SPECTROPOLARIMETRY OF THE CLASSICAL T TAURI STAR T TAURI

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1 The Astronomical Journal, 131: , 2006 January # The American Astronomical Society. All rights reserved. Printed in U.S.A. SPECTROPOLARIMETRY OF THE CLASSICAL T TAURI STAR T TAURI Antoun G. Daou and Christopher M. Johns-Krull 1 Department of Physics and Astronomy, Rice University, Mail Stop 108, 6100 Main Street, Houston, TX 77005; agdaou@rice.edu, cmj@rice.edu and Jeff A. Valenti 1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21210; valenti@stsci.edu Received 2005 July 11; accepted 2005 September 19 ABSTRACT High-resolution (R 60;000) circular spectropolarimetry of the classical T Tauri star T Tau is presented. The star was observed on 1997 November 21 and 22. Analyzing 12 photospheric absorption lines, the mean longitudinal magnetic field is found to be ¼ G. The 3 upper limit of j j105 G. Previously, T Tau was reported to have a mean longitudinal field of G. A strong mean magnetic field (2.4 G) has been reported on the surface of T Tau based on Zeeman broadening measurements in unpolarized light. The present observations indicate that it is very unliely that this field is dipolar in nature. In order to verify the observing techniques and analysis methods used on T Tau, spectra of the Sun obtained by observing the asteroid Vesta are analyzed in the same fashion. Here the mean longitudinal field is ¼ 4 3 G, which is well within the limits of previous observations. As a further chec on our results, we also present data for the magnetic Ap star 53 Cam, which gives a mean longitudinal magnetic field that agrees well with the published field variations for this star. Key words: planets and satellites: individual (Vesta) stars: individual (T Tauri, 53 Camelopardis) stars: magnetic fields techniques: polarimetric 1 Visiting Astronomer, McDonald Observatory, which is operated by the University of Texas at Austin INTRODUCTION T Tauri stars ( TTSs) are young (<10 Myr), roughly solar-mass stars that have only recently emerged from their natal molecular cloud cores to become optically visible. It is generally believed that classical T Tauri stars (CTTSs) have strong magnetic fields that regulate the accretion of dis material onto their surfaces (e.g., Camenzind 1990; Königl 1991; Cameron & Campbell 1993; Shu et al. 1994). These magnetospheric accretion models assume a dipolar geometry, but observations to date suggest only wea dipolar components on the surfaces of TTSs (see Johns-Krull et al. 1999; Valenti & Johns-Krull 2004). Generally, a strong dipolar component of the magnetic field should produce detectable circular polarization in magnetically sensitive photospheric absorption lines. Recently, Smirnov et al. (2003) reported a surface longitudinal magnetic field of G on the CTTS T Tau based on circular polarization measurements. If confirmed, this field detection would be the largest mean longitudinal field ever detected on a late-type star. Magnetic activity generally declines with age, so perhaps it would not be surprising to find such a large longitudinal field on a young star. However, the first tas is to verify the field measurement. Measuring magnetic fields on TTSs is difficult. The stars are relatively faint and show many spectral peculiarities, which complicates the analysis. As a result, little is nown about the magnetic fields on these young stars, despite their theoretical significance. For late-type stars, Zeeman broadening of spectral lines in unpolarized light has been used to measure magnetic field strength and filling factor (see, e.g., Robinson 1980; Saar 1988; Valenti et al. 1995; Johns-Krull & Valenti 1996). This is a difficult measurement for young stars because rotational broadening typically mass any Zeeman broadening. However, observations in the infrared spectral regions are more sensitive to magnetic broadening due to the wavelength dependence of the Zeeman effect (e.g., Johns-Krull et al. 1999). In addition to broadening line profiles, magnetic fields also enhance line equivalent widths by an amount that depends on the saturation of Zeeman components. This effect has been used to measure relatively large magnetic fields on a few T Tauri stars (Basri et al. 1992; Guenther et al. 1999). In late-type stars, this method becomes increasingly difficult due to line crowding. Great care is needed to distinguish Zeeman effects on line equivalent width from unnown blends or errors in nonmagnetic stellar parameters (e.g., T eff,logg, or