Cool Stars Have Magnetic Fields Too

  • Title: Molecules as magnetic probes of starspots
  • Authors: N. Afram and S. V. Berdyugina
  • First Author’s Institution: Kiepenheuer Institut für Sonnenphysik, Freiburg, Germany
  • Paper Status: Submitted to Astronomy & Astrophysics

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    A very spotty Sun during an active period in 2003. Image credit: NASA/SOHO

If you want to make a room full of astronomers laugh, raise your hand after a talk and ask, “but what about magnetic fields?” For all their pervasiveness in astronomy—everything from planets to galaxies can be home to magnetic fields of varying shapes and strengths—relatively little is understood about magnetism in the cosmos. Consider, for example, the solar dynamo. (Dynamo is a fancy term for “thing that creates a magnetic field.”) We can all agree that the closest, best-studied star to Earth has complex magnetic fields which give rise to features like sunspots, but we do not understand the inner workings of our Sun’s dynamo.

While the Sun is an excellent starting point in a quest to understand magnetism, the authors of today’s paper want more. The Sun is but one star, and we know that other stars have dynamos. Properly characterizing stellar magnetic fields is important for exoplanet studies, among other things, because exoplanet observations are hugely affected by the presence of starspots. One way to measure stellar magnetic fields is with the Zeeman effect. Strong magnetic fields affect the spectra of stars, causing a single absorption feature to split into several components. However, this effect is most easily seen in hot stars, yet the cosmos is littered with cooler stars: G dwarfs like our Sun, and even more cooler-still K and M dwarfs.

Spectropolarimetry to the rescue

Today’s paper looks at something only relatively cool stars can have in their atmospheres: molecules. Specifically, the authors investigate how molecular absorption lines change in starspots as a function of magnetic field strength. This offers another advantage over the atomic Zeeman effect, because it measures the magnetic field only in starspots—definitive signatures of magnetism which are the only places certain molecules exist—instead of a “global” magnetic field that could result from several strong components canceling each other out.

The authors consider four molecules (MgH, TiO, CaH, and FeH) in three kinds of stars (G, K, and M dwarfs) that can be observed in three ways. One way is “regular” spectroscopy: measuring the strength of a molecular absorption feature. The other two ways use spectropolarimetry to measure the Stokes V and Q parameters.* Because different flavors of polarized light are sensitive to magnetic fields, astronomers can take advantage of this to observe how absorption lines change in the presence of magnetic fields. An example for an MgH absorption feature in K dwarf star is shown below.

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Magnetic starspots change how MgH absorption would look in a K dwarf star. The solid, dashed, and dotted lines show increasingly higher spot coverage in the left column and increasingly strong magnetic fields in the right column. The top panels are the absorption feature’s overall intensity, the middle panels are its Stokes V parameter (measuring circular polarization), and the bottom panels are its Stokes Q parameter (measuring linear polarization).

Molecular alphabet soup

Of course, not all molecules, starspots, or magnetic fields are created equal. By modeling how different combinations of these would look on different kinds of stars, the authors make clear predictions to guide future observations. For example, are you most interested in Sun-like G dwarfs? Then you should probably focus on MgH and FeH absorption features. Or perhaps your instrument setup is better-suited to observing CaH and TiO. In that case, you might consider studying cooler M dwarfs and adjust your exposure times accordingly. The figure below summarizes the paper’s predictions for which Stokes V (circular polarization, left panel) or Q (linear polarization, right panel) signals are likely to show up for each molecule in a variety of cool, spotted stars.

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Observers should find more signs of magnetic activity in some situations than others. Both Stokes V (left) and Stokes Q (right) signals are predicted to be strongest in CaH and TiO molecules for M dwarf stars, while MgH and FeH are a better bet for hotter G dwarfs.

These predictions pave the way for careful observations of magnetic starspots, because now observers can select targets and exposure times more efficiently. The Sun isn’t the only cool star in town with a detectable magnetic field, which brings us one step closer to unlocking the mysteries of stellar dynamos.

 

*Section 2.1.3 of this article introduces the Stokes parameters in a friendly astronomical context.

About Meredith Rawls

Meredith received her PhD in Astronomy from New Mexico State University in 2016. She was a regular astrobites author from 2013–2016 and assisted with the website and social media from 2015–2018.

15 Comments

  1. I’m impressed that we apparently have (it seems based on this) a significant number of instruments that are able to pick out specific absorption features on cool stars; I would have thought that these would be a relatively difficult features to detect.

    Reply
    • Detecting individual absorption features is a matter of getting enough photons (large telescopes and/or long exposure times) and having an instrument with sufficiently high resolution. You need a pretty high-res spectrograph (or, in this case, spectropolarimeter) to resolve the shape of individual absorption lines, but it is a common technique. If you’re interested to learn more about spectroscopy in general, this PDF offers a pretty accessible (but long) overview: http://www.ursusmajor.ch/downloads/analysis-and-interpretation-of-astronomical-sp.pdf

      Reply
      • Can such devices be coupled to existing telescopes or must we have purpose-built instruments to make such measurements?

        Reply
        • Many telescopes are equipped with spectrographs, but spectropolarimeters are less common. In general, any telescope can be outfitted with a suite of instruments, and then observers select the instrument/telescope combination that suits their research best. No two spectrographs are alike, either: each one is custom-built by teams of experts with unique features and tradeoffs.

          Reply
  2. More photons leads to better absorption features? Will this study be done then with more luminous stars such as those belonging to F, A class?

    Reply
    • Yes, perhaps; however, those stars tend to have fewer spots than cooler dwarfs and they are already better-studied simply by virtue of being brighter. One reason the authors of this paper focused on cooler stars is because they are a dime a dozen in our galaxy but relatively poorly understood since they are intrinsically faint.

      Reply
  3. That’s a fascinating idea to measure the polarization of the light that we receive from a star to determine it’s magnetic field. I never would have thought that light polarization would have been useful in Astronomy before.

    Reply
    • Anything light can do is useful in astronomy! 🙂

      Reply
  4. I’m surprised that we know so little about the Sun’s inner dynamo! I often forget that there are still things we have yet to understand about our host star.

    Reply
  5. Very nice post! One question though: how do magnetic fields change the spectrum of a star?

    Reply
  6. This article is very interesting. Is there anything in the current literature relating this work to stellar flares? This could be interesting to better understand the relationship between magnetism and flaring.

    Reply
    • Space weather broadly encompasses the heart of your question, I think. Linking stellar flares to magnetic activity is definitely an active area of research, particularly when those flares interact with the Earth. For example, we’d like to be able to measure what the Sun is doing and predict when it will have a flare event. We still have lots to learn about even our closest star!

      Reply
  7. I’m a bit confused by what you mean when you say a “global magnetic field … could result from several strong components canceling each other out”. I understand the the overall idea you’re getting at is that we can get a lot more specific information about molecules (components) by looking at star spots rather than the whole star, but I don’t understand what components would be canceling each other out. Do you mean that the different molecules polarize differently such that when they overlap you can’t discern a specific polarization?

    Reply
    • Let me clarify a little. The idea here is that measuring the Zeeman effect in absorption lines that exist all over a star’s surface will tell you the sum total of the star’s magnetic field. If the magnetic fields were all nicely aligned and parallel to one another, that would essentially be great, end of story. However, since differential rotation yanks stellar magnetic fields into a twisted mess, you could be inadvertently measuring weak overall magnetic field that is made up of lots of strong components, but physically oriented in such a way that they cancel out. I hope that helps.

      Reply

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