Investigating magnetic activity in very stable stellar magnetic fields. Long-term photometric and spectroscopic study of the fully convective M4 dwarf V374 Pegasi
The ultrafast-rotating (Prot ż 0.44 d) fully convective single M4 dwarf V374 Peg is a well-known laboratory for studying intense stellar activity in a stable magnetic topology. As an observable proxy for the stellar magnetic field, we study the stability of the light curve, hence the spot configuration. We also measure the occurrence rate of flares and coronal mass ejections (CMEs). We have analysed spectroscopic observations, BV(RI)C photometry covering 5 yrs, and additional RC photometry that expands the temporal base over 16 yr. The light curve suggests an almost rigid-body rotation and a spot configuration that is stable over about 16 yrs, confirming the previous indications of a very stable magnetic field. We observed small changes on a nightly timescale and frequent flaring, including a possible sympathetic flare. The strongest flares seem to be more concentrated around the phase where the light curve indicates a smaller active region. Spectral data suggest a complex CME with falling-back and re-ejected material with a maximal projected velocity of ~675 km s-1. We observed a CME rate that is much lower than expected from extrapolations of the solar flare-CME relation to active stars.
Antisolar differential rotation of the K1-giant σ Geminorum revisited
Kővári, Zs.; Kriskovics, L.; Künstler, A.; Carroll, T. A.; Strassmeier, K. G.; Vida, K.; Oláh, K.; Bartus, J.; Weber, M.
ADS link
Surface differential rotation and other global surface flows on magnetically active stars are among the observable manifestations of the underlying stellar dynamo. Therefore, these types of observations are important for stellar dynamo theory and useful constraints for solar dynamo studies as well.
We revisit the active K1-giant component of the long-period RS CVn-type binary system σ Gem and its global surface flow pattern.
We refine the differential rotation law from recovering the spot migration pattern. We apply a detailed cross-correlation technique to a unique set of 34 time-series Doppler images recovered using data from 1996-97. By increasing the number of the available cross-correlation function maps, we expect a more robust determination of the differential surface rotation law. In addition, we present a new time-series Doppler imaging study of ż Gem using our advanced surface reconstruction code iMap for a data set collected in 2006-07.
Results from the reprocessed cross-correlation study confirm that the star performs antisolar-type differential rotation with a surface shear α of - 0.04 ± 0.01, i.e., almost a factor of two larger compared to the previously claimed value. We also confirm the evidence of a global poleward spot migration, with an average velocity of 0.21 ± 0.03 km/s, in accordance with theoretical predictions. From the new observations, we obtain three subsequent Doppler images. The time evolution of these images confirms the antisolar-type differential rotation of the same amount.
Examples of meridional spot redistributions during the covered
timespan of about 43.5 days at arbitrarily chosen quadrant longitudes
of 25°, 115°, 205°, and 295° (top: left, right, below: left, right,
respectively). In each case, the replacements of the maximum temperature
offsets (dots) are fitted with a 1st degree function.
Study of FK Comae Berenices. VII. Correlating photospheric and chromospheric activity. Correlating photospheric and chromospheric activity
Vida, K.; Korhonen, H.; Ilyin, I. V.; Oláh, K.; Andersen, M. I.; Hackman, T.
ADS link
We study the connection between the chromospheric and photospheric behaviour of the active late-type star FK Comae. We use spot temperature modelling, light curve inversion based on narrow- and wide-band photometric measurements, Hż observations from 1997-2010, and Doppler maps from 2004-2010 to compare the behaviour of chromospheric and photospheric features.
Investigating low-resolution Hα spectra, we find that the changes in the chromosphere seem to happen mainly on a time scale longer than a few hours, but shorter variations are also observed. According to the Hα measurements, prominences are often found in the chromosphere that reach to more than a stellar radius and are stable for weeks, and they seem to be often, but not always connected to dark photospheric spots. The rotational modulation of the Hż emission typically seems to be anticorrelated with the light curve, but we did not find convincing evidence of a clear connection in the long-term trends of the Hż emission and the brightness of the star. In addition, FK Com seems to be in an unusually quiet state in 2009-2010 with very little chromospheric activity and low spot contrast, which might indicate the long-term decrease in activity.
