Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW Hydrae

• We present high spectral resolution (R approximate to 108,000) Stokes V polarimetry of the classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 angstrom and 6678 angstrom. The He I lines in these CTTSs contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large-scale magnetospheric accretion flow. We detect strong polarization in the narrow component of the two He I emission lines in both stars. We observe a maximum implied field strength of 6.05 +/- 0.24 kG in the 5876 angstrom line of GQ Lup, making it the star with the highest field strength measured in this line for a CTTS. We find field strengths in the two He I lines that are consistent with each other, in contrast to what has been reported in the literature on at least one star. We do not detect any polarization in the broad component of the He I lines on these stars, strengthening the conclusion that they form over a substantially different volume relative to the formation region of the narrow component of the He I lines.

Nyckelord

accretion

accretion disks

line: profiles

stars: atmospheres

stars: formation

stars: magnetic field

stars: pre-main sequence

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