Flare Stars Research Articles

Population size estimation based upon zero-truncated, one-inflated and sparse count data

Estimating the size of a hard-to-count population is a challenging matter. In particular, when only few observations of the population to be estimated are available. The matter gets even more complex when one-inflation occurs. This situation is illustrated with the help of two examples: the size of a dice snake population in Graz (Austria) and the number of flare stars in the Pleiades. The paper discusses how one-inflation can be easily handled in likelihood approaches and also discusses how variances and confidence intervals can be obtained by means of a semi-parametric bootstrap. A Bayesian approach is mentioned as well and all approaches result in similar estimates of the hidden size of the population. Finally, a simulation study is provided which shows that the unconditional likelihood approach as well as the Bayesian approach using Jeffreys’ prior perform favorable.

Statistical studies of flare stars and other flashing objects carried out at the Byurakan Observatory

The review briefly presents the statistical studies of flare stars and related objects carried out at the Byurakan Astrophysical Observatory, in particular: i) determination of the evolutionary status of flare stars, ii) an explanation of the observed difference between flare stars of the galactic field (type UV Cet) and flare stars of systems (open clusters, associations), iii) the connection between flare stars and T Tauri stars, iv) short-term and long-term (evolutionary) variability of flare activity, v) original statistical methods developed for the study of flare stars and their systems, vi) recent advances in research on flashing objects. This review does not present such important areas of research on flare stars as the determination of physical parameters, photometry and colorimetry of stars and their flares, detailed studies of individual stars, theoretical works on possible mechanisms of flares. These areas were well presented in many books and reviews by Ambartsumian and Mirzoyan.

Flare stars in nearby Galactic open clusters based on TESS data

The study is devoted to search for flare stars among confirmed members of Galactic open clusters using high-cadence photometry from {\it 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 a sample of cool stars ($T_{\rm eff}<5000$~K), and 29\% -- 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.

Photometric and spectroscopic study of flares on Ross 15

We conducted photometric and spectroscopic observations of Ross 15 in order to further study the flare properties of this less observed flare star. A total of 28 B-band flares are detected in 128 hr of photometric observations, leading to a total flare rate of hr−1, more accurate than that provided by previous work. We give the energy range of the B-band flare (1029.5 – 1031.5 erg) and the flare frequency distribution (FFD) for the star. Within the same energy range, the FFD is lower than that of GJ 1243 (M4) and YZ CMi (M4.5), roughly in the middle of those of three M5-type stars and higher than the average FFDs of spectral types ≥ M6. We performed, for the first time for Ross 15, simultaneous high-cadence spectroscopic and photometric observations, resulting in detection of the most energetic flare in our sample. The intensity enhancements of the continuum and Balmer lines with significant correlations between them are detected during the flare, which is the same as those of other deeply studied flare stars with similar spectral type.

Analysis and modeling of the dynamics of the glow of calcium and hydrogen lines in solar and stellar flares

Abstract We present intensity curves of solar flares obtained in the Hα hydrogen line and CaII H, CaIR 8542Å lines using multichannel spectrographs of Ondřejov Observatory (Czech Republic) for the period 2000–2012. The general behavior of observed intensity curves is practically the same for all flares and is consistent with temporal variations of X-ray emission. However, our results differ significantly from those obtained by other authors for selected flare stars, for example, AD Leo; EV Lac; YZ CMi. We tried to explain the difference in the behavior of Ca II and Hα radiation flux by appearance of a shock wave during a flare and slow heating of the plasma.

Flare Activity and Magnetic Feature Analysis of the Flare Stars. II. Subgiant Branch

Abstract We present an investigation of the magnetic activity and flare characteristics of the subgiant stars mostly from F and G spectral types and compare the results with the main-sequence (MS) stars. The light curve of 352 stars on the subgiant branch (SGB) from the Kepler mission is analyzed in order to infer stability, relative coverage, and contrast of the magnetic structures and also flare properties using three flare indexes. The results show the following. (i) Relative coverage and contrast of the magnetic features along with rate, power, and magnitude of flares increase on the SGB due to the deepening of the convective zone and more vigorous magnetic field production. (ii) Magnetic activity of the F- and G-type stars on the SGB does not show dependency on the rotation rate and does not obey the saturation regime. This is the opposite of what we saw for the MS, in which the G-, K-, and M-type stars show clear dependency on the Rossby number. (iii) The positive relationship between the magnetic features’ stability and their relative coverage and contrast remains true on the SGB; though, it has a lower dependency coefficient in comparison with the MS. (iv) Magnetic proxies and flare indexes of the SGB stars increase with increasing relative mass of the convective zone.

