Characteristics of activity energetics of the UV cet-type flare stars

A quantitative comparison of energetics of different manifestations of the activity of the UV Cet-type flare stars — sporadic flares, quiet chromospheres and coronae and stellar spots — have been carried out. On the basis of a statistical analysis of about 1800 flares registered for 23 flare stars, the energy spectra of flares have been constructed and time-averaged powers of optical radiation of flares —L _flares — have been estimated. By use of spectroscopic observations of 26 flare stars between flares, the total radiation in Balmer lines of quiet chromospheres —L _Bal — have been found. On the basis of these data and the soft X-ray observations of 29 flare stars, the mean of the ratiosL _flares/L _bol,L _Bal/L _bol andL _X /L _bol have been found to be equal to (1–2)×10^?4. By use of photometric observations for 9 stars, the total starspot radiation deficits —L _spots — have been estimated as (4×10^?3?6×10^?2)×L _bol. Discussions of probable underestimates in the valuesL _flares,L _Bal,L _X , andL _spots leads to the conclusion that the total power of non-equilibrium processes that take place in flare star atmospheres is close to the total starspot radiation deficit in these stars.     

The EVE Doppler Sensitivity and Flare Observations

The Extreme-ultraviolet Variability Experiment (EVE; see Woods et al., 2009) obtains continuous EUV spectra of the Sun viewed as a star. Its primary objective is the characterization of solar spectral irradiance, but its sensitivity and stability make it extremely interesting for observations of variability on time scales down to the limit imposed by its basic 10 s sample interval. In this paper we characterize the Doppler sensitivity of the EVE data. We find that the 30.4 nm line of He ii has a random Doppler error below 0.001 nm (1 pm, better than 10 km s⁻¹ as a redshift), with ample stability to detect the orbital motion of its satellite, the Solar Dynamics Observatory (SDO). Solar flares also displace the spectrum, both because of Doppler shifts and because of EVE’s optical layout, which (as with a slitless spectrograph) confuses position and wavelength. As a flare develops, the centroid of the line displays variations that reflect Doppler shifts and therefore flare dynamics. For the impulsive phase of the flare SOL2010-06-12, we find the line centroid to have a redshift of 16.8 ± 5.9 km s⁻¹ relative to that of the flare gradual phase (statistical errors only). We find also that high-temperature lines, such as Fe xxiv 19.2 nm, have well-determined Doppler components for major flares, with decreasing apparent blueshifts as expected from chromospheric evaporation flows.     

Polarimetry of stellar active regions and flares

Observations of regular and irregular polarimetric variability in late-type stars are reviewed, and the related physical and geometrical effects are discussed. There are indications that the irregular part of the variability could be caused by transient events, possibly associated with flares. Polarimetric observations during flares are reviewed, and preliminary results of new observations of a well-known flare star, YY Geminorum, are presented. The results show that the small flare in YY Gem did not cause any significant variations in linear polarization, while the binary eclipse evidently causes an enhancement in the polarization. The reasons for the difficulties in stellar flare polarimetry are discussed. Finally, future prospects for the observations of flaring stars and for the utilization of linear polarimetry as a complementary method to other techniques of surface imaging of stellar activity and flares are presented.     

X-ray Flares Observed from Six Young Stars Located in the Region of Star Clusters NGC 869 and IC 2602

We present, for the first time, an analysis of seven intense X-ray flares observed from six stars (LAV 796, LAV 1174, SHM2002 3734, 2MASS 02191082+5707324, V553 Car, V557 Car). These stars are located in the region of young open star clusters NGC 869 and IC 2602. These flares detected in the XMM-Newton data show a rapid rise (10–40 min) and a slow decay (20–90 min). The X-ray luminosities during the flares in the energy band 0.3–7.5 keV are in the range of 10^29.9 to 10^31.7 erg s^?1. The strongest flare was observed with the ratio ～13 for count rates at peak of the flare to the quiescent intensity. The maximum temperature during the flares has been found to be ～100 MK. The semi-loop lengths for the flaring loops are estimated to be of the order of 10¹⁰ cm. The physical parameters of the flaring structure, the peak density, pressure and minimum magnetic field required to confine the plasma have been derived and found to be consistent with flares from pre-main sequence stars in the Orion and the Taurus-Auriga-Perseus region.     

