The Universe Tonight
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By Dr. Grant Hill
WR-104: Are We Looking Down The Gun Barrel of a GRB?
Our sun is a star, and in terms of size, brightness, age, etc. it is a fairly average one. Much more extreme ones exist though, ranging from small, dim stars barely able to sustain nuclear reactions to ones many times more massive and luminous. In human terms, these massive stars live hard and die young. The nuclear reactions in their cores burn at such a prodigious rate that they evolve in the astrophysical blink of an eye.
Like the sun, these massive stars are losing mass. From the sun, we call this the solar wind. The winds from massive stars are millions of times more powerful than the solar wind though. Like their smaller cousins, massive stars can be found as members of binaries; pairs of stars orbiting each other. This can give rise to a fascinating phenomenon. The winds of two massive stars can collide and produce a shock front. Since the winds from the two stars are unlikely to be exactly equal, the stronger one will push the weaker one back and form a cone shaped shock region that tends to partially enclose the star with the weaker wind. As the two stars orbit each other, we view this cone shaped region from different angles which allows one to characterize the shape and orientation of the shock cone. This provides a host of valuable information about the stars, including the relative strengths of their winds and the orbital inclination of the binary.
I have written code which allows me to model the variations seen in spectroscopy of colliding wind binaries and derive the information described above. I have been applying this code to spectra of binaries that include a Wolf-Rayet star. After a massive (O type) star evolves through nuclear burning, it is thought to become what is known as a Wolf-Rayet star. These WR stars have even stronger winds than their O type ancestors and even shorter lives.
WR 104 is thought to be one such WR+O binary. It is the prototype for a small but growing group of stars that present the remarkably striking appearance of pinwheels. Near infrared images of WR 104, taken at Keck, show the (apparently) face-on spiral turning with an 8 month period. The pinwheel is assumed to be composed of dust produced via the colliding winds in a low-inclination WR+O binary. The dust condenses in the shock region and as it flows along the shock front it cools and glows at infrared wavelengths. These assumptions have been very successful in modeling the imaging, but remain largely untested by spectroscopy.
Strong motivation for further study of this system has emerged. Recent theory suggests that some gamma-ray bursts (GRB's) are core-collapse supernovae viewed nearly pole-on. The WC class Wolf-Rayet star in WR 104 is the type of star thought to be a possible GRB progenitor. If the orbit (and thus stellar rotation axes) is pole-on, the effects on Earth's biosphere could be significant.
For eight years, I have been obtaining optical spectra of WR 104 at Keck and am now applying my code to model the spectra. The results so far, confirm that WR 104 is a binary star with colliding winds. The orbital inclination though appears to be inclined from perfectly face-on. Exactly how inclined and whether this removes any threat to Earth remains somewhat uncertain still.
About The Presenter:
Grant Hill obtained his PhD in astronomy in 1993 from the University of Western Ontario and was awarded the Plaskett Medal for his thesis, entitled "What is Normal Among the A-type Stars?". The basis of this work was development of spectrum synthesis code to determine chemical abundances in broad-lined stars and application of the code to a sample of stars similar to Vega.
Following post-doctoral fellowships at the Universite de Montreal and at the Dominion Astrophysical Observatory in Victoria , a job at McDonald Observatory in Texas beckoned. There he joined the Hobby-Ebberly Telescope project during the late stages of construction and stayed through commissioning and into early science operations.
In 2000, Grant joined the staff of Keck Observatory where he assists visiting observers, helps maintain instruments, and works on his own research. In the past that research has involved stellar pulsation, magnetic fields, and chemical abundances.
His current research, involves massive star binaries in general, and their colliding stellar winds in particular.