Research into Boyajian’s Star, otherwise known as KIC 8462852 or ‘Tabby’s Star,’ has continued in robust fashion even as many of us were distracted by that other curiosity with a faint SETI potential, the interstellar asteroid `Oumuamua. In both cases, a highly interesting object provoked speculation as to its origins, with Boyajian’s Star getting the lion’s share of attention because the unusual dips in its lightcurve proved hard to explain.
Now a team of more than 200 researchers led by Tabetha Boyajian herself is drawing useful conclusions about the star. Also on the team is Penn State’s Jason Wright, whose interest in possible SETI signatures led him to point out that engineering on a vast scale could not immediately be ruled out. The paper now being made available in The Astrophysical Journal Letters shows that the star dims more at some wavelengths than at others.
And that is, to say the least, problematic for the idea that an artificial megastructure orbits Boyajian’s Star. The paper, titled “The First Post-Kepler Brightness Dips of KIC 8462852,” draws on data collected by Boyajian (Louisiana State) and colleagues as a result of a Kickstarter campaign in which some 1700 contributors donated money to observations through the Las Cumbres Observatory, a network of robotic telescopes with northern hemisphere sites in the Canary Islands and Hawaii. Follow-up data were acquired from a number of other instruments.
The observations ran from March 2016 to December 2017, with four main 1-2.5% dips, beginning in May of 2017 and named “Elsie,” “Celeste,” “Skara Brae,” and “Angkor,” persisting on timescales from several days to weeks. What analysis of these dips shows is not consistent with any solid structure around the F3-class Star. As Wright explains on his PSU site:
Eva Bodman has done a lot of work to characterize how much deeper the dips are at blue wavelengths than red ones. If there were opaque objects blocking our view of the light, the star should get equally dim at all wavelengths. Instead, Eva finds that the blue (B) dips are much deeper—about twice as deep—as they are when we look at infrared wavelengths (i’ band, just beyond human vision).
This is consistent with ordinary astrophysical dust, and a major conclusion of our paper: the dips are not caused by opaque macroscopic objects (like megastructures or planets or stars) but by clouds of very small particles of dust (less than 1 micron in typical size). We can also say that these clouds are mostly transparent (“optically thin” in astrophysics parlance).
Image: Analysis of LCO data by Eva Bodman.
This work marks the first real-time detection of a dip in brightness for this unusual star, and as the paper notes, “Triggered spectroscopic and polarmetric observations taken during the dips reveal no large, obvious changes compared to out of dip observations.”
The paper goes on to say:
Invoking dust still challenges our creativity in developing a unified theory to explain all the observations; however, the models of Wyatt et al. (2017) give hope to a swarm of yet unspecified objects in an eccentric orbit (in this case, exocomets, with an alternative being dust-enshrouded planetesimals as proposed by Neslušan & Budaj 2017) causing the brightness fluctuations. Continued monitoring to detect events in the future will help narrow down any periodicity within the dip occurrence, which would strengthen the argument that the source of the obscuring material was in orbit around the star, as opposed to density fluctuations in the ISM, etc.
It would have been exciting, to say the least, to find evidence for an artificial cause of Boyajian’s Star’s peculiarities, but I find this work exhilarating in its own right. What we have here is a highly publicized, privately funded investigation into an enigmatic phenomenon that now seems to be closer to a solution. That extraterrestrial engineering is not involved doesn’t diminish the power of the process, in which scientists examined an observational anomaly from all angles and counted an ETI hypothesis among the possibilities.
Bear in mind that using through the Kepler data on Boyajian’s Star alone would not have been sufficient because ground-based follow-up observations were not contemporaneous. That made the ability to summon up a crowd-funded campaign to observe with various instruments at differing sensitivities, resolutions and wavelengths an essential component in this result.
A friend asked not long after the Boyajian’s Star story broke whether I would be disappointed if it turned out to have a ‘boring natural cause.’ But that’s just it. I don’t find natural causes boring, especially when they push us to the limit to explain them. We still have a mystery here, because the original comet hypothesis — or the idea that some kind of circumstellar material is responsible — gains new life at the same time that regular dimming of the star itself — through mechanisms not yet understood — cannot be ruled out. Getting a handle on unusual astrophysical phenomena has a deep allure as we continue to learn about the cosmos.
