KIC 8462852 (also Tabby's Star or Boyajian's Star) is an F-type main-sequence star located in the constellation Cygnus approximately 1,280 light-years (390 pc) from Earth. Unusual light fluctuations of the star, including up to a 22% dimming in brightness, were discovered by citizen scientists as part of the Planet Hunters project, and, in September 2015, astronomers and citizen scientists associated with the project posted a preprint of an article describing the data and possible interpretations. The discovery was made from data collected by the Kepler space telescope, which observes changes in the brightness of distant stars to detect exoplanets.
Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||20h 06m 15.457s|
|Declination||+44° 27′ 24.61″|
|Apparent magnitude (V)||±0.017+11.705|
|Evolutionary stage||Main sequence|
|B−V color index||0.557|
|V−R color index||0.349|
|R−I color index||0.305|
|J−H color index||0.212|
|J−K color index||0.264|
|Proper motion (μ)||RA: ±2.6 −9.9mas/yr
Dec.: ±2.4 −10.5mas/yr
|Parallax (π)||2.56 ± 0.31 mas|
|Absolute magnitude (MV)||3.08|
|Luminosity (bolometric)||4.68 L☉|
|Surface gravity (log g)||±0.24.0 cgs|
|Rotation||±0.0001 days 0.8797|
|Rotational velocity (v sin i)||±484 km/s|
Several hypotheses have been proposed to explain the star's large irregular changes in brightness as measured by its light curve, but none to date fully explain all aspects of the curve. A prominent hypothesis, based on a lack of observed infrared light, posits a swarm of cold, dusty comet fragments in a highly eccentric orbit. However, the notion that disturbed comets from such a cloud could exist in high enough numbers to obscure 22% of the star's observed luminosity has been doubted. Another hypothesis is that of a large number of small masses in "tight formation" orbiting the star. However, spectroscopic study of the system has found no evidence for coalescing material or hot close-in dust or circumstellar matter from an evaporating or exploding planet within a few astronomical units of the mature central star. It has also been hypothesized that the changes in brightness could be signs of activity associated with intelligent extraterrestrial life constructing a Dyson swarm. However, the scientists involved are very skeptical, with others describing it as implausible.
KIC 8462852 is not the only star that has large irregular dimmings, but all other such stars are young stellar objects called YSO dippers, that have different dimming patterns. An example of such an object is EPIC 204278916.
New dimming events of KIC 8462852 began in the middle of May 2017, and totaled, as of 16 September 2017, four prominent dimmings, the last one being the largest of the year, with a 2.3% (or a 3%) dimming. These events have provided the urgent opportunity for worldwide, real-time, coordinated data collection.
KIC is an acronym for the Kepler Input Catalog, 8462852 being the star's catalog number. Colloquially the names "Tabby's Star" and "Boyajian's Star" refer to the initial study's lead author, Tabetha S. Boyajian; KIC 8462852 is sometimes called the "WTF Star", after the study's subtitle "Where's The Flux?" (a joking reference to the colloquial expression of disbelief "WTF").
KIC 8462852 in the constellation Cygnus is located roughly halfway between the bright stars Deneb and Delta Cygni as part of the Northern Cross. KIC 8462852 is situated south of 31 Cygni, and northeast of the star cluster NGC 6866. While only a few arcminutes away from the cluster, it is unrelated and closer to the Sun than it is to the star cluster.
History of observationsEdit
KIC 8462852 was observed as early as the year 1890. The star was cataloged in the Tycho, 2MASS, UCAC4, and WISE astronomical catalogs (published in 1997, 2003, 2009, and 2012, respectively).
The main source of information about the luminosity fluctuations of KIC 8462852 is the Kepler space observatory. During its primary and extended mission from 2009 to 2013 it continuously monitored the light curves of over 100,000 stars in a patch of sky in the constellations Cygnus and Lyra.
Observations of the luminosity of the star by the Kepler space telescope show small, frequent, non-periodic dips in brightness, along with two large recorded dips in brightness roughly 750 days apart. The amplitude of the changes in the star's brightness, and the aperiodicity of the changes, mean that this star is of particular interest for astronomers. The star's changes in brightness are consistent with many small masses orbiting the star in "tight formation".
