A New Generation of Cool White Dwarf Atmosphere Models. III. WD J2356$-$209: Accretion of a Planetesimal with an Unusual Composition

S. Blouin, P. Dufour, N. F. Allard +3

WD J2356$-$209 is a cool metal-polluted white dwarf whose visible spectrum is dominated by a strong and broad sodium feature. Although discovered nearly two decades ago, no detaile…