Death is a part of existence – even for stars, which eventually run out of the nuclear fuel powering them. The death of massive stars, or those eight times larger than our own sun, can give rise to some of the most astonishing phenomena in our universe, including black holes and supernovae.  Often referred to as ‘rockstars’ by the scientists who study them - because they live fast and die young, typically in fiery explosions - DePaul graduate student Angel Hernandez is seeking to understand exactly how and why these stars die so dramatically. The influence of black hole companions could provide fresh clues, his new study indicates.

Hernandez simulated the death of a massive star with a black hole companion in new research published this summer in the Astrophysics Journal. The goal was to investigate how a massive star and black hole paired together in a binary system could produce gamma-ray bursts – tremendous outputs of high-energy radiation – on the verge of the star’s collapse and eventual death.

He and his research team used a special stellar evolution code to simulate a star in circular orbit with a black hole at varying mass ratios, orbital periods, rotations, and other related variables to analyze the conditions that enable such systems to produce large gamma-ray bursts upon the star’s death. The research addresses persistent questions about which massive stars produce the bursts, and why. While single, core collapses of massive stars are already well understood, binary interactions like the one Hernandez studied have so far eluded scientists.

“We simulated the evolution of a massive star and black hole in orbit around each other and found that in some cases the black hole can spin up the star through tidal interactions, giving the star enough angular momentum to potentially produce a gamma-ray burst when it collapses. For the collapsing star scenario, the key requirement is that the star must retain this spin and avoid losing excessive mass to stellar winds or to the black hole through Roche lobe overflow, a process in which the black hole pulls material off the star,” Hernandez explains.

The team identified a favorable range of orbital periods and metallicities, or compositions of heavy elements, where the star can be spun up, finding that both short and long orbital periods can produce the spin-up necessary for gamma-ray production.

“This finding is important because it suggests a pathway to producing long-lasting gamma-ray bursts, which may explain a subset of bursts that are unusually bright at radio wavelengths,” Hernandez clarifies.

 

Beyond stellar death, Hernandez is also interested in how massive stars are born. He is currently working on a master’s thesis to understand how the most massive stars in our galaxy develop using radio astronomy. He recently attended the National Radio Astronomy Observatory's 21st Synthesis Imaging Workshop in Socorro, New Mexico to acquire the latest imaging techniques. He was also awarded a $10,000 Illinois Space Grant Consortium fellowship through DePaul’s longstanding collaboration with the NASA National Space Grant College program. The funding will help Hernandez conduct advanced research with key institutional partners, such as the Very Large Array (formerly known as the Karl G. Jansky Very Large Array), one of the world’s most widely used radio telescopes. Hernandez uses signals from such telescopes to track microwave radiation emissions that hold important clues to stellar development.

“Since these stars form deep within clouds of gas and dust that block visible light, I use special cosmic lasers called "masers" to see through the haze and trace the motion of the surrounding material. By studying these masers, I can map the powerful jets and outflows that these young stars launch, helping to reveal the violent and dramatic process of their birth,” Hernandez says.

The intensity of these cosmic events is what attracted Hernandez to astrophysics in the first place. He is grateful to past and current research mentors, including DePaul astrophysicist Anuj Sarma, for cultivating a fascination with the most formative explosions in our universe. “Their passion proved contagious,” he shares.

Hernandez hopes to continue researching stars at the doctoral level once he completes his degree. You can read his full research publication here.