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Bradley Schaefer: A Response to Michael Hippke

The question of a gradual dimming of KIC 8462852 continues to make waves, the most recent response being Michael Hippke’s preprint on the arXiv site, discussed in the post immediately below. Bradley Schaefer (Lousiana State University), who published his work on the dimming he found in now digitized photographs in the archives of Harvard College Observatory, disagrees strongly with Hippke’s findings and is concerned that the paper impugns the solid work being performed by DASCH (Digital Access to a Sky Century@Harvard). Below is Dr. Schaefer’s response with details on the astrophotographic photometry at the heart of the issue.

by Bradley E. Schaefer


A few hours ago, Michael Hippke posted a manuscript to arXiv (http://arxiv.org/abs/1601.07314), and submitted the same manuscript to the Astrophysical Journal Letters (ApJLett). This manuscript claims to have found that the DASCH data produces light curves with secular trends (both systematic dimmings and brightenings) over the century-long records. This same DASCH data (from the collection of archival sky photographs now at Harvard Observatory) was used to recognize a dimming of KIC 8462852 (a.k.a. ‘Tabby’s Star’ or the ‘WTF star’) at an average rate of 0.165±0.013 magnitudes per century from 1890 to 1989.

This dimming from the DASCH data is just a long-time scale version of the dimming also seen with the Kepler spacecraft, and these dimmings are still a high mystery and a perplexing problem. Hippke is taking his claimed result (that the majority of DASCH light curves have major and widespread calibration errors resulting in apparent secular trends) as then implying that KIC 8462852 does not have any secular trend. This claim is easily proved wrong.

Hippke made two major errors, both of which are beginner’s mistakes, and both of which will erroneously produce apparent dimmings and brightenings when none exist. First, Hippke explicitly includes red-sensitive and yellow-sensitive photographs together with the blue-sensitive photographs. The different colors will produce systematically different brightnesses (magnitudes). The trouble is further that the red and yellow photographs are predominantly at late times in the century-long light curve (in the 1970s and 1980s), so the inclusion of many magnitudes that are systematically high or low only at the end of the century will artificially make the star appear to brighten or dim over the century.

Second, Hippke explicitly includes magnitudes from photographs with known and recognized defects. The Harvard photographs are not perfect, with some having long-trailed images, some being double exposures with stars overlapping, and some having various plate defects where the emulsion is nicked or such. The DASCH scanning and software has a robust means of identifying problem photographs, and these are objective measures independent of the magnitude. These known-poor-quality magnitudes should not be used for any sensitive purposes. Colloquially put, these are ‘garbage’. Hippke keeps all the many good DASCH magnitudes and he also adds in the garbage magnitudes, so his final light curves have many points that are systematically skewed.

The frequency of the poor-quality magnitudes varies over time, usually with more early-on during the century. And the erroneous magnitudes are variously systematically brighter or dimmer, also varying over the century. The result of Hippke’s good+garbage light curves is that the garbage points tilt the light curve by a bit. This is seen when I take all of Hippke’s same stars and data and go from his sloped light curves (including his garbage points) to flat light curves (with only the good points). The bottom line is that Hippke’s second big mistake was to include the poor-quality photographs. Garbage-in, garbage-out.

So we understand why Hippke’s secular trends are wrong. But we already knew this very well anyway. The reason is that the DASCH people have already measured many (likely up around the millions) of light curves for single main sequence stars (i.e., stars that really should be perfectly constant) and found that their light curves are actually very flat. This is in stunning contradiction to the claims of Hippke that the majority show big secular trends.

Hippke’s paper has a title of “KIC 8462852 Did Likely Not Fade During the Last 100 Years”, yet his paper never discusses or analyses any data from KIC 8462852. One reason is perhaps that he cannot get around the flatness of the five check star light curves. That is, these five stars always appear within 3 millimeters of Tabby’s Star on these 10″x8″ phootgraphs, these stars are all of similar brightness as Tabby’s Star, and they all have similar color as Tabby’s Star.

If there were any systematic problems for the DASCH data with Tabby’s star, then we should see the exact same dimming trend in the check stars as is seen for Tabby’s Star. But we do not. These ‘check stars’ serve as the classic control study in science. They serve as proof that neither the check stars nor Tabby’s Star have any substantial systematic problem. They serve as proof that Hippke’s title is wrong.

