Abundance Analysis Of Barium Stars

Barium (Ba) stars are a distinct class of giants which were first identified by Bidelman & Keenan (1951). The stars of this group are G to K giants, and in their spectrum there are abnormally strong lines of s-process elements, especially Baâ…¡at 4554A, as well as enhanced CH, CN, and C2 molecular bands (Burbidge et al.1957). Besides Ba and Sr, other heavy elements such as Y, Zr, La, Ce, Pr, Nd and Sm are also enhanced.In the research of Ba stars, we have got lots of characters of this kind of star. But there are many open questions here. Such as the reason of the overabundance of heavy elements, what effectes the phenomenon, and the dynamic of Ba stars. Aimed to solve the questions on Ba stars, we start to do this work.Lu (1991) presented a table of 389 Ba stars and quantified their Ba intensities on the scale defined by Warner (1965) who suggested Ba intensity from 1 to 5 according to the strength of the barium line at 4554 A. These Ba stars were identified into mild Ba stars (Ba<2) and strong Ba stars (Ba=2-5).While McClure et al. (1980) revealed that most barium stars, maybe all of them, show variations in radial-velocity suggesting the presence of companions.Then a number of Ba stars have been measured their orbitalelements of binary systems (Carquillat et al.1998, Udry et al.1998a[72],1998b, Jorissen et al.1998).Although there are these pioneer work on abundance analysis of Ba stars, the work on observation is not enough yet. And the detailed analysis on the abundances of this large sample of Ba stars will be important and necessary to reveal their properties and formation scenario, Therefore, we started a project to analyze the chemical compositions of a large sample of Ba starsThe high resolution and high S/N ratios Echelle spectra have been obtained for 57 Ba stars by using the 2.16m telescope of NAOC in Xing Long, China. Liang et al. (2003) and Liu et al. (2009) have analyzed 4 and 8 sample stars before. We analyzed 45 of them in this work. We use the standard ECHELLE package in MIDAS environment for radial velocity and equivalent widths (EWs) measurements of lines. The atmospheric parameters were derived in iterative way with the temperatures obtained from color-temperature calibration; surface gravities form empirical calibration, metallicity and microturbulence velocity. At last, we obtained the abundances with inputting atmospheric parameters and EWs to the atmospheric model.We find that most of these samples are in disk except for HD 26 in halo through their chemical abundances. Their abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Ni are similar to the solar abundances. The neutron capture (n-capture) process elements Y, Zr, Ba, La, Eu are overabundant relative to the Sun generally. In strong Ba stars, the first peak slow neutron capture process (s-process) elements Y and Zr are higher than the a and iron peak elements. The second peak s-process elements Ba, La and the rapid neutron capture process (r-process) Eu are higher than the first one. But in the mild Ba stars the first peak is not obvious in overabundance, and it is similar to the iron peak elements. In this work, we use Y I and Zr I line to calculate the abundances of Y and Zr. Some work suggested that the abundances being calculated by using Yâ…¡and Zrâ…¡are higher than that of Yâ… and Zrâ… (0.1-0.3dex).Then we add the sample stars of Liang et al. (2003), Allen& Barbuy (2006a), Smiljanic et al. (2007), Liu et al. (2009) and Zacs(1994) to enlarge the samples to analyze the Ba stars in details.Based on the high resolution spectra, we define a new criterion for separating strong Ba stars from the mild Ba stars as [Ba/Fe]=0.69, which could be more accurate than the previous criterion from Ba intensity based on tube image.The sample stars of previous work are almost strong Ba stars, and more than half number of ours are mild Ba stars. We can research the Ba stars in details. The relation of [Ba/Fe] vs. [Fe/H] shows that the abundances of strong Ba stars correlate with metallicity, i.e., higher overabundances at lower metallicities, while there is no such trend for mild Ba stars. These could mean that the Ba-rich stars may belong to a relative older disk group than the Ba stars with a lower [Ba/Fe]. There is no obvious correlation between [hs/ls] and [Fe/H].We calculate the UVW of the sample stars to analyze the dynamic of Ba stars. We find that most of these samples are in disk except for HD 26 in halo. The kinematic analysis about U, V, W velocities do not show obvious difference between the strong and mild Ba stars. We study the heavy-element abundances of Ba stars from a theoretical model of wind accretion (Liang et al.2000, Zhang et al.1999), and we can get that the wind accretion model of binary system can explain the abundance patterns of Ba stars. This suggests that their heavy-element overabundances were caused by accretion of the ejecta of AGB stars (the progenitors of the present white dwarf companions) through stellar winds. After that, we talk about the parameters (orbital period and neutron exposure in the AGB progenitor companion) affecting the overabundance of Ba element. The shorter orbital period and stronger neutron exposure will produce higher overabundances, and the process is a complex construction from all the related parameters.