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BE Cet is a young solar analog with an age of 0.6 Gyr and a rotation period of 7.655 days. According to chromospheric and photospheric indices, its activity is higher than the solar one. An analysis of photometric data on the time interval between 1977 and 2019 shows the presence of only 6.76 yr cyclic variations in the mean brightness with an amplitude of 0.02 mag. The obtained cycle is 1-2 yr shorter in comparison with the chromospheric cycle determined earlier, whose length was estimated to be 9 or 7.6 yr. Parameters of the cycle, its amplitude and duration change slightly in different epochs. The short-term light variations due to rotational modulation occur with an increase in amplitude up to 0.05 mag near the activity cycle minimum and a decrease in its maximum. Some events of a rapid increase in brightness of 0.2-0.6 mag may be considered as flares., 6 pages, 3 figures, 1 table

Features of the development of activity cycles in the solar-type stars and fast-rotating cool dwarfs have been considered for 65 stars observed in some decades. Cycles with duration of 7-18 years compared to the solar cycle were found for about 50% of the studied stars. In cooler dwarfs with rotation periods of less than 5 days, cyclic changes in brightness occur on longer scales, up to 80 years. Activity of the highest level is produced on K dwarfs; their main cycles are long and have the highest amplitudes. Both old and young solar-type stars show a similar tendency in increasing the cycle length with a slower rotation. No evidence for a relation between the rotation period and duration of cycles was found for cool dwarfs with $P_{rot} < 5$ days., Accepted by Geomagnetism and Aeronomy, 2019, V.59, No 7. 7 pages, 4 figures, 1 table

The behavior of the average annual luminosity of K-M dwarfs OU Gem, EQ Vir, V1005 Ori and AU Mic was studied at time intervals of several decades. The main sources of photometric data for 1989-2019 were the Hipparcos, ASAS, KWS databases. An analysis of long-term series showed that the average annual brightness of all stars varies cyclical. The durations of possible cycles for OU Gem, V1005 Ori and AU Mic are 40-42 years, for EQ Vir - 16.6 years, and amplitudes of cycles are of 0.09$^m$ - 0.2$^m$. The selected stars belong to the group of red dwarfs for which the average surface magnetic field exceeds of several kilogauss. We examined the type of the relationship between the parameters of the cycle, its duration and amplitude, for 9 stars with < 4 kG. A tendency to increasing of the amplitude and duration of the cycle when the value of decrease has been noted. It may be suggested that with an increasing of the surface magnetic field, it becomes more uniform and the level of activity changes in this case in less degrees than on stars with strong local fields concentrated in large spots., 7 pages, 4 figures, 2 tables. Accepted to Geomagnetism and Aeronomy, 2020, Vol.60, No.7

The cycles of activity similar to the solar one are thought to exist in other stars with outer convection zones. The long-term monitoring of magnetic cycles in stars similar in structure to the Sun is a main diagnostic method for understanding how dynamo generation and amplification of magnetic fields occur. We have performed a search for solar-like magnetic activity on the yellow subgiant β Aql. Direct measurements of the longitudinal magnetic field of β Aql were performed by measuring the Zeeman splitting in spectral lines, using the circularly polarized spectra obtained at the Crimean Astrophysical Observatory over 51 nights during 1997–2015. The magnetic field on β Aql was detected with the confidence level above 3σ over 24 nights. The activity cycle of β Aql was found to be 969 ± 27 days. We assume that the activity of β Aql is similar to that of stars younger than the Sun. The most probable rotation period was found to be Prot = 5.08697 ± 0.00031 days. Assuming the global magnetic field of β Aql as a dipole, we estimated the polar field strength Bpol = 24 G, the angle between the rotation axis and the line of sight i = 25°, and the angle between the rotation and dipole axes β = 96°.

Results are presented from a search for periodic variations in the brightness and color indices of the active star PZ Mon based on many years of photometric data from 1992 to 2015. The photometric period derived from the entire set of observations is 34.16 days, but the period may vary by 1.5% within individual intervals. The color index V-R varies with the same period. These variations are indicative of reddening of the star with decreasing brightness. A correlation between the values exists over the entire observation interval. The variations in B-V occur over an interval of 26-28 days. A nonuniqueness in these variations shows up in a brightness-color index diagram: a reduction in the color index with decreasing brightness is observed in some epochs, which can be explained in terms of a spottedness model by the presence of cold, as well as hot, formations on the star’s surface.

