WR 104 is a fascinating star. Located approximately 8000 light years from Earth in the constellation of Sagittarius, it has an O-type binary partner where the interaction of both stars produce a spiral pattern of hot dust, expanding as a beautiful spiral.
WR 104 is what is known as a Wolf-Rayet star, the final phase of a massive star's life. Wolf-Rayets have short lifespans, suicidally ejecting plasma into space, producing strong stellar winds. These facts, plus some of the factors outlined in this post, make Wolf-Rayets my favorite stellar objects.
Wolf-Rayets are fiery, hot, dynamic stars, but WR 104 also has a sinister side... it could explode as a gamma-ray burst. Not only that, but recent observations of the system suggest it is facing right at us. If that thing blows, and it fires a deadly beam of gamma-rays in our direction, it could do serious damage to our planet.
However, right in time for the Discovery Space Wide Angle all about stars that explode, I had a chat to Keck Observatory scientist Dr. Grant Hill who has other ideas about the damaging potential of WR 104...
A ticking bomb
Last year, reports surfaced pointing to the results of an Australian study of WR 104. The prognosis wasn't good. Keck Observatory had taken some pictures of the pinwheel and it quickly became apparent that we were staring right down the spin-axis of the binary system.
To add to the morbid nature of the study, Dr. Peter Tuthill, discoverer of WR 104 nine years ago, said in a University of Sydney 2008 press release, "I used to appreciate this spiral just for its beautiful form, but now I can't help a twinge of feeling that it is uncannily like looking down a rifle barrel."
Doesn't sound good, does it?
Naturally, one could argue that it could be hundreds of thousands of years before WR 104 detonates, but the fact is that WR 104 is a bomb with a fuse of an unknown length.
I was fortunate enough to attend the American Astronomical Society (AAS) meeting in Long Beach back in January. Happily, in the abstract list, I saw Dr. Grant Hill was doing a talk all about WR 104, so I attended his presentation and wrote an article about it.
Dr. Hill is a researcher based in Hawaii and uses the Keck Observatory to keep an eye on WR 104. He's devoted his career to understanding the nature of Wolf-Rayet stars.
"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 the Near Infrared Camera (NIRC)," Hill told Discovery Space in a recent interview. "Those images show the dust plume which is thought to result from the colliding winds spiraling outward."
"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."
It's all about angles
Although, from the infrared observations, it's hard to see how WR 104 could possibly be facing anywhere other than right at us, Hill has uncovered something of a conundrum.
"Having said that, there are several reasons why I am not so sure," Hill continued. "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."
This analysis makes use of data gathered by another Keck instrument called the Low Resolution Imaging Spectrograph (LRIS). Dr. Hill decided to work with LRIS as it is sensitive to the spectral emission line of ionized carbon. It turns out that the carbon emission is a good diagnostic for the compressed wind regions where the stellar winds collide between WR 104 and its O-type partner (causing the infrared-emitting dust spiral), making it a great target for Hill to gain velocity readings from the spectrograph.
In addition to his finding that the spectral lines indicate WR 104 is tilted away from us by 30-40°, Dr. Hill is working on a model that can simulate the changing shape of the carbon line according to varying 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."
Statistical doubts
There's another nagging doubt in his mind that WR 104 may not actually be facing us, and it's a question of statistics. "I think that about a half a dozen of these spiral pinwheels have now been imaged," Hill pointed out. "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."
Okay, so the infrared observations by the NIRC and the spectroscopic results from LRIS don't appear to agree, and there's the statistical question that 5 times out of 6, the spiral pinwheels are facing us. What possible explanation could there be for WR 104?
Referring to Peter Tuthill's website, Dr. Hill suggested that WR 104 has more of a 3D structure than meets the eye. "Take some three dimensional object like a fish bowl and tilt it. It is hard to figure out exactly what the tilt is," he said.
"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."
Could it be that WR 104 isn't the potential killer it was once though to be? It looks like there's some serious questions hanging over the "WR 104 is facing us" idea, and there's some doubt as to whether the Wolf-Rayet will produce a gamma-ray burst at all.
Having said that, Dr. Hill leaves us with a slightly unsettling thought: "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!"
Who said physicists don't have a sense of humor! (You were joking weren't you? Grant?)
Read the full interview with Dr. Grant Hill on Discovery Space.
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