Add physical, non-interacting (and apparent?) binaries to stellar grid

[karllark]

Adding binaries to the stellar grid would significantly improve the fidelity of the model given than many (50%?) of point sources in galaxies are binaries. Currently, only single stars are modeled.

There are two types of binaries: physical and apparent.

Physical binaries are those that are coeval and orbiting each other. The will have the same age. The physical binaries can be split into two categories: non-interacting and interacting. The non-interacting binaries could be modeled as two single stars with the same age using the single star stellar evolution tracks. The interacting binaries would have to be modeled using stellar evolution codes that explicitly deal with the interaction. That sounds hard.

Apparent binaries are those that are not physically associated, but happen to show up along the same line-of-sight (unresolved).

Adding physical, non-interacting binaries should be the most straightforward. Minimizing the new grids points could be done by applying a filter that only adds the binary model to the grid if the binary SED at any wavelength in a defined wavelength range is different by some TBD amount (1%?) from the SED of just the most luminous source alone. The two stars in the binary would be constrained to have the same age.

Of course, the same code could be used for apparent binaries without the constraint that two sources having the same age. The number of apparent binary SEDs that would much greater than for the physical, non-interacting sources, but that is just a matter of computer memory and computation time.

[galaxyumi]

It is time to think about including binaries seriously.

[karllark]

Definitely can be done fairly straightforwardly for non-interacting true binaries and apparent binaries as detailed above. Just makes the grid larger. I would suggest we start there.

As for interacting binaries where the interaction changes the evolution, that is a harder problem. At least I think think so mainly due to the lack of the right evolutionary tracks for stars in such systems. But I'm not an expert at all.

[karllark]

@galaxyumi : is this something you are interested in working on? I'd by happy to talk with you about the details of how and where in the code it could be implemented.

[lcjohnso]

@galaxyumi @karllark -- I'd love to keep in the loop on this, and I've got a relatively easy/straight-forward first science case in mind if you're looking for a motivating application for the code. Hack day target for this month?

[karllark]

Definitely could be a hackday project. This will add to the number of parameters the BEAST has to fit for. Good to think about what these parameters would be.

First thoughts.

For apparent binaries, it is all 7 parameters. (maybe not distance if we are at M31 or greater distances)

For physical binaries, it is many fewer as the age, metallicity, distance, and line of sight extinction should be the same - so secondary initial mass is really the only free parameter. So maybe physical binaries would be a good place to start. Especially if we only add binaries to the grid if the secondary changes the primary SED in any band above some threshold like 1% (see issue description).

[karllark]

One additional need, what prior to use for the binary fraction? Does the binary fraction vary with primary mass?