LAMOST JO6O728.44+232547.7: A Special A-type Star With Barium And Europium Enrichment

Among all the A-type stars,there are over 30% stars showing special metallicity anomalies.Due to these characteristics,these special A-type stars can be divided into the following types: Am star,Ap star,? Bootis,and Herbig Ae/Be.Generally speaking,the spectral types of a star can be derived based on the spectra properties of Ca II H&K lines,Balmer line and metallic lines of other elements(will be called the metal lines hereafter),and usually the spectral types derived from the three line series are almost the same.But in some special A type stars,the Ca II H&K lines are unusually weak,thus the spectral types that derived from it will be at least 5 sub-type earlier than the metal lines,e.g.the spectral type derived from Ca II H&K is A5,while the result will be F2 for metal lines of other elements.On the other side,Am stars usually show abundance enhancement of its metal elements,especially the Fe and Ni,while some elements(Ca,Sc)will be lower than the Sun.We notice that one star of the LAMOST spectra survey,LAMOST JO6O728.44+232547.7(HD 251812)is an likely Am star,since the spectral type that derived from the Ca II H&K lines and the metal lines are A7 and F2,respectively.To confirm whether it is an typical Am star,in this work we will measure and analyze the metallicity of this star.We collected the LAMOST spectra(both low-resolution and medium-resolution spectra)and the high-resolution spectra from APO.Based on the SIU package of IDL program,we derived the stellar parameters of this star by the following methods,the effective temperature is determined by the irrelevance between the Fe abundance and its excitation potential;the surface gravity is obtained mainly from the ionization balance of Fe I and Fe II,and the microscopic turbulence velocity from the irrelevance of Fe abundance and equivalent width(EW).With repeated iteration method,we finally confirmed the main stellar parameters of this star: effective temperature of 7050 K,surface gravity of 3.9,metallicity of-0.49 and the microscopic velocity of ?2.8 km/s.In the process of measuring the elemental abundances,we selected the spectra lines of each element and excluded the mixed lines to ensure the reliability of the measured abundances.The final abundances of 24 elements such as C,O,Na,Mg,Al,Si,Ca,Ba,and Eu were obtained in this work,and the non-local thermodynamic equilibrium(NLTE)effect was considered for Ba and Eu elements,and the [Ba/Fe] of1.63,[Eu/Fe] of 2.51,and [Ba/Eu] of-0.88 were obtained.The mass,age,and radius of the star were then determined from physical parameters such as effective temperature,metal abundance,apparent magnitude after extinction correction.The mass of the sample star is less than that of the known Am star.For the physical reasons of the high abundance of barium and europium,we obtained the apparent radial velocity of the star based on the medium-resolution spectra and found a variation of about two kilometers.Because less than the apparent radial velocity error of 3km/s,We cannot make scientific judgments.Nevertheless,we still infer that the sample star might belong to a binary system,and its enriched Ba element should come from the massive companion star produced during the AGB phase and contaminated by mechanisms such as stellar wind,while the excess Eu element comes from the nebula that generated the binary system and was contaminated by r-process events.However,there is another possibility that the enriched Ba element and Eu element are produced during the evolution of the primary star,the s-process produces mainly Ba and the i-process produces mainly Eu.The binary properties of the sample star require higher resolution spectroscopic observations to obtain higher precision of the radial velocity.