11 research outputs found
Spectroscopic Abundances and Membership in the Wolf 630 Moving Group
The concept of kinematic assemblages evolving from dispersed stellar clusters has remained contentious since Eggenâs initial formulation of moving groups in the 1960s. With high-quality parallaxes from the Hipparcos space astrometry mission, distance measurements for thousands of nearby, seemingly isolated stars are currently available. With these distances, a high-resolution spectroscopic abundance analysis can be brought to bear on the alleged members of these moving groups. If a structure is a relic of an open cluster, the members can be expected to be monolithic in age and abundance in as much as homogeneity is observed in young open clusters. In this work, we have examined 34 putative members of the proposed Wolf 630 moving group using high-resolution stellar spectroscopy. The stars of the sample have been chemically tagged to determine abundance homogeneity and conïŹrm the existence of a homogeneous subsample of 19 stars. Fitting the homogeneous subsample with YaleâYonsei isochrones yields a single evolutionary sequence of âŒ2.7 ± 0.5 Gyr. It is concluded that this 19 star subsample of the Wolf 630 moving group sample of 34 stars could represent a dispersed cluster with an [Fe/H]=â0.01 ± 0.02 and an age of 2.7 ± 0.5 Gyr. In addition, chemical abundances of Na and Al in giants are examined for indications of enhancements as observed in ïŹeld giants of old open clusters; overexcitation/ionization effects are explored in the cooler dwarfs of the sample; and oxygen is derived from the infrared triplet and the forbidden line at λ6300
Lithium in the Upper Centaurus Lupus and Lower Centaurus Crux Subgroups of Scorpius-Centaurus
We utilize spectroscopically derived model atmosphere parameters and the
\ion{Li}{1} subordinate line and the doublet to
derive lithium abundances for 12 members of the Upper-Centaurus Lupus (UCL) and
Lower-Centaurus Crux (LCC) subgroups of the Scorpius Centaurus OB Association.
The results indicate any intrinsic Li scatter in our 0.9-1.4 stars
is limited to dex, consistent with the lack of dispersion in
stars in the 100 Myr Pleiades and 30-50 Myr IC 2391 and
2602 clusters. Both ab initio uncertainty estimates and the derived abundances
themselves indicate that the 6104 line yields abundances with
equivalent or less scatter than is found from the 6708 doublet as a
result of lower uncertainties for the subordinate feature, a result of low
sensitivity to broadening in the subordinate feature. Because NLTE corrections
are less susceptible to changes in surface gravity and/or metallicity for the
6104 {\AA} line, the subordinate Li feature is preferred for deriving lithium
abundances in young Li-rich stellar association stars with K.Comment: Accepted for publication in Astronomical Journal (abstract shortened
for astro-ph submission
V474 Car: A Rare Halo RS CVn Binary in Retrograde Galactic Orbit
We report the discovery that the star V474 Car is an extremely active, high
velocity halo RS CVn system. The star was originally identified as a possible
pre-main sequence star in Carina, given its enhanced stellar activity, rapid
rotation (10.3 days), enhanced Li, and absolute magnitude that places it above
the main sequence. However, its extreme radial velocity (264 km s)
suggested that this system was unlike any previously known pre-MS system. Our
detailed spectroscopic analysis of echelle spectra taken with the CTIO 4-m
finds that V474 Car is both a spectroscopic binary with orbital period similar
to the photometric rotation period, and metal poor ([Fe/H] 0.99). The
star's Galactic orbit is extremely eccentric (e 0.93) with
perigalacticon of only 0.3 kpc of the Galactic center - and its
eccentricity and smallness of its perigalacticon are only surpassed by
0.05%, of local F/G-type field stars. The observed characteristics are
consistent with V474 Car being a high velocity, metal poor, tidally-locked
chromospherically active binary (CAB), i.e.\ a halo RS CVn binary, and one of
only a few such specimens known.Comment: Accepted to Astronomical Journa
A Revised Age for Upper Scorpius and The Star-Formation History Among the F-Type Members of the Scorpius-Centaurus OB Association
We present an analysis of the ages and star-formation history of the F-type
stars in the Upper Scorpius (US), Upper Centaurus-Lupus (UCL) and Lower
Centaurus-Crux (LCC) subgroups of Scorpius-Centaurus. We find that 1) our
empirical isochrones are consistent with the previously published age-rank of
the Sco-Cen subgroups, 2) LCC and UCL appear to reach the main sequence turn-on
at spectral types ~F4 and ~F2, respectively. An analysis of the A-type stars
shows US reaching the main sequence at about spectral type ~A3. 3) The median
ages for the pre-main sequence members of UCL and LCC are 16 Myr and 17 Myr,
respectively, in agreement with previous studies, however we find that 4) Upper
Sco is much older than previously thought. The luminosities of the F-type stars
in US are typically a factor of ~2.5 less luminous than predicted for a 5 Myr
old population for four sets of evolutionary tracks. We reexamine the
evolutionary state and isochronal ages for the B-, A-, and G-type Upper Sco
members, and the evolved M supergiant Antares, and estimate a revised mean age
for Upper Sco of 11+/-1+/-2 Myr (statistical, systematic). Using radial
velocities and Hipparcos parallaxes we calculate a lower limit on the kinematic
expansion age for Upper Sco of >10.5 Myr (99% confidence). However, the data
are statistically consistent with no expansion. We reevaluate the inferred
masses for the known substellar companions in Upper Sco using the revised age.
Specifically, we estimate the mass of 1RXS J1609-2105b to be 14^{+2}_{-3} Mjup,
suggesting that it is a brown dwarf rather than a planet. Finally, we find the
fraction of F-type stars exhibiting Ha emission and/or a K-band excess
consistent with accretion to be 0/17 (<19%; 95% C.L.) in US at ~11 Myr, while
UCL has 1/41 (2^{+5}_{-1}%; 68% C.L.) accretors and LCC has 1/50 (2^{+4}_{-1}%;
68% C.L.) accretors at ~16 Myr and ~17 Myr, respectively. [Abridged]Comment: Accepted for publication in ApJ. Formatted with emulateapj, 28 pages,
16 figures, 14 table
Spectroscopic study of the Wolf 630 moving group
VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title \u27Spectroscopic abundances and membership in the Wolf 630 moving group.\u27 (bibcode: 2010AJ....140..293B