Sect.4 and discuss our results in Sect.5. 2. Observations and data reduction We observed six stars with the high-resolution near-infrared CRIRES spectrograph (Kaeufl et al. 2004) mounted at the Very Large Telescope to obtain high-resolution, R ≈ 105, spectra in the H and K band. The journal of our observations is presented in Table1. The CRIRES adaptive optics correction improved the seeing to 0:2500 in the H band and 0:1600 in the K band. The stars were placed in the 0:200 slit, which yields a spectral res- olution of R = 105. The dispersed light was recorded by four 512 × 1024 detectors, producing four distinct spectra that each cover ∼1500 km s−1 and are separated by ∼400 km s−1. The data were obtained in ABBA nods to improve background correction. Data were reduced with custom-written codes in IDL. The AB and, separately, the BA pairs were subtracted from each other first. Bad pixels were eliminated using a cosmic ray rejec- tion routine. The spectra for each exposure were then obtained by summing the number of counts over a ±0:500 (±6 pixels) ex- traction region. Nearby regions on the detector were used to sub- tract any residual flux. The odd-even effect on detectors 1 and 4 was corrected on each individual spectrum. The four spectra were then individually normalized. The total spectrum was cal- culated from the median of each pixel whenever multiple ex- posures of the same target in a given wavelength region were available. The wavelength scale was calculated using telluric ab- −1 Fig. 1. Normalized CRIRES spectra in the H band (upper panel) and sorption lines and had an rms precision of 0.05–0.2 km s de- the K band (lower panel). The spectra are shifted vertically with stellar pending on the segment. Telluric absorption was corrected with effective temperature rising upwards. The arrows in the upper panel in- observations of A-type stars immediately following our science dicate the spectral lines used in the magnetic field analysis. The central observations. The wavelength scale was then shifted to the helio- arrow indicates the magnetically sensitive spectral line Fe i 15 648.5 Å centric velocity frame. The reduced spectra are shown in Fig.1. with geff = 3. CHXR 28 is part of a triple system with a tight compan- ion (Ab separated by 0:14200) and a wide companion (B) located 1:8500 from the tight subsystem (Lafrenière et al.
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