Spots and activity cycles of HD 199178

We present our analysis of photometry for the FK Com star HD 199178 (V1794 Cyg). The V-band light curves are used to restore the distribution of temperature inhomogeneities on the stellar surface. The spots on the surface of HD 199178 are concentrated at two preferred longitudes separated by 0.5 in phase (180° in longitude). In addition to the quasi-periodic switching of the most active area between these two longitudes, which occurs in cycles of 2.1–2.4 or 4.1 years, we suspect that the two active areas moved toward each other across the stellar surface, possibly merging into a single formation. The detected cycle in the star’s brightness variations of about 8.0 years is also clearly visible in variations of the star’s spottedness. Themagnetic activity of the FK Com star HD199178 is in many ways similar to that observed for the prototype of this group.     

Spots on the surfaces of the chromospherically active stars GSC 08923-01147 and GSC 08933-01802: Observational data of the ASAS-3 project

Active regions on the surfaces of the chromospherically active stars GSC 08923-01147 and GSC 08933-01802 are found by reconstructing the surface-temperature inhomogeneities using the V light curves, obtained from observations performed during the ASAS-3 project. Up to one-third of the total observed area of the stellar surfaces was covered by spots. In GSC 08923-01147, the spotted fraction decreased from 0.31 to 0.05 during the observation period, while it varied within 0.2–0.3 during four years in GSC 08933-01802. The detected phases of active longitudes tend to concentrate near two positions separated by about half a period. The active regions in GSC 08933-01147 became closer or further apart on a time scale of 4.8 years. The maximum separations of the longitudes Δφ _long were maximum when the amplitude of the brightness variability reached a minimum. During the transition of the star from a minimum-brightness to an enhanced-brightness state, the longitude of the more active region switched. The brightness variability of GSC 08933-01802 was more substantial (ΔV }~ 0.55 ^m −0.06 ^m ). For some time, only one extended active region was present on the stellar surface, while two regions were usually present. No switch of the active longitudes was detected. The more active region monotonically moved over the surface of the star, causing the longitudes of the active regions to drift together. This monotonic change in the position of the more active longitude ceased at the minimum of the brightness-variation amplitude.     

Stellar activity as observed by the KEPLER space telescope: The K Dwarf KIC 8429280

The activity of the K dwarf KIC 8429280 (TYC 3146-35-1) has been studied. Unique high-precision photometric observations of this object obtained with the KEPLER space telescope suggest a pronounced amplitude modulation of the brightness of the star, and have made possible the analysis of surface-temperature inhomogeneities. The evolution of active regions on the surface of KIC 8429280 has been traced during 105 rotation periods. Evidence has been found for the existence of two active longitudes on the surface of KIC 8429280, separated by approximately 180°. The motion of the active longitudes on the surface of KIC 8429280 is complex and unstable. At some times, the active regions moved together in longitude with the rotation of the star, while they moved in opposite directions at other time. The less active region sometimes disappeared completely, and only one active region was observed on the stellar surface. The area of the spotted surface S is 4% of the visible stellar surface for the adopted inclination of the rotation axis of the star to the line of sight, i = 60°. The periodicity for variations in S is no less than 90 d. The timescale for the change in the amplitude of the brightness variations is 30 d. Three epochs of alternation of the active longitudes are close in time to three of four firmly established minima in the amplitudes of the brightness variations. The results of the light-curve analysis for KIC 8429280 are compared to results obtained for the young active solar-type star Corot-Exo-2, which has a similar light curve with a pronounced modulation.     

