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.     

Possibilities for identifying FK com candidates using observations with the Kepler Space Telescope

High-accuracy photometric observations obtained with the Kepler Space Telescope are used to identify candidate FK Com stars-a very rare group of single, rapidly rotating, chromospherically active G-K stars. Published data for more than 40 000 stars are used with available Kepler observations from the Q3 time interval to select four stars with temperature ranges, surface gravities, and rotation periods consistent with those of FK Com stars. These stars also display brightness variations with considerable amplitudes, possibly testifying to the presence of appreciably spotting on their surfaces. The rotation periods of these stars are determined, and the parameters of their differential rotation estimated. The locations (longitudes) of the dominant active regions on the stellar surfaces are identified. In all cases, the active longitude does not remain constant, andmoves across the stellar surface with time. In general, the character of this activeregion movement is the same as that found earlier for FK Com and HD 199178. These displacements are characterized by monotonic motions over hundreds of days, as well as changes in the positions by about 180° (“flip-flops”) or phase shifts not exceeding 0.4 in phase. The number of active-longitude position changes during the studied time interval ranges from one for KIC 11862915 to seven for KIC 5785906 (five phase shifts are also detected for the latter star). The time scale for the position changes of the active longitudes is from 1500 days (about 4 years) to 200 days (0.54 years), comparable to the reported time intervals between flip-flops for FK Com (from 0.8 to 4.4 years). The duration of the stellar activity cycles are estimated by analyzing the amplitude spectrum for changes in the brightness-variation amplitudes for datasets covering a single rotation period. The photometric variations of the stars on various time scales (from the rotation period, which reveals the presence of surface temperature inhomogeneities, to activity cycles lasting for several years) are similar to those derived for FK Com and other stars of this type. The need for spectroscopic observations of the selected candidates to establish whether they are single (do not show signs of binarity), look for emission lines of chromospheric origin, estimate the lithium abundances, and determine the stellar rotation velocities from spectral-line profiles is noted.     

Magnetic activity of the star Corot-Exo-2a

Continuous photometric observations of the young active solar-type star Corot-Exo-2a using the “Corot” space telescope obtained over 142 days were used to analyze the star’s surface temperature inhomogeneities and to monitor their continuous evolution. This analysis was based on the iPH code, which reconstructs the distribution of temperature inhomogeneities on the surface of a star based on its light curve in a two-temperature approximation. We identified five time intervals in the positions of active areas, with corresponding flip-flop events, interpreted as activity periods. Their durations were between 55 and 15 days. The time scale for the active-longitude flip-flops of Corot-Exo-2a is a few tens of days, rather than years, as for other stars studied earlier. We detected motions of the active longitudes, possibly indicating differential rotation of the star. The phenomenon of flip-flops in the positions of active longitudes has a complex character. This is the first case apart fromthe Sun where we are able to follow the appearance and development of temperature inhomogeneities on a stellar surface in such detail. We determined typical timescales for variations of the activity parameter of the star in the ranges 17–20, 28–32, 33–38, and 51–55 days, which characterize changes of the brightness variation amplitude, the spotted surface area, positions of active areas, and brightness variations.     

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.     

Spots and Activity of Am Stars

The results of a detailed analysis of spot activity on metallic-line A (Am) stars are presented. Two ROTD stars, KIC 8323104 and KIC 8703413, whose power spectra are characterized by a narrow peak and numerous lower-amplitude features at lower frequencies are considered. KIC 12253106, which is not an ROTD star, but undoubtedly has spots on its surface, is also studied. The rotation periods of the surface spots have been found, and the evolution of the spot positions explored. The differential-rotation parameter ΔΩ has been estimated. An analysis of the light curve to reconstruct maps of temperature inhomogeneities on the surface of KIC 8703413 has been performed. These maps indicate concentrations of the active regions at two longitudes. One of these longitudes was active about twice as often as the other one during our observations. The cyclical pattern of the variability of the spot positions on the stellar surface is determined, which may be related to oscillations due to Rossby waves.     

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.     

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.     

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.     

