Can I ask the intercession of those who have died whom I believe went to heaven?
While canon law prohibits the public veneration of those not officially recognized by the Church as saints or blessed, private prayer to and for such faithful departed is not prohibited.
Code of Canon Law (CIC) states, “It is permitted to reverence through public veneration only those servants of God whom the authority of the Church has recorded in the list of the saints or the blessed” (CIC 1187).
The Canon Law Letter and Spirit clarifies “…there is no reason – in fact, quite the contrary – why the faithful might not pray, but privately, not only for but also to those who by widespread popular consent have led lives of singular service to God and to the Church…” (p. 674).
The Catechism of the Catholic Church (CIC) recognizes this:
“The witnesses who have preceded us into the kingdom, especially those whom the Church recognizes as saints, share in the living tradition of prayer by the example of their lives, the transmission of their writings, and their prayer today. They contemplate God, praise him and constantly care for those whom they have left on earth. When they entered into the joy of their Master, they were “put in charge of many things.” Their intercession is their most exalted service to God’s plan. We can and should ask them to intercede for us and for the whole world” (CCC 2683).