Figure 3 from Monet & Simon (2016) in this focus issue, showing the light curve of Boyajian's Star over the course of the Kepler mission.

Jason T. Wright (Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA)

Boyajian's Star is a fascinating puzzle. First noticed by citizen scientists perusing Kepler data as part of the Planet Hunters project, this star—KIC 8462852—has stubbornly defied easy or even complicated explanation. In their discovery paper, Boyajian et al. showed that it appears to be an ordinary, field, early-F star in nearly every way, lacking infrared excess and radial velocity variation. Nonetheless, it exhibits at least two unprecedented and—so far—nearly inexplicable photometric behaviors.

The first is a set of days-long dimming events ("dips") of variable depth and duration that occurred sporadically throughout the Kepler prime mission, but greatly intensified in number in the telescope's final observing season of the Cygnus field. They are quite unlike the exoplanetary transits Kepler was designed to find in almost every way, including their lack of periodicity, large depth (up to 22%), long durations, and asymmetric shapes.

These dips bear some superficial resemblance to "dipper" stars like AA Tau, but those are characteristically young stars with circumstellar material that occasionally occults large fractions of the stellar disk. Boyajian's Star shows no evidence of youth and, as Marengo et al. and Lisse et al. in this focus issue have confirmed, has no infrared evidence for a disk. The original suggestion of Boyajian et al. that a swarm of giant comets might explain the dips is explored by Bodman & Quillen.

The second is a long-term dimming. Montet & Simon show that calibration data from Kepler can be used to perform accurate long-term photometry on the entire field, and that Boyajian's Star appeared to dim by 3% over the course of the Kepler prime mission. Also in this focus issue, Schaefer shows that this behavior may go back over a century, and that the star was 15% brighter in 1890 than in 1990. However, Hippke et al. argue that this result is not statistically robust.

The lack of both close-in warm material and a tight binary companion seem to leave only unlikely scenarios as solutions. In this focus issue, Wright et al. connect the dips to earlier predictions from the SETI community that Kepler would be able to detect and distinguish planet- or star-sized artificial structures orbiting its target stars, if they exist. This suggestion, which attracted significant media attention, led to several SETI efforts, described by Schuetz et al., Abeysekara et al., and Harp et al.

New data, new analysis, and new ideas are needed to solve the puzzle. Wright & Sigurdsson attempt to outline and rank families of possible solutions, and suggest that structure in the interstellar medium may be to blame. Makarov & Goldin come to a similar conclusion based on their analysis of pixel-level Kepler data, showing that some of the dips and other features of the Kepler light curve are due to contaminating light from neighboring stars.

It is still unclear whether Boyajian's Star will be the prototype of a new class of variable star, and whether it will reveal new astrophysical processes, either mundane or exotic. It is clear from this focus issue that astronomers are rising to the challenge of one of the most interesting stars in Kepler's panoply.

