New UNLV-led research is helping to unravel clues to a cosmic mystery that has eluded scientists for decades.
Cataclysmic variables (CVs) are binary star systems in which primary stars – incredibly dense and compact white dwarfs – accumulate material from nearby companion stars. The material spirals in towards the white dwarf through what is known as an accretion disk. These deep space systems are responsible for a number of cosmic phenomena, including sudden bursts of light known as classical novae that temporarily appear to resemble new stars before fading away over time.
Modern astronomical instruments have enabled scientists to understand many of the mechanisms that drive CVs. But one phenomenon – periodic brightness variations referred to as superhumps – has proven more difficult to explain. First identified roughly 50 years ago, these brightness modulations appear for periods either slightly longer (positive superhumps) or shorter (negative superhumps) than the duration of the system’s orbital time frame.
In a new paper published March 23 in The Astrophysical Journal Letters, a team of astrophysicists from UNLV and the Space Telescope Science Institute offers a new explanation for the occurrence of negative superhumps in CVs.
For decades, the prevailing theory for their formation was that the disk of collected material around the white dwarf is both circular and tilted to the binary orbit, causing it to precess backwards in a motion similar to that of a spinning top. The problem with this theory, the UNLV team says, is that there hasn’t been a strong explanation for how the disk could be tilted or what could sustain the tilt.
Instead, the team proposes in the new paper that the accretion disk can become eccentric – in other words, it becomes elongated rather than circular. The eccentric accretion disk gradually rotates its orbit over time through a process known as retrograde apsidal precession. And, as a result of pressure, it naturally produces negative superhumps without requiring a disk tilt.
“Cataclysmic variables have been visible to the human eye for hundreds of years, and what began as observations of a blinking light in the sky were later revealed to be one star eating another star,” said David Vallet, study lead author and postdoctoral researcher in the Department of Physics and Astronomy at UNLV. “While observations of superhumps date back to the 1970s, we believe the eccentric disk model clears up prevailing concerns of the tilted disk model and explains the prevalence of negative superhumps across a wide range of binary star masses.”
In certain systems, this new model shows that disk expansion may create conditions in the inner and outer portions of the disk to allow for the temporary coexistence of positive and negative superhumps. According to researchers, the theory may also explain how positive superhumps can occur in high mass ratio systems if the disk density builds up in the outer parts of the disk.
These areas will form the focus of the team’s future research, which will involve using large numerical simulations to model an evolving disk and match a predicted light curve to the observations.
“Every piece of this puzzle increases our knowledge of mechanisms that drive the evolution of our universe,” said Vallet.
Publication Details
“Negative superhumps in cataclysmic variables driven by retrograde apsidal disk precession” was published March 23 in The Astrophysical Journal Letters. Co-authors include David Vallet, Rebecca Martin, and Stephen Lepp with the Nevada Center for Astrophysics at UNLV; and Stephen Lubow with the Space Telescope Science Institute in Baltimore. Martin and Lepp are professors in the UNLV Department of Physics and Astronomy, and Vallet is a postdoctoral researcher.