stellar abundances). Perhaps the most distinctive method for detecting stellar magnetic fields is to loo for net circular polarization in Zeemansensitive lines using high-resolution spectropolarimetry. Generally, Zeeman components are elliptically polarized, with the components of opposite helicity split to either side of the nominal line wavelength. If there is a net longitudinal component of the magnetic field on the stellar surface, net polarization results in a shift between Zeeman-sensitive lines observed in rightcircularly and left-circularly polarized ( RCP and LCP) light. The shift is (e.g., Mathys 1988, 1991) e ¼ 2 4m e c 2 2 g ea ¼ 9:34 ; g ea m8; ð1þ where g eff is the effective Landé g-factor of the transition, is the strength of the mean longitudinal magnetic field (given in ilogauss), and is the wavelength of the transition (in angstroms). T Tau is a CTTS with spectral type K0, v sin i ¼ 20 m s 1 (Basri & Batalha 1990), and a rotation period of 2.8 days (Herbst et al. 1986). T Tau is a triple star system (Koreso 2000), although the optical light is dominated by a single component (T Tau N, also referred to as T Tau A). The accretion dis around this component is thought to be nearly pole-on (Herbst et al.

2 SPECTROPOLARIMETRY OF T TAURI 521 TABLE 1 Observations and Results Star Date (UT) Start Time (UT) N exp Total Exposure Time (s) Observed Predicted a T Tau Nov 21 5: T Tau Nov 22 3: Sun b Nov 24 3: Cam Nov 22 10: Cam Nov 23 12: Cam Nov 24 11: a Taen from Hill et al. (1998) for 53 Cam. b Obtained by observing Vesta. 1986). A low inclination for the stellar rotation axis is favored by the H line profile variations (Johns & Basri 1995), and recently, Aeson et al. (2002) confirmed a low inclination for the dis on the basis of infrared interferometry. Johns-Krull et al. (2000) estimate the accretion rate onto the star to be 3 ; 10 7 M yr 1, while Calvet et al. (2004) find ð3:1 5:7Þ ; 10 8 M yr 1. This discrepancy is liely due to differing assumptions (see Gullbring et al. [1998] and Johns-Krull et al. [2000] for a discussion) rather than variable accretion. The accretion rate for T Tau is typical of all CTTSs within the context of each study. Guenther et al. (1999) find that for T Tau the product of the magnetic field strength B and filling factor f is Bf ¼ 2:35 0:15 G, but they could not constrain the field geometry. Smirnov et al. (2003) used high-resolution spectropolarimetry of T Tau to infer a mean longitudinal magnetic field of ¼ and G at two different epochs on T Tau. The authors note that their measurement is quite close to the detection limit; however, based on the consistency of these two measurements, they argue that 150 G for T Tau. If confirmed, this detection would be the strongest mean longitudinal field ever detected on a cool star. As part of a larger program to study magnetic fields on TTS, we obtained high-resolution spectropolarimetry of T Tau in 1997 November. We analyze these data here in an effort to confirm the field detection of Smirnov et al. (2003). The observations and data reduction are discussed in x 2; the data analysis is presented in x 3, where we attempt to measure the longitudinal fields on T Tau, the Sun, and 53 Cam; x 4 presents a discussion of these measurements; and x 5 gives the conclusions of this study. 2. OBSERVATIONS AND DATA REDUCTION The spectra presented here were obtained using a Zeeman analyzer (ZA) system on the 2.7 m Harlan J. Smith Telescope at McDonald Observatory. This system has been described by Vogt et al. (1980), with subsequent modifications described by Johns- Krull et al. (1999a). The ZA splits incoming stellar light into two parallel converging beams that create two separate images on the spectrograph slit. One beam contains approximately half the unpolarized light and any RCP light, while the other beam contains the remainder of the unpolarized light and any LCP light. The ZA was used with the cross-dispersed coudé echelle spectrometer (Tull et al. 1995). This spectrometer provides a 2 pixel spectral resolution of R ¼ / 60;000 and enough space between the orders to interleave simultaneously stellar spectra of both circular polarization states. We observed each star at least twice in different configurations to help assess and remove systematic errors. A Babinet-Soleil phase compensator ( PC) in front of the ZA reduced the spurious linear polarization induced by oblique bounces off flat mirrors in the coudé mirror train. The amount of phase compensation is a function of position on the sy, and