Stellar activity from observations with the KEPLER space telescope: The M dwarf GJ 1243

We used continuous observations with the KEPLER space telescope during 44.45 days to study the activity of the fully convective low-mass M dwarf GJ 1243. The star’s rotational period is 0.593 days. Our precision photometry of GJ 1243 made it possible to study its surface temperature inhomogeneities and follow their evolution. We find evidence for two active longitudes on the surface of the star, separated by 203° in longitude or 0.56 in phase. The position of spots on the surface was found to be very stable during the analyzed 74 rotation periods. Assuming that the rotational axis is inclined to the line of sight by i = 60°, the total spotted area S is, on average, 3.2% of the total visible stellar surface, and increased by 0.7% in 100 days; if i = 30°, the area is S = 5.6%. The change in S is due to an increase in the area of the smaller active region. We compare the magnetic-activity features of GJ 1243 and another fully convective star, V374 Peg.     

HD 181906: A star with two systems of active longitudes on its surface

We use continuous 156-day COROT photometric observations of the F dwarf HD 181906 to analyze temperature inhomogeneities on the stellar surface and follow their evolution. The analysis used the iPH code, which solves for the temperature inhomogeneities in a two-temperature approximation without any assumptions about the shape or number of spots on the surface. For the first time in studies of active regions and active longitudes, we find that the phases of the active longitudes on the surface of HD 181906 are concentrated close to two systems of active longitudes. In each system, the active longitudes are separated by 180°, with the shift between the systems being 100°. During the observing period, switches between the systems of active longitudes occurred quasi-periodicly on a time scale of 30–35 day about two-thirds of the time, while these switches occurred less frequently about one-third of the time. The positions of active regions switched either every 20–25 day or every 40–45 day. The periodicity of brightness and amplitude variations is of the order of 110 day. Variations in the spot coverage and changes in the active longitudes have shorter time scales (about 55–75 day). All these parameters are variable on time scales of 25–38 day. A wavelet analysis of the periodicity of the brightness variations indicates that all the above processes are quasi-periodic; activity on all time scales became less pronounced in the last third of the observing period.     

<Emphasis Type="Italic">WBVR</Emphasis> observations of the X-ray star V1341 Cyg = Cyg X-2 in 1986–1992

We present the results of WBVR observations of the low-mass X-ray binary V1341 Cyg = Cyg X-2. Our observations include a total of 2375 individual measurements in four bands on 478 nights in 1986–1992. We tied the comparison and check stars used for the binary to the WBVR catalog using their JHK magnitudes. The uncertainty of this procedure was 3% in the B and V bands and 8%–10% for the W and R bands. In quiescence, the amplitude of the periodic component in the binary’s B brightness variations is within 0.265 ^m –0.278 ^m (0.290 ^m –0.320 ^m in W); this is due to the ellipsoidal shape of the optical component, which is distorted with gravitational forces from the X-ray component. Some of the system’s active states (long flares) may be due to instabilities in the accretion disk, and possibly to instabilities of gas flows and other accretion structures. The binary possesses a low-luminosity accretion disk. The light curves reveal no indications of an eclipse near the phases of the upper and lower conjunctions in quiescence or in active states during the observed intervals. We conclude that the optical star in the close binary V1341 Cyg = Cyg X-2 is a red giant rather than a blue straggler. We studied the longterm variability of the binary during the seven years covered by our observations. The optical observations presented in this study are compared to X-ray data from the Ginga observatory for the same time intervals.     

Long-term starspot activity of the eclipsing binaries BH Vir and WY Cnc

We present the results of multicolor photometry of the two spotted, short-period, late-type RS CVn stars BH Vir and WY Cnc. We have derived the parameters of the binaries’ components, and refined their masses, radii, and luminosities. A strong flare of WY Cnc was detected for the first time, and pre-flare variations of the star’s activity studied. Our observations and published data spanning 40 years are analyzed using a zone starspot model. We demonstrate that the spots are always concentrated near the equators and at intermediate latitudes, with maximum spotted areas as large as 29% for BH Vir and 21% for WY Cnc. The temperature differences between spotted regions and the quiet photosphere were 2300 K (BH Vir) and 1800 K (WY Cnc). The detected long-term brightness variations of BH Vir suggest the existence of an activity cycle with a probable period of 22 years. Both stars have starspots concentrated at two active longitudes separated by approximately half of the orbital period during all the studied seasons; these longitudes remained the same (0° and 184°) for BH Vir over 40 years, whereas they migrated in the direction of the stellar rotation at a rate of 3.8°/yr for WY Cnc, suggesting a cycle of 47 years for the migration of the active longitudes.     