Photometric data covering 1994–2009, obtained at the Crimean Astrophysical Observatory or retrieved from the ASAS and SuperWASP catalogs, were used to analyze the brightness variations of the rapidly rotating star V833 Tau, whose activity level is close to saturation. Combined with previously published results, these data represent for the first time all stages in the development of the star’s 19- year activity cycle. The photometric period and rotational-modulation amplitude for different epochs are determined, and the qualitative pattern of the spots is considered. The photometric period is close to the orbital period, but always exceeds it, indicating that surface inhomogeneities are located far from the equatorial plane. With the high spottedness of the star, reaching 28% at the cycle maximum, the rotationalmodulation amplitude is 0.05−0.1 m , and increases during the growth and decline phases of the 19-year cycle. The rotational modulation is due to spots with higher latitudes than in the case of the Sun, and concentrating on active longitudes.

The bulk of available stellar activity observations is frequently checked for the manifestation of signs in comparison with the known characteristic of solar magnetic modulation. The problem is that stellar activity records are usually an order of magnitude shorter than available observations of solar activity variation. Therefore, the resolved time scales of stellar activity are insufficient to decide reliably that a cyclic variation for a particular star is similar to the well-known 11-yr sunspot cycles. As a result, recent studies report several stars with double or multiple cycles which serve to challenge the underlying theoretical understanding. This is why a consistent method to separate 'true' cycles from stochastic variations is required. In this paper, we suggest that a conservative method, based on the best practice of wavelet analysis previously applied to the study of solar activity, for studying and interpreting the longest available stellar activity record - photometric monitoring of V833 Tau for more than 100 years. We find that the observed variations of V833 Tau with timescales of 2-50 yr should be comparable with the known quasi-periodic solar mid-term variations, whereas the true cycle of V833 Tau, if it exists, should be of about a century or even longer. We argue that this conclusion does not contradict the expectations from stellar dynamo theory., Comment: accepted in MNRAS

Manifestations of activity on the YZ CMi (M4.5Ve) flare star which has a rotation period of 2.77 days and belongs to a group of activity-saturated stars, are considered from long-term photometric data together with photographic measurements from 1926 to 2009. Long-term changes in yearly mean brightness of the star are found on a time scale of 27.5 years with an amplitude of $0.2 - 0.3^m$ that indicate variations in development of surface inhomogeneities and a large degree of surface spottedness in the maximum activity. Spots are distributed unevenly over the surface, their concentration is higher at certain longitudes spaced by intervals equal to 0.6 phases of the rotation period. The recovery of active longitude (a possible "flip-flop" effect) occurs over a period of about 6 years. We note differences in activity of this star from typical for the Sun and solar-like stars, which are associated with its dynamic characteristics $P_{cyc}$ and $P_{rot}$ and its inner structure., Corrected version. 8 pages, 5 color figures

Based on the multiband (BVRIJHKL) photometric observations of the active red giant PZ Mon performed for the first time in the winter season of 2017-2018, we have determined the main characteristics of the spotted stellar surface in a parametric three-spot model. The unspotted surface temperature is Teff=4730 K, the temperature of the cool spots is Tspot=3500 K, their relative area is about 41%, and the temperature of the warm spots is Twarm=4500 K with a maximum relative area up to 20%. The distribution of spots over the stellar surface has been modeled. The warm spots have been found to be distributed at various longitudes in the hemisphere on the side of the secondary component and are most likely a result of its influence., Comment: 5 pages, 7 figures

Photoelectric B-band data and calibrated photographic estimates are used to plot a combined light curve of the red dwarf V833 Tau (dK5e) covering 1899–2009. A frequency analysis was applied to the resulting series of yearly mean B magnitudes, yielding possible photospheric activity cycles acting on various timescales. The high-amplitude variations, up to 0.6m(0.8), follow a cycle with a duration of 78.25 yrs. A 18.8-year cycle with an amplitude of 0.39m–0.16m was also detected, and changes of its characteristics for each observing epoch considered. The star becomes redder at epochs of decreased brightness, suggesting that the 18-year cycle is related to spot-formation processes. The existence of the shorter 6.4-year and 2.5-year cycles reported earlier is confirmed.

