Can flare loops contribute to the white-light emission of stellar superflares ?

Abstract

Since the discovery of stellar superflares by Kepler satellite, these extremely energetic events have been studied in analogy to solar flares. Their white-light (WL) continuum emission has been interpreted as being produced by heated ribbons. In this paper we compute the WL emission from overlying flare loops depending on their density and temperature and show that, under conditions expected during superflares, the continuum brightening due to extended loop arcades can significantly contribute to stellar flux detected by Kepler. This requires electron densities in the loops $10^{12}-10^{13}$ cm${}^{-3}$ or higher. We show that such densities, exceeding those typically present in solar flare loops, can be reached on M-dwarf and solar-type superflare stars with large starspots and much stronger magnetic fields. Quite importantly, the WL radiation of loops is not very sensitive to their temperature and thus both cool as well as hot loops may contribute. We show that the WL intensity emergent from optically-thin loops is lower than the blackbody radiation from flare ribbons, but the contribution of loops to total stellar flux can be quite important due to their significant emitting areas. This new scenario for interpreting superflare emission suggests that the observed WL flux is due to a mixture of the ribbon and loop radiation and can be even loop-dominated during the gradual phase of superflares.

7 pages, 4 figures, accepted for publication in The Astrophysical Journal

References in corpus (9)

• A White Light Megaflare on the dM4.5e Star YZ CMi
• Strong dipole magnetic fields in fast rotating fully convective stars
• Unprecedented Fine Structure of a Solar Flare Revealed by the 1.6~m New Solar Telescope
• New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
• Starspot activity and superflares on solar-type stars
• High Dispersion Spectroscopy of Solar-type Superflare Stars. II. Stellar Rotation, Starspots, and Chromospheric Activities
• On the nature of off-limb flare continuum sources detected by SDO/HMI
• An explanation of remarkable emission line profiles in post-flare coronal rain
• The Origin of Superflares on G-Type Dwarf Stars of Various Ages

Cited by in corpus (21)

• Do Kepler superflare stars really include slowly-rotating Sun-like stars ? - Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -
• Statistical Properties of Superflares on Solar-type Stars: Results Using All of the Kepler Primary Mission Data
• EvryFlare I: Long-term Evryscope Monitoring of Flares from the Cool Stars Across Half the Southern Sky
• Optical and X-ray observations of stellar flares on an active M dwarf AD Leonis with Seimei Telescope, SCAT, NICER and OISTER
• Statistical Analysis of Stellar Flares from the First Three Years of TESS Observations
• Giant white-light flares on fully convective stars occur at high latitudes
• Discovery of a Long-Duration Superflare on a Young Solar-Type Star EK Draconis with Nearly Similar Time Evolution for H$\alpha$ and White-Light Emissions
• High-Density Off-Limb Flare Loops Observed by SDO
• Simultaneous Kepler/K2 and XMM-Netwon observations of superflares in the Pleiades
• 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
• Fast optical flares from M dwarfs detected by a one-second-cadence survey with Tomo-e Gozen
• Observations of instability-driven nanojets
• Spectral diagnostics of cool flare loops observed by SST: I. Inversion of the Ca II 8542 Å and H$\beta$ lines
• Observations and modeling of spectral line asymmetries in stellar flares
• From Chromospheric Evaporation to Coronal Rain: An Investigation of the Mass and Energy Cycle of a Flare
• Minute-cadence Observations of the LAMOST Fields with the TMTS: III. Statistic Study of the Flare Stars from the First Two Years
• Possible evidence of induced repetitive magnetic reconnection in a superflare from a young solar-type star
• Spectroscopic observations of flares and superflares on AU Mic
• Soft X-ray heating as a mechanism of optical continuum generation in solar-type star superflares
• A Seven-Day Multi-Wavelength Flare Campaign on AU Mic III: Quiescent and Flaring Properties of the X-ray Spectra and Chromospheric lines
• Specialist Discussion Meeting: 3D structure of the flare chromosphere