A residual H α dynamic spectrum of FK Com. Crosses indicate the observations. The thick dashed line shows the 0 radial velocity shift (i.e. the center of the stellar disk), while the other two dashed lines indicate the edges of the disk.
Coronal Magnetic Reconnection Driven by CME Expansion - the 2011 June 7 Event
van Driel-Gesztelyi, L.; Baker, D.; Török, T.; Pariat, E.; Green, L. M.; Williams, D. R.; Carlyle, J.; Valori, G.; Démoulin, P.; Kliem, B.; Long, D. M.; Matthews, S. A.; Malherbe, J.-M.
ADS link
Coronal mass ejections (CMEs) erupt and expand in a magnetically structured solar corona. Various indirect observational pieces of evidence have shown that the magnetic field of CMEs reconnects with surrounding magnetic fields, forming, e.g., dimming regions distant from the CME source regions. Analyzing Solar Dynamics Observatory (SDO) observations of the eruption from AR 11226 on 2011 June 7, we present the first direct evidence of coronal magnetic reconnection between the fields of two adjacent active regions during a CME. The observations are presented jointly with a data-constrained numerical simulation, demonstrating the formation/intensification of current sheets along a hyperbolic flux tube at the interface between the CME and the neighboring AR 11227. Reconnection resulted in the formation of new magnetic connections between the erupting magnetic structure from AR 11226 and the neighboring active region AR 11227 about 200 Mm from the eruption site. The onset of reconnection first becomes apparent in the SDO/AIA images when filament plasma, originally contained within the erupting flux rope, is redirected toward remote areas in AR 11227, tracing the change of large-scale magnetic connectivity. The location of the coronal reconnection region becomes bright and directly observable at SDO/AIA wavelengths, owing to the presence of down-flowing cool, dense (1010 cm-3) filament plasma in its vicinity. The high-density plasma around the reconnection region is heated to coronal temperatures, presumably by slow-mode shocks and Coulomb collisions. These results provide the first direct observational evidence that CMEs reconnect with surrounding magnetic structures, leading to a large-scale reconfiguration of the coronal magnetic field.
Magnitude-range brightness variations of overactive K giants
Oláh, K.; Moór, A.; Kővári, Zs.; Granzer, T.;
Strassmeier, K. G.; Kriskovics, L.; Vida, K.
ADS link
Decades-long, phase-resolved photometry of overactive spotted cool stars has revealed that their long-term peak-to-peak light variations can be as large as one magnitude. Such brightness variations are too large to be solely explained by rotational modulation and/or a cyclic, or pseudo-cyclic, waxing and waning of surface spots and faculae as we see in the Sun.
We study three representative, overactive spotted K giants (IL Hya, XX Tri, and DM UMa) known to exhibit V-band light variations between 0.65m-1.05m. Our aim is to find the origin of their large brightness variation.
We employ long-term phase-resolved multicolor photometry, mostly from automatic telescopes, covering 42 yr for IL Hya, 28 yr for XX Tri, and 34 yr for DM UMa. For one target, IL Hya, we present a new Doppler image from NSO data taken in late 1996. Effective temperatures for our targets are determined from all well-sampled observing epochs and are based on a V - IC color-index calibration.
The effective temperature change between the extrema of the rotational modulation for IL Hya and XX Tri is in the range 50-200 K. The bolometric flux during maximum of the rotational modulation, i.e., the least spotted states, varied by up to 39% in IL Hya and up to 54% in XX Tri over the course of our observations. We emphasize that for IL Hya it is just about half of the total luminosity variation that can be explained by the photospheric temperature (spots/faculae) changes, while for XX Tri it is even about one third. The long-term, 0.m6 V-band variation of DM UMa is more difficult to explain because little or no B - V color index change is observed on the same timescale. Placing the three stars with their light and color variations into H-R diagrams, we find that their overall luminosities are generally too low compared to predictions from current evolutionary tracks.
A change in the stellar radius due to strong and variable magnetic fields during activity cycles likely plays a role in explaining the anomalous brightness and luminosity of our three targets. At least for IL Hya, a radius change of about 9% is suggested from mbol and Teff, and is supported by independent v sin i measurements.
