A new study by astronomers at the University of Illinois at Urbana-Champaign, published in the journal Physical Review Letters, links the unexplained changes in the brightness of the extraordinary star KIC 8462852 to internal phenomena.
Our Sun is the most familiar example of a main-sequence star. Image credit: NASA.
NASA’s Kepler Space Telescope was designed to search for extrasolar planets by measuring dips in a star’s brightness as orbiting planets move across the stellar disc.
Kepler stares at more than 150,000 stars towards the constellations of Cygnus and Lyrae, and so far has discovered over 5,000 exoplanet candidates. But the telescope also monitors the light fluctuations in all the other stars, even dips not caused by transits, and has found some bizarre situations.
The strangest is the case of KIC 8462852, a main-sequence F-type star located in the constellation Cygnus, approximately 1,480 light-years away.
The timing and duration of diminished light flux episodes Kepler detected coming from this star are a mystery. These dimming events vary in magnitude and don’t occur at regular intervals, making an orbiting planet an unlikely explanation.
The source of these unusual dimming events is the subject of intense speculation. Suggestions from scientists have ranged from asteroid belts to a Dyson sphere, a hypothetical energy-gathering structure built around the star.
Now University of Illinois researcher Mohammed Sheikh and co-authors offer a new solution to the puzzle of KIC 8462852. They suggest the luminosity variations may be intrinsic to the star itself.
“There are a few telltale signs of occultation, or dimming by an independent body blocking the view. The most important is periodicity,” said co-author Prof. Richard Weaver.
“In KIC 8462852, the small and big events are not periodic and this is one of the central mysteries of the light curve.”
KIC 8462852 (center), also known as TYC 3162-665-1, 2MASS J20061546+4427248, Boyajian’s star and Tabby’s star, is approximately 1,480 light-years from Earth. Image credit: Centre de Données astronomiques de Strasbourg / SIMBAD / DSS.
The astronomers applied a statistical analysis to the light curve’s smaller irregular variations. What they found is a mathematical pattern consistent with a well-established avalanche model: the smaller dimming events are the ‘crackling noise’ or small avalanches that are observed during the time intervals between the larger avalanches, equated to the larger dimming events.
The small dimming events come in a remarkably broad range of sizes, which are distributed according to a simple scaling law.
These results suggest the dimming events may be intrinsic to KIC 8462852 and that the star may be near the critical point of an underlying continuous phase transition.
The authors performed the calculations for the analysis of the observational data. They explain the mathematical method, which starts by establishing a median dimming threshold across the light curve.
“The threshold is an artifice we resort to in order to define what an avalanche is in the context of the light curve. In fact, the statistics are pretty robust to where we choose the threshold, so the exact value isn’t important. What is important is that we get enough avalanches to do statistics,” Sheikh said.
“Once the light curve drops below the threshold, we consider such an event the start of an avalanche. While the light curve remains below the threshold the avalanche continues, and it stops when it increases again to a value above the threshold.”
“Avalanches have two main properties, size and duration. The size is the total area enclosed by the light curve (underneath the threshold) and the threshold.”
“The avalanche size is related to the net decrease in energy emitted by the star during the dimming event, when compared to a constant emission rate of the star, or the constant threshold value,” Sheikh said.
“The avalanche duration is the length of the event. We also look at the power spectral density, which is related to how much power per unit of frequency is contained in the light curve.”
“Basically, we are looking at the statistical distributions of the fluctuations. All of these things have power laws associated with them. This gives us an independent way to interpret the events and check consistency with the model.”
Power laws have the interesting feature that they look the same on different scales. So when you zoom in to small scales and short times you get the same types of statistical distributions as when you zoom out to larger scales and longer times. Power laws reflect self-similarity of the system over a wide range of length and time scales — similar to fractals — that look the same when you zoom in or when you zoom out.
Significantly, the statistics of KIC 8462852’s smaller dimming events are consistent with the predictions of a scaling theory.
The scientists have established that a small-event scaling pattern punctuated by larger events is typical of systems near a phase transition.
They have seen this in the intermittent deformation dynamics of nanocrystals, the event statistics of metallic glasses, rocks, and granular materials, and in earthquakes on much larger scales spanning 12 decades in length. Similar types of avalanches are also seen in neuron firing avalanches in the brain, in magnetic systems, and in many other condensed matter systems.
“We know from other systems near nonequilibrium phase transitions that a system can have small events that show power law scaling and large events that have different dynamics,” said co-author Prof. Karin Dahmen.
“Examples of such transitions are magnetic systems that are slowly driven with magnetic field, or the slow deformation of somewhat brittle materials where there is often first little crackling that gets louder and louder until there is a big snap when the material breaks.”
“The small events in our star analysis would be like the little crackles while the large events would be the analogue of the big snap. Our mean field model is actually able to account for both, small events and large ones. It has a built in ‘weakening’ mechanism that explains why there should be two types of avalanches.”
If the dimming events are associated with an oncoming phase transition, what would the star be transitioning toward and in what time frame?
“As more data is being analyzed we hope it will be possible to identify exactly what type of transition this is,” Prof. Weaver said.
“We don’t have a deep enough understanding to get a definitive answer, and more observations are required. We can only speculate on what such a transition would be.”
“It’s important to note — lack of periodicity alone is not enough to rule out occultation,” Prof. Weaver added.
“That’s part of the reason why theories such as comets or planetary debris are so popular.”
“We can’t definitely rule out these things with our findings, but we can say that the power laws we have obtained are more consistent with intrinsic variation.”
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Mohammed A. Sheikh et al. 2016. Avalanche Statistics Identify Intrinsic Stellar Processes near Criticality in KIC 8462852. Phys. Rev. Lett. 117 (26): 261101; doi: 10.1103/PhysRevLett.117.261101
This article is based on press-releases from the University of Illinois at Urbana-Champaign and the Harvard-Smithsonian Center for Astrophysics.