Results of infrared photometry and a model for the dust envelope of the protoplanetary nebula candidate V1027 Cyg

Results of JHKLM photometry for the protoplanetary nebula candidate V1027 Cyg obtained in 1991–2008 are reported. In all bands, the brightness variations did not exceed 0.2 ^m . Estimated linear trends demonstrate no significant changes in the mean brightness and color indices of the object, with the possible exception of the L-M color index, which showed a small decrease. A search for possible periodicities in the brightness variations yielded the most probable period of 237^d. A model for a spherically-symmetric dust shell has been calculated based on the photometric results supplemented with data on the mid- and far-infrared fluxes. The estimated mass-loss rate of the star is 1.3 × 10⁻⁵ M _⊙/year.     

Search for and study of hot circumstellar dust envelopes

Long-term (1984–2008) JHKLM photometry for 254 objects is presented. The observations were carried out in the standard JHKLM photometric system using an original method and a modern IR photometer designed and built at the Sternberg Astronomical Institute. Our program of studies included searches for and studies of relatively hot circumstellar dust envelopes. The most important results obtained using these observations include the following. We have detected relatively hot dust envelopes in a number of objects for the first time, including the RCB star UV Cas, RX Cas, several classical symbiotic stars, etc. A model has been calculated for the dust envelope of FG Sge, which formed around the star as a result of several successive cycles of dust condensation beginning in Autumn 1992. Several dust-condensation episodes have been traced in the envelopes of symbiotic systems (CH Cyg, V1016 Cyg, HM Sge, etc.), as well as the role of the hot component in the formation of the dust envelopes. We have established from variations of the IR emission that the cool components in the symbiotic novae V1016 Cyg and HM Sge, and possibly CH Cyg, are Miras. The binarity of V1016 Cyg and HM Sge has also been firmly established. The variability of a whole series of object has been studied, including the stellar components of close binary systems and several dozen Mira and semi-regular variables. The ellipsoidality of the components in the RX Cas system (a prototype W Ser star) and the cool component in the symbiotic systems CI Cyg and BF Cyg has been firmly established. We have obtained the first IR light curve for the eclipsing system V444 Cyg (WN5+O6), and determined the wavelength dependence of the obtained parameters of the WN5 star. Analysis of the IR light curves of several novae indicate the condensation of dust envelopes in the transition periods of Cygnus 1992, Aquila 1993, and Aquila 1995. The IR light curve of R CrB has been obtained over a long period and analyzed. IR observations of the nova-like variable V4334 Sgr have been carried out over four years, over which the star passed through four stages during its motion along its post-AGB evolutionary track; the star’s bolometric flux and optical depth of its dust envelope have been estimated, and the structure and mass of the dust layer determined. We have analyzed the IR variability of the symbiotic star V407 Cyg over 14 years, and found its cool component to be a Mira with a period of 745 days. The observed pulsations and trend are associated with the luminosity and temperature variations of the Mira, as well as the optical depth of the dust envelope. The size of the dust grains and mass-loss rate of the Mira have been determined. We have obtained JHKL light curves for the Seyfert galaxy NGC 4151 over 23 years. The IR brightness of the galaxy grew from 1985 through 1996 (by ∼0.9 ^m at 1.25 μm, ∼1 ^m at 1.65 μm, ∼1.1 ^m at 2.2 μm, and ∼1.3 ^m at 3.5 μm), while the galaxy simultaneously reddened. The “cool” variable source in NGC 4151 was still in the active state in 1998, although its luminosity had decreased by approximately 15%–20%. If the “cool” component of the variable source in this galaxy is a dust envelope heated by the central “hot” source, it should be optically thin to the radiation of this source: its mean optical depth is in the range 0.05–0.15. Emission from dust particles heated to temperatures of 600–800 K was observed in the near IR at a distance of several parsecs from the nucleus during the period of activity in 1995–1998; the inferred mass of emitting dust was 5–20 M _⊙. In 1994–2003, we observed a tendency for NGC 4151 to become bluer at 1.25–1.65 μm while simultaneously reddening at 2.2–3.5 μm. Beginning in Autumn 2000, the galaxy began to emerge from a minimum, which lasted from March 2000 through April 2001 in the IR; a flare of the galactic nucleus was observed and followed in detail in the IR in this same period. We confirm the IR variability of the nucleus of the Seyfert galaxy NGC 1068, which can be located in various stages of activity. The variability of NGC 1068 is associated with a complex source. A periodic component has been detected in the J brightness variations of the oxygen Mira V2108 Oph; we have shown that this star is immersed in a fairly dense dust envelope, and have calculated amodel for this envelope. We have calculated an (axially symmetrical) dust-envelope model for the carbon semi-regular variable RWLMi with a density distribution characteristic for the “superwind” stage. This envelope model is able to reproduce the observed fluxes over the entire observable spectrum, and displays a good agreement with the observational data. The three hot supergiants V482 Cas, QZ Sge, and HD 338926 may be variable in the IR. Long-term photometry of eight planetary nebulae in the near IR (1.25–5 μm) has enabled us to firmly establish the IR brightness and color variability of these nebulae on time scales from several tens of days to six-to-eight years. We have analyzed long-term JK photometry of the X-ray binary Cyg X-1 (V1357 Cyg). Periodicity with a characteristic time scale of ∼11.5 years is visible in the JK brightness variations of Cyg X-1 in 1984–2007, possibly due to periodic variations of the temperature, radius and luminosity of the optical component of the binary with P ∼11.5 yr. Fourteen-year IR light curves of five RV Tau stars (R Sge, RV Tau, AC Her, V Vul, and R Sct) and the yellow supergiant and protoplanetary-nebula candidate V1027 Cyg have been analyzed. A spherically symmetrical dust-envelope model has been calculated for V1027 Cyg.     