Spots and activity of the M dwarf KIC 1572802

The photometric variability of the M dwarf KIC 1572802 has been studied using the most complete observational data, obtained by the Kepler Space Telescope. Power spectra constructed from 59 488 single brightness measurements over 1460 days (~4 yr) show complex brightness variations. It is suggested that two peaks corresponding to the periods P = 0.37088^d and P = 0.37100^d are related to the presence of active regions at different latitudes on the differentially rotating star. Maps of the surface temperature inhomogeneities are used to derive the positions of these active regions. Analysis of these maps suggests that a switch in the active latitudes occurred 590 days after the beginning of the observations. The variations of the positions of the active regions are also analyzed. These high-temporal-resolution observations revealed a short time-scale change in the active latitudes lasting about 7^d, followed by a “flip-flop,” for the first time. The fraction of the surface of KIC 1572802 covered by spots is S ~ 7%. Comparison with literature data indicate that this S value for KIC 1572802 is substantially higher than the average spottedness of stars with temperatures of 3500–4500 K. This may indicate enhanced activity of KIC 1572802. The parameters of the differential rotation of the star are estimated; the inferred rotational velocity, Ω = 0.0056 ± 0.0010, is substantially lower than the solar value, but comparable to Ω for the cool dwarfs HK Aqr and EY Dra. The value of the Rossby number Ro = 0.011 suggests that KIC 1572802 is in the saturation region of the diagram of Ro vs. X-ray luminosity. If the Ro value for KIC 1572802 is this low, this implies that its magnetic field is of the order of tens or even hundreds of Gauss.     

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.     

Spot activity of the binary CG Cyg: Observations during maximum spottedness

We present the results of new multicolor photometry of the chromospherically active binary CG Cyg acquired in 2005–2009 (136 hours of observations). The light curves for each season reveal rotational brightness modulation due to spots that varies in amplitude and phase from season to season. We have determined the longitudes of spotted areas: for each season, they were located on the primary, close to the line joining the centers of the components. The longitude distribution of spots was analyzed for 44 years of observations of CG Cyg using all data available in the literature. The active longitudes of CG Cyg are not fixed at the quadratures, as was believed earlier: most of the time (1965–2003), the spots were concentrated at two active longitudes at the quadratures, at orbital phases 0.28±0.06 and 0.70±0.08, but, during a shorter time after 2004, they were located along the line joining the component centers, at orbital phases 0.50 ± 0.04 and 0.93 ± 0.03. We detected a switch of the active longitude by 180° during 1.5 months in 2008, accompanied by an increase in the amplitude of the rotational brightness modulation and only a slight increase in the star’s spotted area (by 5%). Our analysis of archive data reveals that switches of the active longitude during time intervals of 1–1.5 months were observed three times during the entire observational history of CG Cyg (in 1991, as well as in our observations of 2003 and 2008). All these switches were accompanied by similar phenomena: an increase in the amplitude of the rotational brightness modulation (by 0.06 ^m , 0.02 ^m , and 0.04 ^m ) and an increase in the spotted area (by 79%, 11%, and 5%). We used a zonal spot model to reconstruct the parameters of the spotted regions on CG Cyg. At all our observing epochs, the spots were located at low latitudes, in a region that was symmetric about the equator, 10° to 14° wide on either side. The spots are cooler than the surrounding photosphere by 2000 K. The spotted area varied only slightly from season to season, comprising 13%–15% of the surface area of the star, close to the historic spottedness maximum for the CG Cyg system.     

Evolution of cool spots on the surface of V410 Tau

The activity of V410 Tau, a T Tauri star with periodic brightness variations, is studied. The evolution of spots on the star??s surface over 46 years is investigated by reconstructing inhomogeneities in the stellar surface temperature based on photometric measurements. Analysis of the distribution of filling factors f as functions of time and longitude on the star??s surface can be used to trace changes in the location of the dominant active area. In some cases, a second active area (longitude) separated by about 180° is detected. No reliable manifestations of switching of the positions of the active areas were detected. The position of the active area was stationary and the position of minimum brightness remained unchanged during a time interval of 4800 days (about 13 years), which ended by the end of 2002. Though the position of the brightness minimum remained virtually constant, the light-curve shape (asymmetry) and the spot ??outline?? in the reconstructed temperature maps exhibited variations. The star possesses considerable cool spots on its surface, whose fractional area is typically 32% and varies between 27% and 40%. Variations of the star??s brightness and spottedness on time scales of 4.8?C5.5 years are most prominent.     

