A 13-year-old boy was killed while riding his bicycle in Hopkinton Wednesday evening, authorities said. The boy was struck by a sedan on Hayden Rowe Street around 5 p.m., the Middlesex district attorney’s office said. The boy, whom officials are not yet identifying, was taken to an area hospital, where he was pronounced dead, said Stephanie Guyotte, a spokeswoman for District Attorney Marian T. Ryan. The circumstances are under investigation, Guyotte said, and there is no word on whether the driver will be charged. It is unclear if the victim was wearing a helmet when he was hit. Guyotte said the driver of the car was hospitalized as a precaution.Nicholas Jacques can be reached at firstname.lastname@example.org.