WR 104: The Dying Star With Deadly Potential

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WR 104: A Wolf-Rayet time bomb?
DCL

The scoop: Last year, a star called WR 104 caused a stir. This massive Wolf-Rayet star is reaching the end of its life and astrophysicists believe it may explode as a supernova some time in the near future (in astronomical terms, this means tens to hundreds of thousands of years, so don't bet on anything happening soon).

However, WR 104 has a sinister side... it could erupt as the most energetic explosion observed in the cosmos: a gamma-ray burst (GRB).

As it's in our galaxy, and looks as if it's pointing right at us, there's concern that it could cause damage to Earth's biosphere should it explode as a GRB. With this in mind, Dr. Grant Hill, a researcher at the Keck Observatory on Big Island, Hawaii, has been investigating whether we are actually staring down the barrel of a potential GRB gun...

Ian O'Neill: Hi Grant!

Grant Hill: Aloha Ian

Ian: How's things going on Big Island? I hope it's less rainy.

Grant: Brightening up now... the tropical storm went past South Point earlier today and is now heading away from us.

Ian: Cool, I must admit though, I quite liked the Hawaii rainstorms. Great rainbows.

Grant: Local expression: No rain, no rainbows.

Ian: Hehe, true.

Are you ready for some questions to be thrown at you?

Grant: Sure, fire away.

Ian: Right, last year, I remember reading first about WR 104 and the potential danger it poses for Earth. A lot of it was media hype, but I found the science behind it fascinating.

How did you get into researching WR 104? Was it a well known star system before the concern that it was "facing right at us"?

Grant: I first got interested in Wolf-Rayet (WR) stars in general about 20 years ago while an undergraduate student. Over the years I was interested in various aspects of them and got interested in WR colliding wind systems almost 15 years ago.

I started observing WR 104 in 2001 shortly after some of the remarkable images of it were published that implied it was a colliding wind system. At that time it was immediately obvious that it appeared to be face-on, but the implications with regard to its possible future as a gamma-ray burst had not been discussed.

Ian: I do admit to having a bit of a love for Wolf-Rayet stars too — they certainly seem to be an exciting phase of a star's life. What kind of star becomes a WR star? They're massive stars aren't they?

Grant: Yes, the most massive stars (O type stars) are thought to evolve into WR stars.

Ian: How do we know WRs have gamma-ray burst (GRB) potential? Have there been any before-and-after observations of a WR turning into a GRB?

Or is it a certain "flavor" of WR star that explodes as a GRB as opposed to an 'ordinary' supernova?

Grant: Bearing in mind that I am not an expert on GRB's and the field is rapidly advancing... My understanding is that we haven't as yet caught a WR star going GRB in the act. No clear cut, before and after in other words. Theory predicts though that massive stars will eventually become supernovae. GRB's come in two flavors. What are called the long duration ones are believed to be rapidly rotating massive star supernovae in which the spin axis of the star is pointed towards you.

Ian: It's certainly an exciting field to be following, especially as it's developing.

I remember in the AAS meeting back in January, when I saw your talk about WR 104, that you were analyzing the star. Optical observations suggested that WR 104 was pointing right at us, but your results suggested different. What instrumentation did you use to collect the data?

Grant: The images of WR 104 which imply the orbit is face on were taken using near infrared light, and using an instrument here at Keck called NIRC (near infrared camera). Those images show the dust plume which is thought to result from the colliding winds spiraling outward.

Your readers can see those images by going to Peter Tuthill's home page. I've been getting optical spectra of WR 104 with an instrument called LRIS (low resolution imaging spectrograph). I've chosen to work in the optical because there is a spectral emission line arising from ionized carbon there which is very sensitive to density and thus a good diagnostic of the compressed wind region which results from two stellar winds colliding.

Ian: I actually wanted to ask about the binary partner too, was it the interacting stellar winds that helped you to deduce that WR 104 was tilted away from us? In fact, can the pinwheel spiral of WR 104 even be produced without a binary partner?

