Tuesday, November 28, 2017

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.

The Quiet, Slowly Dimming Period

Before the first dip in May of 2017, we already had strong reason to believe that the star had been slowly dimming for the better part of a year from the AAVSO data. I posted this tentative conclusion of 1-2% per year dimming in April of 2017. I compared this to the dimming Montet and Simon has mined out of the Kepler Full Field Image data (see Wow! Signal Episode 33), which was a precursor to a series of dips, and wondered if a dip might be coming soon.  

The Sequence of Dips 

Things started to get really interesting in Mid May of 2017, just as the Las Cumbres Observatory was getting into full swing monitoring the star.


The first dip of 2017 started around the 18th of May 2017, and lasted for about 5 or 6  days, during which the star’s brightness declined more than 1%, and even closer to 2% at peak. 1% may not seem like much, but that is a measurable decline in flux and not something that is normally seen with main sequence stars like this, as we discussed in Tabby's Star for the Perplexed. Also, 1% is way too big to be a transiting planet for a star this size.  This dip was later named “Elsie” after a vote by the Kickstarter supporters.

Since no one knew whether there would be more dips or not, an Astronomer’s Telegram went out, and a number of astronomical instruments pointed at the star in those few days. We don’t have all the data made public yet, but we do know that this dip resulted in a slight reddening of the star - that is, the dip was definitely deeper in blue light than in red. This is strong evidence that whatever was blocking the star was not a single solid object, but had very fine dust as a major component. These dust particles would have to be well under 1 micron in size, which is typical of dust seen in the interstellar medium or comet dust, but such small particles in orbit around the star would not last long before the pressure of starlight (which exceeds the force of gravity) drove them away.


  A second dip started on the 11th of June, and was eventually named Celeste. Celeste lasted about 2 weeks, and was also a bit more than a  1% drop in brightness. We don’t yet have a full report of the observations taken during Celeste, but so far, I don’t believe there was much difference from Elsie. Following Celeste, there was as period during which the brightness of the star seemed unsettled. I called this DWAIN, but it wasn’t a dip. What we can see now in retrospect was the brightness of the star - neglecting the dips - was bottoming out.

Skara Brae

The third dip was, to my mind, the strangest. And began about the 2nd of August. This dip was named Skara Brae, and lasted about 16 days until August 18th. In addition to its duration (a typical planetary transit is well under 1 day), what makes Skara Brae stand out is its symmetry, the linear slopes of its sides,  and a high degree of photometric activity in the exact center of the dip, when the brightness was briefly down 3%.


The fourth and largest dip - Angkor - started around the 28th of August, dipped to about 2%, and lasted until the 14th of September - the exact times when a dip starts and ends are a little fuzzy - so it lasted about as long as Skara Brae, and was deeper on average, although it seems to have been a bit more ragged and not quite as cleanly symmetrical - it seemed to bottom out (sharply, like Skara Brae) on around September 10th. One notable thing about Angkor is that it was the first dip that the AAVSO data clearly caught. This is probably because of its depth.
A recent LCO light curve from http://www.wherestheflux.com/

The Post-Dip Brightening and Dimming Again

After Angkor, it became apparent that the star was brightening slowly in the shorter wavelength. In B band, this brightening may have been as much as 2% from the minimum between Celeste and Skara Brae. We don’t have as much data in the longer wavelength I band data from AAVSO, but the I band brightness appears to be roughly flat after Angkor. I say “appears” because the scatter in the I data is around 2%, so it will take a while to see a trend emerge.

I would have enjoyed it if the star had just kept brightening for a long time, but in mid November, Bruce Gary noticed a rapid decrease in brightness over 1 day, and a slow decline thereafter, and for now we have seen the star give up about half of its brightness gain since Angkor.

Some New Papers in Process Made Public

Scientific papers come out fast in preprint these days, as the recent flurry of papers addressing the hyperbolic asteroid Oumuamua made clear. This is partly because astronomers can reduce their data very rapidly, and also because they can collaborate electronically. This has also been happening with recent developments on Boyajian's Star.

In August, a couple of new preprints came out, and both dealt with the long term dimming of the star. As always, there will be links in the show notes. They generally agreed that there had been a slow dimming trend since 2016, but the paper by Simon et. al. dug up a 4000 day span of data from the All Sky Automated Survey, and found that there had also been two periods of brightening. This isn’t surprising that the star doesn’t just dim all the time, but no one had found an example until now.  The second preprint, by Meng, et. al., also looked closely at the recent slow dimming using data from the Swift space telescope as well as ground based observations. They found a reddening in the dimming which was different than reddening we see from interstellar medium, suggesting that whatever is causing the dimming is orbiting around the star.

In September, there was an interesting preprint by Steele, et. al., that looks at the polarization of light during the period of the dips. I'm not yet sure if these measurements constrain any hypotheses that much, but I'm glad someone took a look at it.

Another preprint, by Wyatt, et. al. published in October, looks at the Exocomet interpretation in terms of the Elsie dip. This paper revealed that there is infrared space telescope data from NEOWISE (not yet made public) during Elsie, and that no increase in emission was detected during Elsie. They give the integrated depth of Elsie as 6.5 %-days. Angkor  and  Skara Brae had much greater integrated depths, and so there would be more hope of detecting an infrared excess. If there are such data, they haven’t been released yet.

What We're Looking Forward to Now

As the star comes together with our sun, photometric observations from the ground will get harder to come by until early Spring of 2018. Only observers in the more northern latitudes will have a good look at it when the sun is down. We hope that some space-based observing time will be made available, as it was last Winter, so that we can keep monitoring for more dips.
A lot of people think they know something about the periodicity of the dips, but this remains speculative until someone can clearly demonstrate at least three closely similar events with an even spacing between them. To me, the system is clearly evolving, and fine dust is blocking at least some of the flux, and this dust will not be in a periodic orbit - it must be produced afresh by something else that may or may not be doing so periodically.
In the near term, we await a paper by Boyajian, et. al. that brings together all the observational data from the dips, or at least from Elsie. We have reason to believe that is in work,  but it's likely be time consuming to get it right, and will be worth waiting for.

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