Planet Hunters Talk

KIC 8462852 (Tabby's Star) - Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989

  • johnfairweather by johnfairweather

    Centauri Dreams


    KIC 8462852: A Century Long Fade?

    Posted: 14 Jan 2016 07:31 AM PST

    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.

    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).

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  • ajamyajax by ajamyajax in response to johnfairweather's comment.

    648,000 seems possible to me. (Space is a great place for large numbers that are well beyond those in our everyday lives.)

    "Scientists estimate the Kuiper Belt is likely home to hundreds of thousands of icy bodies larger than 60 miles (100 km) wide, as well as an estimated trillion or more comets."

    "The Oort Cloud lies well past the Kuiper Belt, and theoretically extends from 5,000 to 100,000 times the distance of Earth to the sun, and is home to up to 2 trillion icy bodies, according to NASA."

    From: http://www.space.com/56-our-solar-system-facts-formation-and-discovery.html/

    Posted

  • johnfairweather by johnfairweather

    Someone has just published a paper on how long does it take to build a Dyson Sphere - http://beta.briefideas.org/ideas/424bb64cf38eb9d7db0dae57dec3d28d

    Posted

  • ajamyajax by ajamyajax in response to johnfairweather's comment.

    Hmmm, that looks like our Dr. Chris to me:

    http://blog.planethunters.org/2015/10/16/comets-or-aliens/

    Posted

  • johnfairweather by johnfairweather

    Yes, it is Brooke & Chris.

    The ending reminded me of the ending in Asimov's Foundation, in that there is a probability of everything coming together.

    In this case maybe a society has developed along different lines to Earth's, so that in the 1400 Earth years, would a society stay that stable?

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  • JKD by JKD

    KIC 8462852 there is also a long discussion in „Old Talk“ to be found in „Search 8462852“

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  • johnfairweather by johnfairweather

    Thanks for that, I see the discussion goes back five years.

    Should be a graph somewhere, showing how the magnitude is decreasing. Should be able to work out what size of object could decrease the magnitude, by that amount.

    Posted

  • johnfairweather by johnfairweather

    Centauri Dreams


    KIC 8462852: No Dimming After All?

    Posted: 27 Jan 2016 05:56 PM PST

    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.

    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.

    Posted

  • ajamyajax by ajamyajax in response to johnfairweather's comment.

    Well Michael Hippke and Daniel Angerhausen looked at 28 F-type main sequence stars, and Bradley E. Schaefer looked at just one, correct?
    An objective outlook would favor the wider sampling method, in my view. But agree that more work likely to follow.

    Posted

  • johnfairweather by johnfairweather

    Methinks, I see a conference coming on -

    Bradley Schaefer: A Response to Michael Hippke

    Posted: 28 Jan 2016 01:25 PM PST

    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 usage 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.

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  • CMEdwards by CMEdwards

    Not to further muddle the issue, but a passing reader posting in the comments thread for this editorial

    http://www.centauri-dreams.org/?p=34927

    says that he tried informally repeating Hippke's analysis and found the majority of Hippke's sample were variable stars. He claims that when you kick out all the variable stars, KIC 8462852 is still the only non-variable star in Hippke's data that is fading.

    It's just just a random claim on the internet right now (no details to establish rigor, not even a proper author), but it is the sort of repetition that needs to be done.

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  • johnfairweather by johnfairweather

    You might find some useful papers in this article, under the reference section -
    http://talk.planethunters.org/#/boards/BPH0000007/discussions/DPH0001fdy

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  • ajamyajax by ajamyajax

    Or this:

    "Drifting Asteroid Fragments Around WD 1145+017"

    S. Rappaport, B.L. Gary, T. Kaye, A. Vanderburg, B. Croll, P. Benni, J. Foote

    http://arxiv.org/abs/1602.00740

    (new)

    "Are asteroid fragments drifting around a distant white dwarf star?"

    by Tomasz Nowakowski

    http://phys.org/news/2016-02-asteroid-fragments-drifting-distant-white.html

    Posted

  • Byran by Byran

    Find possible to transit frequency

    http://arxiv.org/pdf/1509.03622v2.pdf
    Printed 26 January 2016

    There is another possible periodicity that is worth discussing briefly. In Table 1, we summarize the times and depths of 10 discrete dips present in the Kepler light curve, also labeled in panel ‘(b)’ – ‘(e)’ of Figure 1. If we examine the two most prominent dips (D1568 and D1520; also see panel ‘(d)’ in Figure 1), we see that they have a separation of 48.8 days. We can also see that the D800 dip (dip #5 in Table 1) is separated from the D1520 dip by 15 of these intervals, if the interval is more precisely defined to be 48.4 days. Furthermore, the very shallow dips early in the Kepler time series at D260 and D360 are very close to 26 and 24 of
    these 48.4-day cycles from the D1520 dip. The other five identified discrete dips (four of which are very shallow), also listed in Table 1, are about a half cycle out of phase with this period to within ±5% of a cycle. In this exercise, we have neglected the fact that the three most prominent dips in the D1500 region are quite highly structured, and they also have additional minima whose times could have been tagged and included in the analysis. At this time we do not ascribe any particular significance to this period, but it is something to bear in mind as more data on this object become available.

    enter image description here

    *We thank Josh Carter for pointing out the possible 48-day periodicity. *

    Possible explanations for the periodicity?

    1. Artifact handling
    2. Unknown type of periodic variable stars
    3. Constellation astroengineering objects (like NASA A-train and GEO ring)

    enter image description here

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