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} $\lambda6104$ subordinate line and the $\lambda6708$ 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 $M_{\odot}$ stars is limited to ${\sim}0.15$ dex, consistent with the lack of dispersion in ${\ge}1.0$ $M_{\odot}$ 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 $\lambda$6104 line yields abundances with equivalent or less scatter than is found from the $\lambda$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 $T_{\rm eff}{\ge}5200$ 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$^{-1}$) 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] $\simeq -$0.99). The star's Galactic orbit is extremely eccentric (e $\simeq$ 0.93) with perigalacticon of only $\sim$0.3 kpc of the Galactic center - and its eccentricity and smallness of its perigalacticon are only surpassed by $\sim$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

Chemically tagging the Hyades Supercluster: A homogeneous sample of F6-K4 kinematically selected northern stars

Stellar kinematic groups are kinematical coherent groups of stars that might have a common origin. These groups are dispersed throughout the Galaxy over time by the tidal effects of both Galactic rotation and disc heating, although their chemical content remains unchanged. The aim of chemical tagging is to establish that the abundances of every element in the analysis are homogeneus among the members. We study the case of the Hyades Supercluster to compile a reliable list of members (FGK stars) based on our chemical tagging analysis. For a total of 61 stars from the Hyades Supercluster, stellar atmospheric parameters (Teff, logg, xi, and [Fe/H]) are determined using our code called StePar, which is based on the sensitivity to the stellar atmospheric parameters of the iron EWs measured in the spectra. We derive the chemical abundances of 20 elements and find that their [X/Fe] ratios are consistent with Galactic abundance trends reported in previous studies. The chemical tagging method is applied with a carefully developed differential abundance analysis of each candidate member of the Hyades Supercluster, using a well-known member of the Hyades cluster as a reference (vB 153). We find that only 28 stars (26 dwarfs and 2 giants) are members, i.e. that 46% of our candidates are members based on the differential abundance analysis. This result confirms that the Hyades Supercluster cannot originate solely from the Hyades cluster.Comment: A&A, in pres

KECK HIRES SPECTROSCOPY OF CANDIDATE POST T TAURI STARS

This work utilizes an age-oriented method to identify isolated post T Tauri stars in a 13-star sample of high resolution spectroscopy from the 10-m Keck telescope. This analysis utilizes qualitative age information derived from lithium abundances, chromospheric emission indices and UVW kinematic motions in analyzing positions in the the H-R diagram. Once qualitatively assigned a ``young\u27\u27 age, pre-main sequence isochrones are utilized to determine stellar mass and age estimates. Through this method, 5 of the 13 stars have been confirmed as being in the post T Tauri class. We also examine irregular photometric variability of PTTs and compare the homology of observed SED\u27s with model atmospheres which reveals a possible near-IR excess. We conclude that our age oriented analysis is a robust means to select samples of nearby, young, isolated post T Tauri stars that otherwise masquerade as normal field stars

Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association

The associations and moving groups of young stars are excellent laboratories for investigating stellar formation in the solar neighborhood. Previous results have confirmed that a non-negligible fraction of old main-sequence stars is present in the lists of possible members of young stellar kinematic groups. A detailed study of the properties of these samples is needed to separate the young stars from old main-sequence stars with similar space motion, and identify the origin of these structures. We used stars possible members of the young (~ 10 - 650 Myr) moving groups from the literature. To determine the age of the stars, we used several suitable age indicators for young main sequence stars, i.e., X-ray fluxes and other photometric data. We also used spectroscopic data, in particular the equivalent width of the lithium line Li I and Halpha, to constrain the range of ages of the stars. By combining photometric and spectroscopic data, we were able to separate the young stars (10 - 650 Myr) from the old (> 1 Gyr) field ones. We found, in particular, that the Local Association is contaminated by old field stars at the level of ~30%. This value must be considered as the contamination for our particular sample, and not of the entire Local Association. For other young moving groups, it is more difficult to estimate the fraction of old stars among possible members. However, the level of X-ray emission can, at least, help to separate two age populations: stars with <200 Myr and stars older than this. Our results are consistent with a scenario in which the moving groups contain both groups of young stars formed in a recent star-formation episode and old field stars with similar space motion. Only by combining X-ray and optical spectroscopic data is it possible to distinguish between these two age populations.Comment: 7 pages, 7 figures. Accepted for publication in A&

