Expanding ties between the state’s two largest medical care providers, fast-growing Steward Health Care System has struck a deal with Partners HealthCare System to send its most severely injured patients from emergency rooms at Steward’s 10 community hospitals to Partners-owned Massachusetts General and Brigham and Women’s hospitals in Boston.

Steward employees learned of the plan in a letter from Dr. Michael Callum, president of Steward Medical Group, which represents about 700 doctors employed by the for-profit hospital chain. Under the arrangement, Callum said, Partners physicians will help several Steward hospitals establish trauma centers to handle less critical cases.