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FU Orionis: Implications of Sudden Brightening for Planet Formation

I would like to thank the many Centauri Dreams readers who contributed to the successful Kickstarter campaign to fund a year’s worth of study of KIC 8462852. As I write, there is less than an hour to go, but we have already gone well over the needed $100,000 mark. Congratulations to Tabitha Boyajian, and thanks for all the work she and her colleagues have put into this effort. Now we have a year of observations ahead using the Las Cumbres Observatory Global Telescope Network. The long-term observations will be crucial because we don’t know what to expect in terms of sudden dimming in this star’s light curve.

What a pleasure it is to write for this audience. Readers here have played a large role in pushing this project over the top, and we’ll follow the work on KIC 8462852 closely in coming days. Meanwhile, have a look at Penn State’s Jason Wright discussing ‘Tabby’s Star.’

Speaking of Unusual Stars…

If KIC 8462852 is a star that some believe is becoming less bright over time (the controversy between Bradley Schaefer and Michael Hippke is well documented here), FU Orionis is the opposite, a star whose notable brightening in 1936 is the most extreme such event we have seen around a star of Sun-like size. This is intriguing stuff, because we seem to be looking at a solar system in its infancy, undergoing a process that our own Sun may have experienced on the way toward the formation of the planets in our system. If so, this stellar activity would have changed the chemistry in the surrounding disk, with implications for how planets form.

The 1936 brightening event took FU Orionis, some 1500 light years away in the constellation Orion, from an apparent visual magnitude of 16.5 to 9.6, and the star is now around magnitude 9, with visible light observations showing a slow fade in progress. Stars like this — V1057 Signi is another — are young pre-main sequence stars in a class named after FU Orionis and nicknamed FUors. All exhibit extreme changes in magnitude and spectral type, evidently caused by the star consuming inner disk materials in a sudden feeding spree.

Joel Green (Space Telescope Science Institute) and colleagues wondered when FU Orionis might return to pre-1936 brightness levels, and what effect such brightening events could have on early system formation. The team used infrared data from the Stratospheric Observatory for Infrared Astronomy (SOFIA) and checked it against Spitzer Space Telescope data from 2004. Green presented the results, showing the star’s behavior over the twelve-year interval, at the just concluded meeting of the American Astronomical Society in San Diego.


Image: This artist’s concept illustrates how the brightness of outbursting star FU Orionis has been slowly fading since its initial flare-up in 1936. The star is pictured with the disk of material that surrounds it. Researchers found that it has dimmed by about 13 percent at short infrared wavelengths from 2004 (left) to 2016 (right). Credit: NASA/JPL-Caltech.

The brightening of FU Orionis observed in 1936 was the result of the infant star devouring gas and dust from its surrounding disk, a three-month process that caused it to become 100 times brighter in short order, heating the disk to temperatures up to 7000 K. It’s clear from the most recent study that the star continues to ingest disk material. According to this JPL news release, it has consumed the equivalent of 18 Jupiters in the eighty years since the first outburst.

Even so, the SOFIA measurements confirm that the total amount of visible and infrared light from FU Orionis has decreased by about 13 percent during the twelve years since the Spitzer data were taken. The dimming is occurring at short infrared wavelengths only, an indication that about 13 percent of the hottest disk material has disappeared even as cooler materials have remained intact. Says Green:

“A decrease in the hottest gas means that the star is eating the innermost part of the disk, but the rest of the disk has essentially not changed in the last 12 years. This result is consistent with computer models, but for the first time we are able to confirm the theory with observations… The material falling into the star is like water from a hose that’s slowly being pinched off. Eventually the water will stop.”

Based on this work, we could expect FU Orionis to return to 1936 brightness levels within the next several hundred years. Still unanswered is the question of what set off the rampant activity in the first place. The fact that a protoplanetary disk can experience such sudden activity is striking. Green’s team believes that a similar event in the early Sun’s history could explain why materials close to the Sun have a different composition than those farther out, accounting for the different abundances of certain elements on Mars than on the Earth. The brightening would have had a much larger effect on the inner disk than on more distant materials.

The presentation is Green, “The Evolution of FU Orionis Disks,” American Astronomical Society, AAS Meeting #228, id.#308.05 (abstract).


Comments on this entry are closed.

  • Christopher Phoenix June 17, 2016, 18:47

    Congratulations to Tabitha Boyajin for her successfully funded Kickstarter, can’t wait to see what further observations of KIC 8462852 will bring! We are all eagerly awaiting any further data on this enigmatic object.

    Regarding FU Orionis–it is interesting to think that this brightening event actually occurred in 436 AD., only we could not see it yet ’cause the light from the event to 1,500 years to reach Earth. Around the time FU Orionis began brightening, the Western Roman Empire was crumbling from weak internal rule and the onslaught of barbarian tribes. Rome had been ransacked by the Visigoths about two decades before, and this trend of military disasters and instability was only going to continue.

    Lots of interesting things were happening elsewhere too, of course, but the decay and fall of Rome has a way of arresting attention! Suffice to say, most of the human race was (and still is) more concerned with terrestrial strife and wars than they were with the sky. But still that light began crawling towards the Earth, to eventually fall into the receptors of instruments not yet built. The invention of the telescope was still over eleven centuries away.

    The events we observe happening around other distant stars are not just remove in space–they are remote in time as well.

  • Michael June 18, 2016, 4:53

    Its great news that the funding is now in place, I still think it is a planet with moons causing the dimming but alas we will have to wait with baited breath.
    I will and no doubt others will be watching this one closely, I doubt we will have to wait long.

  • andy June 18, 2016, 11:38

    I was interested to see that FU Orionis has a supergiant spectral type, looked a bit further and found this is due to radiation from the disc rather than the star itself. They must be pretty impressive close up.

  • Michael June 18, 2016, 13:43

    I am not surprised that there is sporadic increases in brightness as the central star accretes mass it is not the only object gaining mass. There are other clumps of mater increasing in mass and they can be consumed by the greater mass. I suspect this to be a natural process which can lead to close in planets that we see during our observations.

  • Michael Spencer June 19, 2016, 7:21

    “What a pleasure it is to write for this audience”

    When I read these words, Paul, I had to laugh a little, thinking what a pleasure it is to read your site. Your writing style and respect for your audience has affected my own poor efforts. And I’m in awe of your prodigious output, I must say.


  • Paul Gilster June 19, 2016, 8:53

    Very kind, Michael. Thank you. As to ‘prodigious output,’ it varies now and then depending on my energy levels, but I’ll try to keep it coming ;-)

  • Michael June 20, 2016, 10:54

    I think this youtube clip goes a long way to explain planet formation.