Based on our photometric observations in 2015-2016 and archival photometric data for the active red giant PZ Mon, we have found the main characteristics of the stellar surface: the unspotted surface temperature Teff=4730K, the spot temperature Tspot=3500K, and the relative spot area from 30 to 40%. The best agreement with the observations has been achieved in our three-spot model including a cool polar spot with a temperature of about 3500K as well as large and small warm spots with a temperature of about 4500K. The stable polar spot is responsible for the long-period brightness variations. Its presence is confirmed by an analysis of the TiO 7054$~\AA$ molecular band. The small-amplitude 34-day variability is attributable to the warm spots located on the side of the secondary component, which determine the relatively stable active longitude., Comment: 8 pages, 7 figures, accept in Astromonical Letters, 2018

We use original spectra and available photometric data to recover parameters of the stellar atmosphere of PZ Mon, formerly referred as an active red dwarf. The derived effective temperature Teff=4700K and gravity lgg=2.8 suggest that PZ Mon is a K2III giant. Stellar atmosphere parameters (Teff and lgg) alongside with the evolutionary tracks are used to estimate the stellar mass of ~1.5Mo and the radius of ~7.7Ro. The angular radius derived by the infrared flux method when combined with the linear radius suggests the distance of 250+/-70 pc, a factor 2.5 smaller than that suggested by the Hipparcos parallax. The red giant status of PZ Mon is confirmed by the carbon and nitrogen abundance. The spectrum reveals pronounced HeI 5876 A absorption and Halpha emission indicating the robust chromosphere. The IUE spectrum is found to contain transition layer emission line of CIV 1550 A. The CIV and X-ray luminosities turn out typical of RS CVn stars. The extended set of available photometric data confirms the period of 34.14 days presumably related to the stellar rotation. We found variations of the radial velocity with the amplitude of ~8 kms which could be caused by the orbital motion., 9 pages, 10 figures, 4 tables. Accepted in MNRAS

Two A-stars that are moving in the opposite directions with respect to a system with radial velocities +100 and −200 km/s were detected in the θ1 Ori C system (HD 37022). These stars are considered to be escaping stars. An analysis of the high-quality spectra using the cross-correlation method allowed us to determine the physical characteristics of these stars: effective temperature, gravitational acceleration, relative and absolute luminosities, mass, chemical composition, and the locations of the stars on the Hertzsprung-Russell diagram.

The star Θ1 Ori C1, which is the nearest companion of the principal star Θ1 Ori C, is studied using extensive spectral data. Two radial velocity curves are constructed and solved. One of these describes the motion of the principal star and the other, the motion of the companion C1; in both cases the stars rotate about the center of mass of the C + C1 system. The physical parameters (temperature, acceleration of gravity, mass) and chemical composition of the star C1 are determined. A periodicity of 15.4 days associated with the rotation of the principal star is identified in the variations of the radial velocity.

The photometric properties of the late-type star PZ Mon are analysed assuming the presence of spots on its surface. The long-term variability in brightness is considered using photographic data together with photoelectric data in the B band. The combined light curve covers the time span from 1898 to 2004 and shows two deep minima which might be interpreted as an activity cycle with a period of 50 years. The investigation of seasonal variations is based on UBVRI observations carried out with the 1.25 m telescope at the Crimean Astrophysical Observatory in 1992–2004. The derived colours of PZ Mon indicate that the spectral type of the star is K5 and therefore the parameters of spots were calculated adopting a value for the temperature of the photosphere of 4800 K. Applying the simple model of star spots we found that a seasonal variability can be produced by spots which cover 17–30% of the hemisphere. The spot temperatures are constant and close to 2700 K. If the deep minima on the long-term curve are also ...

We observed the symbiotic star CH Cyg with the 70 cm telescope of the Crimean Astrophysical Observatory in 1982, during its active phase, when it was brighter than 6m in the V band. We simultaneously detected the continuum brightness at wavelengths of 3737, 5092, and 5500 A with a time resolution of 20 s. We present light curves for these wavelengths for four nights between July 13 and August 22, demonstrating the character of the star’s rapid variations. Various frequency-analysis methods were applied to the data series at 3737 and 5092 A, where the light from the hot component made an appreciable contribution. For both series, we find some 20 similar frequencies corresponding to periods ranging from 150 to 6000 s. Our estimates of the significance of the identified frequencies indicate that the significance level exceeds 3 σ in all cases, and is even higher in most cases. For many of the oscillations, the amplitudes varied on time scales shorter than one day. Multiperiodicity and variable oscillation amplitudes are characteristic of nonradial pulsations. We conclude that nonradial oscillations typical of a white dwarf’s g modes were excited during the symbiotic star’s state of highest activity.