the PC is under computer control so that it can be adjusted automatically. Between exposures, the PC was advanced half a wave in order to reverse the sense of circular polarization recorded in the two interleaved spectra. All spectra were reduced using an echelle-reduction pacage developed by Valenti (1994), and wavelength solutions were determined from spectra of a thorium-argon lamp. Table 1 gives a journal of the observations discussed here. 3. ANALYSIS 3.1. Global Photospheric on T Tau and the Sun For T Tau, we measured the photospheric using 12 magnetically sensitive absorption lines ( Fig. 1), which form over the entire stellar surface. Lines for both T Tau and the Sun were chosen that have relatively large Landé g-values, are relatively strong and unblended, and are not significantly affected by telluric absorption. We interpolated LCP and RCP spectra onto a common wavelength scale and then cross-correlated each line profile, obtaining for each line a wavelength shift proportional to (eq. [1]). We then used a Monte Carlo analysis to estimate uncertainties in our measured wavelength shifts. We constructed simulated data for each polarization state by adding noise appropriate for our observations to a noiseless Gaussian fit of the observed line profile (sum of LCP and RCP). We analyzed these synthetic data in the same manner as our actual observations, obtaining simulated line shifts. The whole process was repeated 100 times, and the standard deviation of the results was taen as the uncertainty in our line-shift measurement. Table 2 gives the results of our cross-correlation analysis for 12 absorption lines between 6166 and in the spectra of T Tau. On 1997 November 21, the weighted mean for all lines was ¼ G. On 1997 November 22 we obtained only three spectra of T Tau, the middle one having the PC advanced half a wave. This permits two pairwise estimates of that are not independent. The first and second spectra yielded ¼ G, while the second and third spectra yielded ¼ 9 41 G. Averaging these two results and their uncertainties yielded ¼ G for 1997 November 22. The mean for each night is given in Table 1. Our tightest 3 upper limit for a given night is j j126 G, which is below the G detection reported by Smirnov et al. (2003), although the two measurements are consistent given the relatively large uncertainty in both of them. If we average our field measurements for both nights, we find ¼ G. In order to test our methods we used the Sun as a nonmagnetic reference. We observed the asteroid Vesta to obtain a spectrum of

3 522 DAOU, JOHNS-KRULL, & VALENTI Fig. 1. Observed LCP (red) andrcp(blac) spectra for the 12 lines used in the analysis of T Tau. Vertical lines define limits used in the cross-correlation analysis. the Sun as a star. In the optical, we analyzed 22 magnetically sensitive absorption lines between 5866 and ( Fig. 2). Table 3 gives results for each of these lines in the spectrum of Vesta as a solar proxy. We obtained a weighted mean of G for the Sun, as reported in Table 1. This nondetection lies well within the limits of the longitudinal field observed on the Sun (j j < 4 G; Kotov et al. 1998) Accretion Region for T Tau Johns-Krull et al. (1999a) discovered that the He i 5876 emission line can be circularly polarized in spectra of CTTSs, implying coherent magnetic fields at the stellar footpoints of accretion columns. They measured ¼ 2:46 0:12 G for BP Tau. Valenti & Johns-Krull (2004) found night-to-night variations of the He i polarization in four CTTSs (AA Tau, BP Tau, DF Tau, and DK Tau), suggestive of rotational modulation. Symington et al. (2005) surveyed seven CTTSs for He i 5876 polarization, detecting polarization at greater than the 3 level in three stars (BP Tau, DF Tau, and DN Tau). Smirnov et al. (2004) reported detections of circular polarization in the He i 5876 emission line of T Tau on all three nights they observed the star, although with significant variability from one night to the next (field measurements range from +320 to G). We examined our observations of T Tau for evidence of circular polarization in the He i 5876 emission line. The line is reasonably Gaussian, despite being rather broad (see Fig. 4), so TABLE 2 Longitudinal Magnetic Field Calculation Using Net Circular Polarization in Zeeman-Sensitive Lines for T Tau 1997 Nov Nov 22 a 1997 Nov 22 b (8) Species g eff S/N (m8) Ca i Fe i Fe i V i Fe i Fe i Fe i Fe i Ti i Al i Fe i Fe i Mean a Using the first and second spectra taen. b Using the second and third spectra taen.

