Non-interacting physical binary inclusion

[galaxyumi]

We just started an official conversation how to include non-interacting physical binary in the BEAST fitting (see also issue #19). Our motivation for this development is that (1) binary stars exist; (2) they matter for proper modeling of individual massive stars in galaxies and ensemble modeling with MegaBEAST.

The non-interacting physical binaries are coeval and orbiting each other. Thus, we can assume that they have same age, metallicity, and distance, but different masses. We can model them as a combination of two single stars using the single star stellar evolution tracks. There might be a more efficient way to deal with this, but at the moment, we will add the binary model as the new grid points to the spec grid (i.e., before applying dust because they would have the same dust) if a binary SED in all filters is different by some TBD amount (1%?) from the SED of just the most luminous source alone. More specifically, to add new grid points for binary models, we will go though each grid point for single star models and generate binary model grid points by combining with a set of single stars models with the same age, Z, and distance that have distinct brand-band SEDs from a single stars case.

'binary mass ratio' will be a new additional parameter in the model grids. For the single star case, it will be 0.

Please add any relevant papers or useful resources that we need to study!!

[e-sacchi]

Papers I know about this:

• Sana & Evans 2011 (https://ui.adsabs.harvard.edu/abs/2011IAUS..272..474S/abstract) -- O-star spectroscopic binary fraction in nearby open clusters, f ~ 0.44, 0.2 <= M2/M1 <= 1.0
• Sana et al. 2013 (https://ui.adsabs.harvard.edu/abs/2013A%26A...550A.107S/abstract) -- O-stars in the Tarantula Nebula, f = 0.51±0.04, Figure 14 explores mass ratios
• Schneider et al. 2015 (https://ui.adsabs.harvard.edu/abs/2015ApJ...805...20S/abstract) -- maybe a bit more focused on mass-loss and interaction

[christinawlindberg]

https://arxiv.org/pdf/2203.10066.pdf