Update On the Puzzling Boyajian’s Star

What’s causing the mysterious light-curve dips of the so-called “alien megastructure” star, Boyajian’s Star? A recent study analyzes a variety of possible explanations to determine which ones are the most plausible.

An Unusual Light Curve

Earlier this year, astronomer Tabetha Boyajian reported on the unusual light curve of the star KIC 8462852. This star, now nicknamed “Tabby’s Star” or “Boyajian’s Star”, shows unusual dips on day-long timescales that are too large to be explained by planet transits or similar phenomena.

In addition to these short dips in luminosity, recent observations have also indicated that the star has faded by roughly 20% over the past hundred years. What could be causing both the short-term dips in the star’s light and the long-term dimming over a century?

Alien Megastructures? Or Another Explanation?

Boyajian’s Star was vaulted into the media spotlight when astronomer Jason Wright (Pennsylvania State University and University of California, Berkeley) proposed that its unusual light curve could potentially be explained by a surrounding megastructure built by an extraterrestrial civilization.

Now Wright is back with co-author Steinn Sigurd̵sson (Pennsylvania State University). In a new study, Wright and Sigurd̵sson analyze an extensive list of explanations for the puzzling apparent behavior of Boyajian’s Star, based on our latest knowledge about this strange object.

The Realm of Possibilities

Here are just a few possible causes of Boyajian’s Star’s dimming, as well as the authors’ assessment of their plausibility. For the full list, see the authors’ original article, or check out Wright’s own summary of the article here!

1. Pulsations, polar spots, and other stellar variability: unlikely
   The authors show that the variety of timescales observed for dimming events make scenarios involving stellar variations unlikely.
2. Circumstellar material: unlikely
   Material orbiting the star (like comets) would explain some of the light-curve dips, but it can’t explain the long-term dimming observed.
3. Post-merger return to normal: unclear
   Perhaps Boyajian’s Star recently merged with a brown dwarf or other star? Now it could be gradually dimming as it returns to its normal brightness, and restructuring of the star’s material could cause the short-term dips. Though this scenario is possible, the timescales for the brightness changes are shorter than we would expect.
4. Artificial structures: unclear
   Spherical swarms of structures would intercept the star’s light and re-radiate it in infrared. Since long-wavelength observations have found no evidence of such radiation, the authors declare spherical geometries to be unlikely. Other structure geometries can’t yet be ruled out, though.
5. Small-scale interstellar medium (ISM) structure: plausible
   Small-scale density variations in the ISM between us and Boyajian’s Star could cause the dimming we observe, but the fact that nearby stars don’t show similar dimming sets tight limits on the size of such ISM clumps.

Looking to the Future

Of the possible locations for the source of the dimming, Wright and Sigurd̵sson deem the interstellar space between us and Boyajian’s Star to be the most likely culprit. They identify several future lines of research that could help us further eliminate possibilities, however, including a study of the ISM toward Boyajian’s Star, a hunt for similar variations in stars near in the sky to Boyajian’s Star, and infrared observation of the star with JWST to search for heat signatures.

Citation

Jason T. Wright and Steinn Sigurd̵sson 2016 ApJ 829 L3. doi:10.3847/2041-8205/829/1/L3