20 research outputs found
Spot activity of the RS CVn star {\sigma} Geminorum
We model the photometry of RS CVn star Geminorum to obtain new
information on the changes of the surface starspot distribution, i.e., activity
cycles, differential rotation and active longitudes. We use the previously
published Continuous Periods Search-method (CPS) to analyse V-band differential
photometry obtained between the years 1987 and 2010 with the T3 0.4 m Automated
Telescope at the Fairborn Observatory. The CPS-method divides data into short
subsets and then models the light curves with Fourier-models of variable orders
and provides estimates of the mean magnitude, amplitude, period and light curve
minima. These light curve parameters are then analysed for signs of activity
cycles, differential rotation and active longitudes. We confirm the presence of
two previously found stable active longitudes, synchronised with the orbital
period d and find eight events where the active longitudes
are disrupted. The epochs of the primary light curve minima rotate with a
shorter period d than the orbital motion. If the
variations in the photometric rotation period were to be caused by differential
rotation, this would give a differential rotation coefficient of . The presence of two slightly different periods of active regions may
indicate a superposition of two dynamo modes, one stationary in the orbital
frame and the other one propagating in the azimuthal direction. Our estimate of
the differential rotation is much higher than previous results. However,
simulations show that this can be caused by insufficient sampling in our data.Comment: 10 pages, 6 figures. Submitted to A&
The continuous period search method and its application to the young solar analogue HD 116956
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). 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. 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 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.Comment: 14 pages, 11 figures, 3 table
Flip-flops of FK Comae Berenices
Context.FK Coma
Did the ancient egyptians record the period of the eclipsing binary Algol - the Raging one?
The eclipses in binary stars give precise information of orbital period
changes. Goodricke discovered the 2.867 days period in the eclipses of Algol in
the year 1783. The irregular orbital period changes of this longest known
eclipsing binary continue to puzzle astronomers. The mass transfer between the
two members of this binary should cause a long-term increase of the orbital
period, but observations over two centuries have not confirmed this effect.
Here, we present evidence indicating that the period of Algol was 2.850 days
three millenia ago. For religious reasons, the ancient Egyptians have recorded
this period into the Cairo Calendar, which describes the repetitive changes of
the Raging one. Cairo Calendar may be the oldest preserved historical document
of the discovery of a variable star.Comment: 26 pages, 5 figures, 11 table
Time-series analysis of long-term photometry of BM Canum Venaticorum
Long-term photometry is commonly used to monitor chromospheric activity of late-type stars. We study standard Johnson differential V photometry of the RS CVn binary BM Canum Venaticorum (BM CVn) spanning over a quarter of a century. Our main aims are to determine the activity cycles, the rate of surface differential rotation, and the rotation period of the active longitudes of BM CVn. The continuous period search (CPS) algorithm is applied to the photometry. The changes of the mean and amplitude of the light curves are used to search for activity cycles. The rotation period changes give an estimate of the rate of surface differential rotation. The Kuiper method is applied to the epochs of the primary and secondary minima to search for active longitudes. The photometry reveals the presence of a stable mean light curve (MLC) connected to the orbital period P-orb=20.(d)6252 of this binary. We remove this MLC from the original V magnitudes, which gives us the corrected V magnitudes. These two samples of Vand Vdata are analyzed separately with CPS. The fraction of unreliable CPS models decreases when the MLC is removed. The same significant activity cycle of approximately 12.5 years is detected in both V and V samples. The estimate for the surface differential rotation coefficient, k >= 0.10, is the same for both samples, but the number of unrealistic period estimates decreases after removing the MLC. The same active longitude period of P-al=20.(d)511 +/- 0.(d)005 is detected in the V and V magnitudes. This long-term regularity in the epochs of primary and secondary minima of the light curves is not caused by the MLC. On the contrary, the MLC hampers the detection of active longitudes.Peer reviewe
Detection of real periodicity in the terrestrial impact crater record: quantity and quality requirements
Aims. To determine the quantity and quality requirements for the terrestrial impact crater data which
would allow reliable detection of real periodicity.
Methods. Artificial impact crater data samples of different size and accuracy are simulated. Erosion is considered, as well as the effect of the unknown ratio between periodic and aperiodic impacts.
The probabilities for detecting real and false periodicities are solved with the Rayleigh test from these simulated data.
Results. Reliable detection of real periodicity is currently impossible – unless all impacts on Earth have been periodic
Spot activity of
Aims. We investigate the spot activity of the young magnetically active main sequence star LQÂ Hy