Magnetic Fields of Massive Stars

I am in the physics and space sciences senior seminar class this year, taught by Darin Ragozzine, Ph.D. The class focuses on preparing for graduation and opportunities available afterward for someone with a bachelor’s degree in physical sciences.

One aspect of the class is an oral presentation of research done by the student, usually with one of the faculty members. I joined Veronique Petit’s, Ph.D., research group. I will be studying magnetic fields of massive stars; in particular, two stars that closely resemble the star called τ Sco (aka “analogues” of τ Sco), which had been unique until these other stars were discovered. Here I will describe some basic background, past research, and what I hope to contribute to further research with my senior project, entitled Do τ Sco analogues have complex magnetic fields?

Observing spectral lines produced by the light of stars has let us figure out quite a few things about their properties, such as their composition, how much energy they produce, their age, their temperature, etc. Many of these were first observed from our sun; then as our technology got better, we were able to apply the same techniques to distinguish properties for much further objects.

One trait in particular was the sun’s magnetic field, determined by evidence of the Zeeman effect. We had already discovered putting atoms in a magnetic field would cause their spectral lines to split, so when someone noticed the sun’s spectrum was emitting a similar pattern, it was concluded our sun had some magnetic fields, particularly around sunspots. Taking this technique beyond our solar system, it was discovered many other stars also exhibit magnetic fields, too (van Maanen 24).

Years later, we have observed enough stars with magnetic fields to notice that stars with other similar properties also tend to have similar magnetic fields. However, one massive star discovered, called τ Sco, has a very complex magnetic field in comparison to other massive magnetic stars. More observations led to the discovery of some unusual UV and X-ray emissions, leading scientists to conclude τ Sco is showing some stellar wind anomalies in addition to its complex magnetic field.

So the big question: do these two properties both just happen to be unique to this one star, or is one property the cause of the other? Observations of our sun have shown a correlation between a higher number of sunspots and an increase in solar flares and coronal mass ejections — in other words, a higher magnetic field results in a stronger solar wind (Meyer-Vernet). So if that is true here, there is good reason to suspect it will be true for other stars. But with only one star with such strange properties to observe, it is kind of hard to conclude anything about the relation between its magnetic field and stellar wind.

A few years ago, two other stars were suspected to have similar properties to τ Sco—HD 66665 and HD 63425. The initial reason why they were thought to be analogues of τ Sco was due to their very similar UV emissions. Both stars were observed for a period and their spectral lines analyzed to determine if they really do have similar enough properties to be put in the same group as τ Sco. From the results of one particular procedure, called the “least squares deconvulotion,” the probabilities of the noise to signal ratio were taken. All of the probabilities of noise were minimal, meaning magnetic signals were present in all the observations (Petit et al L45-L48).

HD 66665 and HD 63425 were concluded to most likely be analogues of τ Sco. Some questions raised by this research, however, require more observations. The first is why HD 63425 has distinctly shallower lines in several of its optical diagnostic fits where HD 66665 and τ Sco do not (pictured below). A possible explanation is material could be trapped in the star’s magnetic field, affecting the observed data. The second question is whether the data really has a good picture of what the magnetic field looks like, or were all of the observations taken of only one orientation of the stars? “More phase-resolved observations are required in order to assess the potential complexity of their magnetic fields and verify if the wind anomalies are linked to the field complexity,” (Petit et al L49).

More observations have been made now, and I will be analyzing the data to determine if HD 66665 and HD 63425 have magnetic fields as complex as τ Sco, mainly with the use of the computer program Interactive Data Language (IDL). I will be using an equation for the magnetic field as given by Donati and Landstreet in their published work (listed below). This will hopefully lead to a better understanding of these more complex magnetic fields than observed in other massive stars, and eventually determine if they are the cause of the stellar wind anomalies observed at τ Sco.

References

1)      Petit et al. “Discovery of the first τ Sco analogues: HD 66665 and HD 63425.” Mon. Not. R. Astron. Soc. 412, L45-L49. 2011. doi: 10.1111/j.1745-3933.2010.01002.x

2)      Donati, J.F. and Landstreet, J.D. “Magnetic Fields of Nondegenerate Stars.” Annual Rev. Astron. Astrophys. 47: 333-70. 2009. doi: 10.1146/annurev-astro-082708-101833

3)       Van Maanen, A. “The Zeeman Effect on the Sun.” Publications of the Astronomical Society of the Pacific. 34(107). Pg 24.

4)      Meyer-Vernet, Nicole. Basics of the Solar Winds. Cambridge University Press. 2007.