The first major dip, on 5 March 2011, reduced the star's brightness by up to 15%, and the next 726 days later (on 28 February 2013) by up to 22%. (A third dimming, around 8%, occurred 48 days later.) In comparison, a planet the size of Jupiter would only obscure a star of this size by 1%, indicating that whatever is blocking light during the star's major dips is not a planet, but rather something covering up to half the width of the star. Due to the failure of two of Kepler's reaction wheels, the star's predicted 750-day dip around February 2015 was not recorded. The light dips do not exhibit an obvious pattern.
In addition to the day-long dimmings, a study of a century's worth of photographic plates suggests that the star has gradually faded from 1890 to 1989 by about 20%, which would be unprecedented for any F-type main-sequence star. However, teasing accurate magnitudes from long-term photographic archives is a complex procedure, requiring adjustment for equipment changes, and is strongly dependent on the choice of comparison stars. Another study, examining the same photographic plates, concluded that the possible century-long dimming was likely a data artifact, and not a real astrophysical event. Another study from plates between 1895 – 1995 found strong evidence that the star has not dimmed, but kept a constant flux within a few percent, except a 8% dip on 24 Oct 1978, resulting in a period of the putative occulter of 738 days. 
A third study, using light measurements by the Kepler observatory over a four-year period, determined that KIC 8462852 dimmed at about 0.34% per year before dimming more rapidly by about 2.5% in 200 days. It then returned to its previous slow fade rate. The same technique was used to study 193 stars in its vicinity and 355 stars similar in size and composition to KIC 8462852. None of these stars exhibited such dimming.
Light curve galleryEdit
Based on KIC 8462852's spectrum and stellar type, its changes in brightness could not be attributed to intrinsic variability; while a few hypotheses have been proposed involving material orbiting the star and blocking its light, none of these fully fit the observed data.
Some of the proposed explanations involve instrument or data artifacts, interstellar dust, a series of giant planets with very large ring structures, a recently captured asteroid field, the system undergoing Late Heavy Bombardment, and an artificial megastructure orbiting the star.
Younger star with coalescing material around itEdit
Astronomer Jason Wright and others who have studied KIC 8462852 have suggested that if the star is younger than its position and speed would suggest, then it may still have coalescing material around it.
A 0.8–4.2-micrometer spectroscopic study of the system using the NASA Infrared Telescope Facility (NASA IRTF) found no evidence for coalescing material within a few astronomical units of the mature central star.
Planetary debris fieldEdit
High-resolution spectroscopy and imaging observations have also been made, as well as spectral energy distribution analyses using the Nordic Optical Telescope in Spain. A massive collision scenario would create warm dust that glows in infrared wavelengths, but there is no observed excess infrared energy, ruling out massive planetary collision debris. Other researchers think the planetary debris field explanation is unlikely, given the very low probability that Kepler would ever witness such an event due to the rarity of collisions of such size.
As with the possibility of coalescing material around the star, spectroscopic studies using the NASA IRTF found no evidence for hot close-in dust or circumstellar matter from an evaporating or exploding planet within a few astronomical units of the central star. Similarly, a study of past infrared data from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer found no evidence for an excess of infrared emission from the star, which would have been an indicator of warm dust grains that could have come from catastrophic collisions of meteors or planets in the system. This absence of emission supports the hypothesis that a swarm of cold comets on an unusually eccentric orbit could be responsible for the star's unique light curve, but more studies are needed.
A cloud of disintegrating cometsEdit
One proposed explanation for the reduction in light is that it is due to a cloud of disintegrating comets orbiting the star elliptically. This scenario would assume that KIC 8462852's planetary system has something similar to the Oort cloud and that gravity from a nearby star caused comets from said cloud to fall closer into system, thereby obstructing KIC 8462852's spectra. Evidence supporting this hypothesis includes an M-type red dwarf within 132 billion kilometers (885 AU) of KIC 8462852. However, the notion that disturbed comets from such a cloud could exist in high enough numbers to obscure 22% of the star's observed luminosity has been doubted.
Submillimetre-wavelength observations searching for farther-out cold dust in an asteroid belt akin to the Sun's Kuiper Belt suggest that a distant "catastrophic" planetary disruption explanation is unlikely; the possibility of a disrupted asteroid belt scattering comets into the inner system is still to be determined.
