E. S. Dmitrienko

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.

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.

Spots and the activity of M dwarfs from observations with the Kepler Space Telescope

A method for estimating the spottedness parameter S (the spotted area as a fraction of the surface of an active star) proposed earlier is applied to an analysis of activity in 1570 M dwarf stars. The analysis is based on observational material obtained with the Kepler Space Telescope, as well as data on the fluxes of the studied objects in the near and far ultraviolet (NUV and FUV) based on data from the GALEX space telescope. The variations of S with the ages of the stars are studied (four groups with different ages are distinguished), as well as variations of S with their rotational periods. A diagram characterizing the relationship between S and the Rossby number Ro resembles the classical dependence of the X-ray luminosities of active stars on Ro, and a saturation regime is attained at the same value, Ro = 0.13. Moreover, objects with ages of more than 100 million years do not form a single sequence (and stars older than 900 million years possess surface spottednesses of order 1%). The S−Ro dependence obtained could expand possibilities for analyzing the dependence of the X-ray luminosities of active stars on their Rossby numbers, and could also be applied to refine parameters characterizing the action of dynamo mechanisms, such as the dynamo number ND. A comparison of the GALEX NUV and FUV brightness estimates with the activity parameters of the stars suggests that younger, more rapidly rotating active stars are brighter in the NUV, and that the FUV flux grows and the difference of the FUV and NUV brightnesses decreases with increasing spottedness S.

The magnetic field of the pulsating subdwarf Balloon 090100001

We have analyzed polarization observations of the subdwarf Bal 09, which is one of a group of hybrid sdB stars that display simultaneously both short- and long-period pulsations. Certain properties previously unknown for subdwarfs have been established for Bal 09, such as variations of the pulsation amplitude of the main oscillation mode, rotational splitting of multiplets, and variations of this splitting. Information about the stellar magnetic field must be considered if we wish to explain these properties. New observational data enabling estimation of the longitudinal magnetic field of Bal 09 have been obtained on the main stellar spectrograph of the 6-m telescope of the Special Astrophysical Observatory. Studies of the longitudinal component of the magnetic field 〈Bz〉 were carried out using a regression analysis. This method simultaneously yields estimates of the uncertainty in 〈Bz〉. Test measurements of 〈Bz〉 were carried out using the same method. For the star HD 158974, which has zero total magnetic field, the estimated longitudinal magnetic field is 〈Bz〉 = −4 ± 5 G. The standard magnetic field for the Ap star α2CVn was measured to be −363 ± 17 G, in very good agreement with measurements in the literature. The estimated longitudinal magnetic field for Bal 09 is 34 ± 63G—appreciably lower than values established earlier for six subdwarfs, ≈1.5 kG. The results of the regression analysis for both individual spectral subranges and for intervals containing characteristic spectral features did not indicate reliable detections of a magnetic field exceeding the uncertainties in 〈Bz〉. The uncertainty in 〈Bz〉, which was 60–80 G for the entire spectral range and 140–200 G for selected spectral intervals, leads to an estimated upper limit on the longitudinal magnetic field 〈Bz〉 for Bal 09. This estimate for 〈Bz〉 can place observational constraints on theoretical explanations for the amplitude variations of the pulsations, rotational splitting of multiplets, and possible variations of the internal structure of the star.

Spots and Activity of Pleiades Stars from Observations with the Kepler Space Telescope (K2)

Observations of the K2 continuation of Kepler Space Telescope program are used to estimate the spot coverage S (the fractional spotted area on the surface of an active star) for stars of the Pleiades cluster. The analysis is based on data on photometric variations of 759 confirmed clustermembers, together with their atmospheric parameters, masses, and rotation periods. The relationship between the activity (S) of these Pleiades stars and their effective temperatures shows considerable change in S for stars with temperatures Teff less than 6100 K (this can be considered the limiting value for which spot formation activity begins) and a monotonic increase in S for cooler objects (a change in the slope for stars with Teff ~ 3700 K). The scatter in this parameter ΔS about its mean dependence on the (V −Ks)0 color index remains approximately the same over the entire (V−Ks)0 range, including cool, fully convective dwarfs. The computated S values do not indicate differences between slowly rotating and rapidly rotating stars with color indices 1.1 < (V−Ks)0 < 3.7. The main results of this study include measurements of the activity of a large number of stars having the same age (759 members of the Pleiades cluster), resulting in the first determination of the relationship between the spot-forming activity and masses of stars. For 27 stars with masses differing from the solarmass by nomore than 0.1M⊙, themean spot coverage is S = 0.031±0.003, suggesting that the activity of candidate young Suns is more pronounced than that of the present-day Sun. These stars rotate considerably faster than the Sun, with an average rotation period of 4.3d. The results of this study of cool, low-mass dwarfs of the Pleiades cluster are compared to results from an earlier study of 1570 M stars.

