Power Beaming Parameters & SETI re KIC 8462852

When I first got interested in SETI, I naively assumed that we would get a detection fairly soon, and that we would detect not a directed beacon but simple background traffic in a remote civilization. I had no idea at the time how difficult it would be to pick up the kind of radio traffic we routinely generate on Earth from a distant star, and as a matter of fact, my interest in shortwave radio led me to assume that, just as I enjoyed the sport of DX — listening for distant signals — so SETI would simply be an offshoot of this, with a harder-to-get QSL card.

Some time in the mid-1980s I wrote a piece called “Where the Real DX Is” for Glenn Hauser’s Review of International Broadcasting, running through a list of the nearest stars and talking about SETI projects that had been tried up to then. I haven’t gone back to read that article in years and would probably find it an embarrassing chore. But it’s interesting to me that the idea of leakage radiation does have its place, particularly in the area of beamed power.

Power Beaming at Work

What I knew little of back in the 80’s was how beamed power — using high-power microwave beams but including millimeter-wave beams and lasers — might empower a space-based infrastructure. While television and radio signals are all but undetectable from another star (at least, for a civilization at our level of development), intense, focused beams to impart power to a spacecraft present a better target. The interesting question that James Benford (Microwave Sciences) and son Dominic (NASA GSFC) ask in the paper we began looking at yesterday is just how detectable power beaming would be.

fwdlasersail

This takes off on ideas Jim Benford discussed last year in these pages (see Seeing Alien Power Beaming), and both Jim and Dominic recently looked at the question in relation to SETI efforts on a very intriguing star indeed in Quantifying KIC 8462852 Power Beaming. After all, if a distant civilization were building something like a Dyson sphere or ‘swarm’ around a star, it would have to have constructed an infrastructure that might well involve power beaming technologies.

Image: A favorite image from a favorite artist, Rick Sternbach. Here we’re looking at an interstellar lightsail near an exotic world. The uses of beamed power in near and deep space are numerous, and many of them appear to be detectable from a SETI perspective. The image comes from a 1983 issue of Science Digest. Credit: Copyright Rick Sternbach.

The Benford paper runs through a number of beaming concepts ranging from orbit raising, a lower power application, to actual launch to orbit using microwave thermal thrusters. Various high-power beaming applications have been quantified for missions within the Solar System as well, using microwave or laser beams to boost craft to 100 to 200 kilometers per second. A mature infrastructure might involve high-speed unmanned supply craft being decelerated upon arrival by a beam system similar to the one that launched it. I mentioned yesterday that beaming from a solar power station in space to a planetary surface is not a likely observable. Here’s why, whether we’re talking about microwave bands or lasers in the optical range:

The beam must be carefully controlled to deliver power to the receiving rectifying antennas on the ground (Mankins 2014). Any side emissions are economic losses, therefore substantial measures would be taken to reduce side lobes to a minimum. Further, the first several sidelobes are absorbed in the ground. The remaining side lobes are dispersed in angle so that the power density in the far field will be very low. For the worked example in Mankins (Mankins 2014; Dickinson 2016), the back lobe is down 40 dB relative to the ? 1GW main beam.

And note this:

This is in contrast to power beaming transportation applications, in which the varying solid angle of the receiving spacecraft results in the main beam increasingly leaking around the edges of the vehicle being accelerated.

So that takes detection of one of the numerous power beaming scenarios off the table, but many others survive scrutiny. Have a look at this table from the paper, which draws on representative parameters for applications of power beaming presented earlier in the discussion.

Screenshot from 2016-02-25 11:15:46

Here we’re looking at observables in terms of slew rate (movement of the beam), the EIRP and the observation time. EIRP stands for Effective Isotropic Radiated Power, the measured radiated power in a single direction — it’s the product of the antenna gain times the power radiated. For useful background, the standard text is Jim Benford’s own High Power Microwaves (now in its third edition, CRC Press, 2016). A quick note on EIRP from the paper:

Spectral flux density, typically denoted in Janskys, is the power density divided by the bandwidth. While this is commonly used as the observed quantity in radio astronomy, we cannot know the bandwidth of an ETI transmitter. Consequently, in thinking about ETI power beaming mission [sic] we must deal with EIRP, not spectral flux density. Beaming power does not require or even necessarily benefit from narrow bandwidth; energy transference is what matters.