A review of stellar flares and their characteristics

We review the flaring activity of stars across the HR-diagram. Brightenings have been reported along the entire Main Sequence and in many stars off the Main Sequence. Some stars are decidedly young, others are in advanced stages of stellar evolution. Flares are common on stars with outer convection zones and outbursts have been reported also on other types of stars, although confirmations are needed for some of them.Analyses of flare occurrence sometimes find flares to be randomly distributed in time, and sometimes indicate a tendency for flares to come in groups. Preferred active longitudes have been suggested. Recent solar results, where the occurrence rate for flares is found to exhibit a periodicity of 152 days, suggest that stellar flare data should be reanalyzed over long time baselines to see if the present confusing situation can be resolved.The radiation from stellar flares is dominated by continuum emission and about equal amounts of energy have been recorded in the optical, UV, and X-ray regions of the spectrum. In solar flares strong continuum emission is rarely recorded and a large collection of bright emission lines takes prominence. Small flares occur more frequently than large ones and the latter have longer time-scales. Flare energies can exceed 10³⁷ erg. The most productive flare stars are those where the convective envelopes occupy large volumes. Slow stellar rotation rates are believed to reduce the level when the star has been braked significantly from its young rotation rate.     

The statistical analyses of flares detected in B band photometry of UV Ceti type stars

In this study, we present the unpublished flare data collected from 222 flares detected in the B band observations of five stars and the results derived by statistical analysis and modeling of these data. Six basic properties have been found with a statistical analysis method applied to all models and analyses for the flares detected in the B band observation of UV Ceti type stars. We have also compared the U and B bands with the analysis results. This comparison allowed us to evaluate the methods used in the analyses. The analyses provided the following results. (1) The flares were separated into two types, fast and slow flares. (2) The mean values of the equivalent durations of the slow and the fast flares differ by a factor of 16.2 ± 3.7. (3) Regardless of the total flare duration, the maximum flare energy can reach a different Plateau level for each star. (4) The Plateau values of EV Lac and EQ Peg are higher than the others. (5) The minimum values of the total flare duration increase toward the later spectral types. This value is called the Half-Life value in models. (6) Both the maximum flare rise times and the total flare duration obtained from the observed flares decrease toward the later spectral types.     

Flares on opposite sides of the star: Observational aspects

The problem of the flare taking place on opposite sides of a star is considered. Such a screened flare, diffused through the star's atmosphere (chromosphere), may also be registered. The theoretical light curve for diffused flare event is derived, which differs strongly from the usual flare light curves. The light curve of diffused flare is characterized first of all by its very slow rise of brightness. This result opens quite a new direction to understand the nature of the so-called slow flares, observed often among the UV Cet-type stars as well as flare stars in aggregates. All slow flares can be interpreted as quite ordinary flares of quite ordinary flare stars — taking place, however, on the opposite sides of the star. The results of interpretation of some slow flare events of YY Gem and three flare stars in Orion are presented. An attempt is made for the determination of actual amplitudes of screened flares taking place on the opposite sides of a star.     

Parameters of superflares on G-Type stars observed with the Kepler space telescope

We continued the analysis of 279 G-type stars with superflares (energies in the range of 10³³–10³⁶ erg). We calculated the SFL parameter (part of the stellar surface which emits in the flare). The SFL estimates were derived from the relation connecting this value with the amplitude of the flare and its radiation on the assumption of the blackbody character of the emission at times close to its maximum. Most SFL values are in the range of 0–0.1, with values of 0.2–0.4 for some strong flares. Dependence of SFL on effective temperature for stars with superflares is similar to that found earlier for the spottedness parameter S. The SFL distribution reaches its maximum in the temperature range of about 5100–5250 K and decreases with the effective temperature increase. We suggested an assumption on the presence of bimodal distribution in the “SFL–rotation period” relation with a gap for objects with rotation periods P of about 10 days. For stars with P less than 10 days, the given data can indicate a decrease in flare areas with the P increase. Our analysis showed that significant changes both in flare energy and in flare areas can be achieved with small changes in spottedness S for one and the same star.     

