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Solar Physics

, Volume 290, Issue 12, pp 3487–3523 | Cite as

New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

  • Adam F. Kowalski
  • S. L. Hawley
  • M. Carlsson
  • J. C. Allred
  • H. Uitenbroek
  • R. A. Osten
  • G. Holman
Solar and Stellar Flares

Abstract

The heating mechanism at high densities during M-dwarf flares is poorly understood. Spectra of M-dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of \(T\approx10^{4}~\mbox{K}\) in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at \(\lambda\le3\,646\) Å, and 3) an apparent pseudo-continuum of blended high-order Balmer lines between \(\lambda=3\,646\) Å and \(\lambda\approx3\,900\) Å. These properties are not reproduced by models that employ a typical “solar-type” flare heating level of \({\le}\,10^{11}~\mbox{erg}\,\mbox{cm}^{-2}\,\mbox{s}^{-1}\) in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological three-component interpretation. We present a new 1D radiative-hydrodynamic model of an M-dwarf flare from precipitating nonthermal electrons with a high energy flux of \(10^{13}~\mbox{erg}\,\mbox{cm}^{-2}\,\mbox{s}^{-1}\). The simulation produces bright near-ultraviolet and optical continuum emission from a dense (\(n>10^{15}~\mbox{cm}^{-3}\)), hot (\(T \approx12\,000\,\mbox{--}\,13\,500~\mbox{K}\)) chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a \(T\approx10^{4}~\mbox{K}\) blackbody-like continuum component and a low Balmer jump ratio result from optically thick Balmer (\(\infty\rightarrow n=2\)) and Paschen recombination (\(\infty\rightarrow n=3\)) radiation, and thus the properties of the flux spectrum are caused by blue (\(\lambda\approx4\,300\) Å) light escaping over a larger physical depth range than by red (\(\lambda\approx6\,700\) Å) and near-ultraviolet (\(\lambda\approx3\,500\) Å) light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau–Zener transitions that result from merged, high-order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.

Keywords

Flares, dynamics Flares, energetic particles Flares, impulsive phase Flares, models Flares, spectrum Flares, white-light 

Notes

Acknowledgements

Adam F. Kowalski thanks the Science Organizing Committee of the Solar and Stellar Flares meeting in Prague, Czech Republic for the opportunity to present this work. We thank an anonymous referee for clarifications and comments that helped improve this work. Adam F. Kowalski thanks Petr Heinzel and Hans Ludwig for bringing hot star modeling articles of Stark broadening to his attention. We thank Adrian Daw, Eric Agol, Ellen Zweibel, and Jeremiah Murphy for helpful discussions and William Abbett for several IDL routines used in the analysis. AFK also acknowledges helpful discussions at the International Space Science Institute with Lyndsay Fletcher’s Solar and Stellar Flares team and with Sven Wedemeyer-Bohm’s Magnetic Activity of M-type Dwarf Stars and the Influence on Habitable Extra-solar Planets team. This research was supported by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA, and by the University of Maryland Goddard Planetary Heliophysics Institute (GPHI) Task 132.

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Copyright information

© Springer Science+Business Media Dordrecht 2015

Authors and Affiliations

  • Adam F. Kowalski
    • 1
    • 2
  • S. L. Hawley
    • 3
  • M. Carlsson
    • 4
  • J. C. Allred
    • 2
  • H. Uitenbroek
    • 5
  • R. A. Osten
    • 6
  • G. Holman
    • 2
  1. 1.Department of AstronomyUniversity of MarylandCollege ParkUSA
  2. 2.NASA Goddard Space Flight CenterHeliophysics Science DivisionGreenbeltUSA
  3. 3.Department of AstronomyUniversity of WashingtonSeattleUSA
  4. 4.Institute of Theoretical AstrophysicsUniversity of OsloOsloNorway
  5. 5.National Solar ObservatorySunpsotUSA
  6. 6.Space Telescope Science InstituteBaltimoreUSA

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