Grant: Answering the second question first: I'm not aware of any explanation for the pinwheel spiral that does not involve colliding winds from two binary partners.

Regarding the orientation of the orbit of WR 104... looking at the images, it is very hard to believe the orbit is not face on. The thing really does look like it is pointed right at us.

Having said that though, there are several reasons why I am not so sure. First, I have been able to measure velocities for both the WR star and its companion. With those velocities I can calculate the orbit and by extension say something about the orbital inclination if I assume reasonable masses for the stars. Doing that implies the inclination is at least 30 or 40 degrees.

I can also use some computer code I have written to model the changing shape of that carbon line. That modeling includes the orbital inclination. I am working hard right now trying to find the smallest inclination which results in reasonable model fits. So far the lowest I can get it is around 30 or 35 degrees.

Finally, I think that about a half a dozen of these spiral pinwheels have now been imaged. If I recall correctly, all but one of them look to the eye as if they are nearly face on. Statistically that raises a red flag to me.

Having said all that... WR 104 is definitely the best observed of these things and the images sure look like it is pointed right at us.

Ian: So basically, optical images suggest it's face-on, but looking at the spectroscopic velocities suggests it's tilted 30-35 degrees? That's a bit of a conundrum! I've found you can gain a lot more information from spectroscopy. Are there some reasons why this might be the case? Could the optical data be an illusion of some sort?

Grant: Yes it is definitely a bit of a mystery. The latest thing I am trying with the modeling is to make the trailing edge of the pinwheel denser. Maybe that will allow me to use lower inclinations to fit the spectra. Theory predicts the trailing edge of the spiral might be denser. I'm starting to run out of things to try though!

I would not say necessarily that there is more information in the spectroscopy. I think the two (imaging and spectroscopy) are very complementary. The spectroscopy tends to give information along the line of sight and (in this case) about the inner regions of the binary system. The imaging tends to give information in the plane of the sky and (in this case) the outer regions of the binary system.

Peter Tuthill mentions one idea on his web site. Take some three dimensional object like a fish bowl and tilt it. It is hard to figure out exactly what the tilt is. Perhaps including this depth effect in models of the images will allow larger inclinations. Perhaps my including a greater density in the trailing edge of the spiral will allow lower inclinations in my modeling, and the truth is somewhere in between.

Its great fun though working with the data and trying to tease the information out of it!

Ian: It sounds like a great research area — I just love the fact we can get this much information from a star system 8,000 light years away (is that the correct distance?)

I suppose the burning question (pardon the pun) is what if WR 104 really was pointing right at us? Would it really be cause for concern? Or is it really hype?

Grant: Distance estimates vary. The closest I think I have seen is about 5,000 light years. The farthest is about 8,500.

I think that most experts agree that the WR star in WR 104 will go supernova within (astronomically speaking) the near future. By that I mean the next few hundred thousand years.

Will this SN also be a GRB? That may come down to whether or not it is rapidly rotating. That is probably not likely, but can not be ruled out.

Would we see it as a GRB? That comes down to whether it is pointed our way.

Would it be dangerous? Again that depends on just how exactly it is pointed our way and how narrow the beam is. I have seen estimates by others that it is close enough there could be effects (bad ones) on the earth's biosphere.

Ian: Phew, that's a relief — I'll wait until I invest in that lead-lined bunker for a few thousand years then :)

That's awesome, thank you for speaking to me Grant!

Was there anything else you wanted to add? Got lots and lots of great stuff here.

Grant: My pleasure. Maybe I shouldn't say that WR 104 could already have gone supernova. After all the light we are analyzing is thousands of years old! Maybe your readers don't want to hear that little thought though... ha ha.

Ian: Superb! Lol, yes, I will certainly add that little fact... to get 'em thinking ;)

Grant: No problem. Feel free to get in touch again if you want future updates on my efforts.

Ian: I will do that for sure! Let me know of any publications you might be putting out, I'd love to see what else you find.

Grant: Aloha

Ian: Aloha :)

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