KIC 8462852 Analysis - you can participate

Tabby Boyajian has a new initiative related to the analysis of light coming from the star known as Boyajian's Star that I've blogged about quite a bit here and spoken about over at the Wow! Signal.

The new initiative is about making telescope images in various wavelength bands from the Las

One of the Las Cumbres Telescopes

Cumbres Observatory network available in a regular basis for community analysis. These images will be centered around Boyajian's Star.

Citizen scientists will crack open the images, analyze the variations in the star's brightness at different colors, and look for emerging trends. It's a great project for a science or math class, or anyone of any age or background interested in participating. All you need is a computer, an internet connection, and the willingness to learn. Help is available with every  step of the process.

The best way to get started is to go on over to the subreddit set up for this purpose, and ask your questions. Download and install AstroImageJ, grab the training images, follow the how-tos and other guidance we have published, and learn by doing. We are putting more information there almost daily, and we expect the first batch of images in about 3 weeks, weather permitting.

You will be participating in solving a scientific mystery. We don't know where the very fine dust is coming from that is causing the deep dips in brightness Boyajian's Star, and there is very likely other material involved that we haven't yet got much of a clue about.  The way these kinds of mysteries are typically solved is through lots of persistent and consistent effort by lots of people. Why not you?

I'm still perplexed - Tabby's Star Update for November 2017

Update: 29 November 2017

I've been meaning to put out an update for the last several months, and just when I am poised to do so, something else happens. So, here it is is, and I may need another update soon. It' s been an eventful few months, and if you haven't been following closely, you may want to read this.

The tl;dr

Kickstarter-funded observations of the star by the Las Cumbres telescope network began in 2016. There was a Winter interruption when the star was too close to the sun, but observations resumed in the Spring. From about mid-2016 there was a prolonged dimming episode which I am tempted to assume was related to what followed. In May, we saw our first of four dips, during which the overall slow dimming stopped and turned into a slow brightening. After the last dip in mid September, the star brightened for about one month, levelled off in brightness, and lately has been slowly dimming again. There are some new preprints out that contain some interesting tidbits.

July 2017 Update on Tabby's Star

If you want to know what is going on day to day with Tabby's Star, then the site Where's The Flux is an excellent resource. If you want to catch up on the basic info with sourced facts, you might want to check out the Wiki on /r/kic8462852. It includes a timeline of what has happened so far and a list of information sources - both the professional literature and more accessible materials as well.

In this post I'll try to create a bit more context without going overboard on the speculation. People love speculating on this star (as do I), but really very little of it is justified at this point. The hard work of observing and phenomenology has to take precedence. My main focus has been on figuring out the broad strokes of what it is we've been seeing since October of 2015 when this ordinary star suddenly became the focus of intense study.

More on the AAVSO trends for Boyajian's Star

My earlier post on the dimming of KIC 8462852 that might be observable in the AAVSO photometry was looking for a single trend line, which seemed to be just observable above the noise. I hedge there, because there are always assumptions not far below the surface that might spoil the result. The human brain and randomness are old enemies, and often when we want to see a pattern, it's just nature playing tricks on us.

Updates to the Century-Long Dimming of Tabby's Star

In the main post on Tabby's Star, I brought up the subject of Bradley Schaefer's contention that Tabby's Star is slowly dimming over the course of a century or so, primarily based upon his analysis of the Harvard library of photographic plates. Not everyone agreed with this, but for different reasons. This post is to absorb updates to this story, at least for a while

Just to be clear, the initial findings of Boyajian, et. al. concerning the star are based upon the Kepler data, some previous surveys, and follow-up observations, and are not affected by this controversy. It may be an additional piece to the puzzle, but ti may also turn out that Schaefer is wrong. Of course, what we don't know is what happened before the plate library started in 1890, and we have limited information since 1989. If Tabby's Star has in fact been dimming, we don't know how long it has been going on.

What Schaefer did was to look at both the digitized library and his own estimates of the star's brightness (in a standard band called Johnson B) directly from the plates, using known comparison stars on each plate. He found it necessary to reject a certain subset of plates he found unreliable. He fit both sets of data to a straight line, as well as data for certain "check stars" nearby with  similar color. He found that the brightness of the check stars had not changed over the 20th Century, but that Tabby's star had.

Astronomical brightness is measured on a logarithmic scale, with dimmer stars having a higher magnitude. An easy way to remember it is that a star 100 times dimmer will have a magnitude difference of 5. That is, if one star is magnitude 7, then a magnitude 12 star is 100 times dimmer. So, one or two tenths of a magnitude is a noticeable dimming. Vega, a bright star in the Summer Triangle, is almost zero magnitude, and probably the dimmest star you can see with the naked eye would be Magnitude 6. Astronomers use particular filters to measure brightness, and in the standard "Johnson B" filter, Tabby's Star is is in the neighborhood of 12.2. That makes it pretty dim, but that is because of its distance, almost 1500 light years away. Vega, on the other hand, is quite close as stars go at just about 25 light years away.  Tabby's star is actually about 4 times brighter than the Sun.

When Schaefer studied the B magnitude for Tabby's Star, we concluded that it was dimming relative to his check stars. There is no way that a star like this - a so-called "main sequence" star should exhibit large variations in brightness over such a time period - no one knows of any other exceptions, and some well-validated models tell us that is what we should expect.

So, is Schaefer right? One of the criticisms of his work is that he used measurements from DASCH that may have issues - the so-called "flagged" measurements. The reasons a measurement might be flagged vary. Schaefer rejected many of these, but not others.

Let's look at the DASCH light curve for Tabby's Star using the unflagged points only. One you notice right away is that the unflagged points are a small minority of all points. Anyway, here it is:

 

Tabby's Star (KIC 8462852), unflagged measurements only, Kepler Calibration

While the curve looks fairly flat, I get that it does show an average decline similar to Schaefer's result. Note that "Menzel gap" starting in the 1950s, during the period when Harvard was not collecting plates because the funding was directed to other projects. The points on the right of the gap are mostly below the average, and the points to the left tend to be above it a bit, but we don't need much dimming to have a real anomaly on our hands. On top of this, B2015 report a recent B magnitude observation (presumably about 2014, but no date is given) of 12.26, which is well below the line.

Schaefer's check stars don't appear to show this trend when using only unflagged points. In hisl atest preprint in April 2016, he changes the check stars to improve their closeness to Tabby's star both in color, magnitude, and proximity, but the conclusion is the same - Tabby's star varies more than the check stars.

If you look in Table 2 of Schaefer's paper, you will note that he finds that the B magnitude near the end of the 19th century was about 12.265. That is much brighter than the more recent 1987 magnitude of 12.458 - roughly 20%.

The American Association of Variable Star Observers, has been keeping an eye on Tabby's Star since Boyajian, et. al. came out in the fall of 2015.  here is what they have seen so far (early May of 2016):

The B magnitudes they are seeing are about 12.4 - consistent with Schaefer, but a bit above his trend line. There are subtleties here that are a bit tricky. For example, the bluish-white Tabby's Star is far enough away that there is measurable reddening of its light by the interstellar medium, and I'm not sure this is accounted for in the same way in all systems. 

The AAVSO plots don't show the error bars, and we have a data gap of almost 30 years. Is it possible the star's brightness levelled off in the 1980s and hasn't dimmed all, and maybe even brightened slightly since then? We may never know, but by taking more data, we can monitor longer and longer term behavior.

As noted in the last Wow! Signal episode, it's now time for the photometry experts to compare notes and hammer out a consensus on this issue. Is there compelling evidence that Tabby's Star is dimming? I think so, but I could wrong - there's a first time for everything.