Flare Properties of A-type Stars in Kepler Data

Abstract We analyzed the long-cadence (LC) light curves of 5435 stars with effective temperatures of 7000–10,500 K in Data Release 25 of Kepler to extend the samples of A-type flare stars and study their flare properties. A total of 103 flare stars are detected, of which 72 have the Large Sky Area Multi-Object fiber Spectroscopic Telescope spectral types, resulting in 51 A-type stars, including 17 new and 34 known. A total of 352 flares are detected from the 51 stars. The flare durations, amplitudes, and energies in the Kepler band are calculated, with medians of 2.5 hr, 2.1 mmag, and 1035.8 erg, respectively. We give the flare frequency distribution (FFD) of the A-type stars. Within the same energy range, the flare rates of our FFD are lower than those in previous research, which was also based on Kepler LC data, and the FFD slope is consistent with that in the research. The activity–rotation relation of the A-type flare stars is shown for the first time, in which the flare activity decreases as the rotation period increases. The relation is similar to that of the late-type stars in previous studies. For the two relatively bright A-type flare stars, KIC 5360548 and KIC 9468475, three high-resolution spectra are observed on different nights for each of them, and radial velocities are calculated and compared with that of the previous research. The comparisons suggest that there is no evidence of the KIC 5360548 companion, and KIC 9468475 could have a low-mass companion.

Coronal Loops and Optical Radiation from Flaring Stars

The possibility that large-scale coronal loops of solar-type stars and M dwarfs make a significant contribution to the white-light continuum emission is considered. It is shown that that the existence of high-density (>1012 cm–3) coronal loops contradicts the standard model of solar and stellar flares. As a result, the optical flare emission cannot be determined by free–free or free–bound processes within loops. The contribution from the lower atmosphere, which is heated beneath the loops due to the irradiation of soft X-ray emission, should be also negligible. The possible alternative scenarios are discussed.

Statistical Signatures of Nanoflare Activity. II. A Nanoflare Explanation for Periodic Brightenings in Flare Stars Observed by NGTS

Abstract Several studies have documented periodic and quasi-periodic signals from the time series of dMe flare stars and other stellar sources. Such periodic signals, observed within quiescent phases (i.e., devoid of larger-scale microflare or flare activity), range in a period from 1 to 1000 s and hence have been tentatively linked to ubiquitous p-mode oscillations generated in the convective layers of the star. As such, most interpretations for the observed periodicities have been framed in terms of magnetohydrodynamic wave behavior. However, we propose that a series of continuous nanoflares, based upon a power-law distribution, can provide a similar periodic signal in the associated time series. Adapting previous statistical analyses of solar nanoflare signals, we find the first statistical evidence for stellar nanoflare signals embedded within the noise envelope of M-type stellar lightcurves. Employing data collected by the Next Generation Transit Survey (NGTS), we find evidence for stellar nanoflare activity demonstrating a flaring power-law index of 3.25 ± 0.20, alongside a decay timescale of 200 ± 100 s. We also find that synthetic time series, consistent with the observations of dMe flare star lightcurves, are capable of producing quasi-periodic signals in the same frequency range as p-mode signals, despite being purely composed of impulsive signatures. Phenomena traditionally considered a consequence of wave behavior may be described by a number of high-frequency but discrete nanoflare energy events. This new physical interpretation presents a novel diagnostic capability, by linking observed periodic signals to given nanoflare model conditions.

