Magnetic field topology of the RS CVn star II Pegasi

The dynamo processes in cool active stars generate complex magnetic fields responsible for prominent surface stellar activity and variability at different time scales. For a small number of cool stars magnetic field topologies were reconstructed from the time series of spectropolarimetric observations using the Zeeman Doppler imaging (ZDI) method. In this study we follow a long-term evolution of the magnetic field topology of the RS CVn binary star II Peg. We collected high-resolution circular polarisation observations of II Peg using the SOFIN spectropolarimeter at the Nordic Optical Telescope. These data cover 12 epochs spread over 7 years. A multi-line diagnostic technique in combination with a new ZDI code is applied to interpret these observations. Magnetic inversions using these data reveals evolving magnetic fields with typical local strengths of 0.5-1.0 kG and complex topologies. Despite using a self-consistent magnetic and temperature mapping technique, we do not find a clear correlation between magnetic and temperature features in the ZDI maps. Neither do we confirm the presence of persistent azimuthal field rings found in other RS CVn stars. Reconstruction of the magnetic field topology of II Peg reveals significant evolution of both the surface magnetic field structure and the extended magnetospheric field geometry. From 2004 to 2010 the total field energy drastically declined and the field became less axisymmetric. This also coincided with the transition from predominantly poloidal to mainly toroidal field topology. A qualitative comparison of the ZDI maps of II Peg with the prediction of dynamo theory suggests that the magnetic field in this star is produced mainly by the turbulent alpha^2 dynamo rather than the solar alphaOmega dynamo. Our results do not show a clear active longitude system, nor is there an evidence of the presence of an azimuthal dynamo wave.Comment: 20 pages, 10 figures; accepted for publication in Astronomy & Astrophysic

Doppler images of II Pegasi for 2004-2010

Aims. We study the spot activity of II Peg during the years 2004-2010 to determine long- and short-term changes in the magnetic activity. In a previous study, we detected a persistent active longitude, as well as major changes in the spot configuration occurring on a timescale of shorter than a year. The main objective of this study is to determine whether the same phenomena persist in the star during these six years of spectroscopic monitoring. Methods. The observations were collected with the high-resolution SOFIN spectrograph at the Nordic Optical Telescope. The temperature maps were calculated using a Doppler imaging code based on Tikhonov regularization. Results. We present 12 new temperature maps that show spots distributed mainly over high and intermediate latitudes. In each image, 1-3 main active regions can be identified. The activity level of the star is clearly lower than during our previous study for the years 1994-2002. In contrast to the previous observations, we detect no clear drift of the active regions with respect to the rotation of the star. Conclusions. Having shown a systematic longitudinal drift of the spot-generating mechanism during 1994-2002, the star has clearly switched to a low-activity state for 2004-2010, during which the spot locations appear more random over phase space. It could be that the star is near to a minimum of its activity cycle.Comment: Accepted for publication in Astron. and Astrophys., 8 pages, 5 figure

Doppler images and the underlying dynamo. The case of AF Leporis

The (Zeeman-)Doppler imaging studies of solar-type stars very often reveal large high-latitude spots. This also includes F stars that possess relatively shallow convection zones, indicating that the dynamo operating in these stars differs from the solar dynamo. We aim to determine whether mean-field dynamo models of late-F type dwarf stars can reproduce the surface features recovered in Doppler maps. In particular, we wish to test whether the models can reproduce the high-latitude spots observed on some F dwarfs. The photometric inversions and the surface temperature maps of AF Lep were obtained using the Occamian-approach inversion technique. Low signal-to-noise spectroscopic data were improved by applying the least-squares deconvolution method. The locations of strong magnetic flux in the stellar tachocline as well as the surface fields obtained from mean-field dynamo solutions were compared with the observed surface temperature maps. The photometric record of AF Lep reveals both long- and short-term variability. However, the current data set is too short for cycle-length estimates. From the photometry, we have determined the rotation period of the star to be 0.9660+-0.0023 days. The surface temperature maps show a dominant, but evolving, high-latitude (around +65 degrees) spot. Detailed study of the photometry reveals that sometimes the spot coverage varies only marginally over a long time, and at other times it varies rapidly. Of a suite of dynamo models, the model with a radiative interior rotating as fast as the convection zone at the equator delivered the highest compatibility with the obtained Doppler images.Comment: accepted for publication in Astronomy & Astrophysic

The first close-up of the "flip-flop" phenomenon in a single star

We present temperature maps of the active late-type giant FK Com which exhibit the first imagining record of the ``flip-flop'' phenomenon in a single star. The phenomenon, in which the main part of the spot activity shifts 180 degrees in longitude, discovered a decade ago in FK Com, was reported later also in a number of RS CVn binaries and a single young dwarf. With the surface images obtained right before and after the ``flip-flop'', we clearly show that the ``flip-flop'' phenomenon in FK Com is caused by changing the relative strengths of the spot groups at the two active longitudes, with no actual spot movements across the stellar surface, i.e. exactly as it happens in other active stars.Comment: 4 pages, accepted by A&A Letter

Superfluid to normal phase transition and extreme regularity of superdeformed bands

