Observations of Magnetic Fields on Late-Type Stars and Brown Dwarfs

SOLAR POLARIZATION 5 ASP Conference Series, Vol. 405, c 2009 S. V. Berdyugina, K. N. Nagendra, and R. Ramelli, eds.

Observations of Magnetic Fields on Late-type Stars and Brown Dwarfs

C. M. Johns–Krull Rice University, Dept. of Physics & Astronomy, 6100 Main St., Houston, TX 77005, USA

Abstract. Magnetic fields produce activity on the Sun and cool stars. Ac- tivity itself involves both the dynamo production of magnetic fields and the magnetic heating of the outer atmosphere of these objects which produces the radiative signatures of activity. On cool pre-main sequence stars, magnetic fields truncate circumstellar disks and direct accreting disk material near the poles of the central star. In the past several years, measurements of the magnetic field strength and constraints on the field geometry have become available for a number of low mass stars and brown dwarfs. Much of the recent observational effort has centered on TTauri stars, due to the importance of strong stellar fields in explaining this stage of star formation, and on very low mass stars and brown dwarfs where measurements of traditional activity indicators suggest sharp differences in field production and/or heating efficiency compared to what is observed in slightly higher mass stars. It is important to note that a complete picture of the fields on these objects requires both spectropolarimetric and Zee- man broadening measurements made in non-polarized light. Here, I will review the techniques for measuring the magnetic field properties of cool stars and the general trends that have emerged from these observations.