Time-resolved spectroscopy and photometry of M dwarf flare star YZ Canis Minoris with OISTER and TESS: Blue asymmetry in the Hα line during the non-white light flare

Abstract In this paper, we present the results from spectroscopic and photometric observations of the M-type flare star YZ CMi in the framework of the Optical and Infrared Synergetic Telescopes for Education and Research (OISTER) collaborations during the Transiting Exoplanet Survey Satellite (TESS) observation period. We detected 145 white-light flares from the TESS light-curve and four Hα flares from the OISTER observations performed between 2019 January 16 and 18. Among them, three Hα flares were associated with white-light flares. However, one of them did not show clear brightening in the continuum; during this flare, the Hα line exhibited blue asymmetry which lasted for ∼60 min. The line-of-sight velocity of the blueshifted component is in the range from −80 to −100 km s−1. This suggests that there can be upward flows of chromospheric cool plasma even without detectable red/near-infrared (NIR) continuum brightening. By assuming that the blue asymmetry in the Hα line was caused by a prominence eruption on YZ CMi, we estimated the mass and kinetic energy of the upward-moving material to be 1016–1018 g and 1029.5–1031.5 erg, respectively. The estimated mass is comparable to expectations from the empirical relation between the flare X-ray energy and mass of upward-moving material for stellar flares and solar coronal mass ejections (CMEs). In contrast, the estimated kinetic energy for the non-white-light flare on YZ CMi is roughly two orders of magnitude smaller than that expected from the relation between flare X-ray energy and kinetic energy for solar CMEs. This could be understood by the difference in the velocity between CMEs and prominence eruptions.

On the mechanism of polarized metre-wave stellar emission

ABSTRACT Two coherent radio emission mechanisms operate in stellar coronae: plasma emission and cyclotron emission. They directly probe the electron density and magnetic field strength respectively. Most stellar radio detections have been made at cm-wavelengths where it is often not possible to uniquely identify the emission mechanism, hindering the utility of radio observations in probing coronal conditions. In anticipation of stellar observations from a suite of sensitive low-frequency ($\nu \sim 10^2\, {\rm MHz}$) radio telescopes, here I apply the general theory of coherent emission in non-relativistic plasma to the low-frequency case. I consider the recently reported low-frequency emission from dMe flare stars AD Leo and UV Ceti and the quiescent star GJ 1151 as test cases. My main conclusion is that unlike the cm-wave regime, for reasonable turbulence saturation regimes, the emission mechanism in metre-wave observations ($\nu \sim 10^2\, {\rm MHz}$) can often be identified based on the observed brightness temperature, emission duration, and polarization fraction. I arrive at the following heuristic: M-dwarf emission that is ≳ hour-long with ${\gtrsim}50{{\ \rm per\ cent}}$ circular polarized fraction at brightness temperatures of ${\gtrsim}10^{12}\,$K at ${\sim}100\, {\rm MHz}$ in M-dwarfs strongly favours a cyclotron maser interpretation.

A Stacking Search for Gamma-Ray Emission from Nearby Flare Stars and the Periodic Source TVLM 513-46546

Abstract So far, the Sun is the only isolated main-sequence star detected in gamma-rays, particularly during powerful flares. Young ultracool dwarfs are far more active so they are also plausible gamma-ray sources. We performed a spatial stack of 97 of the nearest X-ray and radio flare stars to search for GeV emission using nearly 12 yr of data from the Fermi Gamma-ray Space Telescope. The stacked residual maps showed no significant signal. Modeling the upper limits indicates a peak stellar flux at least a factor of 7 below the noise level. We also analyze the phase-folded light curve of the rapidly rotating radio star TVLM 513-46546, report a tentative (TS = 30) pulsed signal, and refine its period. We examine the possibility of a false-positive signal by analyzing nearby Fermi catalog sources and test fields, and by repeating the analysis using different periods. No other periodic signals are found, despite clear detections of the catalog sources, and the TS value for TVLM 513 increases systematically to the optimal period. The putative gamma-ray signal is nearly in phase with the optical peak, and out of phase with the radio pulses by 0.4 ± 0.05 rotations. These results argue for emission from relativistic protons streaming down flux tubes toward the photospheric active regions. The protons colliding with the atmosphere create neutral pions that decay into gamma-ray photons. This would be the first detection of a normal, isolated star in gamma-rays, and the strongest evidence yet for proton acceleration in stellar magnetospheres.

