T. Hackman

Surface imaging of HD 199178 (V1794 Cygni)

We present surface temperature maps for the FK Comae-type star HD 199178 (V1794 Cygni) calculated from high resolution spectra obtained in 1994 and 1995. The spot pattern evolves, but all maps reveal a large cool spot remaining nearly at the same high latitude. The main spot is 1200-1600 K cooler than the mean surface temperature. The observed slightly flat bottomed absorption lines would usually be interpreted as evidence for a large cool polar spot. We argue that antisolar surface differential rotation offers a better explanation for the box like shape of the line profiles. However, we do not find conclusive evidence for antisolar differential rotation and note that there are still other possible explanations for the slightly flat bottomed line profiles.

Magnetic field topology of the RS CVn star II Pegasi

Context. 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, often yielding surprising and controversial results. Aims. In this study we follow a long-term evolution of the magnetic field topology of the RS CVn binary star II Peg using a more self-consistent and physically more meaningful modelling approach compared to previous ZDI studies. Methods. 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, comprising one of the most comprehensive spectropolarimetric data sets acquired for a cool active star. A multi-line diagnostic technique in combination with a new ZDI code is applied to interpret these observations. Results. We have succeeded in detecting clear magnetic field signatures in average Stokes V profiles for all 12 data sets. These profiles typically have complex shapes and amplitudes of similar to 10(-3) of the unpolarised continuum, corresponding to mean longitudinal fields of 50-100 G. 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 on the time scale covered by our observations. 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. Conclusions. 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 alpha Omega dynamo. Our results do not show a clear active longitude system, nor is there evidence of the presence of an azimuthal dynamo wave.

Flip-flops of FK Comae Berenices

Context. FK Comae Berenices is a rapidly rotating magnetically active star, the light curve of which is modulated by cool spots on its surface. It was the first star where the “flip-flop” phenomenon was discovered. Since then, flip-flops in the spot activity have been reported in many other stars. Follow-up studies with increasing length have shown, however, that the phenomenon is more complex than was thought right after its discovery. Aims. Therefore, it is of interest to perform a more thorough study of the evolution of the spot activity in FK Com. In this study, we analyse 15 years of photometric observations with two different time series analysis methods, with a special emphasis on detecting flip-flop type events from the data. Methods. We apply the continuous period search and carrier fit methods on long-term standard Johnson-Cousins V-observations from the years 1995−2010. The observations were carried out with two automated photometric telescopes, Phoenix-10 and Amadeus T7 located in Arizona. Results. We identify complex phase behaviour in 6 of the 15 analysed data segments. We identify five flip-flop events and two cases of phase jumps, where the phase shift is Δ φ 0.031. Conclusions. The flip-flop cannot be interpreted as a single phenomenon, where the main activity jumps from one active longitude to another. In some of our cases the phase shifts can be explained by differential rotation: two spot regions move with different angular velocity and even pass each other. Comparison between the methods show that the carrier fit utility is better in retrieving slow evolution especially from a low amplitude light curve, while the continuous period search is more sensitive in case of rapid changes.

Dynamical evolution of titanium, strontium, and yttrium spots on the surface of the HgMn star HD 11753

Aims. We gathered about 100 high-resolution spectra of three typical HgMn (mercury-manganese) stars, HD 11753, HD 53244, and HD 221507, to search for slowly pulsating B-like pulsations and surface inhomogeneous distribution of various chemical elements. Methods. Classical frequency analysis methods were used to detect line profile variability and to determine the variation period. Doppler imaging reconstruction was performed to obtain abundance maps of chemical elements on the stellar surface. Results. For HD 11753, which is the star with the most pronounced variability, distinct spectral line profile changes were detected for Ti, Sr, Y, Zr, and Hg, whereas for HD 53244 and HD 221507 the most variable line profiles belong to the elements Hg and Y, respectively. We derived rotation periods for all three stars from the variations of radial velocities and equivalent widths of spectral lines belonging to inhomogeneously distributed elements: Prot (HD 11753) = 9.54 d, Prot (HD 53244) = 6.16 d, and Prot (HD 221507) = 1.93 d. For HD 11753 the Doppler imaging technique was applied to derive the distribution of the most variable elements Ti, Sr, and Y using two datasets separated by ∼65 days. Results of Doppler imaging reconstruction revealed noticeable changes in the surface distributions of Ti ii ,S rii ,a nd Yii between the datasets, indicating the hitherto not well understood physical processes in stars with radiative envelopes that cause a rather fast dynamical chemical spot evolution.

Study of FK Comae Berenices V. Spot evolution and detection of surface differential rotation

Aims. We investigate the spot evolution and the surface differential rotation of the single late-type giant FK Com. Methods. A total of 18 new surface temperature maps of FK Com are calculated with the Doppler imaging technique for the years 1993–2003. Photometric observations from 2002–2004 are also given. The new and previously published spectroscopic and photometric observations are used to study the spot locations and lifetimes, and to estimate the value of the surface differential rotation. Results. The phases of the active regions determined from the Doppler images follow closely the active longitudes determined earlier from the long-term photometric observations. One active longitude can remain active for several years, but the exact spot configuration within the active longitude changes on much shorter time scales, indicating that the spot lifetime is months instead of years. There are periods during which the spot configuration changes even within days. Measurements using spot latitudes from the Doppler images and spot rotation periods from the photometric observations yield a surface differential rotation law of Ω= (151.30 ◦ /day ± 0.09 ◦ /day) − (1.78 ◦ /day ± 0.12 ◦ /day) sin 2 ψ and the relative differential rotation coefficient α = 0.012 ± 0.002 for FK Comae.