A quick digital method based on the PARSEC measuring complex which was developed in the Main Astronomical Observatory (Ukraine) is proposed to search for long-term variability in the brightness of red dwarfs. The capability of this scanning complex was tested earlier when it was used to digitize photographic plates successfully in the process of creating the astrometric catalogue. Application of this complex and special software to compute digitized images and to obtain photometrical data is discussed briefly.

Variability of the photospheric radiation of 40 (dKe-dMe) dwarfs in the solar neighborhood due to variations in the spottedness of their surfaces is analyzed based on the behavior of their mean annual brightnesses over long time intervals. The amplitudes and characteristic time scales of the variations of the mean annual brightness are taken to be indicators of photospheric activity and were used to infer the levels of photospheric activity in the stars studied. The influence of axial rotation on the development of cyclic activity in young red dwarfs and F-M main-sequence stars is analyzed. The durations and amplitudes of the photospheric variability of rapidly rotating (dK0e-dK5e) stars testifies to a higher level of photospheric activity among red dwarfs and solar-type stars. The X-ray luminosities of these stars grow with the amplitude of the variations of the mean annual brightness. However, this is not typical of rapidly rotating M dwarfs, for which the X-ray emission varies by more than two orders of magnitude, although their degrees of spottedness are all virtually the same. A linear relationship between the X-ray and bolometric luminosities is observed for young (dKe-dMe) stars, with their ratios log(L x/L bol) being about −3. These properties can be used to determine whether a red dwarf is a young star or is already on the main sequence.

The results of spectrophotometric observations of CH Cyg in an active phase and during emergence from it are given: the spectral energy distribution in the 3225-7500 A range with 23 A resolution, the absolute monochromatic illuminances at eight selected wavelengths in the 3225-8015 A, and the emission fluxes in the Hα and Hβ lines. The features of the energy distribution in the stellar spectrum and the observed forms of variability, different between the short-wavelength and red parts of the spectrum, are determined by the instability of both of the system's components. In the active phase, CH Cyg's emission in the observed spectral range can be represented as the total emission from a cool M6 III giant and a hot hydrogen gas with a temperature T e = 10,000-15,000 K. In the low-brightness state, the star's energy distribution fully corresponds to that of a cool giant, the spectrum of which varies from M6 III to M7 III. The photometric variability of the cool component in the λ 8015 A range is 0 m .3-0 m .4. An increase in the brightness of the cool giant preceded a bright outburst of the hot component.

New photoelectric UBVRI observations of the eclipsing variable V 1016 Ori have been obtained with the AZT-11 telescope at Crimean Astrophysical Observatory and with the Zeiss-600 telescope at Mount Maidanak Observatory. Light curves are constructed from the new observations and from published and archival data. We use a total of 340, 348, 386, 185, and 62 magnitude estimates in the bands from U to I, respectively. An analysis of these data has yielded the following results. The photometric elements were refined; their new values are \(Min I = JDH 2441966.820 + 65\mathop .\limits^d 4331E\). The UBVRI magnitudes outside eclipse were found to be \(5\mathop .\limits^m 95\), \(6\mathop .\limits^m 77\), \(6\mathop .\limits^m 75\), \(6\mathop .\limits^m 68\), and \(6\mathop .\limits^m 16\), respectively. No phase effect was detected. We obtained two light-curve solutions: (1) assuming that the giant star was in front of the small one during eclipse, we determined the stellar radii, rs=0.0141 and rg=0.0228 (in fractions of the semimajor axis of the orbit); and (2) assuming that the small star was in front of the giant one, we derived rg=0.0186 and rs=0.0180 for the V band. The brightness of the primary star in the bands from U to I is L1=0.96, 0.92, 0.90, 0.89, and 0.88, the orbital inclination is \(i = 87^\circ .1\), and the maximum eclipse phase is α0= 0.66. In both cases, we accepted the U hypothesis, assumed the orbit to be elliptical, and took into account the flux from the star Θ1 Ori E that fell within the photometer aperture. The first solution leads to a discrepancy between the primary radius determined by solving the light curve and the radial-velocity curve and its value estimated from the luminosity and temperature. This discrepancy is eliminated in the second solution, and it turns out that, by all parameters, the primary corresponds to a normal zero-age main-sequence star.