Friday, January 5, 2018

The Elsie Paper

Noteafter this was nearly done,  couple of people pointed out to me the simultaneous release of a preprint by Deeg+ that reaches essentially the same conclusion as the Boyajian+ paper, but uses a different method, and covers all 4 2017 dips.

This post is a slightly updated text version of Wow! Signal Burst 25, which was was being released on the 3rd of January 2018, almost simultaneously with a press release announcing a new paper on Tabby’s Star by Boyajian, et. al., titled The First Post-Kepler Brightness Dips of KIC8462852 

In a previous post, I went over the events of last summer and into the Fall of 2017.  I recommend that one first if you are unfamiliar with those events, and also to Tabby's Star for the Perplexed. We also had a conversation with astrophysicist Eva Bodman on the Unseen Podcast in October 2017 in which we discussed recent developments.

To provide a very quick summary there were 4 brightness dips observed from Boyajian’s Star over the summer, with increasing depth and duration. These were named, in order, Elsie, Celeste, Skara Brae, and Angkor. Following the Angkor dip, there was a prolonged period during which the star’s brightness increased, informally dubbed “the blip” - and now the blip may be repeating.  In Burst 24 we also discussed the published evidence we had to date that whatever is causing both the dips and the long term dimming must have as a major component, very fine dust.

The new paper is about observations taken over that period of time in 2017, but is primarily focused on Elsie. Since no one knew at the time that there would be more dips after Elsie, a lot of resources were martialed to monitor that one dip.

The paper is in 5 Sections and an appendix. It discusses a large number of observations taken over the summer, including SETI observations, and discusses their implications for models explaining the star’s behavior.  What I intend to do in this burst is briefly visit each section and explain what’s going on in more depth than you’ll get in a press release, but probably less than an undergraduate astrophysics class. I’ll try to explain a few concepts that are important to the paper, but I won’t cover every detail.  Any mistakes or missions are my fault alone.

Also, note that at several points in this paper, papers currently in progress are mentioned. More information is coming. In particular, the infrared observations are still being written up. I hope there will be some interesting findings there.


Section 1

Let’s start with the Introduction. This goes over pretty much the same ground as covered in our previous posts addressing Tabby’s Star. It begins with the Planethunters discovery of the oddities of this star based on the Kepler space telescope data, and takes us up to the start of Elsie, although some of the literature reviewed in this section was first released post-Elsie.


Section 2

Section 2 is the big section, covering photometry, infrared photometry, spectroscopy, polarimetry, and radio SETI observations. I’ll explain what each of those is when we get to it.

Let’s start with section 2.1, which is photometry.

Photometry is a measurement of how the brightness of a star varies, and how this variation depends on which band of light wavelengths you use. The brightness in turn depends on a number of things, including how much interstellar dust and gas the light has to travel through to get to Earth. A graph of the measured brightness vs. time is called a light curve. The first part discusses the monitoring that was going on since 2016 using the crowdfunded Las Cumbres telescope network. When photometry using the Las Cumbres network first noticed Elsie beginning on the 19th of May, a number of other telescopes joined in on the observations. Figure 1 in the paper shows Elsie light curves from 12 different observatories. These are all detailed in the Appendix. They all show the same basic shape for Elsie, but vary in depth depending on the color band of light observed.

Section 2.1.1 was one of the most interesting to me, because it describes the dips in more detail than we’ve seen before. For example, the paper notes the very shallow decline in brightness between between Celeste and Skara Brae - which I called DWAIN at the time (Dip Without an Interesting Name), although I have also thought could be interpreted as the long term dimming bottoming out. The paper leaves open the interpretation of DWAIN.

I am particularly intrigued by the third dip known as Skara Brae, and details on the “twinkle” event at the center of Skara Brae, which they describe as a very short duration dip (see Figure 3) that bounces to a higher brightness. They compare the twinkle to the Kepler Day 1540 dip, which does look similar, but they caution against reading too much into that. It’s still not clear to me how any model out there now is going to explain Skara Brae, but most likely I’m not applying enough imagination.

There is also some interesting discussion of the steepest dip, Angkor, and how brightness hovered just below normal before it fully recovered.

Section 2.2 - Spectroscopy  - is what we’ve only had limited information about so far.

In photometry, we look at broad bands of color (or equivalently, wavelength) and measure the brightness of the star in that band. In spectroscopy, we are interested in a very detailed account of how the brightness varies with wavelength -its spectrum - since different substances either emit or absorb light at very specific wavelengths. Astronomers have been doing spectroscopy for generations, and can learn a great deal about a star from studying its spectrum, including how fast it is moving toward or away from Earth - also known as the radial velocity.

You might guess that because we are dividing the light up into many small pieces, we need a more powerful telescope to get good data, and you are correct. The big telescopes used for spectroscopy are far more expensive and harder to schedule, so we can’t just monitor the star all the time with spectroscopes. The smallest telescope detailed here was the 3 meter Shane telescope, and the other was the 10 meter Keck telescopes in Hawaii.