1. Introduction

Magnetic fields on the surface of cool stars are thought to be the link between dynamo activity within the star and a host of observed phenomena at an above their surfaces including: the appearance of starspots; heating of chromospheres, transition regions, and coronae; the channeling of accreting disk material onto the surface of pre-main sequence stars; and possibly the launching of vigorous outflows in young stars. None of these processes is well understood. One avenue to constrain and better understand these phenomena is to obtain actual measurements of the stellar fields. However, this can be challenging since many of the stars which are expected to show large field strengths and magnetic filling factors also are rapidly rotating, making the expected magnetic broadening of spectral lines difficult to detect in the presence of rotational line broadening. On the other hand, with the advent of high resolution infrared (IR) spectrometers, better analysis techniques, and the promise of new magnetic field diagnostics, we can now measure accurate surface magnetic parameters for a large sample of active stars including dwarfs, pre-main sequence stars (PMS), and just recently, brown dwarfs. In the area of stellar activity, these new observations will allow us to separate the coupled questions of dynamo generation of fields and the resulting magnetic heating they produce in the outer atmosphere. In the case of pre-main sequence 485 486 Johns–Krull stars, these observations allow us to test current theories of magnetospheric accretion and disk locking, which affects the rotational evolution of young stars. This in turn may feedback on the dynamo generation of these fields. Field measurements on both PMS and low mass main sequence stars provide tests of emerging dymano models for fully convective stars. Virtually all measurements of stellar magnetic fields make use of the Zeeman effect. In this review, measurements based on Zeeman broadening of magnetically sensitive lines observed in intensity spectra are discussed. When an atom is in a magnetic field, different projections of the total orbital angular momentum are no longer degenerate, shifting the energy levels taking part in the transition. In the simple Zeeman effect, a spectral line splits into 3 components: 2 σ components split to either side of the nominal line center and 1 unshifted π component. This both broadens the spectral line and causes (partially) saturated lines to increase their total opacity resulting in an increase in line equivalent width. This equivalent width effect has been used by a number of authors to infer the mean magnetic field strength, i.e., the product of the field strength, B, and the filling factor, f, of magnetic regions (Basri et al. 1992; Basri & Marcy 1994; Rueedi et al. 1997; Guenther et al. 1999). However, great care must be taken to determine accurate Teff values for stars when using this technique since the temperature has a strong effect on line equivalent widths. The wavelength shift of a given σ component is given by ∆λ = kλ2gB; where k is a constant, g is the Landé-g value of the specific transition, B is the strength of the magnetic field, and λ is the wavelength of the transition. One thing to note from this equation is the λ2 dependence of the Zeeman effect. Compared with the λ dependence of Doppler line broadening mechanisms, we see that observations in the IR are generally more sensitive to the presence of magnetic fields than optical observations. For most studies of main sequence stars, the observed line profile can be expressed as F (λ) = FB(λ)∗f +FQ(λ)∗(1−f); where FB is the spectrum formed in magnetic regions, FQ is the spectrum formed in non-magnetic regions, and f is the flux weighted surface filling factor of magnetic regions. The magnetic spectrum, FB, differs from the spectrum in the quiet region not only due to Zee- man broadening of the line, but also because magnetic fields affect atmospheric structure, causing changes in both line strength and continuum intensity at the surface. Most studies assume that the magnetic atmosphere is in fact the same as the quiet atmosphere; a point we will return to later. If the stellar magnetic field is very strong, the splitting of the σ components is a substantial fraction of the line width, and it is easy to see the σ components sticking out on either side of a magnetically sensitive line (e.g., Fig. 1). In this case, it is relatively straightforward to measure the magnetic field strength, B, from the splitting, and the depth of the σ components relative to the π component gives the filling factor, f. Differences in the atmospheres of the magnetic and quiet regions pri- marily affect the value of f. If the splitting is a small fraction of the intrinsic line width, then the resulting observed profile is only subtly different from the profile produced by a star with no magnetic field and more complicated modeling is required to be sure all possible non-magnetic sources (e.g., rotation and pressure broadening) have been properly constrained. Measuring circular polarization in magnetically sensitive lines is perhaps the most direct means of detecting magnetic fields on stellar surfaces, but it is Magnetic Fields on Late-type Stars 487 also subject to several limitations. When viewed along the axis of a magnetic field, the Zeeman σ components are circularly polarized, but with opposite helicity; and the π component(s) is(are) absent. The helicity of the σ components reverses as the polarity of the field reverses. Thus, on a star like the Sun that typically displays equal amounts of + and − polarity fields on its surface, the net polarization is very small. If one magnetic polarity does dominate the visible surface of the star (such as in a dipolar field geometry), net circular polarization is present in Zeeman sensitive lines. This can be measured in the Stokes V profile, or as wavelength shift between the line observed through right- and left- circular polarizers. The magnitude of the shift represents the surface averaged line of sight component of the magnetic field (which on the Sun is typically less than 4 G even though individual magnetic elements on the solar surface range from ∼ 1.5 kG in plage to ∼ 3.0 kG in spots). Thus, polarization measurements supply information on the geometry of the stellar magnetic field, though polarity cancellation means that substantial portion of the magnetic flux can be missed by polarization measurements alone.

2. Main Sequence Stars

Measuring magnetic fields on late type main sequence stars proved to be challenging using optical diagnostics. Improvements in analysis techniques invariably resulted in a reduction of the measured field strengths. An excellent discussion of this history is given by Valenti et al. (1995). Saar & Linsky (1985) first advo- cated using IR lines for field measurements on cool stars, and Saar (1994, 1996) and Valenti et al. (1995) produced many reliable field measurements for G and K stars. These results showed that the field strength on these stars appeared to be set by pressure equilibrium with the surrounding photospheric gas, and that it was the filling factor of the magnetic regions that correlated with rotation rate and typical signatures of “activity.” These initial results suggested that the strongest fields should be found on the dMe stars, as the early observations of Saar & Linsky (1985) also suggested. Johns–Krull & Valenti (1996) used high resolution high signal-to-noise optical spectra to measure the magnetic field on the flare stars EV Lac and GJ 729. Figure 1 shows the spectrum of EV Lac along with several inactive comparison stars. This figure clearly shows the presence of Zeeman split σ components on either side of the Landé g = 2.5 Fe i line at 8468.4 A.˚ The magnetic field is even more clear in the ratio plot shown in the top panel of Fig. 1. Dividing the active star by an inactive template of the same spectral type removes the TiO lines for which the line data is uncertain. These ratio plots also show that the less Zeeman sensitive Ti i line at 8467.2 A˚ shows magnetic broadening. Fitting a single component magnetic field model to these data, Johns–Krull & Valenti find B = 3.8±0.5 kG with f = 0.50±0.13 for EV Lac (B = 2.6±0.3 kG, f = 0.50±0.13 for GJ 729). However, Fig. 1 (top) shows that a single component magnetic field is not adequate to explain the data; rather, a distribution of magnetic field strengths is suggested. A single magnetic field component is not able to simultaneously reproduce the near and far wings of the Fe i line. Evidence for a similar distribution has been found in IR spectra of AD Leo (Saar 1994). It is likely this distribution represents a true spatial distribution of field strengths 488 Johns–Krull (9/DF f >DPSOLWXGH@ *-% >VHSDUDWLRQ@ %!N*FRPSRQHQW

