Australia Telescope National Facility

A Radio Survey of Older T Tauri Stars
in the $\eta$ Chamaeleontis Cluster
Eric E. Mamajek , Warrick A. Lawson , Eric D. Feigelson, PASA, 16 (3), 257.

Next Section: Acknowledgements
Title/Abstract Page: A Radio Survey of
Previous Section: Observations and Results
Contents Page: Volume 16, Number 3

Discussion

Fig. 1 compares the X-ray and radio levels of the $\approx$ 8-Myr-old $\eta$ Cha T Tauri stars and the 5-Myr-old WTTs in the Upper Sco subgroup of the Sco-Cen OB association (Brown et al. 1996) to the Benz & Güdel (1994) L_X/L_R relation for stellar activity. Why were none of the $\eta$ Cha WTTs detected with log(L_R) > 15.6 erg Hz^-1 s^-1 even when a few quiescent-level radio detections were expected? Flares in stars with lower L_X values should also have been seen; past experience has shown that a fraction of younger T Tauri stars with lower L_X values (10²⁹-10³⁰ erg s^-1) flare during their radio observations (see Fig. 5 of C97) at levels an order of magnitude higher than that predicted by the Benz & Güdel (1994) relation. By examination of Fig. 1, and consideration of the radio detection rates of the surveys mentioned previously, it is surprising that none of our WTTs were detected.

**Figure 1:** Benz-Güdel diagram for Upper Scorpius and $\eta$ Cha weak-lined T Tauri stars. The straight line is the expected locus for simultaneous measurements made of active stars. Upper Sco data is from Brown et al. (1996). See text for details.
$\begin{figure} \begin{center} \epsfysize=9cm \epsffile{MLF1.eps}\end{center}\end{figure}$

To understand why radio emission was undetected in the $\eta$ Cha WTTs, we look to the stars' rotation as a possible answer. First, we estimated v sin i from the FWHM of isolated absorption lines in medium-resolution spectra of these stars (as presented in MLF99), comparing them to values of v sin i measured from high-resolution spectra of RECX 1, 7, 10 and 12 (Covino et al. 1997). Of those four, RECX 7 had the highest v sin i (28-32 km ${\rm s}^{-1}$ ) and the highest FWHM measure among the WTTs in MLF99. Secondly, multi-epoch differential-V CCD photometry of the RECX stars was obtained by W. Lawson and L. Crause during 1999 Feb-Mar using the SAAO 1.0-m telescope. Preliminary results for these stars indicate rotation periods between 1.3-8 days, typical for young stars (Crause et al. 2000). Making use of the approximate luminosities of MLF99, we calculate equatorial velocities

$v_{\rm eq} < 50$ km ${\rm s}^{-1}$ for all these stars. Only two (RECX 7 and 12) have $v_{\rm eq}$ that would place them near the lower limit of v sin i for ultra-fast rotators ( $30 \leq$ v sin i $\leq 150$ km ${\rm s}^{-1}$ , Soderblom et al. 1993).

In conclusion, we find that none of our $\approx$ 8-Myr-old WTTs are radio-emitting at the log(L_R) > 15.6 erg Hz^-1 s^-1 level. The radio emission is below that seen in younger ( $\leq$ 5 Myr-old) samples of T Tauri stars; however most of the stars detected in those studies were rapidly rotating. Data from two studies (MLF99; Crause et al. 2000) indicate that the $\eta$ Cha WTTs are not, as a group, fast-rotators. The results of this radio survey support the view that rotation, not youth, is the dominant factor in accounting for nonthermal radio emission in young stars (Simon 1992).

Next Section: Acknowledgements
Title/Abstract Page: A Radio Survey of
Previous Section: Observations and Results
Contents Page: Volume 16, Number 3

ASKAP

Public