Wednesday, October 6, 2010

My (Former) Research Explained: What I remember about OH/IR stars

What do I remember about OH/IR stars? Not very much, but hopefully enough to give you some insight into what they are. I remember there are some interesting properties about them though and what I learned about OH/IR stars helped me understand some aspects of my current research, surprisingly. Perhaps I don't remember much because my project was doomed to fail about 3 weeks into the summer program. I'll get into that another time but for now I'll get into the stars themselves.
OH/IR stars are a form of astronomical maser. That sentence is a mystical mouthful for such short form. OH refers to the type of molecule found surrounding the star, the hydroxyl molecule which is similar to hydroxide that many are familiar with. The IR refers to the range of light that the molecule emits, which is infrared. The term maser is very similar to laser except that laser is an all encompassing term and maser is specific to light in the microwave range (>1um, <1mm) and it just so happens that infrared light is in the microwave range.

Everyone and their mother has heard of lasers before. If I remember correctly masers were discovered first and then Alfred Laser successfully made the first laser. I'm joking of course. Laser is actually an acronym for Light Amplification by Stimulated Emission of Radiation which I went into some detail a long time ago. As I stated above, masers are lasers whose wavelengths are microwaves. Technically the laser I use is a maser since I work with a 1.064um emission. Amazingly the phenomenon of lasing happens naturally and the hydroxyl molecule is just one example.

OH molecules absorb one frequency of light and through stimulated emission can emit another frequency in the infrared spectrum. In the case of a star, there are so many OH molecules that you get the amplification aspect of the phenomenon and therefore you have a maser. The source for the maser (every laser has a pumping source which starts the avalanche of emission) comes from the star itself. Which reminds me to tell you that the star isn't made of OH molecules. Instead the star is surrounded by OH molecules.
Figure 1: Typical maser emission spectrum. 
Think back to when I talked about stellar evolution. Some stars don't die in a fiery-fiery fire, instead they slowly eject their outer layers leaving only the core behind to become a white dwarf. What becomes of those outer layers? They turn into proto-planetary nebulae and then planetary nebula, from which new stars and planets can be formed. OH stars are in the phase just between red giant and proto-planetary nebulae.

Basically, the star is in a state of great flux and its outer envelope is being pushed away. Imagine a giant sphere of molecules surrounding a star, like a cloud sort of. The masing comes from this cloud. Light from the star gets absorbed by the gas cloud and then reemitted out into space at a different wavelength. What we see comes from radio astronomy (hence the need to use the giant radio telescope in Puerto Rico) and looks like Figure 1 above.

Researchers will detect two wavelengths of light from a single source due to the Doppler effect. Because the outer shell of the star is expanding, one side will be moving towards us and the other side will be moving away from us at great speeds. This expansion will shift the wavelength of the light so there will be 2 wavelengths of light corresponding to which side of the shell we are looking at. From this you can figure out the speed of the expansion of the shell (using the speed of light and your light wavelengths as your references). You can make some other clever deductions as well, but I wouldn't know what those are.

There are lots of different kind of masers because there are lots of different formations of hydroxyl gas in the universe. I only know about this version, but I have heard of megamasers

Not to be confused with this guy...
and maser clouds which are just like interstellar gas pockets which somehow get mased by huge and far space objects, perhaps Quasars. If you want to know more about masers and perhaps with more accuracy then check out the Wikipedia article (which I did NOT use to write this).



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