The Magnetic Fields of Classical T Tauri Stars

By C. Johns–Krull. 2007

308 citations recorded by the source corpus.

We report new magnetic field measurements for 14 classical T Tauri stars (CTTSs). We combine these data with one previous field determination in order to compare our observed field strengths with the field strengths predicted by magnetospheric accretion models. We use literature data on the stellar mass, radius, rotation period, and disk accretion rate to predict the field strength that should be present on each of our stars according to these magnetospheric accretion models. We show that our measured field values do not correlate with the field strengths predicted by simple magnetospheric accretion theory. We also use our field strength measurements and literature X-ray luminosity data to test a recent relationship expressing X-ray luminosity as a function of surface magnetic flux derived from various solar feature and main-sequence star measurements. We find that the T Tauri stars we have observed have weaker than expected X-ray emission by over an order of magnitude on average using this relationship. We suggest the cause for this is actually a result of the very strong fields on these stars, which decreases the efficiency with which gas motions in the photosphere can tangle magnetic flux tubes in the corona.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/e72f18f3231f20f8c6c40a53716b28479332ccf8

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In Astronomy and Astrophysics

1. J. Donati
2. A. Reiners
3. G. Herczeg
4. K. Luhman
5. L. Hillenbrand
6. L. Hartmann
7. J. Bouvier
8. N. Calvet

Citations in the corpus

• Magnetospheric accretion on the T Tauri star BP Tauri (2008, background): “…in cool spots, the average brightness-unweighted magnetic flux we derive (1.8 kG) is smaller than that estimated by Johns-Krull et al. (1999b) and Johns-Krull (2007), suggesting that we still likely miss magnetic flux from cool spots (e.g., from dark non-accreting small-scale magnetic regions).”
• Least-squares deconvolution of the stellar intensity and polarization spectra (2010, background): “…the magnetic field, chemical composition, and stratification in A and B chemically peculiar stars (e.g., Kochukhov 2003, 2006; Ryabchikova et al. 2006) and to model magnetic fields of T Tauri stars (Johns-Krull 2007) and active M dwarfs (Johns-Krull & Valenti 1996; Kochukhov et al. 2009).”
• Accretion onto Pre-Main-Sequence Stars (2016, background): “Moreover, solar-mass T Tauri stars have surface-averaged magnetic field strengths of 1–2 kG (e.g., Johns-Krull 2007, Donati & Landstreet 2009), which is sufficient to produce magnetospheric truncation radii near corotation if the field were purely dipolar (Equation 1).”
• EXCLUSION OF COSMIC RAYS IN PROTOPLANETARY DISKS: STELLAR AND MAGNETIC EFFECTS (2013, background): “Stellar Magnetic Fields Zeeman broadening observations of young T-Tauri stars have revealed strong magnetic fields at the stellar surface with magnitudes of order 100 G - 1.6 kG (e.g., Johns-Krull 2007).”
• Magnetic Fields of Nondegenerate Stars (2009, methodology): “…assumed to disrupt the inner regions of the accretion disc and to ensure approximate corotation between the inner disc rim and the stellar surface (Johns-Krull 2007); this discrepancy is however not surprising, Zeeman broadening methods being sensitive to small-scale fields whereas predictions of…”
• A spectroscopic survey of Herbig Ae/Be stars with X-shooter – I. Stellar parameters and accretion rates (2015, background): “However, for MA to be applicable the star must have a magnetic field with sufficient strength to truncate the disc; for CTTs this is of the order of kilo-Gauss (Ghosh & Lamb 1979; Koenigl 1991; Shu et al. 1994; Johns-Krull 2007; Bouvier et al. 2007).”
• Accretion-powered Stellar Winds. II. Numerical Solutions for Stellar Wind Torques (2008, background): “…suggested that, for an observationally constrained dipole magnetic field strength of 200 G (e.g., Johns-Krull et al. 1999; Bouvier et al. 2007; Johns-Krull 2007a,b; Smirnov et al. 2003a; Yang et al. 2007), it might indeed be possible for the stellar wind to extract enough angular momentum to…”
• UV Excess Measures of Accretion onto Young Very Low Mass Stars and Brown Dwarfs (2008, background): “The Rin ranges from 3–10 R∗ based on magnetic field strengths of higher-mass CTTSs (Johns-Krull 2007).”
• Stellar magnetism: empirical trends with age and rotation (2014, methodology): “For accreting PMS stars, Johns-Krull (2007) found no correlation between any magnetic and stellar/dynamo parameters5, and in particular, found no correlation between the magnetic flux ΦI , estimated from the average surface magnetic field as calculated from ZB measurements, and Prot.”
• The Sun in Time: Activity and Environment (2007, methodology): “Johns-Krull (2007b) recently succeeded in obtaining near-infrared diagnostics to measure the photospheric magnetic field on a class I protostar.”
• MHD simulations of accretion onto a dipolar magnetosphere - II. Magnetospheric ejections and stellar spin-down (2012, background): “Since CTTS are known to be magnetically active (see e.g. Johns-Krull 2007; Yang & Johns-Krull 2011), different magneto-hydrodynamic (MHD) mechanisms of angular momentum removal have been proposed.”
• Observations of Cool-Star Magnetic Fields (2012, background): “Similar work on other T Tauri stars using infrared spectra was done by Johns-Krull et al., much of it is summarized in Johns-Krull (2007) where additional measurements of 14 T Tauri star magnetic fields are presented.”
• Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph (2007, result): “…revealed that surface magnetic strengths on cTTSs (measured from Zeeman magnetic broadening of unpolarised profiles) do not correlate well with those expected from theoretical models, casting doubts on the validity of the disc-locking mechanism (e.g., Bouvier et al. 2007; Johns-Krull 2007).”