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Rotation periods for stars of the TW Hydrae association
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Mon. Not. R. Astron. Soc. 000, 1–9 (2005) Printed 2 February 2008 (MN LATEX style file v2.2)
Rotation periods for stars of the TW Hydrae association:
the evidence for two spatially- and rotationally-distinct
pre-main sequence populations
Warrick A. Lawson
1?
and Lisa A. Crause
2,3?
1School of Physical, Environmental and Mathematical Sciences, The University of New South Wales,
Australian Defence Force Academy, Canberra ACT 2600, Australia
2Department of Astronomy, University of Cape Town, Private Bag, Rondebosch 7700, South Africa
3South African Astronomical Observatory, P. O. Box 9, Observatory 7935, South Africa
Accepted January 2005
ABSTRACT
We have conducted a photometric study of late-type members of the TW Hydrae
association (TWA) and measured the rotation periods for 16 stars in 12 systems. For
TWA stars listed by Webb et al. and Sterzik et al. (TWA 1? 13; led by TW Hya =
TWA 1) we find a median period of 4.7 d. However, for stars that we measured in
the TWA 14 ? 19 group identified by Zuckerman et al., we find a median period of
only 0.7 d. The period distributions of the two groups cannot be reconciled at the 3-σ
significance level. Using photometric arguments supported by the Hipparcos distance
to HD 102458 (= TWA 19A), we find that TWA 14?19 reside at an average distance
of d ≈ 90 pc, spatially at the near boundary of the Lower Centaurus-Crux (LCC)
subgroup of the Ophiuchus-Scorpius-Centaurus OB-star association (OSCA). Proper
motions for HD 102458, TWA 14, 18 and 19B link these stars to the LCC subgroup.
From Hertzsprung-Russell (H-R) diagram placement, we derive an age of ≈ 17 Myr for
the HD 102458 system that may be the representative age for the TWA 14?19 group.
Merging various lines of evidence, we conclude that these stars form a spatially- and
rotationally-distinct population of older pre-main sequence (PMS) stars, rather than
being an extension of the TWA beyond those stars ass
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