Magnetism in the Coolest, Smallest Stars

Special live online demo version; see GitHub page. Use arrow keys to navigate • Figures and movies are copied from many sources; I have made best effort to credit and will rectify any oversights.

Hallinan et al., NRAO/AUI/NSF

Magnetism in the Coolest, Smallest Stars

PKGW, Edo Berger, Ben Cook, John Gizis, Ashley Zauderer

Magnetism is ubiquitous in astrophysics.

… and it’s troublesome.

“The uncertainty scales with the strength of the magnetic field.” — truism

“What happens when you add in a magnetic field?” — generic seminar question

Magnetism plays a role on all sorts of scales:

  • Galaxy clusters
  • Relativistic jets and shocks
  • Accretion disks around black holes, white dwarfs, etc.
  • The interstellar medium within galaxies
  • Stellar systems

The Sun is magnetized.

Average surface field ∼1 Gauss (∼$10^{-4}$ Tesla).

NASA Solar Dynamics Observatory AIA — YouTube

The solar magnetic field is generated deep within the Sun’s interior.

Dynamo: converts mechanical to electromagnetic energy.

Solar dynamo believed to be an “α–Ω” mechanism.

The field is dissipated in the Sun’s outer atmosphere.

This dissipation powers flares, coronal mass ejections (CMEs), and underlies the mystery of coronal heating: how do you get >10$^6$ K from 7000 K?

Magnetic reconnection is a key process.

The dominant dissipation mechanism, but a huge challenge to understand.

NASA SDO — YouTube

The Earth is magnetized too.

Average surface field ∼0.5 G (but radius is much smaller than Sun!).

NASA / Goddard Space Flight Center

NASA — YouTube

Field is much more dipolar than Sun.

Deflects CMEs and the solar wind — vital protection!

Aurorae are reconnection phenomena.

NASA — YouTube

Solar activity has dramatic real-life impacts!

CMEs routinely force satellite safe-moding

Geomagnetic storm of 1989: nine-hour, Quebec-wide blackout

“Carrington Event” of 1859:

  • Telegraphs spontaneously catching fire
  • Aurorae visible in Cuba, Hawaii
  • Modern cost: $2 trillion? (Lloyd’s/AER)

Maunder Minimum / “Little Ice Age” tied to famines, wars, witch hunts (Behringer 1999)


Our understanding of the solar and terrestrial dynamos is far from complete.

Both phenomenologically and theoretically.

NASA / David Hathaway

Kitt Peak Solar Observatory

Very small stars can provide deep insight.

  • Empirically: high levels of magnetic activity
  • Intermediate between Sun-like stars and planets
  • Source of data: dependence of various observables on stellar properties
  • Challenge/opportunity of non-solar dynamo process


We target ultracool dwarfs (UCDs).

The very smallest stars (hydrogen-burning) and brown dwarfs (BDs; no H fusion). Temperatures $\lesssim$3000 K.

Steven Dutch / UW Green Bay

UCDs may not be attention-getters, but they’re everywhere!

A best-effort sample of objects within 8 parsecs (26 lightyears):

The UCD/BD regime represents the transition between stars and planets.

Courtesy Z. Berta.

Jupiter is ∼320 ($10^{2.5}$) Earth masses, 11 Earth radii.

The Sun is ∼1000 Jupiter masses ($10^{5.5}$ Earth masses), 10 Jupiter radii.

Unlike the Sun, UCDs are fully convective.

Recall: the solar dynamo depends crucially on the “tachocline” interface layer between the radiative and convective zones!

Wikipedia / Xenoforme

What happens to the dynamo?!

Emission in many bands traces magnetism. (climate change denialists)

Corona: X-rays, radio

Transition region: ultraviolet

Chromosphere: optical spectral lines (e.g. Hα, 6563 Å)

Photosphere: optical broadband variability

Standard tracers show a drop in magnetism.

PKGW+ (2013)

Berger+ (2010)

Radio observations show $\vec B$ remains robust over the transition to full convection.

Bright, bursty, circularly-polarized emission strongly indicates electron cyclotron maser instability (ECMI).

$$\nu_\textrm{cyc} = \frac{e B}{2 \pi m_e c}$$

Easy handle on the characteristic magnetic field strength:

$$B \approx (360\textrm{ G}) \frac{\nu}{1\textrm{ GHz}}$$

Recall: solar surface field is ∼1 G.

This remains true down to extremely cool (900 K) objects (Route & Wolszczan, 2012).

This elevated radio emission is a surprise.

Most observations have obeyed $L_R \propto L_X^{\sim 3/4}$.

PKGW+ (2013)

Another sign of a different regime of dynamo activity.

We may be able to detect extrasolar planets directly at radio wavelengths.

First claimed detection!

Lecavelier des Etangs+ (2013)

It’s a little dubious.

Observations will speak directly to the habitability of these planets.

The fraction of active stars also increases.

In fact, it seems to shoot up past the transition to full convection.

Yet another new behavior toward the UCD regime.

Magnetism and rotation are correlated.


  • UCDs are rapid rotators
  • More rotation → more magnetic activity, up to a point

This makes sense in standard dynamo models.

Stars “spin down” via magnetized winds, but strength of effect decreases with radius (recent realization — Reiners & Mohanty, 2012)

UCDs upset the rotation/activity relation.

Fast rotation and a falloff of X-ray emission.

Cook, PKGW+ (2013)

The big puzzle: is this really due to rotation, or is it just a correlation effect with mass/temperature?

Mass is not the only factor at play.

We observed objects with similar masses, different rotational velocities; also carefully collected published data.

Statistical analysis, accounting for upper limits: significant correlation in the narrow mass range as well as the broader sample.

Cook, PKGW+ (2013)

This could still be correlation without causation.

Zeeman data track X-ray activity closely.

Measure a mean field strength with complicated spectropolarimetric “Zeeman-Doppler imaging” (ZDI) technique.

Cook, PKGW+ (2013)

Caveat: ZDI data are not sensitive to tangled fields, and only detect ≲15% of the total field.

Perhaps this stems from a bimodal dynamo.

Stronger, larger-scale field → more X-ray?

Dynamo bimodality in fastest rotators (Gastine+, 2013)

Here are some takeaways.

  • Understanding stellar and planetary magnetism is a challenging problem with real-life impacts.
  • Observations of ultracool dwarfs are providing insight.
  • Radio observations probe magnetism in the coolest, smallest objects and may detect planets directly.
  • The falloff in UCD X-ray emission may be related to changes in the topology of the magnetic field.

Thanks for listening! Questions?

Me: Peter K. G. Williams · @pkgw ·

Design credits: Hakim El Hattab (“night” theme), Julieta Ulanovsky (Montserrat font), Steve Matteson (Open Sans font).

Tech credits: git, reveal.js, MathJax, pdf.js, Firefox developer tools.

Acknowledgments: this work is supported in part by the NSF REU and DOD ASSURE programs under NSF grant no. 1262851, the Smithsonian Institution, NSF grant AST-1008361, and NASA Chandra Award Number G02-13007A issued by the Chandra X-ray Observatory Center, operated by the Smithsonian Astrophysical Observatory and NASA under contract NAS8-03060.

Magnetogram History Movie

NASA / David Hathaway