Where next, then, with Boyajian’s Star? The paper concludes:
We emphasize the importance that continued monitoring will bring to our understanding of the physical processes responsible for the light curve features. In general, precise, long-term photometric monitoring to detect future dips is a level-zero requirement. These data also provide the means of informing planned triggered observations such as high-resolution spectroscopy to study the events in more detail. Furthermore, extended photometric monitoring will enable us to characterize the star’s long-term variability (Schaefer 2016; Montet & Simon 2016; Meng et al. 2017; Simon et al. 2017), which is thought to be linked to the dips in some way. All-in-all, the apparent low duty cycle of the dips, unclear predictions on when they will recur, and fairly unconstrained multiyear timescales of the long-term variability will require a committed, intensive monitoring program spanning the next decade and beyond.
The paper is Boyajian et al., “The First Post-Kepler Brightness Dips of KIC 8462852,” Astrophysical Journal Letters 2018 January 3 (preprint).
Those of you who have been following the controversy over the dimming of KIC 8462852 (Tabby’s Star) may remember an interesting note at the end of Bradley Schaefer’s last post on Centauri Dreams. Schaefer (Louisiana State University) had gone through his reasoning for finding a long-term dimming of the star in the DASCH (Digital Access to a Sky Century@Harvard) database. His third point about the star had to do with the work of Ben Montet (Caltech) and Joshua Simon (Carnegie Observatories).
Montet and Simon’s work relied on an interesting premise. Tabby’s Star had been discovered because it was in the Kepler field, and thus we had high-quality data on its behavior, the unusual light curves that the Planet Hunters team brought to the attention of Tabetha Boyajian. As the researchers note in a new paper, Kepler found ten significant dips in the light curve over the timespan of the Kepler mission, dips that were not only aperiodic but irregular in shape, and that varied enormously, from fractions of one percent up to 20% of the total flux of KIC 8462852.
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.
Schaefer noted in his Centauri Dreams post (see Further Thoughts on the Dimming of KIC 8462852) that if Tabby’s Star were actually fading at a rate of 0.164 mag/cen, then it should have undergone fading during the period it was under observation by Kepler (in fact, it should have faded by 0.0073 mag over the Kepler lifetime on the main Cygnus field). Montet and Simon have now presented us with their analysis in a paper just up on the arXiv server.
A fading of the kind Schaefer described would be well above the photometric precision of the Kepler instrument. Montet and Simon realized they could search for long-term trends by using the full-frame images (FFI) collected during the Kepler mission. Eight of these were recorded at the beginning of the mission, with another FFI recorded each month throughout the mission. Given that the mission lasted four years, a star dimming at the rate Schaefer suggests should decrease in brightness by 0.6% over the Kepler baseline. And as the authors point out, using FFI data avoids the removal of the dimming trend by the data processing pipeline.
The results: The study, which worked with KIC 8462852 and seven nearby comparison stars, found that in the first three years of the Kepler mission, Tabby’s Star dimmed at a rate of 0.341%±0.041% per year. Over the next six months, it decreased in brightness by 2.5%, and then stayed at that level during the duration of the primary Kepler mission. The paper continues:
We then compare this result to a similar analysis of other stars of similar brightness on the same detector, as well as stars with similar stellar properties, as listed in the KIC, in the Kepler field. We find that 0.5% of stars on the same detector and 0.7% of stars with similar stellar properties exhibit a long-term trend consistent with that observed for KIC 8462852 during the first three years of the Kepler mission. However, in no cases do we observe a flux decrement as extreme as the 2.5% dip observed in Quarters 12-14 of the mission. The total brightness change of KIC 8462852 is also larger than that of any other star we have identified in the Kepler images.