KIC 8462852: The Infrared Flux We analyzed the warm Spitzer/IRAC data of KIC 8462852. We found no evidence of infrared excess at 3.6 μm and a small excess of 0.43 ± 0.18 mJy at 4.5 μm below the 3 σ threshold necessary to claim a detection. The lack of strong infrared excess 2 years after the events responsible for the unusual light curve observed by Kepler further disfavors the scenarios involving a catastrophic collision in a KIC 8462852 asteroid belt, a giant impact disrupting a planet in the system or a population of dust-enshrouded planetesimals. The scenario invoking the fragmentation of a family of comets on a highly elliptical orbit is instead consistent with the lack of strong infrared excess found by our analysis. IRTF/SPeX Observations of the Unusual Kepler Light Curve System KIC8462852 We have utilized the NASA/IRTF 3 m SpeX instrument’s high-resolution spectral mode to observe and characterize the near-infrared flux emanating from the unusual Kepler light curve system KIC 8462852. By comparing the resulting 0.8–4.2 μm spectrum to a mesh of model photospheric spectra, the 6 emission line analyses of the Rayner et al. catalog, and the 25 system collections of debris disks we have observed to date using SpeX under the Near InfraRed Debris disk Survey, we have been able to additionally characterize the system. Within the errors of our measurements, this star looks like a normal solar abundance main-sequence F1V to F3V dwarf star without any obvious traces of significant circumstellar dust or gas. Using Connelley & Greene’s emission measures, we also see no evidence of significant ongoing accretion onto the star nor any stellar outflow away from it. Our results are inconsistent with large amounts of static close-in obscuring material or the unusual behavior of a YSO system, but are consistent with the favored episodic giant comet models of a Gyr old stellar system favored by Boyajian et al. We speculate that KIC 8462852, like the ∼1.4 Gyr old F2V system η Corvi, is undergoing a late heavy bombardment, but is only in its very early stages. A Search for Brief Optical Flashes Associated with the SETI Target KIC 8462852 The F-type star KIC 8462852 has recently been identified as an exceptional target for search for extraterrestrial intelligence (SETI) observations. We describe an analysis methodology for optical SETI, which we have used to analyze nine hours of serendipitous archival observations of KIC 8462852 made with the VERITAS gamma-ray observatory between 2009 and 2015. No evidence of pulsed optical beacons, above a pulse intensity at the Earth of approximately , is found. We also discuss the potential use of imaging atmospheric Cherenkov telescope arrays in searching for extremely short duration optical transients in general. KIC 8462852: Transit of a Large Comet Family We investigate the plausibility of a cometary source of the unusual transits observed in the KIC 8462852 light curve. A single comet of similar size to those in our solar system produces a transit depth of the order of 10 −3 lasting less than a day which is much smaller and shorter than the largest dip observed ( for ∼3 days), but a large, closely traveling cluster of comets can fit the observed depths and durations. We find that a series of large comet swarms, with all except one on the same orbit, provides a good fit for the KIC 8462852 data during Quarters 16 and 17, but does not explain the large dip observed during Quarter 8. However, the transit dips only loosely constrain the orbits and can be fit by swarms with periastrons differing by a factor of 10. To reach a transit depth of ∼0.2, the comets need to be in a close group of ∼30, if they are ∼100 km in radius or in a group of ∼300 if they are ∼10 km in radius. The total number of comets required to fit all of the dips is ∼70 ∼ 100 km or ∼700 ∼ 10 km comets. A single comet family from a tidally disrupted Ceres-sized progenitor or the start of a Late Heavy Bombardment period explains the last ∼60 days of the unusual KIC 8462852 light curve. KIC 8462852 Faded at an Average Rate of 0.164 ± 0.013 Magnitudes per Century from 1890 to 1989 KIC 8462852 is a completely ordinary F3 main-sequence star, except that the light curve from Kepler shows episodes of unique and inexplicable day-long dips with up to 20% dimming. Here, I provide a light curve of 1338 Johnson B-band magnitudes from 1890 to 1989 taken from archival photographic plates at Harvard. KIC 8462852 displays a secular dimming at an average rate of 0.164 ± 0.013 mag per century. From the early-1890s to the late-1980s, KIC 8462852 faded by 0.193 ± 0.030 mag. The decline is not an artifact because nearby check stars have closely flat light curves. This century-long dimming is unprecedented for any F-type main-sequence star. Thus, the Harvard light curve provides the first confirmation (past the several dips seen in the Kepler light curve alone) that KIC 8462852 has anything unusual. The century-long dimming and the day-long dips are both just extreme ends of a spectrum of timescales for unique dimming events. By Ockham’s Razor, two such unique and similar effects are very likely produced by one physical mechanism. This one mechanism does not appear as any isolated catastrophic event in the last century, but rather must be some ongoing process with continuous effects. Within the context of dust-occultation models, the century-long dimming trend requires 10 4–10 7 times as much dust as for the deepest Kepler dip. Within the context of the comet-family idea, the century-long dimming trend requires an estimated 648,000 giant comets (each with 200 km diameter) all orchestrated to pass in front of the star within the last century. Optical SETI Observations of the Anomalous Star KIC 8462852 To explore the hypothesis that KIC 8462852's aperiodic dimming is caused by artificial megastructures in orbit, rather than a natural cause such as cometary fragments in a highly elliptical orbit, we searched for electromagnetic signals from KIC 8462852 indicative of extraterrestrial intelligence. The primary observations were in the visible optical regime using the Boquete Optical SETI Observatory in Panama. In addition, as a recommended preparatory exercise for the possible future detection of a candidate signal, three of six observing runs simultaneously searched radio frequencies at the Allen Telescope Array in California. No periodic optical signals greater than 67 photons m −2 within a time frame of 25 ns were seen. If, for example, any inhabitants of KIC 8462852 were targeting our solar system with 5 MJ laser pulses, locally illuminating an approximately 3 au diameter disk, the signal could have been detected at the Boquete Observatory. The limits on narrowband radio signals were 180–300 Jy Hz at 1 and 8 GHz, respectively. While the power requirement for a detectable, isotropic narrowband radio transmission from KIC 8462852 is quite high, even modest targeting on the part of the putative extraterrestrials can lower this power substantially. Families of Plausible Solutions to the Puzzle of Boyajian's Star Good explanations for the unusual light curve of Boyajian's Star have been hard to find. Recent results by Montet & Simon lend strength and plausibility to the conclusion of Schaefer that in addition to short-term dimmings, the star also experiences large, secular decreases in brightness on decadal timescales. This, combined with a lack of long-wavelength excess in the star's spectral energy distribution, strongly constrains scenarios involving circumstellar material, including hypotheses invoking a spherical cloud of artifacts. We show that the timings of the deepest dimmings appear consistent with being randomly distributed, and that the star's reddening and narrow sodium absorption is consistent with the total, long-term dimming observed. Following Montet & Simon's encouragement to generate alternative hypotheses, we attempt to circumscribe the space of possible explanations with a range of plausibilities, including: a cloud in the outer solar system, structure in the interstellar medium (ISM), natural and artificial material orbiting Boyajian's Star, an intervening object with a large disk, and variations in Boyajian's Star itself. We find the ISM and intervening disk models more plausible than the other natural models. KIC 8462852 Faded throughout the Kepler Mission KIC 8462852 is a superficially ordinary main sequence F star for which Kepler detected an unusual series of brief dimming events. We obtain accurate relative photometry of KIC 8462852 from the Kepler full-frame images, finding that the brightness of KIC 8462852 monotonically decreased over the four years it was observed by Kepler. Over the first ∼1000 days KIC 8462852 faded approximately linearly at a rate of 0.341 ± 0.041% yr −1, for a total decline of 0.9%. KIC 8462852 then dimmed much more rapidly in the next ∼200 days, with its flux dropping by more than 2%. For the final ∼200 days of Kepler photometry the magnitude remained approximately constant, although the data are also consistent with the decline rate measured for the first 2.7 years. Of a sample of 193 nearby comparison stars and 355 stars with similar stellar parameters, none exhibit the rapid decline by >2% or the cumulative fading by 3% of KIC 8462852. Moreover, of these comparison stars, only one changes brightness as quickly as the 0.341% yr −1 measured for KIC 8462852 during the first three years of the Kepler mission. We examine whether the rapid decline could be caused by a cloud of transiting circumstellar material, finding that while such a cloud could evade detection in submillimeter observations, the transit ingress and duration cannot be explained by a simple cloud model. Moreover, this model cannot account for the observed longer-term dimming. No known or proposed stellar phenomena can fully explain all aspects of the observed light curve.

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