We derive the exact semiclassical expression for the second inertial parameter $\cal B$ for the superfluid and normal phases. Interpolation between these limiting values shows that the function ${\cal B}(I)$ changes sign at the spin $I_c$, which is critical for a rotational spectrum. The quantity $\cal B$ turns out to be a sensitive measure of the change in static pairing correlations. The superfluid-to-normal transition reveals itself in the specific variation of the ratio ${\cal B}/{\cal A}$ versus spin $I$ with the plateau characteristic of the normal phase. We find this dependence to be universal for normal deformed and superdeformed bands. The long plateau with a small value ${\cal B}/{\cal A}\sim A^{-8/3}$ explains the extreme regularity of superdeformed bands.Comment: 30 pages in LaTeX, 6 figures (PostScript). To be published in Yadernaya Fizika (Physics of Atomic Nuclei), special edition dedecated to the 90th birthday of Prof. I. I. Gurevit

The magnetic field and the evolution of element spots on the surface of the HgMn eclipsing binary AR Aur

The system AR Aur is a young late B-type double-lined eclipsing binary with a primary star of HgMn peculiarity. We applied the Doppler imaging method to reconstruct the distribution of Fe and Y over the surface of the primary using spectroscopic time series obtained in 2005 and from 2008 October to 2009 February. The results show a remarkable evolution of the element distribution and overabundances. Measurements of the magnetic field with the moment technique using several elements reveal the presence of a longitudinal magnetic field of the order of a few hundred gauss in both stellar components and a quadratic field of the order of 8 kG on the surface of the primary star.Fil: Hubrig, S.. Astrophysikalisches Institut Potsdam; AlemaniaFil: Savanov, I.. Russian Academy of Sciences. Institute of Astronomy; RusiaFil: Ilyin, I.. Astrophysikalisches Institut Potsdam; AlemaniaFil: Gonzalez, Jorge Federico. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico San Juan. Instituto de Ciencias Astronomicas de la Tierra y del Espacio; ArgentinaFil: Korhonen, H.. European Southern Observatory; AlemaniaFil: Lehmann, H.. Thuringer Landessternwarte; AlemaniaFil: Schöller, M.. European Southern Observatory; AlemaniaFil: Granzer, T.. Astrophysikalisches Institut Potsdam; AlemaniaFil: Weber, M.. Astrophysikalisches Institut Potsdam; AlemaniaFil: Strassmeier, K. G.. Astrophysikalisches Institut Potsdam; AlemaniaFil: Hackman, T.. Thuringer Landessternwarte; AlemaniaFil: Tkachenko, A.. Thuringer Landessternwarte; Argentin

Long-term spot monitoring of the young solar analogue V889 Herculis

Context. Starspots are important manifestations of stellar magnetic activity. By studying their behaviour in young solar analogues, we can unravel the properties of their magnetic cycles. This gives crucial information of the underlying dynamo process. Comparisons with the solar cycle enable us to infer knowledge about how the solar dynamo has evolved during the Sun's lifetime. Aims. Here we study the correlation between photometric brightness variations, spottedness, and mean temperature in V889 Her, a young solar analogue. Our data covers 18 years of spectroscopic and 25 years of photometric observations. Methods. We use Doppler imaging to derive temperature maps from high-resolution spectra. We use the Continuous Period Search method to retrieve mean V-magnitudes from photometric data. Results. Our Doppler imaging maps show a persistent polar spot structure varying in strength. This structure is centred slightly off the rotational pole. The mean temperature derived from the maps shows an overall decreasing trend, as does the photometric mean brightness, until it reaches its minimum around 2017. The filling factor of cool spots, however, shows only a weak tendency to anti-correlate with the decreasing mean brightness. Conclusions. We interpret V889 Her to have entered into a grand maximum in its activity. The clear relation between the mean temperature of the Doppler imaging surface maps and the mean magnitude supports the reliability of the Doppler images. The lack of correlation between the mean magnitude and the spottedness may indicate that bright features in the Doppler images are real.Peer reviewe

Differences in the solar cycle variability of simple and complex active regions during 1996-2018

Aims. Our aim is to examine the solar cycle variability of magnetically simple and complex active region. Methods. We studied simple (alpha and beta) and complex (beta gamma and beta gamma delta) active regions based on the Mount Wilson magnetic classification by applying our newly developed daily approach. We analyzed the daily number of the simple active regions (SARs) and compared that to the abundance of the complex active regions (CARs) over the entire solar cycle 23 and cycle 24 until December 2018. Results. We show that CARs evolve differently over the solar cycle from SARs. The time evolution of SARs and CARs on different hemispheres also shows differences, even though on average their latitudinal distributions are shown to be similar. The time evolution of SARs closely follows that of the sunspot number, and their maximum abundance was observed to occur during the early maximum phase, while that of the CARs was seen roughly two years later. We furthermore found that the peak of CARs was reached before the latitudinal width of the activity band starts to decease. Conclusion. Our results suggest that the active region formation process is a competition between the large-scale dynamo (LSD) and the small-scale dynamo (SSD) near the surface, the former varying cyclically and the latter being independent of the solar cycle. During solar maximum, LSD is dominant, giving a preference to SARs, while during the declining phase the relative role of SSD increases. Therefore, a preference for CARs is seen due to the influence of the SSD on the emerging flux.Peer reviewe