** Shares of Acadia Healthcare ACHC.O rise 10.7% to $24.26
** Activist investor Engine Capital sends letter to co's board asking it to explore a potential asset sale and add new directors to its board
** Engine Capital, which owns about a 3% stake in Acadia, says the healthcare service provider has sustained negative returns due to ineffective execution of its reorganization in 2022
** Acadia did not immediately respond to Reuters request for comment
** Including session's move, ACHC stock was down 39.4% YTD