- ID:
- ivo://CDS.VizieR/J/ApJS/244/37
- Title:
- Kepler flare star parameters from DR25
- Short Name:
- J/ApJS/244/37
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We analyze the light curve of 1740 flare stars to study the relationship between the magnetic feature characteristics and the identified flare activity. Coverage and stability of magnetic features are inspired by rotational modulation of light-curve variations and flare activity of stars are obtained using our automated flare detection algorithm. The results show that: (i) the flare time occupation ratio (or flare frequency) and the total power of flares increase by increasing relative magnetic feature coverage and contrast in F-M-type stars; (ii) magnetic feature stability is highly correlated with the coverage and the contrast of the magnetic structures, as this is the case for the Sun; and (iii) stability, coverage, and contrast of the magnetic features, time occupation ratio, and total power of flares increases for G-, K-, and M-type stars by decreasing the Rossby number due to the excess of the produced magnetic field from dynamo procedure until reaching to the saturation level.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/859/87
- Title:
- Kepler long- and short-cadence flare parameters
- Short Name:
- J/ApJ/859/87
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Flare research is becoming a burgeoning realm of interest in the study of stellar activity due to the launch of Kepler in 2009. Kepler provides data with two time resolutions, i.e., the long-cadence (LC) data with a time resolution of 30 minutes and the short-cadence (SC) data with a time resolution of 1 minute, both of which can be used to study stellar flares. In this paper, we search flares in light curves with both LC data and SC data, and compare them in aspects of the true-flare rate, the flare energy, the flare amplitude, and the flare duration. It is found that LC data systematically underestimated the energies of flares by 25%, and underestimated the amplitudes of flares by 60% compared with SC flares. The durations are systematically overestimated by 50% compared with SC flares. However, the above percentages are poorly constrained and there is a lot of scatter. About 60% of SC flares have not been detected by LC data. We investigate the limitation of LC data, and suggest that although LC data cannot reflect the detailed profiles of flares, they can also capture the basic properties of stellar flares.
- ID:
- ivo://CDS.VizieR/J/ApJ/858/55
- Title:
- K2 ultracool dwarfs survey. III. M6-L0 flares
- Short Name:
- J/ApJ/858/55
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report the white light flare rates for 10 ultracool dwarfs using Kepler K2 short-cadence data. Among our sample stars, two have spectral type M6, three are M7, three are M8, and two are L0. Most of our targets are old low-mass stars. We identify a total of 283 flares in all of the stars in our sample, with Kepler energies in the range log E_Kp_~(29-33.5)erg. Using the maximum-likelihood method of line fitting, we find that the flare frequency distribution (FFD) for each star in our sample follows a power law with slope -{alpha} in the range -(1.3-2.0). We find that cooler objects tend to have shallower slopes. For some of our targets, the FFD follows either a broken power law, or a power law with an exponential cutoff. For the L0 dwarf 2MASSJ12321827-0951502, we find a very shallow slope (-{alpha}=-1.3) in the Kepler energy range (0.82-130)x10^30^erg: this L0 dwarf has flare rates which are comparable to those of high-energy flares in stars of earlier spectral types. In addition, we report photometry of two superflares: one on the L0 dwarf 2MASS J12321827-0951502 and another on the M7 dwarf 2MASS J08352366+1029318. In the case of 2MASSJ12321827-0951502, we report a flare brightening by a factor of ~144 relative to the quiescent photospheric level. Likewise, for 2MASSJ08352366+1029318, we report a flare brightening by a factor of ~60 relative to the quiescent photospheric level. These two superflares have bolometric (ultraviolet/optical/infrared) energies 3.6x10^33^erg and 8.9x10^33^erg respectively, while the full width half maximum timescales are very short, ~2min. We find that the M8 star TRAPPIST-1 is more active than the M8.5 dwarf 2M03264453+1919309, but less active than another M8 dwarf (2M12215066-0843197).
- ID:
- ivo://CDS.VizieR/J/A+A/545/A85
- Title:
- LHS1070 photometry and spectroscopy
- Short Name:
- J/A+A/545/A85
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- LHS 1070 is a nearby multiple system of low mass stars. It is an important source of information for probing the low mass end of the main sequence, down to the hydrogen-burning limit. The primary of the system is a mid-M dwarf and two components are late-M to early L dwarfs, at the star-brown dwarf transition. Hence LHS 1070 is a valuable object to understand the onset of dust formation in cool stellar atmospheres. This work aims at determining the fundamental stellar parameters of LHS 1070 and to test recent model atmospheres: BT-Dusty, BT-Settl, DRIFT, and MARCS models.
- ID:
- ivo://CDS.VizieR/J/A+AS/131/321
- Title:
- Long-term monitoring of active stars. VIII.
- Short Name:
- J/A+AS/131/321
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As a part of an extensive program focusing on the global properties and evolution of active stars, high-precision UBV(RI)c photometry of 31 selected stars, collected at the European Southern Observatory over the 14-29 February 1992 interval, is presented. Significant evolution of the light curves, period variations and evidence for long-term variability of the global degree of spottedness are found. Some spectral classifications are revised and the inferred photometric parallaxes are compared, whenever possible, with the values measured by the Hipparcos satellite. Flare events were detected for the star HD 16157=CC Eri, EXO 055609-3804.4=TY Col and HD 119285=V851 Cen. Optical variability was discovered for the Pop II binary HD 89499. These observations contribute to the establishment of a time-extended photometric database which can give important clues on topics such as the stability of spotted areas, differential rotation, solar-like cycles and the correlation between inhomogeneities at different atmospheric levels.
