in the
![$\eta $](/pasa/16_3/mamajek/paper/mamajek/../img1.gif)
Eric E. Mamajek , Warrick A. Lawson , Eric D. Feigelson, PASA, 16 (3), 257.
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Discussion
Fig. 1 compares the X-ray and radio levels of the 8-Myr-old
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 & GÓÌdel (1994) LX/LR relation for stellar activity. Why were none of the
Cha WTTs detected with log(LR) > 15.6 ergÒšHz-1Òšs-1 even when a few quiescent-level radio detections were expected? Flares in stars with lower LX values should also have been seen; past experience has shown that a fraction of younger T Tauri stars with lower LX 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 & GÓÌdel (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.
![]() |
To understand why radio emission was undetected in the Cha WTTs, we look to the stars' rotation as a possible answer. First, we estimated vÒšsinÒši from the FWHM of isolated absorption lines in medium-resolution spectra of these stars (as presented in MLF99), comparing them to values of vÒšsinÒši measured from high-resolution spectra of RECX 1, 7, 10 and 12 (Covino et al. 1997). Of those four, RECX 7 had the highest vÒšsinÒši (28-32 km
) 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
km
for all these stars. Only two (RECX 7 and 12) have
that would place them near the lower limit of vÒšsinÒši for ultra-fast rotators (
vÒšsinÒši
km
, Soderblom et al. 1993).
In conclusion, we find that none of our 8-Myr-old WTTs are radio-emitting at the log(LR)Òš>Òš15.6 ergÒšHz-1Òšs-1Òšlevel. The radio emission is below that seen in younger (
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
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).
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