Flares of red dwarf stars and the galactic X-ray background

The mean density of the UV Cet-type flare stars in the solar neighbourhood is estimated. If differences of activity levels on different flare stars are taken into account, their summary flare activity is equivalent to 0.03 YZ CMi's flare activity per cubic parsec or to 4×10²⁶ erg s^–1 pc^–3 in U-passband. From the X-ray flare observation on YZ CMi of 19.10.74 we estimate the luminosity of stellar flares in soft and intermediate X-ray. The ratio of X-ray to optical radiation for stellar flares is close to the respective ratio for strong solar chromospheric flares. It is shown the set of red-dwarf flare stars has all essential features of an ensemble of discrete X-ray sources to represent the galactic diffuse X-ray background.     

On the infrared observations of stellar flares

In connection with the appearance of the first results of infrared observations of stellar flares, a more elaborate analysis ofnegative infrared flares as a phenomenon, predicted by the fastelectron hypothesis, has been carried out. As a result, the wavelength regions of negative flares are established for the stars of different spectral types as well as the calculated amplitudes of the negative flares (Tables I and II). The analysis of the infrared observations (c.f. Kilyachkoet al., 1978) lead to the following conclusions:

1. The negative infrared flares discovered around 8000 Å is not in agreement with the theory in the case of the flare star UV Cet. Some traces of negative flares have been noted for a number of less powerful flares of EV Lac.
2. The amplitudes of the recorded positive flares of UV Cet and EV Lac on λ8000 Å are in good agreement with the magnitudes predicted by the fast-electron hypothesis (non-thermal bremsstrahlung).
3. In the future the negative flares around 8000 Å should be looked for in early-type flare stars of types M0-K5.
4. For a positive discovery of negative flares, future observations must be carried out in the wavelength region of 1–3 μm.

     

Coordinated Observations of the Red Dwarf Flare Star EV Lac in 1993

Results of cooperative observations of the flare star EV Lac in September 1993 are presented. One of the about 30 optical flares detected was powerful enough to permit a quantitative analysis of its intrinsic radiation with the colour-colour technique. Sinusoidal brightness variations due to spottedness of the stellar surface was found to have an amplitude V = 0.^m0.24. Behaviour of the K band stellar brightness during strong and weak U band flares are considered. The upper limits of very fast optical brightness variations were estimated during both a moderate flare and quiet state of the star. No decametric bursts were observed during the campaign that could be certainly attributed to flare activity.     

Spatial distribution of solar flares in 23rd solar cycle

H-alpha flares accompanied by the X-radiation f ?? 10^?6 wm^?2 in power are examined; 2331 flares were registered during the first half of the 23rd solar cycle (1997?C2000). The specific power of the X-radiation of the flares monotonically doubles from the minimum to the maximum of the sunspot. An increase in the number of flares in each solar rotation is nonmonotonic and disproportional to the relative number of sunspots. Several longitudinal intervals with increased flare activity can be distinguished in the entire time interval of five to ten rotations. The longitudinal distributions of flares and boundaries of the sector structures of a large-scale magnetic field differ considerably. This confirms the existence of two types of zero lines; the first type is determined by active regions, and the second one is determined by large-scale structures with weak magnetic fields. The flares concentrate near Hale??s zero lines of the first type.     

Stellar flare statistics — Physical consequences

The observational data permit us to establish clear statistical correlations between different parameters of stellar flare activity and the characteristics of quiet stars. These relations are:

1. between energies and frequencies of flares on stars of different luminosities;
2. between total radiation energies of flares and quiet stars both in X-ray and Balmer emission lines;
3. between flare decay rates just after the maxima and flare luminosities at maxima.

     

Stars with Discrepant <Emphasis Type="Italic">v</Emphasis> sin <Emphasis Type="Italic">i</Emphasis> as Derived from the Ca II 3933 and Mg II 4481 Å Lines. VII. HD9531 (SB), HD31592 (SB2), HD129174 (SB?)