THE REALITY OF THE WOLF 630 MOVING GROUP

The concept of relic kinematic assemblages from dispersed stellar clusters has remained contentious since Eggen\u27s initial formulation of moving groups in the 1960\u27s. However, the availability of high quality parallaxes from the Hipparcos space astrometry mission has resulted in distance measurements for thousands of nearby, seemingly isolated stars. With these newly determined distances, a high resolution spectroscopic abundance analysis can be brought to bear on many of the alleged members of these relic associations. If a structure is a relic of an open cluster, the members can be expected to be monolithic in age and abundance inasmuch as homogeneity is observed in young open clusters. In this dissertation I have examined 34 members of the Wolf 630 moving group using high resolution stellar spectroscopy. The stars of the sample have been analyzed through a process known as chemical tagging to determine abundance homogeneity and confirm the existence of a homogeneous subsample of 20 stars. Fitting the homogeneous subsample with Yale-Yonsei isochrones, yields a single evolutionary sequence of ∼ 2.7 ± 0.5 Gyr. Additionally basic N-Body simulations, using the NEMO Stellar Dynamics toolkit, have been used to examine the kinematic evolution of typical star clusters in a model galactic disk potential that has been studded with Giant Molecular clouds. The results of these simulations suggests a high degree of kinematic coherence following spatial dissolution, validating that open clusters can maintain a common kinematic identity following their loss of spatial concordance. It is, therefore, concluded that moving groups can plausibly represent the relics of dissolved open clusters and that a 20 star subsample of the Wolf 630 moving group sample of 34 stars could represent such a dispersed cluster with an [Fe/H]=-0.01 ± 0.02 and an age of 2.7 ± 0.5 Gyr

Abundances of Stars with Planets: Trends with Condensation Temperature

Precise abundances of 18 elements have been derived for ten stars known to host giant planets from high signal-to-noise ratio, high-resolution echelle spectroscopy. Internal uncertainties in the derived abundances are typically <=0.05 dex. The stars in our sample have all been previously shown to have abundances that correlate with the condensation temperature (T_c) of the elements in the sense of increasing abundances with increasing T_c; these trends have been interpreted as evidence that the stars may have accreted H-depleted planetary material. Our newly derived abundances also correlate positively with T_c, although slopes of linear least-square fits to the [m/H]-T_c relations for all but two stars are smaller here than in previous studies. When considering the refractory elements (T_c > 900 K) only, which may be more sensitive to planet formation processes, the sample can be separated into a group with positive slopes (four stars) and a group with flat or negative slopes (six stars). The four stars with positive slopes have very close-in giant planets (three at 0.05 AU) and slopes that fall above the general Galactic chemical evolution trend. We suggest that these stars have accreted refractory-rich planet material but not to the extent that would increase significantly the overall stellar metallicity. The flat or negative slopes of the remaining six stars are consistent with recent suggestions of a planet formation signature, although we show that the trends may be the result of Galactic chemical evolution.Comment: 64 pages (single column), 5 figures, 10 tables. Accepted by Ap

Elemental abundances of intermediate age open cluster NGC 3680

We present a new abundance analysis of the intermediate age Galactic open cluster NGC 3680, based on high resolution, high signal-to-noise VLT/UVES spectroscopic data. Several element abundances are presented for this cluster for the first time, but most notably we derive abundances for the light and heavy s-process elements Y, Ba, La, and Nd. The serendipitous measurement of the rare-earth r-process element Gd is also reported. This cluster exhibits a significant enhancement of Na in giants as compared to dwarfs, which may be a proxy for an O to Na anti-correlation as observed in Galactic globular clusters but not open clusters. We also observe a step-like enhancement of heavy s-process elements towards higher atomic number, contrary to expectations from AGB nucleosynthesis models, suggesting that the r-process played a significant role in the generation of both La and Nd in this clusterComment: 8 pages, 6 figures, accepted for publication in MNRA