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\IBVShead{5989}{28 April 2011}

\IBVStitletl{Rotational Variability in Pre-Main-Sequence Stars:}{ TWA 6 in 2008}

\IBVSauth{LAWSON, W. A.$^1$; CRAUSE, L. A.$^2$}

\IBVSinst{School of Physical, Environmental \& Mathematical Sciences, University of New South Wales, Australian Defence Force Academy, P.O. Box 7916, Canberra, ACT 2610, Australia, e-mail: w.lawson@adfa.edu.au}
\IBVSinst{South African Astronomical Observatory, P.O. Box 9, Observatory 7935, South Africa, e-mail: lisa@saao.ac.za}

\SIMBADobj{TWA 6}
\IBVStyp{ T Tauri }
\IBVSkey{photometry}
\IBVSabs{Differential BVRI CCD photometry of the TW Hydrae association star}
\IBVSabs{TWA 6 obtained in 2008 shows the star to have a photometric amplitude}
\IBVSabs{a factor of 2.5 lower than observed in 2001. We discuss this}
\IBVSabs{difference in terms of a possible solar-type activity cycle.}


\begintext

\vspace*{-0.4cm}

TWA~6 (= 2MASS J10182870--3150029) is a K7 spectral-type member of the TW Hydrae association (TWA), one of the nearest pre-main sequence (PMS) stellar populations to Earth (Webb et al., 1999; Reid, 2003; Zuckerman \& Song, 2004). Located at a distance of $\approx 50$ pc (Mamajek, 2005), TWA~6 has a mass of $\approx 0.7$ M$_{\odot}$ and a luminosity of $\approx 0.25$~L$_{\odot}$ (Skelly et al., 2008). The association has an estimated age of $\approx 10$ Myr, and this is consistent with the age inferred for TWA~6 ($\sim 12$ Myr) following \hbox{Hertzsprung-Russell~diagram} placement of the star and comparison with PMS evolutionary tracks (Skelly et al., 2009). TWA 6 is an example of a weak-lined T~Tauri (WTT) star, with weak Balmer emission lines (the H$\alpha$ equivalent width is $\approx 5$ \AA), and no near-infrared excess that would indicate the presence of an inner circumstellar disk. 

The most-remarkable feature of TWA~6 is its large photometric amplitude that is a consequence of the rotational modulation of cool starspots that cover a few tens of percent of the photosphere. Lawson \& Crause (2005) found the star to have a $V$-band amplitude of 0.49 mag modulated on a rotation period of 0.54 d, in {\it BVI$_{\rm C}$\,} CCD differential photometry of the star obtained with the 1-m telescope at the South African Astronomical Observatory (SAAO) in 2001. (In this paper, the light curve amplitude is given as the peak-to-peak amplitude.) A trial series of previously-unpublished $V$-band observations of TWA~6 obtained at SAAO in 2000 returned the same period, although the star had a slightly-lower $V_{\rm ampl}$ of 0.40 mag. In Table 1, we present these data for the first time. Compared to other WTT~stars, the photometric amplitude of TWA~6 is large; most WTT~stars have rotational amplitudes of $\sim 0.1$ mag, and few have rotational amplitudes $> 0.2$ mag (Lawson et al., 2001; Lawson \& Crause, 2005). Lawson et al. (2001) summarise the observing procedure at SAAO, and the production of the differential light curves.

The combination of a short rotation period for the star's spectral type, with
a \linebreak\hbox{$v \sin i = 72$ km\,s$^{-1}$} placing TWA~6 firmly in the
`fast rotator' regime ($v \sin i > 50$ km\,s$^{-1}$) for T~Tauri stars, and a large photometric amplitude makes TWA~6 an ideal target for Doppler mapping studies, where spectral line profile variations allow for reconstruction of stellar surface features such as starspots, and other structures such as solar-type `plages' and prominences. Skelly et al. (2008) observed TWA~6 using the UCLES echelle spectrograph at the 3.9-m Anglo-Australian Telescope in 2006, with the resulting Doppler~map showing a large polar spot, and other starspot groups extending to the equator. The outcome was a starspot distribution similar to that seen in other young, fast-rotating stars. However, the inferred luminosity variation resulting from the Doppler study was only 0.1 mag, a factor of $4-5$ lower than that observed at SAAO in 2000 and 2001. While Doppler-reconstructed light curves are suspected of under-estimating the level of photometric variability, it is unlikely that the light amplitude is under-estimated by more than a factor of 2. Skelly et al. (2009) discussed this aspect for the Doppler studies of TWA~6 and TWA~17 and concluded that, in 2006, TWA~6 probably did have lower photometric variability than was observed during the 2000 and 2001 observing seasons.