4 Fig. 2. Observed LCP (red) and RCP(blac) spectra for the 22 lines used in the analysis of Vesta. Vertical lines define limits used in the cross-correlation analysis.

5 524 DAOU, JOHNS-KRULL, & VALENTI Vol. 131 TABLE 3 Individual Line Data for Field Measurement of the Sun (8) Species g eff (m8) S/N Ti i Ca i Fe i Ca i Fe i Fe i Fe i V i V i Fe i Cr i Fe i Fe i Fe i Ti i Sc i Fe i Fe i Fe i Fe i Fe i Fe i Mean the analysis technique we used to analyze photospheric absorption lines applies equally well to the He i emission line. Analyzing four spectra obtained on 1997 November 21 yielded ¼ G in the region in which the He i line forms. Again, the three spectra obtained on 1997 November 22 allow two pairwise estimates of that are not independent. The first and second spectra yielded G, while the second and third spectra yielded G. We combine these two results to obtain a final estimate of G on 1997 November Global Photospheric for 53 Cam As a test of our analysis procedure, we measured the longitudinal magnetic field on the magnetic Ap star 53 Cam. In the optical, we analyzed two strong, unblended, magnetically sensitive lines: Si ii (g ea ¼ 1:16) and Fe ii (g ea ¼ 0:80). Fig. 3. Results ( filled squares; error bars are smaller than the size of the symbols) plotted on the longitudinal magnetic field variation of 53 Cam from Hill et al. (1998). Fig. 4. He i 5876 emission line in T Tau. The solid line shows the profile on 1997 November 21, and the dashed line shows the profile on 1997 November 22. Nightly observations on UT dates 1997 November 22, 23, and 24 showed systematic variations in the strong longitudinal magnetic field (Table 1). As shown in Figure 3, the mean each night agrees well with measurements in Hill et al. (1998). Table 1 lists the predicted field on 53 Cam, using the Hill et al. (1998) ephemeris: ¼ 53 þ 4572 sin (2) G,where is the phase defined by ¼ (HJD obs 2;448;498:186)/8:02681 and HJD obs is the heliocentric Julian date of the observation. 4. DISCUSSION 4.1. Magnetic Geometry in the Photosphere of T Tau Guenther et al. (1999) used differences between observed and synthetic equivalent widths of magnetically sensitive and insensitive lines in T Tau to infer a magnetic field strength of B ¼ 2:35 0:15 G, averaged over the entire surface of T Tau. Johns- Krull et al. (2001) detected a similar mean field strength of 2.5 G based on the Zeeman broadening of K-band Ti i lines. In the idealized form of most magnetospheric accretion theories, the stellar field is assumed to be a pure dipole aligned with the stellar rotation axis (e.g., Königl 1991). If we assume that the magnetic field on T Tau is a dipole aligned with the stellar rotation axis and that the mean magnetic field strength is B ¼ 2:4 G, averaged over the entire stellar surface, then we can compute the expected mean longitudinal magnetic field as a function of inclination. Herbst et al. (1986) estimated an inclination of 8 13 for T Tau, which implies an expected net longitudinal magnetic field of at least 950 G for the largest allowed inclination and even larger for smaller inclinations. Our 3 upper limits on the global photospheric ofttauare165and 126 G on two consecutive nights, disproving the model and calling into question at least one assumption (aligned dipole at the stellar surface). One possibility is that the magnetic dipole axis is tilted with respect to the rotation axis by an angle, nown as the obliquity. If the sum of the inclination and obliquity is near 90, then oppositely directed magnetic field lines on the visible hemisphere will produce substantial flux cancellation and produce a small value of (when i þ ¼ 90 there is complete cancellation and ¼ 0). For the low inclinations of T Tau reported by Herbst et al. (1986), the obliquity of the dipole axis would have to be large to yield rotational phases with significant magnetic flux cancellation. However, cannot be zero at all phases no matter the value of because the inclination of T Tau is nonzero. We

