How do I close a checking and money market a/c of my partner when there's no will or power of attorney only a death certificate?

The problem is that no one is appointed as the representative of the estate.  If the account is a joint account, then the money is yours.  If not, you would have to apply to be the intestate administrator of your partner's account, and then pay inheritance tax on the money.  It may not be worth it because it will be costly.