Stellar activity observed by the kepler space telescope: The systems with two planets and two active longitudes KOI 877 and KOI 896

Unique high-accuracy 33.5-day continuous photometric observations of the KOI 877 and KOI 896 systems are used to study their surface inhomogeneities and trace the evolution of spots over two to three rotational periods. These systems are two of five candidate multi-planet objects. We found evidence for the existence of two active longitudes on the surfaces of KOI 877 and KOI 896, separated by 165° and 135°, respectively. The fraction of the surface area covered by the spots is similar to the solar one, and comprises about 0.6–1.1% of the total visible surfaces of the stars. A switch of the active longitudes likely occurred in KOI 877 during the observation period. Calculated maps of surface temperature inhomogeneities may be helpful for more accurate derivations of the parameters of planetary systems.     

Photometric period and rotational brightness modulation of V833 Tau

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.     

Activity observed by the Kepler space telescope: The M dwarf LHS 6351 (KIC 2164791)

Continuous 123.87-day observations with the “Kepler” space telescope are used to study the activity of the fully convective, low-mass M dwarf LHS 6351. The axial-rotation period of the star is 3.36 day. High-precision photometric observations of LHS 6351 enabled studies of its surfacetemperature inhomogeneities and their evolution. The difference in the longitudes of active regions increased from 120° at the beginning to 207° at the end of the observations, for i = 60° (and from 156° to 198° for i = 30°). This variation of the locations of the spots on the stellar surface provides evidence for differential rotation of the star. According to our estimates, the rate of displacement of the active regions is (0.006–0.014) ± 0.002 rad/day. Assuming i = 60°, the total area of spots S decreased, on average, from 1.2% to 0.92% of the total visible surface of the star; if i = 30°, this area decreased from 1.8% to 1.0%. We compared manifestations of the magnetic activity of LHS 6351 with the properties of the fully convective M dwarfs V374 Peg and GJ 1243, studied earlier. We derived the dependence of ΔΩ on the Rossby number for these M dwarfs, and identified two groups of stars with differing mass and differential rotation.     