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.     

Stellar activity observed by the Kepler Space Telescope. The M dwarf of the Kepler-32 system with five orbiting planets

The activity of the central star of the Kepler-32 planetary system is studied using continuous 1141-day observations with the Kepler Space Telescope. The Kepler-32 system includes a slowly rotating Mdwarf (rotational period of 37.8 d) with a mass of 0.54M _⊙ and five planets. One of the unique properties of the system is its compactness: the orbits of all five planets are less than a third of the size of the orbit of Mercury; the planet closest to the star is separated from it by only 4.3 stellar radii. Surface-temperature inhomogeneities of the central star are studied using precise photometric observations of Kepler-32, and their evolution traced. In total, 42 624 individual brightness measurements in the 1141-day (3.1-year) observing interval were selected for the analysis. The calculated amplitude power spectra for the first and second halves of the interval of the Kepler-32 observations indicate appreciable variability of the photometric period, corresponding to the evolution of active regions at various latitudes on the stellar surface. Evidence for the existence of two active regions on the stellar surface separated in phase by 0.42 has been found. Time intervals in which the longitudes of the active regions changed (“flip-flops”) with durations of the order of 200–300 days have been established. The spotted area of the star was, on average, about 1% of the total visible surface, and varied from 0.3 to 1.7%. The results for the dwarf Kepler-32 are compared with those from a spectropolarimetric survey of 23 M dwarfs, including both fully convective stars and stars with weakly radiative cores. For a more detailed comparison, temperature inhomogeneities on the surface of one of the survey stars, DS Leo, was reconstructed using the ground-based observations (316 individual measurements of the V-band brightness of the star during seven observing seasons in an all-sky automated survey). The general properties and evolution of the active regions on DS Leo and Kepler-32 are considered. The positions of the active regions on the surface of Kepler-32 yields no evidence for differential rotation of this star. The possibility of detecting the magnetic field of Kepler-32 is proposed. The analysis of the photometric data for Kepler-32 are also compared to the previous results for the fully convective, low-mass M dwarfs GJ 1243 and LHS 6351. This demonstrates that the observed manifestations of activity on Kepler-32 correspond to those for active G-K stars and to M dwarfs with masses of the order of 0.5M _⊙, rather than Mdwarfs with masses from 0.2 to 0.5M _⊙.     

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.     

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.     

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.     

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.     

Analysis of MOST light curves of the active stars <Emphasis Type="Italic">ɛ</Emphasis> Eri AND <Emphasis Type="Italic">κ</Emphasis> Cet

We have analyzed light curves from the MOST satellite for the two active dwarfs ? 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.     

A search for FK Com candidates using Kepler Space Telescope observations: Analogs of HD 199178

Analysis of collected photometric observations obtained with the Kepler Space Telescope were used to select and study 33 objects with parameters corresponding to those of the FK Com starHD199178; these can be considered candidate stars of this type. In this final study, the four objects with the best light curves, which show the properties of their regular rotational modulation most clearly, were selected for detailed studies. The photometric analysis is based on all data currently available in the Kepler archive (covering almost four years). The rotational periods and estimated parameters of the objects’ differential rotation are determined, and the longitudes of the dominant active regions on the surfaces found. For all four stars, the spot coverage is approximately 1% of the visible stellar surface area. The rotational periods and data on the stars’masses and radii fromtheMAST catalog are used to determine the rotation velocities projected onto the line of sight, which ranged from 12 to 21 km/s. Further studies will enable definite conclusions about how these stars are related to FK Com stars. If they are ultimately classified as FK Com stars, this will considerably increase the number of this rare type of star and the also number of rapidly rotating, single, late-type giants.