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Figure 1. Zeeman broadening in EV Lac, relative to a temperature sequence of inactive dwarfs. TiO cancels in the ratio, which we ﬁt.

on the stellar surface, instead of a distribution of field strengths with depth in the line formation region. Evidence in support of this comes from observations of sunspots where a distribution with depth is indeed believed to be present (e.g., Mathew et al. 2003). Despite this depth dependent field, observations of sunspots near the disk center show that the Zeeman sensitive line profiles are well characterized by a single magnetic field value (Saar & Linsky 1985; Wallace & Livingston 1992). This is clearly not true for the M dwarfs (and the TTauri stars discussed below) suggesting a true distribution of field strengths across the surface. While the earlier studies described above suggested it is the filling factor of magnetic regions that correlates most directly with activity indicators, recent work suggests it is simply the total magnetic flux that correlates best. Figure 2 is taken from Pevtsov et al. (2003) and shows the correlation of the magnetic flux and the X-ray luminosity. Points to the left of the diamonds are various solar features. The diamonds are G, K, and M dwarfs, and the x’s are TTauri stars (TTS). As a group, the TTS fall below the relation traced by the solid line (this result is upheld by Johns–Krull 2007). As described below, the photospheres of TTS are dominated by magnetic pressure, and we suggest the relative lack of X-ray emission in these stars given their magnetic flux to be due to the strong fields inhibiting the convective motions ultimately responsible for stressing the fields at the surface and producing heating. Magnetic Fields on Late-type Stars 489

Figure 2. X-ray luminosity versus total magnetic ﬂux for various solar features and several late type stars.

3. Pre-Main Sequence Stars

TTauri stars (TTS) lie above the main-sequence in an H-R diagram and have one or more other indicators of youth, such as high lithium abundance, a circumstellar disk, or association with a dark cloud. Classical TTS (CTTS) accrete material from a circumstellar disk, which gives rise to excess continuum and line emission emission from blue to infrared (IR) wavelengths. Uchida (1983) and Uchida & Shibata (1984) first suggested that CTTS magnetic fields disrupt the inner accretion disk, lifting material out of the disk plane toward the stellar magnetic poles. As the material accretes along the stellar field lines, it is concentrated onto small fractions of the stellar surface where a shock forms as the disk material strikes the star. Indeed, observations show that the filling factor of accreting regions on CTTS is ∼ 1% (Valenti et al. 1993; Calvet & Gullbring 1998). This insight inspired many detailed magnetospheric accretion models (e.g., Königl 1991; Collier Cameron & Campbell 1993; Shu et al. 1994; Johns–Krull & Gafford 2002). These models invoke different forms of magnetic coupling, but all attempt to match the stellar rotation period with the Keplerian orbital period of material near the inner edge of the disk. This underlying assumption leads to similar self-consistency relationships between stellar mass (M∗), radius (R∗), rotation period (P∗), dipole magnetic field strength at the equator (Beq), and mass accretion rate in the disk (M˙ ). 490 Johns–Krull

Johns–Krull et al. (1999b) used literature values of M∗, R∗, P∗, and M˙ for 14 CTTS to calculate the range of Beq values required by three of the magnetospheric accretion models cited above. Every model yields a similar ordering of CTTS from lowest to highest Beq, so this ordering might be used to test the rotational equilibrium hypothesis underlying all magnetospheric accretion models (including those applied to white dwarfs and neutron stars) since in principle all the variables in the theory can be measured independently for CTTS. TTS typically have v sin i values of 10 km s−1, making it diﬃcult to measure actual Zeeman broadening in optical diagnostics. Magnetic broadening measurements for TTS are best done using the Ti i 2.2 µm lines shown in Fig. 3. Robust Zeeman broadening measurements require Zeeman insensitive lines to constrain nonmagnetic broadening mechanisms. Fortunately, CO lines at 2.31 µm have negligible Land´e-g factors, making them an ideal null reference.

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Figure 3. An IRTF/CSHELL spectrum of the K7 CTTS BP Tau (his- togram) is compared with synthetic spectra based on magnetic (doubled curve) and nonmagnetic (single curve) models. Zeeman insensitive CO lines are well fit by both models. The Zeeman sensitive Ti i lines are much broader than predicted by the nonmagnetic model. The magnetic model reproduces the observed spectrum, using a distribution of magnetic field strengths (inset his- togram) with a mean of 2.1 kG over the entire stellar surface. The effective Landé-g factor is given for each atomic line.

Figure 3 presents an IR spectrum of the CTTS BP Tau. The Zeeman insensitive CO lines are well fitted by nonmagnetic models with rotational broadening that also accurately reproduce optical line profiles. In contrast, the 2.2 µm Ti i lines cannot be fitted by the nonmagnetic model. Observed BP Tau line profiles are much broader. Fields such as these have now been detected on numerous Magnetic Fields on Late-type Stars 491

TTS (Johns–Krull et al. 1999b, 2004; Yang et al. 2005; Johns–Krull 2007). In these cases, as for the M dwarfs, we find that a distribution of field strengths is needed to fit the data – no single value of the magnetic field works. Therefore, we typically fit the line profiles by solving for the filling factors on the stellar surface of components with fields equal to 0, 2, 4, and 6 kG. The intensity- weighted mean magnetic field strength, hBi, over the entire surface is then the sum of the filling factors multiplied by their component field strengths, and in the case of BP Tau is 2.1 kG. Thus, magnetic fields on BP Tau (and almost all other TTS for which there are measurements) are stronger than on the Sun, even though the surface gravity on BP Tau is lower by a factor of ten. As mentioned above, on the Sun and other main-sequence stars, magnetic field strength seems to be set by an equipartition of gas and magnetic pressure. In contrast, the photospheres of BP Tau and other TTS are apparently dominated by magnetic pressure, rather than gas pressure (see also Johns–Krull et al. 2004). This im- plies that the nonmagnetic model atmospheres used to analyze TTS are missing an important pressure support term. Energy transport by convection should also be reconsidered.

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Figure 4. The observed mean magnetic ﬁeld strength, B¯, plotted versus the predicted equatorial magnetic ﬁeld strength from the Shu et al. (1994) treatment of the magnetospheric accretion model.

The original motivation to for measuring the magnetic fields on CTTS was to test the magnetospheric accretion theories described above. Johns–Krull (2007) has now measured the mean magnetic field strength on 15 CTTS for which the relevant data exists to predict the expected field strength from mag- 492 Johns–Krull netospheric accretion models using the equations given in Johns–Krull et al. (1999b). Figure 4 shows these observed field strengths versus the predicted field strengths using the equations specific to the Shu et al. (1994) model (the scatter plot looks the same independent of the specific magnetospheric accretion model used). The expected correlation is clearly absent. However, this does not mean that magnetospheric accretion is to be abandoned. It is likely that the fields on TTS have a more complicated geometry than simple dipoles as generally assumed. Johns–Krull & Gafford (2002) showed how the model of Shu et al. (1994) can be used to abandon the dipole assumption, while still making predictions for the stellar field strength. An additional parameter is needed - namely the filling factor of accretion zones on the stellar surface. Johns–Krull & Gafford (2002) and Valenti & Johns–Krull (2004) showed that the combination of observed field values and filling factors do agree reasonably well with the models, though additional tests are needed.

4. FeH and the Push toward Brown Dwarfs

There is growing interest in, and evidence for, activity on very cool dwarfs. Recently, a controversy has arisen on the subject. The initial paradigm developed for very low mass stars and brown dwarfs assumed that hot gas in the atmospheres of these stars is present only during flares (Rutledge et al. 2000; Mohanty & Basri 2002). Here, hot gas refers to gas at transition region and coronal temperatures. The M9 brown dwarf, LP944-20, observed with Chandra by Rutledge et al. (2000) does show persistent chromospheric (Hα) emission during quiescence (i.e., outside of flares), as do many of its counterparts, but the X-ray emission appeared to be confined to flares. At the cool temperatures associated with these very late-type objects, the traditional atomic Zeeman diagnostics are no longer present. Molecular Zeeman diagnostics are required. Wallace et al. (1998) noted that Zeeman broadening of FeH lines grows with increasing J and Ω in a sunspot spectrum (see also Wallace & Hinkle 2001; Berdyugina et al. 2003). The FeH molecule becomes quite strong in mid to late M spectral types, increasing in strength to about L3 and then weakening from there (e.g., Leggett et al. 2001). An example of the Zeeman sensitivity of FeH in a sunspot and the dMe star AD Leo is shown in Fig. 5. The top panel of this figure compares a lab and a sunspot spectrum in the 9900 A˚ region which is dominated by FeH. While a few of these FeH lines have low Landé-g values and hence low Zeeman sensitivity (e.g., the narrow sunspot lines at 9920.8 and 9922.1 A),˚ the broadening of the majority of the lines in the sunspot spectrum is due to the Zeeman effect in these highly sensitive lines. The bottom panel of Fig. 5 compares spectra of the active M dwarf AD Leo and the inactive M dwarf GJ 725B. The two narrow (Zeeman insensitive) lines seen in the sunspot spectrum are narrow in both M dwarfs. Magnetically sensitive lines that are broad in the sunspot spectrum are also broad in AD Leo, but remain narrow in GJ 725B. For example, the two lines at 9903.1 and 9903.6 A˚ are resolved in GJ 725B, but not in AD Leo because the Zeeman broadening of these lines causes them to overlap. The AD Leo spectrum is well matched by a mixture of the rotationally broadened spectrum of GJ 725B (30%) added to a similarly rotationally broadened spectrum of the sunspot (70%). The Zeeman Magnetic Fields on Late-type Stars 493

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Figure 5. FeH in the lab, a sunspot, and active and inactive M dwarfs. sensitive Fe i line at 8468.4 A˚ yields a similar result. Since FeH can be used to much cooler temperatures, these lines promise a sensitive probe of magnetic fields into the substellar regime. Reiners & Basri (2006, 2007) have recently done just this. The Zeeman splitting patterns of the FeH lines are not well known; however, Reiners & Basri (2006) developed a technique similar in spirit to that described above in the modeling of AD Leo in Fig. 5. Reiners and Basri used previous field determinations on M dwarfs to calibrate the Zeeman sensitivity of the FeH lines. Reiners & Basri (2007) then applied this technique to very low mass objects, including some likely brown dwarfs. In several of these objects they detect strong fields covering most of the surface of the star, showing that these objects can sustain dynamo action, even when they appear to be relatively inactive.

5. Polarization Results

Early results, mainly on main sequence and RS CVn stars, have generally failed to detect circular polarization, placing limits on the net magnetic field strength present of 10 − 100 G (e.g., Vogt 1980; Brown & Landstreet 1981; Borra et al. 1984). The interpretation resulting from these studies is that the late-type stars studied likely have complicated surface magnetic field topologies which display approximately equal amounts of opposite polarity field which results in no de- tectable net magnetic field. A significant advance was achieved when Donati et 494 Johns–Krull al. (1997) used the technique of least squares deconvolution (LSD) to effectively average the profiles of thousands of lines in the observed polarization spectra and detect weak polarization on several late-type stars. Using this LSD technique on rapidly rotating stars, the technique of Doppler imaging (see review by Rice 1996) has been extended to Zeeman Doppler imaging (ZDI) and magnetic maps of cool stars are now becoming available for many stars. Imaged stars include the RS CVn star HR 1099 (Petit et al. 2004), the dMe star V374 Peg (Donati et al. 2006), and the planet host star τ Boo (Catala et al. 2007). Generally, these studies find non-dipolar field geometries and weak overall values for the mean line of sight magnetic field strength, BZ , again due to the cancellation of polarization signals by fields of opposite polarity. Another result is that for stars previously known from Doppler imaging studies to have large polar spots, the ZDI results show that these spots are not the result of global dipolar field (e.g., Petit et al. 2004). There has also been a push to perform polarization measurements on fully convective M dwarfs. Phan-Bao et al. (2006) detect polarization corresponding to field strengths ranging from BZ = −40G to +18 G at different epochs on EV Lac, a star known from Zeeman broadening measurements to have a surface averaged magnetic field strength of ∼ 2 kG (see above and Johns–Krull & Valenti 2000). Thus, both Zeeman broadening and polarization measurements are needed to get a full picture of the magnetic field properties of late-type stars. Much of the most recent work has focused on pre-main sequence stars. As mentioned above, existing magnetospheric accretion models assume that intrinsic TTS magnetic fields are dipolar; however, this would be unprecedented for cool stars. Overall, there are relatively few measurements of BZ for TTSs. Until recently, TTau was the only TTS observed polarimetrically, with a 3σ upper limit of |BZ | < 816 G set by Brown & Landstreet (1981). TTau has been the focus of more recent study: Smirnov et al. (2003) report a detection of a net field of 160 ± 40 G on TTau which was not confirmed by Smirnov et al. (2004) or Daou et al. (2006). Donati et al. (1997) used the rapid rotation of the diskless NTTS V410 Tau to actively isolate strips on the stellar surface and detect net circular polarization from the star; however, no field strength was ascribed to these results. In addition, Donati et al. do not detect polarization on two other rapidly rotating TTSs. Yang et al. (2007) detect a net field of BZ = 149 ± 33 G on TW Hya on one night of their 6 night monitoring campaign on this star, finding only (3σ) upper limits of ∼ 100 G on the other nights. Additional studies also only find upper limits (3σ) of 100–200 G on 4 additional CTTSs (Johns– Krull et al. 1999a; Valenti & Johns–Krull 2004). Also very recently, Donati et al. (2007) use their LSD technique to detect significant polarization in the photospheric lines of the CTTS V2129 Oph on several nights. The strongest polarization detected corresponds to a field of BZ = −180 ± 7 G; however, the average field detected over the 8 nights of their observing is −52 G. In light of the strong magnetic fields measured using Zeeman broadening techniques, the general absence of polarimetric detections strongly suggest the magnetic fields on TTSs are not dipolar, at least at the stellar surface. Again, as higher or- der terms will fall off more rapidly with distance, it is expected that the dipole component of the field will indeed dominate at distances of several stellar radii. However, measuring the fields at these distances is quite difficult. The only direct field measurement above the surface of a TTSs is the recent detection of Magnetic Fields on Late-type Stars 495 circular polarization in the line profiles of FU Ori (Donati et al. 2005). Here, the fields