01 November 2011

Magically Spotted

Title: No magnetic field in the spotted HgMn star μ Leporis
Authors O. Kochukhov et al.

Chemically peculiar stars are, well, peculiar. What makes them so peculiar? When analyzing abundances of stars (say, via spectroscopy) we generally have a good idea of the relative proportions of all the elements. There is no general rule that can be followed, as there are various scenarios which can occur that lead to stars with different relative abundances of particular elements. For instance, stars which are the progeny of the first generation of supernova that exploded in the Universe, will typically be metal poor but have a higher abundance of what are known as α capture elements (O, Ne, Mg, Si, S, Ar, Ca, and Ti). One can easily convince themselves that this is consistent with our view of Type II supernova (the death of a massive star) when considering the by-products of a run-away nuclear reaction chain. However, this is but one scenario of a few that we can use to generalize the relative abundance patterns of stars. When these generalizations are ignored by nature, we get chemically peculiar stars - stars that show a chemical signature (relative abundance pattern) drastically different from what we have come to expect.

HgMn stars are a particular class of chemically peculiar stars. It turns out these stars just happen to be late-B stars, likely a key to understanding the nature of the peculiarities. Specifically, as one might guess, they show anomalous amounts of Hg and Mn (singly ionized to be precise) - these two chemical species are over abundant. Even more interesting, is that they show signs of spot modulation (variations in the light output of the star due to temperature variations on the stellar photosphere). Spot activity is nothing new for anyone familiar with a starspot. Unfortunately, the spots on these stars are not necessarily magnetic in origin (more on this later).

Most chemically peculiar stars are thought to be fairly well understood. The physics involved in creating such star is related to diffusion physics. Plasma is constantly on the move within stars - there is no reason for stellar plasma to remain static. Gravity, temperature gradients, and concentration gradients all force particles to reorganize into the most likely configuration (maximization of entropy!). Thus, plasma particles are shifting around attempting to find this state. Well, one other type of process which can occur is called radiative acceleration (or levitation), and it turns out to solve a few problems.

B stars are hot. Super hot. About 12,000 K hot. This means that photons have a lot of energy and are capable of transferring that energy to anything in their way (say, some gas particle). Depending on the opacity of radiation to a particular chemical species at a certain point in a star, photons can actually "push" gas particles towards the surface of the star, aiding in the particle diffusion process. This substantial side-effect, strong radiation pressure, is what is thought to "correct" our standard approach to particle diffusion in order to remedy the discrepancies with chemically peculiar stars.

Unfortunately, this theory doesn't seem to do a great job of reproducing HgMn the properties of HgMn stars. Magnetic fields have also been shown to help produce chemical inhomogeneities in Ap stars (the "classic" chemically peculiar star). We then have to wonder if magnetic fields are at work in the HgMn stars. To do so, one just has to look for the presence of magnetic fields around a star. The clue which betrays the presence of a magnetic field is the amount of polarized radiation.

I am by no means an expert in E&M, so I cannot provide an intuitive description of the physical process of photon polarization. Instead, I point you to a trustworthy source. To probe a stellar environment for polarized radiation and the presence of a magnetic field, observers use a setup called spectropolarization. Essentially, there is a polarization filter which feeds a spectrograph. From this, one can analyze the profiles of particular lines which are sensitive to magnetic fields (typically lines formed due to ionized atoms). The figure on the right displays what reduced data looks like. Note the squiggly lines on the right. In the presence of a magnetic field, the amplitude of the variations if far larger - thus, the authors concluded there is no sign of a strong field around this star.

Interestingly, though, when they look at the line profiles, while there is no clear indication of a magnetic field stronger than 3 G, there is an odd shape to some of the spectral lines. These odd shapes do not appear to be an artifact of their reduction procedures, as they use the same procedures on a set of theoretical spectra in order to derive theoretical line profiles. Here is what they find:

Figure 2: From Kochukhov et al. (2011), TiII and YII line profiles from both synthetic spectra (dashed) and observed, time averaged spectra (solid).

The dashed line is the theoretical line profile while the solid line is that derived from observations. Indents can be seen near the trough of the line. The authors suggest the shape is derived from the abundance distributions being axisymmetric. This, coupled with rotation, they claim provides the explanation. However, no theoretical line profiles were developed with this hypothesis.

No matter how you look at it, though, there is a lack of a large-scale magnetic field with a strength greater than 3 G. As such, magnetic processes are an unlikely solution to the peculiar abundance pattern on the surface of these stars. To date, not good hypothesis exists to explain the anomalies we observe. The next lines of attack are to look at more rigorous radiative diffusion scenarios (time dependence) and at the effects of hydrodynamical instabilities/circulation, such as meridonal circulation.

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