Can my neighbors make me stop smoking in my townhouse?

Probably the only thing that the neighbors could do about your smoking would be based on whatever covenants there are that govern the townhouse. There could be a provision that would affect your ability to smoke in your townhouse if the smoke can't be kept out of the adjoining unit. If the attic space is shared and the smoke does come through the attic, you may have to look for a way to install a smoke barrier between the units, but your ability to do that also could be governed by the documents. If the documents prohibited smoking in the units or smoking that does not confine the smoke to the unit, the documents are a contract that each owner agreed to when they bought their townhouse. You would want someone to review the documents, but you also might be well served to look at other ways of controlling the smoke beyond just opening the windows.

You need to carefully read any covenants or restrictions concerning the townhouse. I don't think this would be something that requires an attorney. You might want to politely ask your neighbors if they see something that gives them to tell you what you can do on your own property. HOWEVER, you might want to think how your felt if your neighbors were doing something that legitimately affected their enjoyment of their property. What I mean by "legitimately" is that if your smoking IS affecting them, this is not a petty matter from their perspective. Think about that and whether or not is fair for you to do something that you have the right to do, but affects their enjoyment of their property (and potentially their health). Sometimes what we have legal right to do is not morally correct.