Polluted White Dwarfs And Their Pollution Mechanisms

As the end product of the evolution of most(90%)stars in the universe,while dwarf are the most important object for our understanding of star formation and evolution.Under the strong gravity of the while dwarf's surface,the heavy elements sink into the while dwarf's interior,while the light elements float up to the surface.Thus,in the theory,a white dwarf should have no heavy elements on its surface.However,heavy elements have been detected in the atmospheres of white dwarfs.We call this class of white dwarfs with heavy elements on their surfaces polluted white dwarfs.At present,there has been strong evidence that the source of pollution from the planets,its specific pollution mechanism has been a major controversy.But we know for sure that it must be the result of the interaction between the star and the planet.Recently,The discovery of WD 1856+534 system which includes a giant planet orbiting a cool and old white dwarf(WD)with an orbital period of about 1.4 days.The formation of this system may be involved in the common envelope evolution.The emergence of this system provides an important example for the study of the pollution mechanism of polluted white dwarfs.Using the Modules for Experiments in Stellar Astrophysics and the common envelope evolution with?and?algorithms,we investigate the formation of WD1856+534 system which includes a giant planet orbiting a cool and old white dwarf(WD)with an orbital period of about 1.4 days.In?algorithm,the formation of WD1856+534 system is greatly affected by a series of helium flashes which drive star to expand and enhance the internal energy and decrease gravitational energy of the stellar envelope.Therefore,the helium flash changes the stellar structure which is mainly represented by parameter?in?algorithm.The value of?depends on parameter?_th,defined the percentage of the internal energy contributing to the ejection of the envelope.Its value can change from about 0.1 to 30,and even can be smaller than 0.We find that CEE can not produce WD 1856+534 system when?_th=0,while can do it when?_th=1.However,the progenitor of WD 1856+534 system should have a star with initial mass between 1.5 and 2.0 M_?.In?algorithm,the change of separation between red giant and planet during CEE keep within an order of magnitude,and the planet can usually survive.Therefore,WD 1856+534 system can easily be produced in?algorithm.