- ID:
- ivo://CDS.VizieR/J/ApJ/852/72
- Title:
- Luminosity functions of tidal disruption flares
- Short Name:
- J/ApJ/852/72
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The tidal disruption of a star by a massive black hole is expected to yield a luminous flare of thermal emission. About two dozen of these stellar tidal disruption flares (TDFs) may have been detected in optical transient surveys. However, explaining the observed properties of these events within the tidal disruption paradigm is not yet possible. This theoretical ambiguity has led some authors to suggest that optical TDFs are due to a different process, such as a nuclear supernova or accretion disk instabilities. Here we present a test of a fundamental prediction of the tidal disruption event scenario: a suppression of the flare rate due to the direct capture of stars by the black hole. Using a recently compiled sample of candidate TDFs with black hole mass measurements, plus a careful treatment of selection effects in this flux-limited sample, we confirm that the dearth of observed TDFs from high-mass black holes is statistically significant. All the TDF impostor models we consider fail to explain the observed mass function; the only scenario that fits the data is a suppression of the rate due to direct captures. We find that this suppression can explain the low volumetric rate of the luminous TDF candidate ASASSN-15lh, thus supporting the hypothesis that this flare belongs to the TDF family. Our work is the first to present the optical TDF luminosity function. A steep power law is required to explain the observed rest-frame g-band luminosity, dN/dL_g_{propto}L_g_^-2.5^. The mean event rate of the flares in our sample is ~1x10^-4^galaxy^-1^/yr, consistent with the theoretically expected tidal disruption rate.
- ID:
- ivo://CDS.VizieR/J/AJ/140/1402
- Title:
- M dwarf flares from SDSS spectra
- Short Name:
- J/AJ/140/1402
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have identified 63 flares on M dwarfs from the individual component spectra in the Sloan Digital Sky Survey (SDSS) using a novel measurement of emission-line strength called the Flare Line Index. Each of the ~38000M dwarfs in the SDSS low-mass star spectroscopic sample of West et al. (2008, Cat. J/AJ/135/785) 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.
- ID:
- ivo://CDS.VizieR/J/ApJS/207/15
- Title:
- M dwarf flare spectra
- Short Name:
- J/ApJS/207/15
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a homogeneous analysis of line and continuum emission from simultaneous high-cadence spectra and photometry covering near-ultraviolet and optical wavelengths for 20 M dwarf flares. These data were obtained to study the white-light continuum 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. New model constraints are presented for the time evolution among the hydrogen Balmer lines and between CaII K and the blackbody continuum emission. We calculate Balmer jump flux ratios and compare to the solar-type flare heating predictions from RHD models. The model ratios are too large and the blue-optical ({lambda}=4000-4800{AA}) slopes are too red in both the impulsive and gradual decay phases of all 20 flares. This discrepancy implies that further work is needed to understand the heating at high column mass during dMe flares.
- ID:
- ivo://CDS.VizieR/J/A+A/637/A22
- Title:
- M dwarfs rotation-activity relations and flares
- Short Name:
- J/A+A/637/A22
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using light curves obtained by the K2 mission, we study the relation between stellar rotation and magnetic activity with a special focus on stellar flares. Our sample comprises 56 bright and nearby M dwarfs observed by K2 during campaigns C0-C18 in long and short cadence mode. We derive rotation periods for 46 M dwarfs and measure photometric activity indicators such as amplitude of the rotational signal, standard deviation of the light curves, and the basic flare properties (flare rate, flare energy, flare duration, flare amplitude). We found 1662 short cadence flares of which 363 have a long cadence counterpart with flare energies up to 5.6*10^34^erg. The flare amplitude, duration and flare frequency derived from the short cadence LCs differ significantly from the ones derived from the long cadence data. The analysis of the short cadence light curves results in a 4.6 times higher flare rate than the long cadence data. We confirm the abrupt change of the activity level in the rotation-activity relation at a critical period of ~10d when photometric activity diagnostics are used. This change is most drastic in the flare duration and the flare frequency for short cadence data. Our flare studies revealed that the highest flare rates are not found among the fastest rotators and that the stars with the highest flare rates do not show the most energetic flares. We found a ~2 times larger superflares frequency (>5*10^34^erg) for the fast rotating M stars than for solar like stars in the same period range. By fitting the cumulative flare frequency distribution we derived a power-law index of alpha=1.84+/-0.14 consistent with previous M dwarf studies and the value found for the Sun.
- ID:
- ivo://CDS.VizieR/J/A+A/622/A133
- Title:
- M45, M44 and M67 flare stars
- Short Name:
- J/A+A/622/A133
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The presence and strength of a stellar magnetic field and activity is rooted in a star's fundamental parameters such as mass and age. Can flares serve as an accurate stellar "clock"? To explore if we can quantify an activity-age relation in the form of a flaring-age relation, we measured trends in the flaring rates and energies for stars with different masses and ages. We investigated the time- domain photometry provided by Kepler's follow-up mission K2 and searched for flares in three solar metallicity open clusters with well-known ages, M45 (0.125Gyr), M44 (0.63Gyr), and M67 (4.3Gyr). We updated and employed the automated flare finding and analysis pipeline Appaloosa, originally designed for Kepler. We introduced a synthetic flare injection and recovery subroutine to ascribe detection and energy recovery rates for flares in a broad energy range for each light curve. We collected a sample of 1761 stars, mostly late-K to mid-M dwarfs and found 751 flare candidates with energies ranging from 4x10^32^erg to 6x10^34^erg, of which 596 belong to M45, 155 to M44, and none to M67. We find that flaring activity depends both on Teff, and age. But all flare frequency distributions have similar slopes with alpha from 2.0 to 2.4, supporting a universal flare generation process. We discuss implications for the physical conditions under which flares occur, and how the sample's metallicity and multiplicity affect our results.
