Paper: astro-ph/0508004
Date: Fri, 29 Jul 2005 20:58:17 GMT (31kb)

Title: Keck/HIRES Spectroscopy of Four Candidate Solar Twins

Authors: Jeremy R. King (Clemson U.), Ann M. Boesgaard (U. Hawai`i), and Simon
C. Schuler (Clemson U.)

Comments: Accepted for publication in The Astronomical Journal (November 2005
volume)
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We use high S/N, high-resolution Keck/HIRES spectroscopy of 4 solar twin
candidates (HIP 71813, 76114, 77718, 78399) from our Hipparcos-based CaII H & K
survey to carry out parameter and abundance analyses of these objects. Our
spectroscopic Teff estimates are some 100 K hotter than the photometric scale
of the recent Geneva-Copenhagen survey; several lines of evidence suggest the
photometric temperatures are too cool at solar $T_{\rm eff}$. At the same time,
our abundances for the 3 solar twin candidates included in the
Geneva-Copenhagen survey are in outstanding agreement with the photometric
metallicities; there is no sign of the anomalously low photometric
metallicities derived for some late-G UMa group and Hyades dwarfs. A first
radial velocity determination is made for HIP 78399, and UVW kinematics derived
for all stars. HIP 71813 appears to be a kinematic member of the Wolf 630
moving group (a structure apparently reidentified in a recent analysis of
late-type Hipparcos stars), but its metallicity is 0.1 dex higher than the most
recent estimate of this group's metallicity. While certainly ``solar-type''
stars, HIP 76114 and 77718 are a few percent less massive, significantly older,
and metal-poor compared to the Sun; they are neither good solar twin candidates
nor solar analogs providing a look at the Sun at some other point in its
evolution. HIP 71813 appears to be an excellent solar analog of age 8 Gyr. Our
results for HIP 78399 suggest the promise of this star as a solar twin may be
equivalent to the ``closest ever solar twin'' HR 6060; follow up study of this
star is encouraged.

\\ ( http://arXiv.org/abs/astro-ph/0508004 , 31kb)

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Paper: astro-ph/0508009
Date: Sat, 30 Jul 2005 11:46:22 GMT (124kb)

Title: Rotational Evolution of Solar-Like Stars in Clusters from Pre-Main
Sequence to Main Sequence: Empirical Results

Authors: W. Herbst and R. Mundt

Comments: Accepted and Scheduled for ApJ 10 November 2005
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Rotation periods are now available for ~500 pre-main sequence and recently
arrived main sequence stars of solar-like mass (0.4-1.2 M_sun) in five nearby
young clusters: the Orion Nebula Cluster, NGC 2264, alpha Per, IC 2602 and the
Pleiades. In combination with estimates of stellar radii these data allow us to
construct distributions of surface angular momentum per unit mass at three
different epochs: nominally, 1, 2 and 50 My. Our main result is illustrated in
Fig. 18 and may be summarized as follows: (1) 50-60% of the stars on convective
tracks in this mass range are released from any locking mechanism very early on
and are free to conserve angular momentum throughout most of their PMS
evolution, i.e. to spin up and account for the rapidly rotating young main
sequence stars. (2) The other 40-50% lose substantial amounts of angular
momentum during the first few million years, and end up as slowly rotating main
sequence stars. The duration of the rapid angular momentum loss phase is ~5-6
My, which is roughly consistent with the lifetimes of disks estimated from
infrared surveys of young clusters. The rapid rotators of Orion age lose less
than 10% of their (surface) specific angular momentum during the next 50 My
while the slow rotators lose about two-thirds of theirs. A detectable part of
this loss occurs even during the ~1 My interval between the ONC and NGC 2264.
The data support the view that interaction between an accretion disk and star
is the primary mechanism for evolving the broad, bimodal distribution of
rotation rates seen for solar-like stars in the ONC into the even broader
distributions seen in the young MS clusters.

\\ ( http://arXiv.org/abs/astro-ph/0508009 , 124kb)