“The luminous blue variable is a supermassive, unstable star,” said Yan-Fei Jiang, a researcher at the Flatiron Institute in New York City. Unlike our own smaller and steady-burning Sun, luminous blue variables (LBVs) have been shown to burn bright and hot, then cool and fade, only to flare up again. Because of its mercurial behavior, conventional one-dimensional models have been less than adequate at explaining their special physics.
An LBV in our galaxy, named HD168625, is surrounded by a bipolar nebula that is similar to the one around the supernova (SN) 1987A. SN1987A exploded in 1987 in the Large Magellanic Cloud, a satellite galaxy to our Milky Way, and was the nearest observed supernova in about 400 years.
Appearance tends to fluctuate radically
Sparkling with an exceptional blue-toned brilliance and exhibiting wild variations in both brightness and spectrum, the LBV is a relatively rare and still somewhat mysterious type of star. Its appearance tends to fluctuate radically over time, and that has piqued the curiosity of astrophysicists who wonder what processes may be at play.
This YouTube video starts with a wide-field view of the Carina constellation and zooms down to the Hubble Space Telescope view of the massive luminous blue variable star, AG Carinae. One of the brightest stars in our Milky Way galaxy, AG Carinae undergoes eruptions that have ejected a small nebula of gas and dust.
However, thanks to special, data-intensive supercomputer modeling conducted at Argonne National Laboratory’s Argonne Leadership Computing Facility (ALCF) for its INCITE program in 2018, Jiang and colleagues — Matteo Cantiello of the Flatiron Institute, Lars Bildsten of KITP, Eliot Quataert at UC Berkeley, Omer Blaes of UCSB, and James Stone of Princeton — have now developed a three-dimensional simulation. It 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. The simulation was developed also with computational resources from NASA and the National Energy Research Scientific Computing Center.
Significant Loss of the Star’s Mass
Of particular interest to the researchers was the stars’ mass loss rates, which are significant compared to those of less massive stars. LBVs are some of the most luminous stars in a galaxy, and their intense outward radiation helps to shed the outer layers of their envelopes. But exactly how the mass is lost, the mechanisms for the outbursts, and how the outbursts affect the mass loss rate have previously remained a mystery. Understanding how these stellar bodies lose mass, Jiang said, could lead to greater insights into just how they end their lives as bright supernovae.
“In our simulations material is lost because is pushed outwards by the extremely high radiation flux coming from the inner regions of the star, astronomer Matteo Cantiello wrote in an email to The Daily Galaxy. “As this light reaches the turbulent regions close to the stellar surface, regions of the star where the helium opacity is high are literally lifted and ejected from the star. These patches of high opacity correspond to denser regions, created by the turbulent fluctuations initiated by convection. And these higher opacity regions feel the push of light more strongly than lower opacity (more transparent) regions. If this push is stronger than stellar gravity, material moves outwards and eventually can leave the star.”
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.
“Stars usually lose mass via steady ‘stellar winds’, but these outbursts can substantially increase the amount of mass that is normally lost by the star,” Cantiello told The Daily Galaxy.
“These stars can have a surface temperature of about 9,000 degrees Kelvin during these outbursts,” Jiang said. That translates to 15,740 degrees Fahrenheit or 8,726 degrees Celsius.
Also seen for the first time in three dimensions was the tremendous expansion of the star immediately before and during the outbursts — phenomena not captured with previous one-dimensional models. The three dimensional simulations show that it is the opacity of the helium that sets the observed temperature during the outburst.
According to Jiang, 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.
The paper’s co-author and KITP Director Lars Bildsten explained that 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.”
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 of 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, according to Jiang, including models of the same stars but with different parameters such as metallicity, rotation and magnetic fields.
“We are trying to understand how these parameters will affect the properties of the stars,” Jiang said. “We are also working on different types of massive stars — the so-called Wolf-Rayet stars — which also show strong mass loss.”
Image credit top of page LBV star AG Carinae: Wikipedia