45 research outputs found
The He-shell flash in action: T Ursae Minoris revisited
We present an updated and improved description of the light curve behaviour
of T Ursae Minoris, which is a Mira star with the strongest period change (the
present rate is an amazing -3.8+/-0.4 days/year corresponding to a relative
decrease of about 1% per cycle). Ninety years of visual data were collected
from all available databases and the resulting, almost uninterrupted light
curve was analysed with the O-C diagram, Fourier analysis and various
time-frequency methods. The Choi-Williams and Zhao-Atlas-Marks distributions
gave the clearest image of frequency and light curve shape variations. A
decrease of the intensity average of the light curve was also found, which is
in accordance with a period-luminosity relation for Mira stars. We predict the
star will finish its period decrease in the meaningfully near future (c.c. 5 to
30 years) and strongly suggest to closely follow the star's variations
(photometric, as well as spectroscopic) during this period.Comment: 6 pages, 7 figures, accepted for publication in A&
Period-doubling events in the light curve of R Cygni: evidence for chaotic behaviour
A detailed analysis of the century long visual light curve of the long-period
Mira star R Cygni is presented and discussed. The data were collected from the
publicly available databases of the AFOEV, the BAAVSS and the VSOLJ. The full
light curve consists of 26655 individual points obtained between 1901 and 2001.
The light curve and its periodicity were analysed with help of the O-C diagram,
Fourier analysis and time-frequency analysis. The results demonstrate the
limitations of these linear methods. The next step was to investigate the
possible presence of low-dimensional chaos in the light curve. For this, a
smoothed and noise-filtered signal was created from the averaged data and with
help of time delay embedding, we have tried to reconstruct the attractor of the
system. The main result is that R Cygni shows such period-doubling events that
can be interpreted as caused by a repetitive bifurcation of the chaotic
attractor between a period 2T orbit and chaos. The switch between these two
states occurs in a certain compact region of the phase space, where the light
curve is characterized by ~1500-days long transients. The Lyapunov spectrum was
computed for various embedding parameters confirming the chaotic attractor,
although the exponents suffer from quite high uncertainty because of the
applied approximation. Finally, the light curve is compared with a simple one
zone model generated by a third-order differential equation which exhibits
well-expressed period-doubling bifurcation. The strong resemblance is another
argument for chaotic behaviour. Further studies should address the problem of
global flow reconstruction, including the determination of the accurate
Lyapunov exponents and dimension.Comment: 13 pages, 14 figures, accepted for publication in A&A (some figures
are of reduced quality
Variability of M giant stars based on Kepler photometry: general characteristics
M giants are among the longest-period pulsating stars which is why their
studies were traditionally restricted to analyses of low-precision visual
observations, and more recently, accurate ground-based data. Here we present an
overview of M giant variability on a wide range of time-scales (hours to
years), based on analysis of thirteen quarters of Kepler long-cadence
observations (one point per every 29.4 minutes), with a total time-span of over
1000 days. About two-thirds of the sample stars have been selected from the
ASAS-North survey of the Kepler field, with the rest supplemented from a
randomly chosen M giant control sample.
We first describe the correction of the light curves from different quarters,
which was found to be essential. We use Fourier analysis to calculate multiple
frequencies for all stars in the sample. Over 50 stars show a relatively strong
signal with a period equal to the Kepler-year and a characteristic phase
dependence across the whole field-of-view. We interpret this as a so far
unidentified systematic effect in the Kepler data. We discuss the presence of
regular patterns in the distribution of multiple periodicities and amplitudes.
In the period-amplitude plane we find that it is possible to distinguish
between solar-like oscillations and larger amplitude pulsations which are
characteristic for Mira/SR stars. This may indicate the region of the
transition between two types of oscillations as we move upward along the giant
branch.Comment: 12 pages, 13 figures, accepted for publication in MNRAS. The
normalized light curves are available upon reques
A photometric monitoring of bright high-amplitude delta Scuti stars. II. Period updates for seven stars
We present new photometric data for seven high-amplitude delta Scuti stars.
The observations were acquired between 1996 and 2002, mostly in the Johnson
photometric system. For one star (GW UMa), our observations are the first since
the discovery of its pulsational nature from the Hipparcos data.The primary
goal of this project was to update our knowledge on the period variations of
the target stars. For this, we have collected all available photometric
observations from the literature and constructed decades-long O-C diagrams of
the stars. This traditional method is useful because of the single-periodic
nature of the light variations. Text-book examples of slow period evolution (XX
Cyg, DY Her, DY Peg) and cyclic period changes due to light-time effect (LITE)
in a binary system (SZ Lyn) are updated with the new observations. For YZ Boo,
we find a period decrease instead of increase. The previously suggested
LITE-solution of BE Lyn (Kiss & Szatmary 1995) is not supported with the new
O-C diagram. Instead of that, we suspect the presence of transient light curve
shape variations mimicking small period changes.Comment: 11 pages, 15 figures, accepted for publication in A&
Testing SNe Ia distance measurement methods with SN 2011fe
The nearby, bright, almost completely unreddened Type Ia supernova 2011fe in
M101 provides a unique opportunity to test both the precision and the accuracy
of the extragalactic distances derived from SNe Ia light curve fitters. We
apply the current, public versions of the independent light curve fitting codes
MLCS2k2 and SALT2 to compute the distance modulus of SN 2011fe from
high-precision, multi-color (BVRI) light curves. The results from the two
fitting codes confirm that 2011fe is a "normal" (not peculiar) and only
slightly reddened SN Ia. New unreddened distance moduli are derived as 29.21
+/- 0.07 mag (D ~ 6.95 +/- 0.23$ Mpc, MLCS2k2), and 29.05 +/- 0.07 mag (6.46
+/- 0.21 Mpc, SALT2). Despite the very good fitting quality achieved with both
light curve fitters, the resulting distance moduli are inconsistent by 2 sigma.
Both are marginally consistent (at ~1 sigma) with the HST Key Project distance
modulus for M101. The SALT2 distance is in good agreement with the recently
revised Cepheid- and TRGB-distance to M101. Averaging all SN- and Cepheid-based
estimates, the absolute distance to M101 is ~6.6 +/- 0.5 Mpc.Comment: 8 pages, 7 figures, accepted for publication in A&