Image: Photometry of KIC 8462852 as measured from the FFI data. The four colors and shapes (green squares, black circles, red diamonds, and blue triangles) represent measurements from the four separate channels the starlight reaches as the telescope rolls. The four subpanels show flux from each particular detector individually. The main figure combines all observations together; we apply three linear offsets to the data from different channels to minimize the scatter to a linear fit to the first 1100 days of data. In all four channels, the photometry is consistent with a linear decrease in flux for the first three years of the mission, followed by a rapid decrease in flux of ≈ 2.5% over the next six months. The light gray curve represents one possible Kepler long cadence light curve consistent with the FFI photometry created by fitting a spline to the FFI photometry as described in Section 4. The large dips observed by Boyajian et al. (2016) are visible but narrow relative to the cadence of FFI observations. The long cadence data behind this figure are available online. Credit: Montet & Simon.
M. A. Thompson (University of Hertfordshire) and colleagues published a recent study in Monthly Notices of the Royal Astronomical Society reporting their findings using millimetre and sub-millimetre photometry. The paper finds that a dust cloud orbiting Tabby’s Star would have to be no larger than 7.7 Earth masses of material within a radius of 200 AU, adding “Such low limits for the inner system make the catastrophic planetary disruption hypothesis unlikely.”
Montet and Simon don’t necessarily agree, but in any case there are other problems. The authors think the light curve is “…consistent with the transit of a cloud of optically thick material orbiting the star,” and that such a cloud could be small enough to meet Thompson and team’s requirements. The breakup of a small body or a recent collision producing a large dust cloud could also produce a cometary family that transited the host star as a single group. But we’re still not out of the woods:
To explain the transit ingress timescale, the cloud would need to be at impossibly large distances from the star or be slowly increasing in surface density. The flat bottom of the transit would then suggest a rapid transition into a region of uniform density in the cloud, which then continues to transit the star for at least the next year of the Kepler mission. Moreover, such a model does not naturally account for the long-term dimming in the light curve observed in both DASCH and the Kepler FFI data, suggesting that this idea is, at best, incomplete.
A deeply mysterious star, our KIC 846285. Montet and Simon call for alternative hypotheses and new data to help us explain existing observations, and we can be glad to have Tabetha Boyajian’s team on the case thanks to the success of the recent Kickstarter campaign. Observations are already in progress at the Las Cumbres Observatory Global Telescope Network, and the Kickstarter funds will take us deep into 2017. For more on the Las Cumbres work, see Corey Powell’s recent interview with Boyajian for Discover Magazine, from which this:
From our new observations, we’ll be able to tell a lot about the material that’s passing in front of the star: if it’s some kind of dusty thing, some kind of solid thing. [Boyajian’s working hypothesis is that the dimming is caused by a huge swarm of comets, set loose perhaps by some cataclysmic event around the star.] What’s also important is that we will also get a baseline of spectral observations so we can look at if there’s any radial velocity shift or if there’s any variable emission of the lines, things we’d expect comets to have.
The paper is Montet and Simon, “KIC 8462852 Faded Throughout the Kepler Mission,” submitted to the AAS Journals and available as a preprint. The Thompson paper on circumstellar dust in this system is “Constraints on the circumstellar dust around KIC 8462852,” published online by Monthly Notices of the Royal Astronomical Society 25 February 2016.
Is the anomalous star KIC 8462852 undergoing a long-term dimming or not? We’ve looked at Bradley Schaefer’s work on the star and the follow-ups disputing the idea from Michael Hippke and Daniel Angerhausen (NASA GSFC), with collaboration from Keivan Stassun and Michael Lund (both at Vanderbilt University) and LeHigh University’s Joshua Pepper. Dr. Schaefer (Louisiana State University) believes the evidence for dimming is still strong, and in the post below explains why. He has also provided a link to a more detailed analysis with supporting graphs and figures for those who want to go still deeper (further information below). As we embark on the Kickstarter campaign to put ‘Tabby’s Star’ in the sights of the Las Cumbres Observatory Global Telescope Network — an important project to which I have contributed and hope you will as well — we continue to monitor this evolving story. No matter how it turns out, the Kepler data are iron-clad, so the success of the Kickstarter campaign is vital to provide us with the further data we need to make sense of what we are seeing at this unusual star.