Ranking the Observables

The possibilities are numerous, and not all of them designed for deep space. Microwave thermal thrusters, for example, can be fed by a high-power microwave beam in a single-stage rocket with a flat aeroshell on the underside covered by microwave-absorbing heat exchangers. The system is efficient because the energy source remains on the ground — see the paper for further parameters on this as well as orbit raising, where ground-based energy sources lift a satellite to a higher orbit. As the table shows, orbit raising is significantly lower in power than launch from the surface.

But of course beaming applications deeper into the system are a part of a robust infrastructure based on beaming, and here we can look at interplanetary transfers by beam-driven sails, as well as starship concepts involving prolonged acceleration of an interstellar vehicle, like the Robert Forward concepts I mentioned yesterday. Given the power requirements for starships, such beams would be detectable with our current technology, but this would also require that the Earth fall within the bandwidth of the power beam.

These questions of detectability depend, of course, upon the level of technology of the civilization trying to make the detection. In our case, we have the interesting story of the star KIC 8462852 to give us some guidance. We have radio observations of this curious star that found no signs of power beaming in about 180 hours of observations. The idea was to look for incidental radiated power — leakage radiation — produced by an advanced civilization. The observations were made using the Allen Telescope Array in the 1-10 GHz range.

Refer back to the table above and the beaming applications there. These are the authors’ conclusions re the KIC 8462852 observations using the Allen Telescope Array:

  • The 1 Hz channels could see all the applications, but they are not seen.
  • Launch from a planetary surface into orbits is marginally detectable, at the threshold of the Allen Array for the 100 kHz observations, if at the frequencies observed. Orbit raising, which requires lower power, is not detectable.
  • Interplanetary transfers by beam-driven sails should be detectable in their observations, but are not seen. This is for both the 1 Hz and for the 100 kHz observations.
  • Starships launched by power beams with beamwidths that we happen to fall within (to other solar systems, not our own) would be detectable, but are not seen.

We also learn that the limited observation times and wavelength coverage mean that we have no firm conclusions about the star. More systematic studies, which would include observations at higher radio frequencies, would be called for to come up with a stronger result. I should also mention that an optical SETI attempt was made on KIC 8462852 at the Boquete Optical SETI Observatory in Panama. Given the detection limits of the equipment there and other issues discussed in the paper, none of the power beaming applications in consideration by the Benfords could have been detected at the Boquete site.

The authors’ conclusion:

*As discussed above, the beaming power levels are high and transient and easily dwarf any ETI civilization’s diffuse leakage to space (Sullivan 1978). Power beaming described here is larger than that necessary for beaming systems for communication: EIRP = 1018 W for a 1,000ly-range beacon (Benford, Benford & Benford 2010). SETI programs could explore a different part of parameter space by observations suitable to finding leakage from power beams.

We have no way of knowing whether any civilization exists outside of our own, nor can we know if such a civilization would use power beaming. But the virtues of beamed power for fast transportation within our own Solar System — and for lowering the cost to orbit, as well as raising orbits — give us good reason to continue to explore the use of this technology. In doing so, we should keep in mind that we are producing an observable signal that is well beyond those undetectable “I Love Lucy” episodes that are now 65 light years out. I think the Benfords are right when they argue that detectable radiation is a message in itself, whether or not we ever decide to impose a structured message into the beams that power a future infrastructure.

The paper is Benford & Benford, “Power Beaming Leakage Radiation as a SETI Observable,” submitted to The Astrophysical Journal and available as a preprint.

tzf_img_post

KIC 8462852: No Dimming After All?