Although more details are forthcoming about the spectroscopy, one of the things discussed in the paper was an effort to measure any radial velocity difference between the dips and the times when there were no dips. Long story short, no statistically significant difference was found. We already knew from earlier observations that Boyajian’s Star did not have a close companion, and thi confirms it, although a big planet could be still be fairly close to the star.

Also, the team notes that the usual absorption lines you would expect to see from the interstellar medium are there, and they don’t detectably vary during the dips. It is suggested that we need to see a deeper dip in order to really measure this well.

The bottom line for spectroscopy - so far - is that nothing obviously strange happened to the star’s spectrum during the dips.

Section 2.3 is a short section about the photometry in infrared light taken by the NEOWISE space telescope. All we know is that no change in brightness during the dips was observed at all. Once again, we are told this is more information coming in two new papers.

Section 2.4 talks about polarimetry, or measurements of the polarization of light coming from the star. Essentially, the polarization observed is very probably due to interstellar dust and has nothing to do with the star or anything happening close to the star.

Section 2.5 describes radio SETI efforts during the dips. Here we are looking for very powerful, artificial radio signals coming from the star using the SETI Institute’s Allen Telescope Array in California. In the radio band that they searched, they found no such signals.We have discussed on this podcast before why this is not surprising, even if you think an ET civilization is operating near Tabby’s Star.

Section 3

Section 3 is analysis, and Section 3.1 is very important. The results are summarized in Figure 7 of the paper, which shows after some analysis of the photometry data, that there are distinct differences in the depths of the light curves at different color bands.  Between the shortest and longest wavelength bands, the ratio of the dip depths is almost a factor of two - 1.94. The word astronomers will use for this color variation as that the dips are chromatic. That is very hard to explain with a solid object of any sort.

Section 3.2 takes the color ratios observed and uses this to perform a detailed analysis of the particle sizes that could be scattering the star’s light differently at different wavelengths. This gets into some detailed mineralogy that I won’t detail here.

A number of different minerals are considered, but all require that the particle sizes are much less than 1 micron, and optically thin. Optically thin, simply put, means that the light is shining through the material, and is not being so much blocked by it, but scattered. This sort of small dust particle can not orbit the star, because the star’s radiation pressure will blow it out of the system, so whatever is producing the dust is producing it more or less continually. This is one problem I have with the symmetry of the dips - I would expect the cloud of dust to look more like a comet’s tail. I suspect that what we are seeing isn’t just a transit signature, but a burst of dust production that corresponds with a transit.

Section 4

Section 4 is the Discussion Section, in which the team compares the new data to several models. They show, for example, that it’s hard if not impossible for a giant artificial megastructure to produce the kind of color variations seen (or just any very large opaque object).  Just because we think that there is dust blocking the star doesn’t make it easy to figure out where that dust came from, though.

They lend some encouragement to a model that involves a lot of exocomets or dust-shrouded planetesimals - something has to be producing the dust after all, and they strongly hint that we would need target-of-opportunity time on the James Webb Space Telescope to really narrow down the parameters of such a model. This telescope, if we are lucky, will launch some time next year, although Boyajian’s star is not one of its early science objectives.

Also discussed is a simple model to determine whether stellar cool spots might be responsible for the Elsie family of dips. The result is a firm “maybe,” and they don’t rule out more complex models of how the star might be cooling or changing radius.What it will take to rule those models out isn’t clear to me.

They also caution against any assumption that the events seen to date are periodic with the information we have to date. It certainly seems to me that the system we are observing is evolving a great deal, and the recent “blips” throw a new wrinkle into that. Perhaps, they note, observations in June of 2019 might help us to see a repeat of the first big dip in the Kepler light curve.

Section 5

Section 5 is the conclusion section, and takes a look at the bigger picture. The use of crowdfunding is one element they highlight, and they summarize the key results, especially the fact that the 2017 dips were chromatic.  Finally, they make the case for further monitoring over the long term. You can do something about that by going over to wherestheflux.com and making a donation to pay for telescope time.

I have had some e-mails and other communications that worry that Alien Megastructures are now out the door because of the fine dust. As I’ve discussed before, there is a major problem with the classic sort of Dyson Swarm megastructure hypothesis for Boyajian’s Star that have nothing to do with the chromatic dimming - the constraints on excess infrared brightness that Dyson predicted in his 1960 Science paper "Search for Artificial Stellar Sources of Infra-Red Radiation". These constraints get just a bit tighter now with the NEOWISE observations, although the details are in work. However, if ET is operating around this star, what are the chances they would be kicking up a considerable amount of fresh dust? What other observables might their activities have? I don’t have an answer to those questions, but I think they are intriguing questions, and I plan to look further into them