Spots and active longitudes on the star V815 Her

We have analyzed light curves from the MOST satellite for the two active dwarfs ɛ Eri and κ Cet. Our maps of the stellar surface-temperature inhomogeneities were obtained with no a priori assumptions about the shape, configuration, and number of spots. We find variations of the surface-temperature inhomogeneities with time, also on time scales about equal to their rotation periods. We consider a model of a spotted star with two types of surface inhomogeneities—spots and related plage fields—and demonstrate that the best agreement between the theoretical and observed light curves is achieved for small ratios of the plage-field area to the area of cool spots. This conclusion indicates that long-term brightness variations of stars younger than the Sun are mainly due to variable spots on their surfaces, while the contribution from plage fields becomes more significant for older stars.     

Starspots and Activity of the Flare Star GJ 1243

The photometric variability of the uniqueMdwarf flare star GJ 1243 (KIC 9726699) is investigated using the most complete set of observationalmaterial obtained with the Kepler Space Telescope. The analysis is based on 49 487 individual brightness measurements obtained during an interval of 1460 days (nearly four years). The periodicity of the brightness variations with the period P_phot = 0.59261 ± 0.00060^d is confirmed. The temperature inhomogeneities on the stellar surface reconstructed from the light curve are used to drive maps of these surface-temperature inhomogeneities (of the filling factor f). The resulting maps are used to determine the positions of active regions. Analysis of the surface-temperature maps for GJ 1243 led to the conclusion that the positions of spots on the stellar surface displayed appreciable evolution during the analyzed time interval. The maximum value for the lower limit on the differentialrotation parameter ΔΩ is 0.0022 rad/day. This more accurate estimate of ΔΩ is lower than the values presented earlier by Davenport et al. [1] (0.0058 and 0.0036 rad/day), due to the more accurate account of variations in the positions of the most active longitude in the current study. However, the differentialrotation estimate obtained in [1] using a method based on fitting the evolution of spots using twodimensional Gaussian functions essentially coincides with the new estimate presented here. The fractional area of the total spotted surface S of the star during the observing interval considered varied from 7 to 2%. The amplitude of the brightness variability of the star slowly decreased, varying in the range 1.6?0.5%. Overall, the position of GJ 1243 in spottedness–age, spottedness–rotation period, and spottedness–Rossby number diagrams agrees very well with the general character of the dependences displayed in earlier studies of M dwarfs.     

Activity of an M4.5 hyades dwarf with a planetary system

An analysis of the activity of the Hyades M4.5 dwarf EPIC 210490365, K2–25 (2MASS J04130560+1514520), based on observational data obtained with the Kepler Space Telescope is presented. This dwarf has a Neptune-type planet. The continuous evolution of active regions on the surface of K2–25 is traced over 70 days. The brightness changes of the star display a fairly stable nature. The rotation period of K2–25 is 1.878 ± 0.030 day. Maps of temperature inhomogeneities on the surface of K2–25 are constructed for 37 sets of observations. All these maps show concentrations of spots at two longitudes, with more active region having the larger area. The total spotted surface area S is, on average, 2.6% of the total visible surface of the star. The estimated differential rotation speed of the star is ΔΩ = 0.0071 ± 0.002 rad/day. The positions of K2–25 in S–age, S–rotation period, and S–Rossby number diagrams are consistent with the general trends of these dependences established earlier for M dwarfs. The derived Rossby number for K2–25, Ro = 0.36, is used to estimate the star’s X-ray luminosity to be log(R _X) = ?4.20.     