By Bradley Schaefer
The dips shown by KIC 8462852 (Tabby’s Star) are still a profound mystery. Further, I have found that Tabby’s star has faded by ~20% from 1890 to 1989 as measured from the Harvard plates. (This is now Schaefer 2016, ApJLett, 822, L34.) A straight line fit to the light curve gives a slope of +0.164 ± 0.013 magnitudes-per-century. The quoted error bar here is from the measurement error (as taken by a chi-square fit), whereas there is some larger systematic error associated with all the usual small problems in photographic photometry and the sampling of the plates.
To measure the systematic errors, I used 12 uncrowded check stars of the same magnitude and color as Tabby’s star, and all within ~22 arc-minutes. The average of the linear slopes is -0.007 mag/century, with an RMS scatter of 0.044 mag/cen. With the systematic error dominating, the century-long decline of Tabby’s Star is significant at the 4.0-sigma level (i.e., a probability of 0.000064 of such a high slope occurring by chance, even with systematic errors).
Two papers (Hippke et al. arXiv:1601.07314v4 & Lund et al. arXiv:1605.02760v1) have recently appeared with the basic claim that the historic light curves from Harvard (as part of the DASCH [Digital Access to a Sky Century@Harvard] database) have a much larger systematic error, more like ±0.15 mag/cen, with the agreed slope for Tabby’s Star then not being anything special. If the DASCH RMS scatter in the fitted linear slopes is really this large, then the existence of the century-long fading in Tabby’s Star would not be significant.
The two papers of Hippke and Lund have been widely publicized, because both authors have run to the press first. Indeed, Hippke contacted at least one reporter *before* he had submitted the first version of his paper. (At that time, Hippke had known about the existence of the Harvard plates for only two weeks, he had talked with zero people who had ever seen any archival plate, and Hippke still has never laid eyes on any archival plate.) In the usual way of ‘social media’, Hippke’s and Lund’s claims have been highlighted as a refutation of the century-long dimming, and this has been extended to everything about KIC 8462852. For example, on the first day of the launch of the Kickstarter program, the Reddit talk had the statement that the person ‘thought this has all been refuted’.
Well, despite ‘social media’, it is actually Hippke and Lund that are definitely wrong. As I’ll show, any experienced worker can quickly find exactly what mistakes they made, so that their claimed large scatter of slopes arises simply from two distinct mistakes on their part. But I have no ordinary venue to put out any effective counters or proofs. For example, any further submission to ApJ or ApJLett would not have any new data to show, and it would appear only many months from now. Research on Tabby’s Star is moving fast, so Hippke’s and Lund’s claims need to be challenged soon. The best way that I can think of to get the challenge and proofs out is to place them into a detailed document plus an email (*this* email), and to send this out to people who have queried me for an analysis of Hippke’s and Lund’s manuscripts on Tabby’s Star. A link to the detailed document appears below.
I present three reasons to show that Hippke and Lund have incorrect claims:
Reason #1: Hippke & Lund Both Made Two Killer Mistakes
Mistake #1 is that they selected many check stars that have some random nearby star at just the right distance so as to produce overlapping star images on the Harvard plates with large plate scales. The DASCH photometry uses SExtractor, and the algorithm returns something like the combined magnitude when the two star images overlap. This overlap produces an erroneously-bright magnitude for some plates. This occurs for most of the plates after the 1953-1969 Menzel gap (the Damon plates), resulting in an apparent jump across the Menzel gap. When the whole light curve is fit to a straight line, it will also result in an apparently brightening light curve.
Some crowding stars will cause this effect to be mainly visible on the RB & RH series or the AM & AC series, which result in the opposite sign for the jumps and slopes. In the linked PDF file below, I give many detailed examples, tables, and illustrations. That is, jumps in brightness across the Menzel gap and non-zero slopes are produced as pure artifacts of choosing check stars with nearby crowding stars. Now, critically, Tabby’s Star does not have any crowding stars. So it is not correct to choose any crowded-check-stars. No experienced researcher would make such a choice. It turns out that a large fraction of both Hippke’s and Lund’s stars with high claimed slopes are badly crowded. That is, many of their stars have high slopes simply due to this bad mistake.
