Friday, June 14, 2019

More Aladin on a rainy day - Gaia Alert 16bao

Update 17 Jun 2019: Some folks on /r/KIC8462852 think this is probably an eclipsing binary, and they make a decent case, but I still have questions.

Am I really onto something here? Well, let's just look at the facts as I can dig them up and see where it leads.

The star first came to my attention as the source of Gaia Alert 16bao which I have been following for some time now. Gaia has photometric data for this star (I'm pretty sure it's a star, as I can discuss later) going back to October of 2014, and the latest available is May of 2019 as of this writing, for a total span of 1674 days.

Here is the light curve from Gaia alert 16bao:

What we see are as many as 6 sharp dips over the course of about 800 days, and the rest of the time the brightness of the star is about steady. The deepest of this dips is about 1.7 magnitudes, which is very deep - about 79% dimming for a short time - over about 20 hours before recovering over about 10 hours. The sharpness and irregular spacing of the dips is reminiscent of Boyajian's Star. The dips are too deep, aperiodic and irregular for an eclipsing binary.

The Gaia alert gives the J2000 coordinates as RA=297.72688 degrees , and Dec = 23.55513 degrees. We can plug this straight into Aladin Sky Atlas to see what's there.  Right away we can learn several things.

Here's the image I get when I load in the ALLWISE color image:

This is in the galactic plane, so things get a little crowded. Now here's the Panstarrs image: 

It turns out, that this star is an object PanSTARRS has light curve data for, as we can find out by going to the PanSTARRS portal at MAST. I downloaded the data, which is for 5 filters (g,r,i,z,y), and which range from 481 nm in the visual, to 962 nm in the near infrared. Here are the five light curves, which run from June of 2009 to July of 2013 (R scripts available on github):

There is something weird going on with the data here.  I don't know how to diagnose if this is a problem with PanSTARRS, or if the source is doing that. If we look more closely and just g and r bands, we see big "dips" in r, but not in g. It's possible that we just don't have enough detections to tell.

OK, so to sum up: big dips apparent in the Gaia data, uncertain in the PanSTARRS data, but maybe.  The source is too faint for AAVSO,  ASAS-SN, or the Harvard plates (DASCH). I'm not sure if there are any other places to find photometry. If you start overlaying catalogs in Aladin, you don't get far.

Here are some more facts I can find:

  • It's in the Gaia DR2 catalog, and has a parallax estimate of 0.2437 +/- 0.0358 mas. That puts it at about 13384 light years away - almost 10 times further than Boyajian's Star. The Gaia G magnitude is 14.4 at that distance, which makes it quite a bit brighter than Boyajian's Star, which is listed at a G magnitude of 11.8 (bigger magnitudes are dimmer).
  • The star has a proper motion solution as well. All the surveys find it to be a star, including ALLWISE and UKIDSS.
  • It seems to be really red, and maybe even have an infrared excess. It's ALLWISE object number is J195054.46+233301.2, and its W3 and W4 magnitudes (different IR bands, with W4 being much longer wavelength) are 9.946 and 8.591 respectively.  That's a quie a difference in brightness between the two bands.
So, you've probably figured out where I'm going with this: with its big, sharp dips (bigger than Boyajian's Star) that look possibly like transits, a steady brightness when not dipping, and an IR excess (something Boyajian's Star lacks), is this a candidate for a megastructure star? 

Well, there is still plenty of room for doubt. It might be a particularly large and hot Young Stellar Object (YSO).  Stars like this are still forming planets, and there are big disks of dust orbiting around them that can cause an IR excess and deep dips, although I have yet to find an example of a dip as large as 1.7 magnitudes, especially for a big star, like the dips we saw from Gaia in 2016.  

Without more information, we probably can't know much more. The star hasn't had much attention from professional astronomers, and is too faint for amateurs.

We don't have high cadence photometry data for this star like we do from Kepler, but we will, later this year, have some high cadence data from TESS for about 2 months. Perhaps one of the dips will occur at that time, and we can see the what the profile matches, if anything.

All the data I've downloaded is on github.

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