6 No. 1, 2006 SPECTROPOLARIMETRY OF T TAURI 525 observed T Tau on two consecutive nights, setting strong upper limits on at rotational phases () separated by ¼ 0:36, given the 2.8 day period. Assuming that a tilted dipolar field is responsible for the mean field of 2.4 G observed on T Tau, then must be in the range 77N7 90 to be consistent with our 3 upper limits on. Moreover, the timing of our observations must have been fortuitous if our upper limits on two consecutive nights are due to significant cancellation of a strong dipolar field. At minimum inclination (8 ) and maximum obliquity (90 ), a maximum of 41% of rotational phases yield cancellation consistent with our upper limits. This range of allowed phases decreases for increasing inclination or decreasing obliquity, reaching 0% for obliquities below 77N7. Analyzing variations in He i emission-line polarization of four CTTSs as rotational modulation of a misaligned dipole, Valenti & Johns-Krull (2004) found low values of for two stars and high values for the other two stars. These limited empirical results are not adequate to constrain the statistical distribution of for TTSs, so we are forced to mae assumptions regarding the distribution of to estimate the probability that a dipole field geometry is consistent with our upper limits on. Two plausible assumptions are that (1) obliquity is uniformly distributed from 0 to 90 and (2) the dipole axis is oriented randomly in space (meaning that alarge-value is more liely than a small value because there is more stellar surface area that the dipole axis can penetrate at large ). If i ¼ 8, then there is a 2.7% chance that obliquity and rotational phase would conspire to mae the dipole undetectable in our data for uniform and a 4.2% chance given a randomly oriented dipole axis. These probabilities drop to 1.5% and 2.3%, respectively, if i ¼ 13. Averaging the probabilities over the range of allowed stellar inclination from Herbst et al. (1986), the probability that our data are consistent with a pure dipole is 2.0% and 3.1% for the two obliquity weightings, respectively. Smirnov et al. (2004) also observed T Tau on two consecutive nights, achieving 3 upper limits nearly identical to ours. Given the analysis above, the probability that obliquity and rotational phase conspired to mae a strong dipolar field undetectable is also 2% 3%. The rotation period of T Tau is not nown well enough to phase our data with those of Smirnov et al. Assuming random relative phasing, we can combine our data with those of Smirnov et al. (2004) to estimate that the probability that T Tau has a strong but undetected dipolar field is 9 ; In earlier wor, Smirnov et al. (2003) report a field on T Tau with a strength of G; however, this field measurement is only a4 detection and was not confirmed by the later wor of Smirnov et al. (2004) or in this study. Thus, we reject with high confidence the hypothesis that the magnetic field on T Tau is a tilted dipole at the stellar surface. Although we have demonstrated that the surface field on T Tau is probably not dipolar, it is premature to generalize this result to all TTSs. Tayler (1987) suggested that at least some TTSs may contain a substantial fossil magnetic field that was incorporated into the star from the general interstellar magnetic field during the star formation process. In principle, the geometry of the fossil fields on TTSs could be more ordered than the dynamo-generated fields on the Sun and other cool stars, which lac the global organization needed to produce a strong polarization signal (e.g., Vogt 1980; Brown & Landstreet 1981; Borra et al. 1984). Very few TTSs have been carefully surveyed for organized fields (see Johns-Krull et al. 1999; Valenti & Johns- Krull 2004). Measuring and comparing longitudinal