Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Rotation and activity of pre-main-sequence stars   We present a study of rotation (vsini) and chromospheric activity (Halpha EW)
based on an extensive set of high-resolution optical spectra obtained with MIKE
on the 6.5m Magellan Clay telescope. Our targets are 74 F-M dwarfs in the young
stellar associations Eta Cha, TW Hydrae, Beta Pic, and Tuc-Hor, spanning ages
from 6 to 30 Myr. While the Halpha EW for most F and G stars are consistent
with pure photospheric absorption, most K and M stars show chromospheric
emission. By comparing Halpha EW in our sample to results in the literature, we
see a clear evolutionary sequence: Chromospheric activity declines steadily
from the T Tauri phase to the main sequence. Using activity as an age
indicator, we find a plausible age range for the Tuc-Hor association of 10-40
Myr. Between 5 and 30 Myr, we do not see evidence for rotational braking in the
total sample, thus angular momentum is conserved, in contrast to younger stars.
This difference indicates a change in the rotational regulation at 5-10 Myr,
possibly because disk braking cannot operate longer than typical disk
lifetimes, allowing the objects to spin up. The rotation-activity relation is
flat in our sample; in contrast to main-sequence stars, there is no linear
correlation for slow rotators. We argue that this is because young stars
generate their magnetic fields in a fundamentally different way from
main-sequence stars, and not just the result of a saturated solar-type dynamo.
By comparing our rotational velocities with published rotation periods for a
subset of stars, we determine ages of 13 (7-20) Myr and 9 (7-17} Myr for the
Eta Cha and TWA associations, respectively, consistent with previous estimates.
Thus we conclude that stellar radii from evolutionary models by Baraffe et al.
(1998) are in agreement with the observed radii within +-15%. (abridged)

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The categories of this paper are: physics
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The categories of this paper are: hep-th
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The categories of this paper are: econ
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The categories of this paper are: cond-mat
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The categories of this paper are: gr-qc
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The categories of this paper are: alg-geom
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The categories of this paper are: supr-con
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The categories of this paper are: chem-ph
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The categories of this paper are: math-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: cs
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The categories of this paper are: funct-an
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The categories of this paper are: q-bio
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The categories of this paper are: nlin
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The categories of this paper are: dg-ga
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The categories of this paper are: patt-sol