In this paper of the series we analyze three stars listed among stars with discrepant v sin i: HD9531 and HD31592, which also show radial velocity variations inherent to spectroscopic binaries, and HD129174 which is an Mn-type star with a possible magnetic field. In HD9531 we confirm the radial velocity derived fromthe hydrogen lines as well as fromthe Ca II line at 3933 Å as variable. The profile of the calcium line also appears variable, and with the estimated magnetic induction B_e = ?630 ± 1340 G, this suggests that the abundance of calcium possibly varies over the surface of the star. We identified the lines of the secondary component in the spectrum of HD31592 revealing thus it is an SB2 binary with B9.5V and A0V components. While the primary star rotates with v sin i = 50 km s^?1, the secondary star is faster with v sin i = 170 km s^?1. We find that only 60% of the Mn lines identified in the spectrum of HD129174 can be fitted with a unique abundance value, whereas the remaining lines are stronger or fainter. We also identified two Xe II lines at 5339.33 Å and 5419.15 Å and estimated their log g f.     

SID flares and sunspot morphology

The degree of association between geoeffective (SID producing) flares (hereafter called SID flares) and sunspot morphology is examined. It is found that: (1) the frequency of SID flares associated with sunspot groups is linear function of sunspot area and rate of change in area; (2) the SID flare intensity is dependent on the sunspot area and on the magnetic morphology (field geometry); (3) the probability of a sunspot group being magnetically complex (henceforth called complex ratio) is a linear function of spot area, the larger this area the more likely a group is in the βγ or δ magnetic class; (4) the complex ratio exhibits the greatest degree of association to SID flare frequency. We conclude from these results that a higher frequency of D-region ionizing flares (emitting a soft X-ray flux >2 × 10^?3 erg cm^?2 s^?1) is likely to accompany the disk transit of large area, complex spot groups. This combination of morphological factors reflects a shearing of the associated force-free magnetic field, with accumulation of free magnetic energy to power SID flares. Mutual polarity intrusion would be one observational signature of the pre-flare energy storing process.     

Investigation of flare stars in the pleiades by the method of stellar tracks. I

Observations of the Pleiades cluster by the method of stellar tracks, carried out on the 40-inch Schmidt telescope of Byurakan Astrophysical Observatory, have resulted in the detection of 49 flares from 38 stars, 17 of which were not previously known to be flare stars. It is shown that for bright stars (U ≤ 16.0) the detection of flares in observations by the method of stellar tracks is at least three times more efficient than for observations by the method of stellar chains. Another advantage of the first method is that one can detect brief flares that last less than 6 min. The visual stellar magnitude at the minimum for the brightest of the flare stars that we found is 11.92. This raised the upper luminosity limit of known flare stars in the Pleiades by 0.21 magnitude. A comparison of the expected number of bright flare stars in the Pleiades with the number of all bright members of the cluster (falling in the range from V ≈ 12.0 toV ≈16.0) suggested that all these stars evidently must be flare stars. Translated from Astrofizika, Vol. 42, No. 3, pp. 351–358, July–September, 1999.     

Multi-Wavelength Observations of Quasi-Periodic Pulsations in a Solar Flare

We have performed a search for flares and quasi-periodic pulsations (QPPs) from low-mass M-dwarf stars using Transient Exoplanet Survey Satellite (TESS) two-minute cadence data. We find seven stars that show evidence of QPPs. Using Fourier and empirical mode decomposition techniques, we confirm the presence of 11 QPPs in these seven stars with a period between 10.2 and 71.9 minutes, including an oscillation with strong drift in the period and a double-mode oscillation. The fraction of flares that showed QPPs (7%) is higher than other studies of stellar flares, but it is very similar to the fraction of solar C-class flares. Based on the stellar parameters taken from the TESS Input Catalog, we determine the lengths and magnetic-field strengths of the flare coronal loops using the period of the QPPs and various assumptions about the origin of the QPPs. We also use a scaling relationship based on flares from the Sun and solar-type stars and the observed energy, plus the duration of the flares, finding that the different approaches predict loop lengths that are consistent to within a factor of about two. We also discuss the flare frequency of the seven stars determining whether this could result in ozone depletion or abiogenesis in any orbiting exoplanet. Three of our stars have a sufficiently high rate of energetic flares, which are likely to cause abiogenesis. However, two of these stars are also in the range where ozone depletion is likely to occur. We speculate on the implications of the flare rates, loop lengths, and QPPs for life on potential exoplanets orbiting in their host star’s habitable zone.