TESS observations of southern ultrafast rotating low-mass stars

ABSTRACT In our previous study of low-mass stars using TESS, we found a handful that show a periodic modulation on a period &amp;lt;1 d but also displayed no flaring activity. Here we present the results of a systematic search for ultrafast rotators (UFRs) in the Southern ecliptic hemisphere, which were observed in 2-min cadence with TESS. Using data from Gaia DR2, we obtain a sample of over 13 000 stars close to the lower main sequence. Of these, we identify 609 stars that lie on the lower main sequence and have a periodic modulation &amp;lt;1 d. The fraction of stars that show flares appears to drop significantly at periods &amp;lt;0.2 d. If the periods are a signature of the rotation rate, this would be a surprise, since faster rotators would be expected to have a stronger magnetic field and, therefore, produce more flares. We explore possible reasons for our finding: The flare inactive stars are members of binaries, in which case the stars rotation rate could have increased as the binary orbital separation reduced due to angular momentum loss over time, or that enhanced emission occurs at blue wavelengths beyond the pass band of TESS. Follow-up spectroscopy and flare monitoring at blue/ultraviolet wavelengths of these flare inactive stars are required to resolve this question.

Optical and X-ray observations of stellar flares on an active M dwarf AD Leonis with the Seimei Telescope, SCAT, NICER, and OISTER

Abstract We report on multi-wavelength monitoring observations of an M-dwarf flare star AD Leonis with the Seimei Telescope (6150–7930 Å), SCAT (Spectroscopic Chuo-university Astronomical Telescope; 3700–7500 Å), and NICER (Neutron Star Interior Composition Explorer; 0.2–12.0 keV), with the collaboration of the OISTER (Optical and Infrared Synergetic Telescopes for Education and Research) program. Twelve flares are detected in total, including ten Hα, four X-ray, and four optical-continuum flares; one of them is a superflare with a total energy of ∼2.0 × 1033 erg. We found that: (1) during the superflare, the Hα emission line full width at 1/8 maximum dramatically increases to 14 Å from 8 Å in the low-resolution spectra (R ∼ 2000) accompanied by large white-light flares, (2) some weak Hα/X-ray flares are not accompanied by white-light emissions, and (3) the non-flaring emissions show clear rotational modulations in X-ray and Hα intensity in the same phase. To understand these observational features, one-dimensional hydrodynamic flare simulations are performed using the RADYN code. We find the simulated Hα line profiles with hard and high-energy non-thermal electron beams to be consistent with the initial phase line profiles of the superflares, while those with a softer and/or weak-energy beam are consistent with those in decay phases, indicating the changes in the energy fluxes injected to the lower atmosphere. Also, we find that the relation between the optical continuum and Hα intensity is nonlinear, which can be one cause of the non-white-light flares. The flare energy budget exhibits diversity in the observations and models, and more observations of stellar flares are necessary for constraining the occurrence of various emission line phenomena in stellar flares.

NGTS clusters survey – II. White-light flares from the youngest stars in Orion

ABSTRACT We present the detection of high-energy white-light flares from pre-main-sequence stars associated with the Orion Complex, observed as part of the Next Generation Transit Survey (NGTS). With energies up to 5.2 × 1035 erg these flares are some of the most energetic white-light flare events seen to date. We have used the NGTS observations of flaring and non-flaring stars to measure the average flare occurrence rate for 4 Myr M0–M3 stars. We have also combined our results with those from previous studies to predict average rates for flares above 1 × 1035 erg for early M stars in nearby young associations.