As if the Kepler star KIC 8462852 weren’t interesting enough, Bradley Schaefer (Louisiana State) added to the controversy when he discovered what appeared to be a steady dimming of the star over the past century. Schaefer called the result “completely unprecedented for any F-type main sequence star,” and given the discussion about KIC 8462852 as a SETI target, this raised the stakes. Something just as odd as the object’s strange lightcurves was going on here, and it seemed natural to think that the dimming and the lightcurves were related.

But Michael Hippke now begs to disagree. An old friend of Centauri Dreams (see, for example, his Exomoons: A Data Search for the Orbital Sampling Effect and the Scatter Peak), Hippke takes a close look at Schaefer’s work and reaches a different conclusion. As he sees it, the ‘dimming’ of up 0.165 ± 0.013 magnitudes per century in this F3 star may actually be the result of imperfect calibration on the Harvard plates. In other words, while the lightcurve anomalies remain, the dimming may well be a data artifact rather than an astrophysical enigma.

KIC8462852EfraSAC

Image: KIC 8462852 as photographed from Aguadilla, Puerto Rico by Efraín Morales, of the Astronomical Society of the Caribbean (SAC).

First, though, a word about Bradley Schaefer’s work, about which Hippke says “Schaefer had the excellent idea to look into the old plate archives. To solve this mystery, we need all the information we can get, and Schaefer did very careful and high-quality work.”

This parallels comments I’ve heard from other professionals, who praise the quality of Schaefer’s analysis. Submitted to Astrophysical Journal Letters, the Hippke paper looks to contrast the ‘dimming’ of KIC 8462852 with an analysis of other F-type main sequence stars from the same dataset. Along the way, Hippke double-checks Schaefer and finds sound work:

Although the process of data cleansing and binning involves arbitrary choices, we have reproduced this part of the analysis for all variants with virtually identical results. It is therefore important to note that the method and results in Schaefer (2016) appear to be adequately careful and accurate. In the following, we will thus concentrate solely on the interpretation of his result – whether the dimming is “unprecedented”.

Take away its odd lightcurves and KIC 8462852 appears to be a relatively normal star. Thus Hippke’s criteria for study are F-stars from the Kepler field of view, from which photometry is studied for the 3 most quiet F-dwarfs and 25 bright F-dwarfs in the Harvard DASCH (Digital Access to a Sky Century @Harvard) archive. Trends in the data may not, Hippke believes, be slow drifts but ‘structural breaks’ — in other words, changes caused by abrupt changes in technology or calibration techniques. Evidence for this occurs not only for KIC 8462852 but also for KIC 7180968, indicating we are dealing with a phenomenon not isolated to KIC 8462852.

From the paper:

The significant trends (and/or structural breaks) found in 18 of 28 comparison stars support the interpretation that the dimming of KIC 8462852 is not extraordinary. A careful analysis of each dataset is time-consuming, which is why we have not performed this analysis for hundreds of stars. In case of further doubt on the significance of such trends, the analysis presented could simply be expanded to more stars.

This would make an astrophysical interpretation of the ‘dimming’ unlikely because it would require that a number of main-sequence F-dwarfs fluctuate by 10% or more over the course of a century. “It seems more likely,” writes Hippke, “that the change of emulsions, errors in calibration etc. cause these trends.” In an email just received, Hippke notes of Schaefer’s work that “It might just be that his check stars were unusually stable, which obfuscated existing trends in the data.”

Thus the paper favors the notion that changes in technology and imperfect calibration — quality issues in the dataset itself — explain what otherwise appears to be long-term dimming of KIC 8462852. This leaves us, as the author notes, with the short-term dimmings found in KIC 8462852’s lightcurves, a problem that the question of century-long dimming does not address.

What can be done to investigate the dimming issue further? Hippke’s email suggests that other data, particularly plates from the Sonneberg Observatory in Germany, will be useful for comparison. “Unfortunately, these plates are not available online and have only partially been scanned, so checking these data might take several months.”