STARSPOTS DUE TO LARGE-SCALE VORTICES IN ROTATING TURBULENT CONVECTION

We study the generation of large-scale vortices in rotating turbulent convection by means of Cartesian direct numerical simulations. We find that for sufficiently rapid rotation, cyclonic structures on a scale large in comparison to that of the convective eddies emerge, provided that the fluid Reynolds number exceeds a critical value. For slower rotation, cool cyclonic vortices are preferred, whereas for rapid rotation, warm anti-cyclonic vortices are favored. In some runs in the intermediate regime both types of cyclones coexist for thousands of convective turnover times. The temperature contrast between the vortices and the surrounding atmosphere is of the order of 5%. We relate the simulation results to observations of rapidly rotating late-type stars that are known to exhibit large high-latitude spots from Doppler imaging. In many cases, cool spots are accompanied with spotted regions with temperatures higher than the average. In this paper, we investigate a scenario according to which of the spots observed in the temperature maps could have a non-magnetic origin due to large-scale vortices in the convection zones of the stars.

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.

Doppler images of the RS CVn binary II Pegasi during the years 1994–2002

Aims. We publish 16 Doppler imaging temperature maps for the years 1994-2002 of the active RS CVn star II Peg. The six maps from 1999-2002 are based on previously unpublished observations. Through Doppler imaging we want to study the spot evolution of the star and in particular compare this with previous results showing a cyclic spot behaviour and persistent, active longitudes. Methods. The observations were collected with the SOFIN spectrograph at the Nordic Optical Telescope. The temperature maps were calculated using a Doppler imaging code based on Tikhonov regularization. Results. During 1994-2001, our results show a consistent trend in the derived longitudes of the principal and secondary temperature minima over time such that the magnetic structure appears to rotate somewhat more rapidly than the orbital period of this close binary. A sudden phase jump in the active region occurred between the observing seasons of 2001 and 2002. No clear trend over time is detected in the derived latitudes of the spots, indicating that the systematic motion could be related to the drift of the spot-generating mechanism rather than to differential rotation. The derived temperature maps are quite similar to the ones obtained earlier with different methods and the main differences occur in the spot latitudes and relative strength of the spot structures. Conclusions. We observe both longitude and latitude shifts in the spot activity of II Peg. However, our results are not consistent with the periodic behaviour presented in previous studies.

Spot activity of LQ Hydra from photometry between 1988 and 2011

Aims. We investigate the spot activity of the young magnetically active main sequence star LQ Hya. Our aims are to identify possible active longitudes, estimate the differential rotation, and study long and short term changes in the activity. Methods. Our analysis is based on 24 years of Johnson V-band photometry of LQ Hya obtained with the T3 0.4 m Automated Telescope at the Fairborn Observatory. We use the previously published continuous period search (CPS) method to model the evolution of the light curve of LQ Hya. The CPS fits a Fourier series model to short overlapping subsets of data. This enables us to monitor the evolution of the light curve and thus the spot configuration of the star with a higher time resolution. Results. We find seasonal variability in the mean level and amplitude of the light curve of LQ Hya. The variability of the light curve amplitude seems not to be cyclic, but the long-term variations in the mean magnitude may be indicative of an approximately 13 year cycle. However, because of the limited length of the observed time series, it is not yet possible to determine whether this structure really represents an activity cycle. Based on fluctuations of the light curve period, we estimate the differential rotation of the star to be small, and the star is potentially very close to a rigid rotator. We search for active longitudes from the inferred epochs of the light curve minima. We find that on time scales up to six months there are typically one or two relatively stable active areas on the star with limited phase migration. On the other hand, on time scales longer than one year, no stable active longitudes have been present except for the period between 2003 and 2009 and possibly also some time before 1995. Neither do we find any signs of flip-flops with a regular period. The mean time scale of change of the light curve during the observation period is determined to be of the same order of magnitude as the estimated convective turnover time for the star.

The continuous period search method and its application to the young solar analogue HD 116956

Aims. We formulate an improved time series analysis method for the analysis of photometry of active stars. This new continuous period search (CPS) method is applied to 12 years of V band photometry of the young solar analogue HD 116956 (NQ UMa). Methods. The new method is developed from the previous three stage period analysis (TSPA) method. Our improvements are the use of a sliding window in choosing the modelled datasets, a criterion applied to select the best model for each dataset and the computation of the time scale of change of the light curve. We test the performance of CPS with simulated and real data. Results. The CPS has a much improved time resolution which allows us to better investigate fast evolution of stellar light curves. We can also separate between the cases when the data is best described by periodic (i.e. rotational modulation of brightness) and aperiodic (e.g. constant brightness) models. We find, however, that the performance of the CPS has certain limitations. It does not determine the correct model complexity in all cases, especially when the underlying light curve is constant and the number of observations too small. Also the sensitivity in detecting two close light curve minima is limited and it has a certain amount of intrinsic instability in its period estimation. Using the CPS, we find persistent active longitudes in the star HD 116956 and a “flip-flop” event that occurred during the year 1999. Assuming that the surface differential rotation of the star causes observable period variations in the stellar light curve, we determine the differential rotation coefficient to be |k| > 0.11. The mean timescale of change of the light curve during the whole 12 year observing period was T C = 44.1 d, which is of the same order as the predicted convective turnover time of the star. We also investigate the presence of activity cycles on the star, but do not find any conclusive evidence supporting them.