Stationary hotspot support

[amiszuda]

Hi there!

Currently, in PHOEBE 2.4.21, spots added to a star always rotate with the stellar surface according to syncpar and to changelog. This works as expected for synchronous stars; however, for stars with syncpar > 1 spots are automatically asynchronous in the orbital frame, which prevents modelling hotspots near L1 as stationary in the orbital frame. According to the API and source code, there is no documented way to fix a spot in the orbital frame (or is it?). Is there a method to achieve this, or could future updates include support for spots stationary in orbital phase?

Cheers!

[kecnry]

Hey @amiszuda - good to hear from you! Just to make sure I understand what you're asking - you want asynchronous rotation of the stellar surface (for distortion, line broadening, Rossiter-Mclaughlin, etc) but the spot to remain fixed wrt the other star (as if it were synchronous)? We might be able to implement this as a simple switch in the spot, but I worry it might get confusing, especially in the misaligned case (and spot position locations is complicated enough as-is, as shown by the number of bugfixes recently trying to get it right).

#902 proposes a framework, hopefully for the next release, that would allow quite a bit of flexibility (the last example there is of a migrating spot), but also is quite a bit more involved to get setup. Do you think something like this would do the trick? We could even have an example of using it to do exactly the case you have here where it would just "undo" the surface rotation to keep it fixed relative to the other star or completely overwrite the positioning logic. Very curious what you think!

[amiszuda]

Hi @kecnry, nice to hear from you too! Indeed, I want to keep the asynchronous rotation of one star and to add a spot to this star, but constantly facing the L1 point, rather than attaching it directly to the rotating surface.

I believe the #902 framework would be appropriate for this task. When do you expect it to be out, or is the devel version ok to use for now? If I manage to proceed, I’ll certainly be wiling to create an example that can mimic the accretion-induced hotspot.

[kecnry]

You are welcome to try it and any feedback (or examples that we could post) would be super helpful! There is a chance things may change between now and release based on feedback from you and others. This will hopefully be part of the next 2.5 release - that will still be at least a few months I expect, but we are making progress and getting closer.