3D simulation delves into blue star mystery

(Credit: Getty Images)

A new three-dimensional simulation sheds light on the luminous blue variable (LBV), a relatively rare and still somewhat mysterious type of star.

The star sparkles with an exceptional blue-toned brilliance and exhibits wild variations in both brightness and spectrum. Its appearance tends to fluctuate radically over time, and that has piqued the curiosity of astrophysicists who wonder what processes may be at play.

“The luminous blue variable is a supermassive, unstable star,” says Yan-Fei Jiang, a researcher at the Kavli Institute for Theoretical Physics (KITP) at the University of California, Santa Barbara.

This video shows the 3D simulation of the luminous blue variable star. (High resolution video is available here.)

Unlike our own comparatively smaller and steady-burning sun, researchers have shown that LBVs burn bright and hot, then cool and fade so as to be almost indistinguishable from other stars, only to flare up again, he explains. Because of these changes, conventional one-dimensional models have been less than adequate at explaining the special physics of these stars.

However, thanks to special, data-intensive supercomputer modeling conducted at Argonne National Laboratory’s Argonne Leadership Computing Facility (ALCF) for its INCITE program, researchers have now developed a three-dimensional simulation.

15,740 degrees Fahrenheit

The simulation not only shows the stages of an LBV as it becomes progressively more luminous, then erupts, but also depicts the physical forces that contribute to that behavior. Researchers developed the simulation with computational resources from NASA and the National Energy Research Scientific Computing Center.

Of particular interest to the researchers are the stars’ mass loss rates, which are significant compared to those of less massive stars. Understanding how these stellar bodies lose mass could lead to greater insights into just how they end their lives as bright supernova, Jiang says.

Among the physical processes never before seen with one-dimensional models are the supersonic turbulent motions—the ripples and wrinkles radiating from the star’s deep envelope as it prepares for a series of outbursts.

“These stars can have a surface temperature of about 9,000 degrees Kelvin during these outbursts,” Jiang says. That translates to 15,740 degrees Fahrenheit or 8,726 degrees Celsius.

Also seen for the first time in three dimensions is the tremendous expansion of the star immediately before and during the outbursts—phenomena researchers had not captured with previous one-dimensional models. The three-dimensional simulations show that it is the opacity of the helium that sets the temperature researchers observed during the outburst.

Stellar wind

In a one-dimensional stellar evolution code, helium opacity—the extent to which helium atoms prevent photons (light) from escaping—is not very important in the outer envelope because the gas density at the cooler outer envelope is far too low, Jiang says.

The three-dimensional model demonstrates that “the region deep within the star has such vigorous convection that the layers above that location get pushed out to much larger radii, allowing the material at the location where helium recombines to be much denser,” says coauthor says Lars Bildsten, KITP director.

The radiation escaping from the star’s hot core pushes on the cooler, opaque outer region to trigger dramatic outbursts during which the star loses large amounts mass. Hence, convection—the same phenomena responsible for thundercloud formation—causes not only variations in the star’s radius but also in the amount of mass leaving in the form of a stellar wind.

Additional work is underway on more simulations, including models of the same stars but with different parameters such as metallicity, rotation, and magnetic fields, Jiang says.

“We are trying to understand how these parameters will affect the properties of the stars. We are also working on different types of massive stars—the so-called Wolf-Rayet stars—which also show strong mass loss,” Jiang explains.

Coauthors of the study are from the University of California, Berkeley; Princeton University; and the Flatiron Institute. The National Science Foundation and the Gordon and Betty Moore Foundation funded the work, which appears in Nature.

Source: UC Santa Barbara