     

Densities and mass motions in transition-zone plasmas in solar flares observed from Skylab

We studied the EUV line spectra of three flare observed with the NRL slit spectrograph on Skylab. The electron densities in the flare transition-zone plasmas are determined from density-sensitive lines of Si iii and O iv. The electron densities in all three flares studied were greatest during the flare maximum with values of the order of 10¹² cm^–3. The density decreases by a factor of 2 to 3 in the decay phase of the flares. The intensities of EUV lines from the flare chromospheric and transition-zone plasmas all are greatly enhanced. In contrast to lines for Oi, Ci, Feii and other chromospheric ions, the lines of Oiv and Nv and other transition-zone lines are not only enhanced but also very much broadened.Fitting of the N v 1242 Å line with a two-gaussian model shows that for two of the flares studied, there is a red-shifted component in addition to an unshifted component. The shifted component in the N v line profiles is interpreted as due to a dynamic and moving plasma with a bulk motion velocity of 12 km s^–1 for one flare and more than 70 km s^–1 for the other. The broadened line profiles indicate that there are large turbulent mass motions with random velocities ranging from 30 to 80 km s^–1.Ball Corporation. Now with NASA/Marshall Space Flight Center.     

High Temporal Resolution Spectroscopic Observations of the Flare Star V1054 Oph

We present the results of a high temporal resolution spectroscopic monitoring of the flare (UV Cet type) star V1054 Oph (Wolf 630AB), classified as a dM3.5e visual binary system. Intermediate resolution spectra have been taken during four nights (2–5 April 2001) using the IDS spectrograph of the 2.5 m Isaac Newton Telescope (INT) (La Palma, Canary Islands, Spain). The V1054 Oph spectra show very strong emission lines even in its quiescent state. The analysis of the temporal evolution of the observed emission lines (from H_β to H₁₁ and the Ca II H and K lines) reveals four strong flares and several weak flares. We have studied in detail the behaviour of the chromospheric lines during the different phases (pre-flare, impulsive and gradual decay) of these flares. The observed flares last from ～25 to 95 min. The equivalent width of the H_β line changes by a factor up to ～2.3. Broad wings and asymmetric (red-shifted) lines are observed as well.     

Long-term monitoring of the long-period variable Y Cassiopeiae in the 1.35-cm water-vapor line

Observations of the circumstellar maser emission from the long-period variable star Y Cas in the 1.35-cm water-vapor line are presented. The observations were performed with the RT-22 radio telescope at the Pushchino Radio Astronomy Observatory (Astrospace Center, Lebedev Physical Institute, Russian Academy of Sciences) in the period 1982–2005. The variations in the integrated flux F_int in the H₂O line correlate with the visual light curve of the star. The phase delay Δ? between the F_int variations and the light curve is 0.2–0.4P (P is the period of the star). The H₂O maser Y Cas belongs to transient sources: peaks of high maser activity alternate with intervals of a low emission level when the H₂O-line flux does not exceed (0.1–0.5) × 10^?20 W m^?2. A “superperiod” of ～5.7 yr was found in the occurrence of activity peaks. A particularly strong maximum of maser radio emission took place at the end of 1997, when the flux F_int reached 15.6 × 10^?20 W m^?2. A model for the H₂O maser variability in Y Cas is discussed. The variability is caused by a periodic action of shock waves driven by stellar pulsations. The H₂O maser flares may be associated with short-lived episodes of enhanced mass loss by the star or with the propagation of a particularly strong shock wave when a planet orbiting the star passes through its periastron.