Hippke submitted his draft manuscript to ApJLett, to arXiv and to reporters even before he had any checks with experts on archival sky photographs. For example, I first read his email just about the time that he was submitting his manuscript. He did not contact any of the DASCH people, despite them being the target of his attack. Indeed, he has not talked with anyone who has any experience with or knowledge of any archival photographs. This topic has a lot of detail and many quirks, but Hippke apparently did not have the realization or the will to check out his claims. And, in an email from Hippke from early this morning, he explicitly labelled himself as “a novice” for this technical topic. So he is a novice working without bothering to check with anyone knowledgeable. As such, it is not surprising that he made beginner’s blunders.

A broader problem is now that DASCH has the publicly-stated claim that it has major, widespread, persistent calibration and measurement errors. In knowledeable circles, Hippke’s paper won’t come to anything. But these circles are not large, because few people really understand the working of DASCH or plate photometry.

So most people will simply look at the paper’s conclusions, not recognize the horrible beginner’s blunders that create the false conclusion, and only come away thinking that the DASCH light curves are “wrong” or at least “questionable”. Public perceptions do matter for many aspects. Most important for DASCH is their future success rate in funding proposals, the reception of all future papers relating to DASCH, and the future useage of the DASCH data.

Perhaps from a journalistic point of view, any ‘stirring of the pot’ is good copy. But from the point of view of science and knowledge, putting up unchecked and false claims is bad all the way around. Science has a great strength of being error-correcting, with the normal procedure now for the DASCH people to put out a full formal refutation of Hippke’s claims, and such will appear in many months. But with the one-day turn-around of arXiv and with fast journalist response, there will be many months where the reputation of DASCH is maligned. So Hippke’s choice of running to reporters before the paper appeared publicly, and disdaining any experienced advice despite being a self-proclaimed “novice”, is not good science.


For more on this controversy, see KIC 8462852: Where Are We After Eight Months?, Michael Hippke’s follow-up.


Comments on this entry are closed.

  • Larry Kennedy February 11, 2016, 14:43

    @ Tom Mazanec
    A signaling device? Redirecting to collectors out of line or emitting most IR on different line of site?

  • Paul Carr February 12, 2016, 10:41

    This bothers me – this “structural break”. You can always apply a more complex model and get a better fit to noisy data. However, why would the dimming just happen to be accross the Menzel Gap? This same analysis should be applied to the Landolt standard stars (http://adsabs.harvard.edu/abs/1992AJ….104..340L) that DASCH have used to validate their calibration. There should be no such break for these. Also, there is greater long term variation in red giant stars (stars off the main sequence) that DASCH has published. These would be a good test as well.

    • Rafik February 12, 2016, 16:45

      Agree that a comparison of Landolt F stars would be better (as I mentioned in an earlier comment). Hippke has gone with nearby stable F stars, maybe the Landolt F stars are not nearby or on the same plates as KIC8462852.
      The “structural break” may be an artefact of the changes that occurred during the Menzel gap (telescopes, emulsions etc) and the easiest way to check this is to analyse the Sonneberg plates.
      Rather than rush to publish this new version of the paper, Hippke would have been better off waiting until he has analysed the Sonneberg plates to see if there is any dimming trend or event for KIC8462852.

  • Harry R Ray February 14, 2016, 14:48

    POSSIBLE(ONLY,NOT CONFIRMED) 48.4 day PERIODICITY in ALL of the dips. For all of the details, go to the Extrasolar Visions II website, http://www.solar-flux.forumandco.com and click on the latest post by Borisev.

  • Harry R Ray February 14, 2016, 14:54

    Sorry, I meant Borislav, NOT Borisev.

  • ascop21 February 16, 2016, 7:12

    [… Wright and Desch want to focus on the unusual transit signals from five stars — three M dwarf identified by Kepler, one a burned-out but super-dense white dwarf and other made famous last fall when a substantial and currently impossible-to-explain dust cloud was detected nearby it. All the known explanations to explain it were deemed inadequate, which led to (last option) suggestions that perhaps it was an alien “megastructure” or Dyson swarm built by intelligent beings.

    Wright was part of the group trying to explain the vast cloud around the star — KIC 8462852 or “Tabby’s star,” named after Yale University post-doc and co-founder Tabetha Boyajian) and now suspects that a disintegrating planet could be a source (though he says that Desch was the first to make the case.) …]

    from: http://www.manyworlds.space/index.php/2016/02/15/the-potential-prize-inside-shredding-planets/
    A dust cloud around KIC 8462852 ? A disintegrating planet can explain the light curve ?

    • Harry R Ray February 16, 2016, 15:32

      I just read the “Many Worlds” article. Was it printed BEFORE or AFTER Schaefer’s paper? ALSO: How can a disintgrating planet NOT leave SOME KIND of an IR signature?

