[Guest Post] Newly published paper: The KIC 8462852 Light Curve From 2015.75 to 2018.18 Shows a Vari
Below is a message to you from Prof Brad Schaefer describing a paper he lead that was just accepted for publication.
Prof. Boyajian has asked me to write a guest posting for you on a paper we have just written on KIC 8462852. Our paper has just been accepted to the Monthly Notices of the Royal Astronomical Society, and it will be published in about two months. The abstract summary is below, and a PDF file of the paper is at this link. What we have done is collect 19,176 magnitudes from 2015.75 to 2018.18 in four colors (B, V, R, and I). This starts just at the time that the discovery was announced, and this is complimentary to our Kickstarter program because we cover the year-and-a-half before we could start the LCO observations. Our observations are good for time coverage, but we lack the accuracy, time resolution, and speed-of-response that is the hallmark of the Kickstarter observations, as required to get fast response during a dip. Our light curve was taken both by the Kickstarter team at LSU plus by many of the world's best amateur astronomers reporting to the AAVSO. This has been a huge effort.
Our primary observational product is a light curve in four colors from September 2015 until recently. This is shown in the figure:
The line segment in the bottom right shows the times of the four dips in the Elsie group. (Note that the Kickstarter light curve is so much better for the needed fast time resolution.) There are two key science points from this figure: (1) First, we see that the WTF star is fading slowly over the whole 2.43 years, with dips superposed. Any such long-term fading of an isolated F-type main sequence star is whacko unique. This shows that the fading is on-going. And it points to the secular dimming simply being part of a continuum of variability with the Kepler and Elsie dips. (2) The shapes of the B, V, R, and I light curves are the same, except that the structure is much larger in the B-band and gets smaller towards the red bands. This point strongly rules out the possibilities that the occulter of the central star is any planet, any star, or any solid body. (You can see for yourself how passing a solid body, like your hand, in front of an extended light source, like a window or a bulb, will not change the color of the light passing along to your eye.) But the dimming of the light (more in blue and less in red) happens to the degree as exactly predicted if the occulter of the central star is an ordinary dust cloud. Ah, we now have a 'smoking gun' that the dipping and dimming of KIC 8462852 is caused by some huge cloud of celestial dust passing in front of the star. So the picture is that the dips are caused by some dense/small dust clouds passing in front, while over the years, the dust passing in front is getting thicker, with the star's skies getting hazier.
To get this 'chromatic extinction' (blue dims more than red light), the dust clouds must have a large fraction of the dust being comparable in size to the wavelength of optical light. But we have realized that such ~0.1 micron sized dust will all get speedily blown away from the star (on a time scale of months) due to the star's ordinary stellar wind and stellar radiation pressure. That is, the clouds disperse and scatter away fast. So if we see a dense/small cloud now making a dip, then the dust must somehow have been released or created within the last few months. With the dips having happened over the last decade (as seen by our Kickstarter program and by Kepler earlier), and the long-term secular dimming over the last century (as seen with the Harvard plates and the Maria Mitchell plates), we see that there must be a long steady supply of *something* that keeps making dust clouds in front of the star. In our new paper, we work out the astrophysics of this general scenario.
But this has left unexplained as to what is the mechanism that creates the dust clouds. Any such continuous creation of dust clouds along an orbit is completely unprecedented for anything we know of (or previously imagined) for our Solar System or anywhere outside. So at this point, all we can do is point to new ideas that have some plausibility. We can imagine some freak mega-comet that happened to be broken up, stringing out huge fragments along its highly elliptical orbit. And when every individual fragment gets close to the Sun, it will melt the dusty-ice, freeing the dust to form a coma and tail of dust. (From our own Solar System, we have seen comets breaking up and spreading fragments all along the orbit. This makes for meteor showers, and it made for the long string of comets for Comet Shoemaker-Levy 9 before it hit Jupiter. Each of these throws off dust when getting close to the Sun.) If such a freak mega-comet happened to have broken up with its orbit passing exactly between the parent star and our Earth, then we will see long successions of dusty clouds passing in front of the star, making the dipping and dimming light curve. This is all possible, but it is on a huge scale far-dwarfing anything previously seen or imagined. So now we have not only a strongly supported recognition that the occulter is a dust cloud, but also a general picture that maybe a long string of comets is the cause for the dust clouds.
