The topic line for last week’s gathering of Astronomy on Tap Seattle was What the Hell is Polarimetry?, and it seemed that a significant portion of the audience at Peddler Brewing Company in Ballard shared the question.
UW postdocs Jamie Lomax and Kim Bott explained that when light starts from its source the oscillation of its wave—its “wiggle”—goes in all directions until an interaction with something makes it polarized.
“That just means that it’s wiggling in one direction,” Lomax noted. “There’s a preferred plane for that wiggle to happen in, and in polarimetry what we’re doing is measuring that preferred plane and we’re looking for light that has been polarized.”
“It can help you figure out the shape of things without having to resolve the object,” Bott added.
Polarimetry and massive stars
Lomax studies massive stars and has found use for polarimetry in her work. She gave a talk titled, “Seeing Invisible Circumstellar Structures.”
“The holy grail for us in massive star research is to be able to take a massive star at the beginning of its lifetime, figure out how massive it is,” Lomax said, “and map out what its life is going to look like and figure out what supernova it’s going to end its life as.”
“It turns out that is really hard, and it’s complicated by the fact that most massive stars are probably in binary systems,” she added. Since about two-thirds of massive stars are part of a binary system, one might expect that two-thirds of core-collapse supernovae would be from such systems.
“There’s a problem, and that is we’ve only seen maybe two or three core-collapse supernovae where we have evidence that suggests that it’s come from a binary star,” Lomax said.
Part of the problem, she said, is that we don’t yet know enough about the evolution of binary star systems.
“We can try to hammer out the details of how that mass is transferring between the two stars and when the system is losing material to try to figure out how that effects its future evolution,” Lomax said. “Once we start answering questions like that we can start to tease out why we aren’t seeing all of these binary supernovae we think we should be seeing.”
Lomax talked about the star Beta Lyrae, a binary system. The primary star in the system is losing mass that gets gobbled up by the secondary. This transfer of mass also forms a thick accretion disk of gas around the secondary—so thick light from the actual star can’t get through. There’s also evidence that there are jets shooting out of the system, but we don’t know where they are.
“These are all features that we can’t see very well,” Lomax said. “We can’t see the mass transfer stream between the two stars, we can’t see the jets.”
Here’s where polarimetry comes in. If a star is surrounded by a cloud of gas or dust that is circularly symmetrical, when the starlight interacts with that material the light becomes polarized, and the wiggles line up tangentially with the edge of the disk. If the cloud is elongated in some way, the wiggles form in a “preferred” direction.
“That preferred wiggle direction is 90 degrees from the direction of the elongation of the disk, so you can back out geometric information pretty quickly,” Lomax said. “Just by looking at how the light is wiggling I can tell you how the disc is oriented on the sky.”
Lomax figures that if you don’t do polarimetry you’re throwing out free information.
“You can see invisible things—to you—and that gives you extra information about what’s going on in different systems.”
Exoplanets and aliens
Bott’s talk was titled “The Polarizing Topics of Aliens and Habitable Planets.” She studies exoplanets and said polarimetry comes in handy.
“Stars don’t produce polarized light, which is really great if you’re trying to look at something dim like a planet,” she noted. The polarimeter will simply block out the starlight. There are then a number of things that might be spotted on the planet:
- Glint from an ocean
- Rayleigh scattering
- Clouds and hazes
- Biosignatures of gases in an atmosphere
These can help astronomers characterize a planet, judge its potential habitability, and even determine if life might already be flourishing there.
Bott said that polarimeters that are sensitive enough to study planets are a recent advance, and they’re studying big, bright planets to get the hang of it. Looking for rainbows can be revealing about liquids in the atmosphere of a planet.
“The light will bend in the droplets at a slightly different angle depending what the droplet is made out of,” Bott said, so they can tell whether its water, methane, or sulfuric acid.
“We’re trying to create these really robust models that will take into consideration polarized light from Rayleigh scattering in the atmosphere as well as from rainbows,” Bott said, “and if you have a planet where you can see the surface you’d be able to see the signature from glint as well.”
Since different substances bend light at different angles, we can also learn a lot by watching closely as planets move through their phases as they orbit their host stars.
“On Earth we have light going from air and bouncing off of H2O water,” Bott said. “That’s going to produce a maximum in polarized light at a different angle than on, say, Titan, where you have light going from a methane atmosphere and then bouncing off of a hydrocarbon ocean.”
“We can actually, in theory, tell what the ocean and atmosphere are made out of by looking at where, exactly, in the orbit we see this glint,” Bott explained.
As for aliens, life requires more complex molecules, chiromolecules, that are “wound” in a certain direction, like our own DNA. Such molecules would produce circularly polarized light, which if detected could be a sign that such molecules exist on the planet.
Astronomy on Tap Seattle is organized by graduate students in astronomy at the University of Washington. It’s next gathering is scheduled for October 30 at Peddler Brewing Company in Ballard.
Please support Seattle Astronomy with a subscription through Patreon.