The paper is Hippke, “KIC 8462852 did likely not fade during the last 100 years,” submitted to the Astrophysical Journal Letters (preprint). Bradley Schaefer strongly disputes Hippke’s work, so we haven’t heard the end of this.

tzf_img_post

KIC 8462852: A Century Long Fade?

I hadn’t expected a new paper on KIC 8462852 quite this fast, but hard on the heels of yesterday’s article on the star comes “KIC 8462852 Faded at an Average Rate of 0.165±0.013 Magnitudes Per Century From 1890 To 1989,” from Bradley Schaefer (Louisiana State University). Schaefer takes a hard look at this F3 main sequence star in the original Kepler field not only via the Kepler data but by using a collection of roughly 500,000 sky photographs in the archives of Harvard College Observatory, covering the period from 1890 to 1989.

The Harvard collection is vast, but Schaefer could take advantage of a program called Digital Access to a Sky Century@Harvard (DASCH), which has currently digitized about 15 percent of the archives. Fortunately for us, this 15 percent covers all the plates containing the Cygnus/Lyra starfield, which is what the Kepler instrument focused on. Some 1581 of these plates cover the region of sky where KIC 8462852 is found. What Schaefer discovers is a secular dimming at an average rate of 0.165±0.013 magnitudes per century. For the period in question, ending in the late 1980s, KIC 8462852 has faded by 0.193±0.030 mag. From the paper:

The KIC 8462852 light curve from 1890 to 1989 shows a highly significant secular trend in fading over 100 years, with this being completely unprecedented for any F-type main sequence star. Such stars should be very stable in brightness, with evolution making for changes only on time scales of many millions of years. So the Harvard data alone prove that KIC 8462852 has unique and large-amplitude photometric variations.

That’s useful information, especially given the possible objection to the Kepler findings that they might be traceable to a problem with the Kepler spacecraft itself. Evidently not:

Previously, the only evidence that KIC 8462852 was unusual in any way was a few dips in magnitude as observed by one satellite, so inevitably we have to wonder whether the whole story is just some problem with Kepler. Boyajian et al. (2015) had already made a convincing case that the dips were not caused by any data or analysis artifacts, and their case is strong. Nevertheless, it is comforting to know from two independent sources that KIC 8462852 is displaying unique and inexplicable photometric variations.

As Schaefer notes, KIC 8462852 can now be seen to show two unique episodes involving dimming — the dips described here yesterday for the Kepler spacecraft, and the fading in the Harvard data. The assumption that both come from the same cause is reasonable, as it would be hard to see how the same star could experience two distinct mechanisms that make its starlight dim by amounts like these. The timescales of the dimming obviously vary, and the assumption would be that if the day-long dips are caused by circumstellar dust, then the much longer fading that Schaefer has detected would be caused by the same mechanism.

KIC8462852EfraSAC

Image: KIC 8462852 as photographed from Aguadilla, Puerto Rico by Efraín Morales, of the Astronomical Society of the Caribbean (SAC).

Thus we come to the comet hypothesis as a way of explaining the KIC 8462852 light curves. Incorporating the fading Schaefer has discovered, a cometary solution would require some mind-boggling numbers, as derived in the paper. From the summary:

With 36 giant-comets required to make the one 20% Kepler dip, and all of these along one orbit, we would need 648,000 giant-comets to create the century-long fading. For these 200 km diameter giant-comets having a density of 1 gm cm?3, each will have a mass of 4 × 1021 gm, and the total will have a mass of 0.4 M?. This can be compared to the largest known comet in our own Solar System (Comet Hale-Bopp) with a diameter of 60 km. This can also be compared to the entire mass of the Kuiper Belt at around 0.1 M? (Gladman et al. 2001). I do not see how it is possible for something like 648,000 giant-comets to exist around one star, nor to have their orbits orchestrated so as to all pass in front of the star within the last century. So I take this century-long dimming as a strong argument against the comet-family hypothesis to explain the Kepler dips.

