A 30-year-old Lowell man was in critical condition after being thrown from his car during a crash involving a tractor-trailer Sunday morning, according to State Police. The man, who was not identified, was driving in the northbound lane of Interstate 495 in Boxborough just before 7 a.m., when his Toyota Corolla “made contact” with a tractor-trailer and flipped over, police said in a statement. The man was flown by helicopter to the University of Massachusetts Memorial Medical Center in Worcester with life-threatening injuries. State Police said troopers found the tractor-trailer and identified its owner but did not release the name. State Police spokesman Dustin Fitch said Sunday that no determination had been made about filing charges.