XX Tri light, color, and temperature curves. a) V data (dots)
and their fits with a trend (dashed line), with co-added modulations of
timescales of 6 yr and 12 yr (line). Dotted line shows the cyclic term
from the fit of the V ż IC data. b) B ż V (upper dots) and U ż B (lower
dots). The U ż B data were artificially shifted by +0.
m65 for easier presentation.
c) V ż IC data (dots) and their fits with a trend (dashed line),
and with co-added modulation on a timescale of ~6.5 yr (line). The
seasonal average comp-check values are plotted (shifted by +1.
m64 for
presentation purpose) measured in the same time, and d) effective temperatures
from V ż IC at maxima (triangles, overplotted with the 6.5 yr
cycle originating from the V ż IC fit) and minima (squares) of the light
curves.
Lithium enrichment on the single active K1-giant DI Piscium. Possible joint origin of differential rotation and Li enrichment
Kriskovics, L.; Kővári, Zs.; Vida, K.;
Granzer, T.; Oláh, K.
ADS link
y
We investigate the surface spot activity of the rapidly rotating, lithium-rich active single K-giant DI Psc (which is after its first dredge-up phase, near the RGB bump) to measure the surface differential rotation and understand the mechanisms behind the Li-enrichment. Doppler imaging was applied to recover the surface temperature distribution of DI Psc in two subsequent rotational cycles using the individual mapping lines Ca i 6439, Fe i 6430, Fe i 6421, and Li i 6708. Surface differential rotation was derived by cross-correlation of the subsequent maps. Difference maps are produced to study the uniformity of Li-enrichment on the surface. These maps are compared with the rotational modulation of the Li i 6708 line equivalent width.
Doppler images obtained for the Ca and Fe mapping lines agree well and reveal strong polar spottedness, as well as cool features at lower latitudes. Cross-correlating the consecutive maps yields antisolar differential rotation with a shear coefficient α = -0.083 ± 0.021. The difference of the average and the Li maps show persisent features over the two rotations, and this may indicate that the lithium-enrichment is not activity related. A mutual explanation of the anti-solar differential rotation and the elevated lithium abundance is presented: enhanced outward flow, which is capable of carrying lithium rich material as well as angualar momentum towards the surface and the pole, resulting in an anti-solar differential rotational pattern.
Top row: Difference maps of rotation 1 and 2, and the average difference maps, respectively. Middle row: Averaged latitudinal change of temperature difference. Bottom row: measured equivalent widths.
Looking for activity cycles in late-type Kepler stars using time frequency analysis
We analyse light curves covering four years of 39 fast-rotating (Prot~1d) late-type active stars from the Kepler data base. Using time-frequency analysis (short-term Fourier transform), we find hints for
activity
cycles of 300-900 d at nine targets from the changing typical latitude of the starspots, which with the differential rotation of the stellar surface change the observed rotation period over the activity cycle. We also give
a lowest estimation for the shear parameter of the differential rotation, which is ~0.001 for the cycling targets. These results populate the less studied, short-period end of the rotation-cycle length relation.
STFT plot of KIC003541346. Period variations with a time scale of one year is apparent.
Photospheric and chromospheric activity in V405 Andromedae. An M dwarf binary with components on the two sides of the full convection limit
Vida, K.; Oláh, K.; Kővári, Zs.; Korhonen, H.; Bartus, J.; Hurta, Zs.; Posztobányi, K.
ADS link
We investigate the fast-rotating (Porb=Prot=0.465d), active dwarf binary V405 And (M0V+M5V) using photometric BV(RI)C and optical spectroscopic data. The light variation is caused by the combined effect of spottedness and binarity with a small eclipse. We estimate the system parameters from the available light and radial velocity curves. Three flare events occurred during the observations: two were found in the spectroscopic data and one was observed photometrically in BV(RI)C colours. An interesting eruptive phenomenon emerged from the photometric measurements that can be interpreted as a series of post-flare eruptions lasting for at least 3 orbits (rotations) of the system, originating from trans-equatorial magnetic loops, which connect the active regions in the two hemispheres. The two components of V405 And have masses well over and below the theoretical limit of full convection. This rare property makes the binary an ideal target for observing and testing models for stellar dynamo action.