It will be hard to test this comet-string scenario, much less to get proof. The best possibility that I can think of is to catch the WTF star inside a deep dip, to have a spectrum show narrow absorption lines with the right velocity, and to have the composition as expected from cometary material. To do this, we must catch a deep dip in real-time -- and that needs something like our Kickstarter program. So `Help Me Obi-Wan-Kickstarter, You Are My Only Hope'.
Let me introduce the authors of the new paper. As first author, I am at LSU, being both a very-long time amateur and professional astronomer, plus my wife and I contributed to the Kickstarter program early on. The whole LSU Kickstarter team is coauthors, including Boyajian, Ellis, Nugent, and Bentley. Tyler Ellis is Boyajian's graduate student, and he started by taking most of our early observations with the 24-inch telescope in the Baton Rouge southern suburbs at Highland Road Park Observatory (HRPO) in 2016. While we were taking these series of observations, we had the surprising and wonderful case of an undergraduate (Katie Nugent) asking to help and staying up late for the data-taking. We were lucky enough to get good data just before and during Elsie. Katie followed up by turning the next year's observations (for 2017) into a term project. For this, Rory Bentley, another LSU undergraduate helped out, with the observing shared on the 16-inch and 24-inch scopes at HRPO. (Rory has recently graduated from LSU, and is off to UCLA as a starting graduate student.) The other co-authors (Coker, Dvorak, Dubois, Erdelyi, Graham, Harris, Hall, James, Johnston, Logie, Oksanen, Ott, Rau, and Vanaverbeke) are observers observers who contributed a huge number of magnitudes, all with their own private telescopes (ranging from 8-inch to 28-inch in aperture), from the USA, Finland, Belgium, and the UK. These are among the best observers in the world, it is just that they are not paid for this long and sleepless work. I've worked with many of these people and used/needed their data in the past, including Barbara Harris and Shawn Dvorak who both independently discovered the nova eruption of U Sco, and Arto Oksanen who was so-valuably able to follow the nova event on T Pyx all the way through solar conjunction! These are the people who jumped on the WTF star immediately upon the announcement of its discovery back in September 2015, as they did not have to await observing proposals or funding proposals to be approved. And our paper has three astronomers from England (Wyatt, van Lieshout, and Kennedy) who provided calculations and expertise on the modeling of dust in orbit around the star. Our international team has put of-order 5,000 hours of time, all told, into working on this paper.
The KIC 8462852 Light Curve From 2015.75 to 2018.18 Shows a Variable Secular Decline
Bradley E. Schaefer, Rory O. Bentley, Tabetha S. Boyajian, Phillip H. Coker, Shawn Dvorak, Franky Dubois, Emery Erdelyi, Tyler Ellis, Keith Graham, Barbara G. Harris, John E. Hall, Robert James, Steve J. Johnston, Grant Kennedy, Ludwig Logie, Katherine M. Nugent, Arto Oksanen, John J. Ott, Steve Rau, Siegfried Vanaverbeke, Rik van Lieshout, and Mark Wyatt
ABSTRACT: The star KIC 8462852 (Boyajian's Star) displays both fastme scales from a year to a century. We report on CCD photometry of KIC 8462852 from 2015.75 to 2018.18, with 19,176 images making for 1,866 nightly magnitudes in BVRI. Our light curves show a continuing secular decline (by 0.023±0.003 mags in the B-band) with three superposed dips with duration 120-180 days. This demonstrates that there is a continuum of dip durations from a day to a century, so that the secular fading is seen to be by the same physical mechanism as the short-duration Kepler dips. The BVRI light curves all have the same shape, with the slopes and amplitudes for VRI being systematically smaller than in the B-band by factors of 0.77±0.05, 0.50±0.05, and 0.31±0.05. We rule out any hypothesis involving occultation of the primary star by any star, planet, solid body, or optically thick cloud. But these ratios are the same as that expected for ordinary extinction by dust clouds. This chromatic extinction implies dust particle sizes going down to ~0.1 micron, suggesting that this dust will be rapidly blown away by stellar radiation pressure, so the dust clouds must have formed within months. The modern infrared observations were taken at a time when there was at least 12.4%±1.3% dust coverage (as part of the secular dimming), and this is consistent with dimming originating in circumstellar dust.
PS: [Message from host] The LCO observations are happening because of the wonderful backers of the 2016 Kickstarter project. The Kickstarter campaign has ended, but we are still accepting donations to purchase additional observing time on the LCO 0.4m network. Thanks in advance for your support!