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

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 ≈ 10 4 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at λ ≤ 3646Å and 3) an apparent pseudo-continuum of blended high-order Balmer lines between λ = 3646Å and λ ≈ 3900Å. These properties are not reproduced by models that employ a typical solar-type flare heating level of ≤ 10 11 erg cm −2 s −1 in non-thermal 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 non-thermal electrons with a large energy flux of 10 13 erg cm −2 s −1 . The simulation produces bright nearultraviolet and optical continuum emission from a dense (n >10 15 cm −3 ), hot (T ≈ 12 000 − 13 500 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 ≈ 10 4 K blackbodylike continuum component and a small Balmer jump ratio result from optically thick Balmer (∞ → n = 2) and Paschen recombination (∞ → n = 3) radiation, and thus the properties of the flux spectrum are caused by blue (λ ≈ 4300Å) light escaping over a larger physical depth range compared to red (λ ≈ 6700Å) and near-ultraviolet (λ ≈ 3500Å) light. To model the near-ultraviolet pseudocontinuum 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.

Accretion and Magnetic Reconnection in the Classical T Tauri Binary Dq Tau

“…Observationally, the blackbody component of stellar flares dominates over Balmer continuum at the flare peak, where hot-spot temperatures range between 10,000 and 14,000 K, reducing to 7000 to 10,000 K in the decay phase (Kowalski et al 2013). The higher blackbody temperatures compared to accretion result from deposition of energy directly into the photosphere by the electron beam rather than from radiative heating (Kowalski et al 2015).…”

A VERY BRIGHT, VERY HOT, AND VERY LONG FLARING EVENT FROM THE M DWARF BINARY SYSTEM DG CVn

“…This indicates that 10 times more emission is needed to account for the flux in the uvw2 bandpass if a 6000 K blackbody (or any spectrum that is ofsimilar shape as a 6000 K blackbody) is extrapolated to λ=2030 Å. In the impulsive beam heating phase of the F11, F12, and F13 models from Kowalski et al (2015), the average values of the continuum flux ratio 2030/5500 Å are 1.8, 2.0, and 3.1 for these models respectively. So a ratio of 0.8-0.9 (even given a 20% uncertainty from comparing satellite and ground-based broadband data) can be used as a strong constraint on heating models.…”

“…A preliminary multithread modeling approach to F2 uses the F13 beam heated atmosphere from K13, which were calculated with the RADYN code (Carlsson & Stein 1997). If we assume that the emission during F2 can be modeled by a superposition of impulsively heated loops (new kernels using the "average burst" spectrum ( Table 1 of Kowalski et al 2015; F kernel here)) and decay phase emission from previously heated loops (F13 gradual decay spectrum at t = 4 s in Table 1 of K15; F decay ) with anarea coverage that is25 timesthe kernel emission, then we obtain a broadband spectrum that is generally consistent with the coarse Swift UV and optical colors (uvw2/V∼1 compared to the observations ∼0.8-0.9). We can estimate an areal coverage using an actual RHD spectrum…”

“…With that substitution, the kinetic energy then becomes a function of the Figure 8. Excess flare specific luminosity calculated using the RADYN code as described in Kowalski et al (2015); black plusses and solid lines are the RHD calculations. Red dashed-dotted line is the blackbody spectrum at T BB =6000 K. Blue circles are the midpoints of the uvw2 and V filters, respectively, and are used to estimate a flux ratio to be compared against that observed in F2.…”

Preface

“…Milligan et al, 2014;Kerr and Fletcher, 2014;Kleint et al, 2015) that requires multiwavelength spectral observations, and Milligan (2015) reviews the extreme ultra-violet (EUV) spectroscopy of the lower solar atmosphere during flares, from which the properties of flare plasmas can be deduced, using observations from the Extreme ultraviolet Variability Experiment (EVE) experiment onboard the Solar Dynamics Observatory (Woods et al, 2012) and the Extreme ultraviolet Imaging Spectrometer (EIS, Culhane et al, 2007). In the optical range, broadband spectra of stellar flares are being obtained by large telescopes, and Kowalski et al (2015) present comprehensive insights into the white-light flare emission from stellar flares. These data are hard to obtain, but are crucial also in the solar case.…”