Lightcurve of V405 And with the binary model.
At present only two systems are known whose primaries
are about 2.5 times more massive than the secondaries and one
component is above while the other is below the limit of the
full convection. The primary of V405 And deviates most from
the expected radius of its mass from the known sample. On the
other hand, the secondary of V405 And would be fully convective even in the presence of its strong magnetic field.
Mass-radius curve for 5 Gyr of known binaries. The most outlying point is V405 And..
Long-term photometric measurements in a sample of ultrashort-period (P~0.5 days or less) single and binary stars of different interior structures are analysed. A loose correlation exists between the rotational rate and cycle lengths of active stars, regardless of their evolutionary state and the corresponding physical parameters. The shortest cycles are expected on the fastest rotators in the order of 1-2 years, which is reported in this paper.
Correlation between the rotational period and the length of the activity cycle.
Doppler imaging of the double-lined active binary V824 Ara
Kriskovics, L.; Vida, K.; Kővári, Zs.t; Garcia-Alvarez, D.; Oláh, K.
ADS link
We introduce an iterative spectral disentangling technique combined with Doppler imaging in order to recover surface temperature maps for both components of double-lined active binary systems. Our method provides an opportunity to separate spectra of the active components while minimizing the unwanted disturbances on the given line profile from the other component. The efficiency of the method is demonstrated on real data of the double-lined RS CVn-type binary V824 Ara. The resulting Doppler images reveal cool spots on the polar regions as well as low-latitude features on both of the stars. Moreover, both components have hot spots, that are facing each other. This may indicate interconnection between the stellar magnetic fields.
Doppler images of the V824 Ara system plotted on spheres.
Measuring differential rotation of the K-giant ζ Andromedae
Kővári, Zs.; Bartus, J.; Strassmeier, K. G.; Oláh, K.; Weber, M.; Rice, J. B.; Washuettl, A.
ADS link
Kővári, Zs.; Korhonen, H.; Kriskovics, L.; Vida, K.; Donati, J.-F.; Le Coroller, H.; Monnier, J. D.; Pedretti, E.; Petit, P.
ADS link
We continously study the RS CVn-type binary ζ And. The star is a magnetically active K1 giant with its rotation synchronized to the 17.8-day orbital period. Our revised lithium abundance of ALi=1.2 places ζ And in the vicinity of Li-rich RGB stars but it is nevertheless a Li-normal chromospherically active binary star. The star seems to undergo its first standard dredge-up dilution.
Four consecutive Doppler images were obtained from a continuous 67-night observing run at NSO-McMath in 1996/97. An additional single image was obtained from a continuous 19-night run at KPNO in 1997/98. These unique data allow to compute a small time series of the evolution of the star's surface structure. All line-profile inversions are done with a modified TempMap version that takes into account the non-spherical shape of the star. Representative test reconstructions are performed and demonstrate the code's reliability and robustness.
High and low-latitude spot activity was recovered together with an asymmetric polar cap-like feature. The latter dominated the first half of the two-month time series in 1996/97. The second half showed mostly medium-to-high latitude activity and only a fainter polar spot. The coolest areas were restored with a temperature contrast of about 1000±200 K. Some weaker features at equatorial latitudes were also recovered but these could be partially spurious and appear blurred due to imperfect phase coverage. We use our line profiles to reconstruct an average non-sphericity of Rpole/Rpoint=0.96 which would, if not taken into account, mimic a temperature difference pole-to-equator of ≈220 K, especially at the phases of quadrature. Finally, we apply two different methods for restoring surface differential rotation and found a weak solar-type rotation law with a shear Δżż0.95°/day (α=ΔΩ/Ωeq=+0.049±0.003), i.e. roughly a factor of four weaker at a rotation rate roughly 1.5 times faster than the Sun's.
Doppler image of ζ And with the ellipticy taken into account.