If Schaefer’s work holds up, the cometary hypothesis to explain KIC 8462852 is deeply compromised. We seem to be looking at the author calls “an ongoing process with continuous effects” around the star. Moreover, it is a process that requires 104 to 107 times as much dust as would be required for the deepest of the Kepler light dips. And you can see in the quotation above Schaefer’s estimate for the number of giant comets this would require, all of them having to pass in front of the star in the last century.

The paper is Schaefer, “KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989,” submitted to Astrophysical Journal Letters (abstract).

tzf_img_post

Following Up KIC 8462852

As I sat down to write yesterday morning, I realized there was a natural segue between the 1977 ‘Wow!’ signal, and the idea that it had been caused by two comets, and KIC 8462852, the enigmatic star that has produced such an interesting series of light curves. What I had planned to start with today was: “Are comets becoming the explanation du jour for SETI?” But Centauri Dreams reader H. Floyd beat me to the punch, commenting yesterday: “Comets are quickly earning the David Drumlin Award for biggest SETI buzzkill.”

As played by Tom Skeritt, David Drumlin is Ellie Arroway’s nemesis in the film Contact, willing to knock down the very notion of SETI and then, in a startling bit of reverse engineering, turning into its champion as he claims credit for a SETI detection. And of course you remember controversial KIC 8462852 as the subject of numerous media stories first playing up the idea of alien mega-engineering, and then as quickly declaring the problem solved by a disrupted family of comets that moved between us and the star in their orbit.

boyajian

But KIC 8462852 doesn’t yield to instant analysis, and it’s good to see a more measured piece now appearing on New York University’s ScienceLine site. The title, Tabby’s Mystery, is a nod to Tabetha Boyajian, a postdoc at Yale University who noticed that the dips in light in Kepler data from the star were unusual. We have the Planet Hunters group to thank for putting Boyajian on the case, and a productive one it has turned out to be. As writer Sandy Ong notes, KIC 8462852 produced non-periodic dips in the star’s light that in one case reached 15 percent, and in another 22 percent.

Image: Yale’s Tabetha Boyajian, whose work examines possible causes for the unusual light curves detected at KIC 8462852.

‘Tabby’s star’ is one name KIC 8462852 has acquired, the other being the ‘WTF star’, doubtless standing for ‘where’s the flux,’ given the erratic changes to the light from the object. I’ve written a number of articles on this F3-class star and its light curves, noting not only the size of the two largest dips but also the fact that the dips, unlike those of a transiting planet, are not at all symmetric. Ong quotes Boyajian as saying: “The first one is a single dip that shows a very gradual decrease in brightness, then a sharp increase… The second dip has more structure to it with lots of ups and downs.” For more, see KIC 8462852: Cometary Origin of an Unusual Light Curve? and search the archives here, where five or six other articles on the matter are available.

Comets come into the mix because in Boyajian’s paper on KIC 8462852, they are named as a possibility. To refresh all our memories, let’s go back to the paper:

…we could be seeing material close to the pericenter of a highly eccentric orbit, reminiscent of comets seen in the inner Solar System at pericenter. We therefore envision a scenario in which the dimming events are caused by the passage of a series of chunks of a broken-up comet. These would have to have since spread around the orbit, and may be continuing to fragment to cause the erratic nature of the observed dips.

Comets moving close to the parent star would be given to thermal stresses, and could also be disrupted by close encounters with planets in the inner system. For that matter, tidal disruption by the star itself is a possibility. Boyajian and co-authors point out that a comet like Halley would break apart because of tidal forces if approaching as close as 0.02 to 0.05 AU.

We have WISE data from 2010 showing us that KIC 8462852 lacks the infrared signature a debris disk should produce. But the unusual light curves from Kepler began in the spring of 2011, so for a brief window between the two, there was the possibility of a planetary catastrophe, perhaps a collision between a planet and an asteroid, that would explain what Kepler saw. But Spitzer Space Telescope observations in 2015, analyzed by Massimo Marengo (Iowa State) and colleagues, found no trace of infrared excess at the later date, which seems to rule out a collision between large bodies and leaves the hypothesis of a family of comets still intact. See No Catastrophic Collision at KIC 8462852 for a discussion of Marengo’s work.