Mistake #2 is that they have used the KIC magnitudes for calibration, rather than the APASS magnitudes as strongly recommended by DASCH in many places. The KIC calibration is based on the ‘g’ magnitudes as used by the Kepler satellite, whereas the APASS magnitudes directly give ‘B’ magnitudes. The native system of the Harvard plates is ‘B’. So the use of the KIC-calibration will always be problematic for some purposes because there must always be color terms needing correction. It is only a historical relic that the DASCH database allows the use of the KIC calibration. Yet most of Hippke’s and Lund’s results were made with the KIC calibration.
This actually matters. The reason is that the KIC-calibrated light curve for some presumably-constant star often shows an apparent slope (and possibly a jump in brightness across the Menzel gap), whereas the APASS-calibrated light curve for the same star shows a perfectly flat light curve. I show several examples of this effect in the attached PDF file. With this, we see that the use of the KIC-calibration by Hippke & Lund is causing the jumps and slopes as pure artifacts. Their Mistake #2 would not be made by anyone experienced with the Harvard plates (or anyone who reads the DASCH website or papers).
The attached PDF file gives a detailed account of the commission of the errors. Between the two killer mistakes, all of Hippke’s and Lund’s claims are shown to be artifacts of their bad analysis.
Reason #2: Two Measures by Experienced Workers give ±0.044 and ±0.048
Measure #1 is by myself, as given in fine detail in my ApJLett paper. I derive the century-long slopes for 12 uncrowded check stars that have essentially identical magnitude, color, and position as Tabby’s Star. Whatever systematic and measurement errors happen for Tabby’s Star on the DASCH photometry, the identical effects must be present on these 12 stars. No one can do any better than this for a direct measure of the real total errors. With this, the average slope is very close to zero, while the RMS of the slopes is ±0.044 mag/cen. The largest deviation from a flat slope is one at -0.070 mag/cen. I should mention that I have a vast experience with the Harvard plates, with nearly continuous work since 1979, something like 50 papers in refereed journals, plus five papers on the theory of photographic photometry.
Measure #2 is by Josh Grindlay. He is a professor at Harvard; he has been a long time user of the Harvard plates (going back before 1979), and he is the founder and leader of the DASCH program. He had long been using DASCH light curves, so he knew perfectly well that DASCH produces flat light curves for constant stars. With the spectacle of Hippke’s paper, he started a formal measure of many Landolt stars with the DASCH data. (Landolt stars have long served the community as standard stars, and they are most likely closely constant in brightness.) For 31 Landolt stars, Grindlay finds that the average fitted-linear-slope is -0.015±0.048 mag/cen.
So we have the two most experienced workers in the world, and we are getting an RMS in the fitted-linear-slope of 0.044-0.048 mag/cen. For Tabby’s Star, this results in the century-long dimming being near 4.0-sigma in significance. I think that these two solid measures by the most experienced people in the world are to be strongly preferred to a claim coming from people who have yet to lay eyes on any archival photographic plate.
Reason #3: The Dimming of Tabby’s Star Has Been Confirmed
Ben Montet is in the process of investigating the long-term photometric behavior of KIC 8462852 in a high quality independent data set, and his preliminary results support the finding that Tabby’s star is indeed fading.
Tabby’s Star is bright, and the Kepler data is legendary for its photometric accuracy and stability. If Tabby’s Star is fading at the rate of 0.164 mag/cen (which it might or might not still be doing), then it should have faded by 0.0073 mag over the Kepler lifetime on the main Cygnus Field. This should be discoverable by a careful analysis.
Apparently Montet has made such an analysis, and finds Tabby’s Star to be fading at some unspecified fade-rate. So we have an apparent confirmation of the fading of Tabby’s Star over 4.5 years, although certainly we must await a definitive paper coming from Montet. [A group at Pulkova Observatory has claimed to provide a weak confirmation of a fading of Tabby’s Star. This is based on just ten plates from 1922 to 2001.There is indeed a formally fading slope, but the real uncertainties are greatly larger than any claimed slope. This result is not a confirmation.]
