Th. Granzer

Multiple and changing cycles of active stars II. Results

Aims. We study the time variations in the cycles of 20 active stars based on decade-long photometric or spectroscopic observations. Methods. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data. Results. 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.

First time-series optical photometry from Antarctica sIRAIT monitoring of the RS CVn binary V841 Centauri and the δ-Scuti star V1034 Centauri

Context. Beating the Earth’s day-night cycle is mandatory for long and continuous time-series photometry and had been achieved with either large ground-based networks of observatories at different geographic longitudes or when conducted from space. A third possibility is offered by a polar location with astronomically-qualified site characteristics. Aims. In this paper, we present the first scientific stellar time-series optical photometry from Dome C in Antarctica and analyze approximately 13,000 CCD frames taken in July 2007. Methods. The optical pilot telescope of the “International Robotic Antarctic Infrared Telescope”, named “small IRAIT” (sIRAIT), and its UBV RI CCD photometer were used in BV R for a continuous 243 hours (10.15 days) with a duty cycle of 98% and a cadence of 155 sec. The prime targets were the chromospherically active, spotted binary star V841 Cen and the non-radially pulsating �-Scuti star V1034 Cen. Results. We confirm the known 0.2-day fundamental period of V1034 Cen and found a total of 23 further periods between 2.2 hours and 3.5 days. V841 Cen’s V amplitude due to spots appeared to be at a record high of 0. 4 in V in July 2007. We present a spot-model analysis with a light-curve inversion technique and found the star with a spot filling factor of 44% of the visible hemisphere, among the highest ever measured values for active stars, and a temperature difference photosphere minus spot of 750±100 K. Its odd-numbered (for a single site) rotation period was determined with a higher precision than before (5.8854±0.0026 days) despite the comparably short data set. The rms scatter from a 2.4-hour data subset was 3 mmag in V and 4.2 mmag in R. The differential data quality is 3–4 times better than with the 25cm Fairborn Automatic Photoelectric Telescope in southern Arizona and is likely due to the exceptionally low scintillation noise at Dome C. Conclusions. We conclude that high-precision CCD photometry with exceptional time coverage and cadence can be obtained at Dome C in Antarctica and be successfully used for time-series astrophysics.

Orbital period modulation in close binaries due to cyclic 2 -dynamo activity

The question is answered whether dynamo-generated magnetic fields are able to produce such quadrupole terms in the gravity potential which can explain the observed cyclic orbital variation of RS CVn stars. We start with spherical dynamo models with outer convection zones but without any dierential rotation, i.e. with 2 -dynamos which are known as nonoscil- lating. With the known anisotropic -tensor of rapidly rotating stars the magnetic modes with the lowest dynamo numbers are nonaxisymmetric with a slow azimuthal drift. We also find, however, stable (i.e. with the lowest dynamo number) axisymmet- ric oscillating modes but only for a very special, highly inhomogeneous -tensor. The dynamo model is a linear one with an arbitrary field amplitude which can be scaled in order to reproduce the observations. The star proves to be as prolate during the maximum of the toroidal field energy and it proves to be as oblate during the maximum of the poloidal field energy. In the time average the influence of the toroidal field dominates and the star is slightly prolate. From the computed temporal variations of the gravitation quadrupole moment a magnetic field of more than 10 5 G is found in order to produce the period modulation of order 10 5 which has been observed.

A “Kleine-Nacht-Musik” during the first five years of routine operation

We present a summary of five years of continuous operation of the University of Vienna twin Automatic Photoelectric Telescopes (APTs) – Wolfgang and Amadeus. These two telescopes are part of the Fairborn Observatory facility located in the Sonoran desert close to Washington Camp in southern Arizona. The detection and distinction procedure between weather-induced data-quality loss and systematic data-quality loss turned out to be a crucial task. Therefore, special emphasis is laid on the data quality monitoring tools developed throughout the years. Furthermore, we summarize the scientific highlights from the first five years of operation.

