Browsing through astro-ph today I found a curious title: “The Shear Viscosity and Thermal Conductivity of Nuclear Pasta”. Nuclear pasta, really?
At intermediate densities, around 1014 g/cm3 just below nuclear density, matter may form complex nuclear pasta phases. Competition between short range attractive nuclear, and long range repulsive Coulomb, interactions can lead to clusters with many different non-spherical shapes including long rods or flat plates. Because pasta may form at high densities, it could represent as much as half the mass of the neutron star crust. The complex shapes in nuclear pasta have sizes of tens of Fermis [10-15 meters, about the size scale of atomic nuclei].
The article is just full of great quotes; “Semiclassical nuclear pasta model”; “The complex physics of frustration”‘; “[the model] can describe nuclear pasta in a flexible way”. Plus the physics is absolutely wild.
In case you didn’t notice the early-morning twitter messages, I spent a night observing on the GBT about a while ago (I know, I’m slow to update). As part of the REU program, we get a proposal submitted for us that essentially says “Summer Students” and nothing else, and lo, we get 20 hours. It’s definitely the easiest way I’ll ever have to get telescope time. And what’s more, we even get ideas for projects provided to us by staff astronomers from NRAO. All we have to do is figure out the details of the observing and sit in the control room for a few hours. Then maybe do something useful with the data later.
What I worked on was detecting metals such as carbon, oxygen, neon and sulphur in an HII region (DR 21). Detecting hydrogen and helium in the HII region is simple; the lines are strong and spaced far apart from each other. We could see clearly after a three minute scan. The rest of the elements we were looking for were much weaker and much closer together. Weaker, because there’s simply not as much of the heavier elements. They’re closer together in frequency, because the greater number of protons in the each atom means that the transitions have similar energies. There’s a big energy difference between an atom with one proton and an atom with two protons, but not so much when it’’s between 6 and 7, or 8 protons. So you end up with closely spaced lines, which are then blured together by the thermal velocities too. If the region is in thermodynamic equilibrium, some of the atoms will be blue shifted and some will be red shifted just by the doppler shift, due to random motions within the cloud. This broadening is proprtional to temperature, so it doesn’t help that the HII regions are at 10,000 K.
Enough theory, let’s see data. Here’s the spectrum straight off the telescope (but after calibration, really):
There’s the full spectrum, 12.5MHz wide. The two biggest bumps are hydrogen lines. In the center is the helium line, and between the two blue ticks is the smeared carbon, oxygen, and nitrogen lines. Now let’s look at just the interesting bits:
Here I’ve removed the neighboring hydrogen and helium lines (which is simple because the lines are mostly gaussian). The is definitely something non-gaussian in this plot though. The three or four metal lines have been blended together. Extracting information about the individual lines is going to be tricky. You can already see that the lines on the left are stronger, since the peak is asymmetric. Beyond that I haven’t had a chance to look at it much, so I’ll have to save the details of the analysis for later.
I feel I have to write something here to justify having a blog. There’s a bigger post coming about GBT observing, but for now just an anectdote. So last Monday, it became clear that we were getting two “peaks” in our data, and that one of them was coming from antennas sensing one fluctuations in one direction on the sky, and the other peak from the perpendicular direction (remember I’m looking at the variation in hydrogen as a function of position.) Most importantly, these peaks had different magnitudes, which is why we saw two and not one in the first place. These two peaks corrospond to two different amplitudes of variation, one strong and one weak. A little bit more work showed that peaks were in two different directions on the sky, and hence that the turbulence is stronger in one direction than the other (anisotropic, that is). For that to be the case, there would have to be some seriously funky physics going on. Magnetic fields, expanding bubbles, something interesting. It looked like we were going to have a good set of results, and with a month left of summer to nail it down and write the paper.
Late this afternoon I discovered that the angle of these two peaks shifted around as a function of fluctuation size scale (Sorry, it’s difficult to explain that much clearer). That’s completely unphysical. So there’s probably just some calibration issue, and that’s it. No funky physics, no fun paper. Psh.