For those interested in following this matter further, the document I discuss above, my “ANALYSIS OF HIPPKE et al. (2016) and LUND et al. (2016) is available.” Often the refutations of claims are not short, so I have presented the full details in this document. In sum: We have three strong reasons to know that Hippke’s and Lund’s claims are certainly wrong.
If the star KIC 8462852 is on your mind — and the lively and continuing comments threads on the topic in these pages suggest that it is — you’ll want to know about a new campaign to support further study. ‘Tabby’s Star,’ as it is informally known (after Tabetha Boyajian, whose work at the Planet Hunters project brought the star into prominence), continues to vex astronomers with its unusual light curves. What is causing the star to dim so dramatically remains problematic, with suggestions ranging from comet swarms to extraterrestrial engineering.
A Kickstarter project is now in the works to support further investigation, hoping to extend an effort that has already begun. Boyajian’s team has initiated observations on the Las Cumbres Observatory Global Telescope Network, a privately run effort that maintains telescopes around the world to make sure an object can be examined continuously. Four years of Kepler data have shown us that the dips in the light curves from KIC 8462852 are not periodic, which means the monitoring needs to be continuous because we can’t predict when the next dip will come.
So far, the Las Cumbres network has given 200 hours of observing time, work that will support observations through the summer. The Kickstarter campaign intends to raise $100,000 to fund an entire year of observations, which will include a total of two hours per night. The plan is to observe the star at different wavelengths, alerting larger facilities when something interesting is happening. Variations in dimming at particular wavelengths can tell us much about what kind of material is causing the effect, perhaps supporting a hypothesis like cometary gases or dust.
Jason Wright and Kimberly Cartier (Penn State), calling KIC 8462852 ‘the most mysterious star in our galaxy,’ looked toward the next round of study in a recent essay in The Atlantic:
When the star dims again, we will use telescopes around the world to measure how much the star dims at different wavelengths. Since different substances have characteristic absorption patterns, this will tell us the composition of the intervening material. For instance, if it dims much more at ultraviolet wavelengths than in the infrared, we will know dust is to blame. If we see the characteristic pattern of cometary gases, that will help confirm the cometary hypothesis.
And if we see the same brightness changes at all wavelengths? That would indicate that whatever is blocking the starlight is big and opaque—inconsistent with comets, but consistent with the alien megastructure hypothesis.
The intense interest in KIC 8462852 created a flurry of work among amateur astronomers, including dozens working with the American Association of Variable Star Observers. We do have data from the AAVSO effort so far, but as Boyajian notes in the Kickstarter materials, there is a good deal of scatter in the measurements depending on observer and equipment.
Image: A graph showing brightness measurements (in magnitude) for KIC 8462852 contributed by over 50 AAVSO observers. Although the star displays constant brightness during this time, observer-to-observer offsets smear out any signal of a dip. Credit: AAVSO.
The benefits of using the Las Cumbres network are clear:
If just AAVSO data are used alone, the myriad of offsets and random sampling from many observers (even if greatly reduced by giving more specific instructions to the observers) may always obscure the fine details of the brightness behavior of our star. That is, with a patchwork of observers, dips ~5% or smaller may not be recognizable in near-real-time with AAVSO data (and the Kepler data show there are very few dips greater than this level).
Not all is lost however. The LCOGT data will have the dense sampling with a single consistent system, so dips below the 1% level can be spotted. This also means that we can use the LCOGT data as a very valuable training set to help amateur observers to improve their techniques. By educating amateur observers in this way, their data can be made more precise and can be corrected for systematic offsets. And by accomplishing this, we will greatly increase the number of observations to be used in our final analysis.
Thus we get not only continuous monitoring of KIC 8462852 but a much more finely calibrated way to look for future dips in the light curve of the star. You can see, too, that this is the kind of project to which a fast and relatively inexpensive campaign like this one is ideally suited. Getting observation time on large facilities is always tricky because of the high demand, and private observatories are usually paid through grants, most of which don’t make it through the funding process. When you need a lot of telescope time to look at an anomaly like KIC 8462852, crowdsourcing turns out to be the best and perhaps the only viable option.
