Flares are complicated events that are triggered by the reconnection of magnetic field lines. The dramatic reconnection event accelerates particles into the star’s atmosphere, heating many different layers that emit across the electromagnetic spectrum. Time-resolved spectroscopy is a unique tool for understanding the evolution of heating and cooling during stellar flares. Each emission line or continuum enhancement traces a different layer of the atmosphere, so we can estimate the changing temperatures and densities as the flare evolves.

Left panel: Diagram of the magnetic reconnection event responsible for Solar and stellar flares. The regions responsible for different types of emission are labeled. (credit: C. T. Russell; Gennady et al. 2015) Right panel: Image of the Sun in UV light showing magnetic loops high above the surface. (credit: NASA/SDO)

infrared emission lines

While flares are common on cool, red dwarfs, they are often difficult to observe because the hot flare emission is primarily observable in bluer wavelengths, while the cool stellar surface is more easily observed in redder and infrared wavelengths. To test if flares could be observed at infrared wavelengths, I monitored some of the most well-known flare stars with both optical photometry and infrared spectroscopy, looking for signs of emission during strong flares.

On the left, the evolution of emission lines detected throughout the optical and infrared spectrum. The lines that decline quickly cool the plasma efficiently, while those that continue to emit for longer trace regions of the flaring atmosphere that stay hotter longer. On the right, the layers of formation for emission lines in optical and infrared light. From Schmidt et al. (2012).

With nearly 50 hours of monitoring, I found that infrared lines are only found in strong emission during the most powerful flares. With a simple 1D radiative transfer model, I also found that these infrared emission lines traced only the hottest, most diffuse portion of the flaring atmosphere. Infrared lines are not an efficient method for detecting flares, but they do provide a window into a less well-observed portion of the flaring atmosphere.

Balmer lines and the Balmer jump

I’ve also been involved in projects led by Adam Kowalski centered on spectroscopic flare monitoring in blue and UV spectra focused on examining the Balmer lines and the Balmer jump compared to thermal continuum. These two components are typically both present in flares, but more impulsive flares tend to have stronger atomic/balmer emission, while more gentle flares have more thermal continuum. As more data is obtained and models become more sophisticated, we can examine the range of heating mechanisms that generate flare emission.

  1. The Near-ultraviolet Continuum Radiation in the Impulsive Phase of HF/GF-type dMe Flares. I. Data
    Kowalski, Adam F., Wisniewski, John P., Hawley, Suzanne L., Osten, Rachel A., Brown, Alexander, Fariña, Cecilia, Valenti, Jeff A., Brown, Stephen, Xilouris, Manolis, Schmidt, Sarah J., and Johns-Krull, Christopher
    ApJ 2019
  2. Time-resolved Properties and Global Trends in dMe Flares from Simultaneous Photometry and Spectra
    Kowalski, Adam F., Hawley, Suzanne L., Wisniewski, John P., Osten, Rachel A., Hilton, Eric J., Holtzman, Jon A., Schmidt, Sarah J., and Davenport, James R. A.
    ApJs 2013
  3. Probing the Flare Atmospheres of M Dwarfs Using Infrared Emission Lines
    Schmidt, Sarah J., Kowalski, Adam F., Hawley, Suzanne L., Hilton, Eric J., Wisniewski, John P., and Tofflemire, Benjamin M.
    ApJ 2012