Wednesday, March 18, 2009

The polar EU Cancri, Part 1

Time-resolved spectrum of the polar EU Cancri
Image Credit: Yours truly

My contribution to the conference on cataclysmic variable stars involves an accidental study of a polar (rhymes with "coal tar" or "Moltar") with the name of EU Cancri. (Read here for an explanation of how variable stars get their names).

First, let's learn what polars are. Here's an artist's impression of what a polar might look like. Remember, a cataclysmic variable is a white dwarf star whose gravity is stealing material from a nearby companion star. In polars, the white dwarf has a very strong magnetic field, tens of millions of times the strength of Earth's magnetic field. When hot plasma (like on the surface of a star) encounters such a strong magnetic field that strong, the plasma is forced to follow the magnetic field. So, in the polar, the material that gravity pulls off the companion star follows the white dwarf's magnetic field to the white dwarf's magnetic poles, where it slams into the surface of the white dwarf, releasing lots of heat and light, but only at that specific spot.

Anyway, my tale begins in January 2007. I was at the Keck Observatory, the world's largest telescope, to look at normal, single white dwarfs in the old star cluster Messier 67. My goal was to measure how massive the white dwarfs are in Messier 67. So, I had taken pictures of the star cluster and identified potential white dwarfs (white dwarfs are very faint and blue, so I just picked all the faint, blue stars). I then pointed the telescope at my candidate white dwarfs and started taking spectra. Because taking spectra involves breaking the light of stars into its component colors, and because faint stars like white dwarfs don't have much light to begin with, it was going to take several hours with the giant telescope to get the data I needed.

When I looked at the spectra, most were either normal white dwarfs or were quasars; quasars, like white dwarfs, look like faint blue stars, so I tend to find them quite a bit. But one spectrum looked different. It showed signatures of hydrogen and helium, but these lines were in emission instead of absorption (compare the first and last spectra here to see the difference between absorption and emission). Absorption is typical for normal white dwarfs, but emission means that something weird is going on.

Also, when I looked at the spectrum, it had giant humps that came and went over just 45 minutes. Humps in spectra often mean magnetic fields; they come from a process called cyclotron emission. (Electrons in a magnetic field tend to spiral around in circles and emitting light as they do so). So, I suspected that I'd found a polar, even though I'd never seen the spectrum of one before. But I didn't know what else it would be. And it turns out that I was right.

When I studied the spectra more closely, I noticed that the emission lines were moving. Sometimes they were a little bluer, sometimes they were a little redder. This is due to the Doppler shift. The companion star is orbiting the white dwarf every 125 minutes, so over the three hours I looked at the system, it went through one and a half orbits. To put this in perspective, the two stars are about 400,000 miles apart, or about 1.6 times the moon's distance from the Earth, yet they go around each other in just over two hours (while if the moon were that far away, it would take almost two months to go around the Earth)!

The picture at the top of this post is a stack of all the spectra I took, with some almost-true color added. The picture is kinda like a movie. The very top is the beginning, the bottom is the end, and in between are snapshots like individual frames. You can see the emission lines wiggling back and forth, as the companion star first moves away from us (getting redder due to Doppler shift), then toward us (getting bluer due to Doppler shift), and then back away from us again. You can also see the "humps" due to the magnetic field appear and disappear. I repeat the "movie" twice for clarity, so the spectrum really covers about 5 hours.

Anyway, I find it neat that we can see things changing so much over just a few hours. If I were to make a similar "movie" spectrum of a normal white dwarf, the movie could go for centuries with no noticeable changes. And yet here we see drastic changes in just a few hours.

As I said at the beginning, my spectra were obtained mostly by accident, because I wasn't looking for weird objects. But yet I found one. This tends to happen in astronomy; we go to the telescope looking for one type of object, and we find something completely different.

Tomorrow I will talk a bit about why people might be interested in my star, and how my results probably disappoint everyone.

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