(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.
We present the largest sample of flares ever compiled for a single M dwarf, the active M4 star GJ 1243. Over 6100 individual flare events, with energies ranging from $10^{29}$ to $10^{33}$ erg, are found in 11 months of 1-minute cadence data from Kep
ler. This sample is unique for its completeness and dynamic range. We have developed automated tools for finding flares in short-cadence Kepler light curves, and performed extensive validation and classification of the sample by eye. From this pristine sample of flares we generate a median flare template. This template shows that two exponential cooling phases are present during the white-light flare decay, providing fundamental constraints for models of flare physics. The template is also used as a basis function to decompose complex multi-peaked flares, allowing us to study the energy distribution of these events. Only a small number of flare events are not well fit by our template. We find that complex, multi-peaked flares occur in over 80% of flares with a duration of 50 minutes or greater. The underlying distribution of flare durations for events 10 minutes and longer appears to follow a broken power law. Our results support the idea that sympathetic flaring may be responsible for some complex flare events.
We analyzed Kepler short-cadence M dwarf observations. Spectra from the ARC 3.5m telescope identify magnetically active (H$alpha$ in emission) stars. The active stars are of mid-M spectral type, have numerous flares, and well-defined rotational modul
ation due to starspots. The inactive stars are of early-M type, exhibit less starspot signature, and have fewer flares. A Kepler to U-band energy scaling allows comparison of the Kepler flare frequency distributions with previous ground-based data. M dwarfs span a large range of flare frequency and energy, blurring the distinction between active and inactive stars designated solely by the presence of H$alpha$. We analyzed classical and complex (multiple peak) flares on GJ 1243, finding strong correlations between flare energy, amplitude, duration and decay time, with only a weak dependence on rise time. Complex flares last longer and have higher energy at the same amplitude, and higher energy flares are more likely to be complex. A power law fits the energy distribution for flares with log $E_{K_p} >$ 31 ergs, but the predicted number of low energy flares far exceeds the number observed, at energies where flares are still easily detectable, indicating that the power law distribution may flatten at low energy. There is no correlation of flare occurrence or energy with starspot phase; the flare waiting time distribution is consistent with flares occurring randomly in time; and the energies of consecutive flares are uncorrelated. These observations support a scenario where many independent active regions on the stellar surface are contributing to the observed flare rate.
We present a homogeneous survey of line and continuum emission from near-ultraviolet (NUV) to optical wavelengths during twenty M dwarf flares with simultaneous, high cadence photometry and spectra. These data were obtained to study the white-light c
ontinuum components at bluer and redder wavelengths than the Balmer jump. Our goals were to break the degeneracy between emission mechanisms that have been fit to broadband colors of flares and to provide constraints for radiative-hydrodynamic (RHD) flare models that seek to reproduce the white-light flare emission. The main results from the continuum analysis are the following: 1) the detection of Balmer continuum (in emission) that is present during all flares and with a wide range of relative contributions to the continuum flux at bluer wavelengths than the Balmer jump; 2) a blue continuum at flare maximum that is linearly decreasing with wavelength from lambda = 4000-4800AA, matched by the spectral shape of hot, blackbody emission with typical temperatures of T_{BB}~9000-14,000 K; 3) a redder continuum apparent at wavelengths longer than Hbeta (lambda > 4900AA) which becomes relatively more important to the energy budget during the late gradual phase. We calculate Balmer jump flux ratios and compare to RHD model spectra. The model ratios are too large and the blue-optical (lambda = 4000-4800AA) slopes are too red in both the impulsive and gradual decay phases of all twenty flares. This discrepancy implies that further work is needed to understand the heating at high column mass during dMe flares. (Abridged)
The white light during M dwarf flares has long been known to exhibit the broadband shape of a T~10,000 K blackbody, and the white light in solar flares is thought to arise primarily from Hydrogen recombination. Yet, a current lack of broad wavelength
coverage solar-flare spectra in the optical/near-UV prohibits a direct comparison of the continuum properties to determine if they are indeed so different. New spectroscopic observations of a secondary flare during the decay of a megaflare on the dM4.5e star YZ CMi have revealed multiple components in the white-light continuum of stellar flares, including both a blackbody-like spectrum and a hydrogen recombination spectrum. One of the most surprising findings is that these two components are anti-correlated in their temporal evolution. We combine initial phenomenological modeling of the continuum components with spectra from radiative-hydrodynamic models to show that continuum veiling gives rise to the measured anti-correlation. This modeling allows us to use the components inferred properties to predict how a similar spatially resolved, multiple-component white-light continuum might appear using analogies to several solar flare phenomena. We also compare the properties of the optical stellar flare white light to Ellerman bombs on the Sun.
