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11. 8695 flares from 1228 stars in TESS sectors 1 & 2
ID:
ivo://CDS.VizieR/J/AJ/159/60
Title:
8695 flares from 1228 stars in TESS sectors 1 & 2
Short Name:
J/AJ/159/60
Date:
21 Oct 2021
Publisher:
CDS
Description:
We perform a study of stellar flares for the 24809 stars observed with 2 minute cadence during the first two months of the Transiting Exoplanet Survey Satellite (TESS) mission. Flares may erode exoplanets' atmospheres and impact their habitability, but might also trigger the genesis of life around small stars. TESS provides a new sample of bright dwarf stars in our galactic neighborhood, collecting data for thousands of M dwarfs that might host habitable exoplanets. Here, we use an automated search for flares accompanied by visual inspection. Then, our public allesfitter code robustly selects the appropriate model for potentially complex flares via Bayesian evidence. We identify 1228 flaring stars, 673 of which are M dwarfs. Among 8695 flares in total, the largest superflare increased the stellar brightness by a factor of 16.1. Bolometric flare energies range from 10^31.0^ to 10^36.9^erg, with a median of 10^33.1^erg. Furthermore, we study the flare rate and energy as a function of stellar type and rotation period. We solidify past findings that fast rotating M dwarfs are the most likely to flare and that their flare amplitude is independent of the rotation period. Finally, we link our results to criteria for prebiotic chemistry, atmospheric loss through coronal mass ejections, and ozone sterilization. Four of our flaring M dwarfs host exoplanet candidates alerted on by TESS, for which we discuss how these effects can impact life. With upcoming TESS data releases, our flare analysis can be expanded to almost all bright small stars, aiding in defining criteria for exoplanet habitability.
12. Flare stars across the H-R diagram
ID:
ivo://CDS.VizieR/J/MNRAS/447/2714
Title:
Flare stars across the H-R diagram
Short Name:
J/MNRAS/447/2714
Date:
21 Oct 2021
Publisher:
CDS
Description:
From Kepler data, we show that the incidence of flares on stars drops by only a factor of 4 from K-M dwarfs to A-F stars. Allowing for visibility effects, this implies that the true relative number of flare stars does not change very much from cool dwarfs to hot A stars. The idea that flares on A stars can be attributed to a cool companion has to be rejected because it leads to flare amplitudes two orders of magnitude smaller than actually observed. We confirm that spots on flare stars are generally larger than those on non-flare stars and that flare stars rotate significantly faster than non-flare stars. Analysis of 209 flare stars observed in Kepler short-cadence mode allows accurate measurements of flare shapes and duration. We find that about one-third of the flares have a bump or slope discontinuity on the decaying branch and that flares of long duration are to be found in stars with low surface gravities. Flare energies are strongly correlated with stellar luminosity and radius. The correlation with radius leads to a rough estimate of several tens of gauss for the typical magnetic field associated with a flare. The correlation with stellar luminosity can be understood if the typical flare loop length-scales approximately as the stellar radius. We examined the flare frequency as a function of orbital phase in three eclipsing binaries in which a large number of flares are visible. There appears to be no correlation of flaring with orbital phase, which weakens the hypothesis that flares in close binaries could be a result of reconnection of field lines connecting the two stars.
13. Flare stars in nearby Galactic open clusters
ID:
ivo://CDS.VizieR/J/other/CoSka/51.78
Title:
Flare stars in nearby Galactic open clusters
Short Name:
J/other/CoSka/51
Date:
21 Oct 2021
Publisher:
CDS
Description:
The study is devoted to search for flare stars among confirmed members of Galactic open clusters using high-cadence photometry from TESS mission. We analyzed 957 high-cadence light curves of members from 136 open clusters. As a result, 56 flare stars were found, among them 8 hot B-A type objects. Of all flares, 63 % were detected in sample of cool stars (Teff<5000K), and 2 % - in stars of spectral type G, while 23% in K-type stars and approximately 34% of all detected flares are in M-type stars. Using the FLATW'RM (FLAre deTection With Ransac Method) flare finding algorithm, we estimated parameters of flares and rotation period of detected flare stars. The flare with the largest amplitude appears on the M3 type EQ Cha star. Statistical analysis did not reveal any direct correlation between ages, rotation periods and flaring activity.