Giant pulses from the pulsar PSR B0950+08

Analysis of individual pulses of the pulsar B0950+08 at 112 MHz has shown that giant pulses with intensities exceeding the peak amplitude of the mean profile at these longitudes by two orders of magnitude are observed at the longitudes of all three components of the mean pulsar profile (the precursor and two-component main pulse). The maximum peak flux density of a recorded pulse is 15 240 Jy, and the energy of this pulse exceeds the mean pulse energy by a factor of 153. Strong but infrequent pulses at the longitude of the first component (precursor) can reach peak flux densities of 5750 Jy, exceeding the amplitude of the mean profile at this longitude by a factor of 490. It is shown that the emission at the precursor longitudes is virtually absent when giant pulses appear at the main-pulse longitudes, and vice versa: the presence of giant pulses at the precursor longitude results in the absence or considerable attenuation of the emission at other longitudes. The analysis shows that the cumulative probability function of the pulse peak flux densities has a piecewise power-law form. The power-law index for pulses with intensities exceeding 600 Jy appearing at the longitudes of the main pulse in the mean profile varies from n ₁ = ?1.25 ± 0.04 to n ₂ = ?1.84 ± 0.07. The obtained pulse energy distribution also has an inflection at E > 3000 Jy ms and a power-law form with the same index. The distribution of the pulse intensities at the precursor longitude was obtained, and forms a power law with index n = ?1.5 ± 0.1. The studied properties of the pulses at various longitudes of the mean profile are interpreted in the framework of induced scattering of the main-pulse emission by particles of ultrarelativistic, strongly magnetized plasma in the pulsar magnetosphere.     

Heat capacity and thermodynamic properties of GdPO<Subscript>4</Subscript> in the temperature range 0–1600 K

The heat capacity of gadolinium orthophosphate (GdPO₄) measured in the temperature range 11.15–344.11 K by adiabatic calorimetry and available literature data were used to calculate its thermodynamic functions at 0–1600 K. At 298.15 K, these functions are as follows: C _p ⁰(298.15 K) = 101.85 ± 0.05 J K⁻¹ mol⁻¹, S ⁰(298.15 K) = 123.82 ± 0.18 J K⁻¹ mol⁻¹, H ⁰(298.15 K)–H ⁰(0) = 17.250 ± 0.012 kJ mol⁻¹, and Φ ⁰(298.15 K) = 65.97 ± 0.18 J K⁻¹ mol⁻¹ The calculated Gibbs free energy of formation from the elements of GdPO₄ is Δ _f G ⁰ (298.15 K) = −1844.3 ± 4.7 kJ mol⁻¹.     

The dwarf nova MN Dra: Periodic processes at various phases of the supercycle

We analyze photometry of the dwarf nova MN Dra carried out using various instruments at four observatories on 18 nights between May 20 and June 28, 2009. The observations cover a variety of activity states of the system: a superoutburst, three normal outbursts, and quiescence. Analysis of the system’s light curve during the superoutburst decline reveals positive superhumps that recur, on average, with a period of 0.105 days and are due to the direct apsidal precession of the accretion disk. These are observed until the end of the superoutburst, but their period decreases at a rate of −24.5 × 10⁻⁵ of the period per period. Both the positive-superhump period and its derivative are in good agreement with estimates made during previous superoutbursts. At the brightness minimum and in normal outbursts, MN Dra displays brightness variations with a period of 0.096 days, whose amplitude is much larger during the brightness minimum (0.8 ^m –1.5 ^m ) than during normal outbursts (0.1 ^m –0.2 ^m ). We suggest that these brightness variations could be negative superhumps due to nodal precession of the oblique accretion disk.     

Spots and active longitudes on the surface of the young star Par 1724 in Orion

V-band light-curves are used to reconstruct the temperature inhomogeneities on the surface of the young T Tauri star Par 1724. The spots on the surface of Par 1724 are concentrated close to two preferred longitudes separated by 180° (0.5 in phase). These results provide evidence that Par 1724 may be the youngest star fow which a switch of the active longitudes is detected.