Screenshot-from-2015-11-30-090106

Image: Montage of flux time series for KIC 8462852 showing different portions of the 4-year Kepler observations with different vertical scalings. Panel ‘(c)’ is a blowup of the dip near day 793, (D800). The remaining three panels, ‘(d)’, ‘(e)’, and ‘(f)’, explore the dips which occur during the 90-day interval from day 1490 to day 1580 (D1500). Credit: Boyajian et al., 2015.

Perhaps we’re seeing a natural phenomenon we can’t yet identify. Ong cites Eric Korpela (UC-Berkeley) on the matter:

“I like the comet explanation although ‘comet’ might not be the right word,” says Eric Korpela, another astronomer from the Berkeley SETI Research Center. That’s because the core of such an object would have to be as large as Pluto in order to generate this kind of light, he explains.

Korpela and other astronomers believe the dimming may be due to some kind of natural phenomenon we haven’t yet seen anywhere in the universe. “We just haven’t looked at enough stars to know what’s out there,” he says.

What we saw yesterday with relation to the ‘Wow!’ signal is that we will soon have two chances to monitor the comets involved in Antonio Paris’ hypothesis for generating the signal. In like manner, we’ll have further observations of KIC 8462852. Ong notes that the Green Bank instrument in West Virginia is involved, as are the MINERVA array in Arizona, the MEarth project (Arizona and Chile), the LOFAR telescope in the Netherlands, and amateur observations from the American Association of Variable Star Observers, who will bring their own instruments to bear.

Oh to have a healthy Kepler in its original configuration returning new data on KIC 8462852! But despite the outstanding work being performed by the K2 ‘Second Light’ mission, the instrument is now working on different targets, and our ground-based telescopes have to do the job. What we need to know is if and when ‘Tabby’s star’ starts producing further light curves, and just what they look like. SETI observations have already been attempted using the Allen Telescope Array (see SETI: No Signal Detected from KIC 8462852) looking for interesting microwave emissions. None were found. Expect this enigmatic star to remain in the news for some time to come.

The Boyajian paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the Flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint). The Marengo paper is Marengo et al., “KIC 8462852 – The Infrared Flux,” Astrophysical Journal Letters, Vol. 814, No. 1 (abstract / preprint). Jason Wright and colleagues discuss KIC 8462852 in the context of SETI signatures in Wright et al., “The ? Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” submitted to The Astrophysical Journal (preprint).

tzf_img_post

No Catastrophic Collision at KIC 8462852

Last week I mentioned that I wanted to get into Massimo Marengo’s new paper on KIC 8462852, the interesting star that, when studied by the Kepler instrument, revealed an intriguing light curve. I’ve written this object up numerous times now, so if you’re coming into the discussion for the first time, plug KIC 8462852 into the archive search engine to get up to speed. Marengo (Iowa State) is himself well represented in the archives. In fact, I began writing about him back in 2005, when he was working on planetary companions to Epsilon Eridani.

2009-08-20-00004-2

In the new paper, Marengo moves the ball forward in our quest to understand why the star I’ll abbreviate as KIC 8462 poses such problems. The F3-class star doesn’t give us the infrared signature we’d expect from a debris disk, yet the light curves we see suggest objects of various sizes (and shapes) transiting across its surface. What we lacked from Tabetha Boyajian’s earlier paper (and it was Boyajian, working with the Planet Hunters group, that brought KIC 8462 to our attention) was data about infrared wavelengths after the WISE mission finished its work.

That was a significant omission, because the WISE data on the star were taken in 2010, while the first events Kepler flagged at KIC 8462 occurred in March of 2011, with a long series of events beginning in February of 2013 and lasting sixty days. That gave us a small window in which something could have happened — the idea of a planetary catastrophe comes to mind, perhaps even a collision between two planets, or a planet and large asteroid. What Marengo brings to the table are observations from the Spitzer Space Telescope dating from early 2015.