      • ascop21 February 16, 2016, 18:20

        I don’t know, and no dust cloud was found around Tabby’s star (or i miss something). This article is strange for me too. I found the link to this site in a Jason Wright tweet. Disturbs me.

      • EricSECT February 16, 2016, 19:00

        The disintegrating planet hypothesis, AFAIK, falls flat because we would expect to see, but in fact do NOT detect, excess IR around Tabby’s. If the excess IR is <100 K we are not yet able to detect it, and if the IR is somehow being directed away from our line of sight, we could not detect it. Can anyone think of a natural process that would allow one or both of these to occur?

    • Rafik February 16, 2016, 22:39

      A disintegrating planet is the last thing I would suspect.

      1. All disintegrating planets are being torn apart by tidal forces, etc from their parent stars due to being in close orbit. The large Kepler dips at 723 and 1520 days are highly unlikely (impossible?) to be coming from close to KIC 8462852.

      2. No IR excess – which would scream out from a disintegrating planet.

      3. The light curve from a disintegrating planet looks nothing like any of the KIC 8462852 dips – if anything the 723 day dip looks the opposite. How could a disintegrating planet have its “dust tail” precede it in orbit?

      4. There would be some regularity/consistency in the light curve signature from a disintegrating planet. Regular spacing in dips, consistent shape of light curves and regular length and depth of dips. KIC 8462852 is anything but regular in any of these aspects.

      A swarm of giant comets looks so much more appealing than a disintegrating planet, and comets should also leave an excess IR signal.

      Let’s face it we are left with two alternatives:
      1. Some unknown natural phenomena (maybe a co-moving distant cool brown dwarf with it’s own system of giant rings, planets, dust clouds etc that is in the line of sight – still a poor explanation); or
      2. Some unknown unnatural phenomena.

      The answers may come through either a detailed analysis of the Sonneberg plates or through further direct observations of KIC, especially in 2017.

      • Harry R Ray February 17, 2016, 11:15

        3. Some COMPLETELY UNKNOWN INTERNAL PROCESS that reqires NO DUST or NO NON-NATURAL “structures” WHATSOEVER(Trilithium anyone?), but ONLY if it is a slightly evolved subgiant star that would, without this internal process going on, be considerably more luminous than an F3V at a distance of 1,480 light-years. I am LEANING(but NOT strongly) toward this explanation, but waiting for the GAIA data to come in!

  • Paul Carr February 18, 2016, 12:04

    Yet another revision of Hippke+ out today. More co-authors. There is still no mention of the Landolt standards (which are hard to find in DASCH, although I am told they have curves for about a dozen of these).

    The biggest problem here is that the “structural break” is inconsistent. If it really is instrumental, it ought to be a dimming for all similar stars, but it is (in my assessment) an unconvincing brightening for KIC 6366512.

    Schaefer’s biggest problem is that he needs to show more of his work for his manual brightness estimates. None of those were provided in the preprint, or even plotted, and he only identified and plotted 2 of his check stars. He also needs to demonstrate why his binning approach (a kind of easy low pass filter), does not introduce problems.

    • Boris February 20, 2016, 17:23

      Why are they looking for trends in the photometry SuperWASP, but do not look for it in the 4-year-old high precision photometry telescope Kepler?

  • ljk February 18, 2016, 14:19
  • Rafik February 18, 2016, 22:43

    The revised revised Hippke (et al) paper is a bit of a disappointment.

    Rather than go through identifying all the problems with the revised revised paper, I think it is enough to say:
    PLEASE, PLEASE, PLEASE analyse the Sonneberg Plates for KIC 8462852 (especially the B spectrum) and publish the raw data, including data for nearby stable F STARS of SIMILAR temperature, size and magnitude.

    This will settle the question of whether any significant dimming has occurred for KIC 8462852 over the past century and if so when and by how much.

    • Harry R Ray February 21, 2016, 14:33

      I think Schaefer is in the PROCESS of doing that Right NOW, but due to “immediate accessability” issues, don’t look foe any IMMEDIATE RESULTS in the near future. ALSO: Someone ELSE with Schaeffer’s “EYEBALLING” skills should “eyeball” ALL of the stars in the newest Hippke paper, and give us an IMPARTIAL ACCOUNTING!

  • Harry R Ray February 22, 2016, 16:14

    If Schaefer’s claim is confirmed, could a main sequence star cooling down from a VERY RECENT, VERY close by GRB be a natural explanation for it?