A search through the archives here will pull up the numerous articles about KIC 8462852, but I’d also urge you to dig into the comments section, particularly for the more recent posts, as the discussion has been lively. For overall background, you’ll want to have a look at this thorough explanation of what we’ve learned about the star so far from Paul Carr, who is also active in the comments threads here. Paul’s Dream of the Open Channel is a site you’ll want to follow on a regular basis as we continue to look at this puzzling star. He is also an active podcaster on SETI and related matters, with links provided on the Open Channel site.
As for the Kickstarter project, I see that as of Monday morning, it has raised close to $20,000, with 25 days to go toward the $100,000 goal. The Las Cumbres observing campaign offers us precious new data to add to the Kepler cache and the continuing AAVSO effort. As to Kepler itself, we’re now deep into the K2 mission and KIC 8462852 is no longer observable. To find out when the next dips will occur and test theories about what may be happening here — ringed planets? dust clouds? cometary debris? — Las Cumbres looks like our best bet. Let me encourage you to contribute what you can as we get this observational campaign into gear.
The unusual star designated KIC 8462852, and now widely known as ‘Tabby’s Star,’ continues to be an enigma. As discussed in numerous articles in these pages, KIC 8462852 shows anomalous lightcurves that remain a mystery. Recently Michael Hippke explored a related question: Was the star dimming over time, as postulated by Louisiana State’s Bradley Schaefer? The two sharply disagreed (references below), leading Hippke and co-author Daniel Angerhausen to re-examine their conclusions. Now, with further collaboration from Keivan Stassun and Michael Lund (both at Vanderbilt University) and LeHigh University’s Joshua Pepper, Hippke and Angerhausen have a new paper out, peer-reviewed and accepted for publication by The Astrophysical Journal. What follows are Michael Hippke’s thoughts over the controversy as it stands today, with the dimming of KIC 8462852 again in doubt.
by Michael Hippke
Tabetha Boyajian et al. released a paper on the preprint platform astro-ph in September 2015, which quickly got the Internet up to speed. Planet Hunters, an open community that searches data from the Kepler space telescope, found this unusual star that is now known as “Tabby’s Star”. It is observed to undergo irregularly shaped, aperiodic dips in flux of up to ~20%, much more than expected for any orbiting planets.
Media interest skyrocketed in October, when Jason Wright et al. released a preprint in which they discussed — among many other possibilities — the idea that the dips could originate from an alien race building a mega-size construction around the star, perhaps in the form of a “Dyson sphere”. Could it really be true that we found the first ever evidence of a powerful extraterrestrial civilization? A controversial discussion quickly ensued.
Further examination in other electromagnetic wavelengths only brought disappointing null results. The star was unremarkable in the infrared, showed no sign of artificial laser pulses, or radio emissions. The only somehow realistic astrophysical explanation was offered by Bodman & Quillan, who suggest the presence of a large family of comets, and which as of today are considered to be the “best” explanation.
Image: Cascading comets around a distant star (NASA/JPL/Caltech).
In January 2016, Bradley Schaefer released a preprint that examined historical photographic glass plates from the Harvard Observatory taken since 1889. His results seemed to show that “Tabby’s Star” had dimmed by 20% since that time, which was interpreted by many in the media as an indication of a quickly proceeding alien construction.
This was about the time when Daniel Angerhausen (an experienced astrophysicist at the NASA Goddard Space Flight Center) and I got interested. Our basic thought was: “If this were true, it would be the greatest thing in history!” We wanted to see the evidence with our own eyes, because honestly, we had big hopes ourselves. Initially. We think it is very human to be wanting to be part of something big, or at least seeing it happen. So, we had hopes that something big had been found, and we had the chance to see it happen.
So we dug into the Kepler data and found them rock-solid. These strange dips were really there! Also, we downloaded the historic Harvard data and plotted them. Schaefer had binned them in 5-year intervals, but behind these bins, there were actually over a thousand individual data points. When we plotted these on our screen, and overlaid a linear trend, we became very disappointed. The Harvard plates have an uncertainty (over 100 years) of order 0.1 to 0.2mag, and this was also visually evident. We had serious doubts that this dimming trend was valid.