Image: Astrophysicist Massimo Marengo. Credit: Iowa State University.

We learn that the Spitzer photometry from its Infrared Array Camera (IRAC) finds no strong infrared excess — no significant amount of circumstellar dust can be detected two years after the 2013 dimming event at KIC 8462. Here is what Marengo concludes from this:

The absence of strong infrared excess at the time of the IRAC observations (after the dimming events) implied by our 4.5 µm 3? limit suggests that the phenomenon observed by Kepler produced a very small amount of dust. Alternatively, if significant quantity of dust is present, it must be located at large distance from the star.

This seems to preclude catastrophic scenarios, while leaving a cometary solution intact. The paper continues:

As noted by B15 [this is the Boyajian paper], this makes the scenarios very unlikely in which the dimming events are caused by a catastrophic collision in KIC 8462852 asteroid belt, a giant impact disrupting a planet in the system, or a population of dust-enshrouded planetesimals. All these scenarios would produce very large amount of dust dispersed along the orbits of the debris, resulting in more mid-IR emission than what can be inferred from the optical depth of the dust seen passing along our line of sight to the star. Our limit (two times lower than the limit based on WISE data) further reduces the odds for these scenarios.

Screenshot from 2015-11-30 09:01:06

Image: Montage of flux time series for KIC 8462852 showing different portions of the 4-year Kepler observations with different vertical scalings. Panel ‘(c)’ is a blowup of the dip near day 793, (D800). The remaining three panels, ‘(d)’, ‘(e)’, and ‘(f)’, explore the dips which occur during the 90-day interval from day 1490 to day 1580 (D1500). Credit: Boyajian et al., 2015.

Tabetha Boyajian’s paper analyzed the natural phenomena that could account for KIC 8462’s light curve and concluded that a family of exocomets was the most promising explanation. Here the idea is that we have a family of comets in a highly elliptical orbit that has moved between us and the star, an idea that would be consistent with the lack of a strong infrared signature. Marengo has reached the same conclusion now that we are able to discount the idea of a large collision within the system. Both Boyajian and Marengo favor the comet hypothesis because it does not require a circular orbit and allows associated dust to quickly move away from the star.

In Marengo’s analysis, this fits the data, as the two-year gap between the Kepler light curves and the observations from Spitzer provide enough time for cometary debris to move several AU from the zone of tidal destruction from the star. The paper adds:

At such a distance, the thermal emission from the dust would be peaked at longer wavelengths and undetectable by IRAC. A robust detection at longer wavelengths (where the fractional brightness of the debris with respect to the star would be more favorable) will allow the determination of the distance of the cometary fragments and constrain the geometry of this scenario.

So we have a way to proceed here. Marengo notes that the measurements his paper presents cannot reveal the temperature or the luminosity of the dust that would be associated with such a family of comets, but long-term infrared monitoring would allow us to constrain both. The other day I also mentioned the small red dwarf (about 850 AU out) that could be the cause of instabilities in any Oort Cloud-like collection of comets around KIC 8462. Boyajian’s paper makes the case for measuring the motion or possible orbit (if bound) of this star as a way to tighten predictions on the timescale and repeatability of any associated comet showers.

Marengo dismisses SETI study of KIC 8462, with specific reference to Jason Wright’s recent paper on the matter, as “wild speculations,” an unfortunate phrase because Wright’s shrewd and analytical discussion of these matters has been anything but ‘wild.’

The Marengo paper is Marengo, Hulsebus and Willis, “KIC 8462852 – The Infrared Flux,” Astrophysical Journal Letters, Vol. 814, No. 1 (abstract / preprint). The Boyajian paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint). The Jason Wright paper that examines KIC 8462 in the context of SETI signatures is Wright et al., “The ? Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” submitted to The Astrophysical Journal (preprint).

tzf_img_post