With a new dataset (from three individual observing sites), Doppler imaging was used to confirm the differential rotational pattern of ζ And. Each dataset covers one full stellar rotation with good phase coverage, which results in a continuous coverage of almost three stellar rotations (Prot = 17.8 d). Therefore, these data are well-suited for reconstructing surface temperature maps and studying temporal evolution in spot configurations. Surface differential rotation was measured by the means of cross-correlation of all possible image pairs.
The individual Doppler reconstructions agree well in the revealed spot pattern, recovering numerous low-latitude spots with temperature contrasts of up to ż1000 K with respect to the unspotted photosphere, and also an asymmetric polar cap that is diminishing with time. Our detailed cross-correlation study consistently indicates solar-type differential rotation with an average surface shear ż ż 0.055, in agreement with former results.
The differential rotational pattern of ζ And.
Magnetic Topology of Active Regions and Coronal Holes: Implications for Coronal Outflows and the Solar Wind
van Driel-Gesztelyi, L.; Culhane, J. L.; Baker, D.; Démoulin, P.; Mandrini, C. H.; DeRosa, M. L.; Rouillard, A. P.; Opitz, A.; Stenborg, G.; Vourlidas, A.; Brooks, D. H.
ADS link
During 2 - 18 January 2008 a pair of low-latitude opposite-polarity coronal holes (CHs) were observed on the Sun with two active regions (ARs) and the heliospheric plasma sheet located between them. We use the Hinode/EUV Imaging Spectrometer (EIS) to locate AR-related outflows and measure their velocities. Solar-Terrestrial Relations Observatory (STEREO) imaging is also employed, as are the Advanced Composition Explorer (ACE) in-situ observations, to assess the resulting impacts on the solar wind (SW) properties. Magnetic-field extrapolations of the two ARs confirm that AR plasma outflows observed with EIS are co-spatial with quasi-separatrix layer locations, including the separatrix of a null point. Global potential-field source-surface modeling indicates that field lines in the vicinity of the null point extend up to the source surface, enabling a part of the EIS plasma upflows access to the SW. We find that similar upflow properties are also observed within closed-field regions that do not reach the source surface. We conclude that some of plasma upflows observed with EIS remain confined along closed coronal loops, but that a fraction of the plasma may be released into the slow SW. This suggests that ARs bordering coronal holes can contribute to the slow SW. Analyzing the in-situ data, we propose that the type of slow SW present depends on whether the AR is fully or partially enclosed by an overlying streamer.
Hinode/XRT image
for 7 January 2008. Markings
show a stream interaction region
(SIR1), a coronal hole (CH1), a
rarefaction region (RF1), and two
active regions (AR1 and AR2),
followed by a second stream
interaction region (SIR2), coronal
hole (CH2), and rarefaction
region (RF2). SIR1, RF1, and
RF2 locations are shown
projected radially on the Sun.
Long-term photometry of three active red giants in close binary systems: V2253 Oph, IT Com and IS Vir
Oláh, K.; Moór, A.; Strassmeier, K. G.; Borkovits, T.; Granzer, T.
ADS link
We present and analyze long-term optical photometric measurements of the three active stars V2253 Oph, IT Com and IS Vir. All three systems are single-lined spectroscopic binaries with an early K giant as primary component but in different stages of orbital-rotational synchronization. Our photometry is supplemented by 2MASS and WISE near-IR and mid-IR magnitudes and then used to obtain more accurate effective temperatures and extinctions. For V2253 Oph and IT Com, we found their spectral energy distributions consistent with pure photospheric emission. For IS Vir, we detect a marginal mid-IR excess which hints towards a dust disk. The orbital and rotational planes of IT Com appear to be coplanar, contrary to previous findings in the literature. We apply a multiple frequency analysis technique to determine photometric periods, and possibly changes of periods, ranging from days to decades. New rotational periods of 21.55±0.03 d, 65.1±0.3 d, and 23.50ą0.04 d were determined for V2253 Oph, IT Com, and IS Vir, respectively. Splitting of these periods led to tentative detections of differential surface rotations of δ P/Pż0.02 for V2253 Oph and 0.07 for IT Com. Using a time-frequency technique based on short-term Fourier transforms we present evidence of cyclic light variations of length ż 10 yr for V2253 Oph and 5-6 yr for IS Vir. A single flip-flop event has been observed for IT Com of duration 2-3 yr. Its exchange of the dominant active longitude had happened close to a time of periastron passage, suggesting some response of the magnetic activity from the orbital dynamics. The 21.55-d rotational modulation of V2253 Oph showed phase coherence also with the orbital period, which is 15 times longer than the rotational period, thus also indicating a tidal feedback with the stellar magnetic activity.