With this background of variability information in mind, we observed TWA~6 again at SAAO in 2008, obtaining 15 epochs of data over 7 nights, or a time baseline of nearly 12~rotational periods. Our differential {\it BVR$_{\rm C}$I$_{\rm C}$\,} observations are presented in Table 2, where the phase of each observation is calculated assuming JD$_{0}$ = 2454499.1050, and where we adopt the Skelly et al. (2008) spectroscopic period of 0.5409 d. We have not merged the 2008 photometry with our earlier datasets in an attempt to further improve the rotation period, as the interval between the datasets is long, and there is very little information available on the timescale at which spot patterns evolve in WTT stars, i.e. the appearance or disappearance of major starspot groups could have the effect of introducing a significant phase shift in the light curve.

\vskip 0.5cm

\centerline{Table 1. 2000 SAAO $V$-band differential photometry of TWA 6.}
\begin{center}
\begin{tabular}{ccrccccr}
\hline
JD--2450000 & Phase & $\Delta V$ & & & JD--2450000 & Phase & $\Delta V$ \\
\hline
1584.3945 & 0.0000 & 0.156 & & & 1619.2773 & 0.4903 & --0.268 \\
1585.0000 & 0.0438 & 0.160 & & & 1619.3906 & 0.6997 & --0.141 \\	
1586.3945 & 0.6975 & --0.106 & & & 1622.2656 & 0.0150 & 0.124 \\
1587.3867 & 0.5319 & --0.256 & & & 1622.3828 & 0.2316 & 0.009 \\
1588.3047 & 0.2290 & 0.010 & & & 1623.3359 & 0.9937 & 0.126 \\
1590.3086 & 0.9337 & 0.117 & & & 1623.4688 & 0.2393 & 0.126 \\
1590.4922 & 0.2732 & --0.050 & & & 1624.2852 & 0.7486 & --0.108 \\
1591.5664 & 0.2592 & --0.030 & & & 1624.4453 & 0.0447 & 0.144 \\
1596.2852 & 0.9830 & 0.137 & & & 1624.5625 & 0.2614 & --0.017 \\
1596.4414 & 0.2719 & --0.028 \\
\hline
\end{tabular}
\end{center}

\vskip 0.5cm

\centerline{Table 2. 2008 SAAO {\it BVR$_{\rm C}$I$_{\rm C}$\,} differential photometry of TWA 6.}
\begin{center}
\begin{tabular}{ccrrrr}
\hline
JD--2450000 & Phase & $\Delta B$ & $\Delta V$ & $\Delta R_{\rm C}$ & $\Delta I_{\rm C}$ \\
\hline
4522.2950 & 0.1250 &  0.084 &  0.068 &  0.056 & 0.026 \\
4522.3172 & 0.0838 &  0.113 &  0.090 &  0.070 &  0.052 \\
4522.4952 & 0.7546 & --0.017 & --0.022 & --0.018 & --0.011 \\
4522.5040 & 0.7385 & --0.037 & --0.031 & --0.027 & --0.022 \\
4523.3652 & 0.1468 &  0.074 &  0.061 &  0.054 &  0.042 \\
4523.3701 & 0.1376 &  0.082 &  0.066 &  0.045 &  0.031 \\
4524.3934 & 0.2455 & --0.002 &  0.004 & --0.006 & --0.004 \\
4525.3110 & 0.5491 & --0.099 & --0.085 & --0.082 & --0.049 \\
4525.5570 & 0.0943 &  0.108 &   0.093 &  0.089 &  0.079 \\
4526.3160 & 0.6910 & --0.044 & --0.042 & --0.032 & --0.026 \\
4526.4752 & 0.3966 & --0.094 & --0.082 & --0.083 & --0.053 \\
4527.4125 & 0.6637 & --0.048 & --0.046 & --- & --- \\	
4527.6224 & 0.2757 & --0.039 & --0.026 & --0.016 & --0.025 \\
4528.3740 & 0.8866 &  0.042 &  0.039 &  0.032 &  0.026 \\
4528.6040 & 0.4616 & --0.123 & --0.086 & --0.084 & --0.067 \\
\hline
\end{tabular}
\end{center}

\vskip 0.5cm

Phase-folded {\it BVR$_{\rm C}$I$_{\rm C}$\,} light and colour curves of TWA~6 in 2008 are shown in Fig.~1. The overall trend of the light curve amplitudes follows that seen in the {\it BVI$_{\rm C}$\,} observations obtained in 2001 by Lawson \& Crause (2005), where there is a general decrease in amplitude towards longer wavelengths, but in 2008 the light amplitude was distinctly lower than in 2000 and 2001, with $V_{\rm ampl} = 0.19$ mag. We note that if the 2006 Doppler-inferred amplitude was under-estimated by a factor of $\sim 2$, then the 2006 and 2008 light amplitudes are similar.