magnetic field strengths of TTSs as a group and their counterparts on the main sequence could provide important constraints on the origins of their respective magnetic fields Magnetic Geometry in Accretion Columns above TTau As described in x 3.2, the CTTSs AATau, BP Tau, DF Tau, DK Tau, and DN Tau all display significant circular polarization in their He i 5876 emission lines (Johns-Krull et al. 1999; Valenti & Johns-Krull 2004; Symington et al. 2005). The He i lines of all these stars are characterized by a strong narrow component and a wea broad component in the line profile (Edwards et al. 1994; Alencar & Basri 2000). The narrow component is commonly associated with the accretion shoc itself at the stellar surface, whereas the broad component may have contributions from the magnetospheric accretion flow and/or a hot wind component (e.g., Beristain et al. 2001). Since the broad component of the He i emission line forms over a large, extended volume, magnetic field strength will be weaer than at the stellar surface, and field line curvature may enhance polarization cancellation. As a result, circular polarization in the broad component of the He i 5876 emission is predicted to be less than in the narrow component. The He i line of T Tau is dominated by the broad component, with a very wea or absent narrow component (see Alencar & Basri 2000; Fig. 4). Therefore, it is expected that polarization in the He i emission line of T Tau should be quite small. Given the discussion above, the detection by Smirnov et al. (2004) of significant circular polarization in the He i 5876 emission line in spectra of T Tau is quite a surprise. These authors report mean longitudinal magnetic fields ranging from +320 to G over three nights of observing, but they provide no estimate of their measurement uncertainties. In this paper we examined the He i line in spectra of T Tau, finding no significant circular polarization. Our 3 upper limits on net longitudinal magnetic field where the He i line forms are j j G on two consecutive nights, so we are unable to confirm the result of Smirnov et al. (2004). Additional observations are needed of T Tau and other CTTSs with He i emission dominated by a broad component in order to probe magnetic field properties in the extended region around CTTSs, where this line forms. 5. CONCLUSION Using high-resolution circular spectropolarimetry of a dozen magnetically sensitive absorption lines, we find that the mean longitudinal magnetic field in the photosphere of T Tau is ¼ G. A similar analysis of the solar spectrum reflected by the asteroid Vesta yields ¼ 43 G, which is well within the limits of previous solar measurements. We also analyzed observations of the magnetic Ap star 53 Cam, obtaining very significant measurements that agree well with the ephemeris of Hill et al. (1998). These tests increase confidence in our upper limit on the net longitudinal magnetic field on T Tau. Smirnov et al. (2003) report a field of ¼ G on T Tau. Our results are consistent with this measurement within the measurement uncertainties; however, the field on T Tau is liely to be much less than 160 G. We show that it is unliely that a dipole field on the surface of T Tau can be responsible for the strong mean field observed in unpolarized light by Guenther et al. (1999) and Johns-Krull et al. (2001). Our observations combined with the recent observations of Smirnov et al. (2004) suggest that the mean longitudinal magnetic field on T Tau is in fact quite low, as found in previous surveys of other types of late-type stars. We would lie to acnowledge partial support from the NASA Origins of Solar Systems program through grant NAG made to Rice University. We also than the referee, Pierre Bastien, for his careful reading of the original manuscript and suggestions on how to improve the paper.

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