    • Michael February 22, 2016, 17:15

      GRB are very, very big, we would have seen a remnant formation and I can see no gamma ray sources near by.

      • Harry R Ray February 23, 2016, 10:33

        You are ALMOST CERTAINLY right, but what I was thinking about is an extremely contrived scenario(Kinda like the “commet” scenario)that would include the following: ONE: The GRB reminant would have to be DIRECTLY BEHIND KIC8462852 RIGHT NOW! TWO: The black hole PRODUCING the GRB event would have had to, ALMOST IMMEDIATELY injest ALL OF IT’S PROGENATOR STAR, so that no LARGE NEBULA NEVER FORMED! Three: If a tiny shell of gas was ejected BEFORE the black hole COMPLETELY DEVAURING its progenator star, that gas would NOW have to be EXTREMELY DIFFUSE and ALSO be IN FRONT OF KIC8462852! I don’t believe this scenario for a moment, but; because there APPEARS TO BE A HIGHER THAN NORMAL amount of gas between Earth and KIC8462852 in the general region all around KIC8462852, it it something I THINK astronomers would use if KIC8462852 were PROVEN BEYOND A SHADOW OF A DOUBT to be an F3V instead of an F3IV(which I am predicting when the GAIA data becomes available)and NO OTHER NATURAL EXPLANATIONS HOLD UP!

  • Fred Parker March 20, 2016, 14:59

    One postulate to explain this phenomenon is that this could be a swarm of large reflectors in orbit around this star. This would also explain the lack of an increased infrared signature emanating from the star. The reflectors could possibly be used to significantly expand the habitable zone of the star by partially eclipsing the star’s radiant energy from striking a planet that resides on the hotter side of the zone and directing the reflected light to a planet on the colder side of the zone. The reflected light could even be directed to a nearby solar system (such as towards a planet orbiting a very close, neighboring, colder red dwarf star). And perhaps as the human race will most likely do as our sun ages and gets hotter, we will resort to space-based reflectors to redirect the sun’s radiant energy away from earth and possibly towards frozen celestial bodies farther out to make them more habitable. In all of these scenarios, the reflectors would need to be able to pivot slowly to keep the angle of reflection directed towards or away from the desired planet(s) as it orbited its star. This would explain the recorded dips in light as the swarm of reflectors continually made dynamic adjustments.

  • Fred Parker March 20, 2016, 15:01

    The reflector theory would explain both types of dimming -both the short term variations and the longer term attenuation trend. As an example of a short term variation that matches what was observed from the Kepler telescope, if a planet was being cooled by a swarm of reflectors (made perhaps from an ultra-thin Mylar type of highly-reflective material), then as an observer on earth sees this planet orbit in front of its parent star, the observer would also observe the partial eclipse of the star from the swarm of reflectors. This would be highly directional and would create a sharp swing down and then back up in the amount of radiant energy reaching the observer. The opposite would happen for a planet being warmed up. As for the longer-term dimming of the star, the continued construction and deployment of numerous space-based reflectors over a century of time would produce an ever-increasing obscuration of the star.

    • EricSECT March 21, 2016, 6:28

      Fred Parker:
      This hypothesized occulting object would have to be approximately 20 times the area of Jupiter’s disk (and more if not 100% opaque) to cause the two largest dimmings of Tabby’s Star that we’ve observed. I don’t wish to presume any limitations at all for an advanced ETI but that just seems excessively large (for a simple star-shade). Further, since no reflective material can be 100% effective, some amount of waste heat from this gargantuan object as it warms should be re-radiated away as IR. But given : It is unknown if our current IR detection is sensitive enough to see this reflector waste IR from 1500 ly away.

      Should we not also expect a glint of increased flux every now and then as this proposed reflector/star-shade passes behind Tabby’s (as seen from our line of sight) and some energy is spilled out and directed our way by chance?


      A “Niven Ring” seems to me the best fit for the shape of the largest transits. The excess IR that we do not detect but which MUST exist may be direction-ally radiated (somehow!) and/or too low in temperature (<100K) …and/or we are CLOSE but do not quite have the required sensitivity to see it. Have to wait for JWST.

  • Anton May 22, 2016, 8:17

    It’s funny how a “senior professor” seems to be really upset by an amateur Astronomer pointing out an obvious and most likely correct solution to otherwise an unexplained phenomenon and instead of accepting his own mistake goes on a predictable rant to try to defend his original decision,

    Hippke’s explanation is the Ockam’z razor to this whole dimming effect that was observed without any explanation given.