Very massive stars expel more matter than previously thought

Very massive stars (VMSs), which typically have masses about 100 times that of our own sun, are critical components in our understanding of the formation of important astronomical structures like black holes and supernovae. However, there are some observed characteristics of VMSs that don’t fit the expected behavior based on the best models we have of them.

In particular, they hover around a relatively limited band of temperatures, which are hard to replicate with typical stellar evolution models.

A new paper from Kendall Shepherd and their co-authors at the Institute for Advanced Study (SISSA) in Italy describes a series of new models based on updated solar winds that better fit the observations of VMSs in their natural environment, and might aid in our understanding of the development of some of the most fascinating objects in the universe.

The paper was published on the arXiv preprint server.

Finding VMSs is relatively easy—their size alone makes them stand out in the cosmic background. But one particular area is fertile hunting ground for these giants—the Tarantula Nebula. In that nebula, there is a cluster known as the R136 cluster, which houses nine stars with masses greater than 100 times our sun.

Their collective light output outshines our star by a factor of 30 million. But, importantly, the data we’ve collected on them showed some discrepancies between what was modeled for them and what was observed.

Probably the most noticeable difference was in temperature. Standard models of stellar evolution expected wild swings in temperatures of these VMSs, as the stars expand and contract as part of their evolutionary cycle. However, observations from R136 show them holding to a relatively narrow band of temperatures that current models can’t seem to match.

Matching those temperatures is as simple as changing some of the parameters in the model, according to the authors. They implemented a new type of “stellar wind prescription” in the stellar evolution model PARSEC v2.0.

The prescriptions increased the stellar mass loss of the VMSs near the star’s Eddington limit—the point at which the pressure from the star’s outward radiation matches that of the hydrostatic pressure holding its outer layers in. If this limit is exceeded, the outer layers of the star are blown out in forceful solar winds.

In their PARSEC v2.0 models, the authors essentially create intentionally strong solar winds even at luminosities below the Eddington limit, creating some interesting interactions, especially between binary pairs.

While the strong solar winds from a VMS can make mergers less likely due to a star’s decreased size, the wind itself can contribute to the growth of the companion star, making the detailed interaction an interestingly complex math problem.

Another example of the implications of these updated models has to do with black hole formation. They severely limit the formation of black holes around the lower end of the pair-instability mass gap, a weight class where black holes shouldn’t form due to pair instability supernovae, where a star produces enough electron-positron pairs to begin to collapse upon itself, eventually resulting in a supernovae, but not a black hole.

Singular black holes weren’t the only objects affected by the new model—merging black holes are as well. In particular, the models create more systems where two black holes of a similar size (about 30–40 stellar masses) orbit each other. Those types of systems are much more rare in the standard model, but align with recent data collected from gravitational wave calculations.

Ultimately, the paper showcases the importance of understanding how stellar winds affect some of the most massive stars in the universe, and therefore the evolution and creation of some of the most extreme objects in the universe.

While the parameters the authors used in their model fit much of the data better, there is undoubtedly room for improvement.