14. Flare stars in the Pleiades
ID:
ivo://CDS.VizieR/J/other/PZ/23.141
Title:
Flare stars in the Pleiades
Short Name:
J/other/PZ/23.14
Date:
21 Oct 2021
Publisher:
CDS
Description:
A brief account is given of the problems in identifying newly discovered variable stars with already known ones. These problems are most acute in dense stellar fields, for instance, in the Pleiades. To arrive to a homogeneous coordinates system for variable stars in the Pleiades, the author has measured the coordinates for nearly 550 stars in the region centered on Alcyone. This has made it possible to obtain for the first time accurate coordinates for 165 flare stars; then, for 46 stars we have round significant errors in the published coordinates. A cross-identification table between the GCVS (Cat. <II/139>) and the catalogue of Haro et al. (1982, Cat. <II/131>) has been compiled.
15. Flaring activity of M dwarfs in the Kepler field
ID:
ivo://CDS.VizieR/J/ApJ/849/36
Title:
Flaring activity of M dwarfs in the Kepler field
Short Name:
J/ApJ/849/36
Date:
21 Oct 2021
Publisher:
CDS
Description:
Flare events are mainly due to magnetic reconnection and thus are indicative of stellar activity. The Kepler Space Observatory records numerous stellar activities with unprecedented high photometric precision in flux measurements. It is perfectly suitable for carrying out a statistical study of flares. Here we present 540 M dwarfs with flare events discovered using Kepler long-cadence data. The normalized flare energy, as defined by the ratio to bolometric stellar luminosity, L_flare_/L_bol_, is used to indicate the flare activity. We find that, similar to the X-ray luminosity relation, the L_flare_/L_bol_ versus P_rot_ relation can also be described with three phases, supersaturation, saturation, and exponential decay, corresponding to an ultra- short period, a short period, and a long period. The flare activity and the number fraction of flaring stars in M dwarfs rise steeply near M4, which is consistent with the prediction of a turbulent dynamo. The size of starspots are positively correlated with flare activity. The L_flare_/L_bol_ ratio has a power-law dependence on L_H{alpha}_/L_bol_, a parameter indicative of stellar chromosphere activity. According to this relation, a small enhancement in chromosphere activity may cause a huge rise in flare energy, which suggests that superflares or hyperflares may not need an extra excitation mechanism. Through a comparison study, we suggest that flare activity is a more suitable indicator for stellar activity, especially in the boundary region. However, contrary to what is expected, some M dwarfs with strong flares do not show any light variation caused by starspots. Follow-up observations are needed to investigate this problem.
16. Gamma-ray burst flares: X-ray flaring
ID:
ivo://CDS.VizieR/J/ApJ/788/30
Title:
Gamma-ray burst flares: X-ray flaring
Short Name:
J/ApJ/788/30
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a catalog of 498 flaring periods found in gamma-ray burst (GRB) light curves taken from the online Swift X-Ray Telescope GRB Catalogue (Evans et al. 2007A&A...469..379E, 2009, J/MNRAS/397/1177). We analyzed 680 individual light curves using a flare detection method developed and used on our UV/optical GRB Flare Catalog. This method makes use of the Bayesian Information Criterion to analyze the residuals of fitted GRB light curves and statistically determines the optimal fit to the light curve residuals in an attempt to identify any additional features. These features, which we classify as flares, are identified by iteratively adding additional "breaks" to the light curve. We find evidence of flaring in 326 of the analyzed light curves. For those light curves with flares, we find an average number of ~1.5 flares per GRB. As with the UV/optical, flaring in our sample is generally confined to the first 1000 s of the afterglow, but can be detected to beyond 10^5^ s. Only ~50% of the detected flares follow the "classical" definition of {Delta}t/t<=0.5, with many of the largest flares exceeding this value.
