Southcoast Health System, a New Bedford-based community hospital chain, is seeking to combine with one of Rhode Island's biggest health systems, as consolidation continues to remake the health care market. Southcoast and Care New England Health System of Providence said Monday that they have entered exclusive merger talks. They hope to hash out a deal over the next several months. Southcoast and Care New England each run four hospitals. A merger would create a new nonprofit health care company with about $2 billion in revenue, eight hospitals and more than 1,700 doctors and other providers in two states.