Time-frequency diagram of V2253 Oph.
The lithium-rich single K-giants DP Canum Venaticorum and DI Piscium
Kővári, Zs.; Korhonen, H.; Strassmeier, K. G.; Weber, M.; Kriskovics, L.; Savanov, I.
ADS link
We present the first Doppler imaging study of the two rapidly rotating, single K-giants DP CVn and DI Psc in order to study the surface spot configuration and to pinpoint their stellar evolutionary status.
Optical spectroscopy and photometry were used to determine the fundamental astrophysical properties. Doppler imaging was applied to recover the surface temperature distribution for both stars, while photometric light-curve inversions were carried out for studying the long-term changes of the surface activity of DP CVn. Surface differential rotation of DP CVn was estimated from cross-correlating the available subsequent Doppler reconstructions separated by roughly one rotation period.
Both stars appear to have higher than normal lithium abundance, LTE log n of 2.28 (DP CVn) and 2.20 (DI Psc), and are supposed to be located at the end of the first Li dredge-up on the RGB. Photometric observations reveal rotational modulation with a period of 14.010 d (DP CVn) and 18.066 d (DI Psc). Doppler reconstructions from the available mapping lines well agree in the revealed spot patterns, recovering rather low latitude spots for both stars with temperature contrasts of ΔT ≈ 600-800 K below the unspotted photospheric background. Spots at higher latitudes are also found but either with less contrast (DP CVn) or with smaller extent (DI Psc). A preliminary antisolar-type differential rotation with α = -0.035 is found for DP CVn from cross-correlating the subsequent Doppler images. Long-term photometric analysis supports the existence of active longitudes, as well as the differential rotation.
The differential rotational pattern of DP CVn.
Four-colour photometry of EY Dra: A study of an ultra-fast rotating active dM1-2e star
Vida, K.; Oláh, K.; Kővári, Zs.; Jurcsik, J.; Sódor, Á.; Váradi, M.; Belucz, B.; Dékány, I.; Hurta, Zs.; Nagy, I.; Posztobányi, K.
ADS link
We present more than 1000-day long photometry of EY Draconis in BV(RI)_C passbands. The changes in the light curve are caused by the spottedness of the rotating surface. Modelling of the spotted surface shows that there are two large active regions present on the star on the opposite hemispheres. The evolution of the surface patterns suggests a flip-flop phenomenon. Using Fourier analysis, we detect a rotation period of P_rot=0.45875 d, and an activity cycle with Pż350 d, similar to the 11-year long cycle of the Sun. This cycle with its year-long period is the shortest one ever detected on active stars. Two bright flares are also detected and analysed.
Lightcurve of EY Dra with the rotatonal phases.
Anti-solar differential rotation and surface flow pattern on UZ Librae
Vida, K.; Kővári, Zs.; żvanda, M.; Oláh, K.; Strassmeier, K. G.; Bartus, J.
Oláh, K.; Jurcsik, J.; Strassmeier, K. G.
ADS link
We re-investigate UZ Librae spectra obtained at KPNO in 1998 and 2000. From the 1998 data we compose 11 consecutive Doppler images using the Ca I-6439, Fe I-6393 and Fe I-6411 lines. Applying the method of average cross-correlation of contiguous Doppler images we find anti-solar differential rotation with a surface shear of żż-0.03. The pilot application of the local correlation tracking technique for the same data qualitatively confirms this result and indicates complex flow pattern on the stellar surface. From the cross-correlation of the two available Doppler images in 2000 we also get anti-solar differential rotation but with a much weaker shear of żż-0.004.
Animation of average UZ Lib Doppler images from 1998,.
