[BUG]: ZAMS Secondary Masses Exceed Specified Range in population_params.ini

[Paoloboghos]

When generating a population using POSYDON, setting the primary mass range in population_params.ini to secondary_min_mass = 0.8 and secondary_max_mass = 1.6 sometimes results in ZAMS secondary stars with masses larger than the upper bound (e.g., > 1.6 M☉).
This occurs even though both primary and secondary masses should respect the configured mass range or derived mass ratio constraints.

population_params.py

[astroJeff]

@Paoloboghos it's been a while since I've looked at the binary sampler, so I had to look into this more deeply. I was able to reproduce your bug and now understand the issue. These cases occur when the secondary mass goes below the minimum mass ratio limit of 0.05. In your case, you are requesting a primary mass between 40 and 150 Msun but a secondary between 0.8 and 1.6 Msun. This ends up being a problem, since we did not run MESA binary grids with such low mass ratios.

Assuming you want to proceed in modeling such extreme mass ratio binaries, there are two options:

1. We proceed with what we have now. The q>0.05 limit is currently hard coded, but I can add a quick PR to fix this, allowing for lower q's. It will not be a problem to model such binaries in POSYDON, so long as they remain detached. Once one star overfills its Roche lobe, then we will not have a grid to interpolate from. I don't think your binaries will break, but since they are outside the convex hull for interpolation, they will be "shifted" onto the q=0.05 grid.

2. We can help you run an additional grid at very low mass ratio, say q=0.01. I don't recommend this lightly as it will take a good deal of work to get implemented, and a great deal of CPU resources to run.

If you tell me more about what you're trying to do, there may be other options as well.

[tassos25]

Hi Jeff. Option 1 would be sufficient. Paolo is following up on the work of Matthias on Gaia BHs, so the detached evolution is what he needs for his project. On 13 Nov 2025, at 17:27, Jeff Andrews ***@***.***> wrote:  [https://avatars.githubusercontent.com/u/6596095?s=20&v=4]astroJeff left a comment (POSYDON-code/POSYDON#761)<#761 (comment)> @Paoloboghos<https://github.com/Paoloboghos> it's been a while since I've looked at the binary sampler, so I had to look into this more deeply. I was able to reproduce your bug and now understand the issue. These cases occur when the secondary mass goes below the minimum mass ratio limit of 0.05. In your case, you are requesting a primary mass between 40 and 150 Msun but a secondary between 0.8 and 1.6 Msun. This ends up being a problem, since we did not run MESA binary grids with such low mass ratios. Assuming you want to proceed in modeling such extreme mass ratio binaries, there are two options: 1. We proceed with what we have now. The q>0.05 limit is currently hard coded, but I can add a quick PR to fix this, allowing for lower q's. It will not be a problem to model such binaries in POSYDON, so long as they remain detached. Once one star overfills its Roche lobe, then we will not have a grid to interpolate from. I don't think your binaries will break, but since they are outside the convex hull for interpolation, they will be "shifted" onto the q=0.05 grid. 2. We can help you run an additional grid at very low mass ratio, say q=0.01. I don't recommend this lightly as it will take a good deal of work to get implemented, and a great deal of CPU resources to run. If you tell me more about what you're trying to do, there may be other options as well. — Reply to this email directly, view it on GitHub<#761 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AB7CNKMGTD3Q3DN3F7X5LE334SWNNAVCNFSM6AAAAACLN6FIPOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTKMRYGYZDKMJQGQ>. You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>