An artificial megastructureEdit
Astronomers have hypothesized that the objects eclipsing KIC 8462852 could be parts of a megastructure made by an alien civilization, such as a Dyson swarm, a hypothetical structure that an advanced civilization might build around a star to intercept some of its light for their energy needs. According to Steinn Sigurðsson, the megastructure hypothesis is implausible and disfavored by Occam's razor and fails to sufficiently explain the dimming. However, he says that it remains a valid subject for scientific investigation because it is a falsifiable hypothesis. Due to extensive media coverage on this matter, KIC 8462852 has been compared by Kepler's Steve Howell to KIC 4150611, another star with an odd light curve (which proved, after years of research, to be a part of a five-star system). The likelihood of extraterrestrial intelligence being the cause of the dimming is very low; however, the star remains an outstanding SETI target because natural explanations have yet to fully explain the dimming phenomenon.
Consumption of a planetEdit
In December 2016 a team of researchers proposed that KIC 8462852 swallowed a planet, causing a temporary and unobserved increase in brightness due to the release of gravitational energy. Planetary debris still in orbit around the star would then explain its observed drops in intensity.
Nonequilibrium chaotic variations due to near-criticalityEdit
"Avalanche statistics" with a self-similar or power-law spectrum are a universal property of complex dynamical systems operating close to a phase transition or bifurcation point between two different types of dynamical behavior. Such close-to-critical systems are often observed to exhibit behavior that is intermediate between "order" and "chaos". Three other stars in the Kepler Input Catalog likewise exhibit similar "avalanche statistics" in their brightness variations, and all three are known to be magnetically active. It has been conjectured that stellar magnetism may be involved in KIC 8462852.
Large ringed planet followed by Trojan swarmsEdit
Ballesteros et al. (2017) proposed a large, ringed planet trailed by a swarm of Trojan asteroids in its L5 Lagrangian point, and estimated an orbit that predicts another event in early 2021 due to the leading Trojans followed by another transit of the hypothetical planet in 2023. The model suggests a planet with a radius of 4.7 Jupiter radii, large for a planet (unless very young). An early Red dwarf of about 0.5 R☉ would be easily seen in infrared. The current radial velocity observations available (four runs at σv ≈ 400 m/s) hardly constrain the model, but new radial velocity measurements would greatly reduce the uncertainty. The model predicts a discrete and short-lived event for the May 2017 dimming episode, corresponding to the secondary eclipse of the planet passing behind KIC 8246852, with about a 3% decrease in the stellar flux with a transit time of about 2 days. If this is the cause of the May 2017 event, the planet's orbital period is more precisely estimated as 12.41 years with a semi-major axis of 5.9 AU.
Large planet with oscillating ringsEdit
Circumstellar dust ringEdit
Meng et al. (2017) suggested that, based on observational data from the Swift and Spitzer space telescope, but without reference to high-resolution spectroscopic data collected during the 2017 dimmings, only "microscopic fine-dust screens", originating from "circumstellar material", are able to disperse the starlight in the way detected in their measurements.
A conglomeration of magnetic activity, e.g. sunspots, differential rotation, occasional changes in the distribution of the photosphere, and simply random variation in convective efficiency, have also been postulated. But given that no other star has been observed with such a light curve, and that Tabby's Star is at the hot end of the stars that Kepler observes, it may be that Tabby's star is approaching the end of its convective lifetime, an example of selection bias, or both.
Initial follow-up studiesEdit
In the aggregate numerous optical telescopes continually monitor KIC 8462852 in anticipation of another multi-day dimming event, with planned follow-up observations of a dimming event using large telescopes equipped with spectrographs to determine if the eclipsing mass is a solid object, or composed of dust or gas. Additional follow-up observations may involve the ground-based Green Bank Telescope, the Very Large Array Radio Telescope, and future orbital telescopes dedicated to exoplanetology such as WFIRST, TESS, and PLATO.