Young solar type active stars: the TYC 2627-638-1 system
Oláh, K.; Korhonen, H.; Vida, K.; Ilyin, I.; Dall, T. H.; Järvinen, S. P.; Jurcsik, J.; Andersen, M. I.; Djupvik, A. A.; Pursimo, T.; Moór, A.; Datson, J.; Karjalainen, R.; Liimets, T.; Kubát, J.; Kawka, A.
ADS link
We present BV(RI)C and JHKs photometry and low- and high-resolution spectroscopy of the 11th mag G-type star TYC 2627-638-1. Our investigation reveals that the target is separated into two young, early-G-type main-sequence (or late pre-main-sequence) stars, which are most probably bound and form a wide binary system. A substellar body orbits the brighter component as implied by radial velocity variations. The brighter component possibly also has a faint, later type stellar companion. Both components of the wide binary have clear emission cores in the Ca ii H&K lines and filled-in Hż absorption, indicating that both stars are chromospherically active. Our photometric time series reveals clear but only a few hundredths of a magnitude amplitude rotational modulation, which is most likely due to cool starspots. Two distinct periods, near 3.5 and 3.7 days, are found in the brightness variations. Photometry obtained separately of the two components of the wide binary show that these periodicities belong to the brighter star. The fainter component shows a much slower light variation of about 0.3 mag. amplitude. In addition, long-term changes in the brightness of both stars are seen. The spectral energy distribution shows a strong near-infrared excess in the fainter component of the wide binary.
The TYC 2627-638-1 system.
Surface velocity network with anti-solar differential rotation on the active K-giant σ Geminorum
Kővári, Zs.; Bartus, J.; żvanda, M.; Vida, K.; Strassmeier, K. G.; Oláh, K.; Forgács-Dajka, E.
ADS Link
Kővári, Zs.; Bartus, J.; Strassmeier, K. G.; Vida, K.; żvanda, M.; Oláh, K.
ADS link
The active K1 giant ż Gem and its differential surface rotation is revisited. We refine our previous inconclusive result by recovering the spot migration pattern of this long-period RS CVn-type binary through application of the technique of żaverage cross-correlation of contiguous Doppler imagesż to a set of six Doppler images from 3.6 consecutive rotation cycles. We find an anti-solar differential rotation law with a shear of ż≈ -0.022±0.006. We also find evidence of a poleward migration trend of spots with an average velocity of ≈300 m/s.
Pattern of the meridional flow on σ Gem.
Multiple and changing cycles of active stars
Kolláth, Z.; Oláh, K.
ADS Link
Oláh, K.; Kolláth, Z.; Granzer, T.; Strassmeier, K. G.; Lanza, A. F.; Järvinen, S.; Korhonen, H.; Baliunas, S. L.; Soon, W.; Messina, S.; Cutispoto, G.
ADS Link
Long-term observational data have information on the magnetic cycles of active stars and that of the Sun. The changes in the activity of our central star have basic effects on Earth, such as variations in the global climate, so that understanding the nature of these variations is extremely important.
The observed variations related to magnetic activity cannot be treated as stationary periodic variations, therefore methods like Fourier transform or different versions of periodograms only give partial information on the nature of the light variability. We demonstrate that time-frequency distributions provide useful tools for analysing the observations of active stars.
Methods: We tested and used different methods, such as short-term Fourier transform, wavelet, and generalised time-frequency distributions, for analysing temporal variations in timescales of observational data.
With test data we demonstrate that the observational noise has practically no effect on the determination in the long-term changes of time-series observations of active stars. The rotational signal may modify the determined cycles, therefore it is advisable to remove it from the data. Wavelets are less powerful in recovering complex long-term changes than other distributions that are discussed. By applying our technique to the sunspot data we find a complicated, multi-scale evolution in the solar activity.
Short-term Fourier transform (bottom) of the sunspot number time series (top).
We also studied the time variations in the cycles of 20 active stars based on decade-long photometric or spectroscopic observations. A method of time-frequency analysis, as discussed above, was applied to the data.
Fifteen stars definitely show multiple cycles, but the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics and vary in parallel, indicating a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars.
Conclusions: Stellar activity cycles are generally multiple and variable.
STFT analysis of LQ Hya.
Created by E. Forgács-Dajka
SOLar and STellar Activity Research Team - SOLSTART, 2007
Date:
