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Astron. Astrophys. 341, 768-783 (1999)

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1. Introduction

Classical T Tauri stars (henceforth called cTTSs) are young, low mass, optically visible pre-main sequence emission line stars, which, whilst formally classified in terms of the equivalent width of their [FORMULA] emission, have an accretion disk. It has been a long standing mystery as to how the rotation rates of classical T Tauri stars are kept much lower than the breakup velocity, despite the large accretion rates, and despite the fact that cTTSs are still contracting towards the main sequence. Understanding the mechanism by which pre-main sequence stars lose angular momentum is crucial. Weak-line T Tauri stars (wTTSs) were originally defined on the basis of strong X-ray emission and spectroscopic criteria (e.g. equivalent width of the [FORMULA]-line [FORMULA] Å; Appenzeller & Mundt 1996). Later research indicates that they are similar to the cTTSs, except that they lack close circumstellar disks and accretion (Appenzeller & Mundt 1996; Osterloh & Beckwith 1995). Since cTTSs rotate slower than the weak-line T Tauri stars, and the post-T Tauri stars, it is natural to consider processes involving the disk to keep the rotation rate low (Edwards et al. 1993, Bouvier 1994). The interaction between the star and the disk could take place either via a boundary-layer (Lynden-Bell and Pringle 1974) or via magnetic field lines coupling the disk with a highly magnetic pre-main sequence star (Camenzind 1990; Königl 1991; Najita et al. 1995; Shu et al. 1994a; Shu et al. 1994b; Ostriker et al. 1995; Shu et al. 1995). As shown by Cameron & Campbell (1993), Cameron et al. (1995) and Armitage & Clarke (1996), the low rotation velocities of cTTSs can be reproduced if strong magnetic coupling between the star and the disk is assumed. However, this magnetic accretion scenario has recently been criticised by Safier (1998). Magnetic field strengths of the order of 1 kG on the surface of the star are a necessary (but not sufficient, e.g. Safier (1998)) condition for the viability of the magnetic accretion models. Here we seek to determine whether such surface fields are indeed present.

1.1. Indirect evidence for magnetic fields on cTTSs

Although strong magnetic fields are likely to be highly important for cTTSs, and although several attempts to observe fields using optical spectro-polarimetry have been made (Brown & Landstreet 1981; Johnstone & Penston 1986, 1987) no field has been directly detected yet in cTTSs. On the other hand, the non-detection of magnetic fields by means of spectro-polarimetry using the Zeeman analysis technique does not necessarily imply the absence of such fields, because the signals from regions with different polarity might cancel out. We discuss the indirect evidence for strong fields in cTTS in this section.

Circularly polarized radio emission has been detected in HD 283447 (Phillips et al. 1996) and T Tau (Phillips et al. 1996), which is a clear sign of the presence of magnetic fields 1. Many more cTTSs have been observed in the radio regime but the radiation is in general thermal (Bieging et al. 1984). As pointed out by André (1987), the non-detection of polarized radio emission in many cTTSs does not necessarily exclude magnetic fields since the ionized winds of cTTSs are expected to free-free absorb any non-thermal radio emission produced near the surface of the star. Strong circular polarization of opposite helicity has also been detected in the two lobes of T Tau S (Ray et al. 1997). This is expected if the outflows are magnetically collimated. Magnetic fields are also important for the acceleration and collimation of the winds (Camenzind 1990; Paatz and Camenzind 1996; Shu et al. 1995). Another argument for the presence of magnetic fields on cTTSs is that they are relatively bright in X-rays, and that they occasionally show X-ray flares (Preibisch et al. 1993; Montmerle et al. 1983). Again, the brightness of cTTSs in X-rays does not necessarily imply the presence of a magnetosphere, because soft X-ray radiation could in principle emerge from the post-shock region of the accretion shock of the cTTSs (Gullbring 1994; Lamzin et al. 1996). Although some of the short time scale variability in the optical could be due to flares too, Gahm (1994) argues that most events are probably due to variations of the accretion rate rather than due to flares. Other hints of the presence of magnetic fields are the narrow emission line components in Ca ii and He i (Batalha et al. 1996) that might originate in plage regions on the stellar surface. Broad-band photometry indicates the presence of hot as well as cool spots (Bouvier et al. 1995). Cool (Johns-Krull & Hatzes 1997a; Mennessier 1997) and hot spots (Unruh et al. 1998) are also inferred from Doppler imaging (see discussion in Sect. 1.3)

Evidence for magnetic coupling of the star and the disk comes from line profiles: (1) the observed large in-fall velocities can be explained in the context of magnetic accretion (Edwards et al. 1994), (2) the general shape of some line-profiles can be reproduced by models which assume that the lines are formed in magnetospheric in-fall zones (Hartmann et al. 1994, Folha et al. 1997). (3) Another argument for the magnetic coupling of the star and the disk is that there some evidence for a rotational modulation of the profiles and fluxes of the emission-lines (Johns-Krull & Basri 1997b, Hessman & Guenther 1997).

To sum up, we can conclude that there is some evidence for the presence of magnetic fields in cTTSs, and some indirect evidence for magnetospheric accretion, but the direct proof of the existence of a magnetic field with a strength of [FORMULA] kG or more on cTTSs is still lacking. As outlined above, the detection of a kG-magnetic field on a cTTS is necessary, but not sufficient, evidence for magnetic accretion.

1.2. Direct and indirect evidence for magnetic fields on wTTSs

Although wTTSs start spinning up as they contract towards the main sequence, and no longer accrete a substantial amount of matter, the detection of strong magnetic fields in these stars is, presumably, an argument for the presence of strong fields in cTTSs, because wTTSs are rather similar to cTTSs. The evidence for strong fields is better for wTTSs than for cTTSs. Basri et al. (1992) detected a magnetic field of [FORMULA] kG in the wTTS TAP 35 by using the differential enhancement of the equivalent width (EW hereinafter) of photospheric lines to measure the field strength. Additionally, Donati et al. (1997) have detected a field in V 410 Tau using spectro-polarimetric methods. For HD 283572, the other wTTS studied by these authors, the detection was only marginal.

In addition to these few direct detections, there is ample indirect evidence for the presence of magnetic fields of wTTSs: First of all, some wTTSs are non-thermal radio sources. The non-thermal nature of the radiation is evident from the circular polarization and from the large brightness temperature of [FORMULA] (André et al. 1988). Additionally, VLBI-observations show that the diameter of the magnetospheres are of the order of [FORMULA] (André et al. 1992). Although the strength of the magnetic field is not well constrained by the radio observations, because of the large number of free model parameters, the strength of the fields are estimated as 1-10 G for the emitting regions, which implies fields of 0.1 to 1 kG for the surface of the star (André et al. 1992). In general, the radio-emission of wTTSs is consistent with gyrosynchroton radiation from a dipolar magnetic structure, and with a surface field strength of the order of 1 kG (White et al. 1992). The correlation between the X-ray luminosity and the stellar rotation rate implies the presence of a stellar dynamo (Bouvier 1990; Neuhäuser et al. 1995), and flares have been observed in the radio, X-ray and optical regime (Feigelson and Montmerle 1985; White et al. 1992; Guenther and Emerson 1997). Additionally, Montes et al. (1997) suggest that the broad wings of [FORMULA] are caused by micro-flaring activity. Optical broad-band photometry implies that at least some wTTSs are covered with huge dark spots. The sizes of these spots cover typically 5-40% of the stellar surface (e.g. Bertout 1989, Bouvier et al. 1993). The results from Doppler imaging are in good agreement with the photometry. Reconstructions of the surface structure clearly show cool spots on these stars (Joncour 1994b; Joncour et al. 1994b; Joncour et al. 1994a; Strassmeier et al. 1994; Hatzes 1995; Rice et al. 1996).

As already mentioned, the direct and indirect evidence for strong magnetic fields in wTTSs is often used as an important argument for the presence of fields also in cTTSs. However, Neuhäuser and Preibisch (1994) argue that the larger X-ray fluxes (and faster rotation) of wTTSs compared to cTTSs may imply larger magnetic field strength too.

Despite the indirect evidence for the presence of fields, and the detection of fields in V 410 Tau, and TAP 35, it remains to be clearly shown that the typical fields of wTTSs are of the order of 1 kG or more.

1.3. Magnetic field geometry and surface structures

Although the magnetic field strengths of cTTSs and of wTTSs have not yet been unambiguously determined, some attempts to constrain the structure of magnetic fields have been made. Schüssler et al. (1996) calculated the structure of the magnetic field for rapidly rotating pre-main sequence stars, and found that spots should appear concentrated toward the poles, and not at equatorial regions. This expectation is in general agreement with reconstructions of the surface structure of the wTTS V 410 Tau which shows mainly high latitude star-spots reaching up to the pole, while equatorial spots have occasionally been seen (Joncour 1994b; Joncour et al. 1994b; Strassmeier et al. 1994; Hatzes 1995; Rice et al. 1996). Donati et al. (1997) found a Stokes V signal from this star, which indicates that the observed field is dominated by one polarity. This is also in agreement with the VLBI-observations that show large diameters of the magnetospheres of wTTSs which implies that the field has to be a low order multipole (André et al. 1992). High latitude spots, or even a polar cap, has been detected on the wTTS HDE 283572 too (Joncour et al. 1994a).

If solid body rotation is assumed, doppler imaging reconstructions also show a large, cool polar spot on the cTTS Sz 68. However if anti-solar differential rotation is allowed for this cap disappears and only some small spots remain at [FORMULA] (Johns-Krull & Hatzes 1997a). Mennessier (1997) detected cool spots in the cTTS SU Aur, this time at [FORMULA] latitude. These results also agree with the results of the analysis of high resolution line profiles of RY Tau and SU Aur that can also best be fitted if cool polar spots are assumed (Johns-Krull 1996). In contrast to these results, on DF Tau Unruh et al. (1998) find two hot spots with a featureless continuum spectrum that can be interpreted as the sources of the veiling continuum. If the contribution of these spots are removed from the data, cool spots at mid latitudes appear.

Most of the data on the wTTSs thus seem to imply the presence of cool polar spots. In contrast to the wTTSs, the data on cTTSs is far from conclusive, and any assumption about the temperature of the magnetic regions and geometry of the fields in cTTSs is somewhat speculative.

1.4. Approach in this paper

Here we use the increase of the equivalent widths of magnetically sensitive lines caused by the Zeeman broadening in the stellar atmospheres to derive [FORMULA], (the product of B, the magnetic field strength, and f, the filling factor) on a sample of 5 T Tauri stars. In Sect. 2 we discuss the choice of stars, and the observational data. Sect. 3 briefly outlines the method whereas Sect. 4 explains the analysis method in detail. In Sect. 5 we report on the derived values for the magnetic field strength B, the filling factor f, the product of the two [FORMULA], and the inclination of the field lines. Sect. 6 discusses the significance of the measurements obtained, and the possible influence of the veiling and the geometry of the fields on the measurements of the field strength. We also discuss the influence of the presence of the magnetic field on measurements of the veiling, and the temperature of the stars. Sect. 7 summarises our conclusions.

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Online publication: December 16, 1998