The Evenstar is the necklace given by Arwen to Aragorn as a token of her love. Arwen herself is known to be Arwen Evenstar (Evenstar is not a last name, just another name). Arwen's last name is Undomiel.

Arwen's other name was Undomiel, which means Evenstar.

Her name is Arwen Evenstar, if you're talking about the Arwen from Lord of the Rings

if you're referring to Arwen in the Lord of the Rings, she Is Lady Arwen Evenstar, daughter of Lord Elrond. she is the elf Princess.

Undómiel means Evening Star, aka Evenstar. Thus Elrond's daughter was known as Arwen Evenstar.