We selected some comparison stars, which had similar scatter and trends, and decided that we would release our findings as a preprint on astro-ph. KIC 8462852 is a very special case not only scientifically but also in the way it was discussed in the community. It became one of the most publicly discussed astronomical objects. Many articles on this fascinating object (including its original detection) were published on astro-ph before peer review and some even called it a revolution in scientific discussion, making it real time and on various social media channels involving the public.
Bradley Schaefer himself gave at least 2 interviews on the day his (at that time not peer-reviewed) manuscript came out. Following his own arguments that “putting up unchecked and false claims is bad all the way around”, we had no other choice than putting our doubts on his results out immediately, so that the community, the involved media and the interested public did not have to wait many months for the formal rebuttal. We also decided to do this to keep the discussion public and have interested laymen follow it; we even gave a recipe to reproduce the data using the publicly available Harvard DASCH [Digital Access to a Sky Century @ Harvard] data for the interested reader [see KIC 8462852: No Dimming After All?].
The immediate reactions to our preprint were overwhelmingly negative, just as most of the other publications on KIC 8462852 have been. In retrospect, we attribute this to two very human factors. The first was that some of our previous assumptions and methods were indeed inconsistent and the choice of some comparison stars questionable. But errors are human, and everybody makes them at times! While we believe that these errors did reduce the clarity of our result, we still believed that the result itself was correct. This view was not shared by Bradley Schaefer, who published his reply on Centauri Dreams [see Bradley Schaefer: A Response to Michael Hippke]. The other issue we see was that most readers wanted to believe that this thing was real. Schaefer was an authority, while Hippke was described as a “novice”, and “proof by authority” seemed to weigh in.
As this time, we decided to do two things. First of all, we cut all media connections. Second, we started collecting all issues and questions raised by Schaefer, and many others in the community and here on Centauri Dreams. “Tabby’s Star” started off as a community project, and it continued to be one! The pure number of emails we received was enormous. Our favourite mail, which was received via hand-written paper mail actually, was by an American who said that he was once captured by these aliens himself and is a first-hand witness!
Most feedback was incredibly helpful, however. We gained several co-authors who contributed several more analyses, statistical tools and better ways to select comparison stars. We also got an invitation to the ASTROPLATE conference in Prague, where Michael Hippke met most of the glass photography community. We spent a whole week discussing calibration techniques, scanners, and fungus (one of the reasons why digitization of these plates is so important!). These fungi are actually called “gold disease” owing to their look on the glass. We enjoyed long evening talks about the millions of plates that still await their scientific use, and after all of this, had the chance to discuss our own findings in a presentation and discussion.
During all this, and afterwards, we continued the peer-review process at the Astrophysical Journal. We have published several papers in this journal before, and, as always, it was a very professional process. Yet, they appointed two referees instead of the usual one, and we went through several rounds of question-and-answer to nail down every detail. This took considerable time. Now the paper has been accepted for publication (and is updated on astro-ph). From the feedback we got, and given the much more detailed than usual review process, the study is probably one of the most solid and waterproof papers we ever published. We believe that it is established beyond any reasonable doubt that no dimming can be found within the uncertainties of 0.2mag per century.
Now, what does that mean for the mystery? Are there no aliens after all? Probably not! Still, the day-long dips found by Kepler are real. Something seems to be transiting in front of this star. And we still have no idea what it is!
The cool message here, however, is that we now for the first time in human history have technology that can at least in theory detect such things, with upcoming missions such as JWST and PLATO. To solve the mystery, there are now several more projects under way. The American Association of Variable Star Observers (AAVSO) has collected thousands of amateur astronomer observations to discover new dips. Others, such as the Las Cumbres Observatory Global Telescope (LCOGT) have joined the effort. Observing further dips in different colors can reveal information about the chemistry of the transiting object, which might confirm or reject a cometary origin. Who knows, perhaps these telescopes have already captured some exciting new data, and any day researchers might publish a paper that solves the mystery!
The paper is Hippke et al., “A statistical analysis of the accuracy of the digitized magnitudes of photometric plates on the time scale of decades with an application to the century-long light curve of KIC 8462852,” accepted at the Astrophysical Journal (preprint). A Vanderbilt news release is also available.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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