The flare cumulative frequencies of UV Ceti stars from different spectral types

ABSTRACT In this study, as a result of the photometric observations continued on nine UV Ceti type stars, examining the parameters obtained from the models of distributions of the flare equivalent durations in the logarithmic scale versus flare total durations, it is found that the highest flare equivalent durations occurring on the stars (Plateau values) are varying with the B − V colour indexes of those stars in agreement with the variations of both the magnetic field strength (B) of the magnetic loop and especially the electron density of its plasma (ne). The distributions of flare cumulative frequencies were derived by using the flare equivalent durations, instead of the flare energy. The main result of this study is a secondary variation, which can be just modelled with the damping sine waves, behind the main exponential variation of the flare cumulative frequency. The models indicate that this secondary variation in the flare cumulative frequency distributions is in the same shape with the periodic or quasi-periodic oscillations detected in the light curves of the stellar and solar flares. This caused to ask new questions about the flare mechanism working on stellar atmospheres.

EvryFlare. II. Rotation Periods of the Cool Flare Stars in TESS across Half the Southern Sky

Abstract We measure rotation periods and sinusoidal amplitudes in Evryscope light curves for 122 two-minute K5–M4 TESS targets selected for strong flaring. The Evryscope array of telescopes has observed all bright nearby stars in the south, producing 2-minute cadence light curves since 2016. Long-term, high-cadence observations of rotating flare stars probe the complex relationship between stellar rotation, starspots, and superflares. We detect periods from 0.3487 to 104 days and observe amplitudes from 0.008 to 0.216 g′ mag. We find that the Evryscope amplitudes are larger than those in TESS with the effect correlated to stellar mass (p-value = 0.01). We compute the Rossby number (R o ) and find that our sample selected for flaring has twice as many intermediate rotators (0.04 &lt; R o &lt; 0.4) as fast (R o &lt; 0.04) or slow (R o &gt; 0.44) rotators; this may be astrophysical or a result of period detection sensitivity. We discover 30 fast, 59 intermediate, and 33 slow rotators. We measure a median starspot coverage of 13% of the stellar hemisphere and constrain the minimum magnetic field strength consistent with our flare energies and spot coverage to be 500 G, with later-type stars exhibiting lower values than earlier-type stars. We observe a possible change in superflare rates at intermediate periods. However, we do not conclusively confirm the increased activity of intermediate rotators seen in previous studies. We split all rotators at R o ∼ 0.2 into bins of P Rot &lt; 10 days and P Rot &gt; 10 days to confirm that short-period rotators exhibit higher superflare rates, larger flare energies, and higher starspot coverage than do long-period rotators, at p-values of 3.2 × 10−5, 1.0 × 10−5, and 0.01, respectively.

Observing flare stars below 100 MHz with the LWA

ABSTRACT We observed the flare stars AD Leonis, Wolf 424, EQ Pegasi, EV Lacertae, and UV Ceti for nearly 135 h. These stars were observed between 63 and 83 MHz using the interferometry mode of the Long Wavelength Array. Given that emission from flare stars is typically circularly polarized, we used the condition that any significant detection present in Stokes I must also be present in Stokes V at the same time in order for us to consider it a possible flare. Following this, we made one marginal flare detection for the star EQ Pegasi. This flare had a flux density of 5.91 Jy in Stokes I and 5.13 Jy in Stokes V, corresponding to a brightness temperature 1.75 × 1016(r/r*)−2 K.

Chao’s lower bound estimator and the size of the Pleiades

In this note we would like to point out that the lower bound estimator of the frequency of hidden units in a target population, developed by Chao in ecology, was developed independently in astro-physics and has been used to estimate the size of flare stars in the Pleiades.

Spectral obervations of flare stars TZ ARI and ROSS 867

The results of spectral observations of flare stars TZ Ari and Ross 867B are presented. Observations have been carried out by the 2.6m telescope of Byurakan Astrophysical Observatory in 1999, 2001 and 2018. The spectral cameras "ByuFOSC" and "SCORPIO" were used during the observations. A strong change of emission lines detected on TZ Ari in 2018, while at the same time the brightness changes of the star in all measured spectral ranges are within the framework of measurement errors. Slight variations of brightness and EWHα are detected in quiescent state of star during the observations in 1999. A strong flare of Ross 867B were observed, with a duration of more than 20min. In this case if EWHα shows strong change, when its maximum value corresponds to the photometric maximum of are, at the same time EWHα shows decrease, similar to that, observed earlier during the flares of stars EV Lac, HU Del, CM Dra and WX UMa.