I'm going to start with a quote from the movie Gladiator:
My name is Maximus Decimus Meridius, commander of the Armies of the North, General of the Felix Legions, loyal servant to the true emperor, Marcus Aurelius. Father to a murdered son, husband to a murdered wife.
As you can see, he is a commander, a general, a father and a husband, but none of these requires an article.
When a description of a job role is unique, or when you have already named one specific person who holds that role, you can optionally omit the article.
Examples:
This requires the indefinite article because there are many authors.
- He is the author of Farenheit 451.
Because there is only one author of that specific title, the definite article can be used, or it can be omitted:
- He is author of Fareheith 451.
You can also omit the article if the person is specifically named, for example:
- Ray Bradbury, author and screenwriter, died in 2012.