We present sub-second, continuous-coverage photometry of three flares on the dM3.5e star, EQ Peg A, using custom continuum filters with WHT/ULTRACAM. These data provide a new view of flare continuum emission, with each flare exhibiting a very distinc
t light curve morphology. The spectral shape of flare emission for the two large-amplitude flares is compared with synthetic ULTRACAM measurements taken from the spectra during the large megaflare event on a similar type flare star. The white light shape during the impulsive phase of the EQ Peg flares is consistent with the range of colors derived from the megaflare continuum, which is known to contain a Hydrogen recombination component and compact, blackbody-like components. Tentative evidence in the ULTRACAM photometry is found for an anti-correlation between the emission of these components.
We present a statistical parallax analysis of low-mass dwarfs from the Sloan Digital Sky Survey (SDSS). We calculate absolute r-band magnitudes (M_r) as a function of color and spectral type, and investigate changes in M_r with location in the Milky
Way. We find that magnetically active M dwarfs are intrinsically brighter in M_r than their inactive counterparts at the same color or spectral type. Metallicity, as traced by the proxy zeta, also affects M_r, with metal poor stars having fainter absolute magnitudes than higher metallicity M dwarfs at the same color or spectral type. Additionally, we measure the velocity ellipsoid and solar reflex motion for each subsample of M dwarfs. We find good agreement between our measured solar peculiar motion and previous results for similar populations, as well as some evidence for differing motions of early and late M type populations in U and W velocities that cannot be attributed to asymmetric drift. The reflex solar motion and the velocity dispersions both show that younger populations, as traced by magnetic activity and location near the Galactic plane, have experienced less dynamical heating. We introduce a new parameter, the independent position altitude (IPA), to investigate populations as a function of vertical height from the Galactic plane. M dwarfs at all types exhibit an increase in velocity dispersion when analyzed in comparable IPA subgroups.
We report on the analysis of ~22,000 M dwarfs using a statistical parallax method. This technique employs a maximum-likelihood formulation to simultaneously solve for the absolute magnitude, velocity ellipsoid parameters and reflex solar motion of a
homogeneous stellar sample, and has previously been applied to Galactic RR Lyrae and Cepheid populations and to the Palomar/Michigan State University (PMSU) survey of nearby low-mass stars. We analyze subsamples of the most recent spectroscopic catalog of M dwarfs in the Sloan Digital Sky Survey (SDSS) to determine absolute magnitudes and kinematic properties as a function of spectral type, color, chromospheric activity and metallicity. We find new, independent spectral type-absolute magnitude relations, and color-absolute magnitude relations in the SDSS filters, and compare to those found from other methods. Active stars have brighter absolute magnitudes and lower metallicity stars have fainter absolute magnitudes for stars of type M0-M4. Our kinematic analysis confirms previous results for the solar motion and velocity dispersions, with more distant stars possessing larger peculiar motions, and chromospherically active (younger) stars having smaller velocity dispersions than their inactive counterparts. We find some evidence for systematic differences in the mean U and W velocities of samples subdivided by color.
M dwarfs are known to flare on timescales from minutes to hours, with flux increases of several magnitudes in the blue/near-UV. These frequent, powerful events, which are caused by magnetic reconnection, will have a strong observational signature in
large, time-domain surveys. The radiation and particle fluxes from flares may also exert a significant influence on the atmospheres of orbiting planets, and affect their habitability. We present a statistical model of flaring M dwarfs in the Galaxy that allows us to predict the observed flare rate along a given line of sight for a particular survey depth and cadence. The parameters that enter the model are the Galactic structure, the distribution of magnetically active and inactive M dwarfs, and the flare frequency distribution (FFD) of both populations. The FFD is a function of spectral type, activity, and Galactic height. Although inactive M dwarfs make up the majority of stars in a magnitude-limited survey, the FFD of inactive stars is very poorly constrained. We have organized a flare monitoring campaign comprising hundreds of hours of new observations from both the ground and space to better constrain flare rates. Incorporating the new observations into our model provides more accurate predictions of stellar variability caused by flares on M dwarfs. We pay particular attention to the likelihood of flares appearing as optical transients (i.e., host star not seen in quiescent data).
We have identified 63 flares on M dwarfs from the individual component spectra in the Sloan Digital Sky Survey using a novel measurement of emission line strength called the Flare Line Index. Each of the ~38,000 M dwarfs in the SDSS low mass star spe
ctroscopic sample of West et al. was observed several times (usually 3-5) in exposures that were typically 9-25 minutes in duration. Our criteria allowed us to identify flares that exhibit very strong H-alpha and H-beta emission line strength and/or significant variability in those lines throughout the course of the exposures. The flares we identified have characteristics consistent with flares observed by classical spectroscopic monitoring. The flare duty cycle for the objects in our sample is found to increase from 0.02% for early M dwarfs to 3% for late M dwarfs. We find that the flare duty cycle is larger in the population near the Galactic plane and that the flare stars are more spatially restricted than the magnetically active but non-flaring stars. This suggests that flare frequency may be related to stellar age (younger stars are more likely to flare) and that the flare stars are younger than the mean active population.