\IBVSfig{10cm}{5989-f1.eps}{Phase-folded {\it BVR$_{\rm C}$I$_{\rm C}$\,} differential photometry of TWA 6, obtained at SAAO during 2008.}
\IBVSfigKey{5989-f1.eps}{TWA 6}{light curve}

Table 3 summarises light curve amplitudes for TWA~6 measured in 2000, 2001, and 2008, along with the mean $V$-band magnitude. Clearly a more-complete dataset  of amplitudes would be interesting to obtain, to allow investigation of temporal changes in the light curve amplitude, as would the production of Doppler maps at different epochs to investigate the evolution of the starspot distribution. We keep in mind that photometry measures the contrast of spot coverage during a rotation cycle, i.e. the photometric amplitude is determined from the brightness difference between the most-spotted, and least-spotted, hemispheres of the star. If the large polar spot indicated in the Doppler~maps of Skelly et al. (2008) is always present and is roughly constant in extent, then it might contribute little to the luminosity variations, given that the inclination angle of the star is $\approx 50{\deg}$ and the polar cap is always exposed. This suggests that the presence of lower latitude spots may be principally responsible for driving the light curve amplitude. 

\vskip 0.5cm
\newpage
\centerline{Table 3. Summary of SAAO CCD photometric light amplitudes for TWA 6.}
\begin{center}
\begin{tabular}{lrrrrr}
\hline
Year & $V_{\rm mean}$ & $B_{\rm ampl}$ & $V_{\rm ampl}$ & $R_{\rm C, ampl}$ & $I_{\rm C, ampl}$ \\
\hline
2000 & 11.68 & ---     & 0.40 & --- & --- \\
2001 & 11.74 & 0.54 & 0.49 & --- & 0.27 \\
2008 & 11.43 & 0.25 & 0.19 & 0.18 & 0.14 \\
\hline
\end{tabular}
\end{center}

\vskip 0.5cm

Since we obtained multi-color observations in 2001 and 2008, we can estimate the starspot temperature and compare that to the spot temperature of 3300 K derived in the Doppler imaging study of Skelly et al. (2008). Assuming a $T_{\rm eff}$ for TWA 6 of $4000 \pm 200$~K (Skelly et al., 2008), we obtain a spot temperature of $\approx 3400$ K with a spot filling factor of $\approx 50$ \% in 2001, and a spot temperature of $\approx 3700$ K and a filling factor of $\approx 40$ \% in 2008. The mean $V$-band magnitude of the star changes from $2000-2001$ to 2008 both as a consequence of the reduced photometric amplitude, but also from an increase in the maximum $V$-band magnitude from 11.5 in $2000-2001$ to 11.3 in 2008. TWA 6 may have a younger, higher mass analogue in the few Myr-old, early K-type star V410 Tau which also displays significant variations in its long-term light curve behaviour. After a recent Zeeman-Doppler imaging study to reconstruct the surface spot pattern, the photometric variations of V410 Tau have been interpreted as being due to long-term changes in the spot distribution (Skelly et al., 2010, and references therein).

In summary, TWA~6 is a T Tauri star worthy of on-going monitoring for photometric variability. Its brightness ($V_{\rm max} \approx 11.5$), combined with its short period (0.54 d) and stable (on timescales of at least a few weeks), large amplitude ($0.2-0.5$ mag) visual light curve makes TWA~6 an accessible target for observation even for small telescopes equipped with a CCD camera or photoelectric photometer.

The observations reported in this paper were obtained using facilities at the South African Astronomical Observatory. We thank the SAAO for the allocation of telescope time for this and other observing proposals. We also thank the reviewer of this paper for their constructive remarks.


\references

Lawson, W.A., Crause, L.A., Mamajek, E.E., Feigelson, E.D., 2001, {\it MNRAS}, {\bf 321}, 57

Lawson, W.A., Crause, L.A., 2005, {\it MNRAS}, {\bf 357}, 1399

Mamajek, E.E., 2005, {\it ApJ}, {\bf 634}, 1385

Reid, N., 2003, {\it MNRAS}, {\bf 342}, 837

Skelly, M.B., et al., 2008, {\it MNRAS}, {\bf 385}, 708

Skelly, M.B., et al., 2009, {\it MNRAS}, {\bf 399}, 1829

Skelly, M.B., et al., 2010, {\it MNRAS}, {\bf 403}, 159

Webb, R.A., et al., 1999, {\it ApJ}, {\bf 512}, L63

Zuckerman, B., Song, I., 2004, {\it ARA\&A}, {\bf 42}, 685

\endreferences

\IBVSedata{5989-t1.txt}
\IBVSdataKey{5989-t1.txt}{TWA 6}{photometry}
\IBVSedata{5989-t2.txt}
\IBVSdataKey{5989-t2.txt}{TWA 6}{photometry}

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