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Register a new userLithium Abundance in Binaries of the Hyades Open Cluster
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Added by
David Barrado Navascues
Publication date
1997
fields
Physics
and research's language is
English
Authors
D. Barrado y Navascues
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We have derived accurate and homogeneous Lithium abundances in 49 main sequence binary systems belonging to the Hyades Open Cluster by using a deconvolution method to determine individual magnitudes and colors for the primary and secondary components of the binary. The input parameters of the model are the observed Li equivalent width, the actual distance to the binary, the integrated apparent magnitude and the integrated colors of the binaries -BV(RI)_K. We show that the general behavior is the same in binaries and in single stars (Li is depleted faster in K stars than in G stars and there is a deep dip for mid-F stars). However, there is a larger scatter in the abundances of binary systems than in single stars. Moreover, in general, binary systems have an overabundance, which is more conspicuous in close binaries. In fact, there is a cut-off period, which can be estimated as P(orb)~9 d. This value is in excellent agreement with the theoretical predition of Zahn (1994).
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Mixing mechanisms bring the Li from the base of the convective zone to deeper and warmer layers where it is destroyed. These mechanisms are investigated by comparing observations of Li abundances in stellar atmospheres to models of stellar evolution. Observations in open cluster are especially suitable for this comparison, since their age and metallicity are homogeneous among their members and better determined than in field stars. In this work, we compare the evolution of Li abundances in three different clusters: the Hyades, NGC752, and M67. Our models are calculated with microscopic diffusion and transport of chemicals by meridional circulation, and calibrated on the Sun. These comparisons allow us to follow the evolution of Li abundance as a function of stellar mass in each cluster and as a function of the age by comparing this evolution in each cluster. We evaluate the efficiency of the mixing mechanisms used in the models, and we try to identify the lacking mechanisms to reproduce the observed evolution of Li abundance.
Lithium is a fundamental element for studying the mixing mechanisms acting in the stellar interiors, for understanding the chemical evolution of the Galaxy and the Big Bang nucleosynthesis. The study of Li in stars of open clusters (hereafter OC) allows a detailed comparison with stellar evolutionary models and permits us to trace its galactic evolution. The OC NGC 2243 is particularly interesting because of its low metallicity ([Fe/H]=$-0.54 pm0.10$ dex). We measure the iron and lithium abundance in stars of the metal-poor OC NGC 2243. The first aim is to determine whether the Li dip extends to such low metallicities, the second is to compare the results of our Li analysis in this OC with those present in 47 Tuc, a globular cluster of similar metallicity. We performed a detailed analysis of high-resolution spectra obtained with the multi-object facility FLAMES at the ESO VLT 8.2m telescope. Lithium abundance was derived through line equivalent widths and the OSMARCS atmosphere models. We determine a Li dip center of 1.06 $M_odot$, which is much smaller than that observed in solar metallicity and metal-rich clusters. This finding confirms and strengthens the conclusion that the mass of the stars in the Li dip strongly depends on stellar metallicity. The mean Li abundance of the cluster is $log n{rm (Li)}=2.70$ dex, which is substantially higher than that observed in 47 Tuc. We estimated an iron abundance of [Fe/H]=$-0.54 pm0.10$ dex for NGC 2243, which is similar (within the errors) to previous findings. The [$ alpha$/Fe] content ranges from $0.00pm0.14$ for Ca to $0.20pm0.22$ for Ti, which is low when compared to thick disk stars and to Pop II stars, but compatible with thin disk objects. We found a mean radial velocity of 61.9 $pm$ 0.8 kms for the cluster.
