94 research outputs found
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
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
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&
General Model for Light Curves of Chromospherically Active Binary Stars
The starspots on the surface of many chromospherically active binary stars concentrate on long-lived active longitudes separated by 180°. Shifts in activity between these two longitudes, the âflip-flopâ events, have been observed in single stars like FK Comae and binary stars like Ï Geminorum. Recently, interferometry has revealed that ellipticity may at least partly explain the flip-flop events in Ï Geminorum. This idea was supported by the double-peaked shape of the long-term mean light curve of this star. Here we show that the long-term mean light curves of 14 chromospherically active binaries follow a general model that explains the connection between orbital motion, changes in starspot distribution, ellipticity, and flip-flop events. Surface differential rotation is probably weak in these stars, because the interference of two constant period waves may explain the observed light curve changes. These two constant periods are the active longitude period ##IMG## [http://ej.iop.org/images/0004-637X/838/2/122/apjaa65cbieqn1.gif] and the orbital period ##IMG## [http://ej.iop.org/images/0004-637X/838/2/122/apjaa65cbieqn2.gif] . We also show how to apply the same model to single stars, where only the value of P act is known. Finally, we present a tentative interference hypothesis about the origin of magnetic fields in all spectral types of stars.Peer reviewe
Searching for Planets in the Hyades III: The Quest for Short-Period Planets
We have been using the Keck I High Resolution Spectrograph (HIRES) to search
for planetary companions in the Hyades cluster. We selected four stars from
this sample which showed significant radial velocity variability on short
timescales to search for short-period planetary companions. The radial
velocities of these four stars were monitored regularly with the Hobby Eberly
Telescope (HET) for approximately two months, while sparse data were also taken
over ~4 months: we also obtained near-simultaneous photometric observations
with one of the automatic photoelectric telescopes at Fairborn Observatory. For
three of the stars, we detect photometric variability with the same period
present in the radial velocity (rv) measurements, compatible with the expected
rotation rates for Hyades members. The fourth star continues to show rv
variations and minimal photometric variability but with no significant
periodicity. This study shows that for the three stars with periodic behavior,
a significant portion of the rv fluctuations are likely due primarily to
magnetic activity modulated by stellar rotation rather than planetary
companions. Using simple models for the rv perturbations arising from spot and
plage, we demonstrate that both are likely to contribute to the observed rv
variations. Thus, simultaneous monitoring of photometric (photospheric) and
spectroscopic (chromospheric) variations is essential for identifying the cause
of Doppler shifted absorption lines in more active stars.Comment: 25 pages, 5 figures, accepted for publication by A
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