detected are likely in the accretion disk, and the measured fields may not be anchored in the star at all. Johns–Krull et al. (1999a) made the surprising discovery of circular polarization in emission line diagnostics that form predominantly in the accretion shock at the surface of CTTSs. This circular polarization signal is strongest in the narrow component of the He i 5876 A˚ emission line, but it is also present in the Ca ii infrared triplet lines (e.g., Yang et al. 2007; Donati et al. 2007). Valenti & Johns–Krull (2004) detect He i polarization in four CTTSs: AA Tau, BP Tau (see also Chuntonov et al. 2007), DF Tau, and DK Tau. Symington et al. (2005) also detect He i polarization at greater than the 3σ level in three stars (BP Tau, DF Tau, and DN Tau) in their survey of seven CTTSs, and Yang et al. (2007) detect polarization in this line in the CTTS TW Hya. Most recently, Donati et al. (2007) detect He i polarization in the CTTS V2129 Oph. All these stars are characterized by He i emission lines which have strong narrow components (NCs) to their line profiles (see Edwards et al. 1994; Alencar & Basri 2000, for a discussion of NC and broad component, BC, emission in CTTSs). Smirnov et al. (2004) reported detections of circular polarization in the He i 5876 A˚ emission line of TTau on all 3 nights they observed the star, though with significant variability from one night to the next (field measurements range from +350 G to +1100 G with no uncertainty estimates). TTau’s He i line is dominated by BC emission. Daou et al. (2006) observed TTau on 2 nights, finding field values in the He i line of −29 ± 116 G on one night and −43 ± 300 G on the second. TW Hya’s He i line has a significant broad component, but Yang et al. (2007) do not report any polarization in this part of the line. The NC of the He i emission is commonly associated with the accretion shock itself at the stellar surface, whereas the BC may have contributions from the magnetospheric accretion funnel flow and/or a hot wind component (e.g., Beristain et al. 2001). Since the BC of the He i emission line forms over a large, extended volume, its magnetic field strength should be weaker than at the stellar surface. In addition field line curvature may enhance polarization cancellation in the BC. As a result, circular polarization in the BC of the He i 5876 A˚ emission is predicted to be less than in the NC. Therefore, the result of Smirnov et al. (2004) for TTau is quite surprising. Additional observations of TTau and other CTTSs dominated by BC emission are needed to confirm the polarization detections. The polarization in the NC of the He i emission line (and in the NC of the Ca ii IRT line for those studies that include it) is observed to vary smoothly with stellar rotation (Valenti & Johns–Krull 2004; Symington et al. 2005; Donati et al. 2007). The interpretation of the full set of observations for CTTSs is that they are essentially fully covered with strong magnetic fields with a complex geometry, thus explaining the strong fields detected with Zeeman broadening in the IR and the generally low values of BZ observed with spectropolarimetry of photospheric absorption lines. Despite this generally complex field geometry, the accretion disk appears to be truncated by a dipole-like component of the field, and the accreting material is channeled down to the surface along fields of a preferred polarity – namely those field lines tracing this dipolar component. Thus, emission lines forming near the accretion shocks display strong polariza- 496 Johns–Krull tion. Since these lines form over a small portion (< 5%) of the stellar surface, the resulting polarization signal is modulated by the rotation of the star.

6. Summary

Over the past several years, there has been an explosion of magnetic field measurements on late type stars. The picture that is emerging is that strong magnetic fields exist on many examples of late-type stars, particularly the fully convective M dwarfs, very low mass stars and potentially brown dwarfs, and on pre-main sequence stars. The fields on these stars have a generally complex field geometry; however, the dipole component on CTTSs appears to be strong enough to truncate the surrounding accretion disk and redirect accreting material along field lines to the stellar surface as traced by the strong, rotationally modulated fields measured in emission lines such as the He i 5876 A˚ and Ca ii IRT lines.

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