Lithium abundances in open clusters provide an effective way of probing mixing processes in the interior of solar-type stars and convection is not the only mixing mechanism at work. To understand which mixing mechanisms are occurring in low-mass stars, we test non-standard models, which were calibrated using the Sun, with observations of three open clusters of different ages, the Hyades, NGC 752, and M67. We collected all available data, and for the open cluster NGC 752, we redetermine the equivalent widths and the lithium abundances. Two sets of evolutionary models were computed, one grid of only standard models with microscopic diffusion and one grid with rotation-induced mixing, at metallicity [Fe/H] = 0.13, 0.0, and 0.01 dex, respectively, using the Toulouse-Geneva evolution code. We compare observations with models in a color-magnitude diagram for each cluster to infer a cluster age and a stellar mass for each cluster member. Then, for each cluster we analyze the lithium abundance of each star as a function of mass. The data for the open clusters Hyades, NGC 752, and M67, are compatible with lithium abundance being a function of both age and mass for stars in these clusters. Our models with meridional circulation qualitatively reproduce the general trend of lithium abundance evolution as a function of stellar mass in all three clusters. This study points out the importance of mass dependence in the evolution of lithium abundance as a function of age. Comparison between models with and without rotation-induced mixing shows that the inclusion of meridional circulation is essential to account for lithium depletion in low-mass stars. However, our results suggest that other mechanisms should be included to explain the Li-dip and the lithium dispersion in low-mass stars.
We present a high-precision differential abundance analysis of 16 solar-type stars in the Hyades open cluster based on high resolution, high signal-to-noise ratio (S/N ~ 350 - 400) spectra obtained from the McDonald 2.7m telescope. We derived stellar parameters and differential chemical abundances for 19 elements (C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Ba) with uncertainties as low as ~ 0.01 - 0.02 dex. Our main results include: (1) there is no clear chemical signature of planet formation detected among the sample stars, i.e., no correlations in abundances versus condensation temperature; (2) the observed abundance dispersions are a factor of ~ 1.5 - 2 larger than the average measurement errors for most elements; (3) there are positive correlations, of high statistical significance, between the abundances of at least 90% of pairs of elements. We demonstrate that none of these findings can be explained by errors due to the stellar parameters. Our results reveal that the Hyades is chemically inhomogeneous at the 0.02 dex level. Possible explanations for the abundance variations include (1) inhomogeneous chemical evolution in the proto-cluster environment, (2) supernova ejection in the proto-cluster cloud, and (3) pollution of metal-poor gas before complete mixing of the proto-cluster cloud. Our results provide significant new constraints on the chemical composition of open clusters and a challenge to the current view of Galactic archeology.
We report the discovery of the first hot Jupiter in the Hyades open cluster. HD 285507b orbits a V=10.47 K4.5V dwarf ($M_* = 0.734 M_odot$; $R_* = 0.656 R_odot$) in a slightly eccentric ($e = 0.086^{+0.018}_{-0.019}$) orbit with a period of $6.0881^{+0.0019}_{-0.0018}$ days. The induced stellar radial velocity corresponds to a minimum companion mass of $M_{rm p} sin{i} = 0.917 pm 0.033 M_{rm Jup}$. Line bisector spans and stellar activity measures show no correlation with orbital phase, and the radial velocity amplitude is independent of wavelength, supporting the conclusion that the variations are caused by a planetary companion. Follow-up photometry indicates with high confidence that the planet does not transit. HD 285507b joins a small but growing list of planets in open clusters, and its existence lends support to a planet formation scenario in which a high stellar space density does not inhibit giant planet formation and migration. We calculate the circularization timescale for HD 285507b to be larger than the age of the Hyades, which may indicate that this planets non-zero eccentricity is the result of migration via interactions with a third body. We also demonstrate a significant difference between the eccentricity distributions of hot Jupiters that have had time to tidally circularize and those that have not, which we interpret as evidence against Type II migration in the final stages of hot Jupiter formation. Finally, the dependence of the circularization timescale on the planetary tidal quality factor, $Q_{rm p}$, allows us to constrain the average value for hot Jupiters to be $log{Q_{rm p}} = 6.14^{+0.41}_{-0.25}$.
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