Ellipsoidal primary of the RS CVn binary ζ Andromedae - Investigation using high-resolution spectroscopy and optical interferometry

Aims. We have obtained high-resolution spectroscopy, optical interferometry, and long-term broad band photometry of the ellipsoidal primary of the RS CVn-type binary system ζ And. These observations are used to obtain fundamental stellar parameters and to study surface structures and their temporal evolution. Methods. Temperature maps of the stellar surface were obtained from high-resolution spectra with Doppler imaging techniques. These spectra were also used to investigate the chromospheric activity using the Hα line and to correlate it with the photospheric activity. The possible cyclicity in the spot activity was investigated from the long-term broad band photometry. Optical interferometry was obtained during the same time period as the high-resolution spectra. These observations were used to derive the size and fundamental parameters of ζ And. Results. Based on the optical interferometry the apparent limb darkened diameter of ζ And is 2.55 ± 0.09 mas using a uniform disk fit. The expected ∼4% maximum difference between the long and short axes of the ellipsoidal stellar surface cannot be confirmed from the current data which have 4% errors. The Hipparcos distance and the limb-darkened diameter obtained with a uniform disk fit give stellar radius of 15.9 ± 0.8 R� , and combined with bolometric luminosity, it implies an effective temperature of 4665 ± 140 K. The temperature maps obtained from Doppler imaging show a strong belt of equatorial spots and hints of a cool polar cap. The equatorial spots show a concentration around the phase 0.75, i.e., 0.25 in phase from the secondary, and another concentration spans the phases 0.0‐0.4. This spot configuration is reminiscent of the one seen in the earlier published temperature maps of ζ And. Investigation of the Hα line reveals both prominences and cool clouds in the chromosphere. These features do not seem to have a clearly preferred location in the binary reference frame, nor are they strongly associated with the cool photospheric spots. The investigation of the long-term photometry spanning 12 years shows hints of a spot activity cycle, which is also implied by the Doppler images, but the cycle length cannot be reliably determined from the current data.

On the rotation period of Capella

We present differential Hα and Hβ photometry of the very bright RS CVn-binary α Aurigae (Capella)obtained with theVienna automatic photoelectric telescope in the years 1996 through 2000. Low-level photometric variations of up to 0m.04 are detected in Hα. A multifrequency analysis suggests two real periods of 106 ± 3 days and 8.64 ± 0.09 days, that we interpret to be the rotation periods of the cool and the hot component of the Capella binary, respectively. These periods confirm that the hotter component of Capella rotates asynchronously, while the cooler component appears to be synchronized with the binary motion. The combined Hα data possibly contains an additional period of 80.4 days that we, however, believe is either spurious and was introduced due to seasonal amplitude variations or stems from a time-variable circumbinary mass flow. The rotational periods result in stellar radii of 14.3 ± 4.6 R⊙ and 8.5 ± 0.5 R⊙ for the cool and hot component, respectively, and are in good agreement with previously published radii based on radiometric and interferometric techniques. The long-period eclipsing binary Aurigae served as our check star, and we detected complex light variations outside of eclipse of up to 0m.15 in H α and 0m.20 in Hβ. Our frequency analysis suggests the existence of at least three significant periods of 132, 89, and 73 days. One of our comparison stars (HD 33167, F5V) was discovered to be a very-low amplitude variable with a period of 2.6360 ± 0.0055 days.

Antisolar differential rotation with surface lithium enrichment on the single K-giant V1192 Orionis

Context: Stars with about 1

Doppler imaging of stellar surface structure - XXII. Time-series mapping of the young rapid rotator LQ Hydrae

-

First time-series optical photometry from Antarctica. sIRAIT monitoring of the RS CVn binary V841 Ce

2 solar masses at the red giant branch (RGB) represent an intriguing period of stellar evolution, i.e. when the convective envelope interacts with the fast-rotating core. During these mixing episodes freshly synthesized lithium can come up to the stellar surface along with high angular momentum material. This high angular momentum may alter the surface rotation pattern. Aims: The single rapidly rotating K-giant V1192 Ori is revisited to determine its surface differential rotation, lithium abundance, and basic stellar properties such as a precise rotation period. The aim is to independently verify the antisolar differential rotation of the star and possibly find a connection to the surface lithium abundance. Methods: We applied time-series Doppler imaging to a new multi-epoch data set. Altogether we reconstructed 11 Doppler images from spectroscopic data collected with the STELLA robotic telescope between 2007--2016. We used our inversion code iMap to reconstruct all stellar surface maps. We extracted the differential rotation from these images by tracing systematic spot migration as a function of stellar latitude from consecutive image cross-correlations. Results: The position of V1192 Ori in the Hertzsprung-Russell diagram suggests that the star is in the helium core-burning phase just leaving the RGB bump. We measure

Star Spot Patterns on Young Stars: Theoretical Approach (CD-ROM Directory: contribs/granzer2)

A({\rm Li})_{\rm NLTE}=1.27