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

Abstract

(abridged) 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 $\sim$ 10,000 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at lambda $<$ 3646 Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer lines. These properties are not reproduced by models that employ a typical "solar-type" flare heating level in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological interpretation. We present a new 1D radiative-hydrodynamic model of an M dwarf flare from precipitating nonthermal electrons with a large energy flux of $10^{13}$ erg cm${}^{-2}$ s${}^{-1}$. The simulation produces bright continuum emission from a dense, hot 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 $\sim$ 10,000 K blackbody-like continuum component and a small Balmer jump ratio result from optically thick Balmer and Paschen recombination radiation, and thus the properties of the flux spectrum are caused by blue light escaping over a larger physical depth range compared to red and near-ultraviolet 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.

50 pages, 2 tables, 13 figures. Accepted for publication in the Solar Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015): updated to include comments by Guest Editor. The final publication is available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-x

References in corpus (10)

• Kepler Flares I. Active and Inactive M dwarfs
• Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem
• A White Light Megaflare on the dM4.5e Star YZ CMi
• Radiative Hydrodynamic Models of Optical and Ultraviolet Emission from M Dwarf Flares
• The Radiated Energy Budget of Chromospheric Plasma in a Major Solar Flare Deduced From Multi-Wavelength Observations
• Nonthermal Hard X-ray Emission and Iron Kalpha Emission from a Superflare on II Pegasi
• Multiwavelength observations of a giant flare on CN Leonis I. The chromosphere as seen in the optical spectra
• Hydrogen Balmer Continuum in Solar Flares Detected by the Interface Region Imaging Spectrograph (IRIS)
• The periodic variations of a white-light flare observed with ULTRACAM
• Optical Spectral Observations of a Flickering White-Light Kernel in a C1 Solar Flare

Cited by in corpus (25)

• The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares I: Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29
• Hydrogen Balmer Line Broadening in Solar and Stellar Flares
• Optical and X-ray observations of stellar flares on an active M dwarf AD Leonis with Seimei Telescope, SCAT, NICER and OISTER
• A Very Bright, Very Hot, and Very Long Flaring Event from the M Dwarf Binary System DG CVn
• Statistical Analysis of Stellar Flares from the First Three Years of TESS Observations
• Accretion and Magnetic Reconnection in the Classical T Tauri Binary DQ Tau
• Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres
• Modeling Mg II h, k and Triplet Lines at Solar Flare Ribbons
• First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
• The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. II. Hydrogen Broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope
• Spectral Evidence for Heating at Large Column Mass in Umbral Solar Flare Kernels I: IRIS NUV Spectra of the X1 Solar Flare of 2014 Oct 25
• Solar Flare Arcade Modelling: Bridging the gap from 1D to 3D Simulations of Optically Thin Radiation
• A Superflare on YZ Canis Minoris Observed by Seimei Telescope and TESS: Red Asymmetry of H$\alpha$ Emission Associated with White-Light Emission
• Spectral Observations and Modeling of a Solar White-light Flare Observed by CHASE
• Starspots Modelling and Flare Analysis on Selected MV Stars
• Prospects of Detecting Non-thermal Protons in Solar Flares via Lyman Line Spectroscopy: Revisiting the Orrall-Zirker Effect
• Bridging High-Density, Electron Beam Coronal Transport and Deep Chromospheric Heating in Stellar Flares
• Suppression of Hydrogen Emission in an X-class White-light Solar Flare
• Spectral Characteristics and Formation Height of Off-Limb Flare Ribbons
• Transient Mass Loss Analysis of Solar Observations using Stellar Methods
• Discovery of multi-temperature coronal mass ejection signatures from a young solar analogue
• Origin of the Blue Continuum Radiation in the Flare Spectra of dMe stars
• Separating flare and secondary atmospheric signals with RADYN modeling of near-infrared JWST transmission spectroscopy observations of TRAPPIST-1
• Young M dwarfs flare activity model: Towards better exoplanetary atmospheric characterisation
• Solar Flare Heating with Turbulent Suppression of Thermal Conduction