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 = 0.37088d and P = 0.37100d 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 7d, 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.

A Study of the Magnetic Field in FK Com

We present the results of our new spectropolarimetric observations of FKCom aimed to measure the longitudinal component Bz of its magnetic field. The most interesting interpretation of our results suggests that the Bz value has significantly decreased compared to the 2008 observations of this star. Such a decrease of the longitudinal component of the magnetic field can be similar to the secular variations of Bz registered earlier for another chromospherically active star II Peg. On the other hand, assuming the existence of Bz variations with the rotation phase, we suggest that the variations of the phase curve Bz from 2008 to 2012 originated because of the strengthening of the negative polarity spot, its domination, and as a result-a generally more symmetric distribution of magnetic regions.

Spots and activity of solar-type stars from Kepler observations

The spot coverages S for 2846 solar-type stars with effective temperatures from 5700 K to 5800 K and gravities from 4.4 to 4.5 have been measured. An analysis based on the MAST catalog, which presents photometric measurements obtained with the Kepler Space Telescope during Q9 is presented. The existence of two groups of solar-type stars, with S values between 0.001 and 0.007 and with S > 0.007, is inferred. The second group (active stars) contains 279 stars (about 10% of the total number of stars analyzed). The mean S parameter for the entire sample is 0.004, comparable to the mean spot coverage of the Sun. In general, the dependence of S on the rotation period for solar-type stars has characteristics similar to those found earlier for stars with exoplanets. For the vast majority of the stars in the sample, the activity is constant, and independent of age. The activity of the small number of active stars with S > 0.007 decreases with age. The age variations of the chromospheric activity index R′HK are compared to variations of the spot coverage S. The relations analyzed have common characteristic features. It is likely that both the spot activity level and the chromospheric activity level abruptly decrease for stars older than 4 billion yrs.

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(RX) = −4.20.

Activity and cool spots on the surfaces of G-type stars with superflares from observations with the Kepler Space Telescope

The properties of active regions (cool spots) on the surfaces of 279 G-type stars in which more than 1500 superflares with energies of 1033-1036 erg were detected are analyzed. Diagrams plotting the superflare energy against activity parameters of the stars (the area of their magnetic spots) are considered, and a more extensive study of the activity of two stars with the highest numbers of flares is presented. The range of variation of the superflare energies (up to two orders of magnitude) is realized over the entire interval of rotation periods. It is proposed that the plot of superflare energy vs. rotational period is bimodal. There are probably no appreciable differences in the maximum flare energies for the two groups of objects, which have rotational periods of more than and less than 10 days. Three groups of stars with different surface spottednesses can be distinguished in a plot of superflare energy vs. cool-spot area. The range of variation of the flare energy within a group is roughly the same for these three groups. Most of the points on this diagram lie to the right of the dependence corresponding to B = 3000Gand an inclination i = 90° (the first two groups of objects). It is confirmed that the flare activity is not related directly to circumpolar active regions, since the majority of the points on the diagram lie to the right of the dependence for B = 1000 G and i = 3°. Analysis of stars from the sample, including objects with more than 20 superflares, shows that large variations of the energy (by up to two orders of magnitude) can be reached with small variations of the spottedness parameter S for a single star. Appreciable variability of the spottedness (by factors of five to six) was detected for only two objects from the sample (KIC 10422252 and KIC 11764567). These stars displayed an increase in the flare energy by orders of magnitude for any spottedness level. The activity of KIC 11551430 and KIC 11764567 is analyzed in detail using all available photometric data from the Kepler Space Telescope archives. KIC 11551430 was determined earlier to display the highest number of flares (on average, one flare every 7 days); KIC 11764567 displayed the highest number of flares among stars with rotational periods comparable to the solar period (on average, one flare every 25 days). Maps of surface temperature inhomogeneities of these stars are used to determine the longitudes and other characteristics of their active regions.