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.

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.