ROTATION IN T-TAURI STARS .2. CLUES FOR MAGNETIC ACTIVITY

We use rotation rates from Bouvier et al. (1986), hereafter referred to as Paper I, to investigate correlations between rotation of T Tauri stars (TTS) and their radiative energy losses in the x-ray range and in the Ha, Ca II - K and Mg II - k lines. The stellar sample includes both "classical" TTS [CTTS, with EW(Hα)≥10 Å] and "weak-line" TTS [WTTS, with EW(Hα) < 10 Å]. We find evidence for an inverse correlation between x-ray surface flux and rotational period in a sample of 21 spotted TTS; the same correlation holds for both CTTS and WTTS. Using a much larger sample of TTS, we examine how stellar parameters other than rotation (mass, radius, luminosity, Teff, Av, emission-line flux) influence x-ray emission and conclude that TTS x-ray flux primarily depends on rotation. Over the range of rotational periods investigated here (1-10 days), the correlation between x-ray fluxes and rotational periods of TTS is the same as that found for cool dwarfs and active binaries. We propose that the x-ray/rotation correlation is caused by a solar-type magnetic dynamo in TTS as it is in active dwarfs and RS CVn systems, in agreement with previous claims that rotation is the primary parameter governing the level of solar-type magnetic dynamo activity in cool stars. We find no relationship between rotational period and radiative losses in the Ha, Ca II - K, and Mg II - k lines. We show that Ca II - K and Mg II - k losses scale with Ha line strength over three decades of flux from weak-line to classical and extreme TTS, so that these three diagnostics of line emission are equivalent when searching for a correlation between line flux and rotation. A comparison of TTS with other rapidly rotating cool stars shows that WTTS and active dwarfs display similar radiative losses in the above lines, which is consistent with the hypothesis that chromospheric activity in WTTS is ultimately driven by a solar-type magnetic dynamo. In contrast, CTTS exhibit radiative losses in lines that are up to 100 times larger than those measured in active dwarfs and evolved binaries. It is argued that such large line losses cannot be accounted for by a solar-type magnetic dynamo alone. There is growing evidence that CTTS possess circumstellar disks and recent disk models suggest that an additional energy input resulting from the interaction between star and disk may be responsible for the large radiative losses of CTTS. In contrast, WTTS lack evidence for circumstellar accretion disks, a fact consistent with our proposal that a solar-type magnetic dynamo is responsible for their chromospheric and coronal activity.