A Kickstarter fund-raising campaign was led by Tabetha Boyajian, the lead author of the initial study on KIC 8462852's anomalous light curve. The project proposes to use the Las Cumbres Observatory Global Telescope Network for continuous monitoring of the star. The campaign raised over US$100,000, enough for one year of telescope time. Furthermore, more than fifty amateur astronomers working under the aegis of the American Association of Variable Star Observers have been providing effectively full coverage since AAVSO's alert about the star in October 2015, namely a nearly continuous photometric record.
In October 2015, the SETI Institute used the Allen Telescope Array to look for radio emissions from possible intelligent extraterrestrial life in the vicinity of the star. After an initial two-week survey, the SETI Institute reported that it found no evidence of technology-related radio signals from the star system. No narrowband radio signals were found at a level of 180–300 Jy in a 1 Hz channel, or medium-band signals above 10 Jy in a 100 kHz channel.
In 2016, the VERITAS gamma-ray observatory was used to search for ultra-fast optical transients from astronomical objects, with astronomers developing an efficient method sensitive to nanosecond pulses with fluxes as low as about one photon per square meter. This technique was applied on archival observations of KIC 8462852 from 2009 to 2015, but no emissions were detected.
In September 2017, some SETI@Home workunits were created based on a previous RF survey of the region around this star. This was coupled with a doubling in the size of SETI@Home workunits, so the workunits related to this region will probably be the first workunits to have less issues with quantization noise.
A star called EPIC 204278916, as well as some other young stellar objects, have been observed to exhibit dips similar to those observed in KIC 8462852. They differ in several aspects, however. EPIC 204278916 shows much deeper dips than KIC 8462852, and they are grouped over a shorter period, whereas the dips at KIC 8462852 are spread out over several years. Furthermore, EPIC 204278916 is surrounded by a proto-stellar disc, whereas KIC 8462852 appears to be a normal F-type star displaying no evidence of a disc.
2017 dimming eventsEdit
On 20 May 2017, Boyajian and her colleagues reported, via The Astronomer's Telegram, on an ongoing dimming event (named, "Elsie") which possibly began on 14 May 2017; it was detected by the Las Cumbres Observatory Global Telescope Network, specifically by its telescope located in Maui (LCO Maui); this was verified by the Fairborn Observatory (part of the N2K Consortium) in Southern Arizona (and later by LCO Canary Islands). Further optical and infrared spectroscopy and photometry were urgently requested, given the short duration, measured in days or weeks, of these events. Observations from multiple observers globally were coordinated, including polarimetry. Furthermore, the independent SETI projects Breakthrough Listen and Near-InfraRed Optical SETI (NIROSETI), both at Lick Observatory, continue to monitor the star. By the end of the three-day dimming event, a dozen observatories had taken spectra, with some astronomers having dropped their own projects to provide telescope time and resources. More generally the astronomical community was described as having gone "mildly bananas" over the opportunity to collect data in real-time on the unique star, and the 2% dip itself was named "Elsie" (in reference to Las Cumbres and light curve).
Initial spectra with FRODOSpec at the two-meter Liverpool Telescope showed no evidence of any changes visible between a reference spectrum and this dip. Several observatories, however, including the twin Keck telescopes (HIRES) and numerous citizen science observatories, took spectra of the star. This dimming dip had a complex shape, and initially had a pattern similar to the one at 759.75 days from the Kepler event 2, epoch 2 data. Observations were taken across the electromagnetic spectrum.
Evidence of a second dimming event (named, "Celeste") was observed on 13–14 June 2017, and which possibly began 11 June, by amateur astronomer Bruce Gary. While the light curve on 14 and 15 June indicated a possible recovery from the dimming event, the dimming continued to increase afterwards, and, on 16 June, Boyajian wrote that the event was approaching a 2% dip in brightness.
A fourth prominent dimming event (named, "Angkor") began 05 September 2017, and is, as of 16 September 2017, a 2.3% (or a 3%) dimming event, making it the "deepest dip this year".
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|Wikimedia Commons has media related to KIC 8462852.|
- Kepler light curve data at STScI.edu
- on YouTube, a TED Talk by Tabetha Boyajian
- Spectra (6400-6700 Å) from 20 May 2017, by FRODOSpec/Liverpool Telescope
- Spectra (8400-8700 Å) from 20 May 2017, by FRODOSpec/Liverpool Telescope
- Spectra (3926-3994 Å) from 21 May 2017, by Keck Observatory