17. Gershberg Flare Star Catalogue
ID:
ivo://CDS.VizieR/II/55
Title:
Gershberg Flare Star Catalogue
Short Name:
II/55
Date:
21 Oct 2021
Publisher:
CDS
Description:
This catalog groups the flare stars as presented in the IAU Colloquium 15 (combined Colloquium of Commisions 27 and 42) held at Bamberg 31-Aug to 03-Sep 1971 entitled "New Directions and New Frontiers in Variable Star Research" The following definition of the UV Cet-type variables was given in Kukarkin's General Catalogue of Variable Stars (1969): "dMe stars, sometimes subject to flares with the amplitude from 1 to 6mag. Maximum brightness is attained in seconds or dozens of seconds after the commencement of the flare; the star returns to its normal brightness after several minutes, or dozens of minutes. A typical representative is UV Ceti." Now it is impossible to consider this definition as a quite right because: 1) There are a number of M-dwarf stars affected by the flares similar to the UV Ceti flares, but in their quiet state spectra, no emission lines are observed. For example, BD+43 44A, BD+43 44 B, and SZ UMa, the flare activity of these stars were detected in Crimea; and probably BD-04 4048B is suspected as a flare star by HERBIG. 2) The lower limit of flare amplitude cited in Kukarkin's corresponds to visual observations, but the modern photoelectric observations register flares with amplitudes to 0.02-0.05mag. Therefore we suppose, that UV Cet-type variables are K-M dwarfs, which show quick flares with amplitudes exceeding the observational errors, and duration of the flares are from a few seconds up to a few hundred minutes. The "catalog" file includes only such variable K-M dwarf stars, for which existing observations allow to construct flare light curves. All stars in "catalog", except V371 Ori, have photoelectric flare light curves. The strong flare of V371 Ori was observed in radio region, but simultaneous optical observations were carried out photographically and visually only.
18. HATNet variability survey of K and M dwarfs
ID:
ivo://CDS.VizieR/J/AJ/141/166
Title:
HATNet variability survey of K and M dwarfs
Short Name:
J/AJ/141/166
Date:
21 Oct 2021
Publisher:
CDS
Description:
Using light curves from the HATNet survey for transiting extrasolar planets we investigate the optical broad-band photometric variability of a sample of 27, 560 field K and M dwarfs selected by color and proper-motion (V-K>~3.0, mu>30mas/yr, plus additional cuts in J-H vs. H-Ks and on the reduced proper motion). We apply a variety of variability selection algorithms on the light curves to search for periodic and quasi-periodic variations, and for large-amplitude, long-duration flare events. To set the selection thresholds we conduct Monte Carlo simulations of light curves with realistic noise properties.
19. HST monitoring of flaring stars in the Galactic bulge
ID:
ivo://CDS.VizieR/J/ApJ/754/4
Title:
HST monitoring of flaring stars in the Galactic bulge
Short Name:
J/ApJ/754/4
Date:
21 Oct 2021
Publisher:
CDS
Description:
We utilize the Sagittarius Window Eclipsing Extrasolar Planet Search Hubble Space Telescope/Advanced Camera for Surveys data set for a Deep Rapid Archival Flare Transient Search (DRAFTS) to constrain the flare rate toward the older stellar population in the Galactic bulge. During seven days of monitoring 229293 stars brighter than V=29.5, we find evidence for flaring activity in 105 stars between V=20 and V=28. We divided the sample into non-variable stars and variable stars whose light curves contain large-scale variability. The flare rate on variable stars is ~700 times that of non-variable stars, with a significant correlation between the amount of underlying stellar variability and peak flare amplitude. The flare energy loss rates are generally higher than those of nearby well-studied single dMe flare stars.
20. Kepler flare star parameters from DR25
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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